Vibrational heat capacity of Poly(N-isopropylacrylamide)

•Vibrational motion was linked with a low temperature experimental heat capacity of Poly(N-isopropylacrylamide).•Experimental, apparent heat capacity of Poly(N-isopropylacrylamide) was investigated by advanced thermal analysis.•Equilibrium solid (vibrational) and liquid heat capacity and parameters of Poly(N-isopropylacrylamide) were established.•Equilibrium thermodynamic functions: enthalpy, entropy and Gibbs energy were determined.•Quantum design PPMS, DSC, and TMDSC, are useful tools for characterization of Poly(N-isopropylacrylamide).

The heat capacity of poly(N-isopropylacrylamide) (PNIPA) has been measured from (1.8-460) K using a quantum design PPMS (Physical Property Measurement System) and differential scanning calorimeters. The low-temperature experimental heat capacity below the glass transition temperature 415 K (141.85 °C) was linked to the vibrational molecular motions of PNIPA to establish the baseline of the solid, vibrational heat capacity. The vibrational heat capacity of PNIPA was computed based on group and skeletal vibrations. The group vibrational heat capacities were calculated based on the chemical structure and molecular vibrational motions derived from infrared and Raman spectroscopy. The skeletal heat capacity was fitted to a general Tarasov equation using sixteen skeletal modes to obtain three Debye characteristic temperatures Θ1 = 650 K and Θ2 = Θ3 = 68.5 K. The fit agreed well with the experimental data (±0.2%) from T = (1.8-250) K, and the vibrational heat capacity, which was extended to higher temperatures, served as a baseline of the solid heat capacity for quantitative thermal analysis of the experimental, apparent heat capacity data of PNIPA. The liquid heat capacity of fully amorphous PNIPA was approximated by a linear regression and expressed as Cpliquid(exp) = 0.3379 ⋅ T + 126.69 in J K−1 mol−1. This was then compared to the estimated linear contributions of polymers that have the same constituent groups. Using estimated parameters of transitions and solid and liquid heat capacities at equilibrium, the integral thermodynamic functions of enthalpy, entropy and free enthalpy as functions of temperature were calculated.

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