Self-reinforcing LCPs

This article is an overview of the novel technology of self-reinforced LCPs with polyesters, poly(ethylene terephthalate) (PET) and poly(ethylene naphtha-late) (PEN) [10-13, 21, 23], LCP/polyester blends in a polyester matrix form in situ fibrils which improve the mechanical properties. LCPs have an inherently low melt viscosity, and provide LCP/polyester blends that effectively lower the melt viscosity during melt spinning [24], and fast injection-molding cycles. The miscibility between the LCP and polyesters can be controlled by the degree of transesterification [25] in the reactive extrusion step, and fibril formation in LCP-reinforced polyester fibers has been studied. 

This chapter provides an overview of current researches on liquid crystalline polymers (LCPs). Topics include syntheses of main-chain and side-chain LCFs, structured characterization of LCFs and LCP networks and rheology and processing. Applications of LCP/polymer blends as self-reinforced polymers and electro-optical meterials are also discussed. 

Blends. There has been considerable research in recent years on polymer blends that contain an LCP. This subject was recently reviewed by Dutta et al. (67). The addition of an LCP to another thermoplastic melt effectively lowers the melt viscosity and improves processability. In addition, if the flow field contains an extensional stress component, the LCP dispersed phase is extended into a fibrous morphology and oriented in the flow direction. This microstructure can be retained in the solidified blend to provide self-reinforcement. 

Super-strong (SS) polymers (3-11) are specially-designed combined LCPs in which the side chains of one molecule are designed to interdigitate (pack between) the side chains of neighboring molecules, thus leading to molecular self-reinforcement and enhanced molecular ordering (thus, enhanced mechanical properties) compared to backbone LCPs. 

Tm etc. for the mesophase transitions and by Ti for the transition to an isotropic melt. The presence of such a mesophase (for mainchain LCP systems usually a nematic one) offers the possibility to use these polymers as a reinforcing fibre phase in so-called self-reinforcing composite systems. 

Vectra LCPs consist of parallel, closely packed, fibrous chains in injection molded parts which give the polymer a wood-like appearance at fracture surfaces and also give it seU-reinforcing properties that are at least as good as those of conventional fiher-reinforced thermoplastics. ICI supphes LCPs (self-reinforcing polymers) under the trade name Victrex SRP. Excellent physical properties, low comhnstihility and smoke generation, resistance to chemicals and solvents, and stahihty towards radiation are some of the important properties of LCPs. 

Another rod like TP is self-reinforcing polymers (SRPs) that has exceptional high performance properties but very difficult to process. Unlike LCPs they are amorphous, isotropic, and transparent. Research on SRPs was sponsored during the 1960s by Wright-Patterson Air Force Base, Dayton, OH. 

Before going any further, let us adopt the terminology introduced by Samulski [5]. We have already used above the abbreviation PLCs. Samulski contrasted PLCs to MLCs, and defined the latter as low molecular mass LCs— irrespective of the fact whether they can or cannot polymerize. His terminology is unequivocal and succinct. People unfamiliar with it use long and not necessarily well-defined terms, such as liquid-crystalline substances with low molecular weights —when they presumably mean MLCs. Other names such as liquid crystalline polymers (LCPs) for PLCs or LMMLCs for MLCs are also in use. The abbreviation SRPs for self-reinforcing polymers and the name in situ— composites [6] are used as weU. Moreover, PLCs are sometimes also called molecular composites. 

PEI with 35-95 wt% LCP, self-reinforced polymer compositions LCP fibers, tensile strength, modulus, elongation, impact strength, HDT Isayev and Swaminathan 1989... 

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. 

As can be seen from Fig. 11.2, many fibrils are observed in the cross section of tensile fractured as-spun fiber LCP. These fibrils are acting as if they are reinforcements, and therefore LCP is often referred to as self-reinforced polymer. However, this anisotropy is a big practical problem when LCPs are used in injection-molded parts because LCPs produce oriented moldings with anisotropic mechanical and thermal properties. Properties along the flow direction tend to be superior to those across the flow direction. Fillers represented as glass fiber are often added to LCPs to reduce the anisotropy because the addition of any filler disrupts the alignment of the LCP molecules. This is just the reverse of what is experienced with... 

ISAYEV Self-Reinforced Composites Involving LCPs... 

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