J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3331-3333 X-Ray Investigations on Liquid-crystalline Copolysiloxanes for Second-harmonic Generation Erik Wischerhoff and Rudolph Zentel lnstitut fur Organische Chemie , Jd-Becher- Weg 18-20, Johannes-GutenbergUniversitat, 0-55099 Mainz,Germany Ha rtmut Fischer*t H. H. Wills Physics Laboratory, Royal Fort, TyndalI Avenue, Bristol, UK BS8 ITL Ferroelectric liquid-crystalline copolymers of chiral mesogens and non-chiral comonomers have been described and their mesophase properties have been investigated by X-ray diffraction. Tilted smectic mesophases are obtained for a chromophore content of up to 50 mol%. Liquid-crystalline materials exhibiting a chiral smectic C* phase and some other tilted smectic phases with chirality are known to have a helical structure.However, this helix can be unwound by surface effects and electric and magnetic fields. Thus a macroscopic polar structure with ferroelectric proper- ties is obtained.'*2 There is a wide variety of ferroelectric liquid-crystalline compounds, some having low molar mass and others which are p~lymeric.~-~ These materials have attracted interest for display applications and for their piezoelectric properties, in the case of crosslinked systems.' The polar structure in the helix-unwound state can also be useful for second-order non-linear optical effects like second- harmonic generation (SHG), since these effects rely on a non- centrosymmetric structure in the bulk. Compared with poled polymers, which are often used for this purpose, chiral smectic C* materials have the advantage of a stable polar structure.In contrast to crystals, polymeric liquid crystals have the advantage of being liquid-like, so they can be pro- cessed into any required shape. The potential of chiral smectic C* compounds for non- linear optics was recognised several years ago,* but the first experiments produced disappointing results. Although they showed that SHG was possible, the eficiency was rather poor (d values of about 0.01 pm V-'). This problem was caused by the chemical constitution of common liquid-crystalline materials. In order to obtain second-order non-linear suscep- tibilities in organic materials, long electron-donor-acceptor n systems are needed on the molecular level.However, in most cases liquid crystals do not possess strong donors and accep- tors fixed to their aromatic core in the direction of their polar axis, which is lateral to the long axis of the molecules. It was our attempt to combine the good polar order of the chiral smectic C* phase with a suitable molecular structure for SHG by preparing copolymers of mesogens with a strong tendency to exhibit an S,* phase and chromophores with a donor-acceptor .n system lateral to their polar axis (NLO chromophores). The synthetic route to these materials and first non-linear optical (NLO) experiments are reported else- where.g The intensity of the second-haimonic signal in the sz phase is roughly ten times as high as in the isotropic phase, clearly demonstrating the influence of the polar structure on this effect.We present a detailed X-ray investigation of the phase properties of a selected copolymer system with regard to their dependence on the amount of NLO chromophore incorpor- t Present address: Dept. of Chem. Eng., Polymer Chemistry and Technology, Technische Universiteit Eindhoven, PO Box 5 13, 5600 MB, Eindhoven, The Netherlands. ated. The influence of the NLO chromophores on the phase behaviour of a material is of outstanding importance to its application. Second-harmonic generation can be expected only if the S,* phase is not disturbed by the lateral substit- uents of the chromophores. Materials The copolysiloxanes described here have the general formula shown below and were prepared according to ref.9. All materials are statistical copolymers. (X+ Y):Z=1:2,7:la-lf;(X+ Y):Z= 1:1:2a The compositions and molar masses of the materials are given in Table 1. Table 1 Compositions and molar masses polymer X Y molar mass (GPC) ~~~~~~ ~ ~ ~ la 100 0 27 400 lb 70 30 28 O00 lc 50 50 28 220 Id 30 70 28 800 If 20 80 28 750 2a 70 30 21 700 ~~~ ~~~ The GPC values represent the peak maximum of the GPC curve. Polystyrene standards were used as reference. Structural Investigations X-Ray studies were carried out using an Elliot GX 21 rotating anode generator with a copper target equipped with a flat graphite crystal monochromator and a collimator comprising two pairs of crossed slits.The detection device was a Siemens X-1000 area detector coupled with a PC (GADDS software) for collecting, analysing and storing X-ray images. Tem- perature control of the samples was maintained with a LINKAM hot stage. Samples of the copolysiloxanes were sealed in glass capillaries for temperature-dependent studies. Oriented samples were prepared by pulling fibres out of the anisotropic melt. Fig. 1 shows the X-ray diffractograms of the copolymers lb and Id. All the X-ray diffraction patterns show very similar reflections, as described by Poths and Zentel," and may be interpreted in a similar way. In the X-ray pictures of oriented samples the occurrence of microphase separation' '9' between layers of the polysiloxane main chain and the meso- genic side group can be observed clearly. The reflections cor- responding to the mesogen layers are marked ML and the reflections corresponding to the polysiloxane main chain are marked SOL in Fig.1. In both X-ray patterns, an orienta- tion of the mesogen layer is clearly visible, however, the poly- siloxane layer at 7.5 8, remains unoriented. Thus, a phase separation as found by Diele et d." and Sutherland et a1.12 must occur. X-Ray diffraction studies on copolysiloxanes with different amounts of dimethyl siloxane units in the main chain show similar results, as described by Poths and Zentel." When studying ferroelectric LC polysiloxanes without chromophores they have found an increase of the layer thickness of the smectic layers of about 4 8, per inserted dimethyl siloxane unit.This value is in good agreement with Diele's studies and theoretical predictions deduced from atomic dimensions (3.78 A). In our investigation, similar values are observed. The phase structure is not changed with a change in the ratio of functional units to dimethyl siloxane units. This is also an indication of microphase separation. Since the mesogen layers are well separated from the poly- siloxane layers, no change in the phase behaviour is to be expected. The microphase separation instead favours the for- mation of smectic layers. The change from tilted to orthogonal smectic phases can be observed in the X-ray diffractograms by the change of the angle between small-angle reflections with chromophore con- tents.For 2 70 mol% of chromophore, the intensity maxima of the two reflections are 90" apart, indicating orthogonal smectic mesophases. Table 2 gives the mesophases and structural data of the investigated copolymers. There is a significant change at 80 mol% of chromophore. Materials with lower amounts of chromophore show a layer structure and higher ordered smectic phases below their Sz or S, phase. The material with the highest chromophore content only has a smectic A phase; the layer spacing can only be interpreted by assuming an ML ML Fig. 1 X-ray diffractions of the coploymers Ib and Id J. CHEM. SOC. FARADAY TRANS., 1994, VOL,. 90 Table 2 Mesophases and structural data mesogen polymer phase layer spacing/A distance/l( 48.4 4.67 49 4.60 50 4.65 49 4.60 49 4.65 49 4.60 55 4.67 48.4 4.53 55 4.60 71.6 4.53 38 4.63 interdigitated arrangement of the mesogens, which may be induced by charge transfer interactions or strong dipole- dipole interactions of the chromophore groups.The investigations by X-ray reveal smectic mesophases in all copolymer systems up to 80 mol% of chromophore. This finding is remarkable since lateral substituents fixed to the rigid core of liquid crystal molecules are known to hinder strongly the formation of LC phases. In this case we have 80% of the moieties laterally substituted on both sides of the molecule, but a broad mesophase is still present.Tilted mesophases fulfilling the symmetry requirements for a polar structure can be observed up to 50 mol% of chromo- phore. However, approaching very high chromophore con- tents, only smectic A phases are found. In Table 2, the layer spacings of the different materials at different temperatures are listed. The results from X-ray investigations are fully consistent with texture observations under the polarisation microscope and with ferroelectric switching experiments. As a result, the dependence of the mesophase sequence on the chromophore content can be described by the phase diagram shown in Fig. 2. Conclusions The X-ray investigations performed reveal the influence of laterally substituted NLO chromophores on the phase sequence of ferroelectric liquid-crystalline polymers.Tilted smectic mesophases, which are essential for a polar structure needed for SHG, are obtained for a chromophore content of up to 50 mol%. At higher contents of laterally substituted chromophore, the tilted phase disappears, but a broad S, phase remains present. E.W. and R.Z. wish to thank the Deutsche Forschungsge- meinschaft for financial support. I160 ,,,],,,, I,,I I ,,,I,,, ,I,,,, 140 I 120 100Yh' 80 :: 20 I 0 I0 20 30 40 so 60 70 80 mol% chromophore Fig. 2 Phase diagram of the copolymer system J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3333 References 8 A. Taguchi, Y.Ouchi, H. Takezoe and A. Fukuda, Jpn. J. Appl.Phys., 1989,28, L997. 1 R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Phys., 9 E. Wischerhoff, R. Zentel, M. Redmond, H. Coles and 1975,36,69. 0.Mondain-Monval, Macromol. Chem. Phys., 1994,195, 1593. 2 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980,36,899. 10 H. Poths and R. Zentel, Liq. Cryst., 1994,16, 749. 3 Polymers for Advanced Technologies 3, ed. M. Lewin, M. Jaffe, 11 S. Diele, S. Oelsner, F. Kuschel, B. Hisgen, H. Ringsdorf and J. H. Wendorff and E. Tsuchida, Wiley, Chichester, 1992, vol. 5, R. Zentel, Makromol. Chem., 1987,188,1993. pp, 195-276. 12 H. H. Sutherland, Z. Ali-Adib, B. Gasgous and G. Nestor, Mol. 4 H. Poths, A. SchonfeldFR. Zentel, F. Kremer and K. Siemens- Cryst. Liq. Cryst., 1988, 155,327. meyer, Adv. Muter., 1992,4,351. 5 M. Dumon, H. T. Nguyen, M. Mauzac, C. Destrade and H. Gasparoux, Liq. Cryst., 1991, 10,475. 6 J. Naciri, S. Pfeiffer and R. Shashidar, Liq. Cryst., 1991,10, 585. 7 H. Kapitza, R. Zentel, H. Poths, S-U. Vallerien, F. Kremer and Paper 4/03514C; Received 13th June, 1994 E. W. Fischer, Makromol. Chem. Rapid Commun., 1990,11,593.