Orientation and relaxation of orientation of amorphous polyethylene terephthalate) A. Karim Oultache, Xiaohua Kong, Christian Pellerin, Jose Âe Brisson, Michel Pe Âzolet, Robert E. Prud'homme * Department of Chemistry and Centre de recherche en sciences et inge Ânierie des macromole Âcules, Laval University, Que Âbec, Canada G1K 7P4 Received 12 March 2001; received in revised form 25 April 2001; accepted 25 April 2001 Abstract Polyethylene terephthalate) PET) has been uniaxially stretched at different draw ratios and draw rates above its glass transition temperature, in the 80±1058C temperature range. Molecular orientation and relaxation have been followed by birefringence. A decrease intemperaturereducesthemobilityoftheorientedchainsresultinginaslowrelaxationwhileanincreaseinstretchingrateresultsinhigher orientation values and rapid relaxation after the extension. The same relaxation behavior has been observed from birefringence and polarizationmodulationinfraredspectroscopy.Rouserelaxationtimeshavebeenestimatedfromrheologicalmastercurvesandbirefringence relaxation data, while retraction and the reptation times have been deduced from the scaling laws proposed by Doi and Edwards. q 2001 Published by Elsevier Science Ltd. Keywords: Polyethylene terephthalate); Orientation; Relaxation 1. Introduction The deformation of a viscoelastic material near the rubbery plateau is followed by molecular relaxation. Based on the tube concept, Doi and Edwards have postu- latedthepresenceofthreerelaxationsteps[1].The®rstone corresponds to a Rouse-like relaxation between entangle- ments in order to re-establish a constant chain density; it is followed by a relaxation which is associated with the chain retraction inside the deformed tube; the third step corresponds to the chain disengagement from the tube by areptationprocess.Thesestepsarecharacterizedbyrelaxa- tion times t a , t b , and t c . Thevalidityofthismodelhasbeenexperimentallytested on homopolymers, copolymers, and blends by different groups [2±8]. Tassin and Monnerie [2] have studied the relaxation of polystyrene PS) of different molecular weights, using infrared linear dichroism, and they have applied the time±temperature superposition principle to obtain master curves of the relaxation data. Thus, they have indirectly shown that the polymer molecular weight does not intervene in the Rouse relaxation step, but that it affects the retraction and reptation times. In our group, Messe  et al. [3,4] have studied the relaxation of PS and of polystyrene/poly2,6-dimethyl 1,4-phenylene oxide) PS/ PPO) blends by means of birefringence and polarization modulation infrared linear dichroism. They have directly determined relaxation times associated with the Rouse and retraction steps, over a wide range of temperatures. These results are in agreement with times determined for PS by TassinandMonnerie[2],Boue Âetal.[6],WalczakandWool [7], and Abtal and Prud'homme [8]. For PS, the reported Rousetimesareoftheorderof0.5±16s,andtheretraction times of the order of 230±4000 s, depending on tempera- ture, for molecular weights in the 100,000±800,000 range [2,6±8]. The orientation of PET has been intensively investigated intheliterature,butfewerstudiesdealwiththerelaxationof orientationofthismaterial.Teradaetal.[9]andGuptaetal. [10] have studied the crystallization and thermal shrinkage ofPETatdifferenttemperaturesanddrawratios.Theyhave showed that, beyond l 3; PET develops a strain-induced crystallizationwhichimpedestheshrinkageandwherecrys- tals act as cross-links, preventing the amorphous chain segments from assuming a random coil con®guration, in agreement with previous results reported by Nobbs et al. [11]. More recently, Pearce et al. [12] have stretched PET samples at 808C at different draw rates, quenched them Polymer 42 2001) 9051±9058 0032-3861/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII:S0032-386101)00397-4 www.elsevier.com/locate/polymer * Correspondingauthor.Tel.: 11-418-656-3683;fax: 11-418-656-7916. E-mail address: robert.prudhomme@chm.ulaval.ca R.E. Prud'homme).