This research proposal makes the case that the current understanding of quiescent crystallization of flexible linear polymers is at best incomplete. Specific new directions for further progress are offered which will result in a better understanding of the role of chain length during crystallization. The underlying assumption in this proposal is that a merging of the fields of molecular rheology and nucleation theory is a necessary step toward a better understanding of polymer crystallization. The experiments proposed here will for the first time fully test the internal consistency of arguments developed over the last 50 years by practitioners in the field of polymer crystallization. They will provide a quantitative assessment of the dependence of crystal growth rates, initial crystal and amorphous layer thicknesses and resulting melting temperature on the temperature of crystallization and molar mass. Morphological studies will also be carried out to examine whether changes in the mechanism of crystal growth are associated with morphological transitions. Through collaboration with Prof. Liliane Leger's group in France, the dependence of the chain friction coefficient on the rate of chain-pull out will be studied to evaluate the proposal by Marand et al. that the kinetics of crystal growth is strongly influenced by viscoelastic effects. Experimental crystal growth rate and lamellar thickness data as a function of temperature and chain length gathered in this research program will also provide a unique opportunity for theoreticians to test new ideas. The analysis of small angle X-ray scattering data at the spherulite growth front during crystallization (in collaboration with Prof. Hsiao at SUNY) will also provide important new information on the stability and temporal evolution of lamellar crystals and allow evaluation of the recent lamellar stability model proposed by Strobl.
NON-TECHNICAL SUMMARY:
The proposed research will be integrated with an international educational program dedicated to the task of training the next generation of scientists in the field of crystalline polymers. Expertise in this area of materials research is of great importance to the US plastics industry but is rapidly vanishing with the steady thinning of the population of "polymer crystallizers" of the first generation. As plastics are used in more technologically advanced arenas, the industrial sector is demanding a better understanding of their solidification process and structure/property correlations. The specific use of semicrystalline polymers is a direct consequence of the physical properties derived from their morphology. Their morphology, in turn, is dramatically dependent on the conditions under which crystallization takes place. A better understanding of the crystallization mechanism will provide a pathway to optimize the performance of plastics for many technological applications.
