Transitions and relaxations in amorphous polymers

Abstract

AbstractIn amorphous homopolymers, the relaxation modes observed by Thermally Stimulated Current (TSC) around and above the glass transition (Tg) have common features:– The TSC peak isolated around Tg is constituted of elementary processes characterized by relaxation times obeying an Arrhenius equation. Moreover, the activation enthalpies and entropies follow a compensation law. This mode is the dielectric manifestation of the glass transition.Since the polarization can be taken as order parameter, the width of the distribution of the relaxation time characterizes the distribution of the local order.Data obtained on polystyrenes with various orientation show the relationship between order parameter and orientation.The TSC peak observed some 50 °C above Tg is characterized by relaxation times well described by the Fulcher‐Vogel equation. This mode has been associated with the dielectric manifestation of the liquid‐liquid transition (Tll) observed by other non‐flow techniques such as adiabatic or differential scanning calorimetry. This transition is missing in chemical or physical networks.In amorphous copolymers, polymerization conditions play a key role on the relaxation spectra. A comparative study of butyl acrylate/styrene has been done.– The semicontinuous introduction of monomers gives relatively homogeneous statistical copolymers, with a DSC trace analogous with the one of a monophasic polymer. The TSC study confirms this result. Indeed, the TSC peak observed around Tg, is constituted of elementary processes following a single compensation law.The introduction by steps of monomers leads to a block copolymer, with a broad transition region on a DSC thermogram. Two TSC peaks are observed in the same temperature region. Their fine structure shows the existence of two compensation phenomena and so, confirms the presence of two well‐resolved mechanisms. These results are coherent with the existence of a core shell structure.