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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 46 Composites in the Ti-Si-C and Ti-Ge-C Systems Nickl et al. (1972) have investigated composite formation in the Ti-Si-C and Ti-Ge-C systems as a function of reactant concentration and temperature. They have shown that virtually all of the dozen or more two-and three- phase assemblages possible can be prepared by CVD. Some of the composites prepared were SiC + TiC, SiC + TiC + C, and Ti5Si3Cx + TiSi2. During extensive microstructural characterization it was observed that either of the ternary phases Ti3SiC2 and Ti5Si 3Cx tend to deposit as lamellae when codeposited with binary phases such as TiC or TiSi2. The boundaries between ternary and binary phases were coherent or semi-coherent. The tendency for ternary phases to form alternating layers with binary phases was also observed in the Ti-Ge-C system. MACROMOLECULAR COMPOSITE STRUCTURES A molecular composite is a polymeric material consisting of two or more components dispersed at the molecular level. Examples include compatible polymer blends that are noncrystalline, thermodynamically stable single-phase materials, and blends of components that are processed into a homogeneous state but are not at thermodynamic equilibrium. The latter blends are polymer analogs of metastable multiphase alloys. An example of the synergystic effects that result is General Electric's Noryl--a blend of poly(styrene) and poly(phenylene) oxide that has excellent mechanical properties, especially toughness. The field of polymer blends is extremely active, with interest centered on synthetic routes to produce âmiscibility windowsâ wherein the blend can be produced and processed. The material is then quenched to room temperature, where it maintains its single-phase character because of the lack of molecular mobility below the glass transition. The possibility of tailoring the macroscopic properties of macromolecular systems by varying the nanoscale morphology of the bulk polymer is being shown by research on blends of high-performance systems (Karasz, 1986; DeMuese et al., 1988; Guerra et al., 1988). Polymer blends may or may not be miscible (i.e., forming a true thermodynamic solution). Miscibility is a relatively uncommon phenomenon in homopolymer-homopolymer systems because of the absence of a significant configurational entropy of mixing. In contrast, blends containing random or near-random copolymers are much more likely to form miscible systems, because of the possibility of a net lowering of the overall free energy caused by the presence of intramolecular repulsive interactions in the copolymer. The copolymer route allows three morphological types to be processed into the polymer alloy (Figure 12): a homogeneous single phase between the phase boundary and the glass transition temperature (Tg); two-phase nucleation morphologies; and two-phase spinodal decomposition. By
SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 47 changing the copolymer composition only slightly, the Tg will show minimal change, but the lower critical solution temperature (LCST) shows considerable change, allowing the selection of morphology. Figure 12 Schematic phase diagram for polymer blends (Karasz, 1986). While compatible polymer alloys are of interest, the focus here is on the area of self-reinforcing alloys, the so-called molecular composites. The term molecular composite is adapted from the field of macro composites such as fiberglass-epoxy materials, where one component is a rigid reinforcement for a ductile matrix of the other. If one could disperse individual rod-like macromolecules in a flexible-coil matrix, then the intrinsically large length-to-diameter ratio (L/D) inherent to such rigid macromolecules (easily in excess of 100 for reasonable molecular weights) coupled with the strong nonbonded interactions of the reinforcing molecules to the matrix molecules would provide essentially an ideal reinforced composite. It should be possible to use various processing flow fields to tailor the orientation of the rod-like molecules and produce a high degree of mechanical anisotropy. In practice, however, because these systems consist of two rather dissimilar components, they are highly likely to phase-separate, forcing a nonequilibrium approach to their production. The first attempts to produce such materials originated at the Materials Laboratories at Wright-Patterson Air Force Base (Helminiak et al., 1978) but failed to achieve a single-phase material because slow evaporation of the solvent allowed macro-phase separation to occur (Husman et al., 1980). Takayanagi (1983) employed a wet- spinning process to produce an oriented fiber of nylon 6 or nylon 6, 6 and poly(p-phenylene terephthalamide) (PPTA). Although solid formation during coagulation of the wet-spun fiber was considerably faster than the solvent evaporation method, Takayanagi found that phase separation did indeed occur at a fine scale, producing oriented 30-nm microfibrils of the rigid-rod macro
SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 48 molecules in the semicrystalline flexible chain nylon matrix. Lately, researchers have sought various ways to overcome the tendency to phase-separate. The approach is to employ dilute solutions at a concentration below the critical concentration for phase separation so that the rod, coil, and solvent molecules form a homogeneous isotropic phase. This solution is then wet-spun into an oriented fiber and rapidly coagulated to capture the mixture in a homogeneous solid state. As applied to the poly(p-phenylene benzobisthiazole) PBZT/poly-2, 5(6) benzimidazole (ABPBI) system, such processing results in a solid with no detectable phase separation above the 3-nm level. The 30/70 PBZT/ABPBI fibers thus produced have outstanding mechanical properties (modulus 120 GPa, strength 1300 MPa, strain to break 1.4 percent) (Krause et al., 1986). Another standard approach to control molecular dispersion of different macromolecular components is, of course, to utilize block copolymers. Triblock ABPBI/PBZT/ABPBI materials have recently been synthesized containing the same 30 percent PBZT to 70 percent ABPBI composition (Tsai et al., 1985). The advantage here is that the built-in chemical connections between the dissimilar segments significantly raise the critical concentration at which phase separation takes place; moreover, the scale of this phase separation is dictated to be on the order of the respective block radii of gyration, thus preventing macroscale segregation and loss of physical properties. The mechanical properties of the considerably more easily processed PBZT/ABPBI block copolymer fiber are comparable to those of the physical blend. As indicated previously in the section entitled Reductive Pyrolysis, properties of biphasic materials are particularly synergistic for bicontinuous microstructures. It has been demonstrated theoretically that a bicontinuous structure has an intrinsic thermodynamic stability at the 73:27 volume percent ratio of the constituent phases (Thomas et al., 1988). Such a structure has been observed in block copolymers (Figure 13), but so far not in metal-ceramic composites. A rationalization for the morphologies of two-phase structures has been given by Newnham et al. (1978). Four types of microdomain morphologies have been identified for biphasic block copolymers (figure 14). The triply periodic bicontinuous morphology has the special virtue of generating three- dimensional, near-isotropic reinforcement. Other properties will be influenced as well by such structurally symmetrical composites. With further research into synthetic efforts and exploration of novel processing methods, truly outstanding high-performance materials should certainly be realized.
