Melting parameters of poly(glycolic acid)

Equilibrium melting temperature Tm0, heat of fusion ΔHf, and entropy of fusion ΔSf of poly(glycolic acid) (PGA) was determined by using Clapeyron–Clausius equation. Equilibrium melting temperature Tm0 was 504.6 K which was determined by Hoffman–Weeks plots. The pressure dependence of Tm0 was determined by high pressure DTA up to 150 MPa. Volume change ΔVf at melting was determined by using dilatometer. Heat of fusion in PGA was 183.2 (J g−1), which is very close to the value reported by Chujo et al. who determined it by using Tm depression in copolymer with poly(lactic acid). ΔSf of PGA was 0.363 (J g−1 K−1), which is about twice that of PLA, and the reason was discussed on the basis of the elastic modulus below Tm.