Order-to-disorder Transition of Block Copolymers in Thin Film

Abstract

We present computational results for the thickness dependence on the order-to-disorder transition (ODT) behavior in cylinder- and lamella-forming block copolymer (BCP) films confined within both asymmetric preferential and neutral boundary conditions. Field-based simulations utilizing the self-consistent field theory (SCFT) and Langevin field-theoretic simulation (L-FTS), which is one of the partial saddle point approximation methods, are used to figure out the influence of interfacial interaction on the phase transition behavior.

The SCFT calculation adopting a discrete bead-spring model with finite-range interactions is conducted to investigate BCP films confined within asymmetric preferential interfacial interaction. The asymmetric wetting condition which confines the films with selective interactions of the top and bottom interfaces generates parallelly oriented cylindrical and lamellar microdomains.

To investigate the phase transition behavior with respect to the film thickness, order parameters as a function of χN are presented at various film thicknesses. Below an onset thickness above which the (χN)ODT of the films are weakly dependent on the film thickness, the (χN)ODT of cylinder- and lamella-forming BCP films decreases as the film thickness decreases. The onset thickness of the lamellar phase ~20L0 is thicker than that of the cylindrical phase ~10L0, where L0 is the inter-lattice distance for each morphology. This result indicates that the interfacial interactions are more influential to a one-dimensional multilayer structure of lamellar microdomains than a two-dimensional hexagonally packed order of parallel cylinders.

The phase behavior of lamella-forming BCP films confined between weakly preferential top surface and neutral substrate is also investigated using the SCFT. The neutral substrate is implemented by symmetric random copolymer mat supported on a flat surface. This one-neutral case of interaction also leads to thickness-dependent ODTs of the BCP films. The (χN)ODT increases with decreasing film thickness when the film thickness is less than 6L0 owing to the substrate compatibility effect of the neutral bottom mat toward the BCP films. For the purpose of isolating the bottom neutrality effect, we also performed a calculation with neutralized top surface as well as the bottom surface. In this case, the (χN)ODT increases with decreasing film thickness when the film thickness is less than 20L0.

The phase behavior of cylinder-forming BCPs confined within neutral surfaces was unsettled with SCFT which ignores compositional fluctuation effect. We further performed the L-FTS in order to account for compositional fluctuation effect. Ultraviolet divergence in the L-FTS result is eliminated by the renormalization of Flory-Huggins parameters, allowing accurate comparison of (χN)ODT changes. The (χN)ODT decreases as the film thickness decreases at very large value of invariant polymerization index N bar, and BCPs undergo phase transition from disordered phase to cylindrical phase through spherical phase as predicted by the SCFT calculation. By performing L-FTS at experimentally relevant N bar, we showed that (χN)ODT value of cylinder-forming BCPs within neutral interfaces increases as the film thickness decreases and the BCPs undergo a direct phase transition from disordered phase to cylindrical phase, which is a well-known phenomenon in theory and experiment.