A study of temperature-induced aggregation of responsive comb copolymers in aqueous solution

Abstract

Responsive copolymers can be used to confer temperature-responsiveness to emulsions in the form of temperature-induced gelation triggered by conformational changes of the copolymer. In this study the structural changes of a responsive comb copolymer, poly(NIPAM-co-PEGMa) (N-isopropylacrylamide and poly(ethyleneglycol methacrylate), respectively) in D2O are studied using small-angle neutron scattering (SANS) as well as complementary turbidity measurements. The copolymer was synthesised under conditions that favoured a compositionally asymmetric (“blocky”) structure. Scattering data obtained for the poly(NIPAM) and poly(PEGMa) homopolymers at 25 °C are also presented. The scattering profiles for the copolymer in aqueous solution are strongly temperature-dependent. At temperatures less than or equal to 32 °C the scattering profiles have contributions from q−1 and q−2 scattering terms (rods and Gaussian coils, respectively); whereas, at 36 and 40 °C the profiles are dominated by scattering from Gaussian coils. At 50 °C the contributions to the scattering are from q−2 and q−4 (Gaussian coils and Porod scattering from an interface, respectively). The data were fitted successfully using models that combined these contributions. The radius of gyration (Rg) and aggregation number (Nagg) for poly(NIPAM-co-PEGMa) increased, respectively, from 76 A and Nagg = 1 at 25 °C to 257 A and Nagg = 27 at 50 °C. Rods were present (although in a minority) at temperatures less than 36 °C with an average radius of 11 A. The rods disappeared once the temperature increased to around 36 °C, which is similar to the LCST measured for the copolymer in D2O. At 50 °C the data were fitted with a model containing a Porod scattering term and the results indicated the presence of particles with a diffuse layer thickness of 60 A. The average size of the particles was estimated as 1200 A. The insights gained from the data help to explain the temperature-induced gelation observed for solutions and emulsions containing poly(NIPAM-co-PEGMa).