ABSTRACT
An isentropic equation of state (Isen-Eos) in a high-density polyethylene (HDPE) consists of four variables of PS, VS, TS and entropy S where the VS, PS and TS are variables satisfying the isentropic condition that S is constant. The typical values of isentropic derivatives at constant S = 21.35 [J/molK] and T = 313.15 [K] are αS = −1.66 × 10−3 [K−1] and γS = 13.8 [10−3 GPa] where αS and γS are isentropic thermal expansion coefficient and thermal pressure coefficient, respectively, which can compare with isobaric thermal expansion coefficient αP = 0.368 × 10−3 (K−1) and γv = 2.4 [10−3 GPa] at T = 313.15 (K). The PHug − VHug data of the Hugoniot in the shock wave compression in HDPE can be predicted by the PS − VS with S = 24.5 (J/molK) in the isentropic equation of state approximately. The temperature THug and entropy SHug in the Hugoniot in the HDPE have also been estimated by the isentropic equations of state and it has been found THug increases with pressure and reaches to about 700 K at 50 (GPa), while SHug increases with pressure in low pressure range and then reaches to constant SHug = 24.5 (J/molK) over 30∼50 GPa. The Grüneisen parameters in HDPE evaluated by SHug = 24.5(J/molK) and the isentropic equation of state are in the range of 1.0–1.5 over 0.93 < V (cm3/g) < 1.04.
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