2476
Macromolecules 1986, 19, 2476-2480
(19) Yamamoto, H.; Nishida, A.; Hayakawa, T.; Nishi, N.; Yamamoto, R. Bull. Chem. SOC.Jpn. 1986,59, 1641. (20) Yamamoto, H.; Nishida, A. Macromolecules 1986, 19, 943. (21) Blout, E. R.; Karlson, R. H. J. Am. Chem. SOC.1956, 78,941. (22) Hatano, M.; Yoneyama, M. J. Am. Chem. SOC.1970,92,1392. (23) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966; pp 783-786. (24) Houben, J. L.; Fissi, A,; Bacciolo, D.; Rosato, N.; Pieroni, 0.; Ciardelli, F. Int. J. Biol. Mucromol. 1983, 5, 94.
Hartley, G. S. J . Chem. SOC.1938, 633. Le FBvre, J. W.; Northcott, J. J. Chem. SOC.1953, 867. Wyman, G. M. Chem. Rev. 1955,55, 625. Kalinowski, H. 0.;Kessler, H. Top. Sterochem. 1973, 7, 295-383. (29) Yamamoto, H.; Nishida, A. J . Chem. SOC.Jpn. 1985, 2338. (30) Yamamoto, H.; Nakazawa, A. Biopolymers 1984, 23, 1367. (31) Ballard, R. E.; McCaffery, A. J.; Mason, S. F. Biopolymers 1966, 4, 97.
(25) (26) (27) (28)
Photoresponsive Polymers. 8.l Reversible Photostimulated Dilation of Polyacrylamide Gels Having Triphenylmethane Leuco Derivatives2 Masahiro Irie* and Dawan Kunwatchakun The Institute of Scientific and Industrial Research, Osaka University, Zbaraki, Osaka 567, Japan. Received March 7, 1986
ABSTRACT: Upon ultraviolet irradiation (A > 270 nm), acrylamide gels having triphenylmethaneleucocyanide groups (1-4 mol %) dilated in water. The weight of the gel having 3.1 mol % leucocyanide groups increased by as much as 13 times and the size expanded approximately 2.2 times in each dimension. The dilated gel deswelled in the dark to the initial size. The cycles of dilation and contraction of the gel by photoirradiation could be repeated several times. The gel expansion was suppressed by the addition of salts such as NaCl or KBr. The salt effect and semiquantitative theoretical consideration of the behavior of ions suggested that the osmotic pressure differentials between the gel inside and the outer solution, which is produced by photodissociation of triphenylmethane leucocyanidegroups contained in the gel network, are the main driving force of the photostimulated gel dilation.
Introduction It is well-known that a polymer chain conformation depends on the solvent and temperature. In good solvents, polymers have an extended conformation, while they shrink in poor solvents a t low temperature. Polyelectrolytes change their conformation with changes in pH and salt concentration. Recently, several attempts have been reported to induce the conformational changes by “photochemistry” rather than “chemi~try”.~ Many photosensitive molecules are known to be transformed under photoirradiation into other isomers, which return to the initial state either thermally or photochemically. The isomerizations are always accompanied by certain physical and chemical property changes. The property changes of the chromophores, such as dipole moment and/or geometrical structure changes, have been utilized as the driving force to induce conformational changes of the polymer chains in solution by incorporating the chromophores into the polymers. The following three photochemical reactions are useful to control the chain conformations: (1)trans-cis isomerization of unsaturated linkages in the polymer backbone, (2) reversible photogeneration of strong dipoles in the polymer pendant groups, and (3) photoionization of the pendant groups. Representative examples of each system are polyamides with backbone azobenzene residue^,^ poly(methy1 methacrylate) with pendant spirobenzopyran groups? and poly(NJJ-dimethylacrylamide) with pendant triphenylmethane leucohydroxide.6 It seems possible to amplify the photostimulated conformational changes of the polymer chains in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. Attempts to use the structural changes of photoisomerizable chromophores at the molecular level for direct conversion of photon energy into mechanical work have been initiated by Meriar~.~ The system described by Merian was nylon filament fabric, 6
cm wide and 30 cm long, containing 15 mg/g of azo dye. After exposure to a xenon lamp, the dyed fabric was found to be 0.33 mm shorter. On the basis of Lovrien’s idea? van der Veen and Prins reported a photomechanical transd ~ c e r ,consisting ~ of water-swollen gels of poly(2hydroxyethyl methacrylate) mixed with a sulfonated bis(azostilbene) dye, The photostimulated contraction of the gel was 1.2%. Since then, many materials, most of which contained azobenzene chromophores in polymer, have been reported to show photostimulated deformation.1° The most recent study was polyquinoline with backbone stilbene gr0ups.l’ Till now, however, the reported deformations were limited to less than 10%. In addition, MatZijka et a1.12reported that in some cases a photoheating effect instead of photochemical reaction plays a dominant role in the photostimulated deformation. In the present study, we report large reversible photostimulated dilation of polyacrylamide gels by incorporating a small amount of triphenylmethane leuco derivatives in the gel network. Triphenylmethane leuco derivatives are well-known photochromic molecules which dissociate into ion pairs under ultraviolet irradiation, producing intensely colored trimphenylmethyl cations. The cations thermally recombine with counterions as follows:
According to eq 1, triphenylmethane leuco derivatives function as reversible ionizable groups by photoirradiation.I3 It is inferred from our previous experiments in solution6that the electrostatic force of repulsion between
0024-9297/86/2219-2476$01.50/0 0 1986 American Chemical Society
PhotoresponsivePolymers. 8 2477
Macromolecules, Vol. 19, No. 10, 1986
l p
0
,
,
2
,
-'3
I'd 20
Time, h
22
f ~ / L
LO
.
Figure 1. Photostimulated dilation and contraction of poly-
acrylamide gel having pendant triphenylmethane leucohydroxide groups (3.7 mol %) with light of wavelength longer than 270 nm at 25 "C. Initial pH of external water phase was 6.6. W, is the weight of the gel before photoirradiation.
PH
Figure 2. Swelling of polyacrylamide gels having triphenylmethane leucohydroxidegroups (3.7 mol %) in the dark in water at various pH values. Wo is the weight of the gel at pH 8.0.
photogenerated charges is a more effective driving force of conformational changes in polymer chains than trans-cis geometrical isomerization of unsaturated linkages. Therefore, we incorporated the leuco derivatives into polyacrylamidegels with the aim of achieving large reversible dilation of the gels.
51
1 /j
Experimental Section Acrylamide gels containinga small amount of triphenylmethane leucohydroxide (1, X = OH, Y = N(CH&) or leucocyanide (1, X = CN, Y = N(CH3),)residues were prepared by free radical copolymerization of acrylamide and diphenyl(4-vinylpheny1)methane leucohydroxide (1, X = OH, Y = N(CH3),, 2 = CH= CH,) or leucocyanide (1, X = CN, Y = N(CH3),,Z = CH=CH2) in dimethyl sulfoxide (MezSO) in the presence of N,N'methylenebis(acrylamide). The vinyl derivative of the leucohydroxide was synthesized from p-bromostyrene. The leucohydroxide was converted to leucocyanide by the reaction with KCN under acidic conditions.13 Acrylamide (500 mg), N,"-methylenebis(acrylamide) (13 mg), the vinyl derivative of leucohydroxide or leucocyanide, and 2,2'-azobis(isobutyronitrile) (20 mg) were dissolved in Me2S0and heated to 60 "C for 3 h. The gels were removed from the polymerization tubes and soaked in Me2S0 and then in water to remove all residual monomers and initiators. The content of triphenylmethane leuco derivatives in the gel was measured by elemental analysis. The gels were swollen to the equilibrium condition on standing in water overnight. Then the weight or dimension change of the gels induced by irradiation with ultraviolet light (A > 270 nm, Toshiba UV-29 filter) was measured. Irradiation was carried out with a high-pressure mercury lamp (USIO, 1 kW). Results and Discussion Photostimulated Dilation. Figure 1 shows the reversible dilation of a disk-shaped gel (10 mm in diameter and 2 mm in thickness) having triphenylmethane leucohydroxide residues (3.7 mol %) in water (pH 6.6) as measured by the change of weight. Upon ultraviolet irradiation (A > 270 nm), the gel swelled and the weight of the gel increased by as much as 3 times within 1 h. The dilated gel deswelled in the dark to the initial weight in 20 h. The cycles of dilation and contraction of the gel by photoirradiation could be repeated several times. The weight change was not observed for a polyacrylamide gel having no triphenylmethane leucohydroxide in the gel network. A photoheating effect is discounted on the basis of a reference experiment in which the gel was heated to 40 "C. No significant weight increase was observed. These results suggest that the dilation of the gel is induced by a photochemical reaction of the leucohydroxide residues contained in the network. The gel having the leucohydroxide residues swelled even in the dark when the aqueous solution became acidic as shown in Figure 2. The pH was controlled by adding appropriate amounts of aqueous NaOH or HC1. A t pH 3.8, the gel had a strong green color, and an 11-fold weight
0;
i
5
6
i
0
J
PH
Figure 3. Swelling of polyacrylamide gels having triphenylmethane leucocyanide groups (3.1 mol %) in the dark in water at various pH values. Wois the weight of the gel at pH 8.0.
increase was observed compared with the weight at pH 8.0. The increase in weight is due to chemical ionization of weakly basic leucohydroxideresidues by the reaction with protons: R
R
A
The dilation behavior is similar to the well-known swelling of polymer gels having carboxylic acid residues in an alkaline s01ution.l~ In order to minimize the pH sensitivity, the hydroxide residues were replaced by cyanide groups. Figure 3 shows the pH dependence of the swelling of the gel having leucocyanide residues in the absence of light. The weight remained constant until pH 4.0. Below pH 4.0, the gel has a tendency to swell, though the weight increase is much less than that observed for the gel having leucohydroxide residues. In addition, no strong coloration was observed in the low-pH solution. This indicates that the leucocyanide does not dissociate into ions at low pH in the dark. Figure 4 shows the photoresponsive behavior of the gel having leucocyanide residues (1.9 mol %) in water (pH, 6.5). Upon ultraviolet irradiation, the gel weight increased as much as 18 times (1700%). In the dark, the dilated gel contracted again slowly to the initial weight. The large expansion of the gel having leucocyanide residues is partially attributed to the low degree of swelling of the gel in the dark before photoirradiation. The degree of swelling of the leucocyanide gels at pH 6.5 in the dark is less than that of leucohydroxide gels because of a low pH sensitivity and higher hydrophobic nature. The large ionic dissociation quantum yield of the leucocyanide, 4 times larger than that of the leucohydroxide,may also, to some extent,
Macromolecules, Vol. 19, No. 10, 1986
2478 Irie and Kunwatchakun
Figure 4. Photostimulated dilation and contraction of polyacrylamide gel having triphenylmethane leucocyanidegroups (1.9 mol 5%) with light of wavelength longer than 210 nm at 25 OC. Initial pH of external phase was 6.5. W,is the weight of the gel hefore photoirradiation
t
.\
i.
A B Figure 5. Photostimulated size change of polyacrylamide gel having triphenylmethane leucocyanide groups (3.1mol %): (A) before photoirradiation;(B) after photoirradiation with ultraviolet light (A > 210 nm) for 2 h. (B) returned to the size of (A) in the dark.
T i m e . min
Figure 7. Photostimulated (A) color change and (B)dimension change of polyacrylamide gel having triphenylmethane leucocyanide groups (1.9 mol %) in water. lo is the diameter of the disk-shaped gel.
I . . . I I
OO 2 L Content of leucayonde. mole%
Figure 8. Dependence of the content of leucocyanide groups in polyacrylamide groups in polyacrylamide gel on the photostimulated dilation of the gel. Irradiation wavelength was longer than 210 nm. Figure 6. Bubble formation on the surface of polyacrylamide gel having triphenylmethane leucocyanide groups (3.1 mol %) by ultraviolet (A > 210 nm) irradiation. The irradiation beam WBS focused on a small area of ahout 3 mmz of the gel surface. contribute to the large expansion ratio. The photostimulated dilation process could he observed with the naked eye as the small thin disk-shaped gel grew to become a large thick one. The gel expanded equally in all dimensions without changing shape. However, due to the fixed geometry and shape of the gel, some parts have more freedom to deform than others, and stresses developed during irradiation. The outside boundaries tended to curl due to the stresses. Figure 5 shows the size change of the gel with 3.1 mol % leucoeyanide residues before and after photoirradiation. By continuous irradiation with ultraviolet light the gel's diameter expands from 9 to 20 mm and the dilated gel returned to 10-mm diameter in the dark. The gel weight increased as much as 13 times. The photostimulated dilation value, 2.2 times, agrees well with a dilation value of 2.35 times as estimated from the 13-fold weight change assuming isotropic expansion. When the irradiation beam was focused on a small area of about 3 mm2 of the gel surface (2 mm in thickness), a small bubble was formed reversibly as shown in Figure 6. Although the bubble edge was round aa expected from the gel properties, the diameter was close t o the irradiation area. The height of the buhhle was around 2 mm. Kinetic Measurement and Salt Effect. Figure 7 shows the rate of coloration at 660 nm and the gel di-
mension expansion rate under continuous light irradiation. The very thin gel film having an initial diameter of 20 mm was used in this experiment to diminish the experimental error in measuring the size and absorbance. The triphenylmethyl cation is well-known to have strong ahsorption, with A,, at 622 nm.13 The absorption band at the wavelength corresponds to the amount of positive ions generated in the gel. Upon ultraviolet irradiation, the color of the gel changed quickly from pale green to deep green in less than 3 min and then remained almost constant. In the dark, the color again returned to the initial pale green in several hours. The diameter of the gel, increased slowly and reached a saturated value in around 2 h. The photostimulated dilation was 2.2 times. The large difference in the photoresponse time between the coloration and the size change implies that the rate-controlling step of the gel dilation is not the rate of ionization of the leucocyanide residues hut the diffusion of the gel network into water. According to Tanaka et al.,15 the characteristic time of gel expansion is proportional to a2/D, where a and D are the final radius of the gel sphere in equilibrium and the diffusion coefficient of the gel network. D is defined as D = E l f , where E is the longitudinal hulk modulus of the network and f is the coefficient of friction for the network and the gel fluid. The slow dilation process is ascribable to these gel's physical properties. The amount of gel dilation strongly depended on the content of the leucocyanide residues in the gel network. Figure 8 shows the content dependence of the dilation ratio measured in weight, W/ Wo, where Wo and Ware the gel weight before and after photoirradiation, respectively. The
Macromolecules, Vol. 19, No. 10, 1986
Photoresponsive Polymers. 8 2479 Table I Ion Distribution between the Gel Inside and the External
Solution gel inside
0'
I
'
106
IOL
102 Salt concentration, mole/[
l't
;,;,;-f
,
5
1
' 0 -
0 106 10-1 102 Salt concentratlon,mole/i
Figure 9. Salt effect on the dilation of polyacrylamide gels having leucohydroxide (A) and leucocyanide (B) groups. The content of the leucohydroxideand leucocyanide groups is 3.7 and 3.1 mol %, respectively. KBr ( 0 )and NaCl (0) are used as salts.
ratio increased with increasing leucocyanide content and reached a maximum around 2 mol % . Above 2 mol % ,the photoinduced dilation was decreased. At a leucocyanide content of 3.7 mol % , the dilation ratio was less than 30% of the maximum value a t 2.0 mol % . The bell-shape dependence suggests that the leucocyanide residues have two competitivefunctions in the photoinduced dilation process. One essential function of the chromophores is to produce positively charged groups attached to the gel network. The formation of charges, fixed cations and free anions, generates osmotic pressure differentials between the gel inside and the outer solution, which is considered to be the main driving force for gel expansion (see next section). The leucocyanide residues, on the other hand, have a tendency to contract the gel network because of their hydrophobic nature. At higher leucocyanide content, the latter effect is considered to dominate over the expansion force and suppress the dilation. The addition of salt to the external solution also suppresses the gel expansion as shown in Figure 9. The gels were immersed in the salt solutions and swollen to the equilibrium condition on standing in the solution overnight; then the gels were photoirradiated. Both NaCl and KBr reduced the photostimulated swelling ratio to a similar extent. No photostimulated dilation was observed in the presence of M NaCl or KBr for both gels having leucohydroxide or leucocyanide residues. The salt effect gives strong evidence that the expansion of the gels is caused by the ions produced in the polymer network by photoirradiation. Mechanism of Gel Dilation. According to Grignon and ScallanlGand RiEka and Tanaka,17the swelling of gels with fixed charges in the network can be quantitatively understood by the osmotic pressure differentials obtained from the Donnan equilibrium. In the following, the work of Grignon and Scallan will be extended to the present photoresponsive gel system to account for the swelling behavior on a semiquantitative basis. The equilibrium value of the volume fraction of the gel network, a, can be obtained from the pressure-balance equation.
H+ OHK+
Y KwlY
Br-
Zo@t
n
external solution
+ y + n - K,/y
H+ OHK+ -z
Br-
x
Kw/x m x + m - K w / y - z'
Here, II(@)conf represents the swelling pressure due to the conformational entropy of the network, i.e., the mixing entropy and rubber elasticity term. II(@)cont arises from the interaction among polymer segments and solvent molecules. is the osmotic pressure differential resulting from the difference in the ion concentration in the gel and the external solution. The term ll(+)cod takes into account the contribution from the electrostatic forces of the fixed charges, which would enhance the gel expansion. In the photoresponsive gel system, the Donnan term, IIion,.playsthe most important role. nconf and ncont are considered not to be affected by photoirradiation. The term IImdis expected to contribute to the expansion when the charged groups, produced by photoirradiation, are in close proximity to one another in the network and the electrostatic force of repulsion is not effectively screened by the counterions. As this is not the case of the present system,l' the effect can, for the sake of simplicity, be neglected. Hence, attention can be restricted to the nion term. An ion distribution in the gel and in the external solution is given in Table I. Here LCN and L+ represent leucocyanide residues in gel network and the cations, respectively. Co is the initial concentration of the residues in the gel network. Io,4, and t are the absorbed light intensity, ionic dissociation quantum yield, and the irradiation time, respectively. z and y represent the concentration of CN- and H+ions in the gel phase, respectively, and z and x are the respective concentrations in the external solution. In the present example, KBr was used as the salt and the concentrations of K+ in the external solution and in the gel were m and n, respectively. According to Donnan theory, distribution of mobile ions between the gel and the solution is equilibrium for each ion and the distribution constant, A, can be derived as follows: = (1
+ $)'2
(4)
Grignon and Scallan's results suggest that the degree of swelling of the gel with fixed charged groups is mainly controlled by Hionand can be expressed as follows: degree of swelling
a
1-X nion= 1 + X RTI04t
(5)
Equations 4 and 5 indicate that photogeneration of ions, the increase of Io4t, causes the decrease of X, resulting in the increase of the degree of swelling. The suppression of the gel expansion by the addition of salt can be explained in the following way. The increase of the salt concentration increases n and m, giving rise to the increase of X, resulting in the decrease of swelling. The deswelling behavior in the presence of salt is due to the increase of the Donnan distribution constant of ions. The photoresponsive behavior
Macromolecules 1986, 19, 2480-2484
2480
of the gels observed in this experiment are well described by eq 5. In the above treatment, the concentration of ions is taken to be proportional to the irradiatian time. This is not true at the later stage of photoirradiation as shown in Figure 7A. Almost all ions are produced in a very short time and the concentration is considered to be close to Co. Therefore, IIionmay be expressed under continuous light irradiation as follows: nion=
CH2)(copolymer), 94352-04-2; (1, X = CN, Y = N(CH&, Z =
CH=CH2)(H2C=CHCONH2)*(H2C=CHCONHCHNHCOCH=CH,) (copolymer), 94352-06-4; NaCl, 7647-14-5; KBr, ma-02-3.
References and Notes Part 7: Irie. M.: Iwavanapi. - . T.:. Tanieuchi. . Y . Macromolecules 1985, 18, 2418. Preliminary communication: Irie, M.; Kunwatchakun, D. Makromo1.- Chem., Rapid Commun. 1984,5, 829. (a) Irie, M. Molecular Models of Photoresponsiveness; Montagnoli, G., Erlanger, B. F., Eds.; Plenum: New York, 1983; 281. (b) Ciardelli, F.; Carlini, C.; Solaro, R.; Altomare, A.; Houben, J. L.; Fiss, A. Pure Appl. Chem. 1984,56, Pieroni, 0.; 329. (a) Irie, M.; Hayashi, K. J . Macromol. Sci., Chem. 1979,A13, 511. (b) hie, M.; Hirano, K.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (c) Blair, H. S.; Pogue, H. I.; Riodan, J. E. Polymer 1980,21, 1195. hie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176. Irie, M.; Hosoda, M. Makromol. Chem., Rapid Commun. 1985, 6, 533. Merian, E. Text. J. 1966, 36, 612. Lovrien, R. Proc. Natl. Acad. Sci. 1967, 57, 236. van der Veen, G.; Prins, W. Nature (London) Phys. Sci. 1971, 230, 70. Smeta, G. Adu. Polym. Sci. 1983, 50, 18. Zimmermann, E. K.; Stille, J. K. Macromolecules 1986,18,321. (a) Matgjka, L.; Duiek, K.; Ilavsky, M. Polym. Bull. (Berlin) 1979,1, 659. (b) Matgjka, L.; Ilavsky, M.; DuBek, K.; Wichterle, 0. Polymer 1981, 22, 1511. Herz, M. L. J. Am. Chem. SOC.1975,97, 6777. Kuhn, W.; Ramel, A.; Walters, D. H.; Egner, G.; Kuhn, H. J. Fortschr. Hochpo1ym.-Forsch. 1960, 1, 540. Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214. Grignon, J.; Scallan, A. M. J. Appl. Polym. Sci. 1980,25,2829. RiEka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. 1
1-X 1 + XRTC0
According to these equations, the gel expansion is expected to increase with increasing leucocyanide residue content, C,, in the gel network. This was not the case. It was found that the gel expansion ratio decreased when the content was increased above 2.0 mol 5%. This may be interpreted by taking into account the effect of lIcont. At high content of the leucocyanide residues, the contraction effect due to IIcontovercomes the expansion effect due to IIion. The introduction of hydrophobic bulky phenyl groups in the gel network would decrease the compatibility between the polymer segments and water, preventing gel expansion.
Acknowledgment. We express our thanks to M. G . Tilley and L. Katsikas for kindly correcting the English in the manuscript. Registry No. (1, X = OH, Y = N(CH&, Z = CH= CH2) (H2C=CHCONH2).(H2C=CHCONHCH2NHCOCH=
I
Photoresponsive Polymers. 9.l Photostimulated Reversible Sol-Gel Transition of Polystyrene with Pendant Azobenzene Groups in Carbon Disulfide2 Masahiro Irie* and Ryuzo Iga The Institute of Scientific a n d Industrial Research, Osaka University, Ibaraki, Osaka 567, Japan. Received April 1, 1986
ABSTRACT The gel melting temperature of polystyrene-carbon disulfide gel was found to change reversibly upon ultraviolet irradiation by incorporating a small amount of azobenzene groups into the pendant groups. Photoisomerization of the pendant azobenzene groups from the trans to cis form (62% conversion) increased the gel melting temperature by as much as 9 "C when the polymer contained 10.5 mol % azobenzene groups. The gel-sol transition could be induced isothermally at -52 "C for the polymer gel having 10.5 mol % azobenzene groups and at 200 g/L by changing the irradiation wavelength. Ultraviolet irradiation (400 > X > 310 nm) converted the sol to the gel state, whereas visible irradiation (A > 450 nm) induced the transition from the gel to the sol state. The gel melting temperature dependence on the content of cis-azobenzene and the segment density in the polymer gels suggested that the dipole moment increase of the pendant groups as a consequence of the trans-to-cis isomerization reinforces the coil overlap interactions, resulting in stabilization of the gels. The stable gel reverts to a sol state when the coil overlap junctions are destroyed by the cis-to-trans isomerization with visible light.
Introduction By incorporating a small amount of photoisomerizable chromophores into polymers, we have shown that various polymer properties can be controlled reversibly by phot~irradiation.~The solubility of polystyrene in cyclohexane, for example, changes upon irradiation with light of a specific wavelength when the polymer contains less than 7 mol % azobenzene or spirobenzopyran chromo-
phores in the pendant groups; i.e., ultraviolet light caused precipitation of the polymer, while visible light resolubilized it.44 Another example is a reversible shape change of polymer gelsa7 Polyacrylamide gel with 3.1 mol % triphenylmethane leucocyanide groups dilates in water by as much as 2.2 times in each dimension upon ultraviolet irradiation, and the dilated gel in the dark contracts again to the initial size. Reversible photodissociation of the
0024-9297/86/2219-2480$01.50/00 1986 American Chemical Society
