Both Core- and Shell-Cross-Linked Nanogels: Photoinduced Size Change, Intraparticle LCST, and Interparticle UCST Thermal Behaviors

New thermal- and photoresponsive core-shell nanogel particles were obtained from self-assembly in aqueous solution of a double-hydrophilic block copolymer (DHBCP) of which the two blocks could be photo-cross-linked via the reversible photodimerization and photocleavage of coumarin moieties. The diblock copolymer, consisting of poly[N,N-dimethylacrylamide-co-4-methyl-[7-(methacryloyl)oxyethyloxy]coumarin] and poly[N-isopropylacrylamide-co-4-methyl-[7-(methacryloy)oxyethyloxy]coumarin] (P(DMA-co-CMA)-b-P(NIPAM-co-CMA)), was synthesized by using reversible addition fragmentation chain transfer (RAFT) polymerization. At T > LCST of the P(NIPAM-co-CMA) block, core shell micelles were formed and UV light irradiation at lambda > 310 nm resulted in cross-linking of both the micelle core of P(NIPAM-co-CMA) and the micelle shell of P(DMA-co-CMA); subsequent cooling of the solution to T < LCST gave rise to water-soluble, swollen nanogel particles. Upon UV light irradiation at lambda < 260 nm, the decrease of cross-linking density could increase the swelling of nanogel particles by similar to 23% in diameter. By alternating irradiation with the different wavelengths, the average hydrodynamic diameter of nanogel particles was tunable between similar to 58 and similar to 47 nm. Interestingly, upon further cooling of the solution, aggregation occurred for nanogel particles with a moderate cross-linking density (10%-40% dimerization of coumarin moieties). Therefore, such core- and shell-cross-linked nanogel could display both intraparticle LCST (solubility of polymer chains forming the core) and interparticle UCST (solubility of particles). The possible mechanism and the effect of dimerization degree on the UCST behavior were discussed.