Polymers, liquid crystalline engineering properties

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. 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

Essentially, then, no new, large-volume, highly profitable fibers have been developed since the mid-1950s. Instead, the existing ones have become commodities with all the economic impact thereby implied. No major chemical engineering processes have been added, although the previously described ones have been modified to allow for spinning of liquid crystalline polymers or the formation of gel spun fibers. Research activity has been reduced and centered essentially on modifications of fiber size, shape, and properties, and many variants now are successfully marketed. Production volumes have increased enormously for nylon, polyester, and polyolefin. 

Liquid crystal polymers (LCPs) have been a source of considerable interest for some time, as they have been shown to offer particular advantages in terms of their processability and physical properties which make them attractive in a wide range of engineering applications.346 Serrano and his colleagues have reviewed metallomesogenic polymers, including the liquid crystalline properties of several of the platinum poly-yndiyl polymers described above.85,86... 

Zhou and Lenz (1983) and Zhou et al. (1985) discussed the substitution effect from the point of view of steric and polar effects. However, from above discussions one sees a more intricate picture. A full understanding of the effect demands further study. Nevertheless, substitution is so effective that a lot of studies have used this concept in the molecular engineering of liquid crystalline polymers in order to have desired phase properties. 

WITH the easy-processing properties in the liquid crystalline phase, main-chain TLCPs have been widely used as high-strength fiber, fiber reinforcement, in situ reinforcement additive, and injection molded articles, etc, [ 1 -4]. The successful applications are quite dependent on the adhesion at interface of the liquid crystalline polymer and the conventional engineering resin, which is indeed affected by surface tension and/or interfacial tension between the two phases [1-2],... 

The main reason for the current and future ever-growing interest in liquid crystalline polymers lies in their unusual properties [89], The effective alignment of molecular backbones in LCP is claimed to produce properties even superior to engineering thermoplastics. The long-range orientational ordering of the liquid crystalline polymers leads to anisotropic mechanical, optical, magnetic, and electrical properties. 

From a survey of the recent development in the field of self reinforcing polymers it appears clearly that the unique properties of the LCP make them primary candidates as reinforcing agent, added to engineering thermoplastic in the form of second phase. In the design of the blend several parameters have to be considered, but first let consider in close details the major features of liquid-crystalline compounds. 

In spite of the promising performances of the liquid-crystalline polymers, their use is limited to few practical application. This is due to the high pronounced anisotropy of orientation and properties of LCP processed in the pure state. The apparent contraddiction can be avercome by blending the mesomorphous polymer with ordinary engineering thermoplastics. 

Description of the mechanical properties of polymer composites are also made by considering the properties of particulate-long fiber and laminate composites through the different models generated in the literature. It is shown that, many advantages can be derived by use of liquid crystalline compounds as reinforcing fillers to produce blends with engineering thermoplastics. 

Development of new liquid crystalline (LC) polymeric materials has been a subject of intense interest because of the combination of unusual optical, electrical, and magnetic properties of low-molecular-weight liquid crystals and the mechanical performance and processibility of polymers. Application areas of LC polymers are very diverse, from engineering plastics to LC displays and erasable compact disks. However, the development of conducting and liquid crystalline polymers went on separately in the past in spite of the similarity of the molecular structures of typical main-chain liquid crystals and some conductive polymers. 

Polyarylate polymers are aromatic polyesters derived from aromatic dicarboxylic acids and diphenols. In contrast to liquid crystalline aromatic polyester (derived from dicarboxylic hydroxy acids), polyarylates exhibit amorphous character on molding Tg is ca 198°C. Polyarylates present a competitive cost(performance profile in the context of amorphous engineering thermoplastics, delivering an excellent balance of mechanical properties, particularly practical heat resistance which substantially exceeds that of polycarbonates (qv). 

---