To determine the effect of isothermal structural change on the configurational and vibrational parts, the real component of the complex specific heat $C_p^{\prime }$ of poly(styrene) at 3.33 mHz frequency was measured in its vitrification range during very slow cooling, heating, and during the isothermal annealing of its glassy state formed by rapid cooling. As the structure relaxes during annealing, $C_p^{\prime }$ decreases asymptotically with time by at least 1%. This is attributed to mostly a decrease in the configurational contribution to $C_p^{\prime }$ as the relaxation time of the $\alpha$ process increases and contributions from the Johari-Goldstein relaxation decrease. $C_p^{\prime }$ of the structurally equilibrated state is closer to the vibrational $C_p$ than the $C_p^{\prime }$ for 3.33 mHz frequency measured for the slowest cooling at 1 K/h. The apparent specific heat $C_{p,\text{app}}$ was also measured during cooling and heating of the annealed and the unannealed samples, and the data were analyzed in terms of the enthalpy loss and recovery with the spontaneously changing fictive temperature. A procedure is described for determining when the equilibrated state has been reached. The vibrational $C_p$ is not greatly affected by the structural change toward equilibrium. Thus the configurational $C_p$ is equal to the difference between the equilibrium liquid's and the nonequilibrium glass $C_p$. Consequences of thermoviscoelastic effects on the specific-heat spectroscopy have been discussed.

• Received 8 June 2008

DOI:https://doi.org/10.1103/PhysRevB.78.144203