Brownian dynamics simulation of comicellization of amphiphilic block copolymers with different tail lengths.

Brownian dynamics simulations were performed to study the self-assembly of mixed linear amphiphilic block copolymers. The molecules consist of similar hydrophilic heads and hydrophobic tails of different lengths. The model amphiphilic diblock copolymers have been selected to gain insight into the comicellization process in concentrated regimes, and the micelles were not kinetically frozen on the time scale of simulation. The critical micelle concentration (cmc), micelle size distribution, radius of gyration distribution, density profile of comicelles, shape anisotropy, and dynamics of comicellization have been studied as a function of the varying molar fraction of components. The cmc's of systems rich in the molar fraction of each type were found to be close to the cmc of that component. It has also been found that at a certain concentration comicellization affects the cmc in mixed systems. The weight-average aggregate size distribution of mixed copolymers was found to be between the aggregate distributions of short and long copolymers and becomes broader because of mixing. Moreover, values of the most probable aggregate size and radius of gyration of comicelles follow the mixing rule. Results show that small aggregates are mainly made from shorter block copolymers, whereas longer block copolymers form the major portion of large clusters. Furthermore, the cores of the micelles are mainly composed of longer block copolymers, and in all cases, the concentrations of shorter block copolymers are more dominant in the outer part than in the interior regions. In addition, the dynamics of polymeric micelles was studied using tracer and extraction autocorrelation functions and their relaxation times. The tracer correlation time increases with increasing longer copolymer concentration and deviates positively from the mixing rule. We also find that the total extraction correlation time increases exponentially from short to long copolymers, but the presence of long block copolymers linearly increases the short copolymer extraction correlation times. Short block copolymers, however, linearly decrease the long copolymer correlation times.