Polymers thermal diffusivity

Dj for the polymer solution is not well understood. There are several theories of polymer thermal diffusion in liquids. Extensive reviews have been done by Schimpf and colleagues in several publications [4,9]. As an example, the radiation pressure theory of Gaeta and Scala shows that Dj is proportional to the cross-sectional area of the solute molecules [10] ... 

A laser flash technique has been used to determine the diffusivity of pyroelectric polymers such as polyvinylidene fluoride [83], whereas hot-wire techniques have been used to determine the thermal diffusivity of high-density polyethylene, low-density polyethylene propylene, and polystyrene [83], Dos Santos and coworkers [84] utilized the laser flash technique to study the effect of recycling on the thermal properties of selected polymers. Thermal diffusivity expresses how fast heat propagates across a bulk material, and thermal conductivity determines the woiking temperature levels of a material. Hence, it is possible to assert that those properties are important if a polymer is used as an insulator, and also if it is used in applications in which heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications (including food wrapping) were selected for this study. 

Thermal Diffusivity. Much of the literature in the past dealing with the thermal properties and characteristics of polymers, thermal diffusivity was either neglected or treated as an easily derivable quantity or mathematical parameter. However, thermal diffusivity is an important material property, determining temperature distributions in nonsteady-state heat conduction. Like thermal... 

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

Silver vapor cocondensed with matrices of HjO or paraffin wax (C22H4 ) at 12 K gives mainly an atomic dispersion of Ag. However when these Ag atom matrices are warmed briefly (to 77 K for HjO and up to 80 K for C22H46), thermal diffusion takes place with aggregation of the Ag atoms into small clusters of up to Ag4. These thermal aggregation methods can be used to prepare small clusters, but a mixture of metal polymers is usually obtained. 

Optimization requires that a-rtjl have some reasonably high value so that the wall temperature has a significant influence on reactor performance. There is no requirement that 3>AtlR be large. Thus, the method can be used for polymer systems that have thermal diffusivities typical of organic liquids but low molecular diffusivities. The calculations needed to solve the optimization are much longer than those needed to solve the ODEs of Chapter 6, but they are still feasible on small computers. 

Kdhler, W and Schdfer, R, Polymer Analysis by Thermal-Diffusion Forced Rayleigh Scattering. Vol. 151, pp. 1-59. 

Before developing the necessary equations to predict the temperature within a composite part, it is enlightening to consider the thermal diffusion problem in polymer matrix composites. Thermal diffusivity is defined as... 

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. 

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. 

Thermal diffusivity measurements of polymers were made for the first time by... 

Dissolution measurements under the microscope, therefore, are a powerful tool for selecting solvents or plasticizers. Measurements of elastic range extension give insight into structural changes caused by plasticizers. Thermal diffusivity and heat conductivity measurements are recommended to detect side group and other transitions in polymers and other substances. 

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