Mechanical anisotropy of polymer composites

Mechanical Anisotropy of Polymer Composites 9.2.1 Mechanical Anisotropy of Lamellar Structures... 

The mechanical piopeities of stmctuial foams and thek variation with polymer composition and density has been reviewed (103). The variation of stmctural foam mechanical properties with density as a function of polymer properties is extracted from stress—strain curves and, owkig to possible anisotropy of the foam, must be considered apparent data. These relations can provide valuable guidance toward arriving at an optimum stmctural foam, however. 

Despite its two-dimensional nature, theTakayanagi model is successful in explaining the main features of low strain mechanical anisotropy in oriented annealed crystalline polymers. Perhaps even more important it points the way to more sophisticated composite structure models which are at present being developed for crystalline polymers. 

Abstract In fiber reinforced plastics (FRP), as a special type of polymer matrix composite, fibers provide the stiffness and strength while the surrounding plastic matrix transfers the stress between fibers and protects them. In this chapter, the role of fibers in FRP is delineated, their types and properties are discussed and the fabric forms in which they can be formed and nsed to reinforce FRP are presented. A special focus is given to the effect of the chemical structure of fibers on the stability and the level of anisotropy of their mechanical response. Furthermore, the effect of assembling these fibers into yams and fabrics on the response of the FRP is presented as basis for further readings. 

Broadline nmr, relating melt order with polymer composition and temperature, also provided evidence supporting the thermotropic nature of these materials. The physical properties of molded parts of certain compositions showed a definite mechanical anisotropy. Strength and modulus values in the machine direction were considerably higher than those values transverse to the machine direction, that is, perpendicular to the direction of flow. Again, facile and high molecular orientation was noted for these polymers in the flow direction and this became more pronounced as part thickness was reduced. Further, the melt viscosities of the X7G copolyesters, at about equivalent molecular weights, were... 

Conformation and Chemical Shifts.—Chemical shifts have been correlated with conformation for H shifts in polystyrene and poly(vinyl chloride) and C shifts in model compounds of polypropylene. In all these papers, the chemical shift is related empirically to the occurrence of three- and four-bond steric interactions, similar to those used in the rotational isomeric state treatment of polymer statistical mechanics, " and the shift is expressed as a sum of compositional and conformational increments. The origin of the shielding contributions (magnetic anisotropy, electric field effects, etc.) is not stated, except for polystyrene in which the magnetic anistropy of the aromatic rings are incorporated... 

Liquid crystalline polymer composites have become an increasingly important material for technological field involving photonic applications. Several attractive features such as dielectric anisotropy present in the mesophase, reasonably wide temperature range, low thermal expansion coefficient, high chemical resistance, specific thermo-mechanical response etc. make them more attractive. Of comse several limitations have to overcome in order to avail the total benefit of using such materials for opto-electronic applications. 

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