Co-reporter:Guoqiang Zhang, Yue Li, Saide Tang, Rhett D. Thompson, and Lei Zhu
ACS Applied Materials & Interfaces March 22, 2017 Volume 9(Issue 11) pp:10106-10106
Publication Date(Web):February 28, 2017
DOI:10.1021/acsami.7b00095
Polymer/metallic particle nanocomposites or nanodielectrics can exhibit colossal dielectric constants with a relatively low dissipation factor under low electric fields and thus seem to be promising for high-energy density dielectric capacitors. To study this possibility, this work focused on the dielectric performance and loss mechanisms in polypropylene (PP)/aluminum nanoparticle (nAl NP) composites under high electric fields. Phosphonic acid-terminated poly(ethylene-co-1-butene) was grafted to the Al2O3 surface layer on the nAl NPs in order to achieve reasonable dispersion in the PP matrix. The dielectric breakdown study showed that the breakdown strength decreased to nearly 1/20 that of the neat PP film as the nAl content increased to 25.0 vol %. The leakage current study revealed three electronic conduction mechanisms in the PP/100 nm nAl nanocomposites, namely, ohmic conduction at low fields, hopping conduction at intermediate fields, and Fowler–Nordheim (FN) field electron emission above a critical field, depending on the filler content. Compared to the 100 nm nAl NPs, smaller (e.g., 18 nm) nAl NPs needed a much higher electric field to exhibit FN field electron emission. It was the FN electron tunneling that induced a substantial reduction in breakdown strength for the PP/nAl nanocomposites. Meanwhile, electron-tunneling injected space charges (electrons) from nAl NPs into the PP matrix, and internal electronic conduction led to significant dielectric nonlinearity at high poling fields. Although polymer/metallic NP composites are not suitable for high-field electric applications, they can be good candidates for electrical switches and quantum tunneling composites operated at relatively low electric fields.Keywords: aluminum nanoparticles; electron tunneling; field electron emission; nanodielectrics; polypropylene;
Co-reporter:Guoqiang Zhang, Saide Tang, Aixiang Li, and Lei Zhu
Langmuir June 27, 2017 Volume 33(Issue 25) pp:6353-6353
Publication Date(Web):June 5, 2017
DOI:10.1021/acs.langmuir.7b00573
Thermally stable metallic nanoparticles (MNPs) are highly desirable for the melt processing of polymer nanocomposites. However, due to the high surface energy penalty and decreased melting temperature, MNPs are easy to agglomerate and lose their unique properties if there is no protection or confinement layer. In this work, we report a facile and efficient way to synthesize thermally stable MNPs using core-cross-linked polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) reverse micelles as nanoreactors. From infrared results, gold, silver, and palladium ions exhibited distinctive coordination to the 4VP groups with varying chelation strengths. Compared to the non-cross-linked micelles, 1,4-dibromobutane (DBB)-cross-linking of the P4VP cores provided several advantages. First, it prevented severe swelling of the P4VP cores caused by the reducing agents and subsequent merger of swollen micelles. Second, the quaternized P4VP with hydrophilicity enhanced the uptake speed of precursor metal ions into the cores. Third, the cross-linked cores greatly stabilized the MNPs against the high-temperature environment (e.g., 110 °C for 40 h in toluene). In addition, the solubility of the reducing agents also played an important role. Anhydrous hydrazine could swell the P4VP cores and concentric core–shell particle morphology was obtained. On the contrary, triethylsilane could not swell the P4VP cores and thus eccentric core–shell particle morphology was observed. Only the concentric core–shell MNPs exhibited good thermal stability, whereas the eccentric core–shell MNPs did not. This work suggested that these thermally stable MNPs could be good candidates for the melt processing of functional polymer nanocomposites.
Co-reporter:Zhongbo Zhang, Morton H. Litt, and Lei Zhu
Macromolecules December 12, 2017 Volume 50(Issue 23) pp:9360-9360
Publication Date(Web):December 1, 2017
DOI:10.1021/acs.macromol.7b02243
High dielectric constant polymers, exhibiting relaxor ferroelectric (RFE) behaviors (i.e., slim single and double hysteresis loops, SHLs and DHLs), are attractive for high energy density and low loss dielectric applications. Utilizing the principle of nanosized ferroelectric domains (nanodomains), this study has designed and developed novel RFE-like polyamides (PAs) based on 11-aminoundecanoic acid, 12-aminododecanoic acid, and N-methyl-11-aminoundecanoic acid (NM11) as an alternative to the high-cost and difficult-to-synthesize poly(vinylidene fluoride) (PVDF)-based RFE polymers. In the first attempt, quenched and stretched (QS) PA(11-co-12) copolymers exhibited enhanced ferroelectricity as compared with either nylon-11 or nylon-12. Although relatively narrow hysteresis loops could be achieved at 75 °C, the hydrogen-bonding interaction was not weak enough to induce nanodomains in the nylon copolymers. To further reduce the hydrogen-bonding interaction and achieve nanodomains, a PA(11-co-12-co-NM11) 30/60/10 (molar ratio) terpolymer (terPA-NCH3) was synthesized. The NCH3 groups were expected to participate in the isomorphic crystals, blocking the formation of hydrogen bonds and inducing chain twists in the mesophase. Indeed, the RFE-like behavior with slim SHLs and high dielectric constant (60–70) was successfully achieved for the QS terPA-NCH3 at high temperatures (>75 °C). Pathways to achieve RFE-like behavior for nylon-based polymers are discussed and compared with those for PVDF-based polymers. The knowledge obtained from this study can inspire potential applications for nylon polymers in advanced electrical and power applications.
Co-reporter:Zhongbo Zhang, Morton H. Litt, and Lei Zhu
Macromolecules August 8, 2017 Volume 50(Issue 15) pp:5816-5816
Publication Date(Web):July 20, 2017
DOI:10.1021/acs.macromol.7b01137
Novel ferroelectric properties, such as slim double and single hysteresis loop (DHL and SHL) behaviors, are attractive for high energy density and low loss dielectric applications. In this study, temperature-dependent ferroelectric behavior was studied for mesomorphic even-numbered nylons (i.e., nylon-12 and nylon-6) using electric displacement–electric field (D–E) loop measurements. Upon raising the temperature from room temperature to 100 °C, the D–E loops became increasingly narrower, finally leading to slim DHLs with significantly enhanced apparent dielectric constants (i.e., ∼30 and ∼60) and small remanent polarizations (i.e., 3.5 and 8.2 mC/m2) for quenched and stretched nylon-12 and nylon-6, respectively. Combining wide-angle X-ray diffraction and infrared studies, changes in the mesophases and orientation of hydrogen-bonded amide groups after electric poling were used to unravel the structure–ferroelectric property relationship for the even-numbered nylons. At 100 °C, the quenched and stretched nylon-12 and nylon-6 films exhibited a paraelectric mesophase with twisted chain conformation and disordered hydrogen bonds. Upon high field poling (>100 MV/m), transient nanodomains could be generated with additional twists in the main chain. The observed DHL behavior was attributed to the electric-field-induced reversible transitions between the paraelectric (less twisted chains) and ferroelectric (more twisted chains) states in the mesomorphic crystals of even-numbered nylons. The knowledge gained from this study can inspire potential applications of n-nylons for electric energy storage, e.g., high temperature and high energy density multilayer polymer film capacitors.
Co-reporter:Wenchen Yan, Mingwang Pan, Jinfeng Yuan, Gang Liu, Lixuan Cui, Guanglin Zhang, Lei Zhu
Polymer 2017 Volume 122(Volume 122) pp:
Publication Date(Web):28 July 2017
DOI:10.1016/j.polymer.2017.06.055
•PVDF/P4VP Janus particles were prepared via soap-free seeded emulsion polymerization.•P(St-co-AA) core particles were synthesized by soap-free emulsion polymerization.•RCPCs were obtained via hierarchical self-assembly of PVDF/P4VP Janus particles and P(St-co-AA) cores.Nonspherical poly(vinylidene fluoride)/poly(4-vinyl pyridine) (PVDF/P4VP) composite particles were prepared via a facile one-step soap-free seeded emulsion polymerization. A moderate amount of ammonia solution was introduced into the polymerization system to make the composite latex particles uniform and stable, possibly due to the prevention of P4VP nucleation in the aqueous phase. Depending upon the stirring speed, the composite particles exhibited either the dumbbell-like shape (at a low speed) or the snowman-like shape (at a high speed). The size of the P4VP bulges in the composite particles could be easily adjusted by controlling the feed ratio of 4VP/PVDF. Utilizing the strong interactions between pyridine and carboxylic acid units, self-assembly of snowman-like PVDF/P4VP composite particles with carboxylated PS particles induced the formation of raspberry-like colloidal particle clusters (RCPCs) with PVDF bulges protruding outward. This study provides an alternative way to prepare hierarchical RCPCs.Download high-res image (199KB)Download full-size image
Co-reporter:Xinyue Chen;Jung-Kai Tseng;Imre Treufeld;Matthew Mackey;Donald E. Schuele;Ruipeng Li;Masafumi Fukuto;Eric Baer
Journal of Materials Chemistry C 2017 vol. 5(Issue 39) pp:10417-10426
Publication Date(Web):2017/10/12
DOI:10.1039/C7TC03653A
With the recent advancement of power electronics, polymer film capacitors have become increasingly important. However, the low temperature rating (up to 85 °C) and low energy density (5 J cm−3 at breakdown) of state-of-the-art biaxially oriented polypropylene (BOPP) films have been limiting factors for advanced power electronics. Based on our recent work, multilayer films (MLFs), which consist of a high energy density polymer [e.g., poly(vinylidene fluoride) (PVDF)] and a high breakdown/low loss polymer [e.g., polycarbonate (PC)], have shown potential to achieve high energy density (13–17 J cm−3), enhanced breakdown strength, high temperature tolerance, and low loss simultaneously. In this study, the dielectric properties of PC/PVDF 50/50 32- and 256-layer (32L and 256L) films were investigated. The breakdown strength of the 32L film was as high as 800 MV m−1 at room temperature, as compared to 600 MV m−1 of PVDF and 750 MV m−1 of PC. The temperature rating of the 32L film reached 120 °C, higher than that of BOPP. In addition, it was observed that the 32L film with thicker PC layers exhibited a higher breakdown strength and a lower DC conductivity than the 256L film with thinner PC layers at elevated temperatures. These differences were attributed to the difference in the interfacial polarization of space charges, which was further verified by thermally stimulated depolarization current spectroscopy. From this study, we conclude that interfacial polarization endows MLFs with the desirable dielectric properties for next generation film capacitors.
Co-reporter:Guoqiang Zhang, Daniel Brannum, Daxuan Dong, Longxiang Tang, Elshad Allahyarov, Saide Tang, Karl Kodweis, Je-Kyun Lee, and Lei Zhu
Chemistry of Materials 2016 Volume 28(Issue 13) pp:4646
Publication Date(Web):June 15, 2016
DOI:10.1021/acs.chemmater.6b01383
Polymer/inorganic particle nanocomposites (or nanodielectrics) have attracted pronounced attention for electric energy storage applications, based on a hypothesis that polymer nanodielectrics could combine the high permittivity of nanoparticles and the high electrical breakdown strength of the polymer matrix for enhanced dielectric performance. Although higher discharged energy densities have been reported for numerous polymer nanodielectrics, the dielectric loss mechanisms, which are extremely important for ultimate applications, are rarely discussed. In this work, we intend to address the intrinsic dielectric loss mechanisms associated with polymer nanodielectrics using a model system comprised of 70 nm BaTiO3 nanoparticles (BT NPs) in an isotactic polypropylene (PP) matrix. The effect of space charge-induced interfacial polarization on dielectric losses was investigated using bipolar and unipolar electric displacement–electric field (D-E) loop tests. Since the bipolar D-E loops always exhibited greater nonlinearity than the unipolar loops, the dielectric loss was attributed to the internal AC conduction loss from space charges (e.g., electrons) in the BT NPs, including boundary layer and bulk conductions. To mitigate the internal conduction along the PP/BT interface, atomic layer deposition of a nanolayer (5 nm) of amorphous TiO2 was applied to the BT NPs. Due to a higher resistivity, the coated amorphous TiO2 effectively reduced the boundary layer conduction loss. Nonetheless, the bulk conduction loss in BT NPs still needed to be reduced. This study suggests that more insulating high permittivity NPs are demanded for polymer nanodielectrics to enhance the dielectric performance.
Co-reporter:Yue Li, Janet Ho, Jianchuan Wang, Zhong-Ming Li, Gan-Ji Zhong, and Lei Zhu
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 1) pp:455
Publication Date(Web):December 23, 2015
DOI:10.1021/acsami.5b09368
Understanding nonlinear dielectric behavior in polar polymers is crucial to their potential application as next generation high energy density and low loss dielectrics. In this work, we studied nonlinear dielectric properties of a biaxially oriented poly(vinylidene fluoride) (BOPVDF) film under both low and high electric fields. For fundamental nonlinear dielectric constants at low fields (<30 MV/m), Novocontrol high-voltage broadband dielectric spectroscopy (HVBDS) was accurate enough to measure up to the third harmonics. It was observed that the low-field dielectric nonlinearity for the BOPVDF disappeared above 10 Hz at room temperature, suggesting that the low-field dielectric nonlinearity originated from ionic migration of impurity ions rather than dipolar relaxation of the amorphous segments. Above the coercive field (EC ≈ 70 MV/m), bipolar electric displacement-electric field (D–E) loop tests were used to extract the nonlinear behavior for pure PVDF crystals, which had a clear origin of ferroelectric switching of polar crystalline dipoles and domains and nonpolar-to-polar (α → δ → β) phase transformations. By using HVBDS, it was observed that the ferroelectric switching of polar crystalline dipoles and domains in BOPVDF above the EC always took place between 20 and 500 Hz regardless of a broad range of temperature from −30 to 100 °C. This behavior was drastically different from that of the amorphous PVDF dipoles, which had a strong dependence on frequency over orders of magnitude.Keywords: dipolar relaxation; ferroelectric; high-voltage broadband dielectric spectroscopy; nonlinear dielectric property; poly(vinylidene fluoride)
Co-reporter:Kezhen Yin, Zheng Zhou, Donald E. Schuele, Mason Wolak, Lei Zhu, and Eric Baer
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 21) pp:13555-13566
Publication Date(Web):May 10, 2016
DOI:10.1021/acsami.6b01287
Recently, poly(vinylidene fluoride) (PVDF)-based multilayer films have demonstrated enhanced dielectric properties, combining high energy density and high dielectric breakdown strength from the component polymers. In this work, further enhanced dielectric properties were achieved through interface/interphase modulation and biaxial orientation for the poly(ethylene terephthalate)/poly(methyl methacrylate)/poly(vinylidene fluoride-co-hexafluoropropylene) [PET/PMMA/P(VDF-HFP)] three-component multilayer films. Because PMMA is miscible with P(VDF-HFP) and compatible with PET, the interfacial adhesion between PET and P(VDF-HFP) layers should be improved. Biaxial stretching of the as-extruded multilayer films induced formation of highly oriented fibrillar crystals in both P(VDF-HFP) and PET, resulting in improved dielectric properties with respect to the unstretched films. First, the parallel orientation of PVDF crystals reduced the dielectric loss from the αc relaxation in α crystals. Second, biaxial stretching constrained the amorphous phase in P(VDF-HFP) and thus the migrational loss from impurity ions was reduced. Third, biaxial stretching induced a significant amount of rigid amorphous phase in PET, further enhancing the breakdown strength of multilayer films. Due to the synergistic effects of improved interfacial adhesion and biaxial orientation, the PET/PMMA/P(VDF-HFP) 65-layer films with 8 vol % PMMA exhibited optimal dielectric properties with an energy density of 17.4 J/cm3 at breakdown and the lowest dielectric loss. These three-component multilayer films are promising for future high-energy-density film capacitor applications.
Co-reporter:Jinfeng Yuan;Weiting Zhao;Mingwang Pan
Macromolecular Rapid Communications 2016 Volume 37( Issue 15) pp:1282-1287
Publication Date(Web):
DOI:10.1002/marc.201600206
Co-reporter:Tara L. Fox;Saide Tang;Guoqiang Zhang;Jonathan M. Horton;Bin Zhao;Phoebe L. Stewart
Macromolecular Chemistry and Physics 2016 Volume 217( Issue 16) pp:1767-1776
Publication Date(Web):
DOI:10.1002/macp.201600143
Co-reporter:Zhongbo Zhang, Morton H. Litt, and Lei Zhu
Macromolecules 2016 Volume 49(Issue 8) pp:3070-3082
Publication Date(Web):April 12, 2016
DOI:10.1021/acs.macromol.5b02739
Over the past decades, it has been commonly considered that ferroelectricity is closely related to the polar crystalline structure of odd-numbered nylons, and even-numbered nylons should not exhibit ferroelectricity due to their nonpolar crystalline structures. In this work, we ask a fundamental question: Are odd-numbered nylons with polar crystalline structures prerequisites for ferroelectricity? Here, ferroelectric properties are reported for mesomorphic even-numbered nylons (nylons-12 and -6) quickly quenched from the melt, using electric displacement–electric field (D–E) hysteresis loop measurements. From X-ray diffraction and infrared studies, the structure of the mesophases in the quenched samples was considered to contain multiple twists in the chain conformation, resulting in enlarged interchain distance and dangling/weak hydrogen bonds. Upon high field electric poling, the mesophase structure enables dipolar switching of the dangling/weak hydrogen bonds, forming electric-field-induced ferroelectric domains with twisted chain conformations in the crystal. The domain sizes in even-numbered nylons should be smaller than those in odd-numbered nylons, and thus D–E hysteresis loops should be slimmer. This study shows that odd-numbered nylons and polar crystalline structures are not prerequisites for ferroelectricity in nylons. Instead, mesophases with enlarged interchain spacing and disordered hydrogen bonds are the key to ferroelectricity. The knowledge obtained from this study will help us design new nylons and nylon copolymers with defective crystalline structures for enhanced ferroelectric properties.
Co-reporter:Zhe Liu, Guoqiang Zhang, Saide Tang, Zhongbo Zhang, Hong Zhu, Lei Zhu
Polymer 2016 Volume 103() pp:73-82
Publication Date(Web):26 October 2016
DOI:10.1016/j.polymer.2016.09.036
•Sodium titanate nanotubes are embedded in a high dielectric constant (50–70) relaxor ferroelectric polymer matrix.•The percolation threshold is found to be 0.10–0.125 vol fraction.•Below the percolation threshold, both internal and external conduction account for dielectric losses.In this report, we studied dielectric loss mechanisms in a 1–3 type of polymer nanocomposites, i.e., nanofibers or nanotubes in a polymer matrix. Sodium titanate nanotubes (TiNTs, 8–10 nm outer diameter and 300–500 nm long), which were synthesized by the hydrothermal method, were used as the fillers. The polymer matrix was a high permittivity relaxor ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene). The nanocomposites were fabricated using solution-blending followed by hot-pressing above the melting temperature. Because of the high aspect ratio of TiNT fillers, the percolation threshold was found to be around 10–12.5 vol%. Below the percolation threshold, significant dielectric losses (both linear and nonlinear) were identified using bipolar and unipolar electric displacement – electric field (D-E) loop tests. For aggregated/percolated TiNTs, external electronic conduction was the major linear dielectric loss. For isolated TiNTs, internal electronic conduction contributed significantly to the nonlinear dielectric loss. From this study, we conclude that it is better to develop highly insulating nanofibers or nanotubes for the 1–3 type polymer nanodielectrics and the nanofillers content should be kept far below the percolation threshold in order to avoid significant dielectric losses.
Co-reporter:Juan Guo, Kaiyu Fu, Zhongbo Zhang, Lianyun Yang, Yu-Chen Huang, Ching-I. Huang, Lei Zhu, Daoyong Chen
Polymer 2016 Volume 105() pp:440-448
Publication Date(Web):22 November 2016
DOI:10.1016/j.polymer.2016.07.035
•10,12-pentacosadiynoic acid (DA) and melamine (MA) form non-covalently connected micelles (NCCMs) in aqueous solution.•Annealing of the DA/MA NCCMs at 65 °C leads to well-organized cocrystals.•Topochemical polymerization of the DA/MA cocrystals results in reversible thermochromism.In this study, we report a unique two-step approach to prepare pure and well-organized 10,12-pentacosadiynoic acid (DA)/melamine (MA) cocrystals, from which the topochemically polymerized product, poly(10,12-pentacosadiynoic acid) (PDA)/MA cocrystals, demonstrated reversible thermochromism (note that PDA in its pure crystalline form is thermochromically irreversible). In the first step, co-assembling DA and MA was achieve in a selective solvent for MA (i.e., water) using the non-covalently connected micelle method. In the second step, annealing the DA/MA co-assembly at a temperature higher than the melting point of pure DA crystals (62 °C) but below that of the cocrystals (87 °C) led to the formation of pure DA/MA cocrystals with a molar ratio of 2:1. The resultant lath-like DA/MA cocrystals were nano-sized and had an alternating DA/MA lamellar structure. Due to the strong intermolecular hydrogen-bonding and π-π stacking of MA molecules, the topochemically polymerized PDA/MA cocrystals were completely reversible in thermochromism. This is the first example that reversible thermochromism of PDA is realized through hydrogen-bonding assisted cocrystals with a small molecule.
Co-reporter:Lianyun Yang, Janet Ho, Elshad Allahyarov, Richard Mu, and Lei Zhu
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 36) pp:19894
Publication Date(Web):June 29, 2015
DOI:10.1021/acsami.5b02944
Poly(vinylidene fluoride) (PVDF)-based homopolymers and copolymers are attractive for a broad range of electroactive applications because of their high dielectric constants. Especially, biaxially oriented PVDF (BOPVDF) films exhibit a DC breakdown strength as high as that for biaxially oriented polypropylene films. In this work, we revealed the molecular origin of the high dielectric constant via study of a commercial BOPVDF film. By determination of the dielectric constant for the amorphous phase in BOPVDF, a high value of ca. 21–22 at 25 °C was obtained, and a three-phase (i.e., lamellar crystal/oriented interphase/amorphous region) semicrystalline model was proposed to explain this result. Meanwhile, electronic conduction mechanisms in BOPVDF under high electric fields and elevated temperatures were investigated by thermally stimulated depolarization current (TSDC) spectroscopy and leakage current studies. Space charge injection from metal electrodes was identified as a major factor for electronic conduction when BOPVDF was poled above 75 °C and 20 MV/m. In addition, when silver or aluminum were used as electrodes, new ions were generated from electrochemical reactions under high fields. Due to the electrochemical reactions between PVDF and the metal electrode, a question is raised for practical electrical applications using PVDF and its copolymers under high-field and high-temperature conditions. A potential method to prevent electrochemical degradation of PVDF is proposed in this study.Keywords: dielectric constant; electronic conduction; ionic polarization; poly(vinylidene fluoride); space charge injection; thermally stimulated depolarization current
Co-reporter:Junji Wei, Zhongbo Zhang, Jung-Kai Tseng, Imre Treufeld, Xiaobo Liu, Morton H. Litt, and Lei Zhu
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 9) pp:5248
Publication Date(Web):February 18, 2015
DOI:10.1021/am508488w
In this report, a dipolar glass polymer, poly(2-(methylsulfonyl)ethyl methacrylate) (PMSEMA), was synthesized by free radical polymerization of the corresponding methacrylate monomer. Due to the large dipole moment (4.25 D) and small size of the side-chain sulfone groups, PMSEMA exhibited a strong γ transition at a temperature as low as −110 °C at 1 Hz, about 220 °C below its glass transition temperature around 109 °C. Because of this strong γ dipole relaxation, the glassy PMSEMA sample exhibited a high dielectric constant of 11.4 and a low dissipation factor (tan δ) of 0.02 at 25 °C and 1 Hz. From an electric displacement-electric field (D-E) loop study, PMSEMA demonstrated a high discharge energy density of 4.54 J/cm3 at 283 MV/m, nearly 3 times that of an analogue polymer, poly(methyl methacrylate) (PMMA). However, the hysteresis loss was only 1/3–1/2 of that for PMMA. This study suggests that dipolar glass polymers with large dipole moments and small-sized dipolar side groups are promising candidates for high energy density and low loss dielectric applications.Keywords: broadband dielectric spectroscopy; dielectric constant; dipolar glass polymer; electric energy storge; orientational polarization; sub-Tg transition
Co-reporter:Saide Tang, Tara L. Fox, Ting-Ya Lo, Jonathan M. Horton, Rong-Ming Ho, Bin Zhao, Phoebe L. Stewart and Lei Zhu
Soft Matter 2015 vol. 11(Issue 27) pp:5501-5512
Publication Date(Web):26 May 2015
DOI:10.1039/C5SM00193E
Environmentally responsive self-assembly of nearly symmetric mixed poly(tert-butyl acrylate) (PtBA, 22.2 kDa)/polystyrene (PS, 23.4 kDa) brushes grafted onto 67 nm silica nanoparticles in selective homopolymer matrices [PtBA for the grafted PtBA chains and poly(cyclohexyl methacrylate) (PCHMA) for the grafted PS chains] was investigated using both conventional transmission electron microscopy (TEM) and electron tomography (i.e., 3D TEM). A variety of self-assembled phase morphologies were observed for the mixed brushes in selective polymer matrices with different molecular weights, and these can be explained by entropy-driven wet- and dry-brush theories. In a low molecular weight selective matrix, the wet-brush regime was formed with the miscible chains stretching out and the immiscible chains collapsing into isolated domains. In contrast, when the molecular weight of the selective matrix was higher than that of the compatible grafted polymer chains, the dry-brush regime was formed with the mixed brushes exhibiting the unperturbed morphology. In addition to the molecular weight, the size of nanoparticles (or the substrate curvature) was also observed to play an important role. For small particles (core size less than 50 nm), the wet brush-like morphology with a surface-tethered micellar structure was observed. Finally, the wet- and dry-brush regimes also significantly affected the dispersion of mixed brush particles in selective polymer matrices.
Co-reporter:Yue Li, Saide Tang, Ming-Wang Pan, Lei Zhu, Gan-Ji Zhong, and Zhong-Ming Li
Macromolecules 2015 Volume 48(Issue 23) pp:8565-8573
Publication Date(Web):November 23, 2015
DOI:10.1021/acs.macromol.5b01895
Manipulating polymorphism in extended chain-crystals (ECCs), which are commonly achieved by crystallization under high pressures, is important for enriching our understanding of basic polymer crystallization as well as for achieving high performance materials. In this study, the influence of high pressure and ion–dipole interaction on the polymorphism was investigated by comparing neat poly(vinylidene fluoride) (PVDF) and PVDF with 1 wt % cetyltrimethylammonium bromide (CTAB) nonisothermally crystallized from the melt at 210 °C. Under low pressures (≤10 MPa), γ folded-chain crystals (FCCs), rather than α FCCs, were obtained for PVDF/1 wt % CTAB because of the ion–dipole interaction. Under a moderate pressure (100 MPa), pure β FCCs were formed in PVDF/1 wt % CTAB, owing to the synergistic effect of both high pressure and ion–dipole interaction. Under high pressures (≥200 MPa), mixtures of β/γ FCCs and ECCs were obtained for PVDF/1 wt % CTAB. This was different from the neat PVDF, where mixtures of α FCCs and α/γ/β ECCs coexisted when the pressure was between 200 and 400 MPa. The formation mechanisms of various crystalline forms and FCCs versus ECCs during the nonisothermal crystallization are discussed using the T–P phase diagram for PVDF.
Co-reporter:David H. Wang;Brian A. Kurish;Imre Treufeld;Loon-Seng Tan
Journal of Polymer Science Part A: Polymer Chemistry 2015 Volume 53( Issue 3) pp:422-436
Publication Date(Web):
DOI:10.1002/pola.27445
ABSTRACT
Three new isomeric diamines containing three, oxy-linked benzonitriles (3BCN), one of which is asymmetric (meta, para, or m, p), are synthesized in a 3-step sequence. Polycondensation of these diamines and four common dianhydrides (6FDA, OPDA, BTDA, and PMDA) in N,N-dimethylacetamide via poly(amic acid) precursors and thermal curing at temperatures up to 300 °C lead to three series of tough, creasable polyimide (PI) films (tensile moduli = 1.63 − 2.86 GPa). Among these PIs, two PMDA-based PIs possess relatively high crystallinity and two OPDA-based PIs, low crystallinity, whereas all 6FDA- and BTDA-based PIs, and m,m-3BCN-OPDA-PI are amorphous, readily soluble in common polar aprotic solvents. Thermally stable and having high Tg (216 − 341 °C), these PIs lose 5% weight around 493–503 °C in air and 463–492 °C in nitrogen. Dielectric properties have been evaluated by broadband dielectric spectroscopy (BDS) and electric displacement-electric-field (D-E) loop measurements. D-E loop results show an increase in high temperature permittivity (at 190 °C/1 kHz) from 2.9 (for parent PI CP2 with no nitrile group) to as high as 4.9 for these PIs, while keeping their dielectric loss relatively low. Thus, an increase in dipole moment density by the presence of three neighboring CN per repeat unit can increase the overall permittivity, which could be further enhanced by sub-Tg mobility of para-phenylene linkages (BDS results). Published 2014. J. Polym. Sci., Part A: Polym. Chem. 2014 J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 422–436
Co-reporter:Tara L. Fox, Saide Tang, Jonathan M. Horton, Heather A. Holdaway, Bin Zhao, Lei Zhu, and Phoebe L. Stewart
Langmuir 2015 Volume 31(Issue 31) pp:8680-8688
Publication Date(Web):July 14, 2015
DOI:10.1021/acs.langmuir.5b01739
We present an in situ cryo-electron microscopy (cryoEM) study of mixed poly(acrylic acid) (PAA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles in organic and aqueous solvents. These organic–inorganic nanoparticles are predicted to be environmentally responsive and adopt distinct brush layer morphologies in different solvent environments. Although the self-assembled morphology of mixed PAA/PS brush-grafted particles has been studied previously in a dried state, no direct visualization of microphase separation was achieved in the solvent environment. CryoEM allows the sample to be imaged in situ, that is, in a frozen solvated state, at the resolution of a transmission electron microscope. Cryo-electron tomograms (cryoET) were generated for mixed PAA/PS brush-grafted nanoparticles in both N,N-dimethylformamide (DMF, a nonselective good solvent) and water (a selective solvent for PAA). Different nanostructures for the mixed brushes were observed in these two solvents. Overall, the brush layer is more compact in water, with a thickness of 18 nm, as compared with an extended layer of 27 nm in DMF. In DMF, mixed PAA/PS brushes are observed to form laterally separated microdomains with a ripple wavelength of 13.8 nm. Because of its lower grafting density than that of PAA, PS domains form more or less cylindrical or truncated cone-shaped domains in the PAA matrix. In water, PAA chains are found to form a more complete shell around the nanoparticle to maximize their interaction with water, whereas PS chains collapse into the core of surface-tethered micelles near the silica core. The cryoET results presented here confirm the predicted environmentally responsive nature of PAA/PS mixed brush-grafted nanoparticles. This experimental approach may be useful for the design of future mixed brush-grafted nanoparticles for nano- and biotechnology applications.
Co-reporter:Imre Treufeld, David H. Wang, Brian A. Kurish, Loon-Seng Tan and Lei Zhu
Journal of Materials Chemistry A 2014 vol. 2(Issue 48) pp:20683-20696
Publication Date(Web):09 Oct 2014
DOI:10.1039/C4TA03260H
A set of 12 new polyimides (PIs) with one or three polar CN dipoles directly attached to the aromatic diamine part were synthesized and their electric energy storage properties were studied using broadband dielectric spectroscopy (BDS) and electric displacement–electric field (D–E) loop measurements to determine their potential for high temperature film capacitors for aerospace applications. It was found that adding highly polar nitrile groups to the PI structure increased permittivity and thus electrical energy storage, especially at high temperatures, and 3 CN dipoles were better than 1 CN dipole. Below the glass transition temperature (Tg), a weak γ transition was observed around −100 °C and a broad β transition was observed between 100 and 150 °C. It was the β (i.e., precursor dipolar motion before long-range segmental motion, or glass transition), rather than the γ sub-Tg transition that substantially increased the permittivity of PIs. From the BDS results on PIs having 3 nitrile groups, the enhancement in permittivity from permanent dipoles decreased with dianhydride in the order of pyromellitic dianhydride (PMDA) > 4,4′-oxydiphthalic dianhydride (OPDA) > 1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) > 4,4′-benzophenonetetracarboxylic dianhydride (BTDA). Meanwhile, the increase in permittivity also decreased in the order of para–para, meta–para, and meta–meta linkage in the diamine, suggesting that the para–para linkage favored easier dipole rotation than the meta–meta linkage. From the D–E loop study, the PIs with a combination of PMDA dianhydride and a para–para linkage exhibited the highest discharged energy density and a reasonably low loss.
Co-reporter:Chunhui Bao, Saide Tang, Roger A. E. Wright, Ping Tang, Feng Qiu, Lei Zhu, and Bin Zhao
Macromolecules 2014 Volume 47(Issue 19) pp:6824-6835
Publication Date(Web):September 17, 2014
DOI:10.1021/ma501474m
We report a systematic study on the effect of molecular weight (MW) on microphase separation of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes grafted on 172 nm silica particles. The brushes were synthesized by sequential surface-initiated atom transfer radical polymerization (ATRP) of tBA at 75 °C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C from silica particles functionalized with an asymmetric difunctional initiator bearing an ATRP initiator and an NMRP alkoxyamine. The MWs of the two polymers in each sample were controlled to be similar to each other. A series of mixed brush samples with different average MWs, from 13.8 to 33.1 kDa, but comparable overall grafting densities were made, and their microphase separation was studied by transmission electron microscopy (TEM). The TEM samples were prepared by drop-casting the dispersions of mixed brush-grafted particles in CHCl3 and also in water (stabilized by a surfactant) and staining the brushes with RuO4. While CHCl3 is a good solvent for both PtBA and PS, making the brushes spread out on the carbon films of TEM grids, water is a nonselective poor solvent for the two polymers, causing the brushes to collapse uniformly on silica core. All samples exhibited lateral microphase separation, forming nearly bicontinuous rippled nanopatterns. The average ripple wavelength D increased with increasing the MW. For samples directly cast from chloroform, D scaled with MW0.70 in the studied MW range. For uniformly collapsed mixed brushes cast from water, D was proportional to MW0.56. The latter is close to the theoretical prediction of D ∼ MW0.5 for perfect Y-shaped brushes in the melt and in nonselective poor solvents. We further compared the phase separation of mixed PtBA/PS brushes grafted on silica particles and PtBA-b-PS diblock copolymers and found that the microphase separation of mixed brushes was weaker than that of diblock copolymers.
Co-reporter:Lianyun Yang, Brady A. Tyburski, Fabrice Domingues Dos Santos, Maya K. Endoh, Tadanori Koga, Daniel Huang, Yijun Wang, and Lei Zhu
Macromolecules 2014 Volume 47(Issue 22) pp:8119-8125
Publication Date(Web):November 13, 2014
DOI:10.1021/ma501852x
Co-reporter:Mingwang Pan, Lianyun Yang, Jianchuan Wang, Saide Tang, Ganji Zhong, Run Su, Mani K. Sen, Maya K. Endoh, Tadanori Koga, and Lei Zhu
Macromolecules 2014 Volume 47(Issue 8) pp:2632-2644
Publication Date(Web):April 11, 2014
DOI:10.1021/ma500249p
Recently, nanoconfined poly(vinylidene fluoride) (PVDF) and its random copolymers have attracted substantial attention in research. In addition to the drastic change in crystallization kinetics, major interest lies in crystal orientation and polymorphism in order to understand whether enhanced piezoelectric and ferroelectric properties can be achieved. For example, PVDF has been two-dimensionally (2D) confined in cylindrical nanopores of anodic aluminum oxide (AAO) with various pore diameters. The crystal c-axis becomes perpendicular to the cylinder axes, which favors dipole switching in the impregnated AAO membrane. However, no polar phases have been obtained from 2D confinement even down to 35 nm pores after melt recrystallization. In this work, we realized three-dimensionally (3D) confined crystallization of PVDF in 180 nm nanospheres by employing a facile emulsifier-free batch seeded emulsion polymerization to prepare PVDF@polystyrene (PS) core–shell particles. Influences of polymerization temperature, PVDF/styrene feed ratio, and polymerization time were systematically investigated to achieve completely wrapping of PS onto PVDF particles and avoid the formation of Janus particles. Exclusive confined PVDF crystallization was observed in these core–shell composite particles. Intriguingly, after melt recrystallization, polar β/γ phases, instead of the kinetically favored α phase, were resulted from 3D confinement in 180 nm nanospheres. We attributed this to the ultrafast crystallization rate during homogeneously nucleated PVDF crystallization. For the first time, we reported that 3D confinement was more effective than 2D confinement in producing polar crystalline phases for PVDF.
Co-reporter:Jing Wang;Fangxiao Guan;Li Cui;Jilin Pan;Qing Wang
Journal of Polymer Science Part B: Polymer Physics 2014 Volume 52( Issue 24) pp:1669-1680
Publication Date(Web):
DOI:10.1002/polb.23554
ABSTRACT
Polymer nanodielectrics have become attractive for practical applications such as electric energy storage and electromechanical actuation. However, to enhance the apparent dielectric constant of polymer nanodielectrics, a significant amount (>30 vol %) of spherical particles needs to be incorporated into the polymer matrix. As a consequence, melt-processing of polymer nanodielectrics into uniform thin films becomes difficult at such a high filler content, and electric breakdown strength will greatly decrease. In this work, we describe a three-phase composite approach towards high energy density nanodielectrics at low filling ratios. In this approach, a highly polarizable tetrameric metallophthalocyanine (TMPc) initiator is coated onto 68 nm BaTiO3 nanoparticles, from which poly(methyl methacrylate) (PMMA) brushes are grafted using atom transfer radical polymerization for the nanoparticles to be uniformly dispersed in a poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] matrix. For comparison, two-phase P(VDF-HFP)/BaTiO3 composites without the TMPc interfacial layer are also prepared. Owing to the high polarizability of the TMPc interface layer, the three-phase composite films exhibit higher dielectric constant and thus higher energy density than the two-phase composite films at volume-filling ratios below 5 vol %. Therefore, these high energy density three-phase nanodielectrics with a low filling ratio are promising for melt-processing into thin dielectric films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1669–1680
Co-reporter:Lei Zhu
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 21) pp:3677-3687
Publication Date(Web):October 9, 2014
DOI:10.1021/jz501831q
Polymer dielectrics having high dielectric constant, high temperature capability, and low loss are attractive for a broad range of applications such as film capacitors, gate dielectrics, artificial muscles, and electrocaloric cooling. Unfortunately, it is generally observed that higher polarization or dielectric constant tends to cause significantly enhanced dielectric loss. It is therefore highly desired that the fundamental physics of all types of polarization and loss mechanisms be thoroughly understood for dielectric polymers. In this Perspective, we intend to explore advantages and disadvantages for different types of polarization. Among a number of approaches, dipolar polarization is promising for high dielectric constant and low loss polymer dielectrics, if the dipolar relaxation peak can be pushed to above the gigahertz range. In particular, dipolar glass, paraelectric, and relaxor ferroelectric polymers are discussed for the dipolar polarization approach.
Co-reporter:Jung-Kai Tseng, Saide Tang, Zheng Zhou, Matthew Mackey, Joel M. Carr, Richard Mu, Lionel Flandin, Donald E. Schuele, Eric Baer, Lei Zhu
Polymer 2014 Volume 55(Issue 1) pp:8-14
Publication Date(Web):14 January 2014
DOI:10.1016/j.polymer.2013.11.042
In this study, we report layer thickness effect on the electrical insulation property of polysulfone (PSF)/poly(vinylidene fluoride) (PVDF) multilayer films having a fixed composition of PSF/PVDF = 30/70 (vol./vol.). Breakdown strength, dielectric lifetime, and electrical conductivity were studied for 32- and 256-layer films having various total film thicknesses. Among these films, those having thinner PVDF and PSF layers exhibited lower breakdown strength, shorter lifetime, and higher electrical conductivity than those having thicker layers. These experimental results were explained by Maxwell–Wagner–Sillars interfacial polarization due to contrasts in dielectric constant and electronic conductivity for PVDF and PSF, respectively. When both PVDF and PSF layers were thick (ca. > 100–200 nm), more space charges were available in PVDF and no electronic conduction was allowed for PSF. These accumulated interfacial charges could serve as effective traps for injected electrons from metal electrodes under high electric fields. As a result, reduced electrical conductivity and enhanced breakdown strength/dielectric lifetime properties were obtained. When both layers were thin (ca. < 100 nm), fewer space charges were available in PVDF and significant electronic conduction through PSF resulted in low interfacial polarization. Consequently, higher electrical conductivity, lower breakdown strength, and shorter lifetime were observed. These results provide us insights into potential physics to enhance electrical insulation property of polymer films using a multilayered structure having large dielectric constant contrast.
Co-reporter:Lianyun Yang, Elshad Allahyarov, Fangxiao Guan, and Lei Zhu
Macromolecules 2013 Volume 46(Issue 24) pp:9698-9711
Publication Date(Web):December 11, 2013
DOI:10.1021/ma401660k
Recently, double hysteresis loop (DHL) behavior, which is advantageous for the high energy density and low loss dielectric application, was achieved in a poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)-graft-polystyrene [P(VDF-TrFE-CTFE)-g-PS(14%)] graft copolymer due to the nanoconfinement effect. In this work, we continued to investigate the crystal orientation and temperature effects on the DHL behavior of this graft copolymer. Based on the electric displacement–electric field (D–E) study, crystal orientation had a profound effect on its electrical behavior. For the nonoriented sample, dielectric instead of ferroelectric behavior was observed. After uniaxial stretching, DHLs gradually developed in the oriented films upon increasing the extension ratio. For a fully stretched film, the DHL behavior was stable below 75 °C but gradually disappeared above 100 °C due to enhanced dc conduction and impurity ion migrational loss at elevated temperatures. After subtracting the dc conduction, D–E hysteresis loops from the ion loss were determined for the poling cycles below 100 MV/m. The hysteresis loss from ion migration under an applied field was closely related to ion concentration and diffusion coefficient, which were determined by broadband dielectric spectroscopy. Both parameters were used in a theoretical calculation to obtain hysteresis loops from ion migrational loss. By fitting the theoretical loops with those after dc conduction subtraction, ion mobility was found to be dependent upon both poling field and temperature. This study provides a quantitative understanding of the effects of impurity ions and dc conduction on dielectric and ferroelectric properties of polymers at elevated temperatures.
Co-reporter:Saide Tang, Ting-Ya Lo, Jonathan M. Horton, Chunhui Bao, Ping Tang, Feng Qiu, Rong-Ming Ho, Bin Zhao, and Lei Zhu
Macromolecules 2013 Volume 46(Issue 16) pp:6575-6584
Publication Date(Web):August 9, 2013
DOI:10.1021/ma401264m
Three-dimensional (3D) morphology of hierarchically self-assembled mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles cast from chloroform on a carbon-coated transmission electron microscopy (TEM) grid was directly visualized using electron tomography (i.e., 3D TEM). The number-average molecular weights for PtBA and PS were 22.2 and 23.4 kDa, and their grafting densities were 0.51 and 0.34 chains/nm2, respectively. For a dense monolayer of mixed brush-grafted silica particles, a rippled phase structure was observed, and the monolayer consisted of protruded top portions from large nanoparticles and a bottom continuous film. In the top protruded part of large nanoparticles, isolated cylindrical PS microdomains were formed in the PtBA matrix due to the low PS volume fraction (i.e., 37%). The morphology in the lower continuous film was strongly influenced by interparticle interactions via mixed polymer brushes. As a result, bicontinuous nanostructures instead of isolated PS microdomains in the PtBA matrix were formed, even though the PS was a minor phase. In the continuous film, the top and bottom regions gave better microphase separation due to the larger interstitial spaces, and the microphase-separated pattern was dictated by the hexagonal packing of hairy silica particles. For isolated individual particles cast from chloroform, different morphology was observed because of the lack of interparticle interactions. Clear lateral microphase separation was observed only in the bottom part of the particle, likely because the mixed brush layer in the top portion was too thin. In the bottom part, isolated PS microdomains with a truncated wedge shape radiated out from the projected center of the individual particle. This is the first time that detailed 3D morphology was thoroughly characterized for self-assembled mixed brush-grafted nanoparticles cast from a nonselective good solvent.
Co-reporter:Juan Guo, Lianyun Yang, Lei Zhu, Daoyong Chen
Polymer 2013 Volume 54(Issue 2) pp:743-749
Publication Date(Web):24 January 2013
DOI:10.1016/j.polymer.2012.12.009
Metal ion detection using commercially available ionochromic polydiacetylenes has not been selective. In this work, we report selective detection of metal ions in small organic compound (SOC)-stabilized poly(10,12-pentacosadiynoic acid) (PDA) nanocrystals suspended in an aqueous solution. These PDA nanocrystals were topochemically polymerized using a 254-nm UV light from the corresponding SOC-stabilized DA nanocrystals prepared by a solvent/nonsolvent micellization method. In this detection, the stabilizing SOC could selectively regulate the intercalation of specific metal ions into the galleries of PDA nanocrystals. Whenever the specific metal ions were intercalated, a fast (<1 min) ionochromic transition was observed. In addition, different metal ion selectivity could be achieved by utilization of different SOCs. For example, selective detections of Pb2+ and Ag+ with different detection sensitivities were achieved for a PDA/glycine (Gly) nanocrystal suspension and selective detection of Zn2+ was found for a PDA/boric acid nanocrystal suspension. X-ray diffraction and Fourier transform infrared studies revealed that Pb2+ and Ag+ intercalated into the galleries of PDA nanocrystals and deprotonated most –COOH groups in the side chains of PDA, forming a strong ionic interaction. This strong ionic interaction effectively reduced the conjugation length in the PDA main chain, and therefore selectively induced a blue-to-red ionochromic transition.
Co-reporter:Lianyun Yang, Xinyu Li, Elshad Allahyarov, Philip L. Taylor, Q.M. Zhang, Lei Zhu
Polymer 2013 Volume 54(Issue 7) pp:1709-1728
Publication Date(Web):22 March 2013
DOI:10.1016/j.polymer.2013.01.035
In contrast to the comprehensive understanding of novel ferroelectric [i.e., relaxor ferroelectric (RFE) and antiferroelectric] behavior in ceramics, RFE and double-hysteresis-loop (DHL) behavior in crystalline ferroelectric polymers have only been studied in the past fifteen years. A number of applications such as electrostriction, electric energy storage, and electrocaloric cooling have been realized by utilizing these novel ferroelectric properties. Nonetheless, fundamental understanding behind these novel ferroelectric behaviors is still missing for polymers. In this feature article, we intend to unravel the basic physics via systematic studies of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based terpolymers, electron-beam (e-beam) irradiated P(VDF-TrFE) copolymers, and PVDF graft copolymers. It is found that both the crystal internal structure and the crystal–amorphous interaction are important for achieving the RFE and DHL behaviors. For the crystal internal structure effect, dipole switching with reduced friction and nanodomain formation by pinning the polymer chains are essential, and they can be achieved through crystal repeating-unit isomorphism (i.e., defect modification). Physical pinning [e.g., in P(VDF-TrFE)-based terpolymers] induces a reversible, electric field-induced RFE↔FE phase transition and thus the DHL behavior, whereas chemical pinning [e.g., in e-beam irradiated P(VDF-TrFE)] results in the RFE behavior. Finally, the crystal–amorphous interaction (or the nanoconfinement effect) results in a competition between the polarization and depolarization local fields. When the depolarization field becomes stronger than the polarization field, a DHL behavior is observed. Obviously, the physics for ferroelectric polymers is different from that for ceramics/liquid crystals and can be largely attributed to the long-chain nature of semicrystalline polymers. This understanding will help us to design new ferroelectric polymers with improved properties and/or better applications.Figure optionsDownload full-size imageDownload as PowerPoint slide
Co-reporter:Run Su, Jung-Kai Tseng, Mao-Sheng Lu, Minren Lin, Qiang Fu, Lei Zhu
Polymer 2012 Volume 53(Issue 3) pp:728-739
Publication Date(Web):2 February 2012
DOI:10.1016/j.polymer.2012.01.001
In this report, ferroelectric behaviors were investigated as a function of temperature and poling frequency for a uniaxially stretched poly(vinylidene fluoride-co-trifluoroethylene) 50/50 (mol./mol.) film. A variety of ferroelectric behaviors, including normal ferroelectric, antiferroelectric-like, and paraelectric behaviors, were observed by varying temperature, poling frequency, and poling electric field. Especially, the ferroelectric (FE) to paraelectric (PE) transition and ferroelectric behaviors of the high temperature PE phase were studied in detail by electric displacement-electric field loop measurements. At a high poling frequency (e.g., 1000 Hz) and 100 °C (above the Curie temperature at 64 °C), a paraelectric behavior was obtained due to the nucleation of electric field-induced FE nanodomains inside the PE phase matrix. These FE nanodomains were highly reversible and they quickly depolarized upon removal of the poling field. At an intermediate poling frequency (e.g., 10 Hz) and 100 °C, an antiferroelectric-like behavior was observed, which could be attributed to the competition between depolarization and polarization fields upon reverse poling. Finally, at a low poling frequency (e.g., 1 Hz) and 100 °C, a normal ferroelectric behavior with rectangular hysteresis loops was seen because the small, reversible FE domains had enough time to grow into large irreversible ones. The presence of electric field-induced FE domains in the PE matrix was proved by field dependent Fourier transform infrared study. On the basis of this study, understanding of the paraelectric behavior in polar crystalline polymers will help us design new materials to meet the requirements for high energy density and low loss dielectric applications.
Co-reporter:Ganji Zhong, Run Su, Lifeng Zhang, Ke Wang, Zhongming Li, Hao Fong, Lei Zhu
Polymer 2012 Volume 53(Issue 20) pp:4472-4480
Publication Date(Web):12 September 2012
DOI:10.1016/j.polymer.2012.08.014
In this study, electrospun immiscible blend fibers of poly(vinylidene fluoride) (PVDF)/polysulfone (PSF) were prepared. Due to the strong shearing during electrospinning, cocontinuous fibers of PVDF in the PSF matrix were obtained despite the minor composition of PVDF (5–30 wt.%) in the blend. After annealing these electrospun blend fibers above the melting temperature of PVDF (170 °C) and the glass transition temperature of PSF (185 °C), nanosized droplets (primarily 200–300 nm) of PVDF were developed inside the PSF matrix from the breaking up of PVDF nanofibers because of the Plateau–Rayleigh instability. Fractionated crystallization occurred in these PVDF nanodroplets with the heterogeneously nucleated crystallization in the range of 105–135 °C and the homogeneous nucleation at 55–60 °C. The mechanism of homogeneous nucleation was confirmed by the study of crystallization kinetics using differential scanning calorimetry. Only the nonpolar α-phase was observed by wide-angle X-ray diffraction despite of the homogeneously nucleated crystallization at a high supercooling in these PVDF nanodroplets. This study leads to a conclusion that nanoconfined crystallization at a moderate crystallization rate is less important than the local electric field to induce the ferroelectric phases of PVDF.
Co-reporter:Lei Zhu and Qing Wang
Macromolecules 2012 Volume 45(Issue 7) pp:2937-2954
Publication Date(Web):February 10, 2012
DOI:10.1021/ma2024057
The state-of-the-art polymer dielectrics have been limited to nonpolar polymers with relatively low energy density but ultralow dielectric losses for the past decades. With the fast development of power electronics in pulsed power and power conditioning applications, there is a need for next-generation dielectric capacitors in areas of high energy density/low loss and/or high temperature/low loss polymer dielectrics. Given limitations in further enhancing atomic and electronic polarizations for polymers, this Perspective focuses on a fundamental question: Can orientational polarization in polar polymers be utilized for high energy density and low loss dielectrics? Existing experimental and theoretical results have suggested the following perspectives. For amorphous polar polymers, high energy density and low loss can be achieved below their glass transition temperatures. For liquid crystalline side-chain polymers, dipole mobility is so high that they saturate at relatively low electric fields, and only limited electrical energy can be further stored after dipole saturation. Crystalline polar polymers are promising and can be divided into three categories: normal ferroelectric, paraelectric, and novel ferroelectric. For normal ferroelectric crystalline polymers, switching of a high spontaneous polarization results in a large hysteresis. To reduce the hysteresis, ultrafine crystallites or ferroelectric domains are desired to reduce the spontaneous polarization. For paraelectric crystalline polymers, dipoles have the potential to align in an external electric field. However, a high degree of dipole reversibility is required for the high energy density and low loss application. Novel ferroelectric behaviors include relaxor ferroelectric and antiferroelectric-like behaviors are highly desired because of their high degree of dipole reversibility. To achieve the relaxor ferroelectric behavior, structural defects such as bulky comonomers need to be introduced into the crystalline lattice to expand the lateral unit cell dimensions and speed up the mobility and reversibility of crystalline dipoles. So far, true antiferroelectric crystalline polymers have not yet been discovered. Nevertheless, the antiferroelectric-like behavior has been realized by reducing the compensation polarization via nanoconfinement. In the future, more research is needed to develop new paraelectric and novel ferroelectric polymers for high energy density and low loss dielectrics.
Co-reporter:Chunhui Bao, Saide Tang, Jonathan M. Horton, Xiaoming Jiang, Ping Tang, Feng Qiu, Lei Zhu, and Bin Zhao
Macromolecules 2012 Volume 45(Issue 19) pp:8027-8036
Publication Date(Web):September 17, 2012
DOI:10.1021/ma301300k
We report in this article a systematic study on the effect of overall grafting density (σoverall) on microphase separation of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes synthesized from asymmetric difunctional initiator (Y-initiator)-functionalized silica particles. The initiator particles were made by the immobilization of a triethoxysilane-terminated Y-initiator on the surface of 173 nm silica particles via an ammonia-catalyzed hydrolysis/condensation process in ethanol. Mixed PtBA/PS brushes were then grown from initiator particles by surface-initiated atom transfer radical polymerization of tBA at 75 °C and nitroxide-mediated radical polymerization of styrene at 120 °C. By changing the mass ratio of Y-initiator to silica particles in the initiator immobilization step, we found that the overall grafting density of mixed brushes can be systematically tuned while the individual grafting densities of two polymers remained comparable. A series of mixed PtBA/PS brushes with molecular weights of ∼23 kDa for both PtBA and PS and σoverall from 1.06 to 0.122 chains/nm2 were prepared. Their microphase separation behavior was studied by transmission electron microscopy after the samples were drop-cast from chloroform onto carbon-coated TEM grids, annealed with CHCl3 vapor, and stained with RuO4. CHCl3 is a good solvent for both grafted polymers. When the σoverall was 0.34 chains/nm2 and above, the mixed PtBA/PS brushes underwent lateral microphase separation, forming “rippled” nanostructures. The ripple wavelength (D) increased with decreasing σoverall; the normalized D scaled with σoverall–0.47 in the σoverall range of 1.06–0.54 chains/nm2. With further decreasing the grafting density, the phase separation became weaker, and no microphase separation was observed in the sample with σoverall of 0.122 chain/nm2.
Co-reporter:Matthew Mackey, Donald E. Schuele, Lei Zhu, Lionel Flandin, Mason A. Wolak, James S. Shirk, Anne Hiltner, and Eric Baer
Macromolecules 2012 Volume 45(Issue 4) pp:1954-1962
Publication Date(Web):February 3, 2012
DOI:10.1021/ma202267r
Micro/nanolayer coextrusion was used to fabricate polycarbonate (PC)/poly(vinylidene fluoride) (PVDF) layered films with significantly reduced dielectric losses while maintaining high energy density. The high-field polarization hysteresis was characterized for layered films as a function of PVDF layer thickness (6000 to 10 nm) and composition (10 to 70 vol % PVDF), and was found to decrease with decreasing layer thickness and PVDF content. To gain a mechanistic understanding of the layer thickness (or nanoconfinement) effect, wide-angle X-ray diffraction, polarized Fourier transform infrared spectroscopy, and broadband dielectric spectroscopy were employed. The results revealed that charge migration, instead of dipole flipping, was responsible for the hysteresis in multilayered films. The absence of PVDF dipole-flipping was attributed to the nonuniform electric field distribution in the layered structure, where the field in PVDF layers were calculated to be significantly lower than that in PC layers due to large contrast in dielectric constant (∼3 for PC versus ∼12 for PVDF). The charges were likely to be impurity ions in the form of catalyst residue or surfactants from suspension polymerization. The characteristics of the dielectric spectroscopy relaxation indicated that ions mostly existed in the PVDF layers, and PC/PVDF interfaces prevented them from entering adjacent layers. Therefore, as the layer thickness decreases to nanometer scales, the amount of ion movement, dielectric loss, and hysteresis were decreased. This study provides clear evidence of the nanoconfinement effect in multilayered films, which advantageously decreases the hysteresis loss.
Co-reporter:Fangxiao Guan;Lianyun Yang;Jing Wang;Bing Guan;Kuo Han;Qing Wang
Advanced Functional Materials 2011 Volume 21( Issue 16) pp:3176-3188
Publication Date(Web):
DOI:10.1002/adfm.201002015
Abstract
Dielectric polymer film capacitors having high energy density, low loss and fast discharge speed are highly desirable for compact and reliable electrical power systems. In this work, we study the confined ferroelectric properties in a series of poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-polystyrene [P(VDF-CTFE)-g-PS] graft copolymers, and their potential application as high energy density and low loss capacitor films. Thin films (ca. 20 μm) are prepared by different processing methods, namely, hot-pressing or solution-casting followed by mechanical stretching at elevated temperatures. After crystallization-induced microphase separation, PS side chains are segregated to the periphery of PVDF crystals, forming a confining interfacial layer. Due to the low polarizability of this confining PS-rich layer at the amorphous–crystalline interface, the compensation polarization is substantially decreased resulting in a novel confined ferroelectric behavior in these graft copolymers. Both dielectric and ferroelectric losses are significantly reduced at the expense of a moderate decrease in discharged energy density. Our study indicates that the best performance is achieved for a P(VDF-CTFE)-g-PS graft copolymer with 34 wt-% PS; a relatively high discharged energy density of approximately 10 J cm−3 at 600 MV m−1, a low dielectric loss (tanδ = 0.006 at 1 kHz), and a low hysteresis loop loss (17.6%) at 550 MV m−1.
Co-reporter:Mingwang Pan, Lianyun Yang, Bing Guan, Maosheng Lu, Ganji Zhong and Lei Zhu
Soft Matter 2011 vol. 7(Issue 23) pp:11187-11193
Publication Date(Web):14 Oct 2011
DOI:10.1039/C1SM06498C
Nonspherical poly(vinylidene fluoride)/polystyrene (PVDF/PS) composite latex particles were prepared via a facile emulsifier-free batch-seeded emulsion polymerization. It was found that the styrene (St)/PVDF feed ratio was a determining factor for the formation of well-defined Janus particles. Transmission electron microscopy study revealed that the immiscibility between PVDF and St-swollen PS (PS/St) was the driving force for the dewetting and nucleation of PS/St bulges on the PVDF particle surface. When the St/PVDF feed ratio was high, easy fusion of multiple PS/St bulges into one single bulge on the PVDF particle resulted in well-defined PVDF/PS Janus particles. When the St/PVDF feed ratio was low, it was difficult for multiple bulges to fuse together to form PVDF/PS Janus particles.
Co-reporter:Ganji Zhong, Ke Wang, Lifeng Zhang, Zhong-Ming Li, Hao Fong, Lei Zhu
Polymer 2011 Volume 52(Issue 24) pp:5397-5402
Publication Date(Web):10 November 2011
DOI:10.1016/j.polymer.2011.09.045
By utilizing electrospun blend fibers of polystyrene (PS) and poly(ethylene oxide) (PEO) with diameters in sub-microns, nanodroplets of the minor component (PEO) were obtained by annealing the blend fibers above the glass transition temperature (Tg) of the matrix polymer (PS), as a result of the Rayleigh-Plateau instability in the melt. However, direct thermal annealing of the PS/PEO blend fibers led to poor Rayleigh breakup of the PEO fibers in the PS matrix, and fractionated crystallization with both homogeneous and heterogeneous nucleation was observed, probably due to a broad size distribution of PEO particles. On the contrary, after confining the PS/PEO blend fibers with a high Tg polymer, poly(4-tert-butyl styrene) (P4tBS, Tg ∼ 143 °C), well-defined Rayleigh breakup of the PEO fiber was achieved by annealing the P4tBS-coated PS/PEO blend fibers at 150 °C. Consequently, exclusive homogeneously nucleated PEO crystallization was observed at −20 °C. This report could provide a universal method to achieve nano-sized droplets for the study of nanoconfinement effect by utilizing electrospun immiscible polymer blend fibers without addition of any compabitilizers.
Co-reporter:Ganji Zhong, Lifeng Zhang, Run Su, Ke Wang, Hao Fong, Lei Zhu
Polymer 2011 Volume 52(Issue 10) pp:2228-2237
Publication Date(Web):4 May 2011
DOI:10.1016/j.polymer.2011.03.024
Effects of electric poling, mechanical stretching, and dipolar interaction on the formation of ferroelectric (β and/or γ) phases in poly(vinylidene fluoride) (PVDF) have been studied in electrospun fibers of PVDF/polyacrylonitrile (PAN) and PVDF/polysulfone (PSF) blends with PVDF as the minor component, using wide-angle X-ray diffraction and Fourier transform infrared techniques. Experimental results of as-electrospun neat PVDF fibers (beaded vs. bead-free) showed that mechanical stretching during electrospinning, rather than electric poling, was effective to induce ferroelectric phases. For as-electrospun PVDF blend fibers with the non-polar PSF matrix, mechanical stretching during electrospinning again was capable of inducing some ferroelectric phases in addition to the major paraelectric (α) phase. However, after removing the mechanical stretching in a confined melt-recrystallization process, only the paraelectric phase was obtained. For as-electrospun PVDF blend fibers with the polar (or ferroelectric) PAN matrix, strong intermolecular interactions between polar PAN and PVDF played an important role in the ferroelectric phase formation in addition to the mechanical stretching effect during electrospinning. Even after the removal of mechanical stretching through the confined melt-recrystallization process, a significant amount of ferroelectric phases persisted. Comparing the ferroelectric phase formation between PVDF/PSF and PVDF/PAN blend fibers, we concluded that the local electric field–dipole interactions were the determining factor for the nucleation and growth of polar PVDF phases.
Co-reporter:Fangxiao Guan, Jing Wang, Lianyun Yang, Jung-Kai Tseng, Kuo Han, Qing Wang, and Lei Zhu
Macromolecules 2011 Volume 44(Issue 7) pp:2190-2199
Publication Date(Web):March 2, 2011
DOI:10.1021/ma102910v
An antiferroelectric-like polymer approach was proposed for high electric energy storage and low loss performance by using a novel confinement concept. A poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)-graft-polystyrene [P(VDF-TrFE-CTFE)-g-PS] graft copolymer with 14 wt % PS side chains was successfully synthesized. On the basis of the electric displacement−electric field loop study, a novel antiferroelectric-like behavior with extremely low remanent polarization was achieved in this graft copolymer even when the poling field reached as high as 400 MV/m. Compared with a P(VDF-TrFE) random copolymer having the same TrFE content, a similar discharged energy density but a much lower hysteresis loss was observed. This novel antiferroelectric-like behavior at high poling fields was explained by the confinement (or insulation) effect. After crystallization-induced microphase separation, PS side chains were segregated to the periphery of P(VDF-TrFE) crystals, forming a nanoscale interfacial confining (or insulation) layer. Because of the low polarizability of this confining layer, the compensation polarization at the amorphous−crystalline interface was reduced, and thus the local polarization field became weaker than the local depolarization field. Upon discharging, therefore, a fast dipole reversal and an antiferroelectric-like behavior were achieved even at high poling fields. This study will help us design new polar dielectric polymers for high electric energy storage and low loss applications.
Co-reporter:Weiqiang Cao and Lei Zhu
Macromolecules 2011 Volume 44(Issue 6) pp:1500-1512
Publication Date(Web):February 22, 2011
DOI:10.1021/ma1021242
We report the synthesis and functionalization of amphiphilic dendrimer-like star polymers (DLSPs) with a hydrophobic star-shaped poly(l-lactide) (PLLA) core and a hydrophilic poly(amidoamine) (PAMAM) dendron shell. First, carboxylic acid-functionalized PLLA star polymer was synthesized by ring-opening polymerization of l-lactide followed by functionalization with succinic anhydride. Second, 1-, 2-, and 3-generation PAMAM dendrons with a primary amine at the dendron root and benzyl ester protections at the periphery were prepared via a divergent method. By amide coupling between the carboxylic acid-terminated PLLA star polymer and six PAMAM dendrons, amphiphilic DLSPs were successfully synthesized. To enhance bioactivity and bioconjugation capability, the benzyl ester surface groups in these DLSPs were converted to carboxylic acid, primary amine, and triethylene glycol functional groups, respectively. Nuclear magnetic resonance spectroscopy and size-exclusion chromatography were used to confirm quantitative functionalization. These functional DLSPs exhibited a unique unimolecular micelle (14−28 nm) behavior in aqueous solution with a small amount of aggregation (205−344 nm), as studied by dynamic light scattering. In addition, they also exhibited large differences in thermal behaviors depending on the nature of different surface groups. Experimental results showed that these DLSPs had good solubility in aqueous solutions (ca. 10−25 mg/mL) and could greatly enhance the water solubility of hydrophobic drugs. Therefore, these amphiphilic DLSPs are promising candidates for controlled hydrophobic drug delivery.
Co-reporter:Fang-xiao Guan;Jing Wang;Ji-lin Pan;Qing Wang
Chinese Journal of Polymer Science 2011 Volume 29( Issue 1) pp:65-80
Publication Date(Web):2011 January
DOI:10.1007/s10118-010-1020-8
We studied cycle time (0.01–10 s with triangular input waves) and poling history (continuous versus fresh poling) dependent electric energy storage and discharge behaviors in poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] films using the electric displacement — the electric field (D–E) hysteresis loop measurements. Since the permanent dipoles in PVDF are orientational in nature, it is generally considered that both charging and discharging processes should be time and poling history dependent. Intriguingly, our experimental results showed that the charging process depended heavily on the cycle time and the prior poling history, and thus the D–E hysteresis loops had different shapes accordingly. However, the discharged energy density did not change no matter how the D–E loop shape varied due to different measurements. This experimental result could be explained in terms of reversible and irreversible polarizations. The reversible polarization could be charged and discharged fairly quickly (< 5 ms for each process), while the irreversible polarization depended heavily on the poling time and the prior poling history. This study suggests that it is only meaningful to compare the discharged energy density for PVDF and its copolymer films when different cycle times and poling histories are used.
Co-reporter:Chuilin Lai, Ganji Zhong, Zhongren Yue, Gui Chen, Lifeng Zhang, Ahmad Vakili, Ying Wang, Lei Zhu, Jie Liu, Hao Fong
Polymer 2011 Volume 52(Issue 2) pp:519-528
Publication Date(Web):21 January 2011
DOI:10.1016/j.polymer.2010.11.044
Electrospun polyacrylonitrile (PAN) copolymer nanofibers with diameters of ∼0.3 μm were prepared as highly aligned bundles. The as-electrospun nanofiber bundles were then stretched in steam at ∼100 °C into 2, 3, and 4 times of the original lengths. Subsequently, characterizations and evaluations were carried out to understand morphological, structural, and mechanical properties using SEM, 2D WAXD, polarized FT−IR, DSC, and mechanical tester; and the results were compared to those of conventional PAN copolymer microfibers. The study revealed that: (1) the macromolecules in as-electrospun nanofibers were loosely oriented along fiber axes; although such an orientation was not high, a small extent of stretching could effectively improve the orientation and increase the crystallinity; (2) most of macromolecules in the crystalline phase of as-electrospun and stretched nanofibers possessed the zig-zag conformation instead of the helical conformation; and (3) the post-spinning stretching process could substantially improve mechanical properties of the nanofiber bundles. To the best of our knowledge, this study represented the first successful attempt to stretch electrospun nanofibers; and we envisioned that the highly aligned and stretched electrospun PAN copolymer nanofibers could be an innovative type of precursor for the development of continuous nano-scale carbon fibers with superior mechanical strength.
Co-reporter:Weiqiang Cao, Jing Zhou, Alexander Mann, Yong Wang, and Lei Zhu
Biomacromolecules 2011 Volume 12(Issue 7) pp:
Publication Date(Web):May 28, 2011
DOI:10.1021/bm200487h
A folate-functionalized degradable amphiphilic dendrimer-like star polymer (FA-DLSP) with a well-defined poly(l-lactide) (PLLA) star polymer core and six hydrophilic polyester dendrons based on 2,2-bis(hydroxymethyl) propionic acid was successfully synthesized to be used as a nanoscale carrier for cancer cell-targeted drug delivery. This FA-DLSP hybrid formed unimolecular micelles in the aqueous solution with a mean particle size of ca. 15 nm as determined by dynamic light scattering and transmission electron microscopy. To study the feasibility of FA-DLSP micelles as a potential nanocarrier for targeted drug delivery, we encapsulated a hydrophobic anticancer drug, doxorubicin (DOX), in the hydrophobic core, and the loading content was determined by UV–vis analysis to be 4 wt %. The DOX-loaded FA-DLSP micelles demonstrated a sustained release of DOX due to the hydrophobic interaction between the polymer core and the drug molecules. The hydrolytic degradation in vitro was monitored by weight loss and proton nuclear magnetic resonance spectroscopy to gain insight into the degradation mechanism of the FA-DLSP micelles. It was found that the degradation was pH-dependent and started from the hydrophilic shell gradually to the hydrophobic core. Flow cytometry and confocal microscope studies revealed that the cellular binding of the FA-DLSP hybrid against human KB cells with overexpressed folate-receptors was about twice that of the neat DLSP (without FA). The in vitro cellular cytotoxicity indicated that the FA-DLSP micelles (without DOX) had good biocompatibility with KB cells, whereas DOX-loaded micelles exhibited a similar degree of cytotoxicity against KB cells as that of free DOX. These results clearly showed that the FA-DLSP unimolecular micelles could be a promising nanosize anticancer drug carrier with excellent targeting property.
Co-reporter:Jianjun Miao and Lei Zhu
Chemistry of Materials 2010 Volume 22(Issue 1) pp:197
Publication Date(Web):December 1, 2009
DOI:10.1021/cm902731u
Hydrogen bonding is a powerful driving force for the supramolecular self-assembly of discotic mesogens, and molecular shape also plays an important role in such systems. To study these effects, doubly discotic supermolecules have been synthesized by linking a meso-tetraphenylporphine-4,4′,4′′,4′′′-tetracarboxylic acid (Py) core with four triphenylenes (Tp) arms via either amide or ester bonds. The spacer length between the Py core and Tp disks was C6 and C10, and the alkyl arm length in the Tp disks was C5 and C12, respectively. Compared to the ester-linked Py(Tp)4 supermolecules, the amide-linked samples exhibited rich crystalline and liquid-crystalline phases, suggesting that the intracolumnar hydrogen-bonding among trans amide bonds was the primary driving force for the self-assembly. X-ray diffraction (XRD) was used to understand the supramolecular self-assembly of the amide-linked Py(Tp)4 doubly discotic supermolecules. When the spacer length was no shorter than or similar to the triphenylene alkyl arm length, a rectangular boardlike molecular shape was adopted and thus lamellar structures were obtained. When the spacer length was much shorter than the triphenylene alkyl arms, an ellipsoidal overall molecular shape resulted, and thus a regular columnar phase was obtained. From this study, we speculated that hydrogen-bond-induced microphase separation between moieties with different electron affinities in doubly discotic supermolecules may be useful for the practical applications of organic electronics.
Co-reporter:Jianjun Miao and Lei Zhu
Soft Matter 2010 vol. 6(Issue 9) pp:2072-2079
Publication Date(Web):29 Mar 2010
DOI:10.1039/B927347F
A series of doubly discotic supermolecules with four triphenylene (Tp) mesogens attached to a phthalocyanine (Pc) core via flexible alkyl spacers were designed and synthesized. The samples were denoted as Pc(Tp)4. Two alkyl chain lengths (C5 and C12) in the Tp arms and two spacer lengths (C6 and C10) were used to study columnar liquid crystalline (LC) self-assemblies in Pc(Tp)4 samples. The mesophase morphology was studied by polarized light microscopy and X-ray diffraction techniques. When the spacer length was relatively long (C10), a normal hexagonal columnar structure with the unit cell dimension close to those for the parent Tp or Pc liquid crystals was observed. The Pc columns packed either randomly or in a hexagonal superlattice in the Tp matrix. When the spacer length was short (C6), the Tp arms and the Pc core were tightly coupled together, and thus the whole supermolecules stacked together to form a super-LC column. A column-in-column rectangular mesophase with unit cell dimensions close to the size of the entire supermolecule was observed.
Co-reporter:Jianjun Miao
Chemistry – An Asian Journal 2010 Volume 5( Issue 7) pp:1634-1641
Publication Date(Web):
DOI:10.1002/asia.201000017
Abstract
A series of linear doubly discotic triad supermolecules based on a porphyrin (P) core and two triphenylene (Tp) arms linked by amide bonds are synthesized. The samples are denoted as P(Tp)2. Hydrogen bonding along the P stacks is the primary driving force for the supramolecular self-assembly of P(Tp)2 triad supermolecules. Meanwhile, the degree of coupling between P and Tp disks also plays an important role. For samples with the spacer lengths longer than or similar to the alkyl chain lengths in the Tp arms, P and Tp are decoupled to a large degree. This decoupling result in non-uniform tilt angles for P and Tp disks along both the a- and c-axes. Therefore, large unit cells are observed with eight P(Tp)2 supermolecules per cell. For a sample with the spacer length much shorter than the alkyl chains in the Tp arms, P and Tp are strongly coupled. Therefore, both P and Tp have uniform tilt angles along the a- and c-axes. A small unit cell is obtained with only one P(Tp)2 supermolecule per cell.
Co-reporter:Weiqiang Cao, Jing Zhou, Yong Wang, and Lei Zhu
Biomacromolecules 2010 Volume 11(Issue 12) pp:
Publication Date(Web):November 18, 2010
DOI:10.1021/bm101154r
By coupling a well-defined PLLA star polymer with six carboxylic acid-terminated polyester dendrons based on 2,2-bis(hydroxymethyl)propionic acid, a biodegradable dendrimer-like star polymer (DLSP) with multiple carboxylic acid groups at the outer surface was successfully synthesized. Conjugation of amine-functionalized folic acids (FA) onto the DLSP yielded a folate-DLSP hybrid as a carrier for targeted drug delivery. The chemical structures were proven by proton nuclear magnetic resonance and size exclusion chromatography. The DLSPs could form unimolecular micelles with a mean particle size of about 18 nm, as determined by dynamic light scattering. Flow cytometry and confocal microscope studies revealed that the cellular uptake of the folate-DLSP hybrid against KB cells (overexpressed folate-receptor) was much higher than that of the neat DLSP (without FA) due to the folate receptor-mediated binding.
Co-reporter:Jianjun Miao and Lei Zhu
The Journal of Physical Chemistry B 2010 Volume 114(Issue 5) pp:1879-1887
Publication Date(Web):January 19, 2010
DOI:10.1021/jp910053e
A series of liquid crystalline star supermolecules with polyhedral oligomeric silsesquioxane (POSS) as the central scaffold and eight triphenylenes (Tp) as the peripheral arms were synthesized via amidization reactions. The supermolecules were denoted as POSS(Tp)8. Six POSS(Tp)8 samples were prepared with two alkyl chain lengths in the Tp (C5 and C12) and three spacer lengths (C2, C6, and C10) between the POSS core and the Tp arms. Three samples with C5-Tp were amorphous because of too short alkyl chains in the Tp, while the other three samples with C12-Tp self-assembled into hierarchical liquid crystalline mesophases, as studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). When the spacer length was C2, a column-within-column super hexagonal columnar phase was observed, because the POSS core and the Tp arms were intimately coupled together. With increasing the spacer length to C6 and C10, respectively, the POSS core and Tp arms became gradually decoupled. Alternating POSS-Tp lamellar morphology with a rectangular columnar symmetry (by XRD) was observed by TEM for the POSS(Tp)8 sample with a C6-spacer. For the POSS(Tp)8 sample with a C10-spacer, an oblique columnar phase was determined by XRD, and inverted columnar morphology with four Tp columns forming a super column within the POSS/alkyl chain matrix was observed by TEM. This study suggested that molecular topology played an important role in the supramolecular self-assembly of star-shaped POSS(Tp)8 supermolecules.
Co-reporter:Fangxiao Guan, Jilin Pan, Jing Wang, Qing Wang and Lei Zhu
Macromolecules 2010 Volume 43(Issue 1) pp:384-392
Publication Date(Web):October 26, 2009
DOI:10.1021/ma901921h
By using different preparation and processing methods, poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF−HFP)] films with different crystal orientations were fabricated. Anisotropic dielectric properties and different electric energy storages were observed in these films. When the PVDF crystals in a film oriented with their c-axes perpendicular to the applied electric field, they exhibited large polarizability because the CF2 dipole moments were randomly distributed in a plane parallel to the electric field. As a result, high dielectric constant and high electric energy density were achieved. On the contrary, when the crystal c-axes in a film oriented parallel to the electric field (or the CF2 dipole moments perpendicular to the electric field), polarization became difficult. Consequently, low dielectric constant and low electric energy density resulted. The anisotropic polarizability was also displayed at high electric fields as evidenced by the difference in the remnant/maximum polarization and the dipole switching field for different crystal orientations. These results provide us a guidance to achieve optimal crystalline morphology in PVDF random copolymer films for high electric energy storage applications.
Co-reporter:Fangxiao Guan, Jing Wang, Jilin Pan, Qing Wang and Lei Zhu
Macromolecules 2010 Volume 43(Issue 16) pp:6739-6748
Publication Date(Web):July 23, 2010
DOI:10.1021/ma101062j
Poly(vinylidene fluoride) (PVDF) and poly(VDF-co-hexafluoropropylene) [P(VDF−HFP)] films having different polymorphisms and crystallite sizes but a similar crystal orientation (i.e., c-axes parallel to the film surface) were prepared by different film processing methods. Effects of polymorphism and crystallite size on the dipole reorientation behavior and electric energy storage/release were studied by electric displacement−electric field (D−E) loop measurements. Experimental results suggested that coupling interactions among ferroelectric domains, which could be adjusted by different polymorphisms and/or crystallite sizes, determined dipole reorientation/switching behaviors. Note that the ferroelectric domain coupling is realized via induced compensation polarizations from the media (either amorphous or crystalline PVDF) between aligned ferroelectric domains. A high β rather than α/δ content and a large crystallite size facilitated the coupling interactions among ferroelectric domains, and thus dipoles in highly coupled ferroelectric domains could be easily polarized, resulting in a high dielectric constant and a high stored energy density. However, strong coupling interactions impeded an easy dipole reversal to the so-called antiferroelectric-like (or random) state and thus reduced the discharged electric energy due to a high remanent polarization. Instead, the film with a high β content and a small crystallite size showed the highest discharged electric energy density, suggesting that the ferroelectric domain coupling could be weakened by confining them in nanoscale crystallites. These findings provide us useful guidance to achieve optimal crystalline morphology in PVDF copolymer films for high electric energy storage applications.
Co-reporter:Zhongqiang Zhao, Zhongbo Zhang, Sandra Pejić, Guoqiang Zhang, Yufeng Zhu, Hewen Liu, Morton Litt, Genevieve Sauve, Lei Zhu
Polymer (7 April 2017) Volume 114() pp:
Publication Date(Web):7 April 2017
DOI:10.1016/j.polymer.2017.02.095
•The performance of semiconducting polymers is currently limited by their low dielectric constant in the range of 3–4.•Increasing εr > GHz will enhance both charge separation and transport, leading to improved power conversion efficiency.•Electric field-induced tautomerization in fluorescein monopotassium salt copolymers can increase the electronic εr.The low dielectric constant (εr ∼ 3–4) for semiconducting polymers has been a major cause for their poor performance compared with the inorganic semiconductors, which possess high dielectric constants above 10. This study aimed to increase the electronic/atomic dielectric constant at high frequencies (i.e., εr∞) for semiconducting polymers. A new design strategy was proposed based on the electric field-induced tautomeric structures in conjugated fluorescein. To achieve this goal, fluorescein monopotassium salt-containing random copolymers were synthesized with 50 and 75 mol.% functionality. To reduce the strong electrostatic attraction between the K+ cation and the phenolate anion, 18-crown-6 ether was complexed with K+ in the fluorescein copolymers. A relatively high εr∞ of ∼5.5 and high electron mobility of 0.153 cm2/(V·s) were achieved for the 75 mol.% fluorescein K+/18C6 copolymer. The high electron mobility could be attributed to the relatively high static dielectric constant (εrs ∼ 9 at 1 Hz) of the sample. The fluorescein monopotassium salt copolymers behaved as n-type semiconductors with an optical band gap around 2.26 eV.
Co-reporter:Imre Treufeld, David H. Wang, Brian A. Kurish, Loon-Seng Tan and Lei Zhu
Journal of Materials Chemistry A 2014 - vol. 2(Issue 48) pp:NaN20696-20696
Publication Date(Web):2014/10/09
DOI:10.1039/C4TA03260H
A set of 12 new polyimides (PIs) with one or three polar CN dipoles directly attached to the aromatic diamine part were synthesized and their electric energy storage properties were studied using broadband dielectric spectroscopy (BDS) and electric displacement–electric field (D–E) loop measurements to determine their potential for high temperature film capacitors for aerospace applications. It was found that adding highly polar nitrile groups to the PI structure increased permittivity and thus electrical energy storage, especially at high temperatures, and 3 CN dipoles were better than 1 CN dipole. Below the glass transition temperature (Tg), a weak γ transition was observed around −100 °C and a broad β transition was observed between 100 and 150 °C. It was the β (i.e., precursor dipolar motion before long-range segmental motion, or glass transition), rather than the γ sub-Tg transition that substantially increased the permittivity of PIs. From the BDS results on PIs having 3 nitrile groups, the enhancement in permittivity from permanent dipoles decreased with dianhydride in the order of pyromellitic dianhydride (PMDA) > 4,4′-oxydiphthalic dianhydride (OPDA) > 1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) > 4,4′-benzophenonetetracarboxylic dianhydride (BTDA). Meanwhile, the increase in permittivity also decreased in the order of para–para, meta–para, and meta–meta linkage in the diamine, suggesting that the para–para linkage favored easier dipole rotation than the meta–meta linkage. From the D–E loop study, the PIs with a combination of PMDA dianhydride and a para–para linkage exhibited the highest discharged energy density and a reasonably low loss.
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