Polymers liquid crystalline

The primary structure of polymers is the most basic information for estimating physical properties. Table 11.1 summarizes the lists of papers in which NMR is used to characterize the tacticity, regioregularity, end group, branch, degradation and so on. 

A variable-director NMR method was used to investigate the order and dynamics of aromatic polyamides in lyotropic liquid crystalline phases. In 

Nuclear Magnetic Resonance, Volume 31 The Royal Society of Chemistry, 2002 

2-methylpropylacrylateco-acrylonitrile) poly(2-(trimethylsiloxy)ethyl methacrylate), poIy[2-(hydroxyethyl) methacrylate], poly[2-[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate] poly(2-butene-co-ethylene) poly(2-butyl malic acid ester), poly(3,3-dimethyl-2-butyl malic acid ester) poly(2-cyanomethyl-1,3-butadiene-co-styrene), poly(2-cyanomethyl-1,3-butadiene-co-acrylonitrile) 

3- hydroxy-5-oxohexanoate, 8-acetoxy-3-hydroxyoctanoate, 6-acetoxy-3-hydroxyhexanoate, 

The development of high performance polymers, such as high modulus fibers and super-tough polymer blends, has accelerated in recent years 

Heating a thermotropic liquid crystal results in decreasing the molecular order. The general pattern is as follows below, but not all possible phases may appear, and there are many types of smectic crystals. In addition, the LC phase may appear upon cooling rather than upon heating. 

In many cases unique optical textures are observed for the various orientations and structures of the three classes of liquid crystals. Thin films of nematic crystals, for example, can be identified by the pattern of dark tlueads (isogyres) which can appear in the optical microscope in transmission with crossed polarizers. Hot stage polarizing optical nucroscopy is often used to identify the phases and the transition temperatures. In some cases, the optical texture is not uniquely identifiable and x-ray diffraction and thermal analysis by DSC are used to complement the microscopy. 

Liquid crystalline polymers have been discussed in many texts and review papers [65, 400-413] during the last decade, in which the synthesis, processing, morphology, orientation and structure-property relations are described. The major applications of these materials have been as high modulus fibers and films, with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the solid state. 

Thermotropic polymers are melt processable and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

There are thousands of LC polymers [330,335, 336] and the three commercially important classes are  

Main-chain LCPs have been synthesized by ADMET polymerization of diene mesogens [131]. These polymers were subsequently cross-linked to form nematic 

The term mesophase is used in reference to the liquid crystalline phase, and the molecular (sub)structures that give rise to this phase are termed mesogens. Macromolecules with rod-like or a flat, disc-like geometries tend to form such 

LCP phases are subdivided into thermotropic or lyotropic. Lyotropic liquid crystals are formed by macromolecules that show liquid crystalline behavior in solution. This behavior is strongly concentration dependent. Thermotropic liquid crystals are molecules that show liquid crystalline behavior above the melting point of their crystallites. 

If the mesogens are pendant to the polymer backbone, materials are obtained with special magnetic, electrical and optical properties. They provide for nonlinear optics (NLOs) applications in numerous optoelectronic elements. 

The reptation model has also been successfully applied by Jud, Kausch and Williams (1981) to problems relating to crack healing of amorphous polymers. 

Liquid-crystalline behaviour was first reported by the Austrian botanist, Friedrich Reinitzer, in a letter 

The first commercially important liquid-crystalline polymer was Kevlar. Kwolek wrote in 1965 about anisotropic solutions of wholly aromatic polyamides in alkylamide and alkylurea solvents. This development led to Kevlar, i.e. ultra-oriented fibres of poly(p-phenylene terephthalamide) (Fig. 6.17). The solution of the polymer in concentrated sulphuric acid is nematic (the term nematic will be explained in section 6.5.3) and fibres of high modulus and high strength can be spun from the solution. 

In 1972, the first melt-processable (later categorized as thermotropic) liquid-crystalline polymer was reported by Cottis (1972). The term thermotropic refers to the fact that the liquid-crystalline phase is stable within a certain temperature region. The polymer synthesized by Cottis was a copolyester based on p-hydroxybenzoic acid and biphenol terephthalate. This polymer is now available on the market under the name Xydar. In 1973, the first well-characterized thermotropic polymer, a copolyester of p-hydroxybenzoic acid and ethylene terephthalate (Fig. 6.17), was patented by Kuhfuss and Jackson (1976), when they reported the discovery of liquid-crystalline behaviour in this polymer. The availability of this polymer was of great importance to the scientific community but commercialization is yet to come. At the beginning of the 1980s, the 

The main-chain and side-chain polymers as well as the classical small-molecule liquid crystals consist essentially of one-dimensional molecules (rods). Low molar mass compounds of two-dimensional discshaped molecules were discovered by Chandrasekhar (1977) to possess liquid crystallinity. They were referred to as discotic liquid crystals. Later research has involved the synthesis and characterization of polymers with discotic moieties connected via flexible spacer groups. 

Liquid crystals (or plastic crystals as they are sometimes called) are materials that show molecular alignment in one direction but not three-dimensional crystalline order. During the last 20 years, liquid crystalline polymers have been developed where the polymer chains are so straight and rigid that small regions of almost uniform orientation (domains) separated by distinct boundaries are produced. In the case where these domains occur in solution, polymers are termed lyotropic. Where the domains occur in the melt, the polymers are termed thermotropic. 

An important class of lyotropic liquid crystal polymers are the aramid polymers such as 

Important examples of thermotropic liquid crystalline polymers are copolyesters produced by condensation of hydroxybenzoic acid (HBA) 

In addition to these main-chain liquid crystalline polymers, there are also side-chain liquid crystalline polymers, where the liquid crystalline nature arises from the presence of rigid straight side-chain units (called the mesogens) chemically linked to an existing polymer backbone either directly or via flexible spacer units. 

The review by Noel and Navard [3] gives further information on liquid crystalline polymers, including methods of preparation. 

Major polymer applications microwave cookware, fiber optic connectors, capsules for electronic devices, watches, cameras, audiovisual equipment, under-bonnet automotive components, aerospace structures 

Important processing methods injection molding, fiber spinning, extrusion 

Typical fillers glass fiber, wollastonite, carbon black, magnesium carbonate 

Special methods of incorporation materials have skin/core moi-phology in which the relative amoimts of skin and core vaw with processing parameters as does the distribution of reinforcemenr  

Methods of filler pretreatment drying is very important to prevent hydrolysis  

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The thermotropic aromatic main chain liquid crystalline polymers are also prepared by the phase transfer catalyzed aromatic nucleophilic polymerization [87]. Polyetherification of bis(4-chloro-3-nitrophenyl) sulfone with mesogenic aromatic diols is shown below ... 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

Another natural polymer that needs a fresh look into its structure and properties is bitumen [123], also called asphaltines, that are used in highway construction. Although a petroleum by-product, it is a naturally existing polymer. It primarily consists of polynuclear aromatic and cyclocaliphatic ring systems and possesses a lamellar-type structure. It is a potential material that requires more study, and high-performance materials such as liquid crystalline polymer (LCP) could be made from it. 

M. Saminathan, PhD. Thesis, Synthesis and Characterization of Novel Liquid Crystalline Polymers Containing Azobenzene Mesogen, Regional Research Laboratory,... 

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. 

As Carfagna et al. [61] suggested, the addition of a mesophasic polymer to an amorphous matrix can lead to different results depending on the properties of the liquid crystalline polymer and its amount. If a small amount of the filler compatible with the matrix is added, only plasticization effect can be expected and the dimensional stability of the blend would be reduced. Addition of PET-PHB60 to polycarbonate reduced the dimensionality of the composite, i.e., it increased the shrinkage [42]. This behavior was ascribed to the very low... 

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. 

F. N. Cogswell, Recent Advances in Liquid Crystalline Polymers (L. L. Chapoy, ed.), Elsevier Applied Science, London (1985). 

Blends of polypropylene (PP) and liquid crystalline polymer (LCP) processed without melting the LCP were compared with conventional melt processed blends. In a first stage, PP was blended with 20 wt% of LCP in a twin-screw extruder with the take-up speed varied to achieve blends with different LCP fiber dimensions. In the second stage, these blends were processed both below and above the Tm of the LCP by extrusion and injection molding. 

Polymer Blends Containing Thermotropic Liquid Crystalline Polymer... 

A block copolymer composed of liquid crystalline polymer (LCP) segments or that composed of segments having an LCP unit in their main chain or side chain was synthesized [67,68]. The latter showed partial compatibility and second-phase separation even when in a melt liquid crystalline state. 

Shibaev, V. P. and Plate, N. A. Thermotropic Liquid-Crystalline Polymers with Mesogenic Side Groups, Vol. 60/61, pp. 173—252. 

Liquid crystalline polymers (LCPs) are best thought of as being a separate, unique class of TPs. Their molecules are stiff, rodlike... 

Table 6-10 General properties of crystalline, amorphous, and liquid crystalline polymers...

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