MOLECULAR-LEVEL MODELING OF THE VISCOELASTICITY OF CROSS-LINKED POLYMERS - EFFECT OF TIME AND TEMPERATURE

We present a new molecular-level picture of chain dynamics for describ ing the viscoelasticity of crosslinked polymers. The associated mathem atical model consists of a time-dependent momentum balance on a repres entative polymer segment in the crosslinked network, plus phenomenolog ical expressions for forces acting on the segments. These include a co hesive force that accounts for intermolecular attraction, an entropic force describing the thermodynamics governing chain conformations, and a frictional force that captures the temperature dependence of relati ve chain motion. We treat the case of oscillatory uniaxial deformation . Solution of the model equations in the frequency domain yields the d ynamic moduli as functions of temperature and frequency. The model rep roduces all of the qualitative features of experimental dynamic modulu s data across the complete spectrum of time and temperature, spanning the glassy zone, the beta transition, the dynamic glass transition, an d the rubbery zone. All of the model parameters can be evaluated throu gh the use of independent experimental data. Comparison of model predi ctions with experimental data yields good quantitative agreement outsi de of the glass transition region. (C) 1997 The Society of Rheology.