A Fundamental Study of the Bulk Modulus in Polymers and its Dependence on Chemical Structure

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A fundamental study of the bulk modulus of polymers is proposed with the aim of determining how it depends on chemical structure. The hypothesis that the bulk modulus arises primarily from intramolecular relaxations while the shear modulus depends on intermolecular (or intersegmental) relaxations will be tested using model systems in which the strength of the b-relaxation can be systematically varied by changing, for example, tacticity, crosslink density, or conversion. Comparison of the time-dependent bulk modulus with the time-dependent shear modulus will also be made in order to resolve the issue of whether or not the dispersions in these viscoelastic properties involve a similar spectrum of relaxation times and arise from the same molecular origins. Three model systems will be investigated, including i) poly(methyl methacrylate) as a function of tacticity, ii) a heterocyclic network based on hexamethylene diisocyanate and hexyl isocyanate as a function of crosslink density, and iii) a dicyanate ester/polycyanurate thermosetting material as a function of crosslink density and conversion. The measurements will be made using a pressurizable mercury dilatometer which has been built specifically for determination of the time-dependent bulk modulus and bulk compliance in stress- and strain-controlled modes, respectively. Temperature control in the instrument is stable to better than 0.01 K, and measurement temperatures will cover a broad temperature range from far below to above the glass temperature with measurements made in the glass following conventional protocol. Time-temperature superposition will be used to construct master curves from which the relaxation or retardation time spectra can be obtained. The master curves and relaxation (or retardation) time spectra for the model compounds will be compared in a given family to determine the effects of tacticity, crosslink density, and conversion on the bulk modulus, in addition to being compared to the data from the shear modulus. The results for the thermosetting system will also be used to test whether a general model based on concepts of time-temperature and time-conversion superposition is able to quantitatively describe the time, temperature, and conversion dependence of the bulk modulus in a reacting system. The proposed research program will provide excellent training for the graduate student involved in the project and will also involve an undergraduate researcher. Efforts will be made to include women and underrepresented minorities through special recruiting efforts at women's and minority colleges in the Texas and southwest region. An attempt will be made to include undergraduate students who participate in the TTU McNair Scholars Program for first generation college students (often minorities). In addition to providing excellent training for the students involved in the program, the work will also be broadly disseminated within the scientific community by presentation at national forums and publication in both proceedings conferences and archival journals. The results of the project will also be incorporated to the extent feasible in both undergraduate and graduate courses taught by the PI.