Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature Tg of molecular (non-polymeric) glass formers correlates with molecular mass M as Tg(M) ∝ Mα, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of Tg vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers Tg(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the Tg(M) dependence in polymers begin to deviate from the universal Tg(M) correlation of molecular glasses and eventually saturates at some polymer specific Tg∞ value. We conclude that at least a substantial part of Tg(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of Tg(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal Tg(M) dependence common with molecular glasses.