Solvent-induced microdomain orientation in polystyrene-b-poly(l-lactide) diblock copolymer thin films for nanopatterning

A series of degradable block copolymers, polystyrene-b-poly(l-lactide) (PS-PLLA), with PLLA hexagonal cylinder (HC) morphology has been synthesized in this study. Well-oriented, perpendicular PLLA cylinders of PS-PLLA thin films were efficiently achieved by spin coating using appropriate solvents regardless of the use of substrates. After hydrolysis of PLLA, well-oriented HC nanochannel arrays over large area in addition to uniform surface with controlled thickness and domain size were obtained; providing a simple and efficient path to prepare nanopatterned templates for applications. The induced orientation of PS-PLLA microdomains was strongly dependent upon the evaporation rate of solvent and its solubility between constituted blocks. The origins for the formed perpendicular HC morphology were systematically studied. The primary concern of controlled morphology for nanopatterning is to develop ordered microphase-separated morphology by considering the time scale for segregation, namely segregation strength during solvent evaporation. The induced orientation is attributed to the permeation discrepancy between phase-separated microdomains. The perpendicular morphology is initiated from the air surface, and formed in order to create an optimized condition (i.e. the fastest path) for solvent evaporation whereas parallel morphology may impede the evaporation of solvent molecules. Following the nucleation of microphase separation, the perpendicular morphology can be kinetically induced by solvent evaporation.