Extending Dynamic Range of Block Copolymer Ordering with Rotational Cold Zone Annealing (RCZA) and Ionic Liquids

Scalable and low-cost methods to align and orient block copolymer (BCP) films and membranes are critical for their adaptation for nonlithographic applications. Cold zone annealing (CZA) can align BCP microdomains and is scalable via roll-to-roll (R2R) manufacturing. However, the efficacy of orientation by CZA is strongly dependent on the thermal zone velocity (V_cza). Optimization of this rate can be time-consuming and tedious. To address this shortcoming, we report rotational or radial CZA (RCZA) that provides a combinatorial approach to efficiently determine how linear V_cza rate impacts microdomain orientation. RCZA rapidly identifies the optimal CZA velocities for perpendicular orientation of cylinders in polystyrene-block-poly(methyl methacrylate) films that previously required tens of measurements [ Macromolecules 2012, 45, 7107 ], demonstrated here with much finer velocity resolution using three overlapping radial regimes. Notably, the efficacy of CZA for perpendicular alignment rapidly decays for V_cza > 10 μm/s. To overcome this limitation, the addition of 2 wt % 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide sufficiently alters the surface tension and segmental relaxations via reduced viscosity to increase the processing window for perpendicular cylinders, approximately 75% at V_cza 330 μm/s, approaching R2R speeds. Further increasing ionic liquid content to 5 wt % leads to mostly parallel orientation due to surface wetting. Ionic liquids can dramatically increase BCP processing speeds for applications, such as membranes, and RCZA can efficiently map out the optimal processing parameters.