Vibrational Relaxation in Atactic Polystyrene: A Calculation of the Frequency Correlation Functions of Ring Stretching Modes and Their Variation with Temperature

详细信息    查看全文

• 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：1738 - 1745
• 全文大小：173K
• 年卷期：v.40,no.5(March 6, 2007)
• ISSN：1520-5835

文摘

Time correlation functions and their variation with temperature have been calculated for the modesobserved near 1601 and 1583 cm^-1 in the infrared spectra of atactic polystyrene. The correlation functions deviatefrom the Kubo-Rothschild model describing the fast modulation limit but can be modeled by simply assumingthat there is a fast relaxation process characterized by a single relaxation time that is inhomogeneously broadened.It is noted that as long as the relaxations associated with inhomogeneous broadening are all relatively slow processes(in terms of vibrational relaxations), then their only contribution to the band profile would be through a Gaussianterm that modifies the simple Kubo-Rothschild expression. The factors that contribute to this inhomogeneousbroadening are discussed. Dynamic heterogeneity presumably plays a significant role, but in addition the 1583cm^-1 band appears to be influenced by an anharmonic coupling to lower frequency modes that provides anadditional mechanism of relaxation. As a result, this mode is particularly sensitive to temperature, displayingchanges in the relaxation time and second moments that show a transition near 80 C, about 20 C below thethermally observed T_g.