Next-Generation Additives and Reinforcements for Polymeric Materials
Self-Reinforcing Composites
A major issue affecting the processability of polymers containing traditional rigid reinforcements is that the process viscosity dramatically increases by the introduction of the reinforcements. This results in a number of detrimental issues including poor dispersion, reinforcement fragmentation, and breakdown of molecular weight through mechanochemical degradation. The conceptual approach introduced herein involves introducing a low molecular weight crystallizable compound that, at process temperature, becomes miscible with the polymer or monomer thereby lowering the process viscosity. Upon cooling or reaction, phase separation and crystal nucleation and growth occur to provide reinforcement. A nice feature of this reinforcing methodology is that a myriad of scientific issues must be addressed from both the physics and mechanics perspective making it an ideal research topic for a Ph.D. student. Our group has demonstrated this concept to great success with thermoplastics and thermosets alike[1-4] and resulted in 4 patents[1, 5-7].
Soft-Particle Toughening of Polymers & Composites
Recently, additional research has focused on developing additives either phase separate, or self-assemble into rubbery domains engineered (particle size and inter-particle distance) for optimum energy dissipation during fracture[8, 9], and more recently engineering particle shape.
Molecular-Scale Reinforced Polymers
My research in this area focuses on understanding and developing molecular fortifiers (aka anti-plasticizers). These are molecules that upon cooling or reaction do not act as traditional plasticizers, but interact with the polymer and fill free volume of the polymer. This results in enhanced inter and intra-molecular interactions that increases the polymer modulus and yield stress. We have identified a class of phosphate-based molecules are ideal for amine-cured epoxies[10-15]. More recently, we investigated their effects on the aging effects of glasses[16].
Next-Generation Sustainable Process Technologies
My research in this area focuses on developing and/or exploiting new sustainable process technologies to fabricate materials with superior engineering properties. These range from new exploiting property changes that occur in supercritical and superheated fluids, to altering conventional process technologies to generate new materials, to formulating the energetics of resins to enable new fabrication technologies.
Use of Supercritical Carbon Dioxide as a Process and Reaction Medium
The use of supercritical CO2 as a transport media, reaction media, and process has been a mainstay of my research for more than two decades[17-54]. Herein, we have shown that scCO2 can enhance the drawability of fibers, can be used to make unique anisotropic foams, can be used to fabricate fiber-reinforced and nanocomposites with enhanced properties, as well as cellulose-based composites. Over this review period, my research has continued in the fabrication of a variety of high-performance composites locally controlled anisotropic foams[55-61].
Use of Superheated H2O and Other Environmentally Benign Liquids
Over this review period, my research has expanded to include exploiting the properties of superheated water (shH2O) and other superheated liquids (e.g. ethanol) to enhance processing of a variety of polymers including polyamides[62-64], intractable polymers like polyethersulfone[65, 66], and poly(2,6-dimethyl-1,4-phenynele oxide) [67-69]. Continued research has focused on foaming fibers and films using these liquids (see Fig. 2).
Melt-Mastication of Semi-Crystalline Polymers and Composites
We also introduced a new method to process semi-crystalline polymers to achieve significantly enhanced physical and mechanical properties. The method involves, first mixing the polymer above the melting point, followed by cooling to a temperature between the melting and crystalline temperature. During this period, it was shown that flow-induced crystallization occurs but unlike that in extensional flow fields, a granular morphology results. This leads to materials with exceptionally high isotropic properties (approaching that of glass reinforced materials)[70, 71] as well and improved dispersions or nano-reinforcements[72, 73].
Frontally Polymerizable Adhesives and Composites
We utilized concepts of frontal polymerization, to develop a cationically induced frontally polymerized gel that can be initiated with either thermal or UV radiation[74, 75]. This is an exciting technology that enables the fabrication of high Tg adhesives, coatings, and composites without the need for an external heat source for polymerization/curing of the part. Only initiation is required.
Additive Manufacturing Resin Development for Enhanced Mechanical Isotropy
One of our more recent research initiatives has focused on developing new resins that can be used in additive manufacturing. Regardless of the 3D print technology, one key limitation is the lower strength and ductility realized in one orthogonal direction resulting from layered interfaces (strength anisotropy). Ongoing research is addressing these using different strategies in both SLS and SLA formulations.