Deformation and Relaxation of Polymers Studied by Ultrarapid Scanning FT-IR Spectrometry Christian Pellerin, †,§ Robert E. Prud’homme, † Michel Pe ´ zolet,* ,† Benjamin A. Weinstock, ‡ and Peter R. Griffiths ‡ Centre de recherche en sciences et inge ´ nierie des macromole ´ cules, De ´ partement de chimie, Universite ´ Laval, Que ´ bec, Canada G1K 7P4, and Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received February 16, 2003; Revised Manuscript Received April 26, 2003 ABSTRACT: A recently developed ultrarapid scanning Fourier transform infrared (URS-FTIR) spec- trometer has been used to study the irreversible deformation of polymer films with a millisecond time resolution for the first time. The evolution of molecular orientation as a function of draw ratio and relaxation time was studied for films of amorphous poly(ethylene terephthalate) (PET) stretched above its glass transition temperature (T g). Very good agreement was obtained between the results obtained by URS-FTIR and polarization modulation infrared linear dichroism (PM-IRLD) spectrometry. Reversible gauche-to-trans conversions were observed, indicating that the PET chains remain amorphous. The orientation and relaxation of polystyrene (PS) in films of pure PS and of blends of PS with poly(vinyl methyl ether) (PVME) were also studied above T g. A method allowing the determination of the orientation function of PS using a single p-polarized spectrum is described. Results reveal a significant decrease in PS orientation during the first second following the end of deformation, emphasizing the importance of the experimental time resolution in the characterization of the relaxation behavior of polymers. Introduction Molecular orientation has a large impact on several properties of polymers and polymer blends, such as their modulus and gas permeability. For over three decades, efforts have been deployed to improve our understand- ing of the development of orientation during irreversible macroscopic and reversible microscopic deformations of polymers. 1 Several experimental techniques have been used to characterize the static orientation in samples quenched below their glass transition temperature (T g ) or in situ during slow deformations. Recently, interest has shifted toward the time-resolved determination of orientation during rapid deformation and to the direct determination of the relaxation kinetics following stretch- ing. Wide-angle X-ray diffraction (XRD) has recently been used to study the rapid deformation of poly(ethylene terephthalate) (PET) with a time resolution of 40-900 ms, but such an experiment requires a Synchrotron source. 2-5 A simple dynamic birefringence setup has been used to follow deformation and relaxation kinetics in PET 6 and polystyrene (PS) 7 with a time resolution of 1 ms, but the technique suffers from a lack of selectivity since it only provides an averaged value of the orientation function, 〈P 2 (cos θ)〉. In contrast, infrared linear dichroism (IRLD) can often discriminate between the orientation of the amorphous and crystalline phases in semicrystalline polymers and of the different species in multicomponent systems such as polymer blends or copolymers. 1,8,9 Several techniques have been developed to improve the time resolution of IRLD, which is usually of the order of several seconds with the conventional rapid- scan approach. By using a step-scanning interferometer, time-resolved dynamic IRLD has allowed the study of the small-amplitude cyclic deformation of polymers with a time resolution of 1 ms. 10 However, this technique is not useful for the study of the irreversible macroscopic deformation of polymers since a repeatable stimulus must be applied to the sample at each step of the moving mirror in order to record a complete scan. Polarization modulation IRLD (PM-IRLD), which makes use of a photoelastic modulator to switch the polarization plane of the infrared radiation from parallel (p) to perpen- dicular (s) at a high frequency, allows direct measure- ment of dichroic difference spectra in 400 ms. 8,11 It was used to follow directly irreversible deformation and relaxation kinetics for films of PET, 12,13 of PS, 7 and of blends of PS with poly(vinyl methyl ether) (PVME) 14,15 and with poly(phenylene oxide) (PPO). 16 For conventional FT-IR spectrometers, the optical path difference is introduced by a moving optical ele- ment that travels with a reciprocating motion. The force required to retard the optical element and accelerate it in the reverse direction means that spectral data with a resolution of better than 8 cm -1 cannot be measured at a rate of much more than 20 interferograms per second. The typical duty cycle efficiency of conventional FT-IR spectrometers operating at their highest scan speed is less than 20% and is usually much lower. Recently, Griffiths and co-workers have developed an ultrarapid scanning FT-IR (URS-FTIR) spectrometer that allows the measurement of infrared spectra in 5 ms. 17,18 The optical path difference in this instrument is generated by a rotating wedged mirror, allowing operation at a duty cycle efficiency of approximately 70% at any angular velocity. As a result, spectra with a resolution of 6 cm -1 can be generated at intervals of 5 ms. In fact, a time resolution of 1 ms would be possible, with no loss of duty cycle efficiency, if an analog-to- † Universite ´ Laval. ‡ University of Idaho. § Current address: Department of Materials Science and En- gineering, University of Delaware, Newark, DE 19716-3106. * Corresponding author: Tel (418) 656-2481; fax (418) 656-7916; e-mail michel.pezolet@chm.ulaval.ca. 4838 Macromolecules 2003, 36, 4838-4843 10.1021/ma034199m CCC: $25.00 © 2003 American Chemical Society Published on Web 06/03/2003