The effect of nanoparticles on the mechano-optical behavior of PEN was investigated using a real time true stress-true strain birefringence measuring system [Toki S, Valladares D, Sen TZ, Cakmak M. ANTEC Proc 2001:1830; Koike Y, Cakmak M. Polymer 2003:44:4249]. The large deformation stress-optical behavior revealed that there are at most three distinct regimes that may be augmented by an initial additional glassy component at lower temperatures and/or high rates. The presence of nanoparticles did not change the birefringence development of PEN before strain induced crystallization. However, after strain induced crystallization occurred in PEN, the birefringence increase was found to be diminished due primarily to the decrease in the amount of crystallinity and amorphous chain orientation with the addition of nanoparticles. The final structure and deformation behavior of the nanocomposites have been mapped out in a dynamic phase diagram as a function of stretching temperature, rate and nanoparticle content. This dynamic phase diagram shows that the material undergoes three critical structural transitions: (i) nematic-crystalline transition wherein the material stretched below a certain temperature does not undergo orientation-induced crystallization but develops a highly ordered nematic state. (ii) A kinematic transition wherein the material transforms from a 'structured liquid' to a 'true liquid' state exhibited by the disappearance of the initial glassy component as the material becomes devoid of the inherent structure due to segmental correlations. (iii) While unfilled PEN remains amorphous at high temperatures due to high relaxation rates combined with decreased thermal crystallizability. the presence of nanoplatelets was found to facilitate strain induced crystallization even at such high temperatures primarily due to the decrease in orientation relaxation in their presence. (C) 2007 Elsevier Ltd. All rights reserved.