Influence of torsional barriers on the glass transition temperature of linear polymers

The role of the torsional potential on the glass transition of a linear polymer has been investigated by means of Molecular Dynamics simulations. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition. The united atom description has been adopted, with every methyl and methylene group along the chains being regarded as an individual interacting site. Three rotational isomers are allowed by the reference torsional potential, which has been parameterized so that the two torsional barriers can be tuned separately. Then, two sets of torsional parameters have been considered. The first set is characterized by increasing values for the trans-gauche (t-g) barrier and by no gauche-gauche (g-g) barrier. In the second set, the t-g barrier is kept constant, and the g-g barrier takes increasing values. The zero potential situation has also been analyzed. The glass transition temperature (Tg) has been obtained by following the specific volume of the system as a function of temperature. It has been found that Tg increases with increasing values of the t-g barrier. Nevertheless, Tg becomes independent of the t-g barrier for very high barriers. As for the role of the g-g barrier, Tg is almost independent of it, provided that the t-g barrier already exists.