John G. Williams

Biography

Modeling of Thermoset Cure
During cure of thermoset resins, the reaction mixture undergoes a phase change from liquid to solid. This transition is commonly called gelation, and the gel point is defined as the time at which an infinite molecular network is established. Results have shown that the phase change during cure of epoxy resins is rarely associated with gelation but is usually a complex process. Solidification through the formation of a low-molecular-weight glass has also been observed (vitrification). Our research addresses the study of the phase-change mechanism and the properties of the resultant thermoset solid.

Rheology of Curing Resins
During cure of fiber-reinforced plastics based on thermoset resins, heat and pressure are applied to a fiber bed saturated with partially cured resin. During consolidation, excess resin is removed, and the resin cure is completed. Research on this process is complicated by the variation in resin rheology with cure, by the change in the bed surface area and volume with the degree of consolidation, and by the non-uniform heat transfer and temperature in the system. Our aim is to model the consolidation process and to develop closed-loop control of composite processing.

Molecular Basis for Deformation and Fracture in Polymers
The long-term goal of this project is to develop stiffer and stronger materials through control of molecular interactions and entanglements. When a polymeric material reacts to the application of a mechanical load, the molecules are displaced or deformed. This is believed to occur rather unevenly as polymeric molecules are frequently entangled, which restricts molecular to stress. Many polymers are composed of two or more phases, which respond in different ways to the load. The phases might be amorphous and crystalline, matrix and filler, or a blend of two incompatible polymers. How the phases react to application of a load controls their mechanical behavior. This project uses high-resolution atomic force microscopy to map surface displacements in polymeric systems under load. One study has addressed the interphase in composites, which is the region around the fiber where the mechanical properties of the matrix are affected by the presence of the fiber. Current work suggests that this region may extend into the matrix for about one-tenth of the fiber radius. Knowledge of the properties of this modified material is necessary for prediction of the mechanical properties of the composite.