Multistep Thermosensitivity of Poly(N-n-propylacrylamide)-block-poly(N-isopropylacrylamide)-block-poly(N,N-ethylmethylacrylamide) Triblock Terpolymers in Aqueous Solutions As Studied by Static and Dynamic Light Scattering

Abstract

Poly(N-n-propylacrylamide)-block-poly(N-isopropylacrylamide)-block-poly(N,N-ethylmethylacrylamide) (PnPA-b-PiPA-b-PEMA) triblock terpolymers of low polydispersity have been synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization via sequential monomer addition. The temperature-induced formation and dissociation of terpolymer micelles in aqueous solutions in a heating−cooling cycle were investigated by a combination of static and dynamic laser light scattering. In the heating process, the folding of PnPA at ca. 25 °C led to the formation of polymeric micelles with a collapsed hydrophobic PnPA core and a hydrophilic swollen PiPA−PEMA shell. Further increase of the temperature to ca. 32 and 53 °C led to the successive dehydration of PiPA and PEMA on the periphery, respectively. Our results revealed that when PEMA on the periphery of the micelle is too short to stabilize the hydrophobic core, individual micelles tend to aggregate into large micellar clusters, corresponding to the intrachain contraction and interchain association of the PiPA chains. In the cooling process, the complete dissolution temperature was lower than the association temperature in the heating process. At a given temperature, the aggregates in the cooling process had a higher mass than those in the heating process, clearly indicating a hysteresis.