Multiple Transitions in Polyalkyl Methacrylates

Abstract

A series of polyalkyl methacrylates, two hydroxylated methacrylates, polymethoxy ethyl methacrylate, and polymethyl acrylate have been studied by linear expansion and torsion‐pendulum measurements, in the temperature range from −180°C to just above the respective glass transitiontemperatures. The length‐temperature data (L‐T) were analyzed by a least‐squares differentiation procedure, based on moving arcs, to give directly the thermal expansion coefficients as a function of temperature. The L‐Tmeasurements suggest the existence of two Tgg transitions (Tg −100<Tgg <Tg ) in the polymers with the more flexible side chains, and one transition for polymers with bulky side groups. There is also evidence of a third Tgg transition at about −120° to −90°C, which may be associated with adsorbed water molecules. In similar fashion to that of Tg , the location of the Tgg transitions decreases upon increasing the length of the n‐alkyl side chain. The hydroxylated polymers have higher Tg 's than the corresponding alkyl samples, and have thermal expansion coefficients only about one‐half as large, both features being a result of interchain hydrogen bonds. Torsion‐pendulum measurements indicate only one clear‐cut Tgg transition region; hence, it seems that the two Tgg transitions observed in the expansion studies cannot be unequivocally resolved by dynamic measurements. Expansion coefficients of the n‐alkyl series suggest the retention of some excess free volume below Tg for side chains larger than ethyl. Substitution of 3α′ g″ , the expansion coefficient below our second Tgg transition, into the Simha‐Boyer equation (α l ‐α g ) Tg gives a value close to their empirical constant. This supports their suggestion that polymers with pronounced subgroup motions exist in an iso‐free volume state below the subgroup transitiontemperature.