Liquid crystalline polymers TLCPs

Kim et al. (35) studied the thermotropic liquid crystalline polymer (TLCP) nanocomposites with varying extents of nanotubes. The mechanical performance of the nanocomposites has been demonstrated... 

The incorporation of nanotubes into thermotropic liquid crystalline polymer (TLCP) was reported to enhance the thermal decomposition temperatures and the residual yields of the nanocomposites (35). It was reported that the nanotubes act as protective barriers in the nanocomposites against thermal decomposition and are likely to retard the thermal decomposition of the TLCP nanocomposites owing to building of barrier to hinder the transport of volatile decomposed products out of the nanocomposites. 

Structurally, most commercial thermotropic liquid crystalline polymers (TLCPs) consist of rigid mesogenic monomer units connected with either flexible spacers or kink structures ... 

Gopakumar et al. [10] reported the in situ compatibilization of poly(phenylene sulfide) (PPS)/wholly aromatic thermotropic liquid crystalline polymer (TLCP) Vectra A950 blends by reactive extrusion. The authors prepared the in situ compatibilized PPS/TLCP blends in a twin-screw extruder by reactive blending of PPS and TLCP in the presence of dicarboxyl-terminated poly(phenylene sulfide) (DCTPPS). Block copolymer was formed during reactive blending, by transesterification reaction between carboxyl... 

THE THERMAL STABILITY AND DEGRADATION BEHAVIOR OF THERMOTROPIC LIQUID CRYSTALLINE POLYMERS (TLCPs)... 

Vectra A is a commercially available polymer from Hoechst-Celanese. It is a random co-polyester of hydroxy benzoic acid (HBA) and hydroxy napthoic acid (HNA), which is a well-known class of thermotropic liquid crystalline polymer (TLCP) [92-95]. Crystallization of molecules of TLCPs is considerably different from that of polymers like polyethylene or polyethylene terphthalate [1], TLCPs have reduced flexibility compared to the latter, which implies that large translations of their molecules are required for recrystallization [96-99],... 

Three commercially available thermotropic liquid crystalline polymers (TLCPs) were presented as examples in this section. They are Hoechst Celanese Vectra A950 and Vectra B950 as well as Amoco Xydar . Vectra A950 is a random copolymer of 73 mol% 4-hydroxybenzoic acid and 27 mol% 6-hydroxy-2-naphthoic acid, and Vectra B950 is a random copolyesteramide consisting of 60 mol% of 6-hydroxy-2-naphthoic acid, 20 mol% terephthalic acid, and 20 mol% p-aminophenol. Xydar is made from p-hydroxybenzoic acid, isophthalic and/or terephthalic acids, and 4,4 -biphenol. The repeating unit structures of the three LCPs are shown in Figure 6.2. 

The free volume of thermoplastic miscible blends has also been determined as a function of blend composition (Zhou et al. 2003 Campbell et al. 1997 Roland and Ngai 1991). Those studies have shown that the degree of blend miscibility alters the free volume behavior as a function of blend composition. On the other hand, Hsieh et al. (2000) have studied a number of blends containing only thermotropic liquid crystalline polymers TLCP s as the only components. That work showed that regardless of their various miscibilities, TLCP blends tend to display smaller, fewer free volume sites than expected from a weighted average. This observation has been ascribed to the intrinsic affinity of nematic TLCP s. 

The processing of an immiscible or incompatible polymer pair in which the dispersed phase forms in situ reinforcing fibrils can result in an appropriate morphology and improvement in the mechanical properties. This can be achieved by blending thermotropic liquid crystalline polymers (TLCP) with thermoplastics. These blends are often referred to as in situ composites because in the flow fields that occur during polymer processing operations, e.g., in the advancing melt front... 

The shear rheological properties of thermotropic liquid crystalline polymers (TLCPs) have been studied extensively whereas, the extensional behaviour of TLCPs has become the subject of very few studies (Gotsis and Odriozola 2000). The unusual orientation and structure of thermotropic liquid crystal polymers (TLCPs) can significantly affect the rheological and mechanical properties of... 

Thermotropic liquid crystalline polymers (TLCPs) show its liquid crystallinity in melt phase (Brehmer and de Jeu 2012 Shibaev et al. 1984 Popa-Nita et al. 2009). Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the ordering of the LC phase, pushing the material into a cmiventional isotropic liquid phase. At too low a temperature, most LC phases will form a conventional anisotropic crystal. Many thermotropic LCs shows a variety of phases as temperature is altered (National Materials Advisory Board 1990 Popa-Nita et al. 2009 Davidson 1999). The melt temperature and the thermal history stored within the polymer system plays a vital role in determining the liquid crystallinity of TLCPs. 

Thermotropic liquid crystalline polymers (TLCPs) have gained increased commercial attention because of their unique properties. These include their low coefficients of thermal expansion, low viscosity, and high modulus, low permeability to gases, low dielectric constants, and chemical resistance. As the demand for these characteristics increases, it is anticipated that the use of TLCPs will grow, rising at a projected annual growth rate of 25 % from an estimated use of ten million pounds in recent years. In expanding the potential uses for TLCPs, it has been found that TLCP/TLCP blends can possess characteristics which are better than those of either individual TLCP (Utracki and Favis 1989). But the better result is only possible if the LCP fibrillation is prominent in the blend system. 

Recent works in liquid crystalline polymer science, also emphasises more on the use of Thermotropic Liquid Crystalline Polymers (TLCPs) for development of nanocomposites using different nanofillers (Cheng et al. 2012). A discussion has already been made on this in the foregoing discussions. Carbon based nanofillers are more promising in this regards. 

Also, considerable efforts have been spent on the synthesis of thermotropic liquid-crystalline polymers (TLCPs), which spontaneously form ordered structures over a certain range of temperatures. Such polymers are very attractive to industry from the processing point of view because the problem of solvent recovery would not exist. 

Several reinforcement techniques have been introduced for the fabrication of composite fibres, such as (i) the introduction of thermotropic liquid crystalline polymers (TLCP) to produce a matrix-fibril stmcture, (ii) use of multiphase polymer blends and hard/soft segmented thermoplastics, and (iii) bicomponent extrusion, where different polymers are brought in contact as separate streams just before the spinnerette to produce a sheath-core structure (Salem, 2000). However, the inapplicability of these techniques to high-commodity commercial polymers and other serious drawbacks has limited the appeal. For instance, fabrication of TLCP is very expensive and postprocessing may destroy its unique matrix-fibril structure. Incomplete microphase separation in some polymer blends often leads to a less desirable morphology in multiphase fibres and bicomponent spirming is sensitive to differences in viscosity between the polymers. 

Multiwall carbon nanotube (MWCNT) reinforced thermotropic liquid crystalline polymer (TLCP) nanocomposites were prepared by a melt compounding process. Incorporation of small quantity of the MWCNT improved the thermal stability of MWCNT reinforced TLCP nanocomposites. The rheological behavior of TLCP/MWCNT nanocon sites was dq)endent on the MWCNT content. The complex viscosity and storage modulus of TLCP/MWCNT nanocomposites increased with increasing MWCNT, resulting fiom physical interactions such as the nanotube-polymer noatrix interactions and the nanotube-nanotube interactions. This increment effect was more significant at lower frequencies. 

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