Polymer Chain Length and Branching Effects on the Viscolestic Bulk Modulus and Structural Recovery at High Pressure

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Technical SummaryThe goal of this project is to extend fundamental studies of the effect of chemical structure on the time-dependent bulk modulus to include the effects of chain length and branching on the time-dependent bulk modulus and structural recovery at high pressure. In particular, the influence of chain length in narrow molecular weight distribution polystyrenes and the effect of branching using star-branched polystyrenes will be investigated. In the course of these measurements, the concept of fragility, which varies in polystyrene as a function of molecular weight, will be examined. Measuring the bulk deformation response as a function of temperature and pressure (or volume) will also allow provide fundamental new information on how the bulk relaxation time varies with temperature and volume at equilibrium density. The response in the glassy state at high pressure will also be investigated and will be used to test Kubat's suggestion that the bulk modulus is directly related to the ratio of dh/dv in nonlinear structural recovery experiments. In addition to providing new fundamental knowledge on the impact of molecular structure on the time-dependent bulk modulus and on structural recovery at high pressure, the work is also of practical importance, particularly in composite systems, since the bulk modulus is responsible for the magnitude of isotropic residual stresses. An understanding of how the relaxation time in the glass depends on structure is also critical for predictive modeling of structural recovery and concomitant changes in mechanical properties. The broader impact or extrinsic merit of the proposed work includes the training of a graduate student, an undergraduate researcher, and a post-doctoral researcher, and strong efforts will be made to include minority and female students in this project. The work will also be disseminated in a variety of ways, including incorporation into undergraduate and graduate courses taught by the PI; presentations by the PI, post-doctoral researcher, and students at national and local forums; and publication in both conference proceedings and archival journals. Non-Technical SummaryThe bulk modulus is an important property because, for example, it is related to the development of residual stresses in composite materials, which can adversely impact the integrity of composite components. Similarly, physical aging and structural recovery of polymeric glasses, including those used in composites, result in embrittlement, which again, has negative consequences for component lifetime. In this project, both the bulk modulus and structural recovery are studied at high pressure and as a function of polymer molecular structure in order to gain the understanding necessary to model and predict these properties. A novel pressurizable dilatometer capable of making these difficult measurements will be used. In addition to producing new knowledge through this work, a graduate student, an undergraduate researcher, and a post-doctoral researcher will be trained in cutting edge research in the areas of polymer physics, pressure effects, and instrument design; strong efforts will be made to include minority and female students in this project. The work will also be disseminated in a variety of ways, including incorporation into undergraduate and graduate courses taught by the PI.