Anisotropy thermal properties

LCPs are handicapped by a high anisotropy of properties, shrinkage and thermal expansion low weld strength the cost, though justified by the performances and unusual design rules. 

Based on their thermal properties, polymeric and glassy molecular PR materials can be classified as high-7 and low-7), materials. The Tg values of polymer composites can be lowered by doping small molecules as plasticizer. Introduction of a long alkyl side chain can effectively reduce the Tg of fully functionalized polymers. The glassy molecular materials usually possess low Tg if they are amorphous. For the high-7 and low-7), materials, the figures of merit of the NLO chromophores are defined by Eqs. (35) and (40), respectively. The latter are obviously enhanced by contributions from the polarizability anisotropy of the NLO chromophore. 

To some extent, development of smectic materials has been slow and usually come as an off shoot of work on nematics. For example, the binary mixture shown in (38) exhibits a room temperature smectic A, a short range nematic, and is of positive dielectric anisotropy.Such properties may be used in thermally addressed displays where a transition from a scattering to a clear state forms the optical effect. This transition may also be effected by an electric field. Obviously, more work on useful smectics is required although their inherent high viscosity is a problem. 

Predicting fiber orientation. Isotropic constitutive models are not valid for injection-molded fiber-reinforced composites. Unless the embedded fibers are randomly oriented, they introduce anisotropy in the thermomechanical properties of the material. The fiber orientation distribution is induced by kinematics of the flow during filling and, to a lesser extent, packing. An extensive literature deals with flow-induced fiber orientation while much other work has been devoted to micromechanical models which estimate anisotropic elastic and thermal properties of the fiber-matrix system from the properties of the constituent fiber and matrix materials based on given microstructures. Comprehensive reviews of both research areas have been given in two recent books edited, respectively, by Advani and by Papathanasiou and Guell where many references can be foimd. 

Thermal properties of glass-carbon fibres depend on the anisotropy of the binder film properties that is caused by thermal stresses, whose value and direction depend on the temperature range and filler composition and which is of a variable nature. 

As can be seen from Fig. 11.2, many fibrils are observed in the cross section of tensile fractured as-spun fiber LCP. These fibrils are acting as if they are reinforcements, and therefore LCP is often referred to as self-reinforced polymer. However, this anisotropy is a big practical problem when LCPs are used in injection-molded parts because LCPs produce oriented moldings with anisotropic mechanical and thermal properties. Properties along the flow direction tend to be superior to those across the flow direction. Fillers represented as glass fiber are often added to LCPs to reduce the anisotropy because the addition of any filler disrupts the alignment of the LCP molecules. This is just the reverse of what is experienced with... 

A new area of development is to incorporate the filler permanently into the polymer matrix, by use of coupling reactions. This can increase impact strength and thermal properties of polyamides and modify the anisotropy of partially crystalline plastics, such as polyamides and polyesters. In polypropylene, bonding with kaolin can also improve scratch resistance, which is a useful benefit for automobile interior applications. Surface modification of fillers such as silica, mica, and wollastonite allows these to penetrate markets that were formerly the province of reinforcements such as carbon black and glass fibre. 

The mechanical properties of other semicrystalline polymers similarly depend on the conditions of deformation process (4,59,60). The thermal properties of semicrystalline polymers subjected to SSE have a highly pronounced anisotropy. Thus, the measurements of thermal conductivity of linear polyethylene done in a wide temperature range in directions parallel (If ) and normal (K ) to the direction of extrusion, have shown that If" > (1,61). With EDR increase, thelf"/lf ... 

The pecuiiar crystai structure of graphite resuits in a considerabie anisotropy, that is the properties of the materiai may vary considerably when measured along the ab directions (within the plane) or the c direction (perpendicular to the planes). Such anisotropy, especially in electrical and thermal properties, can often be put to good use as will be seen in later chapters. 

The thermal properties of conductivity and expansion are strongly influenced by the anisotropy of the graphite crystal. The thermal conductivity (fQ is the time rate of transfer of heat by conduction. In graphite, it occurs essentially by lattice vibration and is represented by the following relationship (Debye equation) ... 

Thermal properties of these two rock groups are controlled mainly by mineral composition and the influence of fractures. Alignment of mineral axes and fractures can create anisotropy this is characteristic for metamorphic rocks like gneisses (see Fig. 9.6). Table 9.4 shows a compilation of some data from the literature. 

The flow-induced stress is generally considered smaller than the thermal-induced stress. However, it is not possible to neglect the former due to the frozen-in orientation of polymer molecules affects the mechanical anisotropy, thermal and optical properties, and the long-term dimensional stability. 

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