Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC)
measurements. As in TMDSC TMDMA allows the investigation of reversible and non-reversible phenomena during crystallisation
of polymers. The advantage of TMDMA compared to TMDSC is the high sensitivity for small and slow changes in crystallinity,
e.g. during re-crystallisation. The combination of TMDMA and TMDSC yields new information about local processes at the surface
of polymer crystallites. It is shown that during and after isothermal crystallisation the surface of the individual crystallites
is in equilibrium with the surrounding melt.
Quasi-isothermal temperature modulated DSC (TMDSC) were performed during crystallization to determine heat capacity as function of time and frequency. Non-reversible and reversible phenomena in the crystallization region of polymers were distinguished. TMDSC yields new information about the dynamics of local processes at the surface of polymer crystals, like reversible melting. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for polycaprolactone (PCL). This shows that also after long crystallization times the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are living crystals. A strong frequency dependence of complex heat capacity can be observed during and after crystallization of polymers.
The relaxation strength at the glass transition for semi-crystalline polymers observed by different experimental methods shows
significant deviations from a simple two-phase model. Introduction of a rigid amorphous fraction, which is non-crystalline
but does not participate in the glass transition, allows a description of the relaxation behavior of such systems. The question
arises when does this amorphous material vitrify. Our measurements on PET identify no separate glass transition and no devitrification
over a broad temperature range. Measurements on a low molecular weight compound which partly crystallizes supports the idea
that vitrification of the rigid amorphous material occurs during formation of crystallites. The reason for vitrification is
the immobilization of co-operative motions due to the fixation of parts of the molecules in the crystallites. Local movements
(Β-relaxation) are only slightly influenced by the crystallites and occur in the whole non-crystalline fraction.