Co-reporter:Edward B. Trigg, Brandon J. Tiegs, Geoffrey W. Coates, and Karen I. Winey
ACS Macro Letters September 19, 2017 Volume 6(Issue 9) pp:947-947
Publication Date(Web):August 16, 2017
DOI:10.1021/acsmacrolett.7b00450
Linear polyethylenes with functional groups at precise intervals along the backbone possess a number of remarkable properties, but the current synthetic methods that produce these precise polymers are difficult to scale up beyond the laboratory setting. When evaluating alternative synthetic routes, a critical question is how precise must the polymer microstructure be to achieve the properties of interest? As a first step in answering this question, we present morphological characterization of a nearly precise polymer—that is, an acid-containing polymer wherein the acid groups are separated by either n or n + 1 methylene groups. We find that the size scale and uniformity of the amorphous morphologies of the nearly precise acid-containing polymer and its sodium-neutralized ionomer are essentially indistinguishable from the precise polymers based on X-ray scattering. Meanwhile, the nearly precise polymer is strikingly distinct from a pseudorandom copolymer with similar average composition. This result suggests that the properties of nearly precise polymers could likewise be quite similar to truly precise polymers and beckons future work to explore their properties.
Co-reporter:Edward B. Trigg, L. Robert Middleton, Demi E. Moed, and Karen I. Winey
Macromolecules November 28, 2017 Volume 50(Issue 22) pp:8988-8988
Publication Date(Web):November 14, 2017
DOI:10.1021/acs.macromol.7b02094
Recently we reported an unusual multilayered structure in a linear polyethylene containing precisely periodic carboxylic acid groups pendant to every 21st carbon atom (p21AA). Within the ordered domains, p21AA executes tight chain folds at the location of each acid group, and the chain-fold surfaces participate in hydrogen bonds with adjacent fold surfaces to form acid-rich layers. Here, we investigate the bulk morphologies of p21AA after isothermal crystallization and, using X-ray scattering and polarized optical microscopy, find p21AA to be semicrystalline. By analyzing X-ray peak widths, creating real-space models of lamellae, and performing in-situ X-ray scattering during tensile deformation, we find that, contrary to the typical structure of polymer crystallites, the polymer stems lie in the plane of the lamellae such that the acid layers are transverse (within 30° of orthogonal) to the crystallite plane. This surprising structure, not reported before to our knowledge, could be useful for designing semicrystalline membranes because, given the appropriate chemistry, layers of functional groups could provide pathways for small molecule, ion, or proton transport through crystallites. We expect this novel structure to be accessible in similarly designed crystallizable polymers that contain evenly spaced, moderately sized, associating side groups.
Co-reporter:Edward B. Trigg, Mark J. Stevens, and Karen I. Winey
Journal of the American Chemical Society March 15, 2017 Volume 139(Issue 10) pp:3747-3747
Publication Date(Web):March 6, 2017
DOI:10.1021/jacs.6b12817
Precise control over polymer architecture unlocks the potential for engineered self-assembled crystal structures with useful features on the nanometer length scale. Here we elucidate the structure of the ordered phase of a semicrystalline, functional polyethylene having a precise linear architecture, namely, pendant carboxylic acid groups precisely every 21st backbone carbon atom. By comparing the results of atomistic molecular dynamics simulations with experimental X-ray scattering and Raman spectroscopy data, we find that the polymer chains are folded in a hairpin manner near each carboxylic acid group, giving rise to multiple embedded layers of functional groups that have an interlayer distance of 2.5 nm. This is in contrast to other precise polyethylenes, where the chains are mostly trans within the crystals. Such layers could act as two-dimensional pathways for ionic or molecular transport given an appropriate choice of functional group.
Co-reporter:Philip J. Griffin, Vera Bocharova, L. Robert Middleton, Russell J. Composto, Nigel Clarke, Kenneth S. Schweizer, and Karen I. Winey
ACS Macro Letters 2016 Volume 5(Issue 10) pp:1141
Publication Date(Web):September 23, 2016
DOI:10.1021/acsmacrolett.6b00649
We measure the center-of-mass diffusion of silica nanoparticles (NPs) in entangled poly(2-vinylpyridine) (P2VP) melts using Rutherford backscattering spectrometry. While these NPs are well within the size regime where enhanced, nonhydrodynamic NP transport is theoretically predicted and has been observed experimentally (2RNP/dtube ≈ 3, where 2RNP is the NP diameter and dtube is the tube diameter), we find that the diffusion of these NPs in P2VP is in fact well-described by the hydrodynamic Stokes–Einstein relation. The effective NP diameter 2Reff is significantly larger than 2RNP and strongly dependent on P2VP molecular weight, consistent with the presence of a bound polymer layer on the NP surface with thickness heff ≈ 1.1Rg. Our results show that the bound polymer layer significantly augments the NP hydrodynamic size in polymer melts with attractive polymer–NP interactions and effectively transitions the mechanism of NP diffusion from the nonhydrodynamic to hydrodynamic regime, particularly at high molecular weights where NP transport is expected to be notably enhanced. Furthermore, these results provide the first experimental demonstration that hydrodynamic NP transport in polymer melts requires particles of size ≳5dtube, consistent with recent theoretical predictions.
Co-reporter:Wei-Shao Tung, Philip J. Griffin, Jeffrey S. Meth, Nigel Clarke, Russell J. Composto, and Karen I. Winey
ACS Macro Letters 2016 Volume 5(Issue 6) pp:735
Publication Date(Web):May 27, 2016
DOI:10.1021/acsmacrolett.6b00294
The polymer center-of-mass tracer diffusion coefficient in athermal polymer nanocomposites (PNCs) composed of polystyrene and phenyl-capped, spherical silica nanoparticles was measured over a range of temperatures and nanoparticle concentrations using elastic recoil detection. The polymer tracer diffusion coefficient in the PNC relative to the bulk decreases with increasing nanoparticle concentration and is unexpectedly more strongly reduced at higher temperatures. This unusual temperature dependence of polymer diffusion in PNCs cannot be explained by the reptation model or a modified version incorporating an effective tube diameter. Instead we show that our results are consistent with a mechanism based on nanoparticle-imposed configurational entropy barriers.
Co-reporter:L. Robert Middleton, Edward B. Trigg, Eric Schwartz, Kathleen L. Opper, Travis W. Baughman, Kenneth B. Wagener, and Karen I. Winey
Macromolecules 2016 Volume 49(Issue 21) pp:8209-8218
Publication Date(Web):October 18, 2016
DOI:10.1021/acs.macromol.6b00937
We report the morphology evolution under tensile deformation for strictly linear polyethylenes with associating functional groups. In situ X-ray scattering measurements during elongation reveal that periodic acid group placement along the backbone is required to form hierarchical layered morphologies that lead to strain hardening. This phenomenon was observed in both semicrystalline and amorphous precise acid polyethylenes with acrylic, geminal acrylic, and phosphonic acids. Polymers with nonperiodic (pseudorandom) acid placement fail to form layered morphologies and instead retain a liquidlike distribution of acid aggregates. Acid chemistry and acid concentration influence morphological evolution in both periodic and nonperiodic polymers predominately through the modification of Tg and percent crystallinity, which subsequently impact the mechanical properties. Our results indicate that hierarchical acid-rich layered structures, commensurate with improved mechanical properties, form in polymers with strictly periodic chemical structures and sufficient chain mobility for chain alignment during elongation.
Co-reporter:L. Robert Middleton, Jacob D. Tarver, Joseph Cordaro, Madhusudan Tyagi, Christopher L. Soles, Amalie L. Frischknecht, and Karen I. Winey
Macromolecules 2016 Volume 49(Issue 23) pp:9176-9185
Publication Date(Web):November 10, 2016
DOI:10.1021/acs.macromol.6b01918
Melt state dynamics for a series of strictly linear polyethylenes with precisely spaced associating functional groups were investigated. The periodic pendant acrylic acid groups form hydrogen-bonded acid aggregates within the polyethylene (PE) matrix. The dynamics of these nanoscale heterogeneous morphologies were investigated from picosecond to nanosecond timescales by both quasi-elastic neutron scattering (QENS) measurements and fully atomistic molecular dynamics (MD) simulations. Two dynamic processes were observed. The faster dynamic processes which occur at the picosecond timescales are compositionally insensitive and indicative of spatially restricted local motions. The slower dynamic processes are highly composition dependent and indicate the structural relaxation of the polymer backbone. Higher acid contents, or shorter PE spacers between pendant acid groups, slow the structural relaxation timescale and increase the stretching parameter (β) of the structural relaxation. Additionally, the dynamics of specific hydrogen atom positions along the backbone correlate structural heterogeneity imposed by the associating acid groups with a mobility gradient along the polymer backbone. At time intervals (<2 ns), the mean-squared displacements for the four methylene groups closest to the acid groups are up to 10 times smaller than those of methylene groups further from the acid groups. At longer timescales acid aggregates rearrange and the chain dynamics of the slow, near-aggregate regions and the faster bridge regions converge, implying a characteristic timescale for the passage of chains between aggregates. The characterization of the nanoscale chain dynamics in these associating polymer systems both provides validation of simulation force fields and provides understanding of heterogeneous chain dynamics in associating polymers.
Co-reporter:Sharon Sharick, Jason Koski, Robert A. Riggleman, and Karen I. Winey
Macromolecules 2016 Volume 49(Issue 6) pp:2245-2256
Publication Date(Web):March 7, 2016
DOI:10.1021/acs.macromol.5b02445
The effects of molecular weight and microdomain orientation on the ionic conductivities of poly(styrene-b-methyl methacrylate) diblock copolymer/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (SM/IL) mixtures are assessed through complementary experimental and theoretical techniques. Small-angle X-ray scattering revealed that SM/IL mixtures have anisotropic lamellar morphologies, preferentially oriented parallel to the casting substrate. A method for quantifying the morphology factor, or microdomain orientation within the Sax–Ottino model, using 2-D SAXS data is presented and applied to SM/IL mixtures. Ionic conductivity increases by up to an order of magnitude with a 2-fold increase in molecular weight, even when accounting for the morphology type, composition, microdomain orientation, and PMMA/IL glass transition temperature. Self-consistent field theory calculations predict a nonuniform solvent distribution in PMMA/IL microdomains, suggesting that polymer mobility and ion transport are reduced near PS–PMMA microdomain interfaces. Thus, the increase in ionic conductivity with increasing block copolymer molecular weight is associated with having fewer PS–PMMA/IL interfaces per unit volume.
Co-reporter:Philip J. Griffin;Grace B. Salmon;Jamie Ford
Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 2) pp:254-262
Publication Date(Web):
DOI:10.1002/polb.23914
ABSTRACT
The solution morphologies of a midblock-sulfonated pentablock copolymer in miscible polar/nonpolar solvent blends were characterized as a function of solvent composition and polymer concentration using small angle X-ray scattering. Three distinct solution morphologies are observed upon changing the composition of the solvent blend. At low weight fractions of polar solvent, spherical, sulfonated-core micelles are observed, while spherical, sulfonated-corona (inverted) micelles are observed at high weight fractions of polar solvent. Polymer solution scattering is observed at intermediate concentrations of polar solvent. Additionally, the characteristic dimensions of the sulfonated-core micelles were found to change strongly upon variation of the solvent blend composition, indicating that these solutions—and correspondingly the morphology and properties of polymer membranes into which they are cast—can be tuned through simple variations in the solvent blend chemistry. We demonstrate that the solution morphologies and the characteristic micelle dimensions of these complex pentablock copolymer/binary solvent blends can be reliably predicted by considering the relative interactions of each polymer block and the solvent blend using the Hansen solubility parameters. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 254–262
Co-reporter:James W. Borchert, Ian E. Stewart, Shengrong Ye, Aaron R. Rathmell, Benjamin J. Wiley and Karen I. Winey
Nanoscale 2015 vol. 7(Issue 34) pp:14496-14504
Publication Date(Web):29 Jul 2015
DOI:10.1039/C5NR03671B
Development of thin-film transparent conductors (TC) based on percolating networks of metal nanowires has leaped forward in recent years, owing to the improvement of nanowire synthetic methods and modeling efforts by several research groups. While silver nanowires are the first commercially viable iteration of this technology, systems based on copper nanowires are not far behind. Here we present an analysis of TCs composed of copper nanowire networks on sheets of polyethylene terephthalate that have been treated with various oxide-removing post treatments to improve conductivity. A pseudo-2D rod network modeling approach has been modified to include lognormal distributions in length that more closely reflect experimental data collected from the nanowire TCs. In our analysis, we find that the copper nanowire TCs are capable of achieving comparable electrical performance to silver nanowire TCs with similar dimensions. Lastly, we present a method for more accurately determining the nanowire area coverage in a TC over a large area using Rutherford Backscattering Spectrometry (RBS) to directly measure the metal content in the TCs. These developments will aid research and industry groups alike in the characterization of nanowire based TCs.
Co-reporter:Wei-Shao Tung, Russell J. Composto, Nigel Clarke, and Karen I. Winey
ACS Macro Letters 2015 Volume 4(Issue 9) pp:916
Publication Date(Web):August 13, 2015
DOI:10.1021/acsmacrolett.5b00256
Polymer radii of gyration in isotropic single-walled carbon nanotube (SWCNT)/polymer nanocomposites were previously found to increase with increasing SWCNT concentration. Here, the SWCNTs in nanocomposites were aligned by melt fiber spinning, and the polymer chain conformations were found to be anisotropic. Using SAXS and SANS, the anisotropic SWCNT meshes were found to have smaller mesh sizes in the direction perpendicular to the alignment direction than along the alignment direction. At fixed SWCNT orientation, the radius of gyration was probed parallel and perpendicular to the alignment direction, Rgpar and Rgper, respectively, using SANS. With increasing SWCNT concentration, Rgper increases significantly more than Rgpar, such that the extent of anisotropy increases with SWCNT concentration. The anisotropic polymer conformation is larger in the direction perpendicular to the alignment direction, which corresponds to a smaller SWCNT mesh size. Thus, when the SWCNT concentration and alignment combine to produce a SWCNT mesh size that is smaller than the unperturbed Rg, the polymer conformation circumvents the SWCNTs by adopting a larger Rg. Changes in the polymer conformation in nanocomposites with rod-like nanoparticles has important ramifications for entanglement density, polymer dynamics, and mechanical properties.
Co-reporter:Michael V. O'Reilly and Karen I. Winey
RSC Advances 2015 vol. 5(Issue 25) pp:19570-19580
Publication Date(Web):17 Feb 2015
DOI:10.1039/C4RA15178J
A method for densely grafting poly(ethylene oxide) chains to the surface of silica is presented. The PEO-grafted nanoparticles (PEONPs) are dispersed in a PEO single-ion conductor to accelerate ion transport with an additive that is not molecular, oligomeric, or ionic. Grafting high molecular weight brushes to nanoparticles suppresses PEO crystallinity and lowers the glass transition temperature of the nanocomposite ionomers. Ionic conductivity can be improved by up to an order of magnitude at room temperature with increasing PEONP content while viscosity is reduced. Dielectric spectra corroborate the enhanced ionic conductivity as ion relaxation times decrease with PEONP content. The PEONPs are compared with bare silica nanoparticles (SNPs), which demonstrate more homogenous dispersion in the PEO ionomer than PEONPs. Good dispersion in the SNP nanocomposite ionomers results in viscosity improvements by up to 3 orders of magnitude, but reduces ionic conductivity by one order magnitude. PEONPs show potential as additives to other solid electrolytes, while SNPs enable a more robust electrolyte with modest conductivity penalty.
Co-reporter:Michael V. O'Reilly, Hanqing Masser, Daniel R. King, Paul C. Painter, Ralph H. Colby, Karen I. Winey, James Runt
Polymer 2015 Volume 59() pp:133-143
Publication Date(Web):24 February 2015
DOI:10.1016/j.polymer.2014.12.002
•Lithium-conducting PEO ionomers were plasticized with PEG600.•We observe the dissolution of ionic aggregates into ion pairs.•Ionic conductivity was improved by two orders of magnitude.•Tg was reduced non-linearly with increasing plasticizer content.Poly(ethylene glycol) plasticizer is blended with a PEO-based single-ion conductor to lower the glass transition temperature of the ionomer and solvate the lithium counterions. With increasing plasticizer content at room temperature, FTIR spectra indicate that the fraction of ions in isolated ion pairs increases relative to those in ionic aggregates, and X-ray scattering data indicate that the ionic aggregates are further apart than expected by dilution alone. Together these data show that the average size of ionic aggregates decreases as PEG plasticizer is added. The dissolution of aggregates into ion pairs promotes ion conduction. Coupled with faster segmental dynamics and ion mobility from the depressed Tg, the ionic conductivity of plasticized ionomers improved by two orders of magnitude at room temperature. Resolving the alpha relaxation of these ionomer blends reveals that the mechanism for ion transport is segmentally-assisted ion motion.
Co-reporter:Rose M. Mutiso, Karen I. Winey
Progress in Polymer Science 2015 40() pp: 63-84
Publication Date(Web):January 2015
DOI:10.1016/j.progpolymsci.2014.06.002
Co-reporter:C. Francisco Buitrago, Dan S. Bolintineanu, Michelle E. Seitz, Kathleen L. Opper, Kenneth B. Wagener, Mark J. Stevens, Amalie L. Frischknecht, and Karen I. Winey
Macromolecules 2015 Volume 48(Issue 4) pp:1210-1220
Publication Date(Web):February 9, 2015
DOI:10.1021/ma5022117
Designing acid- and ion-containing polymers for optimal proton, ion, or water transport would benefit profoundly from predictive models or theories that relate polymer structures with ionomer morphologies. Recently, atomistic molecular dynamics (MD) simulations were performed to study the morphologies of precise poly(ethylene-co-acrylic acid) copolymer and ionomer melts. Here, we present the first direct comparisons between scattering profiles, I(q), calculated from these atomistic MD simulations and experimental X-ray data for 11 materials. This set of precise polymers has spacers of exactly 9, 15, or 21 carbons between acid groups and has been partially neutralized with Li, Na, Cs, or Zn. In these polymers, the simulations at 120 °C reveal ionic aggregates with a range of morphologies, from compact, isolated aggregates (type 1) to branched, stringy aggregates (type 2) to branched, stringy aggregates that percolate through the simulation box (type 3). Excellent agreement is found between the simulated and experimental scattering peak positions across all polymer types and aggregate morphologies. The shape of the amorphous halo in the simulated I(q) profile is in excellent agreement with experimental I(q). The modified hard-sphere scattering model fits both the simulation and experimental I(q) data for type 1 aggregate morphologies, and the aggregate sizes and separations are in agreement. Given the stringy structure in types 2 and 3, we develop a scattering model based on cylindrical aggregates. Both the spherical and cylindrical scattering models fit I(q) data from the polymers with type 2 and 3 aggregates equally well, and the extracted aggregate radii and inter- and intra-aggregate spacings are in agreement between simulation and experiment. Furthermore, these dimensions are consistent with real-space analyses of the atomistic MD simulations. By combining simulations and experiments, the ionomer scattering peak can be associated with the average distance between branches of type 2 or 3 aggregates. This direct comparison of X-ray scattering data to the atomistic MD simulations is a substantive step toward providing a comprehensive, predictive model for ionomer morphology, gives substantial support for this atomistic MD model, and provides new credibility to the presence of stringy, branched, and percolated ionic aggregates in precise ionomer melts.
Co-reporter:Wei-Shao Tung, Russell J. Composto, Robert A. Riggleman, and Karen I. Winey
Macromolecules 2015 Volume 48(Issue 7) pp:2324-2332
Publication Date(Web):April 1, 2015
DOI:10.1021/acs.macromol.5b00085
The melt diffusion of polymers confined to nanoscale cylinders was investigated by molecular dynamics simulations and depth profiling experiments. In the simulations, entangled polymers are confined within long cylindrical pores having effective diameters (deff) from 0.4Ree to 1.7Ree, where Ree is the square root of the mean-squared end-to-end distance of the polymer in the absence of confinement. The local dynamics of polymers confined to cylinders exhibit anisotropic relaxations. Perpendicular to the cylindrical axis, monomer motion is suppressed by the adjacent wall, while motion along the cylindrical axis is faster relative to the bulk dynamics. These anisotropic relaxations are discussed in light of our prior studies showing that chain conformations parallel to the cylinder axis are elongated relative to the bulk conformation, whereas in the perpendicular direction the chain conformations are compressed. Furthermore, our previous simulations found that the number of entanglements per chain decreases as deff decreases. Here, the effects of confinement on local dynamics, chain size, and entanglement density are combined to calculate polymer diffusion (Drep,z) along the cylindrical pore according to the reptation model. The center of mass diffusion coefficients (DMSD,z) along the cylindrical pore were also determined using long simulation times. Finally, using elastic recoil detection, polymer tracer diffusion coefficients (Dexp) along the cylindrical nanopores were measured for deuterated polystyrene diffusing into membranes preinfiltrated with polystyrene. Relative to the bulk diffusion coefficients, the diffusion coefficients along the cylinder (Drep,z, DMSD,z, Dexp) systematically increase as the extent of cylindrical confinement increases (smaller diameter). Moreover, normalized Drep,z and normalize DMSD,z from simulations are in good agreement when deff/Ree > 0.5, while normalized Dexp is substantially smaller at all degrees of confinement investigated. These are the first side-by-side comparisons of simulations and experiments of polymer diffusion in cylindrical nanopores, and the implications of faster polymer diffusion along the cylinder and parallel to the confining wall are discussed.
Co-reporter:L. Robert Middleton, Steven Szewczyk, Jason Azoulay, Dustin Murtagh, Giovanni Rojas, Kenneth B. Wagener, Joseph Cordaro, and Karen I. Winey
Macromolecules 2015 Volume 48(Issue 11) pp:3713-3724
Publication Date(Web):May 27, 2015
DOI:10.1021/acs.macromol.5b00797
We report tensile testing and in situ X-ray scattering measurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). The number of backbone carbons (x) between pendant acrylic acid groups along the polyethylene chain (x = 9, 15, 21) has a pronounced effect on both their tensile properties as well as their morphologies during deformation. The semicrystalline precise copolymer (p21AA) displays yielding behavior similar to polyethylene. Also, strain hardening in p21AA coincides with the originally isotropic acid-rich layered structures strongly aligning with acid layers perpendicular to the strain direction, demonstrating the facile nature of the H-bonding within the acid aggregates. When the alkyl spacer is only nine carbons (p9AA), the precise copolymer withstands strains of >1000% without failing, because the liquid-like assembly of acid aggregates permits the acid groups to exchange without developing substantial anisotropy in the structure. Both p21AA and p9AA maintain their morphology type during deformation with considerable plastic deformation and only modest increase in their interaggregate distances. In contrast, p15AA exhibits a structural transformation from a nominally spherical to a layered aggregate morphology during tensile deformation as evidenced by higher order peaks at intermediate scattering angles and larger interaggregate spacing, coinciding with substantial strain hardening. The structural changes in p15AA are particularly sensitive to the strain rate, because the relaxation times of the PE segments and the acid aggregates are accessible. Commensurate with this structural transformation, p15AA has the highest tensile strength of the precise poly(ethylene-co-acrylic acid) copolymers.
Co-reporter:Kelly M. Meek, Sharon Sharick, Yuesheng Ye, Karen I. Winey, and Yossef A. Elabd
Macromolecules 2015 Volume 48(Issue 14) pp:4850-4862
Publication Date(Web):July 14, 2015
DOI:10.1021/acs.macromol.5b00926
A polymerized ionic liquid (PIL) diblock copolymer, poly(MMA-b-MEBIm-Br), was synthesized at various compositions from an ionic liquid monomer, (1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (MEBIm-Br), and a nonionic monomer, methyl methacrylate (MMA), via the reverse addition–fragmentation chain transfer (RAFT) polymerization technique. A hydroxide-conducting PIL diblock copolymer, poly(MMA-b-MEBIm-OH), was also prepared via anion exchange metathesis of the bromide-conducting block copolymer. In a former study, the conductivity and morphology of the bromide- and hydroxide-conducting PIL diblock copolymer were examined at one fixed PIL composition: 17.3 mol %. In this study, additional PIL compositions of (6.6, 11.9, and 26.5 mol %) were explored to fully understand the previous unusual conductivity results. Both bromide and hydroxide conductivities were higher in the PIL block copolymer at PIL compositions of 11.9, 17.3, and 26.5 mol % compared to the PIL homopolymer under the same experimental conditions, even though the homopolymer possessed a higher water and ionic content compared to the block copolymers. These unusual results suggest that the confinement of the PIL microdomain within the block copolymer morphology enhances ion transport compared to its predicted value. Morphology factors (or normalized ionic conductivity, f) were as high as >3 at some conditions, which is much higher than the maximum theoretical limit for randomly oriented lamellar domains (f = 2/3). Application of percolation theory revealed a 3–4-fold enhancement of conductivity when comparing the inherent conductivity to the measured PIL homopolymer conductivity. Both morphology factor analysis and percolation theory corroborate with the absolute conductivity results and the hypothesis that PIL domain confinement in PIL block copolymers enhances conductivity over its bulk properties.
Co-reporter:Siwei Liang, Michael V. O’Reilly, U Hyeok Choi, Huai-Suen Shiau, Joshua Bartels, Quan Chen, James Runt, Karen I. Winey, and Ralph H. Colby
Macromolecules 2014 Volume 47(Issue 13) pp:4428-4437
Publication Date(Web):June 26, 2014
DOI:10.1021/ma5001546
Polysiloxane phosphonium single-ion conductors grafted with oligomeric PEO and with ion contents ranging from 5 to 22 mol % were synthesized via hydrosilylation reaction. The parent Br– anion was exchanged to F– or bis(trifluoromethanesulfonyl)imide (TFSI–). X-ray scattering data suggest ion aggregation is absent in these phosphonium ionomers, which contributes to low glass transition temperatures (below −70 °C) with only a weak dependence on both ion content and counteranion type. Conductivities weakly increase with ion content but exhibit a strong dependence on anion type. The highest conductivity at 30 °C is 20 μS/cm for dry neat ionomer, with the TFSI– anion, consistent with its relatively delocalized negative charge and large size that weaken interactions between TFSI– and the phosphonium cation.
Co-reporter:Chia-Chun Lin, Kohji Ohno, Nigel Clarke, Karen I. Winey, and Russell J. Composto
Macromolecules 2014 Volume 47(Issue 15) pp:5357-5364
Publication Date(Web):July 16, 2014
DOI:10.1021/ma501113c
Diffusion of deuterated polystyrene (dPS) is probed in PS matrices containing stringlike chained nanoparticles (cNP) grafted with PS. This investigation connects prior diffusion studies in model spherical and cylindrical NP systems and provides insight for technological applications, which typically involve irregularly shaped NPs such as carbon black. We report that the presence of chained NPs in PS matrices induces a minimum in the diffusion coefficient (D) with increasing cNP concentration when the key length scale, 2Rg/L ≤ 1.5, where Rg is the gyration radius of dPS and L is the mean length of the impenetrable core of the chained NPs. When 2Rg/L > 1.5, D decreases monotonically as the NP concentration increases. Note that in all cases 2Rg is larger than the diameter of these short-stringy NPs. The diffusion minimum is attributed to anisotropic diffusion in the vicinity of the chained NPs and requires that the long dimension of the cNP be comparable to or longer than the tracer molecule. Two normalizations are explored to provide insight about the diffusion mechanism: D/D0 where D0 is the diffusion coefficient in a pure homopolymer matrix and D/De where De is an effective diffusion coefficient that accounts for the distinct dynamics in the PS matrix and PS brush regions. For D/D0, a sharp transition from a diffusion minimum to a monotonic decrease is observed as dPS molecular weight increases, while for D/De the transition is more gradual. These studies show not only that the NPs act as impenetrable obstacles for polymer diffusion but that the polymer brush grafted to the cNP provides an alternative pathway to control polymer dynamics.
Co-reporter:Yuesheng Ye, Sharon Sharick, Eric M. Davis, Karen I. Winey, and Yossef A. Elabd
ACS Macro Letters 2013 Volume 2(Issue 7) pp:575
Publication Date(Web):June 11, 2013
DOI:10.1021/mz400210a
Herein, we report a polymerized ionic liquid diblock copolymer with high hydroxide conductivity and nanoscale morphology. Surprisingly, the conductivity is not only higher (over an order of magnitude) than its random copolymer analog at the same ion and water content, but also higher than its homopolymer analog, which has a higher ion and water content than the block copolymer. These results should have a significant impact on low-cost (platinum-free), long-lasting, solid-state alkaline fuel cells.
Co-reporter:Rose M. Mutiso, Michelle C. Sherrott, Aaron R. Rathmell, Benjamin J. Wiley, and Karen I. Winey
ACS Nano 2013 Volume 7(Issue 9) pp:7654
Publication Date(Web):August 9, 2013
DOI:10.1021/nn403324t
Metal nanowire films are among the most promising alternatives for next-generation flexible, solution-processed transparent conductors. Breakthroughs in nanowire synthesis and processing have reported low sheet resistance (Rs ≤ 100 Ω/sq) and high optical transparency (%T > 90%). Comparing the merits of the various nanowires and fabrication methods is inexact, because Rs and %T depend on a variety of independent parameters including nanowire length, nanowire diameter, areal density of the nanowires and contact resistance between nanowires. In an effort to account for these fundamental parameters of nanowire thin films, this paper integrates simulations and experimental results to build a quantitatively predictive model. First, by fitting the results from simulations of quasi-2D rod networks to experimental data from well-defined nanowire films, we obtain an effective average contact resistance, which is indicative of the nanowire chemistry and processing methods. Second, this effective contact resistance is used to simulate how the sheet resistance depends on the aspect ratio (L/D) and areal density of monodisperse rods, as well as the effect of mixtures of short and long nanowires on the sheet resistance. Third, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance. Our predictions for silver nanowires are validated by experimental results for silver nanowire films, where nanowires of L/D > 400 are required for high performance transparent conductors. In contrast to a widely used approach that employs a single percolative figure of merit, our method integrates simulation and experimental results to enable researchers to independently explore the importance of contact resistance between nanowires, as well as nanowire area fraction and arbitrary distributions in nanowire sizes. To become competitive, metal nanowire systems require a predictive tool to accelerate their design and adoption for specific applications.Keywords: contact resistance; nanowires; sheet resistance; silver; simulations; transparent conductor
Co-reporter:Jae-Hong Choi, Carl L. Willis, Karen I. Winey
Journal of Membrane Science 2013 428() pp: 516-522
Publication Date(Web):
DOI:10.1016/j.memsci.2012.10.051
Co-reporter:Wei-Shao Tung, Nigel Clarke, Russell J. Composto, and Karen I. Winey
Macromolecules 2013 Volume 46(Issue 6) pp:2317-2322
Publication Date(Web):March 8, 2013
DOI:10.1021/ma302517x
Polymer tracer diffusion in multiwalled carbon nanotubes (MWCNT)/polystyrene (PS) nanocomposites as a function of MWCNT concentration was measured from 152 to 214 °C. At 170 °C, we previously reported a minimum in the tracer diffusion coefficient that occurs at ∼2 wt % MWCNT; this concentration corresponds to the onset of solidlike behavior as measured by linear viscoelasticity. The minimum in the tracer diffusion coefficient in the nanocomposites (D) normalized by the tracer diffusion coefficient without MWCNTs (D0) is shallower at higher temperatures, namely, (D/D0)min = 0.9 at 214 °C and (D/D0)min = 0.6 at 152 °C. Using our trap model, this implies that the difference between the tracer diffusion parallel and perpendicular to the MWCNT is smaller at higher temperatures. Finally, at fixed MWCNT concentrations (0.5, 2, and 6 wt %) the temperature dependence of the tracer diffusion coefficient follows the Williams–Landel–Ferry (WLF) equation, indicating that polymer dynamics in nanocomposites is captured by changes in free volume with temperature.
Co-reporter:Wei-Shao Tung, Vikki Bird, Russell J. Composto, Nigel Clarke, and Karen I. Winey
Macromolecules 2013 Volume 46(Issue 13) pp:5345-5354
Publication Date(Web):June 24, 2013
DOI:10.1021/ma400765v
Chain conformations dictate many of the important physical properties of polymers including their dynamics. Using small angle neutron scattering, we probed chain conformations, specifically the radius of gyration (Rg), in both SWCNT/polystyrene (rSWCNT/Rg ∼ 0.4) and MWCNT/polystyrene (rMWCNT/Rg ∼ 1) nanocomposites. We fit the scattering data using a model that includes an ideal Gaussian chain to describe the polymer conformation and a rod network to describe the carbon nanotube (CNT) network. The scattering contribution from the rod network increases with the CNT concentration in both SWCNT and MWCNT nanocomposites, and the contribution is higher for SWCNT nanocomposites due to the smaller mesh size and higher mesh density. When the SWCNT and MWCNT concentrations are below 2 wt %, there is no significant change in Rg. Above 2 wt %, Rg in the MWCNT nanocomposites decreases slightly, while Rg in the SWCNT nanocomposites increases monotonically as a function of CNT concentration, showing a ∼30% increase at 10 wt % SWCNT loading. Although we previously found a minimum in the tracer diffusion coefficient near the critical nanotube concentration, this trend is absent in the concentration-dependent polymer conformation.
Co-reporter:Jae-Hong Choi, Yuesheng Ye, Yossef A. Elabd, and Karen I. Winey
Macromolecules 2013 Volume 46(Issue 13) pp:5290-5300
Publication Date(Web):June 24, 2013
DOI:10.1021/ma400562a
A series of strongly microphase-separated polymerized ionic liquid (PIL) diblock copolymers, poly(styrene-b-1-((2-acryloyloxy)ethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (poly(S-b-AEBIm-TFSI)), were synthesized to explore relationships between morphology and ionic conductivity. Using small-angle X-ray scattering and transmission electron microscopy, a variety of self-assembled nanostructures including hexagonally packed cylinders, lamellae, and coexisting lamellae and network morphologies were observed by varying PIL composition (6.6–23.6 PIL mol %). At comparable PIL composition, this acrylate-based PIL block copolymer with strong microphase separation exhibited ∼1.5–2 orders of magnitude higher ionic conductivity than a methacrylate-based PIL block copolymer with weak microphase separation. Remarkably, we achieved high ionic conductivity (0.88 mS cm–1 at 150 °C) and a morphology factor (normalized ionic conductivity, f) of ∼1 through the morphological transition from lamellar to a coexistence of lamellar and three-dimensional network morphologies with increasing PIL composition in anhydrous single-ion conducting PIL block copolymers, which highlights a good agreement with the model predictions. In addition to strong microphase separation and the connectivity of conducting microdomains, the orientation of conducting microdomains and the compatibility between polymer backbone and IL moiety of PIL also significantly affect the ionic conductivity. This study provides avenues to controlling the extent of microphase separation, morphology, and ion transport properties in PIL block copolymers for energy conversion and storage applications.
Co-reporter:C. Francisco Buitrago, Janelle E. Jenkins, Kathleen L. Opper, Brian S. Aitken, Kenneth B. Wagener, Todd M. Alam, and Karen I. Winey
Macromolecules 2013 Volume 46(Issue 22) pp:9003-9012
Publication Date(Web):November 19, 2013
DOI:10.1021/ma4013169
The room temperature morphologies of twelve precise copolymers based on polyethylene (PE) were studied by solid-state 13C NMR, DSC, and X-ray scattering. These copolymers feature carboxylic acid, phosphonic acid or 1-methylimidazolium bromide pendants on exactly every 9th, 15th or 21st carbon atom along the linear PE chain. The morphologies were categorized by the arrangement of the acid or ionic aggregates into liquid-like, layered, or cubic morphologies. The liquid-like morphology is characterized by an amorphous PE matrix and liquid-like packing of the aggregates, wherein the interaggregate spacing increases with both the PE segment length and the pendant size. The layered morphologies typically have a semicrystalline PE matrix and upon stretching become highly anisotropic. Notably, the orientation of the aggregates and the PE crystallites relative to the stretch direction depends on whether the morphology is dominated by PE crystallization, as found for acrylic acid (AA) and phosphonic acid (PA) copolymers, or by the strong ionic aggregates, as found for the 1-methylimidazolium bromide (ImBr) copolymers. Cubic morphologies in these precise copolymers require geminal substitution, PA pendants, and sufficiently long PE segments to allow the aggregates to order. These precise copolymers provide an unprecedented array of morphologies that enable correlations between chemical structure and nanoscale morphologies.
Co-reporter:C. Francisco Buitrago, Todd M. Alam, Kathleen L. Opper, Brian S. Aitken, Kenneth B. Wagener, and Karen I. Winey
Macromolecules 2013 Volume 46(Issue 22) pp:8995-9002
Publication Date(Web):November 14, 2013
DOI:10.1021/ma4013175
The morphologies at elevated temperatures (T > Tg, Tm) of 12 precise, polyethylene (PE)-based copolymers with acrylic acid (AA), phosphonic acid (PA), and 1-methylimidazolium bromide (ImBr) groups were studied via X-ray scattering. These precise copolymers enable direct comparisons focusing on the length of the spacer between the functional groups and the type of functional group. The polar groups in these materials self-assemble into microphase-separated aggregates dispersed throughout the nonpolar PE matrix. At high temperatures the PE segments are amorphous, such that the aggregates are distributed in a liquid-like manner in 11 of these precise copolymers. The correlation distances between aggregates increase with the following: carbon spacer length between pendant groups, size and volume fraction of the pendant species, and functional group configuration (single vs geminal substitution). In addition, comparisons are made between precise copolymers and pseudorandom copolymers of the same pendant concentration, wherein the interaggregate distances are much better defined with precise copolymers. Finally, the local packing in copolymers with geminal substitution of PA pendant groups is less compact, which might facilitate ion conduction.
Co-reporter:Jae-Hong Choi, Carl L. Willis, Karen I. Winey
Journal of Membrane Science 2012 Volumes 394–395() pp:169-174
Publication Date(Web):15 March 2012
DOI:10.1016/j.memsci.2011.12.036
The morphology of a series of poly(t-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-t-butylstyrene) (tBS-HI-SS-HI-tBS) pentablock copolymer membranes with a range of ion exchange capacity (IEC) was investigated with small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Comparing the sizes of the micelle cores (2R), obtained from SAXS of the micellar solutions in cyclohexane/heptane, to the interparticle distances (d), obtained from SAXS and TEM imaging of the membranes, reveals that membranes with a low SS volume fraction (0.4, 0.7 and 1.0 mequiv./g IECs) show discrete SS microdomains within a HI-tBS matrix, while high SS volume fraction membranes (1.5 and 2.0 mequiv./g IECs) exhibit a bicontinuous microphase separated morphology without long-range order. As ion exchange capacity increases from 1.0 to 1.5 mequiv./g, the morphological transition from discrete SS microdomains to interconnected SS microdomains results in a significant increase in water vapor transport rate. When exposed to liquid water, the extent of water uptake and the increase in primary spacing of the membranes are greater at higher sulfonation levels suggesting that absorbed water plasticizes the hydrophilic SS microdomains.Graphical abstractHighlights► The sulfonated pentablock copolymer membranes are cast from their micellar solutions. ► At low sulfonation levels the solvent-cast membranes show discrete SS microdomains. ► The highly sulfonated membranes exhibit interconnected SS microdomains. ► A significant increase in WVTR is observed above the morphological transition.
Co-reporter:C. Francisco Buitrago, Kathleen L. Opper, Kenneth B. Wagener, and Karen I. Winey
ACS Macro Letters 2012 Volume 1(Issue 1) pp:71
Publication Date(Web):November 16, 2011
DOI:10.1021/mz2000237
A linear polyethylene precisely functionalized with geminal phosphonic acid pendants on every 21st carbon atom exhibits face-centered cubic (FCC) packing of acid aggregates. X-ray scattering from isotropic films result in higher-order scattering peaks used to determine the lattice parameter at room temperature (aFCC = 4.19 nm) and above the melting temperature of the polyethylene matrix (aFCC = 4.06 nm). Upon stretching the precise acid copolymer, an anisotropic scattering pattern featuring two coexisting crystalline orientations results, both having the ⟨110⟩ direction of the FCC lattice along the stretching direction. This is the first report of cubic ordering of aggregates in an acid copolymer and it is the direct consequence of the molecular precision of the polymer.
Co-reporter:David Salas-de la Cruz;Matthew D. Green;Yuesheng Ye;Yossef A. Elabd;Timothy E. Long
Journal of Polymer Science Part B: Polymer Physics 2012 Volume 50( Issue 5) pp:338-346
Publication Date(Web):
DOI:10.1002/polb.23019
Abstract
The morphology and ionic conductivity of poly(1-n-alkyl-3-vinylimidazolium)-based homopolymers polymerized from ionic liquids were investigated as a function of the alkyl chain length and counterion type. In general, X-ray scattering showed three features: (i) backbone-to-backbone, (ii) anion-to-anion, and (iii) pendant-to-pendant characteristic distances. As the alkyl chain length increases, the backbone-to-backbone separation increases. As the size of counterion increases, the anion-to-anion scattering peak becomes apparent and its correlation length increases. The X-ray scattering features shift to lower angles as the temperature increases due to thermal expansion. The ionic conductivity results show that the glass transition temperature (Tg) is a dominant, but not exclusive, parameter in determining ion transport. The Tg-independent ionic conductivity decreases as the backbone-to-backbone spacing increases. Further interpretation of the ionic conductivity using the Vogel–Fulcher–Tammann equation enabled the correlation between polymer morphology and ionic conductivity, which highlights the importance of anion hoping between adjacent polymer backbones. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012
Co-reporter:Yuesheng Ye, Jae-Hong Choi, Karen I. Winey, and Yossef A. Elabd
Macromolecules 2012 Volume 45(Issue 17) pp:7027-7035
Publication Date(Web):August 27, 2012
DOI:10.1021/ma301036b
A series of polymerized ionic liquid (PIL) block and random copolymers were synthesized from an ionic liquid monomer, 1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide (MEBIm-TFSI), and a nonionic monomer, methyl methacrylate (MMA), at various PIL compositions with the goal of understanding the influence of morphology on ion transport. For the diblock copolymers, the partial affinity between the PIL and PMMA blocks resulted in a weakly microphase-separated morphology with no evident long-range periodic structure across the PIL composition range studied, while the random copolymers revealed no microphase separation. These morphologies were identified with a combination of techniques, including differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy. Surprisingly, at similar PIL compositions, the ionic conductivity of the block copolymers were ca. 2 orders of magnitude higher than the random copolymers despite the weak microphase-separated morphology evidenced in the block copolymers. We attribute the higher conductivity in the block copolymers to its microphase-separated morphology, since significant differences in conductivity are still observed even when differences in glass transition temperature are considered. This work demonstrates that local confinement and connectivity of conducting ions in nanoscale ionic domains in PIL block copolymers can accelerate ion transport significantly.
Co-reporter:Wenqin Wang ; Gregory J. Tudryn ; Ralph H. Colby
Journal of the American Chemical Society 2011 Volume 133(Issue 28) pp:10826-10831
Publication Date(Web):June 24, 2011
DOI:10.1021/ja201405v
A series of sulfonate polyester ionomers with well-defined poly(ethylene oxide) spacer lengths between phthalates and alkali metal cations as counterions are designed for improved ionic conductivity. Ion conduction in these chemically complex materials is dominated by the polymer mobility and the state of ionic aggregation. While the aggregation decreases dramatically at room temperature as the cation size increases from Li to Na to Cs, the extents of ionic aggregation of these ionomers are comparable at elevated temperatures. Both the Na and Cs ionomers exhibit thermally reversible transformation upon heating from 25 to 120 °C as isolated ion pairs aggregate. This seemingly counterintuitive aggregation of ions on heating is driven by the fact that the dielectric constant of all polar liquids decreases on heating, enhancing Coulomb interactions between ions.
Co-reporter:Sadie I. White;Patrick M. Vora;James M. Kikkawa
Advanced Functional Materials 2011 Volume 21( Issue 2) pp:233-240
Publication Date(Web):
DOI:10.1002/adfm.201001383
Abstract
Traditionally, bulk nanocomposites of electrically conducting particles and insulating polymers have been categorized as either insulating or conducting when the nanoparticle concentration is below or above the percolation threshold, respectively. Meanwhile, thin-film polymer nanocomposites can exhibit resistive switching behavior appropriate for digital memory applications. Here, we present the first report of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions close to the electrical percolation threshold measured at low voltage, silver nanowire-polystyrene nanocomposites demonstrate reversible resistive switching with increasing voltage at room temperature. Nanocomposites with compositions outside of this range exhibit either irreversible switching, or no switching at all. We propose that resistive switching in these materials is the result of the field-induced formation of silver filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample’s bulk resistivity. These findings break from the usual dichotomy of insulating or conducting properties in polymer nanocomposites and could inspire new devices that capitalize on this responsive behavior in these versatile materials.
Co-reporter:Minfang Mu, Michelle E. Seitz, Nigel Clarke, Russell J. Composto, and Karen I. Winey
Macromolecules 2011 Volume 44(Issue 2) pp:191-193
Publication Date(Web):December 20, 2010
DOI:10.1021/ma1019818
Co-reporter:Michelle E. Seitz ; Christopher D. Chan ; Kathleen L. Opper ; Travis W. Baughman ; Kenneth B. Wagener
Journal of the American Chemical Society 2010 Volume 132(Issue 23) pp:8165-8174
Publication Date(Web):May 24, 2010
DOI:10.1021/ja101991d
The morphology of a series of linear poly(ethylene-co-acrylic acid) zinc-neutralized ionomers with either precisely or randomly spaced acid groups was investigated using X-ray scattering, differential scanning calorimetry (DSC), and scanning transmission electron microscopy (STEM). Scattering from semicrystalline, precise ionomers has contributions from acid layers associated with the crystallites and ionic aggregates dispersed in the amorphous phase. The precisely controlled acid spacing in these ionomers reduces the polydispersity in the aggregate correlation length and yields more intense, well-defined scattering peaks. Remarkably, the ionic aggregates in an amorphous, precise ionomer with 22 mol % acid and 66% neutralization adopt a cubic lattice; this is the first report of ionic aggregate self-assembly onto a lattice in an ionomer with an all-carbon backbone. Aggregate size is insensitive to acid content or neutralization level. As the acid content increases from 9.5 to 22 mol % at ∼75% neutralization, the number density of aggregates increases by ∼5 times, suggesting that the ionic aggregates become less ionic with increasing acid content.
Co-reporter:Sadie I. White;Rose M. Mutiso;Patrick M. Vora;David Jahnke;Sam Hsu;James M. Kikkawa;Ju Li;John E. Fischer
Advanced Functional Materials 2010 Volume 20( Issue 16) pp:2709-2716
Publication Date(Web):
DOI:10.1002/adfm.201000451
Abstract
The design and preparation of isotropic silver nanowire-polystyrene composites is described, in which the nanowires have finite L/D (< 35) and narrow L/D distribution. These model composites allow the L/D dependence of the electrical percolation threshold, ϕc, to be isolated for finite-L/D particles. Experimental ϕc values decrease with increasing L/D, as predicted qualitatively by analytical percolation models. However, quantitative agreement between experimental data and both soft-core and core–shell analytical models is not achieved, because both models are strictly accurate only in the infinite-L/D limit. To address this analytical limitation, a soft-core simulation method to calculate ϕc and network conductivity for cylinders with finite L/D are developed. Our simulated ϕc results agree strongly with our experimental data, suggesting i) that the infinite-aspect-ratio assumption cannot safely be made for experimental networks of particles with L/D < 35 and ii) in predicting ϕc, the soft-core model makes a less significant assumption than the infinite-L/D models do. The demonstrated capability of the simulations to predict ϕc in the finite-L/D regime will allow researchers to optimize the electrical properties of polymer nanocomposites of finite-L/D particles.
Co-reporter:Wenqin Wang, Wenjuan Liu, Gregory J. Tudryn, Ralph H. Colby and Karen I. Winey
Macromolecules 2010 Volume 43(Issue 9) pp:4223-4229
Publication Date(Web):April 14, 2010
DOI:10.1021/ma100379j
A series of Li-, Na-, and Cs-neutralized polyester ionomers with well-defined poly(ethylene oxide) (PEO) spacer lengths between sulfonated phthalates have been investigated by X-ray scattering at room temperature. As the spacer lengths are increased the PEO segments crystallize, as evidenced by multiple crystal reflections that are identical to those of pure poly(ethylene glycol) oligomers. This crystallization also produces multiple small-angle peaks, which correspond to the well-defined thickness of PEO crystallites. The ionomer peak (q = 1−5 nm−1) is absent in the Na- and Cs-neutralized ionomers, while the Li-neutralized ionomers show peaks at q = 2−3 nm−1, reminiscent of conventional ionic aggregates in ionomers. Detailed analysis of the normalized X-ray scattering intensity from these ionomers reveals a variety of ionic states that are highly dependent on the cation size. The states of ionic groups change from a majority of isolated ion pairs to aggregated structures as the cation size decreases from Cs to Li. These findings compare favorably with ab initio calculations.
Co-reporter:Sadie I. White ; Patrick M. Vora ; James M. Kikkawa ; John E. Fischer
The Journal of Physical Chemistry C 2010 Volume 114(Issue 50) pp:22106-22112
Publication Date(Web):November 23, 2010
DOI:10.1021/jp108191q
We describe the temperature-dependent characterization of resistive switching behavior in bulk silver nanowire−polystyrene composites between 10 and 300 K. We propose that the resistive switching behavior is caused by the electroformation of silver filaments between adjacent nanowire clusters, resulting in an extension of the electrical percolation network in the on state. This process is reversible above 200 K, and irreversible below 100 K. The switching fields are shown to depend strongly on sample composition (i.e., proximity to the electrical percolation threshold), as well as measurement temperature.
Co-reporter:Mohammad Moniruzzaman, Asli Sahin, Karen I. Winey
Carbon 2009 Volume 47(Issue 3) pp:645-650
Publication Date(Web):March 2009
DOI:10.1016/j.carbon.2008.10.046
We have fabricated the first organogel/carbon nanotube composites using 12-hydroxystearic acid (HSA) as the gelator molecule, multi-wall carbon nanotubes as the nanofillers, and 1,2-dichlorobenzene as the organic solvent. We have achieved significant improvements in the mechanical and electrical properties of the organogels by incorporating pristine or carboxylated carbon nanotubes. For example, the linear viscoelastic regime of the HSA organogel, an indicator of the strength of the gel, extends by a factor of four with the incorporation of 0.2 wt% of the carboxylated nanotubes. Also, the carbon nanotubes (specially the pristine tubes) improve the electrical conductivity of the organogels, e.g. six orders of magnitude enhancement in electrical conductivity with 0.2 wt% of pristine tubes. Differential scanning calorimetry experiments indicate that the nanotubes do not affect the thermoreversibility of the organogels.
Co-reporter:Minfang Mu, Nigel Clarke, Russell J. Composto and Karen I. Winey
Macromolecules 2009 Volume 42(Issue 18) pp:7091-7097
Publication Date(Web):August 18, 2009
DOI:10.1021/ma901122s
Nanoparticles present a new frontier for understanding polymer dynamics in complex, nanoscale environments. We report that the addition of single-walled carbon nanotubes (SWCNTs) produces a minimum in the diffusion coefficient with increasing nanoparticle concentration, ϕ. Initially, tracer diffusion coefficients (D) are suppressed with increasing ϕ and then increase beyond a critical concentration, ϕcrit < 1 vol %. Shorter tracer chains exhibit a greater slowing down than longer chains, whereas longer matrix chains decrease the value of ϕcrit. The experimental results are discussed in terms of locally anisotropic diffusion perpendicular and parallel to the nanotube filler and simulated using a trap model that defines a trap size and the extent of slowing perpendicular to the cylindrical trap. The simulated diffusion coefficients capture both the initial decrease in D attributed to isolated traps and the recovery of D above ϕcrit corresponding to trap percolation. Nanoparticles influence polymer diffusion in fascinating ways and will refine our understanding of polymer reptation and might also inform the study of biopolymer diffusion in living systems.
Co-reporter:Wenqin Wang, Tsung-Ta Chan, Andrew J. Perkowski, Shulamith Schlick, Karen I. Winey
Polymer 2009 50(5) pp: 1281-1287
Publication Date(Web):
DOI:10.1016/j.polymer.2009.01.007
Co-reporter:Nancy C. Zhou, Christopher D. Chan and Karen I. Winey
Macromolecules 2008 Volume 41(Issue 16) pp:6134-6140
Publication Date(Web):July 29, 2008
DOI:10.1021/ma800805m
This paper reports the first quantitative reconciliation of imaging and scattering data for poly(styrene-ran-styrenesulfonate) (P(S-SSx)) ionomers. We examined the morphology of solvent-cast and spin-cast P(S-SS0.019)-M ionomers using the combination of scanning transmission electron microscopy (STEM) and X-ray scattering, where the scattering data were fit with a liquidlike hard-sphere model. Both the ionic aggregate sizes (R1) and the sample volume per ionic aggregate (VP) as measured by both techniques were in good agreement. In addition, STEM found that P(S-SS0.019) ionomers prepared by spin-casting exhibit nanometer spherical ionic aggregates that are indistinguishable in size, shape, and spatial distribution from the bulk solvent-cast ionomers. Six P(S-SS0.019)-M ionomers fully neutralized with various cations have ionic aggregate compositions that are predominately ionic, and the ionic aggregate radius (R1) increases as the cation radii increases. Finally, the influence of copolymer type was studied by comparing P(S-SS0.070) and P(S-MAA0.072) ionomers. The ionic aggregates in P(S-SS0.070)-Cu are surrounded by a thicker region of limited mobility and are more ionic as compared with P(S-MAA0.072) ionomers. Although STEM and X-ray scattering have been reconciled for these P(S-SSx) ionomers, a broad application of the liquidlike hard-sphere scattering model is not recommended. However, when STEM and X-ray scattering are reconciled, detailed morphological information can be extracted from the scattering data, particularly regarding the composition of the ionic aggregates, which is important for understanding the mechanisms of ion transport.
Co-reporter:Russel M. Walters, Andreas Taubert, Joon-Seop Kim, Karen I. Winey and Russell J. Composto
Macromolecules 2008 Volume 41(Issue 23) pp:9299-9305
Publication Date(Web):November 7, 2008
DOI:10.1021/ma801756g
The surface segregation of cations in a poly(styrene-ran-methacrylic acid) ionomer fully neutralized with Cs was demonstrated using Rutherford backscattering spectrometry (RBS), scanning force microscopy (SFM), and scanning transmission electron microscopy (STEM). Whereas spin-cast films and those annealed below ∼120 °C exhibit a uniform distribution of Cs, a surface excess of Cs was observed for films annealed at higher temperatures. At long times (>30 h) and high temperatures (>145 °C), the surface concentration of Cs approached a constant value of two-thirds of the total Cs in the film. Although Cs-rich vesicular aggregates (∼8−85 nm diameter) were observed in all films, the surface excess of Cs coincided with nanometer-sized features on the surface. Based on these results, a mechanism was proposed that accounts for cation mobility and a driving force for surface segregation. At elevated temperatures, Cs ions initially in cation-acid lone pairs are solubilized by favorable cation-π interactions facilitated by styrene monomers. Above ∼120 °C, these solubilized cations are sufficiently mobile to diffuse. The driving force to the surface arises from the concentration gradient established when Cs at the surface scavenges Cl from the environment to form CsCl. In the polystyrene-based ionomers, surface segregation is not observed if either the cation mobility is reduced by using a divalent cation or the driving force for surface segregation is removed by eliminating atmospheric Cl.
Co-reporter:Fangming Du;Csaba Guthy;Takashi Kashiwagi;John E. Fischer
Journal of Polymer Science Part B: Polymer Physics 2006 Volume 44(Issue 10) pp:1513-1519
Publication Date(Web):13 APR 2006
DOI:10.1002/polb.20801
Recent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1513–1519, 2006
Co-reporter:Nicholas M. Benetatos
Journal of Polymer Science Part B: Polymer Physics 2005 Volume 43(Issue 24) pp:3549-3554
Publication Date(Web):31 OCT 2005
DOI:10.1002/polb.20623
The morphology of ionic aggregates in semicrystalline Zn- and Na-neutralized poly(ethylene-ran-methacrylic acid) (EMAA) ionomer blown films has been explored with scanning transmission electron microscopy (STEM) and small angle X-ray scattering. The ionic aggregates of Zn-EMAA are spherical, monodisperse, and uniformly distributed in as-extruded pellets and blown films prepared at low and high blow-up ratio. Thus, although the biaxial stresses of film blowing are sufficient to alter the PE superstructure, the ionic aggregates in Zn-EMAA are unaffected. In contrast, the morphology of Na-EMAA as detected by STEM changes from featureless in the as-extruded pellets to a heterogeneous distribution of Na-rich aggregates in the blown films. This transformation in Na-EMAA morphology is consistent with our earlier study of quiescent annealing, suggesting that the morphological change is the result of thermal processing rather than the biaxial stresses of film blowing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3549–3554, 2005
Co-reporter:Minfang Mu ; Russell J. Composto ; Nigel Clarke
Macromolecules () pp:
Publication Date(Web):October 19, 2009
DOI:10.1021/ma9014033
Polymer tracer diffusion in multiwall carbon nanotube (MWCNT)/polymer nanocomposites is reported. As previously reported for SWCNT/polystyrene (PS) nanocomposites, the tracer diffusion of 680k deuterated polystyrene (dPS) is strongly suppressed at low MWCNT concentrations and then increases at higher concentrations. In contrast, the tracer diffusion of 10k dPS and 75k dPS is independent of MWCNT loading. These results reveal an important criterion for exhibiting a minimum in the tracer diffusion coefficient (Dmin) with nanoparticle concentration, namely the relative size of tracer molecule and nanoparticle. Specifically, when the radius of gyration of the tracer polymer (Rg) is smaller than the radius of the nanotube particle (RCNT), the tracer diffusion is independent of nanoparticle concentration, while a Dmin is observed when Rg > RCNT. When the tracer molecule is large relative to the nanoparticle, the diffusion in the vicinity of the nanoparticle appears to become anisotropic, which leads to a Dmin with increasing nanoparticle loading.
