The relaxation behavior of simultaneously and sequentially biaxially stretched PET films was studied at the rubbery state stretching temperatures. The primary objective was to investigate the influence of process conditions and the mode of deformation on the structural changes that take place in the stretched films. Using an instrumented biaxial stretcher, the films were stretched and held at the stretching temperature at fixed dimensions while true stress, true strain, in-and out-of-plane birefringences were monitored. The relaxation behavior was found to be dependent on the process prehistory including extent and rate of deformation. The behavior was divided into three regimes: Regime I, where the birefringence and strain always decrease and the material remains in the amorphous state. Regime II, where both the birefringence and strain first decrease while the film remains amorphous; then they start to increase when the first evidence of strain crystallization appears. Regime III, where strain-induced crystallization was already well-established during the biaxial deformation; both strain and birefringence increase during relaxation. This three-regime behavior was directly linked to the formation of strain-induced crystallization. Off-line Raman spectroscopy, DSC measurements, and X-ray WAXS patterns were used to follow the structure evolution and the transitions between these regimes. ■ INTRODUCTIONThe orientation of PET has been extensively investigated in the literature, but there are very few studies that dealt with the relaxation following the orientation processes. 1 Normally, three directions represent the dynamics of the stretching process; directions 1, 2, and 3 respectively correspond to machine direction (MD), which is the direction of the main stretching action, transverse direction (TD), which is perpendicular to the MD in the sample 2D plane, and normal direction (ND), which is the sample thickness direction in 3D. The level of orientation obtained at the end of the stretching is modified by relaxation especially if high stretching temperatures and low deformation rates are utilized. The recovery that occurs when the acting force is removed has been attributed to motions of the polymer chains in the amorphous regions, 2 particularly in the absence of crystals and entanglements. The oriented chains try to recoil into the isotropic state, as this is the most energetically favorable state. The relaxation process may enhance the crystallization of the polymeric materials particularly when deformation process creates taut chains with unfavorable orientations adjacent to each other. More understanding of the relaxation mechanism should help in the efforts to understand the orientation-induced crystallization during processing.To quantify the relaxation parameters in the molten state, Doi and Edwards 3 have used the rheological measurements to postulate the presence of three relaxation steps. The first one corresponds to a Rouse-like relaxation between entanglements in order to re-establish a constant ch...

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