Amorphous specific heat

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 ... 

Extrusion blow moulding of bottles has been successfully accomplished in reeent years by attention to the points mentioned above. It is to be noted here that UP VC has a much lower average specific heat between the proeessing temperature and room temperature than polyethylene and, being essentially amorphous, no latent heat of fusion. This leads to much less heat needing to be removed on cooling of mouldings and very short cycle times are possible. 

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. 

The specific heat of amorphous plastics increases with temperature in an approximately linear fashion below and above Tg, but a steplike change occurs near the Tg. No such stepping occurs with crystalline types. 

Dependence on Density.—If the density of a metal is increased by hammering, its specific heat is slightly decreased. The same change is observed if the change of density is due to a change of crystalline form, or to change from an amorphous state to a crystalline state, and with different allotropic forms (Wigand, loc. cit,). 

Nernst also concludes that the specific heats of liquids tend to very small values at low temperatures, since according to Tammann ( 88) liquids pass into amorphous solids at low temperatures, and the latter are subject to the ergonic distribution. 

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. 

Fig. 3.10. Typical temperature dependence of the specific heat for an amorphous (---------------------), and a...

As an example, in Fig. 3.11, a schematic two-dimension representation of the structure of cristobalite (a crystalline form of Si02) and of vitreous Si02 is shown. A, B and C represent three cases of double possible equilibrium positions for the atoms of the material in the amorphous state [41]. Atoms can tunnel from one position to another. The thermal excitation of TLS is responsible for the linear contribution to the specific heat of amorphous solids. 

The model proposed by Anderson and Phillips gives a phenomenological explanation of the properties of the amorphous materials without supplying a detailed microscopic description [42]. Low-temperature measurements of the specific heat of amorphous solids have however shown that instead of a linear contribution as expected from the TLS theory, the best representation of data is obtained with an overlinear term of the type [43,44] ... 

All amorphous materials, in summary, show a specific heat with a cubic and an overlinear contribution ... 

Fig Curves of specific heat as a function of increasing temperature for quenched (amorphous) poly (ethylene terephthalate). 

The coefficient of linear expansion of unfilled polymers is approximately 10 X 10 5 cm/cm K. These values are reduced by the presence of fillers or reinforcements. The thermal conductivity of the polymers is about 5 X 10 4 cal/sec cm K. These values are increased by the incorporation of metal flake fillers. The specific heat is about 0.4 cal/g K, and these values are slightly lower for crystalline polymers than for amorphous polymers. 

It resembles the crystalline variety in behaviour except in such properties as are influenced by its fineness of division. The electrometric properties of the two forms are identical.2 The density of the amorphous form ranges from 5-85 to 5-87 3 the specific heat is 0-052.4... 

Tellurium melts at 452° C.1 and boils near 1390° C. under ordinary pressure,2 but volatilises at as low a temperature as 430° C. in a cathode-ray vacuum the vapour is yellow in colour.3 Like the density, the specific heat of the solid is inconstant, ranging from 0-0475 for the distilled element to 0-0524 for the precipitated amorphous substance.4 It has been observed 5 that exposure to X-rays increases the specific heat of tellurium by about 8 per cent., possibly owing to a change in the structure of the element. 

Fig. 1. Specific heat of amorphous polyethylene terephthalate as a function of temperature...

Land, Richards and Ward (1959) made NMR studies of amorphous and partially crystalline samples of polyethylene terephthalate, and found no effects that can be correlated with the specific heat measurements. 

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