Optimising the interfacial response of glass fibre composites with a functional nanoscale plasma polymer coating

E-glass fibres have been coated with a functional plasma polymer (PP) of 5-15 nm thickness, to provide the composites with a controlled interphase. Untreated and unsized E-glass fibre bundles were continuously coated with acrylic acid/1,7-octadiene and allylamine /1,7-octadiene plasma copolymers of various compositions to optimize the bond with matrix resin. High fibre volume (Vf) composites were prepared for short beam shear (SBS) tests from which the interlaminar shear strength (ILSS) could be estimated. It was found that ILSS was high for the highest functional coating. However, with a coating of lower functionality, ILSS increased as the thickness of the coating decreased. This was especially noticeable for the allylamine plasma copolymer systems. Nanoindentation showed that the reduced modulus of the allylamine plasma polymer and hence crosslink density was higher than that of the acrylic acid plasma polymer. Thus a model has been proposed whereby the epoxy matrix resin diffuses into the plasma polymer during cure to form a semi-interpenetrating network (IPN) of differing thickness, which controls interfacial behaviour. When the diffusion length is less than or equal to the coating thickness, the properties of the IPN will dictate stress transfer and the mode of failure. For thicker coatings, the unmodified interlayer of plasma polymer will dictate the micromechanics. The interfacial failure of a composite is dominated by either the interface between the fibre and interphase or interphase and matrix or the interphase properties. Interlaminar shear strength data demonstrate that an interphase with high shear strength and of the thickness 2–5 nm is a crucial parameter. This model enables the single filament fragmentation and ILSS test data to be fully correlated with the chemistry of the interphase.