Influence of tacticity on Tβ, Tg, and TLL in poly(methyl methacrylate)s by the method of thermally stimulated current (TSC)

Abstract

AbstractThe thermally stimulated‐current method (TSC) has been employed to determine the temperatures and intensities of Tβ, Tg, and T > Tg for pure isotactic, pure syndiotactic, and five atactic specimens with syndiotactic triad content from 49.5 to 75%; Tg was found to increase linearly with syndiotactic triad content as Tg (°C) = 48.0 + 0.856 (% syn), with R2 = 0.970 standard error 5.6°C; Tg 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 TLL transition (relaxation) which increases linearly with 100‐% isotactic triad content. Isotactic PMMA shows a T′LL relaxation 50 K above TLL. The Tg and TLL 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 Tg and TLL 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 TLL found by TSC compares well with literature values for isotactic PMMA obtained by several methods, and TLL in the atactic region compares well with literature values for atactic material. The ratio TLL/Tg ranges from 1.09 to 1.20 with no dependence on tacticity. Tg 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 TLL and T′LL follow a Vogel–WLF relationship with temperature. The origin of TLL is discussed in terms of the Frenkel hypothesis of segment–segment interaction. Evidence for TLL and TLL from a variety of methods indicates that these two temperatures are not artifacts of the TSC method.