Polymer Semiflexibility Induces Nonuniversal Phase Transitions in Diblock Copolymers

The order-disorder phase transition and the associated phase diagrams of semiflexible diblock copolymers are investigated using the wormlike chain model, incorporating concentration fluctuations. The free energy up to quartic order in concentration fluctuations is developed with chain-rigidity-dependent coefficients, evaluated using our exact results for the wormlike chain model, and a one-loop renormalization treatment is used to account for fluctuation effects. The chain length N and the monomer aspect ratio alpha directly control the strength of immiscibility (defined by the Flory-Huggins parameter chi) at the order-disorder transition and the resulting microstructures at different chemical compositions f(A). When monomers are infinitely thin (i.e., large aspect ratio alpha), the finite chain length N lowers the chi N at the phase transition. However, fluctuation effects become important when chains have a finite radius, and a decrease in the chain length N elevates the chi N at the phase transition. Phase diagrams of diblock copolymers over a wide 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. Our results demonstrate that polymer semiflexibility plays a dramatic role in the phase behavior, even for large chain lengths (e.g., N approximate to 100).