Photoresponsive Properties and Structures of Complexes Composed of Azobenzene-Containing Triblock Copolymer and Salt in Solution

Abstract

We synthesized a photoresponsive azobenzene monomer which was used to synthesize azobenzene-containing triblock copolymers poly(6-[4-(4-ethoxy-phenylazo)phenoxy]hexyl methacrylate)-block-poly(ethylene glycol)-block-poly(6-[4-(4-ehoxypenylazo)phenoxyl]hexyl methacrylate) (PMMAzo-b-PEG-b-PMMAzo) through atom transfer radial polymerization Their photoresponsive properties and structures in solutions were systematically investigated PMMAzo-b-PEG-b-PMMAzo exhibited the photoisomerization response When PMMAzo-b-PEG-b-PMMAzo was irradiated by UV light trans isomers converted to cis isomers On the other hand the reverse cis-to-trans photoisomerization was obtained after visible light irradiation UV-Vis spectroscopy was carried out to examine the photoresponsive properties of PMMAzo-b-PEG-b-PMMAzo When PMMAzo-b-PEG-b-PMMAzo was dissolved in a neutral solvent (DMF) both blocks were compatible with DMF resulting in a random-coiled conformation When a PEG-selective solvent (H2O) was added to DMF the solubility of PMMAzo in the mixed solvents decreased leading to the formation of micelles composed of PMMAzo in the core and PEG in the corona When the solvent selectivity was increased maximum absorbance in the UV spectra was blue-shifted indicating an increase in the amount of H-aggregated azobenzene mesogens The free volume of H-aggregated mesogens was limited during photoisomerization causing a decrease in the photoisomerization rate Furthermore we investigated the effect of the weight fraction of the azobenzene block (fPMMAzo) on the structure and photoresponsive behavior of PMMAzo-b-PEG-b-PMMAzo Results revealed that the copolymer with high fPMMAzo resulted in the aggregation of mesogens and the formation of micelles in selective solvents Additionally the photoisomerization rate increased with an increase in fPMMAzo The present study also investigated the effect of salt addition (KCl and KSCN) on the structure of PMMAzo-b-PEG-b-PMMAzo in solvents When the solvent selectivity was low mesogens were nonassociated and the addition of salts showed little relation to the aggregation state of mesogens However when the solvent selectivity was too high the salting-out effect on PEG chains was limited resulting in little changes in the aggregation state of mesogens In the mixed solvents with appropriate solvent selectivity salts caused the shrinkage of PEG chains because of the salting-out effect Conversely the size of micelles increased Such behavior was attributed to the expansion of PMMAzo chains in the core of micelles As a result the amount of nonassociated mesogens increased

Date of Award	2019
Original language	English
Supervisor	Chieh-Tsung Lo (Supervisor)