| Summary: | 碩士 === 國立中正大學 === 化學工程研究所 === 88 === Abstract
In this thesis, we study the linear and nonlinear rheological properties of H-shaped polymer melts under single-step strain flows. An H-shaped polymer is the simplest branched polymer that shares the essential feature of the rheological properties of commercial branched polymers (i.e., randomly branched polymers), such as LDPE. By studying the entangled polymer dynamics of an H-shaped polymer, we are be able to know better the molecular origins of the rheological properties of branched polymer melts, and, on the other hand, test the validity of existing theories for the constraint release mechanism.
For the linear viscoelastic properties, we make use of a recently developed scheme extremely efficient for the purpose of stochastic simulations. From the simulation results, it appears that double reptation is necessary for the linear relaxation of star polymers. However, unrealistic results are found for H-shaped polymer if double reptation is assumed for the constraint release mechanism. This finding may imply that double reptation is mathematically correct only for those polymer melts with at least one free end.
For the simulation of large step-strain flows, we employ a realistic full-chain reptation model with the linear relaxation spectra taken from recent theories for H-shaped polymer melts, as well as from the simulation result of the full-chain reptation model for linear polymer melts. The simulation results for the nonlinear relaxation modulus and damping function are compared with recent experimental data, and good qualitative agreement is found for these two properties. In particular, this is the first time that the full relaxation spectra of an H polymer melts during large step strains are realistically accounted for.
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