Dual Temperature- and pH-Dependent Self-Assembly of Cellulose-Based Copolymer with a Pair of Complementary Grafts

Thermo- and pH-responsive micellization of HEC-g-(PNiPAAm&PAA) (CNiPAa) was studied. This binary graft copolymer CNipAa was synthesized via successive radical polymerization of N-isopropylacrylamide (NiPAAm) and acrylic acid (AA) from hydroxyethyl cellulose (HEC) as the backbone. The copolymers are able to self-assembly into a trinity of micelles of different structures by combined pH and temperature stimuli: (1) at 30 °C and a low pH range (<4.6), forming the micelles with a core of PAA/PNiPAAm due to hydrogen-bonding interaction between the two grafts; (2) in basic solution and at a temperature above the LCST of PNipAAm, hydrophobic aggregation of PNiPAAm grafts leading to PNiPAAm-core micelles with a compound shell of PAA/HEC; (3) forming a three-layer micellar structure due to the further response of the PNipAAm-core micelles to pH change. These three forms of micelles were examined by TEM, AFM, and DLS/SLS, and the corresponding forming mechanisms were clarified. As far as we know, it is the first report on so diverse micellization behavior of graft copolymers.