Brownian Dynamics Simulation of ABA Block Copolymer in Selective Solvent: Kinetics of HEX Cylinders to BCC Spheres Transition

A Brownian Dynamics simulation was performed on 200 bead spring chains of triblock copolymer, A$_{10}$B$_{10}$A$_{10}$. The repulsive interactions of A monomers (in good solvent) are modeled by the Weeks-Chandler-Anderson potential. The poor solvent attraction of the B monomers is described by a Lennard-Jones (LJ) potential. We have determined the phase diagram of 30{\%} ABA block copolymer in a selective solvent for the A block. At temperature T=1 (in units of $\varepsilon $/k$_{B}$, where $\varepsilon $ is the well depth of the LJ interaction potential and k$_{B}$ the Boltzmann constant) the equilibrium state is HEX cylinder; at T=1.5 the system is in BCC spheres. We follow the time evolution of the HEX to BCC transition by jumping from T=1 to 1.5. The Fourier transform is calculated at each time-step in the simulation and compared to time-resolved small angle x-ray scattering data from triblock copolymer solution (Kraton G1650 in mineral oil). The simulation is also compared with a calculation based on a geometric model of coupled anisotropic fluctuations to describe the transition from HEX cylinders to BCC spheres.