Quantitative aspects of physical ageing in polymers

• Main Author: Kriesten, Ute
• Published: University of Aberdeen 1993
• Subjects: 668.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592904

Physical ageing, which is the spontaneous increase of mechanical relaxation times after quenching from above the glass transition temperature, was the subject of this study. The quench at the beginning of physical ageing serves to 'rejuvenate' the material. This means it reproducibly creates a frozen-in non-equilibrium material structure with an increased mobility which then relaxes towards equilibrium. The creep behaviour is used to represent the molecular mobility, and was measured with small strain torsional creep tests. As the mobility decreases the retardation time increases. A quantitative evaluation of the changes in the creep behaviour as a result of physical ageing was undertaken. The creep data were combined with measurements that tested other thermodynamic quantities also known to exhibit recovery. These are the specific enthalpy measured with DSC (diferential scanning calorimetry) and specific volume measured using a density gradient column. Physical ageing experiments on polystyrene (an amorphous polymer) were intended to choose between the wide variety of existing theories. No appreciable change in the relaxation spectrum was found; howver, a closer look at the common superposition procedures revealed that such change might easily remain undetected. The experiments measuring volume relaxation and enthalpy relaxation show that physical ageing cannot be related in a straightforward sense to enthalpy relaxation or volume relaxation. In polypropylene, a semicrystalline polymer, various recovery effects were observed at room temperature. This is above the material's glass transition temperature and can therefore not be explained in the same terms as physical ageing in polystyrene. The phenomena are however strikingly similar. Not only does the relaxation spectrum of polypropylene shift to longer relaxation times without much change in the spectrum, but also volume relaxation and enthalpy relaxation are observed in magnitudes comparable to polystyrene. The enthalpy relaxation as observed using DSC shows complexities which help to understand the mechanism of structural relaxation in polypropylene.