Molecular dynamics study of isobaric and isochoric glass transitions in a model amorphous polymer

We perform molecular dynamics simulations of the glass transition through i sobaric and isochoric cooling of a model polymeric material. In general, ex cellent agreement between the simulation results and the existing experimen tal trends is observed. The glass transition temperature (T (g)) is found t o be a function of pressure under isobaric conditions and specific volume u nder isochoric conditions. Under both isobaric and isochoric conditions, th e trans-state fraction and the torsional contributions to the energy underg o abrupt changes at the glass transition temperature. We analyze these data to show that the glass transition is primarily associated with the freezin g of the torsional degrees of the polymer chains which is strongly coupled to the degree of freedom associated with the nonbonded Lennard-Jones potent ial. We attribute the greater strength of the glass transition under consta nt pressure conditions to the fact that the nonbonded Lennard-Jones potenti al is sensitive to the specific volume, which does not change during coolin g under isochoric conditions. Comparison of the isochoric and isobaric data demonstrate that the thermodynamic state is independent of cooling path ab ove T (g), while path-dependent below T (g). The simulation data show that the free volume at the isobaric glass transition temperature is pressure de pendent. We also find that a glass transition occurs under isochoric condit ions, even though the free volume actually increases with decreasing temper ature. (C) 1999 American Institute of Physics.