Measurement of Specific Heat of Polytetrafluoroethylene Using Millidegree Temperature Increments in Thin Foil Calorimeter

Abstract

The thin foil calorimeter technique described earlier is extended to detail specific heats of polymers near transition temperatures. A sample film is rolled up with a metal foil heating element into a cylindrical configuration. Starting with the roll at thermal equilibrium, a constant current is passed through the foil. During a finite period after heating begins, the interior of the roll heats adiabatically. The temperature rise rate, sensed by a thin foil thermocouple, is monitored by a microvolt amplifier-recorder system. A typical 35 sec run shows a linear temperature rise of about 7×10−3 K, with a noise level of the order of 15×10−6 K. Resultant specific heat values are estimated to be precise to within 1%. Resistance thermometry gives corresponding mean sample temperatures with a precision of the order of 0.01 K. Although specific heat values obtained for polytetrafluoroethylene mostly agree with published adiabatic calorimeter data, remarkably different results are obtained in the transition temperature region extending from 280 to 310 K. Where adiabatic calorimeter data show a large enthalpy peak extending over a 15 K temperature interval, foil calorimeter data show a relatively minor but sharp discontinuity near 293 K.