Synergistic role of in-situ crosslinkable electrospun nanofiber/epoxy nanocomposite interlayers for superior laminated composites

Abstract

Adopting a multi-scaled/hierarchical toughening approach, we have produced nanofiber-reinforced epoxy laminate composites with superior toughness as a consequence of built-in, thermally catalyzed cross-linking between the nanofiber and the epoxy matrix, in addition to the usual curing within the epoxy itself. The nanofiber composition of P(St-co-GMA)/TBA-PA is designed such that the cross-linking agent PA groups are catalyzed by the thermally stimulated TBA initiators and inherent epoxy-nanofiber interfacial quality is promoted for toughening purposes. These nanofibers are electrospun onto two forms of the same base epoxy—neat resin films and pre-preg plies containing unidirectional carbon fibers. The nanofiber/epoxy nanocomposite specimens are manufactured via an in-house hot-press film molding method. DSC analysis reveal an increase in exothermic curing enthalpy, consistent with cross-linking between the epoxide groups of the fiber and epoxy matrix occurring in-situ, i.e., triggered and advanced during the epoxy curing cycle. Analysis of the curing kinetics, following Ozawa-Flynn-Wall method, shows that the P(St-co-GMA)/TBA-PA nanofibers have a significant autocatalytic effect on the epoxy matrix curing. Increases in tensile strength (30%) and elastic modulus (8%) are measured compared to the un-reinforced epoxy specimens. Furthermore, end-notched flexure tests reveal a 95% increase in GIIC, due to the incorporation of a single P(St-co-GMA)/TBA-PA nanofiber interlayer into laminated carbon fiber-reinforced composite of (0)48 lay-up configuration. These results suggest that the self-initiated cross-linking between the nanofibers and surrounding epoxy matrix synergistically forms interlayer zones that contribute to toughening. Analysis of the fracture surfaces is presented to elaborate on the significant role of the proposed in-situ cross-linked nanofibers on the remarkable improvements in mechanical behavior of these nanocomposites and interlayered laminates.