Light-triggered stimuli-responsive microgels

This Thesis focuses on the design, synthesis, characterization and application of light-responsive microgels based on PVCL and different photoswitchable molecules. The structure of this research was based on four primary sections. In the first section, a new approach to synthesize light and temperature dual responsive microgels via straightforward crosslinking functionalized light and temperature responsive linear pre-polymers in W/O miniemulsion was developed. Via this method, light and temperature dual responsive poly(N-vinylcaprolactam) (PVCL) and spiropyran based microgels were successfully synthesized by crosslinking the spiropyrans containing pre-polymer that was synthesized via RAFT polymerization and post modification. Importantly, large amount (more than 10 mol%) of spiropyran units were incorporated to the structure of microgels, which renders the synthesized microgels a significant reversible photochromic behavior upon different light histories including UV, visible light and dark, and obvious light triggered swelling or deswelling behavior, particularly in the dark/Vis process, in aqueous media. In addition, the microgels still demonstrate the thermal responsiveness presenting on the thermal induced size change and possess a VPTT around 20 °C. In the second section, a library of novel light and thermal responsive DASA-modified PVCL-co-GMA based microgels were designed and synthesized by introducing the donors and the acceptors of DASA molecules to the epoxy functionalized microgels respectively. The synthesized DASA-modified microgels were classified as first generation and the second generation DASA microgels, and each generation DASA microgels have two different kinds of microgels due to the different DASA molecule structures. All the synthesized DASA-modified microgels demonstrate very low PDIs and temperature responsiveness presenting on the thermal induced swelling-deswelling behavior, though two steps of modifications were treated. Additionally, the VPTTs of the DASA-modified microgels were observed as well, and the VPTT shifts to low temperature with the content of DASA units increasing. Importantly, all the synthesized DASA-modified microgels show irreversible photochromic property in in water and toluene, but the reversible photoswitching behaviors of both generation DASA microgels were observed in the solvents of DCM and THF. The second generation DASA-modified microgels demonstrate a faster and more stable photoswitching property compared to the first generation in DCM and THF. Moreover, photochromic behavior of both first and second generation DASA-modified microgel films were obtained as well, and the first generation DASA-modified microgel films even show thermal recovery of the color. Additionally, due to the large polarity change of the microgels surface caused by the visible light triggered DASA structures change from colored triene form to colorless zwitterionic cyclopentenone form, the first generation DASA-modified microgels exhibit light triggered size change in aqueous solution. In the third section, light, pH and temperature multi responsive microgels were successfully fabricated via the mAzo post modification on the prepared PVCL-co-AAc microgels. The synthesized microgels possess homogeneous distribution and exhibit temperature responsiveness presenting on the thermal induced hydrodynamic radius change, and the incorporated hydrophobic mAzo units contribute to the shifts to lower temperature on the VPTT of the microgels. Interestingly, the mAzo-modified microgels demonstrate a charge reversal behavior and a “V” shaped trend of size change from pH 4 to pH 10 was obtained because of the existence of mAzo moieties and carboxyl groups in the microgels. Importantly, the fabricated microgels exhibit reversible photoswitching behavior upon blue, green or red light irradiation and light triggered size change, as a result of the trans-cis isomerization induced by different visible light irradiation. In the fourth section, potential applications of synthesized light-responsive microgels as carriers were explored and some preliminary experiments including Cu2+ uptake and release by SP containing microgels and light triggered FITC uptake and release by DASA-modified microgels. The spiropyran containing microgels show ability to uptake Cu2+ ions in the dark due to the association with the open form of spiropyran through negatively-charged phenolic oxygen atoms. The release of Cu2+ ions was observed when the microgels were irradiated with visible light due to the increasing of spiropyran ring closed form hindering metal ions binding. The DASA-modified microgels containing long alkyl chain demonstrate a significant visible light controlled release of FITC due to the hydrophobicity change of DASA units in the microgels induced by the photoisomerization of DASA molecules, but the DASA-modified microgels with short alkyl chain did not show obvious visible light triggered release of hydrophobic FITC due to the bad stability of DASA triene forms in water