Vibrational Relaxation in Atactic Polystyrene: An Infrared Spectroscopic Study

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• 作者：Paul Painter ; Maria Sobkowiak ; Yung Park
• 刊名：Macromolecules
• 出版年：2007
• 出版时间：March 6, 2007
• 年：2007
• 卷：40
• 期：5
• 页码：1730 - 1737
• 全文大小：200K
• 年卷期：v.40,no.5(March 6, 2007)
• ISSN：1520-5835

文摘

The shapes and widths of infrared bands and Raman lines are an important source of informationon interactions and molecular dynamics and have been used to study vibrational and rotational relaxation processesin the picosecond time range. Most of this work has focused on small molecules, however, and the analysis ofbandwidths and band shapes has been a largely neglected topic in polymer vibrational spectroscopy. Thecharacterization of polymers introduces both advantages and new problems. One simplification is a consequenceof the large mass and moments of inertia of these molecules. Relaxation times associated with the reorientationof chain segments and many side groups are long compared to vibrational relaxations, and their contribution toband and line contours can be neglected. Unfortunately, this simplification is accompanied by new problems:the multiplicity of overlapping bands observed in many systems and the complications introduced by conformationalsensitivity. Some of these experimental problems are addressed in this paper, where an analysis of the ring stretchingmodes in the 1600 cm^-1 region of the infrared spectrum of atactic polystyrene is presented. It is shown that theshape of one of the fundamental modes in this region of the spectrum changes significantly as the polymer isheated through the glass transition temperature. This is interpreted in terms of a coupling of this mode to latticevibrations through a combination vibration. It is thought that the -relaxation process in glassy solids succeedsmuch faster than picosecond scale motions, and vibrational relaxation, as suggested by these results, would appearto be a sensitive indicator of the type of cooperative motions thought to be involved.