Swelling Behaviors of Doubly Thermosensitive Core–Shell\nNanoparticle Gels

Abstract

Novel\nUCST (upper critical solution temperature) type doubly thermosensitive\ncore–shell nanoparticle gels were prepared in order to investigate\nswelling behaviors. The core–shell nanoparticle gels are synthesized\nvia precipitation polymerization and are composed of a chemically\ncross-linked poly­(methyl methacrylate) (PMMA) core and a poly­(2-hydroxyethyl\nmethacrylate) (PHEMA) shell. Both polymers exhibit a UCST phase transition\nin 1-propanol solution; the transition temperatures are 25 °C\nfor PHEMA and 68 °C for PMMA. Photon correlation spectroscopy\n(PCS) measurements indicate that the large UCST gap between the shell-\nand core-forming polymers gives rise to a clear two-step swelling\nprocess upon heating. For theoretical analysis, a corrected modified-double-lattice\n(MDL) model with a new type of interaction parameter (ε̃_<i>ij</i>), called the MDL-T model, is introduced\nto represent the optimized thermal behavior of the interchange energy\nand is combined with the Flory–Rehner (F–R) chain model\nto express the net free energy of mixing. Required model parameters\nare obtained from the experimental swelling data of homopolymer nanoparticle\ngel solutions and are directly applied to core–shell swelling\ncalculations. The model is applied to two LCST (lower critical solution\ntemperature) type doubly thermosensitive core–shell nanoparticle\ngels such as poly­(<i>N</i>-isopropylacrylamide) (PNIPAM)\ncore/poly­(<i>N</i>-isopropylmethacrylamide) (PNIPMAM) shell\nin water and PNIPMAM core/poly­(<i>N</i>-<i>n</i>-propylacrylamide) (PNNPAM) shell in water. A comparative analysis\nof the MDL-T and other lattice based models is carried out, and a\nnoticeable improvement is observed.