Thermal diffusivity, of polymers

As the thickness of the laminate increases, the strength of this thermal spike and the degree of thermal lag during heat-up increases. Figure 8.8 shows the results for a 62.5-mm (500 ply) laminate of the same material. Now the center-line temperature never reaches the autoclave temperature during the first dwell, and the thermal spike during the second dwell is nearly 135°C. The thermal spike is directly related to the release of internal heat during cure. The thermal lag is a manifestation of the low thermal diffusivity of polymer matrix composites. 

X HERMAL FIELD-FLOW FRACTIONATION (ThFFF) separates polymers according to their molecular weight and chemical composition. The molecular weight dependence is well understood and is routinely used to characterize molecular weight distributions (1-4). However, the dependence of retention on composition is tied to differences in the thermal diffusion of polymers, which is poorly understood. As a result, the compositional selectivity of ThFFF has not realized its full potential. How-... 

DETERMINATION OF THERMAL DIFFUSIVITY OF POLYMERS BY DIFFERENTIAL THERMAL ANALYSIS. 

The thermal diffusion of polymers induced by the applied temperature gradient, unlike polymer transport driven by sedimentation or electrical fields, is a poorly understood phenomenon. However, a good deal of information has accrued by virtue of thermal FFF experiments thermal FFF has now become the most useful and convenient tool for studying the mechanism of polymeric thermal diffusion [17-20]. 

Janca, J. Micro-thermal field-flow fractionation New chal- 18. lenge in experimental studies of thermal diffusion of polymers and colloidal particles. Phil. Mag. 2003, 83, 2045. 

Various other methods have been described for the determination of thermal conductivity. Capillarity has been used to measure the thermal conductivity of LDPE, HOPE and PP at various temperatures and pressures [30]. A transient plane source technique has been applied in a study of the dependence of the effective thermal conductivity and thermal diffusivity of polymer composites [31]. 

W. N. dos Santosa, P. Mummery, A. Wallwork. Thermal Diffusivity of Polymers by The Laser Flash Technique. Polymer Testing 2005 24 628-634. 

The thermal difFusivity values of some polymeric substances are shown in Table 2. For comparison, values for several ceramic and metallic materials are included. Thermal difFusivities of polymers are, in general, quite low and of the order of 0.1-0.3 x 10 m /s. Superficially, trends similar to thermal conductivity are seen. 

In addition to giving conformational information, solid state NMR relaxation experiments can be used to probe the thermal motion of polymers in the hydrated cell wall (5). The motion of the polymers can give us clues as to the environment of the polymer. When there are both rigid and mobile polymers within a composite material, NMR spin-diffusion experiments can be used to find out how far apart they are. 

The simplest recording medium is a bilayer structure. It is constructed by first evaporating a highly reflective aluminum layer onto a suitable disk substrate. Next, a thin film (15-50 nm thick) of a metal, such as tellurium, is vacuum deposited on top of the aluminum layer. The laser power required to form the mark is dependent on the thermal characteristics of the metal film. Tellurium, for example, has a low thermal diffusivity and a melting point of 452 °C which make it an attractive recording material. The thermal diffusivity of the substrate material should also be as low as possible, since a significant fraction of the heat generated in the metal layer can be conducted to the substrate. For this reason, low cost polymer substrates such as poly (methylmethacrylate) or poly (vinyl chloride) are ideal. 

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

For illustrative purposes a thermal diffusivity of 1.3 x 10 3 cm2/s is often used as being typical of rubber and polymers. Some values from the literature for various materials are given in Table 2.1. Using the value of 1.3 x 10"3 it can be calculated that a depth of 3 to 11 Jim is being sampled at 2000 cm"1 as indicated in Table 2.2. This is an order of magnitude greater than that sampled by ATR techniques. 

Kirkland et al. [359] reported the possibility of varying the retention behavior in Th-FFF by the application of a solvent mixture, later supported by other workers [58,360]. The retention enhancement so achieved was attributed to a synergistic effect involving the thermal diffusion of both polymer and solvent. 

This same reaction sequence can be used to describe the thermal decomposition of polymers under reducing conditions. In this case, the value of n is equal to 0 and h is usually set equal to 1.0 in the generalized reaction. Under these conditions, the mass transfer is limited to the removal of the volatiles from the porous green body. This mass transfer can be limited by the pore diffusion or the boundary layer. We still must consider that the surface reaction or the steps of heat transfer in the boundary layer or heat conduction in the porous body could also be rate controlling in this case of the thermal decomposition of polymers under reducing conditions. 

Caruthers, J. M. (ed.). Handbook of Diffusion and Thermal Properties of Polymers and Polymer Solutions, AIChE, New York, 1998. 

It is generally accepted that thermal stability of polymer nanocomposites is higher than that of pristine polymers, and that this gain is explained by the presence of anisotropic clay layers hindering diffusion of volatile products through the nanocomposite material. It is important to note that the exfoliated nanocomposites, prepared and investigated in this work, had much lower gas permeability in comparison with that of pristine unfilled PE [12], Thus, the study of purely thermal degradation process of PE nanocomposite seemed to be of interest in terms of estimation of the nanoclay barrier effects on thermal stability of polyolefin/clay nanocomposites. 

According to the pathways described above, the thermal decomposition of polymers often involves the formation of volatile species within a highly viscous polymeric matrix. Transport of these species through the molten polymer mass towards the vapour phase is not a straightforward process, and so the occurrence of mass transfer limitations can be expected. Various authors have observed that the rate of polymer thermal degradation depends on factors such as the surface area and thickness of the polymer sample, showing that the polymer decomposition is controlled by the diffusion and/or the vaporization of the volatile species.13,14... 

Polymers have unique properties important for processing, viz., low thermal diffusivity, high viscosity and viscoelasticity. Because of the low thermal conductivity, efficient plastication cannot be based on thermal diffusivity alone. Polymer melting requires generating heat dissipation through intensive deformation of the highly viscous melts, leading to thin films. 

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