MATERIALS EXHIBITING LARGE NON LINEAR OPTICAL PROPERTIES

Abstract: Classes of materials suitable for the construction of devices which can be switched under the influence of an applied optical field (exploiting non linear susceptibility of the material) are described. Essentially such materials comprise a nematic polymer liquid crystal molecule which exhibits a very high degree of polar order for a given poling electric field, and comprises long rod-like monomers having a high non linear dielectric susceptibility which cooperate to act as one large dipole in the poling field. Both side chain (comb) and main chain (worm) polymer liquid crystals can be employed. A polymethacrylate with a nematic aromatic ester side chain is one class of a side chain polymer having the desired switching characteristic. Worm polymers having the desired switching characteristics are found inter alia among the aliphatic/aromatic polyimides.

Description: Title: Materials exhibiting large non linear optical properties Field of invention This invention concerns materials which exhibit a large dielectric change in the presence of a poling electric field and in particular materials from which high speed optical switching devices may be constructed. Background to the invention Material for high speed optical switching and processing devices needs to have: 1. a high non linear dielectric susceptibility. 2. low losses, and . " 3. very fast response times. In the first place the molecular structure of the material must lack centrosymmetry and poled organic liquids are a preferred medium. Characteristics 2 and 3 above are also more commonly to be found in molecular electronic systems rather than in inorganic dielectric materials. Using this starting point it has been appreciated that increased switching speeds can be achieved by better organisation of the media. Thus for a given value of the poling electric field a high degree of polar order (ie normally thought about in terms of dipoles) is desirable. Thus it has previously been proposed to use a nematic liquid crystal as a host for dye molecules or as the dye molecule itself, since a nematic material has a higher response to an applied (poling) electric field. It has also been proposed (see B V Levine and C G Bethea in Journal. Chem. Phys. _6_5, 1989 (1976)) to use a material known as PBLG having long rod-like molecules since dielectric susceptibility (ie dielectric change brought about by a given poling field) has been noted as being proportional to the length of the molecular structures. The advantage was exhibited for dilute systems of rod-like molecules but as far as can be ascertained the idea was never developed because high enough concentrations of molecules could not be manipulated by field. It has also been proposed to use side chain polymer liquid crystals since a nematic polymer combines the higher response to an applied field with the mechanical strength of polymers generally, and this is essentially the current state of the art. The example of PBLG has been quoted as an illustration of what is possible when elements of a chain are induced to act cooperatively. In this case it is intrinsic stiffness rather than a nematic field and defects that are responsible. It should be noted that the experiment of Levine and Bethea was performed in solution. At higher concentrations the dynamics of response of rigid rods to a poling field is expected to be very slow. Very stiff rods might also be expected to present fewer steric impediments to the anti-ferroelectric ordering expected ultimately in concentrated solution or melts. Anti-ferroelectric correlation is more difficult to sustain in semi-flexible main chain and comb systems because chains in contact at one point are unlikely to be so at other points. It is expected that the dynamics of the build up of the non linear optical response in response to the poling field will be same as the dynamics of the dielectric response. It is a general object of the present invention to provide a material from which very effective high speed optical switching devices may be constructed. It is one specific object of the present invention to provide such a material using an improved main chain polymer (a so-called "worm" polymer) liquid crystal material which exhibits very large dielectric and optical changes in the presence of an electric field, and equivalently, large non linear optical effects. It is another specific object of the present invention to provide such a material using an improved side chain polymer (a so-called "comb" polymer) liquid crystal material which exhibits very large dielectric and optical changes in the presence of an electric field, and . equivalently large non linear optical effects. Summary pf_ the invention According to the present invention a material suitable for the construction of a device which can be switched under the influence of an applied optical field (exploiting the non linear suceptibility of the material) comprises a nematic polymer liquid crystal "molecule having long rod¬ like monomer having a high non linear dielectric susceptibility which exhibits a very high degree of polar order for a given poling electric field, and cooperate to act as one large dipole in the poling field. One possible material considered was PBLG since the molecules making up this material are completely stretched out under suitable solvent conditions giving a non linear optical effect in the presence of an applied electric poling field. There are however, severe drawbacks and better materials can be envisaged. Preferably the liquid is in the form of a polymer melt (ie an undiluted polymer) so that optical density is high. Response times for such material are fast compared with poling times for nematic melts of rigid rods, because of internal freedom of molecules. The invention can be realised in the use of (1) side chain (comb) and (2) main chain (worm) polymer liquid crystals, - to create anomalously high linear and non linear dielectric responses. A nematic field can induce dipoles and non linear elements along a polymer to act cooperatively in both types of polymers. The structural requirements for each case are discussed later. The invention represents a departure from a logical development of current thinking regarding dipolar response since nematic order is quadrupolar (ie axial) in character. Where a side chain (a so-called comb) polymer is employed, the side chain rods must be attached to a polymer backbone in a stereo-regular manner. For the side chain polymers, any backbone having torsional semi-flexibility can be employed. Examples of a backbone component are polymethacrylate, polymethylmethacrylate and related materials in the same general family, and polysiloxane (often substituted, for example poly(di)methylsiloxane) . Various techniques are well known for the synthesis of such materials so as to achieve varying degrees of stereo- regularity (ie tacticity of functional groups attached to the backbone). Such methods include the selection of solvents of suitable polarity when polymerisation is performed and for example anionic polymerisation leading to polydienes. The selection of material to provide the rod side chains for attachment to the backbone component requires that on ' board dipoles are always pointing towards or are always pointing away from the point of attachment of the rod to the backbone. Most simple molecular rods in common use as esogens trivially satisfy both of these requirements, ie their ends are chemically differentiated (thus ensuring they will always attach one way and not the other) and there is an on board component of the dipole moment (as proven by the existence of a lower frequency component of the dielectric relaxation due to the reversal of rod direction in a nematic). Thus any classical aromatic ester (and their variants) may be employed and an example is given in formula drawing I. One family of components from which examples can be selected which exhibit the switching characteristic of the invention under the influence of an applied optical field comprises a polymethacrylate with a nematic aromatic ester side chain. The invention thus proposes that a particular molecular geometry in comb polymer liquid crystals coupled to a strong nematic order can provide a mechanism by which strong "dipolar (dielectric) effects can be observed in a. quadrupolar medium. This is the analogue of the hair pin mechanism proposed later for main chain polymers. A defect mediated response leads to static dielectric susceptibilities of an activated form which become very large, eventually scaling like the molecular weight. The dynamical response to a poling field has been calculated and turns out to be ultimately slow, scaling like the square of the molecular weight when defect diffusion over the length of the chain is the dominant process. This is still exponentially faster than that expected when rods (eg PBLG) are rotated bodily in a nematic field. Put another way nematically and smectically ordered teeth of comb polymers are known to be coupled to the backbone and the invention requires that this coupling be torsional as well so that dipolar corrections can be induced in addition to quadrupolar order. This allows an activated dielectric response to be obtained, mirroring the creation of torsional defects in the tooth order. The susceptibility becomes large, eventually proportional to the degree of polymerisation. A model for the diffusional dynamics of these defects can be calculated exactly and given an exponential variation of the dielectric relation time with temperature. At low temperatures this relaxation time ultimately varies as the square of the degree of polymerisation. In the case of worm polymers it is important that the rod¬ like monomers are sequentially connected with spaces to give an overall semi-flexibility, and there must be an appropriate net monomer dipole moment. Put another way, in a worm polymer there must be: (1) a semi-flexible connection between the stiff (mesogenic) cores . sequentially polymerised to form the backbone, (2) a net on board dipole moment which always points in the same direction from monomer to monomer, along the sequence of the chain, and - (3) an onboard non linear dielectric element which always points in the same direction from monomer to monomer along the sequence of the chain. Requirement (1) is fulfilled by the sequential attachment of aliphatic/aromatic imides, amides, esters and their combinations. Aliphatic content is seen for instance in certain mesogenic polymers. Dipole direction (requirement (2)) is present in p- phenylene polya ide and introduction of aliphatic character (for flexibility) in molecules of such inarerials provides the characteristics required. One example is an aliphatic/aromatic polyimide. Rapid changes of chain direction with respect to the ordering direction, also change the dipole direction for a worm with stereo regularity. Just as for combs with defects, the overall response grows as the defects become scarcer, ultimately being proportional to the molecular weight. The dipolar response (say to a poling field) causes a similar cooperativity in the non linear optical elements and hence also a very large non linear optical change. In summary for both "side chain" and "worm" types, organic molecules with a donor and acceptor widely separated by a conjugated electron system would appear to be very effective, fast and low-loss non linear elements. Their geometry is often cylindrical which makes them nematic or smectic, or at least able to couple strongly to a nematic solvent. Nematics also present an efficient environment in which to apply a poling field to remove the macroscopic centre of symmetry, an essential step for achieving a non linear optical effect. Polymer liquid crystals have been recognised as offering still greater advantages since they can form films and coatings of good mechanical strength and typically have glassy phases which have the effect of poling. Additionally, sometimes the small solubility of non linear optical molecules in nematic solvents can be overcome by using nematic comb polymers not as solvents but making the non linear optical elements themselves the teeth of the comb. A material having the properties of the present invention is capable of anomalously large linear and non linear dielectric response and the invention also provides a mechanism for its rapid achievement. The non linear optical response can be several hundred times that of existing materials. The invention will now be described in more detail partly in relation to the background theory of materials which may demonstrate very large non linear optical responses and subsequently to specific examples of such materials with reference to the accompanying drawings. Reference will In the drawings Figure .1 illustrates the nematic and smectic phases of comb polymers; Figure 2 illustrates isotactic attachments to a backbone; Figure 3 illustrates the resulting defect of width b resulting from competition between a nematic field and a torsional force resisting rotation perpendicular to the backbone; Figure 4 further illustrates the positions of defects in which tooth dipoles are denoted by arrows pointing away from the main chain; Figures 5(a) and 5(d), 5(b) and 5(e) and 5(c) and 5..(f) , illustrate comb configurations and their mirror images; Figures 6(a)-(c) further illustrate comb defects; Figure 7 illustrates a nematic worm polymer. In the formula drawings: Sheet 1 contains the formula I of an example of an aromatic ester whilst II and III are examples of materials possessing certain characteristics required in a worm polymer before it will satisfy the requirements of the invention, and Sheet 2 contains formulae IV and V and illustrates an effective non-linear dielectric element and a worm polymer. which exhibits very large dielectric and optical changes in the presence of an electric field, and equivalently large non linear optical effects. Sheet 3 shows the synthesis scheme for producing polymer V. Nematogenic rods, connected together as th-e teeth of a polymer comb, form nematic phases in competition with whatever nematic character the polymer backbone itself might have. Neutron scattering experiments and investigations of nematic networks demonstrate conclusively that there is strong coupling between the teeth and backbone of the comb. The present invention is concerned with the defects in the ordering of teeth along the backbone when the polymer is in a particular nematic phase or in a smectic phase. To initially fix ideas, it is assumed that the teeth have a positive nematic order and the backbone has a much weaker nematic strength and is attached to the teeth by hinges with a perpendicular tendency, and thus has a negative nematic order. This phase was first investigated by Kirste and Ohm and by Keller et al who found the backbone confined toward the plane normal to the director. The corresponding s ectic ordering of the teeth confines the backbone more strongly, eventually leading to hops between planes. This nematic phase, N-^, and the corresponding smectic phase of combs is shown in Figure 1. In this figure. Figure A shows an -^ phase with main chain forced to explore directions in or close to the plane perpendicular to the ordering director. Torsional energy is sufficiently low that dipolar correlations between teeth are short ranged. In Figure IB smectic combs with "smectic planes" are shown dotted in and the chain is strongly confined but the teeth have no dipolar correlations. Studies hitherto have been concerned with the nematic order, with teeth and chain backbone configurations and with the connection between these properties. This connection is the additional subtlety of polymer liquid crystals since mesogenic order restricts the drive to maximal entropy usual for polymers. The sketch of the phases in Figure 1 shows the teeth order ne atically and, consistent with the quadrupolar type of ordering, no regard is paid to up or down (dipolar order). This disorder represents a torsional entropy along the backbone and, for molecules with a suitable chemical structure, this entropy can be frozen out at lower temperatures. In backbone polymers, de Gennes has proposed "hairpins" as the molecular defect recovering entropy in the main chain nematic case. They have been discussed in the context of entropy and the current context of dielectric response, (Gunn & Warner) . Layer hops in a smectic phase are also . an attempt to recover some of the entropy lost in confinement toward a plane (Renz and Warner). Although there is no consequence for spatial configuration of a polymer from torsional defects, as there is for hairpins and layer hops, there is an. extreme consequence for the low frequency dielectric response as a dipolar correlation within one chain can result, interrupted only by the torsional equivalent of hairpins or layer hops ( ie ' an abrupt change from an up to down arrangement of . teeth) . Figure 2(a) shows -.teeth with an isotactic attachment to the backbone. Thus in Figure 2(a) isotactic nematic comb polymers are shown without defects, ie all teeth are on the one side. In Figure 2(b) there is shown a torsionally defected configuration of isotactic combs and in Figure 2(c) there is shown an isotactic comb molecule with a backbone consisting of C-C linkages and one repeat with n units is shown, the substituent y 2 being the tooth. When the tooth dipole has a component always pointing either toward or away from the backbone, then such defects represent large changes of energy of the polymer with respect to an external electric field E. This is because all the dipoles between two defects point the same way as each other and act as one large dipole in the external field. The specific response of a system of dipoles grows linearly with the dipole magnitude and hence as thermally generated defects became scarcer, so the response of the polymer liquid crystal grows, eventually being proportional to the molecular weight. Cooperative action of dipoles is followed by the like cooperative action of non linear optical elements (donor/acceptor pairs) and hence the response of the material to changes in the applied electric field becomes very large. Experimental results have allowed the conditions for such defects to be described, their energy and hence probability of occurrence to be calculated and the static dielectric response and the non linear optical response to be found, both of which turn out to be .extremely large (eventually going as the degree of polymerisation). It - has also been possible to produce a model of diffusion of defects for the dynamics of such combs. The dynamics allow the low frequency dielectric response to be found. Mapping onto the dynamics of a 1-D Ising modei allows ann exact solution to the time evolution (Warner and Liu) Principal results are: (1) a large dielectric susceptibility ultimately scaling like the degree of polymerisation, and displaying an activated form betraying the mechanism via torsional defects, and (2) long relaxation times for dielectric response characteristic and diffusion of defects along the chain in three regions. The attachment of teeth has a flexibility that depends on the length of the hinge unit. The term hinge is used synonomously with the term attachment spacer. If the hinge is too small, nematic order is entirely destroyed. If it is very long, teeth and backbone are essentially decoupled. An intermediate case is envisaged corresponding to recorded observations of the phases where side chain order is sufficiently coupled to the backbone to at least partially confine the backbone, either to a plane perpendicular to the nematic director or within the smectic layers. However the hinge is sufficiently strong that any nematic tendency of the backbone is insufficient to cause the teeth and backbone to be parallel. The stereo chemistry of where the teeth are attached to the backbone is of vital importance to computation of the dielectric response. Imagine for simplicity a backbone consisting of C-C linkages in the form [-(CX-^)-(CX 2 Y 2 )—(CX n Y n ) ' -] N , where there are n-(CXY)-units in a monomer and one of the Xl-,...N„n or Yi 1 ...Yn, is a tooth. The all trans- configuration of this is sketched in Figure 2(c) with Y 2 the tooth (mesogenic side chain) . , In this example it is also assumed that the substituents X^ and Y- are such that this all-trans configuration is of lowest energy. Inspection of a simple model shows that, for consecutive teeth to be parallel in all-trans configuration, attachments have to be made at every second (or multiple of two) carbon atom, that is n=2,4... with stereo-regularity in between. To twist two consecutive teeth from an inititally parallel state to an antiparallel state requires the bonds between successive carbon atoms to rotate from 0° up to around 60°. ithin this extent the rotational potential of these bonds increases continuously to its maximum. These angles essentially determine the rotational potential of the bonds. Thus the total rotation potential of two relatively twisted teeth, which is determined by the sum of potentials corresponding to these bonds connected them, increases to a maximal value when these teeth are antiparallel. This suggests that, although this process appears to be torsional, it is not. Continuing to twist the teeth can only reduce the relative angles between the bonds and further rotation about the backbone in the same direction until the total angle is 360°, returns the backbone to the low energy all- trans configuration and * a rotation of 360° is equivalent to 0°. Two rotations of 180° have no net effect and two such states, on meeting ' *, annihilate each other, independently . of the sense in which they are initially rotated, ie + or - 180°. Such processes can loosely be described as torsional since a twist between teeth costs energy made up. of distortions of bonds away from the minimum of their rotational energy states, and a less sharp twist will cost less energy and in general this can be modelled by a torsional energy proportional to the square of the rate of change of angle of teeth with angle, varying from 0 to 180°. The points of attachment along the backbone should not be so widely separated that it is easy for the backbone twist, leaving the torsional states of successive teeth essentially randomly up or down. Since torsion and bend moduli for a backbone have the same stereo-chemical- origin, it seems likely that given bend is sufficiently penalised that a chain can be directionally confined by attached teeth (in N-, phases), this restriction on torsional freedom can also be realised. More complicated is the problem that existing synthetic routes produce combs with no clearly defined tacticity. Clearly the above requirements need molecules produced by highly stereo-specific polymerisation methods, particularly those which produce macromolecules with a high degree of isotacticity (ideally 100%). In principle the methods developed for stereo-specific polymerisation of acrylates and methacrylates could be used. As temperature T is reduced in the nematic phase, the order of the teeth approaches 1. " Thus the angle which a rod makes with the direction n is restricted to values close to 0 or 180°. In a nematic mean field small excursions from either pole, are equally likely in each azimuth'al direction, but when considering the transition from one pole to the other, this is most economically achieved by motion in a plane perpendicular to the local backbone tangent, as sketched in Figure 3 which represents a continuous representation of a defect of : width b from the competition between the nematic field and the torsional force resisting rotation perpendicular to the backbone. Q is the angle between tooth and n. The angle of inclination of the rods denotes the torsion of the backbone in this plane but in general the rods explore orientations with two Euler angles. In the presence of bend, torsion of the backbone actually has a more complicated definition. At low temperatures the arc length separating defects becomes large. This means that if a field is applied along the direction it couples strongly to a defect, since it represents a large section of chain with its dipoles point along or against the field. Low temperatures also imply that: (1) the characteristic region of transition in a defect is much less than the distance between them, and hence (2) defects mostly have a clearly defined identity of their own. This in turn suggests using the position of the defects as a coordinate to describe the dielectric response (see Figure 4) . Figure 4 shows the coordinate system of defects as 0 width. E is the applied field and s-,, s 2 ... are the positions of defects. Tooth dipoles are denoted by arrows pointing away from the -main chain. Figure 5 illustrates configurations and their mirror - images, equal in pair in energy as far as defects are concerned but different in their electrical energies and thereby biased in number by the field. One limited form of the motion of comb nematics will be considered, ie the dynamics of the teeth at low temperatures where the nematic order is strong. The flipping of teeth is not expected to have a strong influence on the spatial motion of the chain, but it will be the ■ relaxation relevant to describe the time dependent aspects of the unusual dielectric response developed in the previous section. In the first place dynamics can be considered by considering the diffusional motion of the torsional defects since it is their motion which leaves a trail of flipped dipoles behind. It has previously been suggested that at low temperatures they form a natural set of coordinates for thermodynamics. It seems they are also natural for dynamics. This model can be likened to the dynamics of a one-dimensional Ising chain. At low temperatures all teeth are essentially up or down, such regions of one polarity being connected with one of the other polarity by defects. At low enough temperatures (high nematic order) there are few defects and their width, which is independent of temperature, is small compared with their separation. This allows defects to be considered as separate entities. For a defect to progress an elementary process such as a tooth flip must occur as shown in Figure 6(a). Figure 6(a) shows a defect advancing one step by an elementary flip (time 1/a). Figure 6(b) shows how the meeting of two defects annihilates the section of the given polarity between them Figure 6(c) shows the generation of a defect pair creates an island of down in a sea of up. If this process occurs at a rate of N per second, then in a time t there will be Nt steps. Since the steps are equally likely in both directions the defect can be thought of as moving randomly. The problem is how the chains evolve to a state where configurations are biased by the applied field toward their energetically more favourable mirror images. Using these models mathematical analysis can be applied to demonstrate the diffusion response and the movement of the defects. As applied to main chain (worm) polymers the invention will be described in relation to Formulae drawings II and III. As previously mentioned, it is necessary for their to be: (1) a semi-flexible connection between the stiff (mesogenic) cores sequentially polymerised to form the backbone, (2) a net on board dipole moment which always points in the same direction from monomer to monomer, along the sequence of the chain, and (3) an onboard non linear dielectric element which always points in the same direction from monomer to monomer along the sequence of the chain. Requirement (1) is fulfilled by the sequential attachment of aliphatic/aromatic imides, amides, esters and their combinations. Aliphatic content is seen for instance in the mesogenic polymer given in formula drawing II. Requirement (2) is present in the p-phenylene polya ide given in formula drawing III. The introduction of aliphatic character (for flexibility) in molecules of the general families mentioned above, as for example in the polymer of formula drawing II, provides: all these characteristics (1), (2) and (3) listed above. A material possessing both requirements (1) and (2) comprises an aliphatic/aromatic polyimide. Effective non linear elements are created by the -positioning of J)onor Acceptor pairs along the polymer "backbone". D and A pairs separated by conjugated bridges are most suitable, the classic example being dimethyla ino-nitrostilbene (commonly referred to as DANS), formula IV (a). Formula IV(a) shows a conjugated bridge with the D_ and A marked. Other common I) and;A units are listed in table IV(b). A polymer having I) and A pairs suitably separated and directed along the backbone is shown in drawing V, and this material exhibits all of the requirements (1) to (3) above. The material shown in Formula V may be synthesised according to the scheme shown in Formula sheet -3. This involves successive synthesis steps as shown. The first synthesis step of material (A) was performed as follows: Sodium metal (0.23g, O.Olmol) and dry ethanol (5cm ) were mixed in a dry, 25cm flask equipped with double surface condenser. After the sodium had completely dissolved a solution of 4-hydroxybenzylcyanine (1.33g, O.Olmol) in absolute ethanol (5cm ) was added with stirring followed by 11-bromoundeca-l-ol (3.76g, 0.015mol) . Once the addition was complete the reaction mixture was refluxed for 6hrs. Ethanol was removed by rotary evaporation, ether (50cm ) was added to the residue and the resulting solution was washed with 10% aqueous sodium hydroxide 3 solution (3x5cm ), aqueous sulphuric acid solution (1M, 3 3 3x5cm ) and water 3x10cm . Evaporation of the dried (magnesium sulphate) ether solution gave the crude product which was purified by passing through a column of silica 3 (6cmx2cm ) using ether/ethyl acetate (7:3) as the eluent to give (2.03g, 0.0067m l, 69%) of a white crystalline solid, m.p. 79C; found C,75.01;H,-9.55;N,4,46%;C ' 19 H 29 N0 2 requires C,75.25;H,9.57;M,4.62% M + (πιass spec ) 303. . v2239cm ~1 (C=N). 1 H-NMR(CDCI 3 ) , [ppm w.r.t. internal TMS] 7.3(d,2H); 6.9(d,2.H), 4.0(t,2H); 3.8(s,2H); 3.7(t,2H); 2.2(s,lH) ; l,4(b,18H) . The second synthesis step of material (B) was performed as follows: 4-Carboxybenzaldehyde (2g, 0.013mol) was added to a solution of sodium (0.6g, 0.026mol) in absolute ethanol 3 (25cm ). (4g, 0.013mol) was added and the resulting mixture was stirred (magnetic follower) under a nitrogen atmosphere for 3 days. The mixture was poured into aqueous hydrochloric acid solution (0.1M, 200cm ) and the solid which precipitated was recovered by filtration, 3 wwashed with acetone (5x50cm ) and dried in vaccuo (20C/10 2mbar) to give another white crystalline solid (4.32g, O.Olmol, 72%). This white crystalline solid which.is found to melt into a liquid crystalline phase at 127c, found C,74.48; H,7.76; N,2.95% C 27 H 33 N0 4 requires C,74.48 H,7.58; N,3.20%. v 2215cm ~ ~~ (C=N) ; vl685cm _1 (C=0) ; v3600- 2200cm ~ (H-bonding). 1 H-NMR (acetone-d g ) , [ppm w.r.t. internal TMS] 8.1(d,2H); 8.0(d,2H); 7.9(s,lH); 7.7(d,2H); 7.3(S,1H); 7.1(d,2H); 4.1(t,2H); 3.5(t,2H); 3.3(s,lH), 1.9(b,18H). The purity of this compound was checked by HPLC. A single narrow peak was displayed. The final synthesis step to produce the desired final material (C) was performed as follows: A solution of the product of the second synthesis (product B), (4.35g, O.OlOmol), N,N-dicyclohexylcarbo-dimide (2.06g, O.OlOmol) and 4-pyrrolidinopyridine (0.015g, O.OOOlmol) in dried dichloromethane (20cm ) was refluxed . for 24 hrs under a nitrogen atmosphere. The mixture was filtered, washed with acetone (5x100cm ) and dried under _2 vaccuo (20C/10 mbar) to give a pale yellow solid (product (C)).- Found C,77.89; H,7.34; N,3.56% (C 27 H 3 *, 0 3 ) n requires C,77.69; H,7.43; N,3.36%. V2210cm ~ (C=N); vl710cm (C=0). 1 H-NMR(CDCI 3 ) , [ppm w.r.t. interal TMS] 8.1(d,2H); 7.6(d,2H); 7.4(s,lH); 6.9(d,2H); 4.3(t,2H); 4.0(t,2H); 1.2(b,18H). Rapid changes of chain direction with respect to the ordinary direction, also change the dipole direction for a worm with stereo regularity. Just as for combs with defects, the overall response grows as the defects become scarcer, ultimately being proportional to the molecular weight. The dipolar response (say to a poling field) is matched by a similar cooperativity in the non linear optical elements and hence also a very large non linear optical response becomes possible. A sketch of a nematic worm polymer, with dipoles along its backbone and a net overall dipole moment, which determines the material response is shown in Figure 7. Thus in Figure 7 thermally induced hairpins are denoted by circular dots along the chain at positins s*,, s 2 etc. The * random walk in the transverse direction caused by thermal undulations is also evident. The dipole direction on each monomer is denoted by an arrow and the external field E points along the director n, also the z axis. The field is favourable to the section (0, s^), unfavourable to (s- , s 2 ) etc and thus biases hairpin conformations. The net dipole moment associated with the conformation shown in (a) is shown at (b). It is proportional to the net z dimension, the oppositely directed sections of chain between hairpins having a nugatory contribution. This is the moment that helps determine the response of the material to the field. In summary for both worm and comb types of polymers, organic molecules with a donor and acceptor widely separated by a conjugated electron system would appear to be very effective as fast, low-loss, non linear elements. Their geometry is often cylindrical which makes them nematic or smectic, or at least able to couple strongly to a nematic solvent. Nematics also present an efficient environment in which to apply a poling field to remove the macroscopic centre of symmetry, an essential step for achieving a non linear optical effect. Polymer liquid crystals have been recognised as offering still greater advantages since they can form films and coatings of good mechanical strength and typically have glassy phases which have the effect of stabilising poling. Additionally, sometimes the small solubility of non linear optical molecules in nematic solvents can be overcome by using nematic comb polymers not as solvents but making the non linear optical elements themselves the teeth of the comb. Low temperature techniques have been used in the literature to find the response of a hairpin gas in main chain nematics. If the dipole moment of a monomer, that is of a tooth, is d and the space between teeth along the backbone is g, then the dipole moment per unit length of the main chain is d/q. The energy of the polymer in an electric field E directed along _n is: V n =-[s 1 -(s 2 -s 1 )+(s 3 -s 2 )+... (+) L-s n ) ]d.E The field acts parallel to the dipoles in the interval and so on. The defect positions are given by s*^ . . . s r .. The final sign in V_ is + or - according to whether the number of defects is even or odd. L is the total length of the chain. A sequence starting the other way, that is anti-parallel in (0,S*L) etc can be obtained by changing the sign of E . A material having the properties of the present invention is capable of anomalously large linear and non linear dielectric response and the invention also provides a mechanism for its rapid achievement. The non linear optical response can be several hundred times that of existing materials, and is thus eminently suitable for the construction of high speed optical switching devices.

Claims: CLAIMS 1. A material suitable for the construction of a device which can be switched under the influence of an applied optical field (exploiting the non linear susceptibility of the material) comprises a nematic polymer liquid crystal molecule which exhibits a very high degree of polar order for a given poling electric field, and comprises long rod¬ like monomers having a high non linear dielectric susceptibility, and cooperate to act as one large dipole in the poling field. 2. A material as claimed in claim 1, wherein the liquid is in the form of a polymer melt so that optical density is high. 3. A material as claimed in claim 1 or 2 comprising a side chain (comb) polymer liquid crystal. 4. A material as claimed in claim 1 or 2 comprising a main chain (worm) polymer liquid crystal. 5. A material as claimed in any of claims 1 to 4, when influenced by a nematic field to induce dipoles and non linear elements along a polymer. 6. A material as described in claim 3, wherein the side chain rods are attached to a polymer backbone in a stereo- regular manner. 7. A material as claimed in claim 6 in which the backbone ~ . - 2C- - has torsional semi-flexibility. 8. A material as claimed in claim 3, wherein the backbone component is one of polymethacrylate, polymethylmethacrylate and related materials in the same general family, polysiloxane or poly(di)methylsiloxane. 9. A material as claimed in claim 3, wherein material for attaching the rod side chains to the backbone component is selected so that on board dipoles are always pointing towards or are always pointing away from the point of attachment of the rod to the backbone. Most simple molecular rods in common use as mesogens trivially satisfy both of these requirements, ie their ends are chemically - differentiated (thus ensuring they will always attach one way and not the other) and there is an on board component of the dipole moment (as proven by the existence of a lower frequency component of the dielectric relaxation due to the reversal of rod direction in a nematic) . Thus any classical aromatic ester (and their variants) may be employed and an example is given in formula drawing I. 10. A material as claimed in claim 3, comprising a polymethacrylate with a nematic aromatic ester side chain. 11. A material as claimed in claim 3, containing nematically ordered teeth in a comb polymer and a polymer backbone to which they are torsionally coupled so that dipolar corrections can be induced in addition to quadrupolar order, to allow an activated dielectric, response to be obtained, mirroring the creation of torsional defects in the tooth order, and thereby create a large susceptibility. 12. A material as claimed in claim 3, containing smectically ordered teeth in a comb polymer and a polymer backbone to which they are torsionally coupled so that dipolar corrections can be induced in addition to quadrupolar order, to allow an activated dielectric response to be obtained, mirroring the creation of torsional defects in the tooth order, and thereby create a large susceptibility. 13. A material as claimed in claim 4, wherein the' rod¬ like monomers are sequentially connected with spacers to give an overall semi-flexibility, and there is- an appropriate net monomer dipole moment. 14. A material as claimed in claim 4, wherein there is (1) a semi-flexible connection between the stiff (mesogenic) cores sequentially polymerised to form the backbone, (2) a net on board dipole moment which always points in the same direction from monomer to monomer, along the sequence of the chain, and (3) an onboard non linear dielectric element which always points in the same direction from monomer to monomer along the sequence of the chain. 15. A material as claimed in claim 14 wherein item (1) is fulfilled by the sequential attachment of aliphatic/aromatic imides, amides, esters and their combinations . 16. A material as claimed in claim 4 comprising an aliphatic/aromatic polyimide. 17. A material as claimed in claim 4, wherein non linear elements are created by the positioning of I)onor Acceptor pairs along the polymer "backbone" so that the on board non-linear dielectric elements always point in the same direction from monomer to monomer, along the sequence of the chain. 18. A material as claimed in claim 17 to which the D and D pairs are separated by conjugated bridges for example dimethylamino-nitrostilbene. 19. A material as claimed in claim 4 having the general formula as shown in formula V. 20. A material as claimed in claim 19 synthesised according to the scheme shown in Formula sheet 3. 21. A material as claimed in claim 1, formed from organic molecules with a donor and acceptor widely separated by a conjugated electron system so as to provide effective, fast and low-loss non linear elements. 22. A high speed optical switching device constructed from a material as claimed in any of the preceding claims. 23. Methods of making a material as claimed in claim 1 substantially as herein described.

Inventors: WARNER MARK, WARNER, MARK
Applicants: INTELLPROP CORP, WARNER MARK, INTELLPROP CORPORATION, WARNER, MARK
Application year: 1989
Main classification: G02F1
IPC codes: C09K9, C09K19, G02F1