Excess heat capacity for low-molecular-weight amorphous polystyrene below the glass-transition temperature: influence of end groups

Abstract

In this work, we analyze the absolute heat capacity below the glass-transition temperature Tg for two types of amorphous polystyrene with differing low-molecular weights. Both amorphous polystyrenes have excess heat capacities that cannot be reproduced by skeletal and group vibrations from 70 K to Tg, which differs from the results for amorphous polystyrene with large molecular weights. The excess heat capacity is analyzed using the Schottky model. The ratio of the degrees of freedom of the Schottky model NSC for PS-A300 and PS-A500 (NSC,PS-A300/NSC,PS-A500) is 1.6, which reflects the ratio of the end groups per molecule ((i.e., the number of monomer units of PS-A500/the number of monomer units of PS-A300 = 5/3) = 1.7), and the resulting excitation energy (=31 meV) is consistent with that determined using inelastic neutron scattering (=30 meV). The results of this investigation indicate that, compared with the high-molecular-weight amorphous polystyrene, the two low-molecular-weight amorphous polystyrenes studied herein have excess heat capacity below the glass-transition temperature. These results were obtained by analyzing the molecular vibrations of the high-molecular-weight polystyrene. The excitation energy obtained by regression analysis of the excess heat capacity determined by using the Schottky model is consistent with the excitation energy obtained by inelastic neutron scattering. This evidence strongly suggests that the excess heat capacity is due to vibrations of methyl end groups.