Glass transitions in polymer-plasticizer systems

Abstract

Glass temperatures (Tg) of pure and plasticized elastomers have been obtained from dilatometric measurements and from the temperature dependence of angular response to torsional stress. The depression of Tg by a diluent, determined by either method, is a convenient measure of its plasticizing ability. Some compounds which are normally crystalline above T g of the pure polymer, but nevertheless compatible with the polymer, depressed the glass temperature. The polymer-diluent compatibility at low temperatures results either from supercooling, the depression of the melting point of diluent by polymer, or possibly a combination of both phenomena. T g decreased as the diluent loading increased to a more or less well defined limit, beyond which further dilution had negligible effect. For diester type plasticizers both ΔTgmax and the limiting diluent concentration were linear functions of compatibility, as defined by the extent to which standard polymer vulcanizates were swollen by plasticizer. The viscosities and apparent activation energies for flow (Ev) of hydrocarbon oils correlated with their low temperature plasticizing efficiencies. Pour points and glass temperatures may also be significant properties of the diluents. It is concluded that low Ev and high solvency for polymer are desirable properties for low temperature plasticizers.