Free Surface-Induced Glass-Transition Temperature Suppression of Simulated Polymer Chains

Glass-transition temperatures (T-g's) in poly-(ethylene oxide) chains of 250 monomers as typical examples have been extensively simulated with the systematically coarse-grained potentials. The studied model systems include two 1-chain systems without and with the periodic boundary condition and two 8-chain systems in the NVT and NPT ensembles, which are labeled as nopbc, pbc, NVT, and NPT, respectively. The absence of crystallization and instead the occurrence of glass transition are first confirmed for all these systems during quick cooling from a high temperature to a low temperature. One unambiguous trend is then obtained for the dynamical T-g: T-g(nopbc) < T-g(pbc) < T-g(NVT) < T-g(NPT). The T-g reductions in the former three systems compared to the last one accord with the greater chain motions at both rubbery and glassy states, which, in turn, is caused by the free surfaces in different degrees. The free surfaces and glass transition for these systems can be obviously revealed by the density maps. It is also found that the dynamical T-g for any system is insensible to either initial configuration or ultimate conformation. In other words, this work provides some new insights into glass transition of confined polymers.