The effect of molecular orientation on the mechanical properties of fiber filled amorphous polymers

The technology of using glass fibers to increase the modulus and the strength of polymeric systems is well developed. Much less widely exploited has been the enhancement of properties by orienting the molecules of the polymer itself. The present work deals with trying to combine the beneficial effects of these two mechanisms: using glass fiber filled polystyrene and introducing molecular orientation into the polystyrene matrix. For rheological reasons in the rubbery state it does not appear to be possible to introduce high levels of orientation, but even a small amount of orientation has a large effect on the subsequent mechanical properties. . The amount of molecular orientation that can be induced decreases with increasing fiber loading, apparently due to a shear thinning effect between the fibers. Thus there is a "trade-off" between the level of loading and the amount of molecular orientation that can be induced. Analysis of the mechanical properties was made in terms of various theoretical models (those of Brody and Ward, Goddard, and Smallwood-Guth for the elastic modulus; and that of Kelly and Tyson, as modified by Lees, for the tensile strength). These comparisions showed that the brittleness of unoriented polystyrene is such that the matrix does not effectively distribute the stress along the length of the fiber and thus the benefit of the length is not realized; whereas the higher ductility of oriented polystyrene allows such transfer and a consequent improvement of properties.