Second order nonlinear optical response of nematic liquid crystalline main chain polymers

ELSEVIER

Synthetic

Metals

101 (1999)

244-245

Second Order Nonlinear Optical Response of Nematic Liquid Crystalline Main Chain Polymers A.T.H. Kochare, S.V. Fridrikha,

M. Warn&,

C.E. Schwarzwglderb,

SC. Morattib,

R.H. Frienda

Tavendish Laborafoy, University of Cambridge,Madingley Road,CambridgeCB3 WE, U.K. bMeIville Laborafoy for Polymer Synfhesis,University of Cambridge,CambridgeCB2 3RA, U.K. Abstract We have synthesised and carried out preliminary characterisation of NLO-active semiflexible main chain polymers with a head-to-tail structure which exhibit a nematic liquid crystalline mesophase. We predict that the nematic order facilitates a large number of dipoles to couple collectively to a dc~ poling field, From this process we ex ect an enhanced x@) response. We attribute a slow response to a poling field and subsequent relaxation to hairpin diffusion a Pong the polymer chains. Values of ru-r,3 of 50 pm/V were observed. Keywords: Non-linear

optical

methods,

Liquid

crystalline

phase transitions,

1. Introduction The ease of synthesis and processing makes olymeric NLO devices attractive for applications. organic NLO chromophores have been incorporated into polymers by means of guest-host systems, as polymer side chains and b including them into the polymer main chain [l]. NL B response, temporal and thermal stability vary considerably for the different ap roaches. In this work we Pecus on liquid cr stalline main chain alders. Fese polymers.consi$ of &O-phores connected ead to tall to form a mam cham polymer (see Fig. 1). The materials are engineered such that they show a nematic li uid crystalline meso hase. In this mesophase the nematic fle3 d stretches out the c5 ams along a director n [2]. ? director n (axial order) DC field E (polar order) A= dipole I

I

1. Illustration of two polymer chains with identical geometrical configurations but opposite dipole moments in the nematic state Fig.

In Fig. 1 two such chains are de icted. A and B differ only by the orientation of their dipo Pe moments In an unbiased state the statistical distribution of chains A and B averages ole momt to zero despite their poling field E couples to a dipole compared to the individual molecular dipole momenfs p by a factor of the average distance between hairpins over the length of a monomer [3]. The chains A become biased with respect to B, breaking the centrc-symmetry and causing a x(*’ response to develop. The xc*) response of the olymer compared to the fluid of monomers is enhanced iYv

* correspondmg

author:

e-mail

ak213@cam.~c..uk I

0379-6779/99/$ - see front matter 0 1999 Elsevier PII: SO379-6779(98)01358-7

Science S.A.

Spin-casting,

Ellipsometry

[3] where u is the energy of a hairpin reversal of the chain dlrection [4].

where

a hairpin

is a

2. Experimental the measurements described below is a consisting of an azobenzene based NLOhydroxybenzoic acid (HBA, II).

Fig. 2. Chemical structure of the random copolymer used for CI+Z measurements. The spacer length n equals 11 units. The overall composition of the copolymer is 1~1 (x = y = 0.5).

The synthesis is described elsewhere 1.51. The molecular weight was determined b GPC analysis to be M,=10,842 and M,=17,869. T,=35 4 was measured with a PerkinElmer DSC. Samples where pre ared on IT0 covered lass substrates by spin-casting tK e polymer from 1,‘i ,2,2 tetrachloroethane solution. Film thicknesses were ~ determined with a dektak profilometer. Polymer films were dried at 54°C at 10.’ mbar for 8h. Altium counterelectrodes weTe thermally evaporated. The time resolved NLO characterization was determined by an elli sometric technique first described by Teng and Man [6 P The ac modulation voltages rep uired for this technique were smaller by a factor of 1a -100 compared to the dc poling fields. The samples were temperature controlled b a Linkham hot stage. Identical samples were examined un B er a poling field in an optical microscope. 3. Results Fig, 3 shows the time resolved response of the polymer 6l.m to a dc oling field and the subse uent decay of the NLO signal a Hter switching off the field. B n application of the dc field both a fast and a slow response can be observed. The fast res onse can only be resolved to the 3 s limit of our setup. R the particular experiment de icted in Fig. 3 the slow component is still increasing a Fter 30 min polin (conditions: polin field 24OV/pm, 65” C). After about 3 fz min the poling fie 3 d was switched off. The relaxation also exhibits a fast and a slow component. The two fast

All rights reserved.

A.T.H. Koch et al. I Synthetic

/

0100 0

20

I

I,

I 40

60

I

I, 60

time (min) Fig. 3. Tie resolved NLO response of polymer film as determined by ellipsometric technique after Teng and Man 141. Poling conditions: E,,=240V/pm, E,=7V/pm rms, f,=2.69 kHz, T=65”C, film thickness d=265 nm. The inset shows the slow relaxation and a fit to a stretched exponential curve. The dotted lines are a guide to the eye,

components for the poling and relaxation are of almost e ual magnitude. The inset of Fig. 3 shows that the slow re1 axation fits well to a stretched exponential function (best fitting parameters: p = 0.122, r = 9.9 s): The NLO response at twice the frequency W of the ac modulation field yields a signal proportional to xc3’ [7]. The response at frequency w is a convolution of xc*’ and xc31 effects for E,,r 0. From measurements of the response at 2 w for a poled and subsequently quenched sample of the same e as described above we estimate that a lar e fraction (5 “ig %) of the fast si nal rise and decay is due to x?3). Under crossed po‘i arisers in the optical microscope the polymer films shows a typical nematic schlieren texture with a domain size of the order 1 ~.lmin the tern erature range between 60” and 190” C An applied dc fiel $ of the same magnitude and at the same temperature as in the NLO measurements of Fig. 3 does neither visibly alter the domain size nor even distort them significantly at the tern erature of 65” C. At tern eratures higher than 85” C and ong poling poling tines cross ?inking of the polymer disto!ts measurements. Hence this temperature regime is difficult to access for long poling times. 4. Discussion

The observation that the domains remain largely unaffected by a poling field sug ests that the slow -response and relaxation result from 9ocal processesinside the domains rather than domain rearrange’ments.This behaviour could be rationalised by assumingthat the NLO res onse is ovemed by the dynamics of hairpins. The dif Pusion of ft.airpins along chains [3] facilitates chain B in Fig. 1 to res ond to a dc poling field by reversing the direction of its enB-to-end vector. Since this vector is proportional to the chain’s net dipole moment,electric field energy will be gainedby this mechanism. Hence after long oling times,the majority of chains will have their enB-to-end vectors oriented parallel to the vector of the oling field increasing the net dipole momentof a domain. Kotation of the whole domain takes place only when the dipole moment is sufficiently lar e for the electric field energy to reach a critical value 5eterminedby the Frank elasticity and the domain size [8]. This processeventual1 yields homeotropic aIignment- a behaviour which we A? m eed observe on fast timescalesat higher temperatures.Thus the relaxation of the xc21 NLO signal also has two contributing components:the domain rotation and the local relaxation of the end-to-end vectors. It has to be emphasisedthat for the data described in Fig. 3 domain rotation plays no significant role as

concluded from optical microscopv.

Metals

I01

(1999)

244-245

245

Any (electronic) xt3)res onsewill be instantaneouswithin the time resolution oP our experiment. Furthermore xf3) dependson quadru olar rather than dipolar order. In the absence of any cKanges in axial order (no domain rearrangements)the slow responseand relaxation is likely to be a pure x c2’effect and hencerelated to the growth and relaxation of dipolar order. The like1 mechanismof such relaxation is hairpin motion. Under tKese assumptionswe canthen usethe sim lified derivations in [9] to estimater33r,3 to be 50 pm/V. itl easurementsof r,,/r,, are underway and are expectedto be muchlarger than the isotropic value of 3 due to the nematicnature of the polyF= Very long relaxation times (decadeso minutes)may be due to strong dipole-dipole interactions between mesoens.We estimatedthe energy of theseinteractions to be of t5 e order kT at room temperature. The interactions may be both between the m&gen of the samechain and between the neiphbourinechains.The dvnamicsof the latter caseresults in% broad” distribution ‘of relaxation times [lo]. The stretched exponential relaxation observed in our systemsis one of the signaturesthis broad distribution. Main chain nematic polymers have much larger Frank elasticity constants and surface anchoring ener ‘es than low molecular wei ht nematic liquid crystals T11, 121. Besidesthe dipole-CF ipole interactions, this might explain the fact that electric fields as strong as 10 V/m are insufficient to rotate the domains and to impose a homeotropicalignmentat (modest)temperaturesup to 65” C. In the absenceof such ali ent a big fraction of domains has a director perpendicuff”” ar or nearly perpendicular to the electric field and hence does not contribute to the NLO ed samples,e.g. by optical alignment, ~~$$?~eIPd:gher responses. 5. Conclusions

We have demonstratedthat liquid crystalline main chain polymerswith a head-to-tail order of the mesogens possess a hi h nonlinear optical activity in the nematicphase.The NL8 responseunder a poling field and its relaxation consisto a fast xt3)anda s ow x -Iregime.The latter mi ht be attributed to reorientation of end-to-end vectors of po7ymer chains. This processis hindered by strong dipole-dipole interactions between the meeogenswhich give rise to a broad distribution of the relaxation times reflected by a stretched exponential behaviour. Even with strong electric fields (10’ V/m) nematicdefectsare not annealed out at the (modest)temperatureof 65” C which hinderspolin Further difficulties with the 4 surface arise from the big anchoring energies, rank elasticity constants and crosslinking under field for temperatureshigher than 65” C. 6. Acknowled

ements

We thank A. % onald and E.M. Terentjev for fruitful discussions.We also thank the En ’ eering and Physical SciencesResearchCouncil (EPSRC)ror funding this project. A.T.H. Koch thanks the “Stiftung Stipendienfonds des Verbandes der chemischen Industrie (VCI)” and the “Bundesministeriumfur Bildung, Wissenschaft, Forschung und Technologie(BMBF)” for financial support. [l] F. Kajzar, J.D. Swalen (eds.), Organic Thin Films for Waveguiding Nonlinear Optics, Gordon and Breach (1996) [Z] J.F. D’Allest;l? Maissa rf ni., Phys. Rev. L&t., 61 (1988) 2562 [3] J.M.F. Gunn, M. Warner, Phys. Rev Lett., 58 (1957) 393 [4] P.-G. de Gennes, Polymer Liquid Crystals, eds. A. Cliferi, W.R. Krigbaum, R.B. Meyer, Academic Press, New York (1952) [5] CC. Teng, H.T. Man, Appl. Phys. Letters, 56 (1990) 1734 [6] C. Heldmann, M. Warner, Macromolecules, 31 (1998) 3519 [7] P.A. Chollet, Y. Levy in [l] [8] S.V. Fridrikh, EM. Terentjev, Phys. Rev. Lett., 79 (1997) 4661 [9] C.P.J.M. van der Vorst, C.J.M. van Weerdenburg, PIE, 1337 (19901246 [lo] see for example: K.D. Singer, L.A. King, J, Appl. Phys., 70 (1991) 3251 [ll] R.G. Petschek, E.M. Terentjev, Phys. Rev. A, 45 (1992) 930 [12] EM Terentjev, J. Phys. II France, 5 (1995) 159