DIRECTED CYSTALLIZATION PRECURSOR STRUCTURES IN POLYMER MELT BY FLOW

The proposed research aims to investigate several fundamental issues of flow-induced crystallization in polymers with an emphasis on events that occurred at the earliest stages of phase transition. These issues include: (1) the pathway to forming nanostructured scaffolds comprised of microfibrils (shish) and trans-crystalline lamellae (kebabs) in undercooled polymer melt by flow prior to full scale crystallization; (2) the relationships among the internal material parameters (chain length, length distribution and chain branching, or the relaxation time spectrum), the external flow parameters (temperature, strain and strain rate, or stress), and the formation of the initial precursor structures; and (3) the nature of the flow-induced crystallization precursor structures in shish and kebab microstructures. The proposed research plan will include: (i) the use of polymer blends with model compounds to enable the investigation of the flow-induced precursor structures, (ii) the use of combined methods including rheology, in-situ Raman spectroscopy and synchrotron SAXS/WAXD techniques and ex-situ microscopic methods, and (iii) the comparison of results from weak shear-flow and strong elongation-flow studies. The scientific merits of the proposed study are as follows. The subject of flow-induced crystallization remains to be one of the most important problems in polymer processing today, and it is still not yet fully understood. The major hurdles in the past were mainly due to the lack of suitable in-situ characterization tools to determine the structures at the earliest stages of flow-induced crystallization. Using the recently available synchrotron X-ray techniques, these hurdles have been effectively overcome. The in-depth understanding obtained in the proposed study shall increase the ability of the polymer industry to correlate the structure, morphology, process and property relationships for crystalline polymers. The broader impacts of this proposal are several. The immediate benefit is that the proposed activities will produce tight links between two scientific institutions (Stony Brook University and the National Synchrotron Light Source). Students and scientists will be trained in both institutions. They will be exposed to relevant industrial problems and will have a chance to tackle these problems with direct interactions from industrial scientists. They will also be involved in the Stony Brook MRSEC summer education program, where they will supervise undergraduate students and industrial scientists to carry out simple synchrotron X-ray experiments.