Polymer structure and property studies in elongational rheology, spherulite deformation, and biaxial strain induced crystallization

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Virginia Polytechnic Institute and State University

A small scale, highly accurate elongational viscometer was developed explicitly for the rheological investigation of well characterized polyethylene samples. Elongational stress growth measurements as well as dynamic shear experiments demonstrated that the rheological response of molten polyethylene was sensitive to both molecular weight distribution and shear modification. The effects of molecular weight distribution and shear modification were rationalized from a molecular point of view. A model is proposed which is based on the concept of a molecular network and incorporates polymer chain entanglement disruption and regeneration.

Direct observations of polymeric semicrystalline morphologies in a copolyester by scanning electron microscopy were made possible by the development of a novel chemical etch. Spherulitic textures were consistent with classic spherulite growth mechanisms and structure theories. Uniaxial deformation of a single spherulite was successfully studied in a model system consisting of isolated spherulites embedded in an amorphous polymer matrix. By isolating the spherulite, the mechanical influence of surrounding and often impinging spherulites found in most semicrystalline polymers on the mechanical response of an individual spherulite was avoided. The mechanical response of the amorphous matrix was characterized and found to correlate with the effectiveness of a cold draw neck in elongating an embedded spherulite. The observed mechanism and morphology of isolated spherulite deformation were rationalized within the context of existing theories of spherulite-to-microfibrillar transitions.

Optically active poly(L-lactic acid) and racemic poly(lactic acid) were synthesized in a ring opening polymerization scheme with stannous octoate as a catalyst and lactic acid as a molecular weight controlling initiator. Binary polymer blends composed of these isomeric polymer pairs were found to be miscible at 40,000 molecular weight and immiscible at 120,000 molecular weight. Strain hardening and the level of strain induced crystallization which occurred in the biaxial deformation of poly(lactic acid) blend films were found to be contingent on the concentration of optically active poly(L-lactic acid). Temperature, molecular weight, and biaxial strain rate were also found to have an influence on strain hardening and strain induced crystallization of these thermodynamically ideal polymer blends.