The discovery of liquid crystals is credited with the Austrian scientist Friedrich Reinitzer who noted that certain cholesteryl esters are characterised by double melting behaviour. Later, Lehman observed that cholesteryl esters exhibit pronounced anisotropy (as seen under a polarised light microscope) after the first melting when the melt becomes cloudy and he coined the term liquid crystals. The liquid crystalline state or mesophase is an intermediate state between the fully anisotropic crystal and the isotropic liquid.  So, the liquid crystals are partially ordered exhibiting some mobility of the liquid state. The characteristic feature of liquid crystals is the presence of a 'mesogen', the structural entity that is responsible for the LC behaviour.  A mesogen, in general, is a rigid or a disc- like or lathe like molecule, the presence of which produces a pronounced anisotropy in shape. This generates organised fluid phases either on melting (thermotropic) or on dissolution (lyotropic). When a mesogen gets repeated in the main chain or side chain of a polymer, liquid crystalline polymers (LCPs) are obtained.   LCPs are a class of unique materials exhibiting high performance properties close to that of theoretical values. The technological importance of liquid crystalline structures is driven by (a) their low melt viscosity of mesophases preferentially the nematic one which will allow the production of thin walled precision products and unique rheological properties manifested by these nanoscopically tailored materials and (b) the fact that LC phase can be trapped or stabilised in the glassy phase of the polymer so that the electro-optical and magnetic properties can be conveniently manipulated for applications in  areas such as imaging technology, non-linear optics, telecommunications etc1-3. 

2. The Genesis of Patent Protection in LCP area 

The first patents on LCPs originated with the discovery of the LC behaviour of polyaramide which later became famous as Kevlar®???????In 1960s, workers at Du Pont started work on aromatic extended chain polyaramides that resulted in the development of high strength / high modulus fibres???he patents that describe this product form the first of the kind on LCPs4-8. The original discovery of the LC behaviour of polyaramide was made by Kowlek in du Pont who noted that the solutions of the polymer are cloudy. That this cloudy nature is due to the pronounced anisotropy of the system was later identified by using a polarised light microscope where they obtained nematic textures characteristic of the LC phase. The technological application of the polymer was made possible due to the series of findings such as its spontaneous orientation under elongational flow. When a lyotropic solution of Kevlar® experiences shear in extrusion through a spinneret hole, the polymer undergoes spontaneous orientation of the macromolecules in the direction of the flow and this formed the basic finding for the processing of LCPs. This has been revealed in another patent to Du Pont16-17 It is interesting to note that the scientific research findings were published by Du Pont only after 19779-15. 
The synthesis of some of the first aromatic polyesters were described in patents issued to ICI Ltd. 18-20 and to Carborundum Ltd21., but their liquid crystalline properties were not known at that time. 
The development of LCPs was hampered by the fact that the wholly aromatic homopolyesters such as poly(4-oxybezoate),  poly(paraphenylene terephthalate) are intractable with melting points above the decomposition temperatures of the polymers (>450?C). So the central problem of thermotropic LCP design has been to disrupt the regularity of the intractable para-linked aromatic polymer chain to the point to which melt processability is achieved without destroying the liquid crystalline behaviour. This was first achieved by Kuhfuss and Jackson who made the first patents on commercial application of LC polyesters by reacting polyethylene terephthalate (PET) with 4-hydroxybenzoic acid ???????????? 

3. The Growth of  Patent Protection of LCPs  and Their Commercialisation

 Following these early patents, one notes a flurry of a large number of patents 24-36 for various LCPs, particulary LC polyesters. Three routes were developed to achieve melt processable high performance LCPs.: (1) introduction of disruptors (flexible spacers or rigid kinks) into the straight polymer chains;  (2) substitution of the aromatic rings; and  (3) copolymerization. It should be noted that usually it is necessary to use a combination of at least two of these approaches to lower the   melting point sufficiently for melt processability and to achieve high mechanical properties. The requirements for a rod like mesogenic moiety melt processability limits the choice to polymers based on linear aromatic esters or esteramides.

Most of the LCPs are prepared by an ester exchange reaction between acetoxyaryl groups and the carboxylic acid groups with the elimination of acetic acid at temperatures above the Tm of the polyester. This polymerisation technique, however, is limited by the viscosity of the melt and this becomes severe as the Tm rises above 300 ?C. In many cases, Tms are in that order and hence difficulties are still reported with the processing of these polymers. In order to overcome this problem, ICI has reported in a patent an aqueous dispersion polymerisation technique whereby an inert heat transfer medium such as a high temperature solvent is used36. Cottis et al.37,38 and Duska et al. 39 have patented methods for the synthesis of high molecular weight aromatic polyesters. 

4. Patents on Structural Modifications to Enhance Property

 Properties of the LCPs can be influenced positively by introducing appropriate structural modifications. Copolymerisation is one of the best methods known to alter the transition   temperatures of main chain LCPs.   The homo polymers are structurally homogeneous and tend to   give perfectly oriented conformation. The copolymerisation introduces inhomogenity in the parallel chain orientation. Copolymers that contain only rigid rod segments are of particular interest because highly oriented samples are expected to exhibit higher moduli than those derived from copolymers that contain flexible or angular segments. Inflexible and relatively rigid and angular unit such as   4,4'- disubstituted   diphenyl ether groups introduce an angle   of approximately 120? into the rigid chain, and such units appear to be particularly effective in reducing or suppressing the tendency to crystallise. Introduction of a substituent into the aromatic ring of the mesogen destroys the plane of symmetry and leads to the random occurrence of head-to-head and head-to-tail isomerism  disrupt  the ability  of the chain segments to pack  into  crystallites. The substituents used were H, Cl, Br, CN, NO2, CH3, OCH3, alkyl and ethyleneoxy groups. The presence of substituents results in a decrease of both mesophasic (Tm) and isotropization  (Ti) transition temperatures. Long chain alkyl and oxyalkyl groups are the most commonly used lateral substituents with the chain length upto about ten carbon atoms.  The transition temperatures decreases and the melting and clearing temperatures alternate typically. This is known as odd-even effect. The tendency for predominantly nematic behaviour gives way to purely smectic behaviour in the higher homologues. Yet another spacer often used is oligosiloxane spacer units in which the uniform distribution results in smectic phase formation whereas random distribution give nematic phase. Thermotropic melts are generally nematic. The randomness of the units in the copolymers and the molecular weights of the polymers have a marked influence on the   phase behaviour of polymers.  Usually, transition temperatures reach a plateau at average chain lengths of approximately 10-15 repeating units.  The entropy change and order parameters at the N/I transition follow a similar trend.
  Calundann et al.40 patented the effect of parrallel offset structures such as a naphthalene moiety on the transition temperatures of the LC copolyesters. Jackson and Kuhfuss have noted that they could successfully make PET (polyethylene terephthalate) liquid crystalline with 20 % hydroxy benzoic acid (HBA)21,22,41 whereas when terephthalic acid was substituted with 2,6 naphthalene dicarboxylic acid in a NDA (2,6-naphthalene dicarboxylic acid)  / HBA / EG (ethylene glycol) system, 35% HBA was required for achieving liquid crystallinity.  Kwolek4, Cottis et al. 42 and Calundann et al. 43,44 in separate patents showed that incorporation of 4,4- biphenylene units in the polyester reduces the transition temperatures by introducing a torsional deformation in the polymer which disturbs the packing ability of the polyester chain.   Calundann further showed that introduction of 2,6-naphthalene moiety in the polyester chain reduces the transition temperature drastically21,22,45-46. A series of thermotropic copolyesters based on 4-hydroxybiphenyl4'-carboxylic acid (HCBA) , HBA and HNA (6-hydroxy-2-naphoic acid) were reported in a patent47. Through a series of patens from 1978-1981 Calundann et al.48-50 and Irwin51 showed that a thermotropic copolyester has a high Tm when a 2,6-dioxyanthraquinine is moiety (2,6-AQ) is present. Schaef