Polymers specific heat

Fig. 4. Typical dependence of polymer specific heat on temperature in the vicinity of...

SpeciTc Heat. The specific heat of a cellular polymer is simply the sum of the specific heats of each of its components. The contribution of the gas is small and can be neglected in many cases. 

The specific heats of polymers are large - typically 5 times more than those of metals when measured per kg. When measured per m, however, they are about the same because of the large differences in density. The coefficients of thermal expansion of polymers are enormous, 10 to 100 times larger than those of metals. This can lead to problems of thermal stress when polymers and metals are joined. And the thermal conductivities are small, 100 to 1000 times smaller than those of metals. This makes polymers attractive for thermal insulation, particularly when foamed. 

Polymer compounds vary considerably in the amount of heat required to bring them up to processing temperatures. These differences arise not so much as a result of differing processing temperatures but because of different specific heats. Crystalline polymers additionally have a latent heat of fusion of the crystalline structure which has to be taken into account. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

Polymer Melt temperature i°C) Mould temperature (X) SG Specific heat (Jkg- K ) Heat required to melt Heat removed on cooling ... 

The negligible water absorption avoids the need for predrying granules. The low specific heat (compared with polyethylene) enables the polymer to be rapidly heated in injection cylinders, which therefore have a higher plasticising capacity with polystyrene than with polyethylene. The setting-up rates in the injection moulds are also faster than with the polyolefins so that faster cycles are also possible. 

Physical and Chemical Properties - Physical State at 15 V and I atm. Solid Molecular Weight Values for anhydrous salt run from 120 to high polymer values Boiling Point at I atm. Not pertinent (decomposes) Freezing Point Not pertinent Critical Temperature Not pertinent Critical Pressure Not pertinent Specific Gravity 1.8-2.5 at 25 °C (solid) Vtpor (Gas) Specific Gravity Not pertinent Ratio of Specific Heats of Vapor (Gas) Not pertinent Latent Heat of Vaporization Not pertinent Heat of Combustion Not pertinent Heat of Decomposition Not pertinent. 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

This result should be vahd for sufficiently high density 0 where correlations, brought about by the mutual avoidance of the chains, are negligible. Due to the recombination-scission process a polydisperse solution of living polymers should absorb or release energy as the temperature is varied. This is reflected by the specific heat Cy, which can be readily obtained from Eq. (9) as a derivative of the internal energy U... 

A MC study of adsorption of living polymers [28] at hard walls has been carried out in a grand canonical ensemble for semiflexible o- 0 polymer chains and adsorbing interaction e < 0 at the walls of a box of size C. A number of thermodynamic quantities, such as internal energy (per lattice site) U, bulk density (f), surface coverage (the fraction of the wall that is directly covered with segments) 9, specific heat C = C /[k T ]) U ) — U) ), bulk isothermal compressibility... 

Though short fiber-reinforced mbber composites find application in hose, belt, tires, and automotives [57,98,133,164] recent attention has been focused on the suitability of such composites in high-performance applications. One of the most important recent applications of short fiber-mbber composite is as thermal insulators where the material will protect the metallic casing by undergoing a process called ablation, which is described in a broad sense as the sacrificial removal of material to protect stmcrnres subjected to high rates of heat transfer [190]. Fiber-reinforced polymer composites are potential ablative materials because of their high specific heat, low thermal conductivity, and ability of the fiber to retain the char formed during ablation [191-194]. 

The transition between crystalline and amorphous polymers is characterized by the so-called glass transition temperature, Tg. This important quantity is defined as the temperature above which the polymer chains have acquired sufficient thermal energy for rotational or torsional oscillations to occur about the majority of bonds in the chain. Below 7"g, the polymer chain has a more or less fixed conformation. On heating through the temperature Tg, there is an abrupt change of the coefficient of thermal expansion (or), compressibility, specific heat, diffusion coefficient, solubility of gases, refractive index, and many other properties including the chemical reactivity. 

Figure 3.10 shows the typical dependence on temperature of the specific heat of an amorphous and a crystalline polymer. For both materials, the specific heat has a steep dependence on temperature, but the behaviour is more complex in the case of the amorphous material. 

The overall specific heat of a polymer is given by a combination of the various contributions to the specific heat of longitudinal and transversal phonons. At temperatures below 1K, the linear contribution due to the TLS must be added. 

Specific heat cP [J/g/K] of technical solids various solids [56] metals, alloy and non-metals [58] metals [71] polymers [47]... 

We shall report hereafter three examples of measurement of heat capacity the first (of a crystal) with a negligible addendum the second (of a polymer) with a heavy addendum the third is the measurement of the carrier specific heat of a heavily doped semiconductor. 

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