6: AZOBENZENE CONTAINING BLOCK COPOLYMER MICELLES in Java

Maker QR Code ISO/IEC18004 in Java 6: AZOBENZENE CONTAINING BLOCK COPOLYMER MICELLES
CHAPTER 6: AZOBENZENE CONTAINING BLOCK COPOLYMER MICELLES
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because of the dissociation of vesicles. On the basis of the transmittance change, this photoinduced dissociation process was completed after about 20 s. Simply by switching the irradiation to visible light (440 nm, 24 mW cm 2), the opposite process occurred immediately, the transmittance dropped as a result of the reformation of vesicles in the solution. These reversible changes were con rmed by SEM observations made on samples cast from the solution before UV irradiation (marked by A), after 40 s UV irradiation (B) and after 40 s visible irradiation (C). The solution under visible light has a transmittance slightly higher than that of the initial vesicular solution, re ecting the difference between the photoinduced vesicle formation kinetics and the initial preparation procedure. Several subsequent alternating UV and visible light irradiations gave rise to reversible changes in transmittance without signi cant differences. Also presented in the gure is the result of a control test aimed at providing more unambiguous evidence. The higher transmittance was obtained with the same BCP solution without addition of water (i.e., polymer chains remained in dissolved state with no aggregation), subjected to exactly the same UV and visible light irradiation; the occurrence of the reversible trans cis photoisomerization of azobenzene moieties on PAzoMA did not change the transmittance of the solution. The rst light-controllable BCP micelle exhibiting reversible dissociation and formation in solution was thus demonstrated with the help of an azobenzenecontaining BCP. As discussed above, the reversible switch of the dipole moment of the azobenzene chromophore appears to be the decisive factor that explains the morphological changes of the micelles and, in particular, the reversibility of these changes. A quick look at the azobenzene moiety in P(tBA-co-AA)-b-PAzoMA (structure b in Fig. 6.2) can already nd an explanation. The azobenzene has substitutions on the para positions, a methoxy end group (OCH3) and oxygen linked to the methylene group of the exible spacer; its trans conformation should have a very small dipole moment because of this near-symmetrical structure, similar to azobenzene without substitution; whereas the bent cis isomer should have a notable dipole moment because of the charge separation. This indeed was con rmed by a Density Functional Theory (DFT) calculation on the model compound of 4,4u-dimethoxyazobenzene that is close to the azobenzene moiety on PAzoMA. As shown in Fig. 6.5, the computed dipole moment of the cis isomer averaged over four energetically stable conformations is 4.4 D, which is larger than unsubstituted azobenzene (B3 D). It is now understandable why this azobenzene BCP displays light-switchable polymer micellar aggregation.
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6.5. FACTORS INFLUENCING THE REVERSIBLE DISSOCIATION AND FORMATION PROCESSES
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Through the real-time monitoring of the transmittance change of a vesicular solution in response to UV and visible light exposure, the effects of a number of variables on the processes of photoinduced dissociation and formation were investigated. In this section, these factors are discussed on the basis of obtained results.
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6.5. FACTORS INFLUENCING THE REVERSIBLE DISSOCIATION AND FORMATION
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x y
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Energy x (deg.) (deg.) (kJ/mol) (D) 0.3 0.0 0.00 0.3 0.00 178.7 178.7 0.4 1.2 1.80 1.8 178.5 178.3 1.6 1.2 1.80
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(D) 0.00 0.00 1.53 1.53
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(D) 1.36 5.37 4.35 4.36
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= 4.4 D
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Figure 6.5. Lowest energy conformation of the cis isomer of 4,4u dimethoxyazobenzene. The calculated relative energies and dipole moment components of the four stable conformations are shown, with the total dipole moment being computed as the average over the four conformations. Source: Tong et al., 2005. Reprinted with permission.
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6.5.1. Effect of Solution Stirring
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When UV light is applied to a vesicular (or micellar) solution for the photoinduced dissociation of the polymer aggregates, it is easy to imagine that the mechanical stirring of the solution may affect the rate of dissociation. Besides the change in thermodynamic stability as discussed above, other factors may intervene. First, with the copolymer of P(tBA-co-AA)-b-PAzoMA, the membrane of vesicle is formed by a SCLCP bearing azobenzene mesogens, for which the trans cis isomerization could result in a photochemical LC-to-isotropic phase transition that may have an optical plasticization effect on the compact hydrophobic membrane. Should this happen, the softened vesicles would become mechanically weaker and thus be more easily broken by the shearing force of the stirring. Secondly, under the used low intensity UV light exposed to a large volume of solution (B1.5 mL), the strong absorption of the chromophore means that it is likely that only azobenzene moieties in the vesicles located nearby the surface of the solution would be excited and undergo the trans cis photoisomerization; the stirring should help the diffusion of polymer vesicles toward the surface region. A greater shearing force or/and a faster mixing of the solution at a higher stirring rate should therefore speed the photoinduced dissociation process throughout the solution. This indeed is the case. Using the same UV irradiation intensity (18 mW cm 2), changes in transmittance of the vesicular solution with no stirring and with stirring at different rates by adjusting the rotation speed of the magnetic bar between about 50 and 300 rmp were recorded and compared in Fig. 6.6a, Note that before the UV irradiation was turned on, the vesicular solution had the same transmittance regardless of the stirring rate, indicating that the shear effect could not break the polymer vesicles. It is clear that the process of the vesicle photodissociation became faster as the stirring rate increased. Actually, most interesting is the result obtained for the still solution. After the UV irradiation was applied on, it showed no change in transmittance
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