We explore by molecular-dynamics simulations the glass transition of dense polymer melts and the mechanical response of the glassy melt to an applied deformation. The models employed in this study are bead-spring models with variable stiffness along the chain backbone. The determination of the glass transition temperature Tg is achieved by two methods: via the evolution of the volume or energy on cooling the melt and via the T-dependence of the elastic constants of the glassy melt on heating toward Tg. Both method yield consistent results. We also analyze the influence of chain flexibility, chain length N, pressure, and cooling rate on the glass temperature of our model. We find e.g. that Tg(semi-flexible chains) > Tg(flexible) and the relationship Tg(N) = Tg(∞) - cst/N.
For T < Tg and N > Ne, Ne being the entanglement length, we apply different deformations (uniaxial and triaxial elongations, compressions, pure shear, deformation-relaxation cycle,...) to our model polymer melts. We reproduce typical experimental stress-strain curves and observe the formation of cavities. See the figure on the right.
The ultimate aim of this investigation (B. Schnell, PhD ULP Strasbourg) is to understand the link between the creation of stable cavities and the microscopic structure of the polymer melt.