| Elastomer modification of epoxy resins has shown a great deal of success for increasing fracture toughness. There are inherent characteristics of vinyl esters (VEs), however, that make them less conducive for elastomer toughening. Therefore, the first objective of this work was to identify these characteristics.; The next objective of this work was to develop a toughening method that circumvents obstacles associated with rubber toughening. We proposed that by imbedding electrospun micro- and nano-fibers as toughening composite interlayers, we could avoid the dependence of the initial compatibility of the modifier and cure behavior of the VE.; As was found with rubber modified VEs, the diffusion of styrene into the electrospun fibers was observed, resulting in void formation around the fibers and reduced resin and composite properties. The final objective of this work involved designing the interface between the electrospun fibers and VE to mitigate this problem. We investigated the effects of a reactive and non-reactive "shell" to distinguish which mechanism, a cross linked or reactive interface, limits styrene diffusion. It was found that styrene was able to permeate the non-reactive cross linked polysiloxane. However, the reactive interface provided a chemical link between the two phases that did not allow for void formation.; The final part of this work entailed evaluating the effects on composite properties of surface modified PS electrospun fiber mats used as interlayers in carbon fiber composites. The interlayer toughened composites were compared to rubber toughened composites. The resins and composites containing fibers with surface treatments yielded the worst properties. The most significant losses included flexural and shear properties. The rubber toughened composite experiences a crack-tip blunting and increased stress concentration zones during delamination. The interlayer composite proved to increase the surface area of fracture during delamination by deflecting the crack in multiple directions. Also, the increased interlayer thickness allowed for fuller development of the stress concentration around crack tip. (Abstract shortened by UMI.)... |
