Nonlinear Optical Properties of Organic and Polymeric Materials

5 Characterization of Liquid Crystalline Polymers for Electro-optic Applications G. R. M E R E D I T H , J. G. V A N D U S E N , and D. J. W I L L I A M S

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Xerox Corporation, Webster Research Center, Webster, NY 14580

In this paper we discuss properties of a novel medium displaying nonvanishing χ . It is a medium whose properties are a synthesis of different classes of materials: liquid crystals, polymers and molecules possessing extremely large second-order nonlinear p o l a r i z a b i l i t y . We begin by briefly reviewing the special optics related properties of liquid crystals, polymers and liquid crystalline polymers (lc polymers), then show the synthesis of ideas which suggests pursuit of molecular doping of the latter. We discuss the physics of guest molecule alignment in a nematic host and the relationship of the response to DC poling fields and the induced χ tensors. Subsequently aspects of second harmonic generation (SHG) experimental characterization of χ are considered. Dynamical and temperature dependent aspects of SHG are presented which indicate that the lc polymer host plays more than a passive role in the poling -freezing process. We therefore present experimental investigations of associated properties of these lc polymers.

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Liquid Crystals And Optics. In the fields of electro-optics and nonlinear optics, technologies develop as a consequence of the a v a i l a b i l i t y of specialized properties of certain materials. For example, the high degree of cooperative alignment of nonsymmetrically polarizable molecules in liquid crystals and the ease with which the direction of their alignments can be changed have lead to an inexpensive display technology (1). The a b i l i t y to drastically alter the macroscopic optical properties with relatively low e l e c t r i c a l fields have made liquid crystals attractive for other optical devices (e.g. light valves) as well. These effects are associated with DC or low frequency Kerr behavior (2). Also, because of the cooperativity, nonlinear optical phenomena such as the AC Kerr effect, s e l f focusing and nonlinear refraction associated with phase conjugation are very pronounced in liquid crystal forming media, particularly in the v i c i n i t y of the liquid crystal - isotropic transition or Frederiks point (3,4). 1

Current address: Eastman Kodak Research Laboratories, Rochester, N Y 14650. 0097-6156/83/0233-0109$07.50/0

© 1983 American Chemical Society

In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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NONLINEAR OPTICAL PROPERTIES

The unique p r o p e r t i e s o f l i q u i d c r y s t a l s have a l s o provided o p p o r t u n i t y f o r study o f novel n o n l i n e a r o p t i c a l processes. An example i n v o l v e s the a b i l i t y t o modify the p i t c h o f c h o l e s t e r i c l i q u i d c r y s t a l s . Because a pseudo-wave vector may be a s s o c i a t e d w i t h the p e r i o d o f p i t c h , a number o f i n t e r e s t i n g Umklapp type phasematching processes (processes i n which wave v e c t o r conservation i s r e l a x e d to a l l o w the vector a d d i t i o n to equal some combination o f the m a t e r i a l pseudo-wave v e c t o r s r a t h e r than zero) are p o s s i b l e i n these pseudo-one-dimensional media. Shen and coworkers have i n v e s t i g a t e d these employing o p t i c a l t h i r d harmonic generation (JL) and four-wavemixing ( 6 ) . Other than employing l i q u i d c r y s t a l s as media f o r i n t e r e s t i n g o p t i c a l e f f e c t s , l i n e a r and n o n l i n e a r o p t i c a l experimentation can provide a probe i n t o t h e i r s t r u c t u r e as w e l l . Spectroscopy o f d i c h r o i s m a s s o c i a t e d w i t h l i g h t absorption by guest molecules d i s s o l v e d i n l i q u i d c r y s t a l hosts has been used both to c h a r a c t e r i z e the guest and t o i n f e r p r o p e r t i e s o f the host (7_,8). Spectroscopy and l i g h t s c a t t e r i n g a s s o c i a t e d with the host i s a more d i r e c t but perhaps more d i f f i c u l t probe of the l a t t e r . One i n t e r e s t i n g round o f experimentation concerned o p t i c a l second harmonic generation (SHG) and p r o o f through i t s observation whether c h o l e s t e r i c media were a c e n t r i c on the s c a l e o f molecular o r g a n i z a t i o n . In nematic media i t i s b e l i e v e d t h a t a d i r e c t o r d e f i n e s the a x i s o f a x i a l alignment, there being no p r e f e r e n t i a l polar alignment (9_). C h o l e s t e r i c s can be viewed as having the same microscopic p r o p e r t i e s w i t h the d i r e c t o r s simply r o t a t i n g on a macroscopic s c a l e along the a x i s a s s o c i a t e d w i t h p i t c h . In i n i t i a l experiments probing t h i s i s s u e SHG was observed from c h o l e s t e r i c media (1Q) but was l a t e r suggested to have a r i s e n as a consequence o f the presence o f noncentrosymmetric m i c r o c r y s t a l s (11). When care was taken to assure the e x c l u s i o n o f m i c r o c r y s t a l s no d e t e c t a b l e harmonic l i g h t was produced ( i l , 1 2 ) . A decade l a t e r the very weak SHG a s s o c i a t e d with s p a t i a l d i s p e r s i o n i n a nematic l i q u i d c r y s t a l was reported ( 1 3 ) . Recently i t has been shown that i f c e r t a i n symmetry c o n d i t i o n s are met, noncentrosymmetric l i q u i d c r y s t a l l i n e order on the s c a l e o f i n t e r m o l e c u l a r o r g a n i z a t i o n can be achieved (14). The s p e c i a l c h a r a c t e r i s t i c required i n smectic C phase t o generate f e r r o e l e c t r i c l i q u i d c r y s t a l l i n e media has been achieved by the i n c l u s i o n o f r e l a t i v e l y s m a l l c h i r a l centers on mesogenic molecules. R e a l i z a b l e macroscopic d i p o l e p o l a r i z a t i o n d e n s i t i e s are consequently s m a l l . Use o f t h i s v e h i c l e t o achieve second-order n o n l i n e a r l y p o l a r i z a b l e media, as described below, i s not expected t o be rewarding. However, a t l e a s t one e l e c t r o - o p t i c device u s i n g such m a t e r i a l s has been devised (15.). Another nonlinear optical phenomena, c l o s e l y r e l a t e d to the main t o p i c o f t h i s paper, which has been performed i n l i q u i d c r y s t a l s i s DC e l e c t r i c f i e l d induced second harmonic generation (1_6). In those experiments the DC f i e l d weakly


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m o d i f i e s the e q u i l i b r i u m d i s t r i b u t i o n of molecules to produce a net noncentrosymmetric p o l a r alignment. SHG i s subsequently observed. In these s t u d i e s one may vary the p o l a r i z a t i o n s o f o p t i c a l f i e l d s t o observe the departure of a n i s o t r o p i e s from those o f i s o t r o p i c media ( v i d e i n f r a ) . The v a r i a b l e p i t c h o f c h o l e s t e r i c s was a l s o used t o achieve pseudo-phase-matching o f the type mentioned above f o r t h i s process ( U ) . Polymers And O p t i c s . The important s p e c i a l f e a t u r e s of polymers i n o p t i c a l t e c h n o l o g i e s a r e probably most s t r o n g l y a s s o c i a t e d w i t h their fabrication characteristics. Inexpensive l e n s e s , prisms, f i b e r o p t i c s , a n t i r e f l e c t i o n c o a t i n g s , e t c . r e s u l t from the ease w i t h which p l a s t i c components may be produced. T h i s i s not t o say that t h e r e have n o t been s p e c i a l polymers produced which have h i g h l y unusual or p o t e n t i a l l y b e n e f i c i a l o p t i c a l p r o p e r t i e s . For example, the p o l y d i a c e t y l e n e m a t e r i a l s , discussed i n f u l l e r d e t a i l i n other a r t i c l e s o f t h i s volume, provide extended molecular orbital c o n j u g a t i o n systems. The r e s u l t i n g d e r e a l i z a t i o n o f IT — e l e c t r o n s can l e a d t o very l a r g e t h i r d - o r d e r nonlinear p o l a r i z a b i l i t y and important o p t i c a l p r o p e r t i e s a s s o c i a t e d w i t h i t (1_S). Special advantages o f t h i s system l i e i n the extremely r a p i d speed a s s o c i a t e d w i t h p u r e l y e l e c t r o n i c response t o e l e c t r i c f i e l d s . Second-order n o n l i n e a r i t y has a l s o been designed i n t o t h i s c l a s s o f polymers (1_â). In t h i s case, though, the second-order n o n l i n e a r i t y i s a s s o c i a t e d w i t h the pendant groups, probably being a consequence o f l o c a l n o n l i n e a r p o l a r i z a b i l i t y . The polymeric property makes f o r a rugged medium i n a d d i t i o n t o o f f e r i n g the p o t e n t i a l o f patternwise ( p h o t o ) p o l y m e r i z a t i o n and s t r u c t u r e c r e a t i o n . An e x t r a chore, though, i n t h i s approach i s t h a t one must i d e n t i f y a s p e c i f i c n o n l i n e a r monomeric species which c r y s t a l l i z e s i n a s t r u c t u r e i n which a significant fraction o f the molecular nonlinear p o l a r i z a b i l i t y i s preserved (a problem discussed i n s e v e r a l papers i n t h i s volume) and grow s u i t a b l e o p t i c a l q u a l i t y c r y s t a l s before p o l y m e r i z a t i o n . The use o f p o l y m e r i z a t i o n to r i g i d i z e s t r u c t u r e s i s a l s o b e n e f i c i a l when these s p e c i e s a r e drawn a s t h i n f i l m from Langmuir troughs, a v e r s a t i l e alignment t o o l ( 2 0 , 2 1 ) . Other aspects o f the theory o f polymers i n n o n l i n e a r o p t i c s may be found i n the paper o f F l y t z a n i s i n t h i s volume. A s p e c i f i c s e t o f experiments which must be mentioned, being d i r e c t l y a s s o c i a t e d with the main t o p i c of t h i s paper, i s the work of Bergman, e t . a l . (22) d e a l i n g with the second-order n o n l i n e a r o p t i c a l p r o p e r t i e s o f p o l y v i n y l i d e n e f l u o r i d e (PVF ). Nonvanishing the second-order n o n l i n e a r e l e c t r i c d i p o l e s u s c e p t i b i l i t y , i s expected i n PVF2 s i n c e i t e x h i b i t s other p r o p e r t i e s r e q u i r i n g noncentrosymmetric microscopic s t r u c t u r e . These p r o p e r t i e s appear 2



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due to the c r y s t a l l i n i t y of PVF . xK*J a s s o c i a t e d with SHG was t h e r e f o r e measured i n a p p r o p r i a t e l y s t r e s s o r i e n t e d and poled samples. Since the nonlinear p o l a r i z a b i l i t y i n t h i s case i s a s s o c i a t e d o n l y w i t h saturated bonds, the magnitude o f achieved (or a c h i e v a b l e ) i s not s i g n i f i c a n t l y l a r g e (~2 X 10"^ esu subsequent to p o l i n g i n a 30V/jmm f i e l d ) i n comparison t o the n o n l i n e a r i t i e s discussed i n t h i s symposium. In t h i s paper we w i l l describe another polymeric s t r u c t u r e , based on more recent knowledge of c h a r a c t e r i s t i c s which enhance n o n l i n e a r p o l a r i z a b i l i t y , e x h i b i t i n g larger and p r o j e c t e d to d i s p l a y even higher values than we have reported (23). Downloaded by UNIV LAVAL on June 15, 2014 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch005

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In the study o f molecular p r o p e r t i e s v i a o p t i c a l spectroscopy, polymers have been e x t e n s i v e l y employed f o r the purpose o f eliminating molecular r o t a t i o n and inducing preferential o r i e n t a t i o n . Used as minimally p e r t u r b i n g m a t r i c e s , polymers enable various spectroscopic p o l a r i z a t i o n c o r r e l a t i o n studies. U n i a x i a l s t r e s s o r i e n t i n g o f f i l m s i s a l s o a convenient means of a c h i e v i n g p a r t i a l a x i a l alignment of nonsymmetrical guest molecules. Recently Havinga and VanPelt (24) have taken another approach f o r s p e c t r o s c o p i c study o f l a r g e dye molecules d i s s o l v e d i n polymer matrices. They used large e l e c t r i c f i e l d s i n c o n j u n c t i o n with thermal m o d i f i c a t i o n o f v i s c o s i t y t o o r i e n t and " f r e e z e - i n " net polar alignments o f the guest molecules. L i q u i d C r y s t a l l i n e Polymers. Recently i t has been shown t h a t i f mesogenic s p e c i e s are attached t o a polymeric backbone through s u f f i c i e n t l y long and f l e x i b l e spacer groups, such a medium w i l l e x h i b i t l i q u i d c r y s t a l l i n e behavior (25,2&). The v i s c o s i t y o f such media i s g r e a t l y increased over t h a t of normal l i q u i d c r y s t a l s and s w i t c h i n g times i n l i q u i d c r y s t a l l i n e d i s p l a y devices are tremendously increased, although recent work has shown t h a t these times can be made s h o r t . That goal i s d i r e c t l y opposite to our g o a l of i n c r e a s i n g r i g i d i t y f o r alignment p r e s e r v a t i o n ( v i d e i n f r a ) . A l s o , an important c o n s i d e r a t i o n f o r some o p t i c a l a p p l i c a t i o n s i s t h a t of beam a t t e n u a t i o n by s c a t t e r i n g . Associated w i t h the increased response time f o r r e o r i e n t a t i o n o f the l i q u i d c r y s t a l d i r e c t o r i s a decrease i n the f l u c t u a t i o n a l l i g h t s c a t t e r i n g which can be q u i t e l a r g e i n normal l i q u i d c r y s t a l s ( 2 1 ) . M o l e c u l a r l y Doped Thermotropic L i q u i d C r y s t a l l i n e Polymer. The idea o f the n o n l i n e a r o p t i c a l medium which i s the s u b j e c t o f t h i s paper r e s u l t s from a s y n t h e s i s of the ideas of the d i s c u s s i o n above and a few concepts from n o n l i n e a r o p t i c a l molecular and c r y s t a l p h y s i c s . As d i s c u s s s e d s e v e r a l places i n t h i s volume, i t i s known that c e r t a i n c l a s s e s o f molecules e x h i b i t tremendously enhanced second-order


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n o n l i n e a r p o l a r i z a b i l i t i e s , β. One o f the main o b s t a c l e s preventing the immediate t e c h n o l o g i c a l use o f these compounds i s the o r i e n t a t i o n a l c a n c e l l a t i o n c h a r a c t e r i s t i c o f p o l a r t h i r d rank t e n s o r s . Only i n noncentrosymmetric environments are t h e i r major components nonvanishing, and furthermore, o n l y f o r h i g h l y c o r r e l a t e d o r i e n t a t i o n s o f the microscopic s p e c i e s , do the β tensors c o n s t r u c t i v e l y sum t o y i e l d a l a r g e value o f χ ^ . Therefore, much research centers around the i n v e s t i g a t i o n o f (noncentrosymmetric) molecular c r y s t a l s as p o s s i b l e new n o n l i n e a r o p t i c a l media which w i l l possess the s p e c i a l advantages designed i n t o the molecular c o n s t i t u e n t s . Since many o f the enhanced β s p e c i e s are n e a r l y planar aromatics w i t h l a r g e d i p o l e moments, i t s been speculated that m i n i m i z a t i o n o f the t o t a l c r y s t a l energy f a v o r s a n t i p a r a l l e l molecular alignment t o maximize the (negative) c o n t r i b u t i o n s o f d i s p e r s i o n and d i p o l e - d i p o l e i n t e r a c t i o n s . Obviously, the l a r g e d i p o l e moments a l t e r n a t i v e l y a l l o w the p o s s i b i l i t y t o achieve s i g n i f i c a n t ( p a r t i a l ) e l e c t r i c f i e l d induced polar molecular alignments. F o l l o w i n g Havinga and VanPelt ( 2 4 ) , one might use a very l a r g e s t a t i c f i e l d t o achieve an alignment o f l a r g e β, l a r g e μ molecules d i s s o l v e d a t h i g h c o n c e n t r a t i o n i n a polymer m a t r i x . By t h e i r method, r e o r i e n t a t i o n i s performed a t high temperatures i n a low v i s c o s i t y s t a t e w i t h subsequent c o o l i n g i n the f i e l d t o a higher v i s c o s i t y s t a t e . The f i e l d i s removed, l e a v i n g a noncentrosymmetric medium which they employed only f o r s p e c t r o s c o p i c purposes. This medium should have nonvanishing i f the polymer can, as they


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showed, permanently prevent r e l a x a t i o n o f the alignment. The magnitude o f would be determined by the degree o f alignment, the c o n c e n t r a t i o n and the magnitude o f β (and some l o c a l f i e l d phenomena). We have proceeded one step f u r t h e r ( 2 2 ) , expecting t o use the inherent ( a x i a l ) alignment p r o p e r t i e s o f l i q u i d c r y s t a l s t o enhance the net guest alignment achievable w i t h an a p p l i e d f i e l d . Besides t h i s , use o f polymers which a r e simultaneously l i q u i d c r y s t a l s increases the o p t i c a l f l e x i b i l i t y o f the r e s u l t a n t medium s i n c e they are biréfringent. I t t u r n s out, though, t h a t the physics o f our medium i s more complex than t h i s simple p i c t u r e . The host does not act as a simple matrix with o n l y the v i s c o s i t y v a r y i n g with temperature. S i g n i f i c a n t v a r i a t i o n o f the alignment i s observed through SHG as f u n c t i o n o f temperature. Consequently, i n t h i s paper we w i l l d i s c u s s our conception o f the alignment physics and r e p o r t c h a r a c t e r i z a t i o n s o f aspects of these l i q u i d c r y s t a l l i n e polymers r e l a t i n g t o t h i s problem.



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Guest Alignment P h y s i c s


Nematic P o t e n t i a l . (28) For a guest molecule d i s s o l v e d i n l i q u i d c r y s t a l l i n e h o s t , d e t a i l s of molecular shape and guest-host i n t e r a c t i o n s determine the r e s u l t a n t o r i e n t a t i o n a l d i s t r i b u t i o n . Of course, the guest i n t r o d u c t i o n may modify host p r o p e r t i e s and guestguest i n t e r a c t i o n s may become important a t the high concentrations we've used. Obviously, a l s o , the guest alignment c h a r a c t e r i s t i c s need not mimic those o f the host. S i n c e i n these s t u d i e s we have used a rather large rod-like guest molecule, 4 — dime thy lamino — 4' — n i t r o s t i l b e n e (DANS), we expect i t t o a s s o c i a t e n e a r l y e q u a l l y w e l l w i t h host mesogenic u n i t s or other guests (which i s e s s e n t i a l l y v e r i f i e d by the r e l a t i v e l y high concentrations a c h i e v a b l e before c r y s t a l l i z a t i o n occurs and by the p r o p e r t i e s reported below). Therefore, we adopt a mean-field d e s c r i p t i o n where the nematic p o t e n t i a l , U ( 0 ) , determines the o r i e n t a t i o n a l d i s t r i b u t i o n f u n c t i o n , P ( 0 ) , o f guest molecules i n a nematic domain. P(0) i s the r e l a t i v e p r o b a b i l i t y t h a t a guest molecule w i l l l i e w i t h i t s ζ a x i s o r i e n t e d a t angle θ t o the nematic d i r e c t o r (or l a b o r a t o r y reference a x i s ) . I t i s assumed, perhaps i n c o r r e c t l y f o r t h i n f i l m s , that Ρ i s independent o f φ i n s p h e r i c a l p o l a r c o o r d i n a t e s . Also i t i s assumed that the e f f e c t s o f r o t a t i o n a l o r i e n t a t i o n o f a molecule such as DANS about i t s "long a x i s " ( d i r e c t i o n f o r maximum i n t e r a c t i o n w i t h the h o s t ) , z, which i s a l s o h e r e i n assumed t o be a good approximation to the d i r e c t i o n o f the permanent d i p o l e moment ^, are averaged away. In Figure 1 we show t h a t f o r i s o t r o p i c medium the renormalized distribution function, P'(0)=P(0)/sin0

(1

i s f l a t , i . e . t h e r e i s no prefered o r i e n t a t i o n a l d i r e c t i o n . On the other hand, the nematic p o t e n t i a l causes a d i s t r i b u t i o n which i s peaked a t 0=0 and β —m and which i s symmetric about Θ-ΊΤ/2 s i n c e U(0) is. The r e l a t i v e values o f P'(0) a r e s t a t i s t i c a l l y determined by r e l a t i v e values o f e x p { — U ( 0 ) / k T } . The nature o f these d i s t r i b u t i o n s i s commonly probed by experiments which determine s p e c i f i c moments. Perhaps the e a s i e s t to study i s d i c h r o i s m which i n d i c a t e s the d e v i a t i o n o f the second moment from that o f an i s o t r o p i c d i s t r i b u t i o n . The a s s o c i a t e d order parameter i s S

(2)

2

=

(2

which o b v i o u s l y i s zero i n i s o t r o p i c and u n i t y i n t o t a l l y p a r a l l e l o r i e n t a t i o n a l d i s t r i b u t i o n s . Fourth moments can be determined from Raman (2£) or fluorescence (30) d e p o l a r i z a t i o n among other



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Figure I. Schematic representation of statistical orientational distribution functions in isotropic (I) and nematic (N) potentials. Dashed curves depict adjusted distributions under electric field poling.


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techniques. However, because o f t h e u n c e r t a i n t y i n e l e c t r i c l o c a l f i e l d s these o p t i c a l techniques c a r r y some unavoidable u n c e r t a i n t y , which i s diminished i f magnetic phenomena are used as probes o f the o r i e n t a t i o n a l moments (Q.). P o l i n g Response. A p p l i c a t i o n o f a p o l i n g f i e l d i n the i s o t r o p i c case, even f o r very l a r g e f i e l d s and f o r d i p o l e s o f the magnitude o f μ i n DANS (7.4 D), o n l y perturbs P'(0) weakly s i n c e t h e /z_*E e l e c t r o s t a t i c energy i s s m a l l compared to kT a t o r above 300 K. T h i s behavior i s represented i n an exaggerated manner by the dashed curve I i n F i g u r e 1. A s i m i l a r e f f e c t o c c u r s i n the l i q u i d c r y s t a l when


U(0)'=U(0)-rE

(3

r e p l a c e s U(0) i n determining P'(0). T h i s s i t u a t i o n i s a l s o d e p i c t e d by the exaggerated dashed curve Ν i n Figure 1. (Obviously we must assume that p e r t u r b a t i o n o f the l i q u i d c r y s t a l l i n e host d i s t r i b u t i o n f u n c t i o n i s minimal. Otherwise one must express U(0) as f u n c t i o n o f E. ) The net p o l a r alignment can be c a l c u l a t e d (3JL) : = / cos0 exp{-U'(0)/kT} sintf άθ / / exp{-U'(0)/kT} sintf ύθ

(4a

= / costf exp{/iEcos0/kT} P ( 0 ) άθ / Q

/ exp{/xEcos0/kT} P ( θ ) άθ

(4b

Q

= / [cos 0 (μΕ/kT) + cos 0 (/xE/kT) /6 + ...] P ( 0 ) άθ / 2

4

2

Q

2

2

4

4

/ [1+COS 0 (jutE/kT) /2+cos 0 (juE/kT) /24+...] P ( 0 ) d0

(4c

Q

Here we have used the z e r o - f i e l d nematic d i s t r i b u t i o n f u n c t i o n P ( # ) f o r convenience o f n o t a t i o n . The degree of net p o l a r alignment can be seen t o be enhanced i n the l i q u i d c r y s t a l over t h e i s o t r o p i c case. The l i m i t i n g cases are i s o t r o p i c d i s t r i b u t i o n s and the I s i n g model ( i n which only 0=0 and θ-ττ are a l l o w e d o r i e n t a t i o n s ) . By r e t a i n i n g o n l y the l e a d i n g terms i n the l a s t equation one sees that i n the h i g h temperature l i m i t Q

^^Isotropic = = μΕ/kT I s i n g

(5a (5b

These are the f a m i l i a r o r i e n t a t i o n a l c o n t r i b u t i o n s t o the DC d i e l e c t r i c response. T h i s l i m i t , ΙμΕ/kTl^l, can be c o n s i d e r e d a l t e r n a t i v e l y t o be a r e s t r i c t i o n t o nonsaturated alignments. For p h y s i c a l systems with o r i e n t a t i o n a l d i s t r i b u t i o n s i n t e r m e d i a t e between the i s o t r o p i c and I s i n g l i m i t i n g models the p o l i n g responses


5.

MEREDITH ET AL.

Characterization of Liquid Crystalline Polymers since f o r |/xE/kT|

117

(4c) above

2

(6

E = o

(2)


v

1

Form o f χ . In our m a t e r i a l s we are i n t e r e s t e d i n p r o p e r t i e s o f the induced χ ^ tensor i n a poled sample, that i s , i n behavior o f the t h i r d moments . These may be c a l c u l a t e d i n the same manner as =. Before proceeding w i t h t h i s task i t i s b e n e f i c i a l t o make use o f group theory and p h y s i c a l arguments t o i n v e s t i g a t e the number o f independent terms which must be considered or c a l c u l a t e d . Since the p o l i n g f i e l d e s t a b l i s h e s C ^ y symmetry, the χ ^ tensor may be shown t o have the f o l l o w i n g form i n contracted index n o t a t i o n (through tensor a n a l y s i s i t i s known that i n a l l C : p>4 p o i n t v

(2)

groups χ ' has the same form ( 1 2 , 3 3 ) and one may look-up the χ tensor form o f C^ (4mm) or C g (6mm) ( 3 4 ) ) : ν

v

I j I

1 2 3

(?)

'

v

1

2

3

4

5

0

0

0

0

XXZ

0

0

0

0

XXZ

0

0

ZXX

ZZZ

0

0

0

zxx

ν

6.

The s e q u e n t i a l order o f the ZXX, XZX and XXZ ( l a b o r a t o r y reference frame) p o l a r i z a t i o n l a b e l s i s important only t o the extent t h a t frequency d i s p e r s i o n a f f e c t s the magnitude o f the elements o f X

(

2

M34)

C o n s i d e r i n g d i s p e r s i o n , there a r e two i m p o r t a n t l y d i f f e r e n t types of n o n l i n e a r i t y which might be e x h i b i t e d by a poled m o l e c u l a r l y doped polymer. F i r s t , the p a r t i a l p o l a r alignment g i v e s r i s e t o a nonvanishing component o f second-order n o n l i n e a r e l e c t r o n i c p o l a r i z a b i l i t y which w i l l allow p u r e l y o p t i c a l processes such as SHG or o p t i c a l wave mixing t o occur. Second, i n the e l e c t r o - o p t i c e f f e c t s the e f f e c t i v e r e f r a c t i v e i n d i c e s of the medium are a l t e r e d by the a p p l i c a t i o n o f a s t a t i c e l e c t r i c f i e l d . I f o n l y the e l e c t r o n i c n o n l i n e a r i t y makes a c o n t r i b u t i o n , then not o n l y w i l l the s o - c a l l e d index interchange symmetry ( i . e . , the p o s s i b l e independence o f magnitude o f X j j ^ on the i j k s e q u e n t i a l order r e f e r r e d to a t the end o f the l a s t paragraph) be more l i k e l y to h o l d under nonresonant c o n d i t i o n s , but X

SHG

w

i

l

1 a

l

s

o

b

e

e x

P

e c t e d

t

o

b

e

approximately

(2) ν

y

equal to χ . However,it i s p o s s i b l e that i n the polymeric s t a t e where sub-T motion can occur t h e s t a t i c f i e l d d r i v e n molecular r o t a t i o n , enhanced by the large value o f μ, coupled w i t h the l a r g e a n i s o t r o p y o f α (the l i n e a r e l e c t r o n i c p o l a r i z a b i l i t y ) w i l l provide E Q

g



118

a s u b s t a n t i a l response. This response i s r e l a t e d t o the DC Kerr e f f e c t which i s an e l e c t r o - o p t i c e f f e c t (2 ). The l a t t e r i s constrained t o be second-order o r higher i n t h e strength o f the DC f i e l d p a r t i c i p a t i n g i n the e l e c t r o - o p t i c e f f e c t by the u s u a l complete v a n i s h i n g of z e r o - f i e l d p o l a r alignment i n f l u i d s . This would not be the case i n t h e poled polymer system. I n t h i s paper we w i l l concentrate on the f i r s t , t o t a l l y e l e c t r o n i c n o n l i n e a r i t y , but mention that we a r e pursuing e v a l u a t i o n o f the e l e c t r o - o p t i c response.


κ

(2)

(2)

E l e c t r o n i c χ ' And I t s Molecular Source. The p u r e l y e l e c t r o n i c χ ' can be r e l a t e d t o the purely e l e c t r o n i c & tensors o f the molecules i n the sample through a d e s c r i p t i o n o f the manner i n which t h e i r c a n c e l l a t i o n i n the absence o f e l e c t r i c f i e l d i s negated by p o l i n g . Since μ.·β? i s very l a r g e i n DANS (25), f o r d e s c r i p t i o n o f our high concentration samples i t i s j u s t i f i a b l e t o n e g l e c t the c o n t r i b u t i o n of t h e host l c polymer t o χ ^ . ( £ i s a p o r t i o n o f H which transforms as a vector i n the three dimensional r o t a t i o n group and i s the o n l y p o r t i o n which may consequently c o n t r i b u t e t o p o l i n g induced i n f l u i d s ( 3 3 ) . ) Also s i n c e i s the o n l y s i g n i f i c a n t element of £ i n DANS (25), we only need t o know the d i s t r i b u t i o n o f molecular ζ axes t o approximately d e s c r i b e χ ^ . For t h i s purpose, s i n c e only ν

v

2

a n d

a

the two s e t s X ^ ^ Z Z ?{ZXX} one r e q u i r e s knowledge only o f

r

e

n

o

n

v

a

n

i

s

h

i

n

g

a

s

shown above,

3

=

(7

and 2

2

= -0CH3

Figure 5. Liquid crystalline polymer family of this study.

4.0

Ί

Γ

Ί

Ί

Γ

Γ

3.5

C - HOMOPOLY 6

3.0

2.5

2.0

1.5

1.0

0.5

280

300

J 320

I

I 340

J

L 360

I

I

380

L 400

T(K) Figure 6. Differential scanning calorimetry scans of lc polymers. Curves are displaced vertically. Conditions: heating, 20 ΚI min; and cooling, 320 ΚI min.


130


Table 1. Properties of l c Polymers Sample

3pac^r

Mv, l (DSC) g

c

Homopolymer (y=1)

47K

27°C

95°C

95-96°C

Copolymer

33K

25°C

101°C

101.5°C

Copolymer

37K

44°C

101°C

102°C

57°C

103°C

105°C

92°C

99°C

Copolymer Homopolymer (y=1) Downloaded by UNIV LAVAL on June 15, 2014 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch005

IJDSC) T (Optical)

46K

44-47°C

In order to increase the s t a b i l i t y o f the f i e l d induced alignment of DANS i t may be necessary to increase the r i g i d i t y of the alignment medium. Decreasing η and m raises T presumably because shorter spacer groups increase the coupling o f the interacting pendant groups to the backbone with a resulting increase i n backbone r i g i d i t y . Conversely, i t i s d i f f i c u l t to predict i f an increase i n T resulting from t h i s approach w i l l adversely a f f e c t the a l i g n a b i l i t y of the mesogens or the loss of polar DANS a l i g n a b i l i t y above T . The loss of alignment of the doped species above Τ i s contrary to the situation described by Havinga and VanPelt (24) where the decreased internal viscosity above T i s accessed to a i d i n the poling procedure. Since we have observed maximum polar alignment to be achieved at or below T , i t i s d i f f i c u l t to predict the effect of a more r i g i d medium. Obviously, some freedom of motion i s required for t h i s alignment to occur below T . Figure 7, which i s the temperature dependent d i e l e c t r i c loss behavior at 1 KHz, shows that indeed there are sub — T motions active i n these polymers at room temperature. I t i s apparently the fortunate weak coupling of such motions to the DANS alignment motions which both allows the slow (several hours) poling to proceed and allows the alignment s t a b i l i z a t i o n factors to operate before relaxation can occur. g

g

g

g

g

g

g

Conclusion A novel second-order nonlinear optical medium which should offer considerable fabrication f l e x i b i l i t y has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation o f complex dynamical and thermal behavior indicates that an important role i s played by the polymer l i q u i d c r y s t a l l i n e host. Additional properties of modified members of this family of l c polymers were consequently investigated. The explanations of guest alignment s t a b i l i z a t i o n and thermal dependence of the a l i g n a b i l i t y remain unresolved issues.


5.

ι Downloaded by UNIV LAVAL on June 15, 2014 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch005


MEREDITH ET AL.

I 120.00

I

1

1

I

I

160.00

1

1

I

I

200.00

1

1

I

I

240.00

1

1

I

I

280.00

131

1

Γ

I

I

320.00

I 360.00

TEMPERATURE Figure 7. Temperature dependence of dielectric loss of 1c polymers. Curves are displaced vertically.


132


Acknowledgment We thank J. Pochan, F. Roberts, R. LaDonna, R. Hudson, W. Herbert and N. Nowacki for experimental contributions. Literature Cited. 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26.

Margerum, J. D.; M i l l e r , L. J. J. Colloid Interface S c i . 1977, 58, 559. Hellwarth, R. W. Prog. Quantum Electron. 1977, 5, 1. Khoo, I. C. Phys. Rev. 1982, A25, 1040 and 1637. Wong, G. K. L.; Shen, Y. R. Phys. Rev. 1974, A10, 1277. Shen, Y. R. "Nonlinear Spectroscopy"; Bloembergen, N., E d . ; North-Holland: New York, 1977, p. 201. Ye, Peixuan; Shen, Y. R. Appl. Phys. 1981, 25, 49. Levenson, R. Α . ; Gray, H. B.;Ceasar, G. P. J. Am. Chem. Soc. 1970, 92, 3653. Cox, R. J. Mol. Cryst. L i q . Cryst. 1979, 55, 1. Stephen, M. J.; Straley, J. P. Rev. Mod. Phys. 1974, 46, 617. Freund, I.; Rentzepis, P. M. Phys. Rev. Lett. 1967, 18, 393. Durand, G.; Lee, C. H. Mol. Cryst. 1968, 5, 171. Goldburg, L. S.; Schnur, J. M. Rad. Electron. Eng. 1970, 39, 279. Arakelyan, S. M.; Lyakhov, G. Α . ; Chilingaryan, Yu. S. Sov. Phys. Usp. 1980, 23, 245. Meyer, R. B. Mol. Cryst. L i q . Cryst. 1977, 40, 33. Clark, N. A.;Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899. Saha, S. K.;Wong, G. K. Appl. Phys. Lett. 1979, 34, 423. Saha. S. K. Opt. Comm. 1981, 37, 373. Sauteret, C.; Hermann, J. P.; Frey, R.; Pradere, F . ; Ducuing, J.; Baughman, R. H.; Chance, R. Phys. Rev. Lett. 1976, 36, 956. Garito, A. F.; Singer, K. D.; Hayes, K; Lipscomb, G. F.; Lalama, S. J.; Desai, K. N. J. Am. Opt. Soc. 1980, 70, 1399. Tieke, B.; Graf, H. J.; Wegner, G.; Naegele, B.; Ringsdorf, H.; Bauerjie, Α . ; Day, D.; Lando, J. B. Colloid Poly. Sci. 1977, 255, 36. P i t t , C. W.; Walpita, L . M. Thin Solid Films 1980, 68, 101. Bergman, J. G.; McFee, J. H.; Crane, G. R. Appl. Phys. Lett. 1971, 18, 203. Meredith, G. R.; VanDusen, J. G.; Williams, D. J. Macromol. 1982, 15, 1385. Havinga, Ε. E.; VanPelt, P. Ber. Bundsenges. Phys. Chem. 1979, 83, 816. Finkelman, H.; Ringsdorf, H.; Wendorf, J. H. Makromol. Chem. 1978, 179, 273. Shibaev, V. P.; Kostromin, S. G.; Plate, N. A. Eur. Polym. J. 1982, 18, 651.



5.

C.

MEREDITH ET AL.


27. Whinnery, J. R.; Hu, Chenming; Kwon, Y. S. IEEE J. Quantum Electron. 1977, 13, 262. 28. Wojtowicz, P. J. "Introduction to Liquid Crystals"; Priestley, Ε. B.; Wojtowicz, P. J.; Sheng, Ping, Eds.; Plenum: New York, 1975. 29. Jen, S.; Clark, N. A.; Pershan, P. S.; Priestley, Ε. B. Phys. Rev. Lett. 1973, 31, 1552. 30. Penchev, I.; Dozov, I.; Kirov, N.; Afanasyeva, N.; Ruoliene, J. Spec. Lett. 1982, 15, 265. 31. B ö t t c h e r , C. J. F. "Theory of E l e c t r i c Polarization"; Elsevier: New York, 1952, p. 28f. 32. Meredith, G. R. J. Chem. Phys. 1982, 77, 5863. 33. Jerphagnon, J.; Chemla, D.; Bonneville, R. Adv. Phys. 1978, 27, 609. 34. Butcher, P. N. "Nonlinear Optical Phenomena"; Ohio State University Engineering: Columbus, 1965. 35. Oudar, J. L . J. Chem. Phys. 1977, 69, 446. 36. Flytzanis, C. "Quantum Electronics: A Treatise"; Rabin, H.; Tang, C. L., Eds.; Academic: New York, 1975, Vol. I, p. 9. 37. Kurtz, S. K. "Quantum Electronics: A Treatise"; Rabin, H.; Tang, L., Eds.; Academic: New York, 1975, Vol. I, p. 227. 38. Byer, R. L . "Nonlinear Optics"; Harper, P. G . ; Wherrett, B. S., Eds.; Academic: New York, 1977, p. 47. 39. Zernike, F.; Midwinter, J. E. "Applied Nonlinear Optics"; Wiley: New York, 1973. 40. Singer, K. D.; Garito, A. F. J. Chem. Phys. 1981, 75, 3572. 41. VanDusen, J. G . ; Williams, D. J.; Meredith, G. R. Polymer Preprint 1982, 23, 149. RECEIVED June 14, 1983


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Nonlinear Optical Properties of Organic and Polymeric Materials

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