Influence of tacticity on T_β, T_g, and T_LL in poly(methyl methacrylate)s by the method of thermally stimulated current (TSC)

Abstract

Abstract The thermally stimulated‐current method (TSC) has been employed to determine the temperatures and intensities of T β , T g , and T > T g for pure isotactic, pure syndiotactic, and five atactic specimens with syndiotactic triad content from 49.5 to 75%; T g was found to increase linearly with syndiotactic triad content as T g (°C) = 48.0 + 0.856 (% syn), with R 2 = 0.970 standard error 5.6°C; T g for the syndiotactic specimen is 136.6°C measured, 133.6°C calculated. Several atactic specimens exhibit a second glass temperature 15 to 35 K above the regression line ascribed to some pure syndio content, and/or some isotactic–syndiotactic stereocomplexes. All specimens exhibited the liquid–liquid or T LL transition (relaxation) which increases linearly with 100‐% isotactic triad content. Isotactic PMMA shows a T ′ LL relaxation 50 K above T LL . The T g and T LL values obtained correlate extremely well with values from differential scanning calorimetry (DSC) determined in a separate study, as well as with most literature data. Intensities of T g and T LL by TSC are greatest for isotactic, next for syndiotactic, with a broad, low minimum for atactic materials. The intensity of a β relaxation increases slowly from isotactic to syndiotactic. The T LL found by TSC compares well with literature values for isotactic PMMA obtained by several methods, and T LL in the atactic region compares well with literature values for atactic material. The ratio T LL / T g ranges from 1.09 to 1.20 with no dependence on tacticity. T g follows simple Arrhenius behavior with enthalpies of activation about one‐half of the values normally calculated from dielectric and mechanical loss. The frequency dependences of T LL and T ′ LL follow a Vogel–WLF relationship with temperature. The origin of T LL is discussed in terms of the Frenkel hypothesis of segment–segment interaction. Evidence for T LL and T LL from a variety of methods indicates that these two temperatures are not artifacts of the TSC method.