Microphase Separation in Sulfonated Block Copolymers Studied by Monte Carlo Simulations
P. Knychała1 , M. Banaszak1 , M. J. Park2 , N. P. Balsara3
1 Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614, Poznan, Poland
2 Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
3 Department of Chemical Engineering University of California, Berkeley, California 94720
Macromolecules, 42 (22), pp 8925-8932 (2009)
The underpinnings of microphase separation in symmetric poly(styrenesulfonate-block-methylbutylene) (PSS-PMB) block copolymer melts were examined by Monte Carlo lattice simulations. The main challenge is understanding the effect of ion pairs in the PSS block on thermodynamics. We assume that experimentally determined Flory-Huggins interaction parameters are adequate for describing intermonomer interactions. Our model does not account for either electrostatic or dipolar interactions. This enables comparisons between simulated and experimentally observed microphases reported by Park and Balsara [Macromolecules 2008, 41, 3678] without resorting to any adjustable parameters. The PSS block in both experiments and theory is partially sulfonated. We quantified the effect of sequence distribution on phase behavior by using alternating and blocky PSS chains in the simulations. Depending on temperature and sequence distribution, simulations show perforated lamellae, gyroid, and hexagonally packed cylinders in addition to the lamellar phase found in simple symmetric block copolymers that do not contain ions. This is driven by extremely repulsive interactions between styrenesulfonate monomers and the uncharged species in the melts. The symmetry of the microphases and the locations of the order-disorder and order-order phase transitions are in qualitative agreement with experimental observations.