Monte Carlo simulation of temperature-induced reversible morphological changes between sphere and vesicle formed by AB diblock copolymers

Abstract

Temperature-induced reversible morphological changes of micelles formed by AB diblock copolymers in a selective solvent for A blocks was studied through Monte Carlo simulation. When only hydrophobic B blocks are temperature-sensitive, the micelle morphology changes from sphere at high temperature to vesicle with a decrease in temperature. When the system was heated back to high temperature, the vesicles transform into spheres, indicating reversible morphological changes with temperature. However, the simulation results show that the pathways of morphological changes between sphere and vesicle depend on temperature change rate. The pathway of the sphere changing into vesicle under a quenching process is different from the pathway of the vesicle changing into sphere when the system was directly heated to the higher temperature. Moreover, the time consumed in the simulation for vesicle transformation into sphere upon heating is significantly shorter than that for the reverse process upon quenching. By contrast, when changing the temperature gradually, the pathways of morphological transitions from sphere to vesicle and from vesicle to sphere are almost identical. Moreover, the time consumed in the simulation for the annealing and gradual heating processes are nearly the same. This pathway differences observed when directly and gradually changing the temperature of the system are further elucidated by investigating the contact number variation between B blocks and solvents with simulation time. On the other hand, when the hydrophilic blocks and hydrophobic blocks are both temperature-sensitive, the introduction of the temperature-sensitivity of hydrophilic blocks hinders the morphological transition from sphere to vesicle under annealing conditions, and only larger spheres can be obtained after the annealing process when the degree of temperature-sensitivity of blocks A is rather high. Meanwhile, the simulation results show that the morphological transitions are still reversible under a slow temperature change rate when the hydrophilic blocks and hydrophobic blocks are both temperature-sensitive.