Glass transition temperature of PIB, PDMS and PMMA from small-time simulations

We have applied some new techniques to obtain predictions of the glass transition temperatures Tg of poly(isobutylene), poly(dimethyl-siloxane), and poly(methyl methacrylate) from small-time atomistic molecular dynamics simulations. The different fragilities of these materials are reflected in the results of the simulations. One approach involved measurement of the apparent softening of the ``cage\u27\u27 in which a monomer is bound, while another involved studying autocorrelation of a convolution of the velocity with a smoothing function in order to detect the frequency of escapes from the ``cage.\u27\u27 To check the accuracy of the short-time methods, the Tg of the polymers was also found using conventional diffusion simulations in which the rate of increase of the root mean squared displacement of an atom, monomer, or molecule is measured at very long times. The economical short-time simulations yielded results for Tg that were identical to those of the computer-intensive long-time simulations