Introduction From the glass transition, it is well known and generally accepted to describe heat capacity by complex numbers. A typical frequency dependence as for other relaxation processes is observed ± a sigmoid step in the real and a peak in the imaginary part of the heat capacity [1±3]. Recent measurements also indicate a frequency depen- dence of the heat capacity of semicrystalline polymers outside the glass-transition range [4, 5]. These observa- tions are related to the occurrence of an excess heat capacity that can be observed in a rather wide temper- ature range between the glass transition and the melting temperature 1 . The origin of this excess heat capacity and 1 The term ``excess heat capacity, C p,excess '' is used here as the dierence between the modulus of measured complex heat capcaity and the baseline heat capacity. Colloid Polym Sci 279:800±806 2001) Ó Springer-Verlag 2001 ORIGINAL CONTRIBUTION C. Schick A. Wurm A. Mohamed Vitri®cation and devitri®cation of the rigid amorphous fraction of semicrystalline polymers revealed from frequency-dependent heat capacity Received: 6 December 2000 Accepted: 29 January 2001 C. Schick &) á A. Wurm á A. Mohamed University of Rostock Department of Physics 18051 Rostock, Germany e-mail: christoph.schick@physik.uni- rostock.de Abstract The relaxation strength at the glass transition shows signi®cant deviations from a two-phase model for semicrystalline polymers. The introduction of a rigid amorphous fraction RAF), which is noncrys- talline but does not participate in the glass transition, allows a de- scription of the relaxation behavior. The question arises when does this amorphous material vitrify. Temperature-modulated dierential scanning calorimetry measurements allow the online study of heat capacity changes during isothermal crystallization. For bisphenol-A polycarbonate PC) and poly3-hy- droxybutyrate) PHB) at reasonably high modulation frequencies 10 mHz), no contribution from reversing melting to the measured heat capacity was detected at the crystallization temperature; there- fore, 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 be- tween 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 vitri®cation of the RAF is the immobilization of cooperative motions owing to the ®xation of parts of the molecules in the crystallites, which is favorable at the fold surfaces. In this way, crys- tallization in PC and PHB limits itself by vitrifying the crystallizable material next to the growing crys- tals. On heating, devitri®cation of the RAF occurs when the crystals, which were formed last, melt in the temperature range of the lowest endotherm. Key words Glass transition á Temperature-modulated dierential scanning calorimetry á Rigid amorphous; Polymers; PC; PHB