A theory of the stress‐induced crystallization of crosslinked polymeric networks

A non‐Gaussian theory of the stress‐induced crystallization of polymeric networks is presented. It is predicted that for uniaxial extension of crosslinked polyethylene, a perfectly oriented, extended‐chain crystal is formed initially, changing to one‐fold crystal oriented perpendicular to the stretch direction at low extension and to a two‐fold crystal having nearly perfect orientation at high extension. The stress is predicted to decay initially and then to rise as the network chains switch from an extended‐ to a folded‐chain morphology, the rise being delayed and finally suppressed by additional crosslinking. The final, equilibrium birefringence is calculated and found to be negative at low extension and positive at high extension. The initial rate of crystallization is calculated using irreversible thermodynamics and is found to increase with extension and decrease with increasing crosslinking and temperature. All of the theoretical predictions are in qualitative agreement with experiment.