Polyethylene heat capacity

Between SO and 90° K no new data have been publidied since 1962. Canyii the extrapolation of the amorphous polyethylene heat capacity of Tucker and Reese (1967) to higher temperature, identical heat capacities for completely crystalline and completely amorp4ious polsmier are reached with the 60° K value. Measurements on samples 6, 14 and 15 agree within 1% up to 90° K, and average values of these measurements are listed in Table III.8. Excluding the 90° K value c f sample 17 which is singularly hi, the constancy of heat capacity with crystallinity continues up to 120° K, Samples 6, and 14 to 18 show a 1—2% maximum deviation from the average heat capacity. 

The amorphous polyethylene heat capacity was evaluated between 410° K and 450° K making use of data from samples 2 to 5, 11 to 13, 17, 19, 20, and 29. The spread of data at any one temperature was 5 to 8%. The averages at each temperature were used for a least squares calculation. The equation ... 

A comparison of the heat capacity data for the difCerMit simple polyoxides below the gjass transition temperature, in the region where the heat capacities are independent of crystallinity, is presented in Table III.21. Polyethylene heat capacities are listed for comparison. An... 

SolubiHty parameters of 19.3, 16.2, and 16.2 (f /cm ) (7.9 (cal/cm ) ) have been determined for polyoxetane, po1y(3,3-dimethyl oxetane), and poly(3,3-diethyloxetane), respectively, by measuring solution viscosities (302). Heat capacities have been determined for POX and compared to those of other polyethers and polyethylene (303,304). The thermal decomposition behavior of poly[3,3-bis(ethoxymethyl)oxetane] has been examined (305). 

Much more information can be obtained from the DSC experiment than simply an observation of the transition from a solid to a liquid phase. A plot of heat flow against temperature is a true depiction of the continuity of the heat capacity at constant pressure (Cp). If the entire temperature range of a given process is known, the physical state of a material will reflect the usefulness of that material at any temperature point on the plot. For polyethylene terephthalate (see Fig. 4.9), a stepshaped transition is interpreted as a change in Cp resulting from a... 

The state of polarization, and hence the electrical properties, responds to changes in temperature in several ways. Within the Bom-Oppenheimer approximation, the motion of electrons and atoms can be decoupled, and the atomic motions in the crystalline solid treated as thermally activated vibrations. These atomic vibrations give rise to the thermal expansion of the lattice itself, which can be measured independendy. The electronic motions are assumed to be rapidly equilibrated in the state defined by the temperature and electric field. At lower temperatures, the quantization of vibrational states can be significant, as manifested in such properties as thermal expansion and heat capacity. In polymer crystals quantum mechanical effects can be important even at room temperature. For example, the magnitude of the negative axial thermal expansion coefficient in polyethylene is a direct result of the quantum mechanical nature of the heat capacity at room temperature." At still higher temperatures, near a phase transition, e.g., the assumption of stricdy vibrational dynamics of atoms is no... 

In the case of polyethylene it is interesting to attempt to calculate the heat capacity per mole of chain atoms using the Einstein function... 

Turning now to room temperature specific heat data, the author [Dole (1959)] has pointed out that the heat capacities per mole of chain atoms are practically the same for polyethylene, 6-6-nylon, and 6-nylon. In the case of polypropylene, however, the methyl side chain apparently contributes as much to the specific heat as the chain atoms. Drawing the polymer into fiber form seems to make very little difference in the specific... 

Fig. 3, Comparison of heat capacities of crystalline Q (C ), open circles, and of a low density polyethylene Cv), solid circles, with values calculated from molecular frequencies and the Einstein function...

B. If the process were adiabatic, what would be the temperature rise The heat capacity of polyethylene is 2 kJ/kg °C. 

Good models are needed because information on important properties such as heat capacity and elastic modulus can be derived from the force constants, The elastic modulus data is particularly useful since it allows the ultimate tensile strength of polyethylene to be determined. Based on present estimates of this, it is apparent that it is still possible to improve existing materials. 

The term QAP is the power required to pressurize the melt (pumping power) and represents less than 5% of the total power required for the process (9). Hence, as can be seen from Equation 3, the power requirement is essentially determined by the product CQAT. Typical processing temperatures are listed in Table III (10, 11. 12). Thus, it is found that high-density polyethylene requires the most power per pound of product while polystyrene requires the least. It should be noted that the average heat capacity values in Table II include the heat of fusion for the semicrystalline polymers such as high- and low-density polyethylene. 

Also plotted in Fig. 1.2 is the experimental heat capacity of the liquid (at omi-stant pressure) In simple cases, such as polyethylene, the heat capacity of the liquid state could be understood by introducing a heat capacity contribution for the excess volume (hole theory) and by assuming that the torsional skeletal vibration can be treated as a hindered rotator A more general treatment makes use of a separation of the partition function into the vibrational part (approximated for heat capacity by the spectrum of the solid), a conformational part (approximated by the usual conformational statistics) and an external or configurational part. 

The resultant thermal curve is similar in appearance to a DTA thermal curve, but the peak areas are accurate measures of the enthalpy changes. Differences in heat capacity can also be accurately measured and are observed as shifts in the baseline before and after an endothermic or exothermic event or as isolated baseline shifts due to a glass transition. Because DSC provides accurate quantitative analytical results, it is now the most used of the thermal analysis techniques. A typical DSC thermal curve for polyethylene tereph-thalate, the polymer used in many soft drink bottles, is shown in Fig. 16.22. 

HEAT CAPACITY MEASUREMENTS ON POLYETHYLENE IN THE TEMPERATURE RANGE OF 2.4 TO 30 K. PH.D. THESIS. 

Figure 2.23 displays the change of enthalpy and free enthalpy of polyethylene from zero kelvin, based on heat capacity data, described in Sect. 2.3.7, and heat of fusion measurements (see, for example. Sect. 4.3.7). There is no jump in free enthalpy at the melting temperature, only a change in the slope. The double arrow marks TS as the difference between H and G as shown in Fig. 2.22, Eq. (3). 

The first step in the analysis must thus be to establish the crystallinity dependence of the heat capacity. In Fig. 2.44 the heat capacity of polyethylene, the most analyzed polymer, is plotted as a function of crystallinity at 250 K, close to the glass transition temperature (T = 237 K). The fact that polyethylene, [(CH2-)xl, is semicrystalline implies that the sample is metastable, i.e., it is not in equilibrium. Thermodynamics... 

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