Vitrification and devitrification of the rigid amorphous fraction of semicrystalline polymers revealed from frequency-dependent heat capacity
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The relaxation strength at the glass transition shows significant deviations from a two-phase model for semicrystalline polymers. The introduction of a rigid amorphous fraction (RAF), which is noncrystalline but does not participate in the glass transition, allows a description of the relaxation behavior. The question arises when does this amorphous material vitrify. Temperature-modulated differential scanning calorimetry measurements allow the online study of heat capacity changes during isothermal crystallization. For bisphenol-A polycarbonate (PC) and poly(3-hydroxybutyrate) (PHB) at reasonably high modulation frequencies (10 mHz), no contribution from reversing melting to the measured heat capacity was detected at the crystallization temperature; therefore, changes in the baseline heat capacity can be studied. The amount of RAF obtained at the crystallization temperature was compared with that obtained from the step in heat capacity at the glass transition at lower temperatures. No changes in the amount of the RAF occur in the temperature range between crystallization and the glass transition. Consequently, the rigid amorphous material is totally established during the isothermal crystallization of PC and PHB. The reason for the vitrification of the RAF is the immobilization of cooperative motions owing to the fixation of parts of the molecules in the crystallites, which is favorable at the fold surfaces. In this way, crystallization in PC and PHB limits itself by vitrifying the crystallizable material next to the growing crystals. On heating, devitrification of the RAF occurs when the crystals, which were formed last, melt in the temperature range of the lowest endotherm.
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