Low-temperature excess specific heat and fragility in semicrystalline polymers

The specific heats of polyethylene (PEO) polymers with two different degrees of crystallinity and of a PEO-NaSCN polymeric complex have been measured between 1.5 and 25 K. The behavior of specific heat with decreasing crystallinity is consitent with the universal feature expected for a solid in which the topological disorder introduces an excess of low-energy vibrational modes: the peak C-p/T-3, as compared to that revealed in the most crystalline and the amorphous phases characterizing the polymer. The excess specific heat over that provided from the Debye theory has been described by an empirical phonon-fracton density of vibrational states. A density of fracton states with a dimensionality equal to 1 appears to well account for the additional localized vibrations in a polymeric network without sidebonds, such as pure PEO.A comparison between the parameter of ''fragility'' obtained from modelingthe mechanical alpha(a) relaxation of the amorphous fraction of the polymers and the excess specific heat has been tried. Within the range of fragilities explored, no definite correlation between the fragility and the the density of soft vibrations has been found.