Chapter 23
Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: August 6, 1998 | doi: 10.1021/bk-1998-0695.ch023
The Influence of the Molecular Structure on the Second-Order Nonlinear Optical Properties of Pyroelectric Liquid Crystalline Polymers 1,3
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M . Trollsås , F. Sahlén , P.Busson ,J. Örtegren , U. W. Gedde , A. Hult , M . Lindgren , D. Hermann , P. Rudquist , L .Komitov ,B. Stebler , and S. T. Lagerwall 2
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Department of Polymer Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden National Defence Research Establishment, S-581 83 Linköping, Sweden Physics Department, Chalmers University of Technology, S-412 96 Göteborg, Sweden 2
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Pyroelectric Liquid Crystal Polymers (PLCP), a novel class of material with intrinsic polar order developed in our laboratory, are discussed. Thin films (2-4μm) of PLCP were prepared by photopolymerization of ferroelectric liquid crystalline monomers in the chiral smectic C (SmC*) phase. The poly(acrylate) materials have been structurally modified in order to improve their nonlinear optical and thermal properties. Further are their longterm nonlinear optical properties discussed based on results from second harmonic generation (SHG), spontaneous polarization ( P ) , and dielectric spectroscopy. s
Ferroelectric liquid crystals (FLC) is a group of organic materials that possesses spontaneous polar order (/). This unique property has increased the interest to use liquid crystals in the field of second-order non-linear optics (NLO). The intrinsic thermodynamically stable polar order separate this group of materials from other polar organic materials which usually are poled by external electrical fields which is subsequently locked in by either lowering the temperature below the glass transition temperature of the material or by a chemical cross-linking reaction (2). 3
Current address: IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, C A 95120-6033. Current address: Institut für Polymere, ETH-Zentrum CNBE 94, Universitätsstrasse 6, CH-8092 Zürich, Switzerland. Corresponding authors.
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© 1998 American Chemical Society
In Organic Thin Films; Frank, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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FLCs aimed for second-order nonlinear optics were first synthesized by Walba et al (3) who synthesized a low molar mass F L C I (Figure 1) which generated a second harmonic signal (^22) of 0.6 ± 0.3 pm/V in the chiral smectic C (SmC*) phase. The N L O units (NLO-chromophore) which generate the SHG-signal, and are aligned perpendicular to the long axis of the molecules in the direction of the polarization, were later successfully modified by Schmitt et al (4) in an attempt to increase the second harmonic signal, Π. The thermal stability of the polar order in these materials is limited, since the polar order only is present in the chiral smectic C phase. In addition to this, the mechanical stability of these low molar mass compounds is a limiting factor. Subsequently to Walba, Zentel et al (5) synthesized a ferroelectric liquid crystalline side-chain polymer and recently Keller et al (6) presented the first ferroelectric liquid crystalline main-chain polymer. The thermal and mechanical stability and the long-term properties of NLO-materials are known to be improved by cross-linking (2). A pyroelectric liquid crystalline polymer (PLCP) was therefore synthesized. This was realized by the synthesis of a crosslinkable monomer mixture that possessed chiral smectic C (SmC*) mesomorphism over a wide temperature range. This ferroelectric mixture was polar aligned and subsequently photocross-linked into the pyroelectric liquid crystal polymer ΠΙ, (Figure 2) (7). The crosslinked material, which did not exhibit a ferroelectric behavior displayed a clear Pockels effect and a small but clear second harmonic signal (^/-coefficient = 0.02 pm/V) (7). To increase the NLO-activity in this new material the molecular structure had to be modified. The possibilities for modification of the monomer system are, however, limited due to the demands of ferroelectricity and cross-linking. However, new monomer systems have recently been developed (8). The spontaneous polarization of these new materials have been measured and the results have been compared with the results from second harmonic generation. This has generated an understanding of the influence of the molecular structure on the properties of these complex materials. The cross-linked materials have also been investigated by dielectric spectroscopy (9) and the long-term properties of the polar order is under investigation by second harmonic generation (SHG) (10). In this paper these results are combined and discussed in order to generate an understanding of how the molecular structure ought to be altered in order to improve the over all properties of these materials. Experimental The synthesis of 4"-[(ll-acryloyloxy) undecyloxy]-4'-biphenyl 4-[(R)-(+)-2-octyloxy]3- nitrobenzoate A l , 4 - ( l 1 - a c r y l o y l o x y u n d e c y l o x y ) phenyl 4 - ( 4 - ( l l acryloyloxyundecyloxy) phenyl) benzoate A2, 4"-{(R)-(-)-2-octyloxy}-3"-nitro phenyl 4- { 4 ' - [ l 1-acryloyloxyundecyloxy] phenyl} benzoate A l b , 4 ' - { 1 1 acryloyloxyundecyloxy}-3"-nitro phenyl 4-{4-[l 1-acryloyloxyundecyloxy] phenyl} benzoate A2b, and 4"-{(R)-(-)-2-[(10-Acryloyloxy)decyl]oxy}-3-nitro phenyl 4-{4'[(1 l-acryloyloxy)-undecyloxy] phenyl} benzoate A2c have been described elsewhere ( 7*The SH-signal for a 2μηι sample of a 35/65 mol-% A1/A2-mixture was reported to be about 1000 times smaller than the signal from a quartz reference (7). In order to compare the signals, that of the sample cell should be extrapolated to the signal that would be obtained from a sample where the interaction length is half the coherence length of the interaction. The ^/-coefficient based on quartz should therefore be d\\iM
