Although this term could be used to describe composites formed through the tailoring of interphases, especially through the use of plasma and etching treatments, in this section we refer to the in-situ formation of the reinforcement and matrix through in-situ polymerization. The initial work in this area dates to the early 1970s with the pioneering work of Takayanagi in Japan. The overriding motivation in the formation of such composites is control of microstructure, and hence performance, at the molecular level. These can have low viscosities for ease of mold filling, with improved thermal and moisture resistance and improved mechanical properties, and can take full advantage of the capabilities of composites to act as multi-functional elements. They have the potential for formation of through-the-thickness reinforcement using methods such as injection molding, RTM, and SRIM in the matrix phase. Table 6.5 gives some of the systems under development. A detailed review of work conducted in the U.S., primarily at Wright Patterson Air Force Base (WPAFB), is given in Wiff et al. (1988, 225).
Review of Important Aspects of Research in the Area of Molecular Composites
Although attractive, this is still at a very experimental stage in the U.S. but is actively being pursued in Japan. The entire topic of functionally gradient materials is an area of currently high activity at both industrial and government laboratories.
Figure 6.10 presents the basic premise for a functionally gradient material. RIPT, a MITI lab, is conducting research in this area with the participation of industrial scientists from both Japanese and international companies. Currently, the emphasis is on the development of materials with new functions through the control of structure at macroscopic and molecular levels such as through the use of:
Figure 6.10. Schematic of Functionally Gradient Composites
Scientists at AIST's laboratories are further investigating materials processing technology in this area based on perceived long-range needs. Two projects are specifically worthy of note. In the first, investigators are studying the control of minute structures and composition through the creation of crystals in a reaction field combined with plasma and high-speed ions for composites with high wear resistance on one surface, and good toughness on the other (Figure 6.11)
Figure 6.11. Inorganic Material Processing for Tailored Composite Surfaces
In the second, advanced reaction fields are being used to align spin in polymers and thus control blends at the monomer level through the use of photochemical processing (Figure 6.12). Although this work is currently directed at the polymer level, there is evidence that if it is successful, it will be quickly transitioned into the composites arena as well.
Figure 6.12. Polymer Preparation Using Photo-Chemical Means for Fine Alignment