Title

Thermoplastic matrix composites via in situ electropolymerization

Date of Completion

January 1990

Abstract

A model electrochemical process for synthesizing thick thermoplastic matrix coatings of poly(maleimide(and N-substituted maleimides)-co-styrene) onto conductive graphite fiber reinforcements has been developed. This represents a new route to pre-impregnating graphite fibers with high performance thermoplastic matrices.^ New and innovative methods of processing thermoplastic composites are needed to overcome the problem of inadequate wetting of individual graphite fiber strands in a fiber bundle of 3K or more filaments. Thermoplastic resins possess very high melt viscosity and cannot diffuse uniformly onto the fiber surfaces. The fibers are invariably poorly wetted resulting in poor adhesion between the reinforcement and the matrix. Thermoplastic matrices are also highly solvent resistant and can only dissolve in expensive high boiling solvents. The problems of solvent removal and the inability to wet individual fibers uniformly and the concomitant poor interfacial properties of the resulting graphite fiber-thermoplastic matrix composites underscores the need to invent new processing techniques. In-situ electrocopolymerization of thermoplastic matrices onto graphite fiber reinforcements enjoys the numerous advantages derived from electrochemical polymerization, such as excellent wettability of the fibers, very good control of coating thickness and matrix properties. Electropolymerization is very cost effective and easy to control. By adopting the electrochemical technique to apply advanced thermoplastic matrices directly onto conductive graphite fiber reinforcements, we overcome the major processing problems associated with the traditional hot melt and solution coating processes.^ Electrochemical reduction of maleimides and N-substituted maleimides have only resulted in the formation of monomeric succinimide (MW $\sim$ 200), because of the hydrogenation of the ethylenic bond which terminates chain growth. In the present study electron deficient maleimides were electrochemically copolymerized with electron rich styrene comonomer. The resulting thermoplastic matrices were ideal for composite technology because of their intrinsic high thermal stability, TD $\sim$ 450$\sp\circ$C, and glass transition temperature $\sim$220-270 $\sp\circ$C. The copolymer matrices also are easily and reversibly processable.^ Aqueous electrocopolymerization of maleimides and styrene ensured a radical chain copolymerization and resulted in high molecular weight resins, Mn $\sim$ 65,000. High molecular weight resins are desirable for composite processing because of the associated improved properties.^ Electrosynthesized prepregs composed of 35-65 $\sp{\rm w}$/o of resin were compression molded into composite panels. These composites showed excellent impact strength $\sim$220 KJ/m$\sp2$, very good flexural strength $\sim$950-1700 MPa and moderate shear strength $\sim$60 MPa.^ In-situ interaction of the thermoplastic matrices with calculated amount of crosslinkable bismaleimide and a multifunctional system such as hydroxyethyl methacrylate (HEMA) respectively, have been attempted. It is recognized that a novel new process and high performance resin systems has been invented but aggressive development of the technology is imperative for commercial exploitation. ^