Improving the fracture toughness of vinyl ester composites using electrospun interlaminar layers

Vinyl ester resins are commonly used as a matrix material for low-cost composites, however, they exhibit low fracture resistance when compared to high performance thermosetting resins. The objective of this work is to increase the fracture toughness of vinyl ester composites by adding an interlaminar toughening layer. This layer is comprised of polysulfone in the form of a randomly oriented nanofibrous mat produced by electrospinning. In previous work, we observed void formation in the interlayer around the individual fibers. Void formation is attributed to the swelling of the fibers with styrene prior to cure and subsequent removal of styrene from the fibers during cure. The formation of voids in the interlayer decreases the modulus and interlaminar strength of the composite. The work presented in this paper focuses on changing the wetting nature of the fibers through radiation grafting of silanol monomers on the fiber surface. By forming crosslinked polysiloxane around the polysulfone fibers, the driving force for the swelling of the fibers with styrene is greatly reduced. Additionally, the polysiloxane is reactive with the vinyl ester matrix and gives connectivity between the two phases. We utilized radiation grafting with oxygen plasma to attach silanol monomers to the surface of the fibers. We showed that plasma treatment of polysulfone electrospun fibers incorporates oxygen containing groups onto the surface of the fibers. These groups act as grafting sites for the silanol. Additionally, we determined that oxygen plasma breaks down the polysulfone structure and reduces the molecular weight. After silanol grafting, we dried the fibers at elevated temperatures (125-150°) which results in the crosslinking of the grafted monomers. Thermogravimetric analysis showed that we grafted approximately 13.4 wt. % of crosslinked polysiloxane to the fibers. The polysiloxane-grafted fibers were successful in reducing void formation between the two phases, which greatly improved the interlaminar strength retention.