The crystallization and morphology of polyethylene and its blends/

The techniques of neutron and x-ray scattering have been used to study the morphology and crystallization behavior of polyethylene and blends of polyethylene. Synchrotron radiation was used to study the crystallization behavior of blends of high density polyethylene/ low density polyethylene (HDPE/LDPE) and linear low density/ low density polyethylene (LLDPE/LDPE). Simultaneous real time small and wide angle scattering from blends slowly cooled at (0.5\sp\circC/min) seem to indicate that the lamellae are formed in bundles of primarily one component. For blends quickly cooled from the melt (quenched to 60\sp\circC) on the other hand, the lamellae are randomly mixed together. HDPE/LDPE and LLDPE/LDPE blends show qualitatively the same crystallization behavior throughout the composition range except for 10%/90% LLDPE/LDPE. At this composition, extensive cocrystallization may be occurring in even slowly cooled samples. Small angle neutron and x-ray scattering was used to determine the location of the short chain branches in selectively deuterated LLDPE. Specially prepared LLDPE with the main chain deuterated was used in these experiments to provide contrast for neutron scattering. Despite density contributions to the neutron scattering from crystalline and amorphous regions, differences between the x-ray and neutron scattering suggest that the concentration of branches may be enhanced at the crystal- amorphous boundary. The extent of this branch-rich region was estimated to be about 30A. Lastly, the chain orientation of ultra high molecular weight PE (UHMWPE) was examined by small angle neutron scattering. A circularly averaging technique was applied in order to avoid sample alignment problems. Between extension ratios of 12 and 60, hot drawn (125\sp\circC) gel crystallized UHMWPE does not show appreciable change in the perpendicular radius of gyration. However, changes in the asymptotic behavior of the scattering intensity from I $\sim$ q\sp{-1.56} at 12x to I $\sim$ q\sp{-1.2} at 60x indicate a change in geometry toward more rod like segments in the higher drawn material

Pathways to Macroscale Order in Nanostructured Block Copolymers

Polymeric materials undergo dramatic changes in orientational order in response to dynamic processes, such as flow. Their rich cascade of dynamics presents opportunities to create and combine distinct alignments of polymeric nanostructures through processing. In situ rheo-optical measurements complemented by ex situ x-ray scattering reveal the physics of three different trajectories to macroscopic alignment of lamellar diblock copolymers during oscillatory shearing. At the highest frequencies, symmetry arguments explain the transient development of a bimodal texture en route to the alignment of layers parallel to the planes of shear. At lower frequencies, larger-scale relaxations introduce rearrangements out of the deformation plane that permit the formation of lamellae perpendicular to the shear plane. These explain the change in the character of the pathway to parallel alignment and the emergence of perpendicular alignment as the frequency decreases

Effect of Systematic Hydrogenation on the Phase Behavior and Nanostructural Dimensions of Block Copolymers

Unsaturated polydienes are frequently hydrogenated to yield polyolefins that are more chemically stable. Here, the effects of partial hydrogenation on the phase behavior and nanostructure of polyisoprene-containing block copolymers are investigated. To ensure access to the order–disorder transition temperature (<i>T</i>_ODT) over a wide temperature range, we examine copolymers with at least one random block. Dynamic rheological and scattering measurements indicate that <i>T</i>_ODT increases linearly with increasing hydrogenation. Small-angle scattering reveals that the temperature-dependence of the Flory–Huggins parameter changes and the microdomain period increases, while the interfacial thickness decreases. The influence of hydrogenation becomes less pronounced in more constrained multiblock copolymers