Polystyrene, azobenzene groups

The photostimulated conformation change of polystyrene with pendant azobenzene groups in cyclohexane has also been studied by the time-resolved light scattering technique [26]. 

Similar phase separation was observed in theta solvents containing polymers with pendant photochromic chromophores. In a theta solvent, the interaction between the polymer and the solvent is in balance with intra- and iftter-polymer interactions. The isomerization of the pendant diromophores alters this balance. The system studied was a cyclohexane solution of polystyrene with pendant azobenzene groups [64]. 

Cyclohexane becomes a theta solvent for polystyrene at 35 °C. Moderate molecular weight polystyrene (M = 5 x 10 ) with pendant azobenzene groups is soluble in... 

Fig. 31. Changes in transmittance at 650 nm of a cyclohexane scdution containing polystyrene having pendant azobenzene groups (content, 6.1 mol%) on alt nate irradiation with ultraviolet (410 nm > A > 350 nm) and visible (A > 470 nm) light at 30 °C [64]...

First, we consider a mono-dispersed polystyrene of molecular weight M containing pendant azobenzene groups. When these groups are in the trans form, the polymer solution phase separates at t, which corresponds to Tg of Fig. 29. The isomerization of the chromophores from the trans to the cis form causes the phase separation temperature to rise to t which corresponds to Tg of Fig. This means that phase separation of the solution is induced between t, and t by ultraviolet irradiation, which causes the trans-cis isomerization. 

Fig. 32. Molecutar weight (M) depmdence of critical miscible temperature, T. T Q and Te(T) indicate the values of T, for polystyrene with cis and trans azobenzene groups at M = oo, respectively. See text for M, t, t Tm, and T ...

Irie M, Iga R. 1986. Photoresponsive polymers 9. Photostimulated reversible sol gel transition of polystyrene with pendant azobenzene groups in carbon disulfide. Macro molecules 19(10) 2480 2484. 

Fig. 5.1 Coil contraction and precipitation of polystyrene bearing pendant azobenzene groups.

Ire and Iga demonstrated that photostimulated sol-gel transitions are possible with gels of a polystyrene copolymer in carbon disulfide solution that carry some azobenzene groups. The transitions are reversible and take place upon ultraviolet light irradiation. The polymer undergoes cis to trans isomerization of the pendant azobenzene groups with a 62% conversion for polymers containing as much as 10.5% of azo groups ... 

Figure 10. Trans to cis isomerization of pendant azobenzene groups after irradiation of cyclohexane solution of polystyrene with azobenzene groups (4.3 mole%). Oscilloscope traces depicting the change of the optical absorption at (a) 360 and (b) 440 nm.

Figure 11 Oscillograms illustrating the increase of the light scattering intensity (a) and of the optical absorption (b), both at 514 nm, after irradiation of polystyrene with pendant azobenzene groups in cyclohexane solution (0.13 g/1) with a laser flash of 347 nm light at 25 C.

TABLE V. Conformational relaxation of polystyrene with pendant azobenzene groups. 

Figure 15. Gel-sol transition of polystyrene with pendant all trans ( ) and 62 % cis azobenzene groups (o ) in 082 measured by the ball-drop method. Arrows indicate melt temperatures of the gels. The content of azobenzene groups is 10.5 mole%. Polymer concentration was 200 g/1.

The precipitation behavior observed in this system is similar to the fractional precipitation of a polymer from a solution by the addition of a miscible nonsolvent. Cyclohexane is a good solvent for polystyrene with trans-azobenzene pendant groups, whereas it is a poor solvent for photoirradiated polystyrene with c/.v-azobenzene chromophores. The fractionation was controlled mainly by the molecular weight of the polymer and not by the content of the azobenzene groups. 

Frenz C, Fuchs A, Schmidt HW, Theissen U, Haarer D. 2004. Diblock copolymers with azobenzene side groups and polystyrene matrix synthesis, characterization and photo addressing. Macromol Chem Phys 205 1246 1258. 

Hayakawa T, Horiuchi S, Shimizu H, Kawazoe T, Ohtsu M. 2002. Synthesis and characterization of polystyrene b poly(l,2 isoprene ran 3,4 isoprene) block copolymers with azobenzene side groups. J Polym Sci Part A Polym Chem 40 2406 2414. 

Sung et al. measured, by repeated laser pulse irradiation, the trans to cis photoisomerization of azobenzene chiomoidiores incorporated at three specific sites on the polystyrene chain the chain center (C-PS), a chain end (E-P or a side group (S-PS). Typical changes in trans content during pulse irradiation are shown in Fig. 14.Sungetal. considered that the microenvironment oftheazobaizene residue... 

Masahiro, I. and Hisami, T. (1983) Photoresponsive polymers. 5. Reversible solubility change of polystyrene having azobenzene pendant groups. Macromolecules, 16, 210-214. 

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