Co-reporter:Adam B. Burns and Richard A. Register
Macromolecules October 24, 2017 Volume 50(Issue 20) pp:8106-8106
Publication Date(Web):October 5, 2017
DOI:10.1021/acs.macromol.7b01632
Crystallization within lamellar block copolymer microdomains, at temperatures above the glass transition of the amorphous block—a form of “soft”, one-dimensional confinement—is investigated in a series of hydrogenated diblocks of norbornene and hexafluoroisopropanol-substituted norbornene, abbreviated hPN-hPHFAN. Crystallization results in a large (up to 23%) increase in the lamellar period (d), but in contrast to a multitude of previous reports—where large changes in d signify breakout of the crystallites from the microdomains, such that the initial and final domain structures and orientations bear no coherent relationship to each other—the hPN crystallites in hPN-hPHFAN remain confined within the microphase-separated lamellae, as revealed by small-angle X-ray scattering on highly oriented flow-aligned specimens. As the d-spacing dilates, the lamellae contract affinely in the lateral directions (as measured by confocal optical microscopy on a millimeter-scale flow-aligned specimen); dimensional changes in the macroscopic specimen (millimeter scale) precisely track the changes in d (≈50 nm scale). The increase in d, and concomitant changes in the macroscopic specimen dimensions, are fully reversible on melting, over multiple melting–recrystallization cycles. The crystallites are found to be oriented with the b-axis parallel to the microdomain interfaces and the crystal stems tilted with respect to the lamellar normal. In this orientation, the crystal thickness is limited by the thickness of the hPN domains, such that the domain spacing increase is driven by the enhanced thermodynamic stability of thicker hPN crystals.
Co-reporter:William D. Mulhearn and Richard A. Register
Macromolecules December 26, 2017 Volume 50(Issue 24) pp:9666-9666
Publication Date(Web):December 12, 2017
DOI:10.1021/acs.macromol.7b02454
The low-strain mechanical properties of linear polyethylene (PE) can be substantially altered by the incorporation of a short block of a polymer with a high glass transition temperature (Tg) into a majority-PE block copolymer. In particular, the yield stress and the tensile modulus can be sharply increased with the addition of a relatively small fraction of glassy block, especially when combined with a thermal history that promotes high crystallinity and crystal thickness of the PE block. For example, the incorporation of 15 wt % of a hydrogenated poly(norbornyl norbornene) block (Tg = 115 °C) into a PE diblock copolymer, cooled from the melt at ∼1 °C/min, doubles the yield stress (from ∼30 to ∼60 MPa) and tensile modulus (from ∼1.5 to ∼3.5 GPa) relative to the values for a PE homopolymer treated with the same thermal history. These property enhancements are closely associated with the composition of the amorphous layer between crystal lamellae and the spatial distribution of the glassy block within this layer. Finally, the ductility of these polymers at high strains is governed by the presence or absence of tie molecules, which can be correlated with the chain dimensions of the PE block in the melt and the distance between crystal lamellae.
Co-reporter:William D. Mulhearn and Richard A. Register
Macromolecules August 8, 2017 Volume 50(Issue 15) pp:5830-5830
Publication Date(Web):July 25, 2017
DOI:10.1021/acs.macromol.7b01295
The thermodynamic interaction strengths between linear polyethylene (PE) and members of a family of hydrogenated polynorbornenes prepared by ring-opening metathesis polymerization can be tuned across a wide range via the choice of substituent appended to the polynorbornene backbone at the 5-position. Isopropyl and certain n-alkyl groups yield polynorbornenes that are highly miscible with PE, capable of producing symmetric diblock copolymers with homogeneous melts at molecular weights in excess of 100 kg/mol. In contrast, phenyl-substituted polynorbornenes are quite immiscible with PE, exhibiting microphase separation in the melt at diblock molecular weights as low as 10 kg/mol. Interaction strengths within this series of polymers do not quantitatively obey regular mixing; entropic contributions to the mixing energy arising from mismatches in free volume and chain stiffness cannot account for the observed deviations. Instead, the interactions can be satisfactorily described by an empirical mixing rule of the form X = (Δγ)1.5, where X is the interaction energy density and γ is a pure-component quantity, operationally analogous to a solubility parameter, with a distinct value for each polymer. These empirical γ parameters are obtained by regression against the entire set of experimental pair interaction energies.
Co-reporter:William D. Mulhearn and Richard A. Register
ACS Macro Letters August 15, 2017 Volume 6(Issue 8) pp:808-808
Publication Date(Web):July 18, 2017
DOI:10.1021/acsmacrolett.7b00443
A new diblock copolymer chemistry, hydrogenated poly(n-hexyl norbornene)-block-poly(cyclohexyl norbornene), is demonstrated to undergo a lower critical ordering transition (LCOT) upon heating. Diblock copolymers exhibiting LCOT behavior are rare, and to our knowledge, the system described in this work is the first all-hydrocarbon species to do so. Furthermore, the tendency toward demixing for this polymer at high temperatures does not arise from a large free volume mismatch between its components, the mechanism most commonly invoked to explain an LCOT in block copolymers or a lower critical solution temperature in polymer blends. We compare the LCOT-type polymer with a homologous family of other norbornene-based polymers exhibiting the more common upper critical ordering transition and demonstrate that the mismatch in thermal expansion coefficients between blocks, which is related to the mismatch in free volumes, does not dictate the type of phase behavior.
Co-reporter:William D. Mulhearn and Richard A. Register
ACS Macro Letters 2017 Volume 6(Issue 2) pp:
Publication Date(Web):January 20, 2017
DOI:10.1021/acsmacrolett.6b00969
Methods for the preparation of narrow-distribution ROMP polycyclopentene are developed to suppress the rate of acyclic metathesis: reaction between the active metal-carbene chain end and an acyclic olefin in the reaction medium. In particular, we investigate interchain metathesis, which generates linear polymers with “scrambled” chain lengths, and we demonstrate the formation of ring polymers by intrachain backbiting and quantify their content in the reaction product. By controlling the relative rates of propagation versus these side reactions, we prepare ROMP polycyclopentene with low dispersity to substantially higher molecular weights than have been reported previously. Polymerization kinetics are quantitatively described by a kinetic model, which accounts for the reversible binding of added trimethylphosphine to the active chain end.
Co-reporter:Adam B. Burns and Richard A. Register
Macromolecules 2016 Volume 49(Issue 24) pp:9521-9530
Publication Date(Web):December 9, 2016
DOI:10.1021/acs.macromol.6b02175
The mechanical properties of thermoplastic elastomers (TPEs) consisting of 6-arm star block polymers with glassy, crystalline, or composite crystalline–glassy physical cross-linking (hard) domains were investigated and compared to the analogous linear triblock or pentablock polymers. The 6-arm stars exhibited qualitatively similar solid-state morphologies and phase behavior to their linear counterparts, as demonstrated by small-angle X-ray scattering and differential scanning calorimetry. Consequently, the architecture had minimal impact on the small-strain behavior in uniaxial extension at room temperature. As the applied strain increased, the star polymers exhibited more pronounced strain hardening than the corresponding linear TPEs, resulting in an increase in the ultimate strength of 20% for the polymers with crystalline end blocks and 30% when the end blocks were glassy. Each of the three star polymers exhibited superior recovery (i.e., lower residual strain) and lower hysteresis than the corresponding linear TPEs when subjected to repeated strain cycles. The enhancement in the recovery was most significant for the polymers with glassy hard domains. The TPEs with crystalline or crystalline–glassy domains recovered more rapidly than the corresponding linear block polymers but showed only modest improvements in the recovery measured after the specimens were allowed to rest for 5 min. These results indicate that the covalent junction at the core of the star strengthens and accelerates the recovery of the network but does not greatly suppress plastic deformation of the crystallites. Overall, this work demonstrates that the mechanical performance of block polymer TPEs can be improved by using a star macromolecular architecture.
Co-reporter:Adam B. Burns and Richard A. Register
Macromolecules 2016 Volume 49(Issue 1) pp:269-279
Publication Date(Web):December 29, 2015
DOI:10.1021/acs.macromol.5b02546
Block copolymers with crystalline, glassy, and rubbery blocks were synthesized by anionic polymerization of butadiene, styrene, and isoprene followed by chlorosilane coupling and hydrogenation. The performance of two pentablock copolymers, with the block sequence crystalline–glassy–rubbery–glassy–crystalline, as thermoplastic elastomers (TPEs) was evaluated against triblock copolymers having either crystalline or glassy end blocks. Judicious choices of block lengths yielded homogeneous melts for both pentablocks; consequently, the low-shear-rate viscosities of the pentablocks were more than 2 orders of magnitude lower than that of the glassy–rubbery–glassy triblock, which remained microphase-separated in the melt. In the pentablocks, physical cross-linking was achieved by crystallization of the end blocks followed by vitrification of the adjacent glassy blocks, forming composite crystalline–glassy hard domains. All of the polymers studied exhibited desirable mechanical behavior for TPEs, including low Young’s modulus, high extensibility, and low permanent set. Increasing the glassy block fraction (at constant hard block content and molecular weight) systematically improved the mechanical performance by reducing the Young’s modulus and increasing the ultimate strength; however, the strain recovery was still limited by the crystalline component. Taken in the context of prior work on semicrystalline TPEs, this work highlights the influence of the crystalline morphology on the mechanical properties.
Co-reporter:Adam B. Burns and Richard A. Register
Macromolecules 2016 Volume 49(Issue 6) pp:2063-2070
Publication Date(Web):March 10, 2016
DOI:10.1021/acs.macromol.5b02764
The synthesis of isoprene-based star polymers by anionic polymerization, chlorosilane coupling, and catalytic hydrogenation was studied in detail, with the goal of obtaining and preserving a well-defined star architecture. The coupling reaction between polyisoprenyllithium (PI-Li) and chlorosilanes, which can be intractably slow in apolar media, was dramatically accelerated by the concurrent addition of tetrahydrofuran (THF) with the coupling agent. In the presence of THF, PI-Li produces a yellow color allowing the living end concentration to be tracked visually, enabling near-stoichiometric coupling, as demonstrated by the synthesis of 6-arm star polymers and star block copolymers. Moreover, glassblowing techniques are not required. Star polymers coupled with chlorosilane terminating agents lost arms during catalytic hydrogenation. Several common hydrogenation catalysts were evaluated. The degree of degradation depended on the identity of the catalyst and the temperature of the reaction—more active catalysts and higher temperatures led to more scission—but not the reaction time. Degradation closely tracked the hydrogenation reaction indicating that scission is a catalytically activated process. This phenomenon is attributed to a decreasing affinity of the polymer for the catalyst with increasing saturation. Degradation of the star architecture depended strongly on the steric environment at the core. It was shown that the steric strain can be reduced by tuning the topology of the coupling agent to limit the number of arms per Si atom. The steric constraints at the core were further relaxed by adding a short run of butadiene units to the arms just prior to coupling, which nearly eliminated degradation during hydrogenation of a 6-arm star.
Co-reporter:Raleigh L. Davis
Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 2) pp:135-140
Publication Date(Web):
DOI:10.1002/polb.23806
ABSTRACT
This work explores coatings with thermally switchable wetting behavior, based on block copolymers that possess both hydrophilic and hydrophobic segments. The amphiphilic block copolymers were synthesized by coupling allyl-ended poly(ethylene oxide) (PEO) and hydride-ended poly(dimethylsiloxane) (PDMS) oligomers via a Pt catalyst. One near-symmetric diblock possessed an order-disorder transition temperature (TODT) of 64 °C. When cooled through TODT in ambient air, the PDMS domains wet the film's surface, producing a hydrophobic coating with a water contact angle (CA) = 90°. However, when cooled in humidified air, hydrophilic PEO domains form at the surface, yielding CA = 30–40°. The coatings can be reversibly switched between the two states by reheating above TODT, in the appropriate environment, and then cooling, rapidly generating the desired room-temperature surface wettability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 135–140
Co-reporter:Dong-Gyun Kim, Tamami Takigawa, Tomomasa Kashino, Oleksandr Burtovyy, Andrew Bell, and Richard A. Register
Chemistry of Materials 2015 Volume 27(Issue 19) pp:6791
Publication Date(Web):September 4, 2015
DOI:10.1021/acs.chemmater.5b03030
Vinyl addition polymers of substituted norbornene (NB) monomers possess very high glass-transition temperatures, making them useful in diverse applications; however, until very recently, the lack of an applicable living polymerization chemistry has precluded the synthesis of such polymers with controlled architecture, or copolymers with controlled sequence distribution. In the present work, block and random copolymers of NB monomers bearing hydroxyhexafluoroisopropyl and n-butyl substituents (HFANB and BuNB) are synthesized via living vinyl addition polymerization, using (η3-allyl)Pd(i-Pr3P)Cl activated by [Li(OEt2)2.5]B(C6F5)4 as the initiator. Both series of polymers are cast into the selective skin layers of thin film composite (TFC) membranes, and these organophilic membranes are investigated for the concentration of n-butanol from dilute aqueous solution via pervaporation. The block copolymers show well-defined microphase-separated morphologies, both in bulk and as the selective skin layers on TFC membranes, while the random copolymers are homogeneous. Both block and random vinyl addition copolymers are effective as n-butanol pervaporation membranes, with the block copolymers showing a better flux-selectivity balance; the optimal block copolymer, containing 19 wt % BuNB, showed a process separation factor of 21 and a flux of 4300 g m–2 h–1 with a 1.00 wt % aqueous n-butanol feed, at a selective layer thickness of 1.3 μm. While polyHFANB has much higher permeability and selectivity than polyBuNB, incorporating BuNB units into the polymer (in either a block or random sequence) limits the swelling of the polyHFANB and thereby improves the n-butanol pervaporation selectivity. An analogous block copolymer derived from ring-opening metathesis polymerization, which shows much greater swelling than the vinyl addition polymers, shows a correspondingly higher flux and lower selectivity.
Co-reporter:Dong-Gyun Kim, Andrew Bell, and Richard A. Register
ACS Macro Letters 2015 Volume 4(Issue 3) pp:327
Publication Date(Web):March 2, 2015
DOI:10.1021/acsmacrolett.5b00079
The vinyl addition polymerization of substituted norbornene (NB) monomers, via (t-Bu3P)PdMeCl activated by [Li(OEt2)2.5]B(C6F5)4, is investigated. NB monomers bearing alkyl, aryl, fluoroaryl, and even hexafluoroisopropanol substituents yield polymers exhibiting monomodal and narrow molecular weight distributions, with molecular weight controlled by reaction time and monomer to initiator ratio, demonstrating the living nature of these polymerizations. These polymers are soluble in common organic solvents and possess excellent thermal stability. Block copolymers are also prepared via sequential monomer addition; these are the first examples of well-defined block copolymers of substituted NB monomers enchained by vinyl addition polymerization.
Co-reporter:Raleigh L. Davis, Brian T. Michal, Paul M. Chaikin, and Richard A. Register
Macromolecules 2015 Volume 48(Issue 15) pp:5339-5347
Publication Date(Web):July 24, 2015
DOI:10.1021/acs.macromol.5b01028
Application of shear stress at the surface of a block copolymer thin film has been shown to substantially orient the microdomains in the direction of the applied shear. The present work systematically examines the influence of key material, film, and process parameters on this alignment behavior using a series of cylinder-forming polystyrene–poly(n-hexyl methacrylate) copolymers. A parallel plate rheometer applies a radially dependent stress gradient to the film’s surface through a viscous nonsolvent overlayer. The degree of alignment is assessed using atomic force microscopy and examined as a function of the applied stress. To quantitatively compare the alignment process across different block copolymer films, a melting–recrystallization model is fit to the data, which allows for the determination of two key alignment parameters: the critical stress needed for alignment and an orientation rate constant. For films containing a monolayer of cylindrical domains, as polystyrene weight fraction or overall molecular weight increases, the critical stress increases moderately, while the rate of alignment drastically decreases. As the number of layers of cylinders in the film increases, the critical stress decreases modestly, while the rate remains unchanged. Substrate wetting condition has no measurable influence on alignment response over the range of conditions investigated. Collectively, these results provide useful quantitative rules that enable predictions of the level of alignment which will occur under particular shearing conditions.
Co-reporter:So Youn Kim;Jessica Gwyther;Ian Manners;Paul M. Chaikin
Advanced Materials 2014 Volume 26( Issue 5) pp:791-795
Publication Date(Web):
DOI:10.1002/adma.201303452
Co-reporter:So Youn Kim, Adam Nunns, Jessica Gwyther, Raleigh L. Davis, Ian Manners, Paul M. Chaikin, and Richard A. Register
Nano Letters 2014 Volume 14(Issue 10) pp:5698-5705
Publication Date(Web):September 11, 2014
DOI:10.1021/nl502416b
While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.
Co-reporter:Raleigh L. Davis, Sahana Jayaraman, Paul M. Chaikin, and Richard A. Register
Langmuir 2014 Volume 30(Issue 19) pp:5637-5644
Publication Date(Web):2017-2-22
DOI:10.1021/la501247x
Flowcoating is a popular technique for generating thin (5–200 nm), substrate-supported polymer films. In this process, a reservoir of coating fluid is held between the horizontal substrate and a nearly horizontal blade above the substrate; a film of fluid is drawn out of the reservoir by moving the substrate. Accelerating the substrate produces a film with a thickness gradient, particularly useful for high-throughput measurements where film thickness is an important parameter. The present work compares experimental film thickness profiles with a model based on a Landau–Levich treatment to identify the experimental parameters which govern film thickness. The key parameters are the capillary number and the radius of curvature of the reservoir’s static meniscus, which is set by the blade angle, gap height, solution reservoir volume, and contact angles of the fluid with the blade and substrate. The results show excellent quantitative agreement with the first-principles model; the model thus provides a design approach which allows a user to produce polymer thin films of virtually any desired thickness profile.
Co-reporter:Raleigh L. Davis, Paul M. Chaikin, and Richard A. Register
Macromolecules 2014 Volume 47(Issue 15) pp:5277-5285
Publication Date(Web):August 1, 2014
DOI:10.1021/ma5012705
Thin films of cylinder-forming polystyrene–poly(n-hexyl methacrylate) diblock copolymers, PS–PHMA, are attractive as nanolithographic templates, an application which demands good control over the thin-film structure—yet the quality of order achieved to date in PS–PHMA films has fallen short of this ideal. In the present work, a series of PS–PHMA diblocks of varying composition, all forming PS cylinders, are synthesized and their morphology studied as a function of film thickness in both nonsheared and shear-aligned films. In nonsheared films, the cylinder axis orientation relative to the surface switches from parallel to perpendicular as a function of film thickness; this oscillation is damped out as the fraction of the cylinder-forming block (PS) increases, away from the sphere–cylinder phase boundary. In aligned films, thicknesses which possess the highest coverage of parallel cylinders prior to shear show the highest quality of alignment postshear, as measured by the in-plane orientational order parameter of the cylinder axis. In well-aligned samples of optimal thickness, the quality of alignment is limited by isolated dislocations, whose density is highest at high PS contents, and by undulations in the cylinders’ trajectories, whose impact is most severe at low PS contents; consequently, polymers whose compositions lie in the middle of the cylinder-forming region exhibit the highest quality of alignment.
Co-reporter:Ha-Kyung Kwon, Vanessa E. Lopez, Raleigh L. Davis, So Youn Kim, Adam B. Burns, Richard A. Register
Polymer 2014 Volume 55(Issue 8) pp:2059-2067
Publication Date(Web):10 April 2014
DOI:10.1016/j.polymer.2014.02.047
Anionic polymerization was employed to synthesize well-defined diblock copolymers of polystyrene and poly(2-ethylhexylmethacrylate), PS-PEHMA. Diblock morphologies in bulk and in substrate-supported thin films were characterized by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively. PS-PEHMA diblocks exhibited thermotropic order-disorder transitions; one diblock showed a thermoreversible transition between lamellae and a higher-temperature morphology assigned as perforated lamellae. Unlike PS-poly(alkylmethacrylate) diblocks where the alkyl group is n-butyl or n-pentyl, PS-PEHMA diblocks showed a typical decreasing Flory interaction parameter with increasing temperature. Thin films of PS-cylinder-forming PS-PEHMA diblocks showed a strong preference for the cylinders to lie in the plane of the film; films of incommensurate thickness readily formed terraces. Films of commensurate thickness were easily aligned over macroscopic areas through the application of mechanical shear.
Co-reporter:Bryan S. Beckingham and Richard A. Register
Macromolecules 2013 Volume 46(Issue 8) pp:
Publication Date(Web):April 2, 2013
DOI:10.1021/ma4002512
Random copolymerization of A and B monomers represents a versatile method to tune interaction strengths between polymers, as ArB random copolymers will exhibit a smaller effective Flory interaction parameter χ (or interaction energy density X) upon mixing with A or B homopolymers than upon mixing A and B homopolymers with each other, and the ArB composition can be tuned continuously. This approach can also be used to tune the segregation strength in A–ArB “block–random” copolymers. Simple models of polymer mixing thermodynamics suggest that the effective interaction energy density in such block–random copolymers should follow XA–ArB = fB2XA–B, but this prediction has not been tested quantitatively. The present work systematically assesses the validity of this rule for thermally stable hydrogenated derivatives of styrene–isoprene block copolymers, through measurements of the order–disorder transition (ODT) temperature on near-symmetric diblock and diblock–random copolymers of varying composition and suitable molecular weight (M). Both hydrogenated derivatives wherein the styrene aromaticity is retained, and derivatives wherein the styrene units are saturated to vinylcyclohexane, are examined, and both are found to closely obey the XA–ArB = fB2XA–B prediction, thereby confirming the utility of this simple relationship in designing block copolymers with targeted interaction strengths using only these two common monomers. The reduction in XA–ArB over XA–B permits the synthesis of polymers having much larger M and domain spacing d while maintaining a thermally accessible ODT; measured domain spacings are found to closely follow the expected scaling, d ∼ X1/6M2/3.
Co-reporter:Bryan S. Beckingham, Adam B. Burns, and Richard A. Register
Macromolecules 2013 Volume 46(Issue 7) pp:2760-2766
Publication Date(Web):March 26, 2013
DOI:10.1021/ma400311p
The mixing interactions among polyolefins and other hydrocarbon polymers are of strong fundamental and practical interest, especially in mixtures involving the simplest member of the family, polyethylene (E). The present work examines the interaction energy densities between E and random copolymers of styrene and hydrogenated isoprene (SrhI), and between E and random copolymers of vinylcyclohexane and hydrogenated isoprene (VCHrhI), by measuring the order–disorder transition temperatures of near-symmetric E–SrhI and E–VCHrhI diblock–random copolymers. The E–SrhI case is of special interest, since the solubility parameters δ fall in the order δS > δE > δhI; if regular mixing were obeyed, zero interaction energy between E and SrhI could be obtained for a suitable SrhI composition. However, large positive deviations from regular mixing are observed in the E–SrhI system, while smaller but significant negative deviations are observed in the E–VCHrhI system. Notwithstanding these irregularities, a ternary mixing model (“copolymer equation”), using independently determined values of the three component interaction energy densities, provides a good representation (within ≈15%) of the experimental interaction energies.
Co-reporter:Bryan S. Beckingham and Richard A. Register
Macromolecules 2013 Volume 46(Issue 9) pp:3486-3496
Publication Date(Web):April 15, 2013
DOI:10.1021/ma400397h
The extent of block microphase separation in nonfrustrated A–B–C triblock copolymers forming a “three-domain, four-layer” lamellar morphology is examined. Specifically, the extent of separation between the B and C blocks is probed, for the case where the B and C blocks are sufficiently compatible that they would not be microphase-separated if they were connected as a diblock. However, attachment of the A block, and consequent localization of the A–B block junction to the A–B lamellar interface, induces extensive separation between the B and C blocks. This separation is revealed both through the small-angle X-ray scattering pattern in the melt, and by distinct glass transitions observed in the solid state for the B block at low B–C segregation strengths, and for both the B and C blocks at higher segregation strengths. The particular polymers studied here have polyethylene as the A block; except for the most weakly segregated triblock, upon cooling from the melt, crystallization of the polyethylene block is confined within the lamellar structure established in the melt, with the polyethylene crystals stacking orthogonally to the microdomain lamellae.
Co-reporter:Michael T. Showak, Adam B. Burns, Andrew J. Stella, and Richard A. Register
Macromolecules 2013 Volume 46(Issue 23) pp:9288-9295
Publication Date(Web):November 22, 2013
DOI:10.1021/ma401834t
Model crystallizable copolymers of norbornene and two 5-alkylnorbornenes were synthesized to investigate the extent and consequences of defect inclusion into hydrogenated polynorbornene (hPN) crystals. Living ring-opening metathesis polymerization yielded narrow-distribution polymers of targeted molecular weights, with modest down-chain compositional gradients controllable through the polymerization conversion; hydrogenation yielded semicrystalline copolymers. When the comonomer was 5-methylnorbornene (MeN), extensive inclusion of MeN units into the hPN crystal was observed; the copolymers showed substantial crystallinities even above 30 mol % MeN, and the dependence of the melting point Tm on crystal thickness followed that for hPN homopolymer. By contrast, when the comonomer was 5-hexylnorbornene, the more usual case of strong exclusion of the counits from the crystal was observed. hPN shows a transition between two crystal polymorphs below Tm, at a temperature Tcc; comonomer incorporation reduces Tcc more rapidly than it reduces Tm, expanding the region over which the high-temperature rotationally disordered polymorph is stable and providing insight into the dependence of the free energy for the two polymorphs on crystal thickness.
Co-reporter:Saswati Pujari, Michael A. Keaton, Paul M. Chaikin and Richard A. Register
Soft Matter 2012 vol. 8(Issue 19) pp:5358-5363
Publication Date(Web):03 Apr 2012
DOI:10.1039/C2SM25270H
Lamellar block copolymers, with the lamellae standing perpendicular to the substrate, are attractive candidates as templates for nanostructure array fabrication. However, no process currently exists to impose long-range in-plane alignment of such perpendicular lamellae on simple unpatterned substrates—to align the lamellar normal over macroscopic distances. Here, we have generated such aligned films of perpendicular lamellae in a polystyrene-poly(methylmethacrylate) diblock, PS-PMMA, by neutralizing the substrate with a random terpolymer brush and shearing the film with a moving polydimethylsiloxane (PDMS) pad in contact with the film surface. At sufficiently high shear stresses, the lamellae align over the entire (cm2) area of the pad; the perpendicular orientation of the lamellae is preserved, although for films thicker than the lamellar spacing, a “capping layer” of PS forms in contact with the PDMS pad. However, when compared with typical shear-aligned block copolymers having a morphology of in-plane cylinders, a significantly higher stress is required to align the lamellar PS-PMMA, and the orientational order is poorer and the dislocation density higher; a limiting order parameter ψ2 ≈ 0.8 is achieved at high stresses.
Co-reporter:Sheng Li;Adam B. Burns;Andrew Bell
Macromolecular Chemistry and Physics 2012 Volume 213( Issue 19) pp:2027-2033
Publication Date(Web):
DOI:10.1002/macp.201200294
Abstract
Novel polymers are synthesized from 5-phenyl-2-norbornene (PhNb) and its saturated side group analog, 5-cyclohexyl-2-norbornene, using ring-opening metathesis polymerization (ROMP). Polymers of both endo-rich and all-exo PhNb show glass transition temperatures (Tg) = 88 ± 1 °C, indicating a negligible effect of monomer stereoisomerism on segmental packing or the energy barriers to motion at the glass transition, despite the substantial size of the side group. Post-polymerization hydrogenation of the PhNb polymers using catalysts with different selectivities reveals that saturation of the backbone produces a 17 °C decrease in Tg (for both aromatic and cycloaliphatic side groups), whereas saturation of the side groups produces a 14 °C increase in Tg (for both saturated and unsaturated backbones).
Co-reporter:Sheng Li and Richard A. Register , Jeffrey D. Weinhold and Brian G. Landes
Macromolecules 2012 45(14) pp: 5773-5781
Publication Date(Web):June 29, 2012
DOI:10.1021/ma300910m
Crystallization in polydisperse ethylene–octene multiblock copolymers, polymerized via chain shuttling chemistry, is examined using two-dimensional synchrotron small- and wide-angle X-ray scattering on flow-aligned specimens. The multiblocks are composed of alternating crystalline (hard) blocks of low 1-octene content and amorphous (soft) blocks of high 1-octene content; the block lengths and the number of blocks per chain are characterized by most-probable distributions. These polymers self-assemble into lamellar domain morphologies in the melt, and the melt morphology is retained in the solid state. Despite extensive mixing between hard and soft blocks, the high crystallinity (>50%) of the hard blocks leads to an alignment of the crystallites within the domain structure, with the orthorhombic polyethylene c-axis generally perpendicular to the lamellar domain normal. The interlamellar domain spacings exhibited by the multiblocks, which exceed 100 nm, are estimated to be 5 times larger than those in near-monodisperse block copolymers having a similar chemical composition and a number-average molecular weight equivalent to the multiblock’s “constituent diblock” repeating unit. This swelling factor exceeds the value of 3 previously reported for analogous polydisperse olefin diblock copolymers, due to the lower segregation strength and enhanced phase mixing of the multiblocks studied here.
Co-reporter:John P. Bishop
Journal of Polymer Science Part B: Polymer Physics 2011 Volume 49( Issue 1) pp:68-79
Publication Date(Web):
DOI:10.1002/polb.22124
Abstract
At a temperature Tcc well below its melting point Tm, hydrogenated ring-opened polynorbornene (hPN) is known to exhibit a crystal–crystal transition; above Tcc, the hPN chains are rotationally disordered. This transition is examined in a series of hPNs polymerized with different Mo- and Ru-based catalysts, each of which imparts a slightly different tacticity to the polymer. Tcc is found to correlate well with the ratio of meso to racemo dyads (m:r); small changes in m:r (from 0.8 to 1.1) are sufficient to raise Tcc by nearly 20 °C. For the homogeneous Mo-based “Schrock-type” catalyst examined, such a change in m:r is easily achieved by simply adding the reversibly binding ligand trimethylphosphine during polymerization. Tcc approaches Tm with increasing m:r, indicating that r dyads stabilize the rotationally disordered structure. When heated above Tcc, hPN crystals thicken at a rate much greater than conventional three-dimensionally ordered crystals, but below the rates shown by the two-dimensional hexagonal (columnar) phase formed by some polymers, reflecting the intermediate level of order and chain mobility present in the high-temperature hPN crystal phase. Solid-state processing of hPN between Tcc and Tm yields highly aligned macroscopic specimens. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010
Co-reporter:Sheng Li, Sasha B. Myers, and Richard A. Register
Macromolecules 2011 Volume 44(Issue 22) pp:8835-8844
Publication Date(Web):October 17, 2011
DOI:10.1021/ma201951j
Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) are synthesized, and their crystallization behavior and morphology are examined using small-angle (SAXS) and wide-angle X-ray scattering (WAXS). In symmetric hPN/LPE diblocks with molecular weights above 50 kg/mol, the hPN block has previously been shown to crystallize first and set the solid-state microstructure. Two-dimensional WAXS on hand-drawn fiber specimens reveals that the LPE crystals formed in confinement stack orthogonally to the hPN crystals. By adjusting total molecular weight, the order of block crystallization may be reversed, even while holding the block length ratio fixed. At a diblock molecular weight of 20 kg/mol, simultaneous time-resolved SAXS/WAXS reveals that the LPE block crystallizes first, even when LPE is the minority component, and restricts hPN to crystallize between the LPE lamellae. The relative orientation of the LPE and hPN crystals in the lower molecular weight diblocks is examined by modeling changes in the SAXS primary peak intensity on cooling two diblocks through the hPN crystal–crystal transition, where hPN densifies as it adopts a rotationally ordered crystal structure. Only a perpendicular stacking of hPN and LPE crystals consistently yields the large reduction in primary SAXS peak intensity observed for both diblocks. Thus, even though the templating block switches from hPN to LPE as the diblock molecular weight is reduced, the orthogonal stacking motif is retained for both high- and low-molecular-weight copolymers.
Co-reporter:Bryan S. Beckingham and Richard A. Register
Macromolecules 2011 Volume 44(Issue 11) pp:4313-4319
Publication Date(Web):May 9, 2011
DOI:10.1021/ma200913k
“Block-random” copolymers—wherein one or more blocks is itself a random copolymer—present a useful and convenient variation on the typical block copolymer architecture, as the interblock interactions and physical properties can be tuned continuously through the random block’s composition. However, typical living or controlled polymerizations produce compositional gradients along the “random” block, which can in turn influence the phase behavior. Organolithium initiation in a cyclohexane/triethylamine mixture is shown herein to yield narrow-distribution copolymers of styrene and isoprene of any desired composition, with no measurable down-chain gradient. These random copolymers (SrI) are also successfully incorporated into well-defined symmetric block copolymers (I-SrI diblocks). Isoprene-selective hydrogenation yields thermally stable hI-SrhI diblocks, which self-assemble into well-defined lamellar morphologies with sharply defined order–disorder transitions, whose temperatures TODT scale predictably with diblock molecular weight. The use of SrhI in lieu of a styrene homopolymer block allows the diblock molecular weight and domain period to be substantially increased for a given value of TODT. The measured interaction energy density between hI and SrhI is consistent with the mean-field “copolymer equation”, providing a first step toward the design of styrene–isoprene block-random copolymers of desired molecular weight and TODT.
Co-reporter:Katsuyuki Wakabayashi and Richard A. Register
Industrial & Engineering Chemistry Research 2010 Volume 49(Issue 23) pp:11906-11913
Publication Date(Web):April 12, 2010
DOI:10.1021/ie100109e
The phase behavior in blends of magnesium salts of long-chain fatty acids with magnesium-neutralized ethylene-based ionomers is described. Anhydrous magnesium stearate and magnesium stearate−palmitate, collectively termed MgSt(Pm), show a common phase behavior, consisting of a crystalline lamellar mesophase (LAM) bilayer phase at low temperatures, a liquid crystalline hexagonal (HEX) phase at intermediate temperatures, and a molten disordered (DIS) phase at high temperatures. Once heated into the HEX or DIS phase, neat MgSt(Pm) is kinetically prevented from reforming the LAM phase upon cooling. Blending an ionomer into MgSt(Pm) destabilizes the HEX phase in favor of DIS; an intimately mixed DIS phase is formed above the LAM and ionomer melting points, where the ionomer acts as an effective solvent for the MgSt(Pm). Quenching this solution forms co-crystals that incorporate alkyl tails from the fatty acid salt and ethylene sequences from the ionomer. Heating the blend to below the LAM melting point melts the co-crystals and allows pure, well-ordered LAM MgSt(Pm) to crystallize from the solution. A generalized phase diagram for MgSt blended with ionomers is presented to describe the observed behavior.
Co-reporter:Sasha B. Myers and Richard A. Register
Macromolecules 2010 Volume 43(Issue 1) pp:393-401
Publication Date(Web):November 17, 2009
DOI:10.1021/ma9019587
We report the morphology and orientation of the crystals which form within the microdomain structures of diblock copolymers of linear polyethylene and glassy poly(vinylcyclohexane), LPE/PVCH, across the spherical, cylindrical, and lamellar morphologies. Compared with short-branched polyethylene (such as hydrogenated polybutadiene), confinement of LPE within spheres, within cylinders, or between PVCH cylinders directly limits the crystal thickness and thereby the crystal melting point. Conversely, crystals formed within LPE lamellae are stacked orthogonally to the LPE/PVCH microdomain layering, so there is no direct limitation imposed on crystal thickness by confinement. As with LPE homopolymer, LPE crystals within lamellae thicken when annealed below the melting point, ultimately forming crystals whose thickness is significantly larger than their lateral extent, set by the bounding PVCH layers. The ribbon-like crystals which form within LPE cylinders or lamellae have a strong orientational coupling to the microdomains; prealignment of the cylindrical or lamellar mesophase by extensional flow yields macroscopic specimens with pronounced b-axial and a-axial orientations, respectively, after subsequent quiescent crystallization.
Co-reporter:John P. Bishop, Richard A. Register
Polymer 2010 Volume 51(Issue 18) pp:4121-4126
Publication Date(Web):19 August 2010
DOI:10.1016/j.polymer.2010.06.061
The chain microstructure (cis/trans ratio) in ring-opening metathesis polymerization (ROMP) of norbornene and methyltetracyclododecene (MTD) using the Schrock-type initiator Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCMe3)2 is shown to be a strong function of monomer concentration, providing a convenient means for tuning the average trans content in the resulting polymers. Moreover, since this is a “living” ROMP initiator, chains formed in batch polymerizations show a continuous gradient of trans content down the chain, with the trans content increasing with conversion. This gradient can be quite substantial; for a typical norbornene polymerization, trans content varies from 30% at one end of the chain to nearly 80% at the other end. The results are explained based on a literature kinetic description for the behavior of this initiator, where cis and trans units are added to the chain by syn and anti rotamers of the active site, respectively. A quantitative mathematical description is developed for the chain microstructure, and associated kinetic parameters are measured for norbornene and MTD at room temperature. The model accurately describes both the variation in average trans content with starting monomer concentration, and the gradients in trans content measured via samples taken at different conversions throughout the polymerization. In contrast, ROMP of MTD using the first-generation Grubbs’ initiator Ru(CHPh)Cl2(PCy3)2 shows no down-chain gradient in trans content.
Co-reporter:John P. Bishop and Richard A. Register
Macromolecules 2010 Volume 43(Issue 11) pp:4954-4960
Publication Date(Web):May 7, 2010
DOI:10.1021/ma100314z
We report the synthesis and characterization of thermoplastic elastomers (TPEs) containing both crystalline and glassy hard segments, with the aim of capturing the mechanical properties of conventional all-amorphous triblock TPEs, while forming the solid-state structure by crystallization from a single-phase melt. To accomplish this, we used living ring-opening metathesis polymerization (ROMP) and subsequent hydrogenation to synthesize symmetric pentablock copolymers with the architecture crystalline−glassy−rubbery−glassy−crystalline. Analogous crystalline−rubbery−crystalline triblocks show a high initial modulus, yielding, and poor recovery, resulting from platelike crystalline hard blocks. By contrast, with the pentablock architecture and appropriate selection of block lengths, crystallization from a single-phase melt causes a layer rich in the glassy block to form around the crystallites, limiting their lateral growth and generating composite hard domains with both crystalline and glassy components. The pentablocks show the low initial modulus, strain-hardening behavior, and small permanent set desired for TPEs, while retaining an easily processed single-phase melt.
Co-reporter:John M. Papalia, Andrew P. Marencic, Douglas H. Adamson, Paul M. Chaikin and Richard A. Register
Macromolecules 2010 Volume 43(Issue 17) pp:6946-6949
Publication Date(Web):August 6, 2010
DOI:10.1021/ma101458m
Co-reporter:Sheng Li and Richard A. Register, Brian G. Landes, Phillip D. Hustad and Jeffrey D. Weinhold
Macromolecules 2010 Volume 43(Issue 10) pp:4761-4770
Publication Date(Web):April 28, 2010
DOI:10.1021/ma100609k
The morphologies of polydisperse ethylene−octene diblock copolymers, synthesized via a novel coordinative chain transfer polymerization process, are examined using two-dimensional synchrotron small-angle and wide-angle X-ray scattering on flow-aligned specimens. The diblock copolymers comprise one amorphous block with high 1-octene content and one semicrystalline block with relatively low 1-octene content, and each block ideally exhibits the most-probable distribution. Near-symmetric diblocks with a sufficiently large octene differential between the amorphous and semicrystalline blocks show well-ordered lamellar domain structures with long periods exceeding 100 nm. Orientation of these domain structures persists through multiple melting/recrystallization cycles, reflecting a robust structure which self-assembles in the melt. The domain spacings are nearly 3-fold larger than those in near-monodisperse polyethylene block copolymers of similar molecular weights. Although the well-ordered lamellar domain structure established in the melt is preserved in the solid state, the crystallites are isotropic in orientation. These materials display crystallization kinetics consistent with a spreading growth habit, indicating that the lamellae do not confine or template the growing crystals. The exceptionally large domain spacings and isotropic crystal growth are attributed to interblock mixing resulting from the large polydispersity; short hard blocks dissolved in the soft-block-rich domains swell the domain spacing in the melt and allow hard block crystallization to proceed across the lamellar domain interfaces.
Co-reporter:Young-Rae Hong, Douglas H. Adamson, Paul M. Chaikin and Richard A. Register
Soft Matter 2009 vol. 5(Issue 8) pp:1687-1691
Publication Date(Web):02 Mar 2009
DOI:10.1039/B820312A
Applying sufficiently strong shear to thin films of a sphere-forming polystyrene–polyisoprene diblock copolymer is shown to induce an order-order transition to cylinders. The transformation is not continuous or epitaxial, as the intercylinder spacing is ca. 10% greater than the spacing between close-packed lines of spheres. The transition is facilitated when the block copolymer has a composition which places it close to the “zero-field” (no shear) sphere/cylinder phase boundary; the shear-induced transformation is more difficult and less effective for a polymer further from this boundary. Applying a modest shear stress to a polymer close to the boundary distorts the hexagonal lattice formed by the spheres without forming cylinders; the mechanical anisotropy produced by this distortion is sufficient to permit a film containing only a single layer of spherical domains to align in shear.
Co-reporter:Robert C. Scogna
Journal of Polymer Science Part B: Polymer Physics 2009 Volume 47( Issue 16) pp:1588-1598
Publication Date(Web):
DOI:10.1002/polb.21762
Abstract
A material strained beyond its yield point typically suffers substantial irrecoverable deformation. Surprisingly, this is not the case for ethylene/methacrylic acid (E/MAA) copolymers and ionomers, for which significant permanent deformation does not result until the applied strain exceeds 50–150%, far beyond the yield strain of 5–10%. At room temperature, strain recovery is complete on the order of hours or days following the removal of the applied load. Interestingly, the onset of permanent deformation coincides with a broad maximum or shoulder in the plot of stress versus strain. Two-dimensional X-ray scattering studies of both initially isotropic samples and highly aligned blown films reveals that this “second yield shoulder,” commonly observed in the stress–strain curves of ethylene/α-olefin copolymers, is fundamentally associated with polyethylene crystal fracture, resulting in fragments of reduced lateral extent. Connections formed between these crystalline fragments lock in the deformed conformations of the amorphous intercrystalline segments, preventing the specimen from retracting to its initial dimensions. Additional recovery is possible through heating; complete melting of the deformed specimens results in full recovery up to applied strains of 200%, beyond which strain-induced chain disentanglement begins. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1588–1598, 2009
Co-reporter:Jeffrey D. Quinn
Journal of Polymer Science Part B: Polymer Physics 2009 Volume 47( Issue 21) pp:2106-2113
Publication Date(Web):
DOI:10.1002/polb.21807
Abstract
“Block-random” copolymers—where one or more blocks are themselves random copolymers—offer a flexible modification to the usual block copolymer architecture. For example, in a poly(A)-poly(A-ran-B) diblock consisting of monomer units A and B, the interblock segregation strength can be continuously tuned through the B content of the random block, allowing the design of block copolymers with accessible order-disorder transitions at arbitrarily high molecular weights. Moreover, the development of controlled radical polymerizations has greatly expanded the palette of accessible monomer units A and B, including units with strongly interacting functional groups. We synthesize a range of copolymers consisting of styrene (S) and acetoxystyrene (AS) units, including copolymers where one block is P(S-ran-AS), through nitroxide-mediated radical polymerization. At sufficiently high molecular weights, near-symmetric PS-PAS diblocks show well-ordered lamellar morphologies, while dilution of the repulsive S-AS interactions in PS-P(S-ran-AS) diblocks yields a phase-mixed morphology. Cleavage of a sufficient fraction of the AS units in a phase-mixed PS-P(S-ran-AS) diblock to hydrogen-bonding hydroxystyrene (HS) units yields, in turn, a microphase-separated melt. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:2106–2113, 2009.
Co-reporter:Robert C. Scogna, Richard A. Register
Polymer 2009 50(2) pp: 585-590
Publication Date(Web):
DOI:10.1016/j.polymer.2008.12.003
Co-reporter:Sasha B. Myers and Richard A. Register
Macromolecules 2009 Volume 42(Issue 17) pp:6665-6670
Publication Date(Web):July 2, 2009
DOI:10.1021/ma901076v
The synthesis, morphological characterization, and deformation behavior of a triblock copolymer containing linear polyethylene (LPE) end blocks and a hydrogenated polyhexylnorbornene (hPHN) midblock are presented. The block copolymer forms a homogeneous melt; upon cooling, LPE crystallization creates a lamellar structure consisting of alternating “hard” semicrystalline LPE domains and “soft” rubbery hPHN domains, imparting thermoplastic elastomer (TPE) behavior to the material. At 20 wt % LPE, the triblock shows a low modulus, high ultimate strain, and excellent strain recovery compared with other crystalline-block TPEs. Though the LPE crystals fragment upon deformation, the interposing hPHN layers hinder LPE fragments originating in different layers from fusing together, preserving the memory of the original domain morphology to high strains.
Co-reporter:John P. Bishop
Macromolecular Rapid Communications 2008 Volume 29( Issue 9) pp:713-718
Publication Date(Web):
DOI:10.1002/marc.200800019
Co-reporter:Jeffrey D. Quinn
Polymers for Advanced Technologies 2008 Volume 19( Issue 6) pp:556-559
Publication Date(Web):
DOI:10.1002/pat.1103
Abstract
The hole-transporting ability of carbazole-containing polymers makes them attractive for applications in organic electronics. Nitroxide-mediated radical polymerization (NMRP) offers the potential to create complex macromolecular architectures, including block copolymers, but N-vinylcarbazole, the simplest and most widely studied carbazole monomer, is known to polymerize poorly by NMRP. Here, we investigate the NMRP of N-ethyl-2-vinylcarbazole (2 VK), a monomer structurally more similar to the styrenes which are known to polymerize well by NMRP, and whose polymer offers a hole mobility an order of magnitude higher than poly(N-vinylcarbazole). Polymerization of 2 VK from a unimolecular alkoxyamine shows a molecular weight which increases steadily with time, though termination or decomposition of the “living” radical ends is clearly evident from the progressive formation of a low-molecular-weight tail as conversion increases. However, a sufficient fraction of the chains remain living that P2VK-PS diblock copolymer can be formed by chain extension of the P2VK macroinitiator with styrene. Copyright © 2008 John Wiley & Sons, Ltd.
Co-reporter:Sasha B. Myers and Richard A. Register
Macromolecules 2008 Volume 41(Issue 14) pp:5283-5288
Publication Date(Web):June 18, 2008
DOI:10.1021/ma800844g
A two-step site transformation technique is presented to convert from living ring-opening metathesis polymerization (ROMP) to living anionic polymerization of unsaturated monomers. This method permits the synthesis of well-defined diblock copolymers of controllable molecular weight and narrow molecular weight distribution, combining ROMP and anionically polymerizable monomers. The technique employs a functional terminating agent to add a styryl group to the end of the ROMP chain. The end group is then metalated to yield an active site for anionic polymerization, so that the ROMP chain acts as a macroinitiator from which the anionic block is grown. Both polycyclopentene/polystyrene and polynorbornene/polystyrene diblocks were successfully synthesized, and the method should be applicable to a broad range of monomers.
Co-reporter:Sasha B. Myers and Richard A. Register
Macromolecules 2008 Volume 41(Issue 18) pp:6773-6779
Publication Date(Web):August 19, 2008
DOI:10.1021/ma800759b
Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) are synthesized, and their crystallization is examined. LPE and hPN homopolymers are both highly crystalline, with similar melting points near 150 °C and χhPN/LPE = 0.02. For symmetric diblocks, even when moderately segregated, crystallization of hPN breaks out of the melt microdomains to create volume-filling spherulites. The LPE block subsequently crystallizes between the hPN crystallites, with the hPN and LPE crystal stems oriented orthogonally. At shallow undercoolings, crystallization proceeds in two distinct steps, but at deeper undercoolings, only a single process is observed, following the temperature dependence of hPN crystallization. At these deeper undercoolings, crystallization of hPN initiates LPE crystallization so that the two processes are nearly simultaneous. By changing the block ratio, the crystallization behavior can be broadly tuned: in hPN-rich diblocks, the two crystallization processes are well-separated at any temperature, while in LPE-rich diblocks, the LPE block crystallizes first.
Co-reporter:Christopher K. Yee-Chan;Robert C. Scogna
Journal of Polymer Science Part B: Polymer Physics 2007 Volume 45(Issue 10) pp:1198-1204
Publication Date(Web):9 APR 2007
DOI:10.1002/polb.21116
In the idealized two-phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully-amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (Tg) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible-chain polymers, such as those formed from vinyl-type monomers. Here, we examine Tg in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. Tg is indeed shown to be elevated relative to fully-amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007
Co-reporter:Li-Bong W. Lee;David M. Dean
Journal of Polymer Science Part B: Polymer Physics 2005 Volume 43(Issue 4) pp:413-420
Publication Date(Web):4 JAN 2005
DOI:10.1002/polb.20350
Blown films of different types of polyethylenes, such as branched low-density polyethylene (LDPE) and linear high-density polyethylene (HDPE), are well known to tear easily along particular directions: along the film bubble's transverse direction for LDPE and along the machine direction (MD) for HDPE. Depending on the resin characteristics and processing conditions, different structures can form within the film; it is therefore difficult to separate the effects of the crystal structure and orientation on the film tear behavior from the effects of the macromolecular architecture, such as the molecular weight distribution and long-chain branching. Here we examine LDPE, HDPE, and linear low-density polyethylene (LLDPE) blown films with similar crystal orientations, as verified by through-film X-ray scattering measurements. With these common orientations, LDPE and HDPE films still follow the usual preferred tear directions, whereas LLDPE tears isotropically despite an oriented crystal structure. These differences are attributed to the number densities of the tie molecules, especially along MD, which are considerably greater for linear-architecture polymers with a substantial fraction of long chains, capable of significant extension in flow. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 413–420, 2005
Co-reporter:John M. Sebastian
Journal of Applied Polymer Science 2001 Volume 82(Issue 8) pp:2056-2069
Publication Date(Web):13 SEP 2001
DOI:10.1002/app.2051
We present a method for accurately determining the true molecular weights of narrow-distribution block copolymers, using only a basic gel permeation chromatograph (GPC) equipped with a refractive index detector and calibrated with polystyrene standards. Our approach is based on the well-known observation that GPC calibration curves for different homopolymers in good solvents are essentially parallel, allowing the curves for different polymers to be described by simple hydrodynamic equivalence ratios rB versus polystyrene. We present values of rB, in both toluene and tetrahydrofuran, for various polydiene and hydrogenated polydiene homopolymers commonly incorporated into commercial styrenic block copolymers. These values of rB must be combined to yield the hydrodynamic equivalence ratio of the block copolymer, from which the block copolymer's true molecular weight can be determined. Three combining rules proposed in the literature are tested against a series of symmetric polystyrene–polybutadiene diblock copolymers of varying molecular weight. A simple linear combining rule accurately represents the results. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2056–2069, 2001
Co-reporter:Daniel A. Vega;John M. Sebastian;Yueh-Lin Loo
Journal of Polymer Science Part B: Polymer Physics 2001 Volume 39(Issue 18) pp:2183-2197
Publication Date(Web):3 AUG 2001
DOI:10.1002/polb.1192
Triblock copolymers in midblock-selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bridging of chains between micelles can lead to macrophase separation. Adding a styrene–isoprene diblock to a styrene–isoprene–styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single-phase gels with a disordered arrangement of micelles. The plateau modulus of these triblock gels scales with the 2.2 power of polymer content, indicating the importance of entanglements in dictating the modulus. Comparing gels made from the midblock-saturated derivative of the same polymer [styrene-(ethylene-alt-propylene)-styrene] in squalane reveals that the modulus differences in the gels are a direct consequence of the difference in the entanglement molecular weight of the midblock homopolymer in bulk. Finally, the broad relaxation spectrum of these triblocks is well-described by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2183–2197, 2001
Co-reporter:Yueh-Lin Loo;Douglas H. Adamson
Journal of Polymer Science Part B: Polymer Physics 2000 Volume 38(Issue 19) pp:2564-2570
Publication Date(Web):15 AUG 2000
DOI:10.1002/1099-0488(20001001)38:19<2564::AID-POLB80>3.0.CO;2-Y
Ruthenium tetroxide (RuO4) is a versatile agent for imparting mass density contrast to saturated hydrocarbon polymers. We have successfully employed RuO4 to examine the microdomain and crystallite morphologies of poly(ethylene-block-vinylcyclohexane) semicrystalline–glassy diblock copolymers via transmission electron microscopy (TEM). Ultrathin sections cut from blocks stained with RuO4 showed excellent contrast in TEM, revealing the individual polyethylene crystallites that formed within the preexisting block copolymer microdomains. The size and orientation of the crystallites, which previously could only be inferred from scattering data, are readily apparent in the micrographs. Moreover, such imaging directly reveals the lateral extent of the crystallites and the number of crystallites lying within the cross section of each microdomain. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2564–2570, 2000
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