LOW-TEMPERATURE EXCESS SPECIFIC-HEAT AND FRAGILITY IN SEMICRYSTALLINEPOLYMERS

The specific heats of polyethylene (PEO) polymers with two different d egrees of crystallinity and of a PEO-NaSCN polymeric complex have been measured between 1.5 and 25 K. The behavior of specific heat with dec reasing 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 r evealed in the most crystalline and the amorphous phases characterizin g the polymer. The excess specific heat over that provided from the De bye theory has been described by an empirical phonon-fracton density o f vibrational states. A density of fracton states with a dimensionalit y equal to 1 appears to well account for the additional localized vibr ations in a polymeric network without sidebonds, such as pure PEO.A co mparison between the parameter of ''fragility'' obtained from modeling the mechanical alpha(a) relaxation of the amorphous fraction of the po lymers and the excess specific heat has been tried. Within the range o f fragilities explored, no definite correlation between the fragility and the the density of soft vibrations has been found.