Sulfur-driven reactive processing of multiscale graphene/carbon fiber- polyether ether ketone (PEEK) composites with tailored crystallinity and enhanced mechanical performance

Abstract

The development of high-performance polyether ether ketone (PEEK) composites often encounters manufacturing challenges such as porosity, difficulty in wetting fibers, low crystallinity, and poor interfacial adhesion, stemming from PEEK's high melt viscosity and chemically stable structure. While numerous studies have aimed to enhance fiber-resin compatibility in PEEK using novel sizing agents made of thermally stable miscible thermoplastics, this study explores a promising alternative: reactive processing to chemically modify the PEEK matrix. Utilizing elemental sulfur to foster chain scission and crosslinking within the resin and to modify the surface chemistry of carbon fibers, our research investigates the effectiveness of sulfur as a simple additive in carbon fiber reinforced PEEK (CF-PEEK) composites to improve crystallinity and mechanical performance. Leveraging PEEK's high processing temperatures, the study explores in-situ chemical modification during the melt phase, incorporating sulfur via spray coating alongside graphene nanoplatelets (GNPs) functionalized with cellulose nanocrystals (CNCs). This approach evaluates sulfur's impact across different filler scales and establishes the reaction conditions necessary for chemical modifications of PEEK and carbon fibers. Our findings indicate that trace amounts of sulfur (0.05 wt% or less) increase the flexural strength from 780 to 800 MPa reaching ∼900 MPa without affecting interlaminar shear performance. The trace amount of sulfur can also improve the degree of crystallinity by 10% in multiscale CNC:GNP–CF–PEEK composites and diminish the effects of poor dispersion and agglomeration in added GNPs. Sulfur's ability to reduce melt viscosity through a chain scission mechanism synergizes with GNP's capacity to boost crystallization rates via improved surface nucleation.