Vibrational and configurational heat capacity of poly(vinyl acetate) from dynamic measurements

The complex heat capacity $Cp*$ of poly(vinyl acetate) has been measured at $20.95mrad∕s$ modulation frequency during the cooling as well as on heating at 24, 8, and $2K∕h$ and during cooling at $0.5K∕h$⁠. The study is complemented with (the rate-dependent) $Cp,app$ measured during cooling and heating at 60, 24, and $8K∕h$⁠. At low temperatures, the real component of $Cp*$ yields the unrelaxed $Cp$ or $Cp,vib$⁠, the vibrational part of $Cp$⁠. It is found to be indistinguishable from $Cp,glass$ and lies on a line extrapolated to its equilibrium melt’s temperature. At $T$ near $Tg,ΔCp(=Cp,melt−Cp,glass)$ shows no detectable contribution from $Cp,vib$⁠. The finding conflicts with a modified entropy theory calculation [E. A. DiMarzio and F. Dowell, J. Appl. Phys. 50, 6061 (1979)], which had predicted that $∼27%$ of $ΔCp$ of poly(vinyl acetate) at $T$ near $Tg$ is vibrational in origin and the remainder configurational. At $T<Tg$⁠, the real component of $Cp*$ varies more slowly with $T$ than $Cp,app$⁠.