(726a) Polymer Semiflexibility Induces Non-Universal Phase Transitions in Block Copolymers

The order-disorder phase transition and the associated phase diagrams of semiflexible diblock copolymers in the presence of concentration fluctuations are investigated using the wormlike chain model. A free-energy functional up to quartic-order in the concentration fluctuations is developed with chain-rigidity-dependent expansion coefficients, which are evaluated using our exact results for the wormlike chain model, and one-loop renormalization is used to account for the fluctuation effects. We find that degree of polymerization N and the monomer aspect ratio \alpha directly control the order-disorder transition and the resulting microstructures at different chemical compositions f_A. We find that when monomers are infinitely thin (i.e. large aspect ratio), finite degree of polymerization N lowers the critical phase transition \chi N. However, fluctuation effects become important when chains have finite radius, and a decrease in the degree of polymerization N elevates the phase transition \chi N. Phase diagrams of diblock copolymers at a full range of N and \alpha are calculated based on our fluctuation theory. We find that both finite N and \alpha enhance the stability of the lamellar phase above the order-disorder transition.