Ionic Liquid Enhanced Long-Range Ordering in Lamellar Block Copolymer Films*

Symmetric PS-b-PMMA block copolymers (BCP) films cast on Silicon wafers are known to form a lamellar structures upon annealing, with PMMA and PS block wetting the substrate and air interface respectively. However, attaining complete parallel ordering becomes increasingly difficult as the molecular weight (Mw) and thickness of the BCP films increase, due to meandering of lamellae in the film. In order to enhance the degree of BCP ordering, previous studies have focused on enhancing the thermodynamic driving force, χN, where χ is the Flory-Huggins interaction parameter and N is the number of segments. Increasing χ with decreasing temperature or increasing N, can enhance thermodynamic driving force but slows kinetics drastically and results in higher defect formation due to lower mobility above glass transition temperature T_g and the entanglement of polymers when Mw increase. In this work we use IL to enhance χ, and attain lamellar films with lower defects and enhanced parallel stratification in high Mw and thickness films. Such enhancement occurs due to the enhanced χ, coupled with enhanced mobility due to a lower Tg of the IL/BCP system. These defect free films may be useful in applications such as batteries, solid state dielectric capacitors, barrier materials etc.χ