Improving Mechanical Performance of Glassy and Semicrystalline Polymers: Molecular Perspectives

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NON-TECHNICAL SUMMARY:Bio-renewable polymers such as poly(lactic acid) (PLA) hold the promise to replace fossil-based thermoplastics such as poly(ethylene terephthalate) (PET). PLA has not replaced PET despite similar physical properties such as relatively high glass transition temperature Tg (60 to 70 degree Celsius) and slow crystallization kinetics. Unlike PET, crystalline PLA are brittle at room temperature, making the material hardly usable. Amorphous PLA is also brittle because of rapid physical aging that can take place in PLA. To make the replacement competitive, which is of considerable economic benefit to the national prosperity and welfare, several scientific advances must be made. First, a better understanding of mechanics of semicrystalline polymers must be achieved from a molecular viewpoint. Second, a clearer relationship must be established between processing conditions and resulting mechanical properties of semicrystalline polymers. The proposed research will investigate why crystallization does not enhance ductility for polymers with Tg above room temperature. In particular, research will be conducted to evaluate the conjectures that (a) crystalline regions of such semicrystalline polymers are mechanically weaker than the non-crystalline regions and (b) fast crystallization is necessary is entrap chain entanglement and preserve (instead of deplete) chain networking. Moreover, efforts will be made to establish the processing, structure and property relationships that will make PLA superbly tough and heat resistant. Specifically, scientific principles will be identified to achieve crystallization while preserving chain uncrossability.TECHNICAL SUMMARY:Past studies of mechanics of semicrystalline polymers and "flow-induced" crystallization mainly focused on polymers whose glass transition temperature are below room temperature and crystallization kinetics are relatively fast, e.g., isotactic polypropylene and polyethylene. The existing knowledge is thus not transferrable for class B semicrystalline polymers (with Tg above room temperature) such as PLA because mechanics of PLA at room temperature involves deformation of glassy amorphous regions that separate the crystalline regions. For example, the well-known respective models of Peterlin and Flory-Yoon do not directly apply. The much higher stress due to glassy polymer deformation raises the question of whether crystalline regions are structurally as strong as the amorphous regions. Since chains do not topologically cross in the crystalline regions, the cohesion of crystalline regions cannot take advantage of strong covalent bonds and is thus limited. The planned research will apply new knowledge in melt rheology and molecular mechanics of polymeric glasses, in combination with the mature characterization tools such as WAXS, SAXS and DSC to establish an insightful perspective that recognizes the need to explicitly describe and control the state of the amorphous phase in terms of the population of tie chains relative to dangling and free chains. Moreover, in situ polarized optically microscopy will be employed to identify the locations (either inside spherulites, or within amorphous regions or at inter-spherulitic boundaries) where mechanical weakness resides, providing the needed information for a molecular model of mechanics of glassy semicrystalline polymers. The proposed work will explore the unique effect of elastic pre-melt-deformation on polymer crystallization and resulting mechanical properties of PLA. Here each stretched entanglement strand is expected to nucleate a nano-crystal that is free of chain folding and acts like rigid nano-fillers in close packing, linked by non-crystalline stretched chain networking. Specifically, cold-crystallization of pre-melt-stretched PLA could result in crystalline yet optically clear samples that are not only superbly ductile but also have outstanding dimensional stability (e.g., zero shrinkage at 100 degrees Celsius)..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.