Compact-coil conformation

This copolymer exhibited a reversible photodecrease of the viscosity in 1,4-dioxane solution. A decrease of 24-30% was found in the reduced viscosity after the solution was irradiated with ultraviolet light. In tetrahydrofuran, the viscosity decrease was 1-8 %. The contraction of the dimensions of the copolymer coil was explained as follows. Cis form azobenzene has a dipole moment of 3.1 D, while the dipole moment of the trans form is less than 0.5 D. Therefore, the trans to cis isomerization induces strong dipoles in the pendant groups. These dipoles tend to orient in parallel and attract each other in less polar solvents, making compact coil conformations preferrable, as shown in Fig. 2(2)-b. In the dark, the viscosity of the copolymer... 

These band-structure calculations are in agreement with the observed UV-vis-ible-NIR spectra. In their compact coil conformation, emeraldine salt typically exhibit three peaks a n-n (band gap) band at ca. 330 nm and two visible-region bands at ca. 430 and 800 nm that may be assigned as n —> polaron band and polaron -> 7t band transitions, respectively61 (see Figure 5.6). 

It is well-known that at-PMMA maintains a compact-coil conformation below pH 4.5 and an extended-coil conformation above pH = 5.5. The conformational transition takes place in the intermediate pH region. Thus, the kinetic effects will reflect the influence of the compact coil of PMAA. Changes in the pseudo-first-order rate constants (/Jobsd) neutral hydrolysis of p-methoxyphenyl dichloroacetate... 

The influence of protein denaturants (such as alcohols, urea and urea derivatives, and quaternary ammonium salts) on the stability of the compact-coil conformation of PMAA is also described. These protein denaturants, when added to aqueous solutions of PMAA, considerably limit the rate of the neutral hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole. 

The cis trans isomerization will induce strong dipoles in the side azobenzene groups, these ones tend to orient parallel suffering, in low polar solvents, a mutual attraction consequently, the macromolecular chain adopts a compact coiled conformation, case 2b in Table 4.4. In the presence of the polar solvents, the change of hydrophobe properties becomes the main reason of the conformational changes, case 2a in Table 4.4. 

PolyaniUne (emeraldine salt) is known to possess mainly two conformations. One is an extended coil conformation and another is a compact coil conformation. Generally, the former shows the higher conductivity than the latter. The conformational change of PMAS (half ox) from extended coil to compact coil conformation... 

Compact coil conformation Scheme 3.17 Conformational change of PMAS (half ox)... 

The second system described by Lovrien, which belongs to mechanism (2), is poly(methacrylic acid) with pendant azobenzene groups. In aqueous solution, the viscosity was found to increase by ultraviolet irradiation. Matejka et. al, have developed the system to styrene -maleic anhydride copolymer with pendant azobenzene groups. The copolymer exhibited a pronounced photodecrease of the viscosity in 1,4-dioxane solution, i.e. a reversible decrease by 24 - 30 % in the reduced viscosity of the solution after ultraviolet irradiation. In THF, the viscosity decreased by 1 - 8 %. The contraction of the dimensions of the copolymer coil is explained as follows trans to cis isomerization induces a strong dipole in the azo bond. These dipoles become mutually oriented and attract each other so that compact coil conformations are preferred. In the dark, the viscosity of the copolymer solution returned to the original value as shown in Figure 3. The increase rate of the viscosity, however, was much slower, by a factor of 1/2.5 to 1/7, than the rate of cis to trans isomerization of the pendant azobenzene chromophores measured by optical absorption in the dark. The discrepancy requires further examination of the postulated mechanism of the conformational change. 

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