The effect of molecular weight and rubbery state uniaxial stretching conditions on the mechano-optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6) films was investigated using a real time spectral birefringence stretching machine. The stress optical behavior exhibits a multi stage behavior that depends on process conditions as well as molecular weight. At low stretching temperatures and high rates the stress optical behavior was found to start with an initial glassy photo elastic behavior. Decreasing stretching rate or increasing processing temperature was found to eliminate this glassy photoelastic regime leading to the observation of a linear initial stress optical behavior past a temperature of 95 degrees C (T-parallel to). The stress optical constant (SOC) was about the same for both M.W. materials stretched at temperatures past T-parallel to, at 2.7 GPa(-1) for HPA6 and 2.61 GPa(-1) for LPA6. Following this initial regime, the behavior is controlled by the competition between orientation and relaxation during deformation. If the chain orientation relaxation is not suppressed by increasing the stretching rate and/or the molecular weight or by decreasing temperature, the material strain crystallizes. (C) 2010 Elsevier Ltd. All rights reserved.