Hydroperoxide-initiated grafting of poly(styrene-stat-acrylonitrile) onto ultra-high modulus polyethylene fibers

Abstract

There is evidence from modeling experiments that designed interphases, e.g. varying graft molecular weight and/or an adjustable interphase modulus, will provide a method of tailoring the mechanical response of a composite. There is no experimental evidence yet to substantiate these claims in fiber-reinforced composites. Hence, a grafting method that allows better design of the interphase, i.e. an intelligent interphase, is described. Our approach is to engineer intelligent interphases using a hydroperoxide-initiated grafting technique that provides a novel method of grafting by a free radical-type polymerization. This paper describes the hydroperoxidation and grafting of poly(styrene-stat-acrylonitrile) (SAN) onto ultra-high modulus polyethylene (UHMPE) fibers. Differential scanning calorimetry and fiber tensile tests were used to monitor the effect of the surface treatment on bulk crystallinity and fiber tensile properties. These characterization methods showed that the hydroperoxidation treatment was not deleterious to the fiber bulk properties. The nitrogen in poly(styrene-stat-acrylonitrile) was used as a marker for ESCA in order to verify grafting. ESCA analysis also demonstrated that the grafting reaction was a surface phenomenon, not a bulk treatment. Tapping-Mode™ atomic force microscopy (AFM) was used to image the surface morphology of the UHMPE fibers and images of the SAN-grafted fibers revealed a distinct topography comprised of nodules, 3-12 nm high and 10-25 nm apart. A simple 'hard-sphere' model is presented to explain the polymer graft conformation based on the surface morphology as imaged by AFM. The model corroborates our conclusions that the hydroperoxidation grafting yields linear chains tethered at the amorphous surface domains.