Strength and toughness of crystalline polymer systems

This review is devoted to the advances in the modification of the strength and toughness of semi-crystalline polymers. Past efforts to obtain ultra-strength polymeric fibers are discussed, including the role of entanglements. The role of crystal plasticity in achieving high toughness is addressed. Two possibilities of the modification of crystal plasticity are described: increase in the crystal thickness and a reduction in the number of mobile dislocations in polymer crystals. Cavitation during deformation arising from mechanical mismatch between differently oriented stacks of lamellae contributes to toughness by activation of other mechanisms of plastic deformation of a material and to the plastic response of a polymer. It is shown that internal cavitation, although augmenting the toughness, greatly reduces the strength of the material. Two examples of efficient toughening in multicomponent polypropylene systems connected with cavitation are described, and attempts to produce highly oriented materials with thickness larger than fibers are reported. Modes of deformation for which a compressive component of stress is present suppress internal cavitation of crystalline polymers, and reveal the decisive role of crystal plasticity in polymer deformation. Cavity-free deformation by rolling in a channel on a circumference of another roll is a new method for the continuous production of highly oriented polymer bars, combining advantageous tensile and impact properties of bars with their large cross-section. Another example of a cavity-free deformation is the drawing of initially amorphous poly(ethylene terephtalate) associated with oriented crystallization. Such a material can be subjected to another drawing, which also proceeds in a cavity-free manner. The resulting strongly oriented material exhibits a very high tensile strength. The role of the amorphous phase in the plastic deformation of crystalline polymers has been analysed. In a cavity-free deformation producing sharp crystalline texture, the amorphous phase shows a 2D pseudo-hexagonal packing in register over the whole deformed material. The orientation of this 2D hexagonal packing is connected with the orientation of crystals following from crystallographic slips and imminent crystal rotation. (C) 2003 Elsevier Ltd. All rights reserved.