Dual Thermo- and pH-Responsive Block Copolymer of Poly(N-isopropylacrylamide)-block-Poly(N,N-diethylamino Ethyl Acrylamide): Synthesis, Characterization, Phase Transition, and Self-Assembly Behavior in Aqueous Solution

Doubly thermo- and pH-responsive poly(N-isopropylacrylamide)-block-poly(N,N-diethylamino ethyl acrylamide) (PNIPAM-b-PDEAEAM) with different compositions were synthesized by reversible addition–fragmentation chain-transfer (RAFT) polymerization. The properties (solubility, self-assembly behavior) of these polymers and corresponding homopolymers in solution depend on the value of pH and the protonation degree of the diethylamino moiety. The effect of the nature of the polymer chain ends also appears to be critical to fully understand this behavior. At pH 10, both PNIPAM and PDEAEAM blocks are thermoresponsive and make a cooperative contribution to phase transition, and stable spherical nanoobjects are detected as temperature rises over transition point. At pH 4, PNIPAM-b-PDEAEAM with longer PDEAEAM length displays a two-step thermoresponsive behavior; upon heating, PNIPAM chains undergo shrinkage and subsequent reorganization causing the formation of large aggregates with a positively charged shell made of PDEAEAM block in its protonated state. The aggregation processes are also particularly sensitive to kinetic considerations, and therefore, measured size of aggregates depends strongly on the history of polymer solution.