Journal of Intellectual Property Rights Vol 4 Se ptember 1999 pp 274-285

Liquid Crystalline Polymers: A Review of Their Patents

C K S Pillai

Regional Nesearch Laborat01Y, Thiruvananthapuram 695019

Liquid C1ystall ine Jlolymers (LCPs) cover a field of emerg ing fronti er area materia ls having g rea t commercial potential. Patents form a large part of their intellectual protection. 'I'll(' revi rw dea ls with the potential features and g rowth of the paten ts fli ed mainly in the area of the LCPs. T he firs t patents on LCPs are , however, on polya ram ides (known by trade name Kevlar~ issued to Du Pon t during th e pe riod 1%2-1977. T he publ ica tions on this polymer appea red only from 1977 onward s. Patents on a number of LCPs bega n appearing during this tim e. A number of fi rms utili zed these patent s for commercializa tion oflhe polymers. Only 12-15 patents were involved in utiliza tion of patents in co mmercialization of th e products. The late r period has, howeve r, witn essed a proliferation of patents to th e tun e of about 60-65 numbers mainly on modification brought about on stru ctu ral aspects. Most of th e patents have been fli ed in USA with Germany following second with oth er countries having only a few nu mber of patent s to their credit. In th e area of commercialization , USA is leading with Hoechst-Celanese toping the lis t. In this paper an attempt has been made to rrview the entirr patent literature on LCPs.

Liquid crysta ll ine polymers (LCPs) are unique polymeric materials with high per­fonn ance and function al applications. They rose in to prominence because: (a) th e orien­tational order of th e LC phase can be re­tai ned in th e polymer after processing thereby th e peJiormance properties und er­went a quantum jump to values close to those of th eoretical predictions th ereby ob­tai n ing extre mely h igh modulu s , h igh strength, high heat resistance, etc. which

were not imaginable in earlier situ ation s,I-7 and (b) that th e LC phase can be trapped or stabilized in the glassy phase of the polymer so that th e electro-optical and magnetic properties can be convenien tly manipulated for applications in areas such as imaging technology, non-linear optics, telecommun i­cations, etc8- 9. Moreover, they have excel­lent dimensional stability, thermal stability, and flam e resistance. Coupled with t:,e ab­sence of creep and shrinkage, these proper-

PIllA !: UQUIf) CRYSTALliNE POLYMERS: A REVlEW OF ... ... . 275

ties 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 r ig id or a disc-like or lathe-like molecule, the presence of which produces a pronounced anisotropy in shape. This gen­erates organized fluid phases either on melt­in g (th e rmotrop ic) or on disso lution (lyotropic). Making use of th e anisotropy in processing, it was possible to achieve im­pressive properties. fo r the production of h igh performance polymers. As most of th e commercial LCPs are thermotropic polyes­ters, the present review will be restricted to thermotropic co polyesters. The first patent on LCPs, however, originated with the dis­covery of th e LC behaviour of polyaramide which later became famous as Kevlar®H}'19.

LCPs are characterized by a number of inter­esting and unique features7

; such as sponta­neous o ri e n tation, p roc essi ng e ase, exc eption al mechanical properti es , low creep and shr inkage. low warpage, low coef­ficient of thermal expansion, etc. Moreover, they have excellent dimensional stabilil ty, thermal stability, and flame resistance. Cou­pled with the absence of creep and shrink­age, these properties make them id eal candidate fo r the high peliorman ce and functional appl ications.

'nlennotropic Liquid Crystal Polymers

In thermotropic polymers, the mesophase is induced by a change in terlperature. Ac-

d· I FI ' I . h 21-22 cor mg to t le ory s attlce t eory an axial ratio of only 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 semiflexible monomers on po-

lymerization eith er as main chain and / or as th e sid e chain polymers.

Wholly aromatic homopolyesters such as poly (4-oxybezoate) , poly (paraphenylene terephthalate) are highly C1y stalline, but are intractable with melti ng points above the decomposition temperatures of th e poly­mers (450°C). Melti ng pe>ints of 610 and 600°C were measured by DSC (scan rate of 80°C/min to minimize degradation) for poly(p-hydroxy benzoic acid) and p0\y(P­phenylene terephthalate), respectiveli ' . So the central problem of thermotropic LCP design is 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.

Potential Features of Patent Protection in LCPs Area

In 1960s workers at Du Pon t started work on aromatic extended chain polyaramides that resu lted in the dev e lopment of hi g h streng th/ high modulus fibres having the trade name Kevlar®Iff.19. 24.25. The patents

that describe this Rroduct form the first of the kind on LCPs l 14. When a lyotropic so­lution of Kevlar® experiences shear in ex­trusion through a sp inn e ret hole , the polymer develops spontaneous orientation ofthe macromol ecules in the direction of the flow and th is formed the basic fi nding for the processing of LCPs. This has been revealed in ;mother patent to Du Pont21-25. It is inter­esting to note that the scientific research fi ndings were published by Du Pont only after 197715-19. Th e synth esis of some of the fi rst aromatic polyesters were described in patents issued to lCI Ltd, 26-28 and to Carbo­rundum Ltd29, but their liquid crystalline properties were not g iven due consideration at that time. The first patent on commercial

276 J INTELLEC PROP RIGHT'S, SEPTEMBER 1999

application of LCPs originated from Kuhfuss and Jackson who synthesized LC polyesters by reacting polyethyl ene terephthalate (PE1) with 4- hydroxybenzoic acid (HBA) ::IO­::I I

Commercialization of LCPs and their Patent Protection

Following these early patents, th ere was a flurry of a large number of patents for vari­ous LCPs, particularly LC polyesters. Fig. 1 providcs a range of liqu id clystal polymers developed by industlY and th e reference numb ers against th em in d icate th e pat­ents::l2-::I4

Most of the LCPs are prepared by an este r exchange reaction betwee n acetoxyalyl groups and the carboxylic acid groups with the elimination of acetic acid at tempera­tures above the transition temperature (rm) of the polyester. This polymerization tech­nique, however, is limi ted by the viscosity of the melt and this becomes severe as the I'm rises above 300 °C. In many cases, I'm are in that order and hence difficul ties are still rc­ported with th e processing of these poly­mers. In order to overcome this problem, ICI has reported in patent an aqueous disper­sion polymerization technique whereby an inert heat transfer medium such as a high

. d44 C . t temperature solvent IS use . OttlS e al. 45,46 and Duska et at. 47 have patented methods for the synthesis of high molecular weight aromatic polyesters. These methods have been useful in the synth esis of thermot­ropic polyesters.

Patents on Structural Modifications to Enhance Property

Properties of the LCPs can be influenced positively by introducing appropriate struc­tural modifications. Copolymerization is one of the best methods known to alter the tran-

sition temperatures of main chain LCPs. Substitution on the aromatic ring is another method . Calundann et at. 48 patente the effect of parallel offset sb'uctres such as a naphthalene moiety on the transition tem­peratures of the LC copolyesters. Jackson and Kuhfuss have noted that they could suc­cessfully make PET (polyethyl ene te re­phthalate liquid crystalline with 20% hydroxy benzoic acid (HBA) ::IO,::I I,4!) whereas when terephthalic acid was substituted with 2,6 naphthalene dicarboxylic acid in a NDA (2,6-naphthalene dicarboxylic acid) /HBA/ EG (ethylene g lycol) system, 35% HBCA was required for achievin g- liquid crystallin ity.

12 !5() Kwolek ,Cottis et ai. and Calundann et at.51 ,52 in separate patents showed that in­corporation of 4,4-biphenylene units in the polyester reduces th e transition tempera­ture by inb'oducing a torsional deformation in the polymer which disturbs the packing ability of the polyester chain . Calundann further showed that in trodu ction of 2,6-naphthalene moiety in the polyester chain reduces the transition temperature drasti­call1 2,::I::I, 5::1·55 . A series of thermotro pic co poly este r s ba sed on 4-hydroxy­biphenyI4'.carboxylic acid (HCBA), HBA ancl6-hydroxy-2-naphthoic ac id (HNA) were reported in a patent56. Through a series of pate nts from 1978-1981 Ca lund an na et at. 57,58 and Irwin59 showed that a thermot­ropic copolyester has a high I'm wh en a 2 6-dioxyanthraquinine moiety (2,6-AQ) is

, 60. present. Schaefgen studied LC polyesters of substituted hydroquinon e(HQ) deriva­tives with TA (MeHQ/ TA and CIHQ/ TA) and reported I'm below 400°C. When methyl group was substitu ted by ph enyl moiety PhH Q/ TA system the transition temBera­ture was brought down to 3460C , 1,62.

Similarly, HQ/ PhTA (4,4'-biphenylen e,lr~~­phthalate) system gave a I'm of 1256°C ' . Th e effects of various comonomers with at

PILLA!: LIQUID CRYSTALLINE POLYMERS: A REVIEW OF.. .. ... 277

Cdm", f-@-~~-q~J) ~~~)(g-@-g)~-o-O) ~~)(g-o-~)(o-o-NH)

Dupoot f-br-~-o-~)~*o-L-g) y y

x and y = H, CI, Br, CH3

z=~-<O>-o@- ~--<O>-©-Ph

f~-g-o-~)

E""","KOd~-o-~ (~-<Q)JOCH'C~O-)

~:0dum(o-\QKQ}°X~-o-~) f-o-~ (\gJ) ICI ~-<Q)-~ ~@-o) (\gJ) ~R-)

R = aromatic diol, carboxylic acid, hydroxy acid (less tha1l2%)

Fig. 1- LCPs developed by industries illustrating the strategies for controlling Tm

32-34

35-38

38

41

30,31

42

43

278 J INTELLEC PROP RIGHTS, SEPTEMI3EI~ 1999

least one comonomer having bulky susti­tutents on LC polymers were reported in a series of patents6~71. These patents form the basis of copolyesters based on BP(4,4'­biphenoJ), TA and HBA, MPBP(3-phenyl-4,4'-biphenol), DPBP (3,3'- bisphenylbiphe­nol) and TA or NDA, substituted HQ, PhHQ and TA, PhHQ, 2 (l-phenylethyl-HQ and TA, etc. In a series of patents from 1975-1978, Kleinschuster et ai. ~7.~8.72-74 showed that the homopolyester of 4,4'-OBBA and CIHQ forms a nematic melt from which fibres can be spun having a tenacity of 10-16 g/denier and a modolus of 170-230 g/ denier. In con­trast, the product of 4,4'-OBP with CITA has to be copolymerized with CIHQ and MeHQ to get liquid crystalline behaviou/4. Sev­eral copolyesters of 3,4'-OBBA (80 mole % and TA (20 mole %), HQ (40 mole %), BP (60 mole %) and 3,4'-OBBA have been reported . I' 7576 th tf . . 111 patent Iterature' a orm anIsotropIc melts at relatively low temperatures. The polyesters 3,4'are from HQ,3,4'-OBBA(70 mole %) and HQ, TA (30 mole%) and 3,4'­OBBA (80 mole % and TA (20 mole %) , HQ (40 mole %), BP (60 mole %) and 3,4'-OBBA and CIHQ, 4'-OBBA(30 mole %) and TA (70 mole%) which gave transition temperatures 341, 320, 300 and 285°C respectively. Sev­eral copolyesters of substituted HQs and 3,4'-BDA (4,4'- benzophenonedicarboxylic acid) and TA have been also reported in patent Iiterature78. Irwin et al. 79,80 prepared a unique th ermotropic polyester based on 3,4'-DHA and TA by melt polycondensation of using the diacetate method . 111e differ­ence is that this polyester exists in helical conformation in contrast to the other wholly aromatic polyesters. It has a mesophase of 75°C with a Tm of 285°e. Several other patents have appeared following the obser­vation that these polyesters have excellent physical and chemical properties81.85 0 156-159). Rogoro et al. 86 0138) showed that in-

troduction of alkoxy g roups in poly (p­phenylene terephthalate) or poly (oxyben­zoate) or poly(p- ph enylene terephtha­lamide) reduces the transition temperature to below 200°e. A series of polyesters based on 1,4- bis (3,5-dimethyl-4-hydroxybellzoyl­benzene 4,4'- BBA and TA were repor;:ed in a patentS7 that showed formation of anisot­ropic melts in the range 355-360°e. Vrie­sema and Miniaci re ported in anoth er patent88 reduction of the processing tem­perature of Xyd er when 5-50 mole % of 2,5-FDA was incorporated .

Patents filed in India

A few patents on LCPs have been filed in India based on work carried out in tLe Re­gional Research Laboratory, Thiruvanan­thapuram. One of the serious problems of the LCP industry is the high cost of the monomers. Two approaches were adopted ill this laboratory to tide over this probl em: (a) Use of low cost commercially viab le co­monomers: 4-hydroxyphe nyl acetic ac id (HPAA) and 4-hydroxyphenyl propionic acid (HPPA) are two comparatively cheap comonomers available in the market. Their terpolyesters might be of g reat value; and (b) Use of naturally available comOllomers with or without modiiication.

A new monomer 8-(3-hydroxyphenyl) oc­tanoic acid (HPOA) prepared by phase transfer catalyzed permanganate oxidation of cardanol derived from CNSL was found to be effective in reducing the transition tem­perature of polyparahyroxy benzoate. Th e process of synthesis of this monol11 r, its homopolyeste r and liquid crystall in e copolyester have been filed for patents89,91. The significance of this monomer is its ori­gin from natural material and hence its ex­tremely low cost and also possesses both a molecular 'kink' and a fl exible segment in its

PILlAI: LIQUID CRYSTALLINE POLYMERS: A REVIEW OF .. .... . 279

structure. Its copolyester with p-HBA [p-hy­droxybenzoate-co-hyd roxyphenyl oc­tanoate) 1 was liquid crystalline with nematic texture and gave a transition temperature of 256°C92. However, its thermal properties were not encouraging. Another polymer [poly (2-bromo-4-hydroxybenzoate-co-3-hy droxyphenyl octanoate) 1 containing a -Br group was synthesized. This polymer has a good range of nematic phase for processabil­ity studies. This polymer shows compara­tively good thermal stability.

Excellent melt processability was achieved for terpolyesters containing naphthalene moiety in their structures. The process for th preparation of the polyester is described in pate nts recently filed9:{,94. Thus, poly [naph thalen etereph thalate-co- (4-

oxyphenyl) propionate] gave a typical ne­matic phase with transition temperature at 240°C and a thermal stability (T5) of 400 °C.

A series of novel LC polymers containing azophenyl group was synthesized by per­forming a diazotization reaction between cardanol or pentadecyl cardanol and paraaminobenzoic acid and polymerization of the resulting monomer to get poly 4 [-4-hydroxy 2-pentadecyl phenyl) azo benzoic acid95. Azo based LC polymers 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 th UV range of 100-200 nm. The LC polymer prepared

Aq.KMn04

O Aliquat 336 .. 0 . ~ CH2C12 ~

HO C1sHn·n HO (CH.zhCOOH

Cardanol HPOA

HO-Q-COOH

-fO-<Q)-Jtr~A(CHcl)* Copolyester

Fig. 2- Synthesis of LC copolyester from HPOA

280 J INTELLEC PIWP RIGHTS, SEPTEMBER 1999

did not absorb in this region indicating pos­sibly that this polymer or related structures might show useful non-linear optical proper­ties. A series of related po~mers were syn­thesized and characterized 6,97. Two patents have been fil ed on th e process of synthesis of this polymer98,99

Current Interests and App1ications

The main applications of LCPs are in areas that exploit combinations of the key proper­ties such as strength, easy fl ow, excellent dimensional stability, th e ab ility to incorpo­rate high levels of fill ers and excellence of chemical resistance. The curren t interests and applications are su mmarized below: 1. In injection mould ed products in elecb-on­ics (eg. surface mount units, connectors , printing wiring boards etc. and computer field s, th e similarity in thermal expansion of metal and LCP is expected to result in good compo nent in tegrity and minim al strai n when components containing metals (eg. solder) and LCPs are in contact an d are subject to thermal cycling or shock. In addi­tion the low warpage and easy flow will allow precision moulding of complicated parts and th e 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 th e better chemical resistant and breaking sb·engths are of im­portance. 3. LCP blend s-(a) Addition of small amounts of LCPs to a conventional polymer improves flow, (b) blends with cheaper polymers to g ive LCP properties at lower costs (eg.LCP­nylon), and (c) blend s with other LCPs to produce better property profiles.

Th e LCPs possess a unique molecular mor­phology which in turn controls the micro and macro morphology giving rise to a prop­erty protile much superior to th e conven-

tional polymers. The molecular morphology is directly related to the polymer architec­ture which can be tailor-made to suit a par­ticular property profile lI sing both molecular design and process design aspects of control of polymer development. The exceptional balance of properties with the ability to tailor the properties of th e LCPs for a particular end-use is creating new opportunities for materials sc ientists and technologists. The present industrial scene is dominated by Amoco an d Hoechst- Celanese (see Ta Ie 1 fo r the names of ind ustries dealing with LCPs) . The industries still face a num ber of probl ems such as (i) the in tractability and insolubility of the aromatic homopolyesters an d homopolyamides, (ii) the high cost of the monomers, (iii) poor weld line stre gth of th e injection mould ed product, and (iv) poor strength in the transverse directioll .

Table 1- LCPs manufacturing compa­nies in ilie world

United Western Japan States Europe

Allied-sig- Al<so Asahi nal Darco ATO-Ch em Denki Kagaku Dupont BASF Idemitsu Eastman DSM Mitsubishi Amoco Hoescht- Mitsubushi Ga '

Celanese Co. Hocscht- ICI Mitsui Toats 1

Celanese Monte- Polyplastics cli son RJl0ne- Sumitomo Poulene

Teijin ToraY,Toyobo, Uni tika Kura ray

PILLAI: LIQUID CRYSTALLINE POLYMERS: A REVIEW Or. .... . . 281

The LCPs find applications in a number of areas such as fibres, films, plastics and res­ins, non-lin ear optics, information storage d evices, holography, chromatography, biomedical materials, etc. References 100-124 give the latest patents fil ed recently.

The Future of Uquid Cryst.:'l1 Polymers

Liquid crystal polymers offer tremendous potential for applications in both functional and high peformance polymers. The well known applications of LC rigid main chain polymers are in the aerospace and military departments as high peJiormance fibres, films, coatings and intricate machin e parts.The use of LCPs in electronic industry and in the chemical indusb-ies are expected to continu e. The main applications of side chain polymers, in particular silicone poly­mers, is in gas liquid chromatography. There are promising developments in ferro­electric side chain LC polymers to be used in display devices where the switching time is critical. Other potential applications for LCPs are in th e area of telecommunication as wage guides, NLO devices, optical win­dows, optical gratings etc. The felToelectric 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 have major applications particularly those connected with artificial organs and replace­me nts wh e re 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 is already picking up and is going to contribute to novel devel­opments. The future of LCPs are certainly bright owing to their unmatched perform-

ance. However, a steep growth depends on the availability of state-of-the-art technolo­gies for the production of speciality chemi­cals to feed the LCP industreis at a cost effective scale.

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284 J INTELLEC PROP RIGHTS, SEPTEMBER 1999

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90 Pillai C K S, Sherrington D C, Sneddon A, A process for the preparation of poly (l,3-phenyl octanoate) - a ho­mopolyester, Indian Pat 678/deI/92, dated 29 July 1992.

91 Pillai C KS, Sherrington D C, Sneddon A. A process for the preparation of poly(1 ,3-phenyl octanoate) a liquid c rysta llin e co po lyste r, Indian Pat 679/ del/ 92 (to Council of Scientific and Industrial Research, New Delhi), dated 29 July 1992.

92 Pillai C K S, Sherrington D C & Sned­don A, Polymer, 33 , 1992, 3968.

93 Prasad V S & Pillai C K S, A process for the preparation of melt processable liq­uid crystalline poly(4-phenylene tere­phthalate -co- oxyphenyl accetate/ propionate) s, poly (4 ,4'-biphenylene terephthalate -co-oxyphenyl acetate/ propionate)s, and poly(2 ,6-naphthyl e ne terephthalate -co ­oxyphenyl acetate/ propion ate)s, (patent applied in 1998).

94 Vijayanathan Veena, Prasad V S, Pillai C K S, A process for the preparation of melt processable liquid crystalline poly (4- phenylene naphthalene -2,6-carboxylate - co - 8 (3-oxyphenyl) oc­tanoate) s poly(4-phenylen e, 2-methoxynaphthalene-2,6- carboxy­late-co-8 (3-oxyphenyl) octanoate) s, poly(4-phenylene 2- phenyl naphtha­len e-2,6-ca rboxylat e -co- 8 (3 -

oxyphenyl) octanoate) s and related terpolyesters (patent applied in 1998).

95 Saminathan M, Pillai C K S & Pavithran C, A Process for the prepa­ration of 4-[ (4-hydroxy-2-pentadecyl phenyl) azo 1 benzoic acid, a bifunc­tional monom er, Indian Pat 12791/de1/92 (to Council of Scientific and In~u strial Research, New elhi) dated 31 December 1992.

96. Saminathan M, Pillai C K S & Pavithran C, A Process for the prepa­ration of Poly-4-[ (4-hydroxy-2-pen­tadecyl phenyl) azolbenzoic ac id, a bifunctional monomer, a processable liquidcrystalline homopolyester, In­dian Pat 12792/ de1/1992 (to Council of Scientific and Industrial Research, New Delhi) dated 31 December 1992) .

97 Saminathan M, Pillai C K S & Pavithra n C, Macromolecules, 26,1993, 7103.

98 Saminathan, M & Pillai C K S, Com-municated, Macromol ecules, 32, 1999.

99 Saminathan M & Pillai C K S, Po fyrner (Accepted) .

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