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 ‘-(α,α‘-dimethyl-α‘ ‘- 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 °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.