Molecular-dynamics simulation study of the glass transition in amorphous polymers with controlled chain stiffness.

We report computation results obtained from extensive coarse-grained molecular-dynamics simulations of amorphous ensembles of polymer chains at constant density. In our polymer model, we use bending and torsion potentials acting along the polymer backbone to control the chain stiffness. The static and dynamic properties of the polymer bulk have been analyzed over a large temperature interval in search for the onset of the glass transition. The glass transition temperatures Tg, for different types of chain stiffness, have been determined from the dependence of the self-diffusion coefficient D on the temperature T as the limiting value where the diffusion vanishes. Increasing the chain stiffness induces an increase of the glass transition temperature. The Tg values estimated from diffusion are confirmed by analyzing the relaxation times of the autocorrelation functions for the torsion angle and for the end-to-end vector. The dependence of the diffusion coefficient D on the chain length N is strongly affected by temperature for chains with bending and torsion stiffness. For systems with relatively short chains (N<or=50), the exponent nu from D proportional to N(-nu) increases from the value nu approximately 1 expected in the Rouse regime to nu approximately 2 as the temperature is lowered towards Tg.