Heat Capacity and Thermodynamic Properties of Poly(chlorotrifluoroethylene) from 2.5 to 620 K.

Heat capacities and thermodynamic properties of a number of poly(chlorotrifluoToethylene) samples subjected to various thermal treatments, to achieve crystallinities ranging from approximately 10 to 90%, have been studied from 2.5 to 370 K by automated adiabatic calorimetiy and from 250 to 620 K by differential scanning calorimetry. Small heat capacity discontinuities in the temperature range from 320 to 350 K were observed in all samples with crystallinities greater than 40%. Spontaneous adiabatic temperature drifts associated with these anomalies were prasitive (exothermic) for quenched samples and negative (endothermic) for annealed samples. Therefore these anomalies were believed to be associated with a relaxation phenomenon similar to that of a glass transition. For highly quenched low crystallinity films, a much larger heat capacity discontinuity of greater than 15% was observed, amidst a crystallization exotherm. In addition to the above phenomena, annealing of the sample at any temperature between 240 to 400 K would produce a shift in the population distribution of crystallites from reorganization or melting and recrystallization. As a result, the apparent heat capacity became somewhat lowered at the annealing temperature and somewhat raised at about 20 K above the annealing temperature.

Heat Capacity and Thermodynamic Properties of Poly(Vinyl Chloride).

Heat capacities, C p , of three different samples of poly(vinyl chloride), PVC, have been determined from 6 to 375 K by adiabatic calorimetry. These three samples were derived from either bulk- or suspension-polymerization processes and were measured either as received or after pelleting under pressure. The heat capacities of the samples are almost identical if the thermal and pressure histories are the same. Below the glass transition temperature, T g , of about 355 K, C p of PVC was found to be exceptionally linear over a wide temperature range. C p of annealed PVC may be represented by (10 + 0.166 T) J K-1 mol-1 to within 1 percent of the measured values from 80 to 340 K. Approximately 200 J mol-1 of energy were stored in the samples by the pelleting processes. The stored energies begin to release at about 30 to 40 K below the glass transition temperature. T g for powdery or relaxed samples occurs around 352 to 356 K for the suspension-polymerized PVC sample and 348 to 351 K for bulk-polymerized sample.

Heat Capacities of Polyethylene IV. High Molecular Weight Linear Polyethylene.

A high molecular weight linear polyethylene sample has been studied by adiabatic calorimetry from 10 to 380 K. Two broad temperature regions of unusual spontaneous temperature drift have been observed. The phenomena occurring around 240 K are similar to that observed in other polyethylene samples studied in this series, and are presumed to be caused by the relaxational processes in the amorphous phase. The weak exothermic behavior occurring around 160 K is presumed to be caused by the stabilization of the quenched sample.

A Self-Balancing Nanovolt Potentiometric System for Thermometry and Calorimetry.

The principle of a self-balancing potentiometric system is described. The principle is applied to the modification of an existing manually operated thermo-free, low voltage potentiometer consisting of Diesselhorst ring elements. The modification involves the addition of reed relays which enable the potentiometer voltage to be set by digital signals. By incorporating a digital voltmeter, or an analog-to-digital converter, and a nanovolt amplifier with the modified potentiometer, self-balancing of the potentiometer may be achieved through either hardware logic implementation or direct digital control from a minicomputer. The resolution of this self-balancing potentiometric system for a full scale input of 100 mV is about one to 10 parts in 108. With real-time digital processing of the data, resolution of about 1 nV or better has been achieved for slowly changing input signals. The overall accuracy of the system is better than 10 ppm for voltage measurements and about 1 ppm for voltage ratio or resistance measurements.

Heat Capacities of Polyethylene III. One Linear and One Branched Sample from 5 to 350 K.

Heat capacities of two polyethylene samples, one linear with a density of 0.973 g cm-3 and one branched with a density of 0.91 g cm-3, have been determined by adiabatic calorimetry from 5 to 360 K in a different experimental arrangement then employed for studies of other polyethylene samples in this series. The heat capacity behavior of these two samples confirms expectations for samples with corresponding densities.