Conformational transitions of poly(N-isopropylacrylamide) chains loopily absorbed at the surfaces of poly(N-tert-butylacrylamide) latex particles in water

Results are presented for a light-scattering study of the conformational transitions and corresponding dynamics of poly(N-isopropylacrylamide) (PNIPAM) chains absorbed from water onto the surfaces of poly(N-tert-butylacrylamide) (PNTBA) latex particles. Similarities between the structures of PNIPAM and PNTBA meant that the PNIPAM chains were strongly adsorbed at the PNTBA surfaces. As the temperature was increased to the theta-temperature and above, the loopily adsorbed-to-globule conformational transition was observed. This was followed on cooling by the observation of an unexpected transition, the globule-to-extended chain conformation, that occurred under better than theta-solvency conditions. The latter transition was explained as resulting from a reduction in the number of contacts between the polymer chains and the latex particle surfaces as the globular conformation was adopted. An additional finding was that these extended conformations were unstable and subsequently relaxed toward the original loopily adsorbed conformation. The relaxation process for the extended-to-loopily adsorbed conformational transition was found to occur slowly and to depend on the temperature. It is proposed that kinetic constraints played an important role in this transition. The transition kinetics were analyzed with the aid of a stretched exponential function. The relaxation time thus calculated was found to increase with increasing temperature and obeyed an Arrhenius law over the temperature range studied. The apparent activation energy of the extended-to-loopily adsorbed conformational transition was estimated to be similar to 80 kJ mol(-1).