Swelling Behaviors of Doubly Thermosensitive Core-Shell Nanoparticle Gels

Novel UCST (upper critical solution temperature) type doubly thermosensitive coreshell nanoparticle gels were prepared in order to investigate swelling behaviors. The coreshell nanoparticle gels are synthesized via precipitation polymerization and are composed of a chemically cross-linked poly(methyl methacrylate) (PMMA) core and a poly(2-hydroxyethyl methacrylate) (PHEMA) shell. Both polymers exhibit a UCST phase transition in 1-propanol solution; the transition temperatures are 25 degrees C for PHEMA and 68 degrees C for PMMA. Photon correlation spectroscopy (PCS) measurements indicate that the large UCST gap between the shell- and core-forming polymers gives rise to a clear two-step swelling process upon heating. For theoretical analysis, a corrected modified-double-lattice (MDL) model with a new type of interaction parameter (epsilon(ij)), called the MDL-T model, is introduced to represent the optimized thermal behavior of the interchange energy and is combined with the FloryRehner (FR) chain model to express the net free energy of mixing. Required model parameters are obtained from the experimental swelling data of homopolymer nanoparticle gel solutions and are directly applied to coreshell swelling calculations. The model is applied to two LCST (lower critical solution temperature) type doubly thermosensitive coreshell nanoparticle gels such as poly(N-isopropylacrylamide) (PNIPAM) core/poly(N-isopropylmethacrylamide) (PNIPMAM) shell in water and PNIPMAM core/poly(N-n-propylacrylamide) (PNNPAM) shell in water. A comparative analysis of the MDL-T and other lattice based models is carried out, and a noticeable improvement is observed.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) for the Center for Next Generation Dye-sensitized Solar Cells (No. 2008-0061903)