The mechano-optical behavior of PET is, for the first time, investigated under temporally varying rates to influence the basic mechanisms of structural organization leading to strain-induced crystallization. For this purpose, four rate profiles, Linear, Sigmoidal, Logarithmic and Exponential, were chosen and films were stretched in Uniaxial Constrained Width mode using newly developed biaxial stretching machine. This machine allows real time direct measure of true stresses, strains, in-plane and out-of-plane birefringences during the deformation. Substantial differences in the mechano-optical behavior and resulting structural mechanism were observed in all of the chosen rate profiles. Linear profile, taken as a standard, yields three stress-optical regimes (SOR) during deformation. At early stages of deformation the birefringence remains linear with stress and material remains amorphous. This is designated as Regime I representing classical stress-optical behavior observed in large number of non-crystallizable polymers. In Regime II, a fast increase of birefringence accompanies formation of crystalline structure with establishment of long-range connected network. In the final Regime III birefringence levels off as the chains approach their finite extensibilities. All three regimes observed in Linear profile are also observed in Logarithmic and Exponential cases. However, Sigmoidal deformation shows only the first two regimes even though the film was stretched to the same total engineering strain as applied to all profiles. Logarithmic profile was found to induce early strain crystallization leading to early development of strain hardening. Exponential profile on the other hand retards the formation of "potentially constraining" long-range physical networks. This allowed the development of higher birefringence and crystallinity levels using this mode. A logarithmic birefringence-work relationship with two distinct stages was found to apply to all temporally varying profiles. (c) 2007 Elsevier Ltd. All rights reserved.