Light-Induced Conformational Changes of Polymers in Solution and


is attained by incorporating photochromic chromophores ... hv)4. 0097-6156/87/0358-0107$06.00/0. © 1987 American Chemical Society ... 1) trans-cis ge...
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Chapter 10

Light-Induced Conformational Changes of Polymers in Solution and Gel Phase

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Masahiro Irie Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567, Japan

Attempts have been made to control the polymer chain conformation reversibly by photoirradiation. The aim is attained by incorporating photochromic chromophores into the pendant groups or main chain. It was found from the results in solution that the intramolecular electrostatic force of repulsion between photogenerated pendant cations i s a more effective driving force for the conformational change than trans-cis geometrical isomerization of unsaturated linkages in the polymer backbone. The large conformational change at the molecular level i s amplified into the shape change of polymer gels at the visible macro level. Poly­ acrylamide gels having photoionizable triphenylmethane leucocyanide groups dilated 2.2 times i n each dimension by ultraviolet irradiation. Electric field effect on the gel was also examined. By applying alternating electric field(0.5 Hz), the rod-shaped gel showed oscillating motion. A p o l y m e r c h a i n c o n f o r m a t i o n i s w e l l known t o d e p e n d o n t h e e n v i r o n ­ ment, s u c h a s s o l v e n t o r t e m p e r a t u r e . I n good s o l v e n t s , polymers have a n extended c o n f o r m a t i o n , w h i l e t h e y s h r i n k i n poor s o l v e n t s a t low \ jmperature. P o l y e l e c t r o l y t e s change t h e i r c o n f o r m a t i o n w i t h c h a n g e s i n pH a n d s a l t c o n c e n t r a t i o n (J.). Our i n t e r e s t i st o c o n t r o l the polymer chain conformation by "photochemistry" rather than by changing t h e environment(2,3). I t i s o b v i o u s l y a t e d i o u s method t o change t h e environment t o c o n t r o l t h e c h a i n c o n f o r m a t i o n . The p h o t o c h e m i c a l m e t h o d i s much s u p e r i o r i n t h e r e s p o n s e t i m e , r e v e r s i b i l i t y and easy procedure. Among n u m e r o u s p h o t o c h e m i c a l r e a c t i o n s , p h o t o c h r o m i c r e a c t i o n s are u s e f u l f o r t h i s purpose. The photochromic r e a c t i o n i s d e f i n e d as a r e v e r s i b l e change i n a c h e m i c a l s p e c i e s between two forms h a v i n g different absorption spectra, hv A

Β hv)4

0097-6156/87/0358-0107$06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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108

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Besides the a b s o r p t i o n s p e c t r a l change, the i s o m e r i z a t i o n s are always a c c o m p a n i e d by c e r t a i n p h y s i c a l p r o p e r t y c h a n g e s , s u c h as d i p o l e moment a n d / o r g e o m e t r i c a l s t r u c t u r a l c h a n g e s . The p r o p e r t y c h a n g e s may be u t i l i z e d a s a d r i v i n g f o r c e t o i n d u c e t h e c o n f o r m a t i o n a l c h a n g e s by i n c o r p o r a t i n g t h e c h r o m o p h o r e s i n t o t h e p o l y m e r s . The a i m t o c o n t r o l t h e p o l y m e r c h a i n c o n f o r m a t i o n b y p h o t o i r r a d i a t i o n was a t t a i n e d b y u s i n g f o l l o w i n g p h o t o c h r o m i c r e a c t i o n s ; 1) t r a n s - c i s g e o m e t r i c a l i s o m e r i z a t i o n o f u n s a t u r a t e d l i n k a g e s i n t h e p o l y m e r b a c k b o n e , 2) r e v e r s i b l e g e n e r a t i o n o f s t r o n g d i p o l e s i n t h e p o l y m e r p e n d a n t g r o u p s , a n d 3) p h o t o i o n i z a t i o n o f t h e p e n d a n t g r o u p s . R e p r e s e n t a t i v e examples of each system are polyamide w i t h backbone azobenzene r e s i d u e s (4-8). p o l y ( m e t h y l methacrylate) w i t h pendant s p i r o b e n z o p y r a n g r o u p s ( 9 - 1 1 ) . and poly(N,N-dimethylacrylamide) w i t h p e n d a n t t r i p h e n y l m e t h a n e l e u c o h y d r o x i d e g r o u p s (12_). The first p a r t d e s c r i b e s some d e t a i l s o f t h e s e e x a m p l e s . I t seems p o s s i b l e t o a m p l i f y t h e p h o t o s t i m u l a t e d c o n f o r m a t i o n a l changes i n s o l u t i o n a t the m o l e c u l a r l e v e l i n t o shape changes of polymer g e l s o r s o l i d s a t the v i s i b l e macro l e v e l . The first p r o p o s a l t o use t h e s t r u c t u r a l changes a t t h e m o l e c u l a r l e v e l f o r d i r e c t c o n v e r s i o n o f p h o t o n e n e r g y i n t o m e c h a n i c a l work has been made b y M e r i a n (13.) i n 1 9 6 6 . S i n c e t h e n , many m a t e r i a l s , m o s t o f w h i c h c o n t a i n e d a z o b e n z e n e c h r o m o p h o r e s , h a v e b e e n r e p o r t e d t o show photostimulated deformation(14). T i l l now, h o w e v e r , t h e r e p o r t e d d e f o r m a t i o n s were l i m i t e d t o l e s s t h a n 10%. In a d d i t i o n , Matejka e t . a l . h a v e p o i n t e d o u t t h a t i n many c a s e s p h o t o - h e a t i n g e f f e c t i n s t e a d of photochemical r e a c t i o n p l a y s a dominant r o l e i n the deformation(15,16). I n due c o n s i d e r a t i o n o f t h e s e r e s u l t s , we h a v e d e c i d e d t o e m p l o y e l e c t r o s t a t i c forces to achieve a large r e v e r s i b l e deformation of gels. The e l e c t r o s t a t i c f o r c e i s e x p e c t e d t o be a m o r e e f f e c t i v e d r i v i n g f o r c e f o r the c o n f o r m a t i o n a l changes of polymer c h a i n s than t r a n s - c i s g e o m e t r i c a l i s o m e r i z a t i o n of unsaturated l i n k a g e s . The second p a r t d e s c r i b e s the p h o t o s t i m u l a t e d d i l a t i o n of polymer g e l s . D u r i n g t h e c o u r s e o f e x p e r i m e n t s t o r e v e a l an e l e c t r i c field e f f e c t o n t h e b e h a v i o r o n t h e p h o t o g e n e r a t e d m o b i l e i o n s , we f o u n d a p e c u l i a r phenomenon, r e v e r s i b l e b e n d i n g m o t i o n o f t h e r o d - s h a p e d gels. The r e s u l t w i l l a l s o be b r i e f l y d e s c r i b e d . Photostimulated Conformational level

Changes i n S o l u t i o n -

Molecular

Figure 1 i l l u s t r a t e s the proposals t o induce the conformational c h a n g e s o f p o l y m e r c h a i n s by u s i n g p h o t o c h r o m i c r e a c t i o n s . The mechanism (1) u t i l i z e s t r a n s - c i s g e o m e t r i c a l i s o m e r i z a t i o n as a t o o l t o enforce the c o n f o r m a t i o n a l changes. When t h e t r a n s - c i s p h o t o i s o m e r i z a b l e chromophores are i n c o r p o r a t e d i n t o the polymer backbone, the i s o m e r i z a t i o n from t r a n s t o c i s form k i n k s t h e e x t e n d e d polymer c h a i n s , r e s u l t i n g i n a compact c o n f o r m a t i o n . The c o m p a c t c o n f o r m a t i o n r e t u r n s t o t h e i n i t i a l e x t e n d e d c o n f o r m a t i o n by t h e t h e r m a l o r p h o t o c h e m i c a l i s o m e r i z a t i o n o f t h e chromophores from the c i s t o t r a n s form. Polyamides w i t h azobenzene g r o u p s i n t h e p o l y m e r b a c k b o n e a r e among t h e e a r l i e s t i n w h i c h t r a n s - c i s i s o m e r i z a b l e chromophores are used t o r e g u l a t e the conformation of polymer c h a i n s ( 4 , 5 ) .

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Light-Induced

10. IRIE

The a z o b e n z e n e follows(17):

Conformational Changes

c h r o m o p h o r e i s known t o c h a n g e t h e g e o m e t r y

' ο

Μ

9,0Α.,^ Downloaded by UNIV OF MISSOURI COLUMBIA on April 8, 2017 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch010

109

λ/|

Ν

—?

«

Α2

1

as

ο

5.5Α

3 0 0 < Μ < 4 0 0 nm 450 nm < λ2 A typical

example o f t h e polymer

h a v i n g azobenzene

r e s i d u e s i s (4,5):

HOOC |!| \ = / COOH (1)

*N-fW -hr Η C

n

The i n t r i n s i c v i s c o s i t y , ( r j j o f p o l y a m i d e ( l ) i n N , N - d i m e y t h y l a c e t a m i d e was f o u n d t o d e c r e a s e f r o m 1.22 t o 0.50 d l / g o n u l t r a v i o l e t i r r a d i a ­ t i o n (410>λ^>350 nm) a n d t o r e t u r n t o t h e i n i t i a l v a l u e i n 30 h i n t h e d a r k a t 20°C. T h e s l o w r e c o v e r y o f t h e v i s c o s i t y i n t h e d a r k was a c c e l e r a t e d b y v i s i b l e l i g h t irradiation(λ >470 n m ) . On a l t e r n a t e i r r a d i a t i o n o f u l t r a v i o l e t and v i s i b l e l i g h t , t h e v i s c o s i t y r e v e r s i b l y c h a n g e d b y a s much a s 6 0 % . M e c h a n i s m ( 2 ) e m p l o y s a n e l e c t r o s t a t i c f o r c e o f r e p u l s i o n among photogenerated charges as a d r i v i n g force f o r a conformational change. T r i p h e n y l m e t h a n e l e u c o d e r i v a t i v e s a r e t h e most c o n v e n i e n t chromophores t o produce p o s i t i v e charges i n t h e pendant groups o f p o l y m e r s , because t h e quantum y i e l d o f t h e p h o t o d i s s o c i a t i o n i s r e a s o n a b l y h i g h ( Φ>0.2) a n d t h e p h o t o g e n e r a t e d p o s i t i v e c h a r g e s have a r a t h e r l o n g l i f e t i m e ( x ^ m i n ) . Upon u l t r a v i o l e t i r r a d i a t i o n , the chromophore d i s s o c i a t e s i n t o an i o n p a i r w i t h p r o d u c t i o n o f an intensely colored triphenylmethyl cation. The c a t i o n r e c o m b i n e s thermally with the counter ion(18); #

2

(2)

X

T r i p h e n y l m e t h a n e l e u c o h y d r o x i d e r e s i d u e s were i n t r o d u c e d i n t o t h e p e n d a n t g r o u p s b y c o p o l y m e r i z i n g t h e v i n y l d e r i v a t i v e ( 2 , X= OH, R= C H = C H ) w i t h N , N - d i m e t h y l a c r y l a m i d e ( 1 2 ) . I n t h e d a r k b e f o r e i r r a d i a ­ t i o n , a methanol s o l u t i o n c o n t a i n i n g t h e copolymer e x h i b i t e d a p a l e green c o l o r . Upon u l t r a v i o l e t i r r a d i a t i o n ( λ>270 n m ) , t h e s o l u t i o n 2

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

110

PHOTOPHYSICS OF POLYMERS

became d e e p g r e e n , w h i c h c o l o r d i s a p p e a r e d s l o w l y i n t h e d a r k w i t h a h a l f l i f e t i m e o f 3.3 m i n ( F i g u r e 2 A ) . The a p p e a r a n c e o f a d e e p g r e e n c o l o r means t h a t t h e p e n d a n t t r i p h e n y l m e t h y l l e u c o h y d r o x i d e residues d i s s o c i a t e i n t o t r i p h e n y l m e t h y l c a t i o n s and h y d r o x i d e i o n s . The photogenerated p o s i t i v e charges recombine w i t h the d i s s o c i a t e d hydroxide ions t o reproduce the c o l o r l e s s leuco form. Concurrently w i t h the c o l o r a t i o n , the reduced v i s c o s i t y of the s o l u t i o n , r| p/c, s h o w e d a r e m a r k a b l e i n c r e a s e f r o m 0.55 t o 1.6 d l / g a s d e p i c t e d i n F i g u r e 2B. A f t e r r e m o v a l o f t h e l i g h t , ^gp/c r e t u r n e d t o t h e i n i t i a l v a l u e w i t h a h a l f - l i f e t i m e o f 3.1 m i n . The v i s c o s i t y c h a n g e i n d i c a t e s t h a t t h e polymer c h a i n expands upon u l t r a v i o l e t i r r a d i a t i o n and s h r i n k s i n t h e d a r k . The g o o d c o r r e l a t i o n b e t w e e n t h e v i s c o s i t y c h a n g e a n d t h e a b s o r p t i o n i n t e n s i t y a t 620 nm i m p l i e s t h a t e x p a n s i o n o f t h e p o l y m e r c o n f o r m a t i o n i s i n d u c e d by the e l e c t r o s t a i c r e p u l s i v e f o r c e s among t h e p e n d a n t t r i p h e n y l m e t h y l cations.

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S

The c o n c e n t r a t i o n d e p e n d e n c e o f Hsp/c c o n f i r m e d t h e a b o v e e x p a n s i o n mechanism. In the dark b e f o r e p h o t o i r r a d i a t i o n , the dependence was l i n e a r ; t h e r e d u c e d v i s c o s i t y d e c r e a s e d w i t h d e c r e a s i n g t h e c o n c e n t r a t i o n of the polymer. During u l t r a v i o l e t i r r a d i a t i o n , t h i s d e p e n d e n c e showed an a n o m a l o u s b e h a v i o r ; flgp/c s t e e p l y i n c r e a s e d a t low polymer c o n c e n t r a t i o n . The v i s c o s i t y d u r i n g p h o t o i r r a d i a t i o n was 4 t i m e s l a r g e r t h a n t h e v i s c o s i t y i n t h e d a r k a t 0 . 0 4 g / d l . At low polymer c o n c e n t r a t i o n , s c r e e n i n g of the e l e c t r o s t a t i c p o t e n t i a l by t h e c o u n t e r i o n s b e c o m e s weak a n d c o n s e q u e n t l y t h e i n c r e a s e o f the r e p u l s i v e f o r c e s of the p o s i t i v e charges along the polymer chain expands the dimension of the c h a i n . The p h o t o - e f f e c t due t o t h e e l e c t r o s t a t i c f o r c e s i s much l a r g e r t h a n t h e e f f e c t o b s e r v e d f o r p o l y amides having azobenzene r e s i d u e s i n the backbone. I t i s worthwhile t o n o t e t h a t t h e p h o t o s t i m u l a t e d i n c r e a s e o f r|sp/c was s t r o n g l y s u p p r e s s e d by t h e p r e s e n c e o f s a l t ( 1 0 M LiBr). The r a t i o o f t h e s p e c i f i c v i s c o s i t y d u r i n g p h o t o i r r a d i a t i o n t o t h a t i n t h e d a r k , Hp/^d t increased with i n c r e a s i n g content of triphenylmethane l e u c o h y d r o x i d e r e s i d u e s i n t h e p e n d a n t g r o u p s and r e a c h e d a maximum o f 3.3 a t 0.18 m o l e f r a c t i o n . A b o v e t h e content,the r a t i o d e c r e a s e d , t h e d e c r e a s e b e i n g due t o t h e l o w s o l u b i l i t y o f t h e residues i n methanol. The c o n c e p t t o a d o p t t h e e l e c t r o s t a t i c r e p u l s i v e f o r c e a s a d r i v i n g f o r c e f o r a photostimulated expansion of the polymer chain i s u s e f u l and w i d e l y a p p l i c a b l e t o o t h e r p o l y m e r s y s t e m s . Polys t y r e n e and p o l y a c r y l a m i d e h a v i n g p e n d a n t l e u c o h y d r o x i d e and l e u c o c y a n i d e groups were found t o change t h e i r s o l u t i o n v i s c o s i t y i n m e t h y l e n e c h l o r i d e and i n w a t e r , r e s p e c t i v e l y . Photostimulated

D i l a t i o n of Polymer Gels

- Macro

Level

I t i s i n f e r r e d f r o m t h e a b o v e r e s u l t s on t h e c o n f o r m a t i o n a l changes i n s o l u t i o n t h a t the e l e c t r o s t a t i c f o r c e of r e p u l s i o n b e t w e e n p h o t o g e n e r a t e d c h a r g e s , m e c h a n i s m ( 2 ) , i s a more e f f e c t i v e d r i v i n g f o r c e f o r c o n f o r m a t i o n a l changes than t r a n s - c i s g e o m e t r i c a l i s o m e r i z a t i o n of unsaturated l i n k a g e s , mechanism(1). In attempting t o a m p l i f y t h e l a r g e c o n f o r m a t i o n a l c h a n g e s due t o t h e e l e c t r o s t a t i c r e p u l s i v e f o r c e s i n s o l u t i o n at the molecular l e v e l to the v i s i b l e m a c r o l e v e l , we i n t r o d u c e d t h e m e c h a n i s m ( 2 ) i n t o t h e g e l p h a s e . A c r y l a m i d e g e l s ( 3 ) c o n t a i n i n g a s m a l l amount o f t r i p h e n y l -

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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IRIE

111

Light-Induced Conformational Changes

F i g u r e 1. S c h e m a t i c i l l u s t r a t i o n o f p h o t o s t i m u l a t e d t i o n a l chages o f polymer c h a i n s .

x

OA

^

Dark

Dark

conforma­

A

^0.2

0


2 7 0 n m ) , t h e g e l q u i c k l y b e n t i n 1 m i n . The g e l e n d m o v e d t o t h e d i r e c t i o n of a p o s i t i v e electrode(Figure 6B). During the bending motion, the center o f g r a v i t y o f the g e l remained a t t h e i n i t i a l p o s i t i o n . T r a n s l a t i o n a l movement o f t h e e n t i r e g e l t o t h e n e g a t i v e e l e c t r o d e was n o t o b s e r v e d . By c h a n g i n g t h e p o l a r i t y o f t h e e l e c t r i c f i e l d , t h e g e l a g a i n becomes s t r a i g h t a n d t h e n bends t o t h e a n o t h e r d i r e c t i o n ( F i g u r e 6 C ) . T h e r e s p o n s e t i m e o f t h e g e l s h a p e c h a n g e was around 2 min. A f t e r s w i t c h i n g o f f the l i g h t , the g e l slowly returned to t h e i n i t i a l s t r a i g h t shape i n t h e e l e c t r i c f i e l d . The r e s u l t suggests t h a t p h o t o d i s s o c i a t i o n of t h e l e u c o c y a n i d e groups i n t h e g e l i s indispensable t o the g e l bending motion. In o r d e r t o d e t e r m i n e q u a n t i t a t i v e l y t h e response time o f t h e m o t i o n , o n e e n d o f t h e r o d - s h a p e d g e l was f i x e d t o t h e w a l l a n d t h e m o v i n g d i s t a n c e o f t h e o t h e r f r e e e n d , 1, f r o m t h e i n i t i a l p o s i t i o n was m e a s u r e d a s a f u n c t i o n o f i r r a d i a t i o n t i m e . F i g u r e 7 s h o w s t h e p h o t o s t i m u l a t e d b e n d i n g m o t i o n o f t h e g e l ( 2 6 mm i n l e n g t h a n d 2 mm i n s e c t i o n d i a m e t e r ) i n an e l e c t r i c f i e l d ( 1 0 V/cm). The f r e e e n d m o v e d t o w a r d t h e p o s i t i v e e l e c t r o d e w i t h a i n i t i a l s p e e d o f 0.40 mm/ sec. By c h a n g i n g t h e p o l a r i t y o f t h e e l e c t r i c f i e l d , t h e e n d m o v e d to another d i r e c t i o n . Upon s w i t c h i n g o f f t h e e l e c t r i c f i e l d , t h e b e n t g e l r e t u r n e d t o t h e i n i t i a l p o s i t i o n w i t h a s p e e d o f 0.075 mm/ sec. The b e n d i n g r a t e d e p e n d s o n t h e a p p l i e d f i e l d . Upon i n c r e a s i n g the f i e l d , t h e r e s p o n s e t i m e i n c r e a s e s i n p r o p o r t i o n t o t h e a p p l i e d field. A l t h o u g h t h e b e n d i n g r a t e became v e r y s l o w , t h e b e n d i n g m o t i o n was o b s e r v e d i n a v e r y weak f i e l d o f 1.25 V/cm. In t h i s case, e f f e c t i v e v o l t a g e a p p l i e d t o t h e g e l was o n l y 0.25 V. I n t h e a b o v e e x p e r i m e n t s , d e i o n i z e d w a t e r was u s e d a s t h e external solution. As d e s c r i b e d i n t h e p r e v i o u s s e c t i o n , t h e addition of s a l t s t o the solution decreased the photostimulated d i l a t i o n of the gels. I f the bending motion i n the e l e c t r i c f i e l d was d u e t o t h e o s m o t i c p r e s s u r e m e c h a n i s m , t h e a d d i t i o n o f s a l t w o u l d a l s o s u p p r e s s t h e m o t i o n . T h i s i s n o t t h e c a s e . On t h e c o n t r a r y , t h e r e s p o n s e t i m e o f t h e b e n d i n g m o t i o n was a c c e l e r a t e d b y t h e addition of salts t o the external solution. The b e n d i n g r a t e i n t h e s o l u t i o n c o n t a i n i n g 2 χ 1 0 ~ 3 m o l e / l N a C l was 1.5 mm/sec, w h i c h i s 4 time f a s t e r than t h e r a t e i n t h e absence o f NaCl. The r e s u l t i n d i ­ c a t e s t h a t t h e b e n d i n g m o t i o n i n t h e e l e c t r i c f i e l d i s n o t due t o t h e o s m o t i c p r e s s u r e mechanism. When a s o l u t i o n c o n t a i n i n g s a l t s i s u s e d , i t i s d i f f i c u l t t o examine p u r e e l e c t r i c f i e l d e f f e c t w i t h o u t b e i n g d i s t u r b e d by t h e e l e c t r o c h e m i c a l r e a c t i o n s on t h e e l e c t r o d e s . The r e a c t i o n s on t h e e l e c t r o d e s p r o d u c e pH g r a d i e n t i n t h e s o l u t i o n . Although the leuco­ c y a n i d e g e l s a r e r a t h e r i n s e n s i t i v e t o t h e pH c h a n g e , t h e c o r r e l a t i o n b e t w e e n t h e b e n d i n g m o t i o n a n d t h e pH c h a n g e was e x a m i n e d b y a d d i n g a pH i n d i c a t o r , p h e n o l r e d , i n t o t h e e x t e r n a l s o l u t i o n . The b e n d i n g m o t i o n was f o u n d t o b e much f a s t e r t h a n t h e c o l o r c h a n g e o n t h e e l e c t r o d e . The r e s u l t s u g g e s t s t h a t t h e r a p i d b e n d i n g m o t i o n i s i n d e p e n d e n t o f t h e pH c h a n g e . T h i s was f u r t h e r c o n f i r m e d b y a p p l y i n g a l t e r n a t i n g e l e c t r i c f i e l d , a s shown i n F i g u r e 8. The r o d s h a p e d g e l , one e n d o f w h i c h i s f i x e d on t h e w a l l , v i b r a t e s i n r e s p o n s e t o t h e a l t e r n a t i n g e l e c t r i c f i e l d o f 0.5 H z . u n d e r u l t r a v i o l e t i r r a d i a t i o n .

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F i g u r e 7. P h o t o s t i m u l a t e d b e n d i n g m o t i o n o f a r o d s h a p e d a c r y l a m i d e g e l (26 mm i n l e n g t h a n d 2 mm i n s e c t i o n d i a m e t e r ) h a v i n g 3.1 m o l e % t r i p h e n y l m e t h a n e l e u c o c y a n i d e g r o u p s i n a n e l e c t r i c f i e l d (10 V/cm) i n w a t e r . T h e e l e c t r i c f i e l d was r e m o v e d a f t e r 120 s e c . ( R e p r o d u c e d f r o m R e f . 26. C o p y r i g h t 1986 A m e r i c a n C h e m i c a l S o c i e t y . )

F i g u r e 8. P h o t o s t i m u l a t e d v i b r a t i n a l m o t i o n o f a r o d s h a p e d a c r y l a m i d e g e l h a v i n g 3.1 m o l e % t r i p h e n y l m e t h a n e leucocyanide g r o u p s i n a n a l t e r n a t i n g e l e c t r i c f i e l d ( 0 . 5 H z , ± 8V/cm) i n water. I n t h i s c a s e , t h e pH o f t h e s o l u t i o n r e m a i n e d i n t h e n e u t r a l v a l u e (around 6.5). The b e n d i n g b e h a v i o r o f t h e g e l s u g g e s t s i n h o m o g e n e o u s e x p a n s i o n of t h eg e l i nt h ee l e c t r i c f i e l d . The n e g a t i v e e l e c t r o d e s i d e o f t h e g e l expands l a r g e r than t h e o t h e r s i d e . Since thee l e c t r i c f i e l d i s a p p l i e d p e r p e n d i c u l a r t o t h e g e l a x i s , m o b i l e n e g a t i v e i o n s , CN~", a r e a t t r a c t e d t o t h ep o s i t i v e electrode side i n theg e l . Consequently, excess p o s i t i v e charges a r e l e f t on t h e o t h e r s i d e . Internal r e p u l s i v e f o r c e between t h e p o s i t i v e charges, which a r e f i x e d i n t h e gel network, i sconsidered t o cause t h e expansion o f t h e negative electrode side o f t h eg e l .

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

IRIE

Light-Induced Conformational Changes

Other Properties

of Photoresponsive

121

Polymers

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Because o f l i m i t a t i o n o f t h i s chapter, o n l y a few p r o p e r t i e s o f photoresponsive polymers are described. Table I I includes several other properties so f a r reported(27-50). A l l o f these p h y s i c a l and c h e m i c a l p r o p e r t i e s a r e found t o be c o n t r o l l e d r e v e r s i b l y by p h o t o i r r a d i a t i o n . I t i s now g e n e r a l l y a c c e p t e d t h a t p h o t o c h r o m i c r e a c t i o n s are u s e f u l as a t o o l t o photo-control the p r o p e r t i e s o f s y n t h e t i c p o l y m e r s . The p h o t o r e s p o n s i v e p o l y m e r s have p o t e n t i a l a p p l i c a t i o n s f o r many p h o t o a c t i v e d e v i c e s , s u c h a s s e n s o r s , s w i t c h e s , m e m o r i e s , photo-mechanical transducers and so on. Table I I .

Photocontrol o f P h y s i c a l and Chemical P r o p e r t i e s o f Polymer S o l u t i o n s and S o l i d s

Solution

Solid

V i s c o s i t y (3-12,27,28) pH-value (5,29) S o l u b i l i t y (30-32) M e t a l Ion C a p t u r e (7,34)

Membrane P o t e n t i a l ( 3 5 ) W e t t a b i l i t y (36) Shape (19,20,37-45) S o l - G e l T r a n s i t i o n (46,47) Tg ( 4 8 ) C o m p a t i b i l i t y o f Polymer Blends (49) Absorptive A b i l i t y (50)

Acknowledgments A c k n o w l e d g m e n t i s made t o t h e D o n o r o f t h e P e t r o l e u m R e s e a r c h F u n d , administered by the American Chemical S o c i e t y , f o r p a r t i a l support o f this activity.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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RECEIVED March 13, 1987

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.