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 (R_g) and aggregation number (N_agg) for poly(NIPAM-co-PEGMa) increased, respectively, from 76 Å and N_agg = 1 at 25 °C to 257 Å and N_agg = 27 at 50 °C. Rods were present (although in a minority) at temperatures less than 36 °C with an average radius of 11 Å. 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 Å. The average size of the particles was estimated as 1200 Å. The insights gained from the data help to explain the temperature-induced gelation observed for solutions and emulsions containing poly(NIPAM-co-PEGMa).