19 August 2010

RECENT ADVANCES IN LIQUID CRYSTALLINE POLYMERS

INTRODUCTION


Liquid crystalline polymers (LCPs) are highly anisotropic fluids that exist between the boundaries of solid and conventional isotropic liquid phase which are formed as a consequence of molecular asymmetry. The liquid crystal phase is the result of long range orientation among the molecular constituents occurring within certain ranges of temperatures (thermotropic), or solution (lyotropic),or pressure (barotropic). They rose into prominence because of two potential findings, which resulted in their use in high performance and functional applications: (a) the orientational order of the LC phase can be retained in the polymer after processing, whereby the performance properties underwent a quantum jump to values close to those of theoretical predictions, thereby obtaining extremely high modulus, high strength, high heat resistance, etc. (b) that the LC phase can be trapped or stabilized 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, etc. Moreover, they have excellent dimensional stability, thermal stability, and flame resistance. Coupled with the absence of creep and shrinkage, these properties make them ideal candidates for the high performance applications.


LCPs and small molar liquid crystals are characterized by 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 board-like molecule, the presence of which produces a pronounced anisotropy in shape. This generates organized fluid phases either on melting (thermotropic) or on dissolution (lyotropic).Making use of anisotropy in processing, it was possible to achieve impressive properties for the production of high performance polymers.


CLASSIFICATIONS


LCs can be classified based on molecular structure, molecular size and phase structure.


1. Classifications based on molecular structure



Based on the shape of the molecules, LCs can be differentiated into calamitic liquid crystals occurring in rod-like molecules, mainly constituting the classical LCs, discotic liquid crystals derived from disc-like molecules and sanidic liquid crystals occurring in board-like molecules.


2. Classification based on molecular size


Under this class, LCPs are of two types: Main Chain LCPs (MCLCPs) and Side Chain LCPs (SCLCPs). In MCLCP, the mesogen, the structural entity that is responsible for the LC behaviour, is in the main chain of the polymer, whereas in the SCLCP, the mesogen is attached to main chain as a pendent group, directly or through a spacer.


3. Classification based on phase structure


Based on phase structure, LC Systems are classified into :

Nematic: Contrary to isotropic liquids, one or two molecular axes are oriented parallel to one another resulting in an oriental long range order.

Smecic: Long range orientational order with partial long range positional order, giving rise to layer structures with a large number of possible variations.

Cholesteric: Cholesteric is a variant of nematic state occurring in chiral compounds.

Cubic: Structures with micellar lattice units or complicated interwoven networks.

Columnar: Structures with columns consisting of parallel arranged disc-like molecules.



4. Classifications based on phase appearance


The LC phases are generally induced by a change in temperature (thermotropic), solvent (lyotropic) or pressure (barotropic).



WHY LCPs ARE UNIQUE?


LCPs are often formed from highly aromatic stiff monomers. Hence it is not surprising that they can form very stiff polymers. Kevlar fibres, for example, have a specific modulus considerably in excess of steel. The advantage that the liquid crystal phase brings is that processing from a naturally ordered fluid gives rise to a more perfect final structure than processing a normal chaotic fluid. The highly aromatic LCPs are also very brittle, have very low failure strains, but can be very strong.


The mechanical properties of LCPs are highly anisotropic. There is also a strong skin core effect, which makes the properties at the surface very different to the bulk. One of the most useful properties of LCP moldings is their very thermal expansion coefficient. This results in low mould shrinkage and hence very high tolerance components can be moulded. This is one of the most exploited properties of LCPs. Another feature when considering their use in blends is their viscosity. The viscosity of LCP can increase with temperature where as the usual trend is for viscosity to reduce with temperature.


Besides the above mentioned features, LCPs possess spontaneous orientation, chemical and thermal stability, low shrinkage and warpage, total reusability, exceptional fire retardance, homogeneous composites, long range deformation for the LC elastomers, low coefficient of thermal expansion etc. In the case of Kevlar , a liquid crystalline polymer, the strength achieved is close to the theoretical strength whereas in the case of other polymers ,there is a vast gap between the two values.


The additional behaviour of LCPs can be understood , where the tensile properties of a number of fibres are compared with that of Kevlar. LCPs do not encounter the normal brittle or catastrophic failures. These exceptional behaviours stem from the hydrogen bonded sheet like structures.


LCPs overtake aluminium and steel in specific strength and specific modulus. These data are strong enough to show the importance of LCPs.



THERMOTROPIC LIQUID CRYSTAL POLYMERS


In thermotropic polymers, the mesophase is induced by a change temperature. According to the Flory’s lattice theory, an axial ratio of anly 6.42 is required for a polymer to be stiff enough to form LC phase. This is achievable by non-mesogenic rigid rod and/or semi flexible monomers on polymerization, either as main chain and/or as the side chain polymers.



Main Chain Liquid Crystal Polymers (MCLCPs)


The basic structures in general in main chain LCPs are benzene rings interlinked at para positions. The majority of synthetic LCPs consists of a succession of rigid units that are individually characterized by possessing chain continuing bonds that are either collinear or parallel and oppositely directed. The examples are the p-phenylene and 1,5-naphthalene units respectively. A Wide range of p-substituted aromatic rings have found their use in LCP synthesis. The rigid structural cyclic units involved need not be aromatic in character and trans 2,4-substituted cyclohexane units are of this class. These ring structures are conformationally rigid, and are main chain components of nearly all the LC synthetic polymers of this type known. The trans conformations required to meet the criterion are maintained either by the presence of double bonds, or as in the case of the amide and ester groups, by a substantial energetic preference for that conformation.



Design of Thermotropic LCPs


The core aspect of LCPs is the mesogen, the structural entity that is responsible for the LC bahaviour. Apart from the mesogenic group, there are a number of additional groups such as linking groups, flexibilising groups, functional groups etc. are required to build up LC polymer. Thus, wholly aromatic homopolyesters such as poly(4-oxybenzoate), poly(paraphenylene terephthalate), are highly crystalline and are intractable with melting points above the decomposition temperatures of the polymers(>450C). So the critical problem of thermotropic LCP design is to disrupt thr regularity of the intractable para-linked aromatic polymer chain to the point to which melt processability is achieved without destroying the liquid crystalline behaviour. There are three possible ways:(1) introduction of disrupters(flexible spacers or rigid kinks) into the straight polymer chains;(2) substitution of the aromatic rings; and (3) co-polymerisation. 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 mschanical properties. The requirements for a rod like mesogenic moiety melt processability limits the choice to polymers based on linear aromatic esters or esteramides.


Synthesis


The majority of LCPs, especially those of commercial significance are prepared by the ester exchange reaction between an acetoxy aryl group and the carboxylic acid group with the elimination of acetic acid at temperatures above the Tm of the polymer. This method is, however, limited by the viscosity of the melt and hence higher viscosities may be attainable. This limit becomes more severe as the value of Tm for the polymer rises above 300 C. A stepwise preparation through a pre-polymer is expected to solve some of these problems. More recently, a technique involving the use of a high temperature inert solvent is reported to give improved products.


The structure-property relationship



Thermotropic melts are generally nematic. The randomness of the units in the co-polymers 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. An illustrative example is the case of poly(4,4’-dioxy-2,2’-dimethyl azoxy benzene dodecanedioyl). Thermotropic LC extrusions show a ‘skin-core’ morphology, as observed in poly(p-phenylene terephthalamide)fibresin which the structure of the melt is frozen into the crystalline solid on extrusions. The orientation of the micro fibrils depends on the thickness of the extrudate.


Side Chain Liquid Crystal Polymers (SCLCPs)


It is well known that low molar mass liquid crystals (MLCs) can be ordered unidirectionally in an electric field or in a magnetic field. LCPs can also be orderedsimilarly. This concept has received much attention in recent years, and the side chain LCPs (SCLCPs) have beev found to be particularly interesting in this aspect. Side chain LCPs emerged as an area of practical importance due to their outstanding optical properties, which are useful in electeo-optical device applications. In SCLCPs, the mesogenic groupsare linked via spacers to an existing polymer backbone. Polymers with mesogenic side chains are usually thermotropic and the mesogenic group maintaina degree of orientational freedom depending on the coupling strength to the backbone. The alignment of mesogens may be altered by the application of electric field, and the alignment takes place ona atime scale order of magnitude faster for SCLCPs than for MCLCPs.This allows the optical properties of SCLCPs to be altered readily by the use of an external electric field.


Although the LC properties of the polymers parallelthose of the low molecular weight analogues, the differences are mainly due to,


(a). the high melt viscosity which restricts the structural re-arrangements influenced by the external field; and,

(b).the interaction of polymer backbone chain, which try to assume a random coil conformation, and the mesogenic group, which try to organize in the LC phase. The backbone mesogen interaction may be decoupled to some extent by introducing spacer groups between the backbone and the mesogenic groups.



LYOTROPIC LIQUID CRYSTALLINE POLYMERS


In 1960s, researchers at Dupont discovered that certain polyamide such as poly parabenzamide exhibit anisotropic properties in concentrated solution and the technological outcome was the super fibre called Kevlar aramid fibres. In fact ,Flory predicted that a solution of hard, asymmetric, rigid rod like particles should separate in to two phases, an ordered mesophase and an isotropic phase, above a threshold concentration. In case of rod like polymers, axial ratio(L/D) of the macromoleculeswas found to govern the concentration at which phase separation occurs. In those cases, the requisite rigidity and large axial ratio are achieved by virtue of stable helical conformation of macromolecules. Lyotropic mesophase is thus induced by a concentration change in a solvent system.


There are two pre-requisite for liquid crystal formation in polymer solution:

  1. Sufficient inherent rigidity of molecular structure,
  2. Sufficient solubility


CURRENT INTERESTS AND APPLICATIONS


The main application of LCPs will be in areas that exploit combinations of the key properties such as strength, easy flow, excellent dimensional stability, the ability to incorporate high levels of fillers and excellence of chemical resistance. The current interests and applications can be summerised as below:

1. In Injection moulded products in electronics (eg: surface mount units, connectors, printing wiring boards, etc. and computer fields). The similarity in thermal expansion of metal and LCP is expected to result in good component integrity and minimal strain when components containing metals and LCPs are in contact and are subject to thermal cycling or shock. In addition, the low warpage and easy flow will allow precision mouldings of complicated parts, and the ability to withstand strong solvents will be of great interest.

2. In industry for making chemically resistant parts (eg:tower packing saddles to replace ceramics). Here, better chemical resistance and breaking strength are of importance.

3. LCP blends:

(a) Addition of small amounts of LCPs to a conventional polymer improves flow.

(b)Blends with cheaper polymers to give LCP properties at lower costs (eg:LCP-

Nylon).

(c) Blends with other LCPs to produce better property profiles.


Blends with Liquid Crystalline Poly (Ester Amides) (PEAs)


Aromatic poly (ester amide) polymers are now well known to exhibit liquid crystalline behaviour. The interest in these polymers stems from the fact that they are a class of high performance polymers that have unique thermo-mechanical properties. They have special structure due to the regular enchainment of ester and amide groups in the same polymer chain that give them properties intermediate of polyesters and polyamides. The synthesis of PEAs is thus a convenient method for introducing the symmetry of intractable fully aromatic homo polyamides and homo polyesters thus making them melt processable. PEAs are particularly interesting because the Hydrogen bonding in the amide group is expected to increase the stability of the mesophase. Moreover, both LC polyester and polyamides are shown to exhibit banded structures in their oriented state and so PEA can be expected to exhibit similar structures that imply a high level of supra molecular ordering. Aromatic PEAs are usually prepared by the exchange reactions of the acetate in the melt or by the interfacial poly condensation method. .

The blending Ws carried out using a Brabender plasticorder fitted with a roller blade counter rotating at a speed of 30 rpm. They were melt blended for about 8 minutes. By this blending liquid crystalline polyester amide improves the processability and thermal stability of EVA. Resistance to thermo-oxidative degradation in air of EVA improves on modification with 5% PEA.


The following table gives a summary of applications by area wise.


Sl.No

Area

Property/ Application



1





2






3









4






5








6






7



Fibres





Films






Plastics and resins









Non-linear Optics






Information Storage Devices







Holography






Chromatography

Closely packed ensembles of parallelly extended polymer chains exhibit the highest achievable specific strength for ultrahigh-strength fibres. Eg. Polyaramides (Kevlar) and rigid aromatic polyesters


Solvent resistance, low flammability and excellent solder resistance permit its use in a wide range of electronic applications, information industry and as a coating for optical fibres. The low water absorption and excellent barrier properties offers potential use in film packaging.


Negligible die swellability, very low mould shrinkage, high heat distortion temperature and high continuous use temperature etc. permit LCPs for use in moulding of high performance structural parts and coatings. The low creep, low linear coefficient of thermal expansion and very low viscosity of the anisotropic melt allow the fabrication of intricate structures as thin as 0.4mm possible. Eg. Wholly aromatic polymers.


LCps containing dipolar groups in a push-pull electronic arrangement are found to exhibit NLO behaviour, SCLCPs with donor and acceptor built –in to bring about the dipole oscillation, could be a valuable substitute for the currently used inorganic NLO single crystals such as Lithium Niobate, KH2PO4 etc.


Spontaneous alignment of the mesogens in a highly ordered manner and the hemeotropic alignment under applied external fields can be used to record information in an aligned liquid crystal using laser that can heat line spots and melt spots on the clear film. When they cool down they form spots rich in defects, which when viewed between crossed polarizers the spot stand out in high contrast as bright against a black background.


Side chainLCPs containing photosensitive group such as stilbene units, azobenzene units etc. showing cis-trans isomerism find such applications where the three dimensional virtual image is stored by shining light on interference pattern that encode the image in a photosensitive film as a refractive index grating.


Mesomorphic stationary phase are useful for Gas-Liquid chromatography separations. The unique flexibility of the polysiloxane as well as its low surface tension ensure quite good column efficiency. The remarkable GC selectivity of these mesomorphic stationary phases was demonstrated for polycyclic aromatic hydrocarbons, polychlorinated biphenyls, fatty acid methyl esters etc.



OPPORTUNITIES FOR TECHNOLOGY DEVELOPMENT


(a). Use of low cost commercially viable co monomers: 4-Hydrixyphenyl acetic acid (HPAA) and 4- Hydroxyphenyl propionic acid (HPPA) are two comparatively cheap monomers available in the market.

(b). Use of naturally available co monomers with or without modification.


A new monomer 8-3-(hydroxylphenyl) octanoic acid (HPOA) prepared by phase transfer catalysed permanganate oxidation of a natural material commonly known as cardanol was found to be a suitable candidate to be used as chain disruptor and its co-poly ester with p-hydroxy benzoic acid gave nematic LCs. HPPA and HPAA have the added advantage of producing biodegradable liquid crystalline polymers which can be used in medicine where biomaterials with strength are required artificial substitutes. Moreover, HPOA is found to possess structural features such as kink structures and a flexibilising soft segment in the same molecule, so that a lowering of transition temperatures sufficiently lower for melt processing the polymer can be achieved. HPPA is also found free in nature and its polyesters are biodegradable and offers opportunities for making biomedical implants such as artificial screws, pins, etc. which requires certain amount of mechanical properties


To be melt processable, the LC polyester should exhibit a nematic mesophase ,should have a wide melting range and also should possess comparatively good thermal stability. The problem of facing the LC industry is the poor weld line strength, because flow fronts in an injection moulding system do not knit together because of the rigidity of the molecules. Tke inherent flexibility of the disrupters affect positively this factor as well.



Synthesis and characterization of melt processable LC polymers containing phenyl azo mesogen


A series of novel LC polymers containing azophenyl group was synthesized by performing a diazotization reaction between cardanol and para aminobenzoic acid and polymerization resulting monomer to get poly4[-4-hydroxy 2-pentadecyl phenyl]azo benzoic acid. These are in general, insoluble and introduction of pentadecyl group has significantly improved its solubility. Moreover, azo based LC polymers are well known to give non-linear optical behaviour, and for such applications , the polymers should not absorb in the UV RANGE OF 100-200 nm. The LC polymers prepared did not absorb in this region, indicating possibly that this polymer might show useful non-linear optical properties. The field of side chain liquid crystal polymers have attracted considerable interest in both academic research not only due to low molecular mass liquid crystals, and that of macromolecules, but also because of the superior properties they possess over the main chain liquid crystal polymers.



THE FUTURE OF LC POLYMERS


Liquid crystal polymers offer tremendous potential for applications in both functional and high performance polymers. The well known applications of LC rigid main chain polymers in the aerospace and military departments as high performance fibres, film, coating and intricate machine parts might recede due to the end of cold war. The use of LCPs in electronic industry and in the chemical industries are expected to continue. The main applications of side chain polymers, in particular silicone polymers, is in gas liquid chromatography. There are promising developments in ferro-electric side chain LC polymers to be used in display devices where the switching time is critical. Other potential applications for LCPs are in the area of telecommunication as wave guides, NLO devices, optical windows, optical grating etc. The ferroelectric LC polymers and NLO active polymers are envisaged to be futuristic advanced materials. The problems to be overcome in these polymers are their switching time and dipole relaxation respectively.



Biodegradable liquid crystals are going to be a major application, particularly those connected with artificial organs and replacements, where screws and pins of high strength are required. Vast developments are possible in the area of self assembly systems. Blending of liquid crystalline polymers with thermoplastics I already picking up and is going to contribute to novel developments. The future of LCPs are certainly bright owing to their unmatched performance. However, a steep growth depends on the availability of state-of-the –art technologies for the production of speciality chemicals to feed the LCP industries at a cost effective scale.



CONCLUSION


Based on the information that I have found, Liquid Crystal Polymers ( LCPs) are used in many different commercial applications ranging from high modulus fibres (Kevlar) to microwave cookware. Much of the driving force behind the development of LCPs had been the increasing desire of replace metal components with engineering polymers, for example in automotive and aerospace sectors. Key properties turned out to be low thermal expansion coefficients leading to high quality moldings, rather than exceptional modulus or strength. Since they have achieved limited market penetrations, focus of interest in them has switched from their use as stand alone materials to their use in blends. In recent years, more attention has been paid to in situ composites, i.e, polyblends containing a liquid crystalline polymer in which LCP can be form fibrils reinforcing the resin matrix. These blends have got improved melt process ability and enhanced mechanical properties.

v Biodegradable liquid crystals are going to be a major application, particularly those connected with artificial organs and replacements, where screws and pins of high strength are required.

v Blending of liquid crystalline polymers with thermoplastics is already picking up and is going to contribute to novel developments.

v The future of LCPs are certainly bright owing to their unmatched performance.

v All things considered, I would say that the future of LCPs are certainly bright owing to their unmatched performance.


REFERENCES.


  1. L.L. Chapoy, Ed: “ Recent advances in LC polymers”, Elsevier, London, 1985.

  1. C.S. Brown and P.T. Alder, “ Polymer blends are alloys”, a review on blends containing LCPs.

  1. C.K.S. Pillai “LC polymers: recent developments”, pop. plast.and packaging.13,1998.

  1. Jackueline I. Kroschuitz “ High performance polymers and composites”, Encyclopedia reprint series.
  2. I.M. Ward “ Structure and properties of oriented polymers”.

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