Molecular Aspects of Flow Effects on Crystallization in iPP

California Institute Of Technology, Pasadena CA

Investigators

Abstract

The microstructure of a semicrystalline polymer depends strongly on processing history. Imposing flow fields on a crystallizing polymer melt can accelerate the rate of crystallization and result in the formation of oriented crystallites. In spite of intensive research over the past three decades, the fundamental basis of the effects of processing on structure development remains elusive. This research probes the underlying molecular-level processes: the interplay of the relaxation dynamics of the molten chains, the distortion of segmental orientation created by flow, the kinetics and anisotropy of nucleation from a distorted melt, and their effects on the subsequent kinetics and morphology of crystallization. Under current DMR support, surprising discoveries have reshaped current thinking. The rate limiting factor in the formation of the oriented precursors that template the row-nucleated morphology is the rate of molecular motion in the subcooled melt, which is unanticipated by any existing model of flow-induced crystallization. The role of macromolecular dynamics, particularly the distribution of relaxation times, was demonstrated using relatively narrow distribution materials and their binary blends. Small amounts of long chains (with known molar mass and stereoregularity) added to a base resin of much shorter chains (approximately one-fifth as long), greatly enhanced the flow-induced formation of oriented precursors. The concentration dependence of the effect of long chains indicates that long-chain/long-chain overlap plays a central role. In view of the profound effect of low concentrations of long chains, proposed research for the next three years focuses on the dependence of the long-chain effect on their relative relaxation time and stereoregularity compared to the bulk, searching for mechanistic clues. To clarify the interplay between the earliest stages of flow-induced structure formation (the creation of point-like precursors) and the subsequent formation of threadlike precursors, homogeneous and heterogeneously nucleated model polymers will be compared. The kinetics of disappearance of the threadlike precursors when held at elevated temperature will be used to test our current hypotheses regarding their structure. The diverse and ubiquitous application of polymers is largely due to "semicrystalline polymers," which account for two-thirds of the annual production of synthetic polymers. Over 100 billion pounds a year are made of polyethylene and polypropylene alone! Their properties can be tuned over an enormous range by controlling the extent and form of crystallization. For example, the development of highly oriented crystallites during spinning gives certain polyethylene fibers a modulus approaching that of steel. However, fundamental understanding needed to achieve intended properties by design remains elusive because of the complex effects of molecular structure and processing history on material properties. New methods to observe the earliest events during crystallization are revealing surprising truths. Contrary to existing models, oriented nucleation induced by flow occurs as fast as the motion of the chain-like molecules permits-the usual activation barrier vanishes. And the polymer chains that matter most in this process are the longest ones, needing to be at least four times longer than average. Such advances in molecular-level understanding guide the design of semicrystalline polymers to optimize their processing characteristics and ultimate material properties, with benefits to the automotive, electronics, food and health care industries.