No abstract

In-situ rheo-optical methods are used to guide electron microscopy (TEM) and X-ray scattering (SAXS) studies of structure development during flow-induced alignment in a lamellar block copolymer melt (nearly symmetric polystyrene-polyisoprene diblock, ODT = 172 °C). The progress of shear-induced alignment is recorded in real-time using flow birefringence; at selected points during alignment samples are taken for ex-situ characterization by TEM and SAXS along all three axes (v, ∇v, ∇ × v) of the flow geometry. Three different trajectories are examined: perpendicular alignment and two qualitatively different routes to parallel alignment in the high-frequency regime (ω > ω′ c ). In general, the initial "fast" process not only enhances the projection of the orientation distribution that corresponds to the final state but also increases other projections of the distribution; the late-stage "slow" process eliminates these other projections and perfects a single alignment. For example, the highest frequency path to parallel alignment begins by transforming poorly organized regions into layers that are predominantly oriented along the parallel and transverse directions. The transition to the slow process is marked by the development of a characteristic texture in which tilt wall boundaries normal to the flow direction separate bands that form a repeating "chevron" pattern (layers tilted up and down about the ∇×v axis). The coarsening of this pattern dominates the slow process, during which the transverse projection is also eliminated.

Molecular aspects of polymer melt rheology play an extremely strong role in governing the processing−structure−property relations of semicrystalline polymers, the dominant materials in the plastics industry. Recent advances in experimental apparatus and methods have revealed that the dramatic changes in crystallization kinetics and morphology induced during shear follow a kinetic pathway. The rate of formation of oriented precursors is not limited by the usual activation barrier to nucleation but instead occurs many orders of magnitude faster, at a rate that tracks the dynamics of the polymer chains in the melt. Model polymers and their binary blends have shown that the relevant melt dynamics that control formation of the oriented threadlike nuclei are those of the longest chains in the melt and that the effect of the long chains is cooperative, greatly enhanced by long chain−long chain overlap. Thus, insights gained into the role of chain dynamics in the molecular mechanism of shear-enhanced crystallization may soon combine with parallel advances over the past decade regarding the dynamics of polydisperse melts to provide the underpinnings for truly predictive models of flow-enhanced crystallization of polymers.