Microfiber and graphene reinforced polymer matrix composites and methods of preparation thereof

A method for forming a microfiber and graphene reinforced polymer matrix composite, comprising:

• distributing graphite microparticles into a first molten thermoplastic polymer phase comprising at least one thermoplastic polymer;
• exfoliating the graphite microparticles in the first molten thermoplastic polymer phase by applying a first succession of shear strain events to the first molten thermoplastic polymer phase so that the first molten thermoplastic polymer phase at least partially exfoliates the graphite microparticles into single- and multi-layer graphene nanoparticles to obtain a graphene-reinforced polymer matrix composite, wherein the shear strain event is equal to or greater than the Interlayer Shear Strength (ISS) of the graphite microparticles;
• and
• distributing microfibers and a second thermoplastic polymer to the graphene-reinforced polymer matrix composite while continuing to apply a second succession of the shear strain events until graphene fractures of the exfoliated single- and/or multi-layer graphene nanoparticles are formed across the basal plane defined by the a-axis and b-axis of the exfoliated particles, wherein the edges of the graphene fractures comprise reactive free radical graphenic carbon bonding sites that react with the one or more molten thermoplastic polymers to provide a composite where thermoplastic polymer chains are directly covalently bonded to, and inter-molecularly cross-linked by, the single- and/or multi-layer graphene nanoparticles.