Well-Ordered Nanoporous ABA Copolymer Thin Films via Solvent Vapor Annealing, Homopolymer Blending, and Selective Etching of ABAC Tetrablock Terpolymers

Solvent vapor annealing treatments are used to control the orientation of nanostructures produced in thin films of a poly(styrene)-block-poly(isoprene)-block-poly(styrene)-block-poly((±)-lactide) (PS-PI-PS-PLA) and its blends with PLA homopolymer. The PS-PI-PS-PLA tetrablock terpolymer, previously determined to adopt a core(PLA)-shell(PS) cylindrical morphology in the bulk, gave perpendicular alignment of PLA cylinders over a limited range of thicknesses using a mixed solvent environment of tetrahydrofuran and acetone. On the other hand, perpendicular alignment was achieved regardless of film thickness by inclusion of 5 wt % homopolymer PLA in the PS-PI-PS-PLA tetrablock. Tapping mode atomic force microscopy (AFM) was used to visualize film surface morphologies. Subsequent reactive ion etching (RIE) and basic hydrolysis of PLA produced 15 nm pores in a PS-PI-PS triblock thin film matrix. Nanoporosity was confirmed by scanning electron microscopy (SEM) images and the vertical continuity of pores was confirmed by cross-sectional SEM analysis.