Time-Resolved Infrared Spectroscopic Studies of Poly(ethylene terephthalate) Deformation Christian Pellerin,* ,² Michel Pe ´ zolet, ‡ and Peter R. Griffiths § De ´ partement de chimie, UniVersite ´ de Montre ´ al, Montre ´ al, QC, H3C 3J7, Canada; De ´ partement de chimie, UniVersite ´ LaVal, Que ´ bec, QC, G1K 7P4, Canada; and Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343 ReceiVed May 9, 2006; ReVised Manuscript ReceiVed July 18, 2006 ABSTRACT: Polarization modulation infrared linear dichroism (PM-IRLD) and ultrarapid-scanning Fourier transform infrared spectroscopy (URS-FTIR) have been used to characterize the evolution of molecular orientation and microstructure during and following the step deformation of amorphous poly(ethylene terephthalate) (PET) above and below its glass transition temperature. The combined use of these techniques allowed a high sensitivity and an unprecedented 10 ms time resolution for the characterization of irreversible polymer deformation using infrared spectroscopy. PM-IRLD results show that the 1410 cm -1 band of PET, often used as a thickness standard, presents a significant dichroism even at low draw ratios. Using this band, the relaxation kinetics of the phenyl ring was directly shown, for the first time, to be similar to that of the glycol group in amorphous PET. These results suggest that the relaxation proceeds mainly via cooperative motions involving at least one repeat unit and not only through rotations around the flexible CH 2 -CH 2 and CH 2 -O bonds. The real-time study of the cold drawing of glassy amorphous PET by URS-FTIR showed that a large gauche-to-trans conversion (from ∼15% to 60% of trans conformers) of the glycol groups occurs during the neck propagation. These trans conformers possess a very large and stable molecular orientation. Nevertheless, spectral analysis revealed that the “mesomorphic” phase, rather than the truly (all-trans) crystalline structure, is produced during cold drawing of PET at room temperature. Introduction Infrared spectroscopy is widely used to characterize the structure-properties relationships in natural and synthetic polymers. 1 One of the most important structural parameters controlling the macroscopic properties of materials, such as spider silk and high-modulus polymeric fibers, is their degree of molecular orientation. Conventional infrared linear dichroism (IRLD) has been used for many years to probe the static molecular orientation in samples quenched below their glass transition temperature (T g ) or in situ during slow deformations. 2-5 The main advantage of IRLD over other characterization techniques, such as birefringence and wide-angle X-ray dif- fraction, is that it can often be used to probe the orientation of multiple species or phases simultaneously. Indeed, its use has often revealed distinct orientation behavior for the different components and phases in semicrystalline polymers, polymer blends, and block copolymers, thus enabling a better under- standing of their properties. 3,4 Recently, more attention has been paid toward the real-time determination of orientation during rapid deformations and the direct determination of the relaxation kinetics following stretch- ing. With a time resolution of the order of several seconds, conventional IRLD is too slow to perform such time-resolved experiments at deformation and relaxation rates relevant to processes such as industrial film blowing or spider silk spinning. However, technical developments in the past few years have significantly improved the time resolution of infrared spectro- scopy for orientation characterization. In particular, polarization modulation infrared linear dichroism (PM-IRLD) 4,6 and ul- trarapid-scanning Fourier transform infrared spectroscopy (URS- FTIR) 1,7 have significantly enhanced the sensitivity and time resolution of infrared spectroscopy for the study of molecular orientation. PM-IRLD uses a photoelastic modulator to rotate the polarization plane of the infrared radiation from parallel (p) to perpendicular (s) to the draw direction at a frequency of ∼100 kHz. Using dual-channel acquisition electronics, this technique allows the direct measurement of the dichroic difference, thus allowing a much improved sensitivity as compared to conven- tional IRLD, 8,9 in addition to a time resolution as good as 400 ms. 10,11 URS-FTIR is a recently developed technique that uses a wedged rotating disk mirror to introduce the optical path difference in the interferometer. 1,7,12 Because there is no need to accelerate and decelerate a reciprocating moving mirror, this technique allows a greatly enhanced duty cycle and time resolution as compared to conventional FTIR spectrometers. Indeed, a time resolution of 5 ms per spectrum has been recently demonstrated for gas adsorption studies. 13 In this work, we have used both PM-IRLD and URS-FTIR to study the evolution of molecular orientation and microstruc- ture during and following the irreversible deformation of thin films of amorphous poly(ethylene terephthalate) (PET), a ubiquitous engineering polymer. The possibilities and limitations of these techniques for such studies will be contrasted. Experimental Section PET films of 7 μm thickness were prepared by blow molding and generously provided by Dr. K. C. Cole of the Industrial Materials Institute of the National Research Council of Canada. Differential scanning calorimetry characterization indicated that the samples were amorphous. Strips of 20 mm × 6 mm were deformed at room temperature and at 90 °C using a custom-built mechanical ² Universite ´ de Montre ´al. ‡ Universite ´ Laval. § University of Idaho. * Corresponding author: Tel (514) 340-5762; Fax (514) 340-5290; e-mail c.pellerin@umontreal.ca. 6546 Macromolecules 2006, 39, 6546-6551 10.1021/ma0610459 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006