Two-stage thermally induced stable colloidal assemblies from PAAc/PNIPAAm/mPEG graft copolymer in water

A facile approach of forming stable polymeric complexes by the two-stage phase transition of the graft copolymer comprising poly(acrylic acid) (PAAc) as the backbone and poly(N-isopropylacrylamide) (PNIPAAm) and monomethoxy poly(ethylene glycol) (mPEG) as the grafts in water was illustrated. Rather loose and hydrated polymeric aggregates with a characteristic hydrophobic multicore structure, achieved by the first-stage thermally induced phase transition of PNIPAAm grafts, underwent the core structure rearrangement (including multicore fragmentation and fusion) upon the second-stage dehydration of mPEG grafts at 90 °C. This was followed by hydrogen-bond pairings of mPEG and PNIPAAm chain segments with unionized AAc residues acting as an effective protective shell against potential hydration of the hydrophobic inner cores during the annealing process. The polymer complexes thus obtained show surprisingly enhanced hydrophobicity of inner cores at 25 °C in water and excellent stability of the morphological structure in response to the temperature disturbance.