Damian A. Hajduk

Crystallization of tethered polyethylene in confined geometries

Ordered poly(ethylene)-poly(vinylcyclohexane) (PE-PVCH) block copolymers are employed to study the crystallization of tethered PE in confined geometries. The high Tg of the PVCH component of these materials forces PE chains to crystallize in well-defined geometries dictated by the mesophase structure of the block copolymer. Effects of chain tethering on crystallization are examined through comparison of singly-tethered PE chains in PE-PVCH (EV) diblocks and doubly-tethered PE in PVCH-PE-PVCH (VEV) triblocks. Crystallinity is independent of the block copolymer mesophase structure in both the EV and VEV systems, although crystallinity in VEV depends on the molecular weight of the PE block of the copolymer. Melting temperature data indicate that spatial confinement reduces crystallite size in EV and VEV, and that the double tethering of PE chains in VEV reduces crystallite size further through topological constraints. Crystal nucleation and growth depend strongly on the type of microstructure in both EV and VEV block copolymers. Differences in the overall rate of crystallization are correlated with the dimensional continuity of the PE microdomains.

Microstructure of triblock copolymers in asphalt oligomers

A model asphalt has been separated into two parts, asphaltene and maltene, through solvent extraction by n-heptane. The interactions of asphaltene and maltene with the triblock copolymer poly( styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) were investigated by transmission electron microscopy (TEM ), small-angle X-ray scattering (SAXS), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). Asphaltene was found to be essentially immiscible with both blocks of SEBS, while maltene was miscible with SEBS. An unusual sequence of morphological transformations of SEBS microstructure with respect to the addition of maltene was observed. The morphology transformed from hexagonal cylinder, to perforated layers, to lamellae and then back to the original hexagonal cylinder. The observed transformation reflects a limited solubility for both S and EB domains: at lower concentration maltene is a preferential additive for S domains, while increasing concentration the swelling of EB-rich microdomains by maltene becomes significant. The basic understanding of the interactions of the components of asphalt with SEBS gives a simple path to characterize and predict the microstructure of triblock copolymers in asphalt oligomers.

Shear-induced ordering kinetics of a triblock copolymer melt

Disorder-to-order transitions, in which an isotropic system acquires a spatially periodic structure, are common to a number of phenomena in materials science. Here, we employ small-angle neutron scattering to probe the effects of reciprocating shear on the isotropic-to-lamellar transition of a triblock copolymer composed of poly(ethylene-co-propylene) (PEP) and poly(ethylethylene) (PEE). Prior work has shown that the transition temperature decreases with increasing shear rate, implying that the isotropic state can be produced at low temperatures through application of a flow field [T. Tepe et al., J. Rheol. 41, 1147 (1987)]. Removing this field results in a “jump” to conditions under which the lamellar phase is stable with a time scale set by the relaxation of concentration fluctuations. We describe the ordering process which results, concentrating on the possible existence of a stability limit for the initial (isotropic) state.

Influence of shear on a lamellar triblock copolymer near the order–disorder transition

The effects of a large strain (600%) reciprocating steady shear on the lamellar orientation and the order–disorder transition of a nearly symmetric poly(ethylene propylene)–poly(ethylethylene)–poly(ethylene propylene) (PEP–PEE–PEP) triblock copolymer have been studied using small-angle neutron scattering and rheological measurements. Shearing in the ordered state produced parallel lamellae under all steady-state conditions investigated, and the lamellar-to-disorder transition temperature decreased dramatically with increasing shear rate. Both phenomena are qualitatively different from the documented behavior of PEP–PEE diblock copolymers. However, cooling the disordered material while shearing at a relatively low deformation rate initially induces perpendicular lamellae, analogous to the diblock response. These observations are considered in the context of theory and other experimental studies.