Molecular dynamics simulations of polyisobutylene in solution comparable to previous nuclear spin relaxation experiments at four different temperatures have been performed to better understand its local dynamics. A sterically crowded elastomer with a low glass transition temperature, polyisobutylene shows interesting comparisons to more crystalline polymers such as polyethylene and poly(ethylene oxide). The more rapid polymer dynamics processes (local dynamics) encompass librational motion about specific conformations and the motions resulting from conformational transitions. These processes are known to have profound effects on a polymer's macroscopic properties, some examples include elasticity, glass transition temperature, and temperature dependence of melt viscosities. The simulated system includes one polyisobutylene 80mer (MW of 4505 g/mol) and 489 toluene molecules. All simulations were run using GROMACS 5.1. Three different all-atom force fields are compared, and optimizations to the hydrogen van der Waals radius were made for the OPLS-AA/L force field for better comparison to electronic structure calculations run on smaller molecule analogs of polyisobutylene. Simulation-calculated results such as correlation times and motional anisotropies produce expected trends that are in agreement with those observed in NMR spin relaxation experiments. Population distributions for single and paired torsion angles, as well as probabilities of torsional angular change at different time increments show the effects of polyisobutylene's structure on the local dynamics.

Local dynamics of polyisobutylene in solution: An all-atom molecular dynamics study

255th National Meeting (2018)

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