Swelling Behaviors of Doubly Thermosensitive Core–Shell Nanoparticle Gels

Abstract

Novel UCST (upper critical solution temperature) type doubly thermosensitive core–shell nanoparticle gels were prepared in order to investigate swelling behaviors. The core–shell 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 °C for PHEMA and 68 °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 (ε̃ij), called the MDL-T model, is introduced to represent the optimized thermal behavior of the interchange energy and is combined with the Flory–Rehner (F–R) 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 core–shell swelling calculations. The model is applied to two LCST (lower critical solution temperature) type doubly thermosensitive core–shell 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.