Jonathan H. Laurer

Thermoplastic elastomer gels. I. Effects of composition and processing on morphology and gel behavior

Thermoplastic elastomer gels (TPEGs) composed of a poly[styrene-b-(ethylene-co-butylene)-b-styrene] triblock copolymer and a low-volatility, midblock-compatible mineral oil have been investigated at different oil concentrations to ascertain the effect of composition on TPEG morphology and mechanical properties. The impact of thermal processing is also examined by comparing gels thermally quenched to 0°C or slowly cooled to ambient temperature. Transmission electron micrographs reveal that gels with 70 to 90 wt % oil exhibit styrenic micelles measuring ca. 24 nm in diameter. Variation in composition or cooling rate does not have any perceivable effect on micelle size or shape, whereas the rate at which the gels are cooled influences the extent of microstructural order and the time to rupture (tR) at constant strain. Dynamic rheological testing confirms the presence of a physically crosslinked network at TPEG compositions ranging from 70 to 90 wt % oil, independent of cooling rate. Results presented here suggest that the dynamic elastic shear modulus (G′) scales as tαR where α varies from 0.41 to 0.59, depending on cooling rate.

Thermoplastic elastomer gels. II. Effects of composition and temperature on morphology and gel rheology

Thermoplastic elastomer gels (TPEGs) composed of a poly[styrene-b-(ethylene-co-butylene)-b-styrene] triblock copolymer and a low-volatility, midblock-compatible mineral oil have been investigated here to ascertain the effects of composition on TPEG morphology, and temperature on mechanical properties. Cryofracture-replication transmission electron micrographs reveal the existence of spheroidal bumps due to copolymer micelles, as well as a network of irregularly shaped, high-aspect-ratio features. Since the density of this network decreases with increasing oil concentration, these features are attributed to copolymer grain boundaries. Micellar periodicities are discerned from small-angle X-ray scattering as a function of copolymer concentration and compared with previously reported data from related systems. Dynamic rheological tests performed up to 140°C indicate that the linear viscoelastic regime for these TPEGs decreases with both increasing copolymer concentration and temperature. A concentration-dependent thermal transition, signified by an abrupt reduction in the dynamic elastic modulus (G′), has also been identified.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2513–2523, 1998

Phase-separation studies of heat-cured ATU-flexibilized epoxies

Abstract Chemical incorporation of a flexible molecule into a heat-cured epoxy offers an important route by which to toughen the epoxy and tailor its ultimate material properties. Acrylate-terminated urethane (ATU) flexibilizers containing polypropylene glycol have been employed in the present work to modify epoxies composed of a diglycidyl ether of bisphenol-A and bis(4-aminocyclohexyl) methane. Differential scanning calorimetry, dynamic mechanical analysis and electron microscopy reveal that the flexibilizer and epoxy undergo phase separation during cure when the flexibilizer loading or molecular weight is sufficiently high. Partial phase miscibility, deduced from both glass transition temperature shifts and stress—relaxation behaviour, is interpreted here in light of thermodynamic and free-volume considerations.

Macromolecular self-assembly in dilute sequence-controlled block copolymer/homopolymer blends

Abstract Conventional block copolymers consist of two long contiguous monomer sequences (‘blocks’) that can, in the same fashion as low-molar-mass surfactants, self-assemble into various microstructural elements (e.g., micelles at low copolymer concentrations) to minimize repulsive contacts in the presence of a parent homopolymer. In this work, we explore the existence of segment-specific interactions, as well as the possibility of tailoring these blend morphologies (and producing altogether new ones), with novel sequence-controlled block copolymers. These copolymers are comprised of at least one block that is a random segment composed of both constituent monomer species. Transmission electron microscopy is employed here to examine the bilayered membranes and channel structures that form in two different series of such copolymers in dilute copolymer/homopolymer blends.

Phase Behavior of Triblock Copolymers upon Incorporation of Non-Parent, Midblock-Associating Additives

Addition of a block-selective homopolymer to a microphase-ordered block copolymer is known to result in preferential swelling of the chemically compatible microdomain. In this work, we examine the miscibility between a triblock copolymer and a relatively low-molecular-weight, chemically dissimilar, midblock-associating homopolymer and demonstrate that the homopolymer molecules residing in the swollen midblock matrix self-assemble to avoid repulsive interactions with neighboring microdomains. We extend this investigation to include systems composed of a very low-molecular-weight, midblock-associating additive (an oil). At high oil concentrations, the glassy copolymer endblocks micellize, resulting in the formation of a thermoplastic elastomer gel.