Liquid Crystalline Polymer Blends and Molecular Composites

Liquid Crystalline Polymer Blends and Molecular Composites 159... 

G. T. Pawikowski et al., Molecular composites and self reinforced liquid crystalline polymer blends. Annual Rev. Mat. Set, 21 159-184 (1991). 

The objective of this work has been to generate films, tapes or ribbons which might serve as a prepreg from blends of either an Ultem or a PEEK or a high molecular weight PPS with various liquid crystalline polymers, to identify the parameters that control the formation of reinforcing microfibrils of LCP phase, and to study the mechanical properties of the composite films. 

Specific blends, which could offer an interesting combination of properties with proper com-patibilization, include PPS/PSE, PEl/PPS, PA/PSE, PA/PEI, and PC/PPS. Patent activity has been noted for most of these blend combinations as well as other selected blends involving engineering polymers as noted in Table 17.3. A number of recent patent and published papers have discussed blends of engineering polymers with various specialty polymers including high temperature polymers, liquid crystalline polymers (LCP s), conductive polymers, and as matrix materials for molecular composites. These will be discussed in the following sections. 

One of the basic problems confronting molecular composites is the difficulty of finding miscible combinations of rigid rod polymers with flexible chain polymers. Poly(p-phenylene benzobisthiazole)/poly(-2,5(6)-benzimidazole block copolymers have been reported by Tsai et al. [1985] and are noted to exhibit better processability and mechanical properties than the simple blends of the block copolymer constituents. Chang and Lee [1993] prepared poly(p-benzamide)/Pl block copolymers and reported on the liquid crystalline behavior. Such approaches would appear to have future implications. As an example PA e.g., PA-66) block copolymers with rigid rod polyamides could be prepared and used in blends with PA-66 to yield the desired molecular composite. 

All the PCL/PC blends with 20-80 wt % PCL were crystalline and cloudy at room temperature. The blends became optically clear when heated (to about 60 °C) and the PCL melted. They then became cloudy again at temperatures of about 260 °C with the onset of liquid-liquid phase separation occurring as a result of LCST behaviour. For PCL-700 (Table 1) the cloud-point curve was fairly flat over much of the composition range but rose slightly (about 5 °C) in blends containing 80 wt % PCL [21]. In comparable experiments with PCL-300 (Table 1) the cloud point curve varied more strongly with composition and was raised with a critical point at about 272 °C. These observations were consistent with the normal enhanced miscibility of lower-molecular-weight polymers. 

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