TLCPs polymers

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

Strong elongational deformation and use of matrix polymers whose viscosity is higher than that of TLCP phase are better to ensure uniform and fine fibril formation. But application of compatibilizing techniques to in situ composite preparation can be useful to get the most desirable products. These can reduce the high costs of the liquid crystalline polymers and expensive special engineering plastics used for the in situ composite preparation and reduce the processing cost, whereas they can increase the performance of produced in situ composites, hence, their applications, too. 

This chapter focuses on recent developments in rheological and mechanical examinations of TLCP blends. Although most of the data presented are taken from actual works done at the Institute of Polymers and Processing of Zhejiang University, Hangzhou, China, this is merely a matter of accessibility and in no way implies that this data is the best or only data of this nature available. 

One of the most attractive features of TLCPs is their ability to alter the rheology of bulk thermoplastic polymers. Most reports in the academic literature are concerned with viscosity reduction. For example, Siegmann et al. [1] observed a steep viscosity drop when a TLCP... 

Nobile et al. [3] reported that viscosity of a polycar-bonate-TLCP blend can increase or decrease in the same system at the same temperature, depending on the shear condition. At very low shear rates the viscosity was found to increase with TLCP loading, whereas at high shear rates a significant drop was observed. But in all of these cases, the way in which the TLCPs alter the bulk polymer flow is not yet well understood. 

The polymers studied and the experimental procedure were described in the last section. The samples were made of the same PC-TLCP composite and the pure TLCP Vectra A950, respectively. To study the influence of sample forms and geometries on mechanical constants, plate samples (80 x 80 x 2 mm-) were also injection molded with a film gate. Tensile samples were then cut from the plate in the longitudinal (//) and transverse... 

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 ... 

For commercial TLCPs having their mesogenic moieties contained within the main chain of the polymer, this structure is often nematic. Nematic order in the liquid crystalline phase is shown schematically in Figure 3. The nematic phase is the LCP phase of lowest order, consisting of aggregates of rod-like molecules. Within each aggregate, or domain, the molecules have a distribution about a common axis, denoted by n.. No order exists, however. 

The physical mixing of two or more polymers to crate a material with properties different from each of the components has become an increasingly popular route to new materials development. The resulting blend or alloy greatly reduces the associated time and costs while permitting improved processibility and enhanced properties tailored to specific application areas. Many commercial examples of two-phase polyblends consist of a matrix polymer impact modified by the addition of rubber particles. Recently, however, TLCPs have received increasing attention in the scientific and technical literature as in situ reinforcements in polymer blends and microcomposites. The matrices examined in the literature include polyimides, PES, PEI, PEEK, polycarbonate, PET, PPS, and polyarylate. 

L.-C. Wu, P. Chen, J. Chen, J. Zhang, and J.-S. He. Noticeable viscosity reduction of polycarbonate melts caused jointly by nano-silica filling and TLCP fibrillation. Polym. Eng. Sci., 47 757-764,2007. 

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... 

Thus,the relationship between the modulus of as-spun fibers and the modulus of injection molded test pieces is not clear,but the flexural modulus is depending upon the BB mol% of polyarylates and the BB mol% might be used as the parameter of the rigidity of polymer chains.We observed many plate-like fibrils and a few needle like fibrils on the cross section of the flexural fractured test pieces of Me-HQ/BB/Cl-PEC(m/n=9/l).Then we prepared TLCPs with more rigid chemical structure,Me-HQ/BB/Cl-PEC(m/n=9.5/0.5),which showed a similar flexural modulus of 39 GPa as Me-HQ/BB/Cl-PEC(m/n 9/l),but X-ray diffraction pattens and SEM of these 2 polymers were quite different.The injection molded test pieces of Me-HQ/BB/Cl-PEC(m/n-9.5/0.5) showed lower degree of orientation and fewer fibrils than those of m/n=9/l as shown in Fig.7 and8(8). 

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],... 

For the wholly aromatic TLCPs derived from TA, phenyl hydroquinone, and 3,4 -dicarboxyl phenylether [59], the thermal stability measured by TGA slightly decreases with the increasing ether content. All polymers exhibit good thermal stability up to about 400°C in N2 and 25°C lower in air, with a 20°C min heating rate. The thermal stability of the TLCPs decreases when the phenyl hydroquinone is replaced by (1-phenylethyl) hydroquinone. 

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