Solution properties of a thermosensitive triblock copolymer of N-alkyl substituted acrylamides.

An ABC triblock copolymer PnPA(124)-b-PiPA80-b-PEMA(44) was synthesized by a sequential three-step reversible addition-fragmentation chain transfer (RAFT) polymerization of N-n-propylacrylamide (nPA), N-isopropylacrylamide (iPA), and N,N-ethylmethylacrylamide (EMA). The thermoresponsive characteristics of the aqueous solution of the triblock copolymer have been studied in comparison with the corresponding mono- and diblocks by UV-visible and NMR spectroscopies, differential scanning calorimetry, and dynamic light scattering. The hydrophilicity of the blocks influences each other, rendering each block a phase transition temperature different from that of the corresponding homopolymers. The cloud points of the three steps of the phase separations were determined to be 31.5, 46.8, and 57.6 degrees C for steps one to three, respectively, while heating the aqueous solution of the triblock copolymer. At approximately 31 degrees C, the PnPA block started to dehydrate, causing the formation of micelles with the PnPA block as the core, dehydrated PiPA as the shell, and water-soluble PEMA block as the corona. The collapse of the dehydrated PiPA block was restricted by the shielding and stretching effect of the neighboring soluble PEMA block, and the resultant micelles had a high water content in their hydrodynamic volume. Continued increase in temperature to approximately 47 degrees C caused further dehydration of the micelles, and the final collapse of the third block of PEMA at approximately 58 degrees C led to further aggregation of the micelles.