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P-v-T Properties and Thermal Constants of Some Polymers

In order to determine the P (pressure)-v (specific volume)-T (temperature) properties of general-purpose polymers, we have constructed a plunger type of apparatus. It is used to investigate experimentally the temperature and pressure dependence the specific volume of various polymers, and the relaxation of loading pressure for molten polymer. Furthermore, P-v-T properties of polymers were discussed in connection with thermal constants such as thermal conductivity, thermal diffusivity and specific heat. The results show that the specific volume of PE at 150∼200°C is about 18∼22% larger than that of one at room temperature because of thermal expansion. The compressibility of the specific volume with increasing loading pressure was measured and the degree of compaction Δv/ΔP (ratio of specific volume to pressure) of the molten polymers was measured to be 8.9∼9.1×10-7 in the PE, 8.5∼9.9×10-7 in the PP and 6.4∼6.9×10-7m3/kg/MPa in the PS, respectively. The volume compressibility of the crystalline polymers are larger than that of the amorphous polymer. The pressure relaxation of the molten polymer in a closed barrel under isothermal-constant volume conditions is dependent on the diffierence between the plunger loading pressure and the pressure produced by thermal expansion of the polymer itself. Using a=1/ρ·λ/Cp, and if thermal conductivity λ and specific heat Cp are known at each temperature, the thermal diffusivity a can be calculate as a function of specific volume v (=1/ρ). Thus the P-a-T properties of polymers can be determined experimentally. The behavior of thermal diffusivity a in a P-a-T diagram of polypropylene (PP) was found to resemble that of specific volume v in a P-v-T diagram.