Heat capacity of poly(N-vinylpyrrolidone)

As a first step in the analysis of the water state in biocompatible poly(N-vinylpyrrolidone) (PVP), the heat capacity of dried PVP was measured over a temperature range 15–520 K by adiabatic calorimetry and differential scanning calorimetry (DSC) to determine the recommended values. Based on these values, the heat capacity of the sold state was analyzed using an approximate group vibrational spectrum and skeletal vibrations. The 2-pyrrolidone-ring group vibrations of 34 modes were determined from the ab initio calculations based on density functional theory regarding the hydrogen-terminated dimer of PVP. The six skeletal vibrational modes are well represented by a Tarasov function with theta temperature of Θ1 = 646.0 K and Θ3 = 81.9 K, which were obtained from a three-step mesh optimization. The heat capacity of the liquid was fitted to a linear function: CPliquid = 0.2909 T + 101.9 (in J K−1 mol−1). The change in heat capacity, ΔCP0, of amorphous PVP at the glass transition temperature (440 K) is 29.3 J K−1 mol−1. The values of ΔCP0, Θ1 and Θ3 agreed with the deep-learning predictions based on the ATHAS data bank with a relative error within 20%. Using the obtained characteristic parameters and group vibrations, the calculated heat capacities as a function of temperature (which correspond to the DSC baseline) were determined.