Liquid crystal polymers mechanical properties

A very high, price and performance family of polymers called liquid crystal polymers (LCPs) exhibit extremely high mechanical and thermal properties. As their ease of processing and price improve, they may find appHcation in thin-waH, high strength parts such as nails, bolts, and fasteners where metal parts cannot be used for reasons of conductivity, electromagnetic characteristics, or corrosion. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

The class of polyester-based liquid crystal polymers (LCPs) represent one of the most attractive materials in the field of engineering thermoplastics because of their superior mechanical properties, heat resistance, accuracy of dimensions, moldability and the excellent balance of these properties [1-5]. LCPs have been recently expanding their applications, in particular, those for precision electronic parts appropriate for surface mount technology (SMT). 

Connectors, switches, electric distributors, fuse boxes and other electric fittings need a subtle balance of electrical and mechanical properties, durability, cost and aesthetics. This broad field creates fierce competition not only between engineering thermoplastics and SMC/BMC for the main applications but also with polypropylene and polyethylene or PVC for the lower performance parts and, at the opposite end of the scale, with high-tech plastics such as polyetherketone, polyetherimide, liquid crystal polymers. .. For example, without claiming to be exhaustive ... 

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

Similady, liquid-crystal polymers exhibit considerable order in the liquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (a ram id) with the repeating unit designated as (2). It is spun into... 

Cogswell (1985) expressed it in the following words "To make the connection from the basic material properties to the performance in the final product, industrial technologists had to learn a new science". It is more or less so, that - for liquid crystal polymers -properties like stress history, optical and mechanical anisotropy, and texture seem to be independent variables this in contradistinction to the situation with conventional polymers. 

The effects of molecular order on the gas transport mechanism in polymers are examined. Generally, orientation and crystallization of polymers improves the barrier properties of the material as a result of the increased packing efficiency of the polymer chains. Liquid crystal polymers (LCP) have a unique morphology with a high degree of molecular order. These relatively new materials have been found to exhibit excellent barrier properties. An overview of the solution and diffusion processes of small penetrants in oriented amorphous and semicrystalline polymers is followed by a closer examination of the transport properties of LCP s. 

Block-like and segmented polymers represent chemically bound "multicomponent" systems that, in our opinion, are able to mimic some of the non-bonded interactions occurring in blends of either compatible or not compatible pol3nners, and to describe phenomena connected with phase segregation and the onset of peculiar micromorpho-logical properties. Additionally, liquid crystal polymers, even though "monocomponent" from a macrochemical point of view in that constituted of only one polymeric material, in reality do behave, under certain selected thermodynamic conditions, as mechanical mixtures of at least two components. 

Chapters 7 and 8 and parts of chapter 11, in particular, have been influenced strongly by the two more-advanced textbooks on the mechanical properties of solid polymers by Professor I. M. Ward, and the section of chapter 12 on liquid-crystal polymers has drawn heavily on the more-advanced textbook by Professors A. Donald and A. H. Windle. These books are referred to in the sections on further reading in those chapters and I wish to acknowledge my debt to them, as to all the books referred to there and in the corresponding sections of other chapters. 

Liquid Crystal Polymers - LCPs. Basic/mechanical/thermal properties... 

Liquid Crystalline Polymers are an important class of polymeric materials because they may exhibit optical properties similar to low-molar-mass liquid crystals and high mechanical properties of polymers. These polymers are broadly classified based on their molecular architecture, i.e. attachment of the mesogen to the polymeric backbone, as main-chain liquid crystal polymers (i) or side-chain liquid crystal polymers (2). In main-chain liquid crystal polymers, mesogens are incorporated into the backbone. The mesogens may be of different shapes and sizes, and are usually rodlike or disklike. Such polymers have not been used for optoelectronic applications because it is very difficult to reorient these materials by electric field. Instead, these materials find applications that use their exceptional mechanical properties. Even side-chain liquid crystal polymers, whose mesogen is attached to the polymer backbone through a flexible spacer switch too slowly for... 

In Figure 16, results of Jackson and Kuhfuss show the effect of molding temperature and polymer morphology weight on the mechanical properties for poly(ethylene terephthalate) modified with p-hydroxybenzoic acid. "The temperature at which the PHB copolyesters are injection molded affects the orientation of the liquid-crystal polymer chains and, therefore, affects the mechanical properties. 

Brostow W, editor. Mechanical and thermophysical properties of polymer liquid crystals. Polym liquid crystal series, vol. 3. London Chapman Hall 1998. ISBN 0-412-60900-2... 

C.G. Bazuin, in Polymer Liquid Crystals - 3 - Mechanical and Thermophysical Properties (W. Brostow, Ed.), Chapman Hall, London, 1998, Chapter 3. 

An interesting new development uses liquid crystal polymers (LCPs) to produce materials with exceptional physical and mechanical properties. Generally the LCP resin comprises of a polymer chain with structural units (mesogenic groups) which can be incorporated into the polymer backbone, or incorporated as a pendant group, or both. One approach is to use the epoxy resin system [18] ... 

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