Study of polymer orientation and relaxation by polarization modulation and2D-FTIR spectroscopy

Polarized Fourier transform infrared (FTIR) spectroscopy has been used exte nsively to study polymer orientation. The dichroic ratio and the dichroic d ifference are normally obtained from spectra recorded sequentially with the infrared radiation polarized parallel and perpendicular to a reference dir ection. To improve the sensitivity of this technique and to be able to foll ow accurately the dynamics of orientation, FTIR spectroscopy has been coupl ed with the polarization modulation (PM) technique. With this technique, th e dichroic difference spectrum is recorded directly, thus minimizing instru mental and sample fluctuations. The results obtained demonstrate the high e fficiency of polarization modulation infrared linear dichroism (PM-IRLD) to determine quantitatively the time dependence of the orientation function o f several chemical groups during the orientation and relaxation processes. Examples of the application of this technique to study in situ the dynamics of orientation will be presented for both stretched films of polystyrene ( PS) and poly(vinyl methyl ether) (PVME) blends and optically oriented copol ymers containing azobenzene side chains. The use of the PM-IRLD technique h as allowed the direct determination of the relaxation kinetics of the compo nents in polymer blends. In the case of the azopolymers, the time-dependent spectra obtained by PM-IRLD have also been analyzed by two-dimensional Fou rier transform infrared (2D-FTIR) spectroscopy. The results obtained with t his technique show that the spectral resolution is significantly enhanced i n the asynchronous maps that also provide valuable information about the re lative movement of the different chemical groups of the polymers. (C) 1998 Elsevier Science B.V. All rights reserved.