The profound effect of crystallinity on polymer glass transition temperatures (Tgs) necessitates obtaining wholly amorphous samples for the purpose of studying their Tgs. Such samples are usually prepared through a melt-quenching process which could possibly introduce large cooling rate effects on their Tgs and is not feasible for all polymers. In this study, a series of wholly amorphous linear aliphatic co- and tetra- polyesters were synthesized via bulk melt step-growth polymerization, and their glass transition temperatures were determined using Differential Scanning Calorimetry (DSC). With the irregularity introduced by the inherent expected randomness from step-growth copolymerization, the crystallinities of copolymers were significantly reduced to near or wholly amorphous states, leaving the Tgs of the polymer samples essentially unaffected by crystallinity. The glass transition temperatures of the polyesters increase linearly with the ratio of ester groups per methylene group. Extrapolation of the ratio to zero ester group content gives a Tg of 177K for amorphous polyethylene (PE). The calculated Tgs of this polyester series using Van Krevelen’s group contribution method manifest similar linearity with the ester group content and are 3 to 17 K higher than the measured Tgs. 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 decrease linearly with increasing ester group content. This suggests the dominant effect of intramolecular interactions on the Tgs of linear aliphatic polyesters over the entire range of ester group content explored experimentally in this study.

Glass transition temperatures of amorphous linear aliphatic polyesters

251st National Meeting (2016)

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