Temperature triggered aggregation toward nanoparticles formation from tri-arm poly(HEAAm-b-NIPAAm) in aqueous solutions

Multi-arm block copolymers with stimuli-responsive behavior are a potential candidate in the field of nanoscience and nanotechnology due to their unique design, specific property-regulating ability and appealing applications. Here, tri-arm star diblock copolymers, poly(2-hydroxyethylacrylamide)- block -poly( N -isopropylacrylamide), P(HEAAm– b –NIPAAm) consisting of PHEAAm and PNIPAAm as inner and outer blocks were synthesized via two-step ATRP. The synthesized polymers were characterized using ATR-IR, 1 H-NMR and GPC techniques. It is revealed that introducing a higher number repeating units (300) of HEAAm into copolymers could facilitate the formation of micelles in the cross-linked points and the occurrence of phase transition at higher temperatures. DLS measurements prove that tri-arm block copolymer with the lower number repeating units (50) of HEAAm core could not prevent random aggregation, whereas longer PHEAAm 300 core produces ca. 200 nm aggregate steadily at the adjoining knot at above LCST. Lowering of UV–Vis absorbance at 40 °C proved the entrapment of Rhodamin B into PNIPAAm 300 aggregates. Atomic force microscopy image of P(HEAAm 300 – b –NIPAAm 21 ) shows a loop (network structure) with the terminal PNIPAAm consistent aggregate size ca. 237 ± 42 nm. Graphical abstract (A) Schematic chemical structure of tri-arm poly(HEAAm-b-NIPAAm) diblock copolymer, (B) AFM image of nanoparticles formation, (C) nanoparticles distribution in aqueous solution at 34 °C (LCST) and at 50 °C, and (D) aggregation and expansion mechanism