The glass transition temperatures of amorphous linear aliphatic polyesters

A series of wholly amorphous linear aliphatic co- and tetra-polyesters were synthesized via bulk melt step-growth polymerization. Their glass transition temperatures were determined using DSC and were essentially unaffected by crystallinity. The glass transition temperatures of the polyesters increase linearly with the ratio of ester groups per methylene group. Extrapolations of the ratio to zero ester group content gave a reliable value for the T_g of amorphous polyethylene (PE). The experimental T_gs manifest a steeper slope on the T_g vs. ester group content plot compared to those calculated using Van Krevelen's group contribution method. The intramolecular equilibrium flexibilities were evaluated through the calculation of conformational entropies of individual polymer chains approximated by considering solely the short-range interactions between neighboring groups, as embodied in their RIS conformational models. Their calculated conformational entropies, S_conf, decrease linearly with increasing ester group content, leading to the observation that S_conf ∝1/T_g.