A stereochemical microstructure-based attempt to explain the dielectric behavior of poly(vinyl chloride) (PVC)

摘要

A poly(vinylchloride) (PVC) obtained by bulk polymerization at 70 degreesC was modified through substitution reaction with sodium benzenethiolate (NaBT) in cyclohexanone solution at 25 degreesC, at various conversions up to 15 mol %. According to earlier work, this allows to change, in a controlled way, the stereochemical microstructure in terms of mainly the content of mmr (meso, meso, racemic) tetrad termini of isotactic sequences of at least one heptad in length, and of its likely chain conformations. The samples were all studied by Dielectric Relaxation Spectroscopy, which allows to measure the real and the imaginary parts of the complex dielectrical constant in a frequency range between 1 and 10(6) Hz, and a temperature range between 100 and 600 K. Most of the changes in the physical quantities that can be analyzed by this technique were found to parallel the changes in stereochemical microstructure with substitution extent. The temperature of the maximum of the dipolar losses, which relates to the temperature range where a relaxation occurs, appeared to increase somewhat up to 0.7% molar conversion. Then the tendency is towards stabilization. The relaxation time r(T) as deduced from the Havriliak-Negami (NH) function, tends to decrease between 0 and 0.7 substitution percent, and to increase afterwards. The evolution of the Ngai's beta parameter with substitution degree is also highly illustrative. It clearly decreases up to 0.7% and then it increases rapidly up to 7% and slowly afterwards. These changes are discussed by taking into account that 0.7 and 7% substitution extents agree with the removal of mmr under GTTG(-)TT conformation and of the same structure under GTGTTT conformation, respectively. From the results, some original correlations between stereochemical microstructure and molecular dynamic behaviors are proposed. (C) 2004 Wiley Periodicals, Inc.