Impact of end-group association and main-chain hydration on the thermosensitive properties of hydrophobically modified telechelic poly(N-isopropylacrylamides) in water

We examine the influence of the macromolecule chain length on the cloud point temperature (T-cp) and the temperature of the coil-to-globule transition (T-M) in aqueous solutions of hydrophobically modified (HM) telechelic poly(N-isopropylacrylamides) (PNIPAM) ranging in concentration from 0.01 to 35 g L-1 (0.1-310 mmol of NIPAM L-1). The telechelic HM-PNIPAM samples with n-octadecyl termini were obtained by RAFT polymerization of NIPAM in dioxane in the presence of S-1-n-octadecyl-S'-(alpha,alpha'-dimethyl-alpha'-N-n-octadecylacetamide )trithiocarbonate as a chain transfer agent. Their molar mass (M-n) ranged from 12 000 to 49 000 g mol(-1) with a polydispersity index lower than 1.20. The cloud point temperatures, measured by monitoring the temperature-induced changes in scattering intensity, decreased significantly with increasing polymer concentration, this effect being more pronounced with decreasing polymer molar mass. In contrast, the temperature of the PNIPAM chain coil-to-globule collapse (30 +/- 1 degrees C) was only slightly affected by solution concentration and polymer molecular weight. These results are interpreted in terms of the coexistence of two phenomena: association of the n-octadecyl terminal groups and hydration of the PNIPAM chains.