Single-molecule behavior of asymmetric thermoresponsive amphiphilic copolymers in dilute solution.

A bead-and-spring model has been used to simulate the behavior of thermoresponsive asymmetric diblock amphiphilic copolymers with aid of Monte Carlo simulations. The alteration of the thermodynamic conditions was mimicked by using a Lennard-Jones potential, which was related to the measured temperatures by comparison with experimental data for aqueous solutions of two sets of diblock copolymers, namely methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide), one with different lengths of the hydrophilic block (MPEG(n)-b-PNIPAAM(71)) and one with varying lengths of the hydrophobic block (MPEG(57)-b-PNIPAAM(m)). The influence of the length of both the thermoresponsive and the hydrophilic block on the size and conformation of single molecules at various temperatures was studied by means of simulations. The temperature-induced contraction of the copolymer (MPEG(n)-b-PNIPAAM(71)) entities is only modestly affected by changing the length of the hydrophilic block, whereas for the MPEG(57)-b-PNIPAAM(m) copolymer both the transition temperature and the magnitude of the compression of the molecules are strongly influenced by the length of the thermosensitive block. When the MPEG chain fully covers the hydrophobic core, the copolymer moieties are stabilized, whereas poorly covered cores can promote interchain aggregation at elevated temperatures.