Oriented Microstructures of Polystyrene-b-Poly(L-lactide) Thin Films Induced by Crystallizable Solvents

摘要

Nanopatterning, the creation of patterns with nanoscale features, has drawn great attention in recent years due to its promising applications in nano-technologies. Large-sized, well-oriented periodic arrays would be needed for practical applications. A variety of new patterning technologies (including top-down and bottom-up methods, in particular, nanopatterning from the self-assembly of block copolymers) have been developed, which try to create well-defined nanopatterns over large areas. Recently, an easy way to create large-sized, well-oriented cylindrical and lamellar microdomains of semi-crystalline block copolymers in thin films has been proposed by De Rosa and co-workers. The induced microstructure orientation of strongly segregated block copolymers is described in full as directional eutectic solidification, the directional solidification of microphase separation induced by directional crystallization of crystallizable solvents at eutectic point. This mechanism differs from epitaxial crystallization that induces specific molecular chain orientation due to lattice matching. In our previous studies, large-sized, well-oriented lamellar microstructure of weakly segregated poly(L-lactide)-b-poly(ε-caprolactone) (PLLA-PCL) thin films were also obtained by using different crystalline substrates (benzoic acid (BA) and hexamethylbenzene (HMB)) whereas disordered textures are formed on amorphous substrates due to the weak segregation strength in the studied temperature range. We hypothesize that the lamellar microstructure orientation in weakly segregated block copolymers is mainly due to directional crystallization of PLLA on the substrates (crystallization-induced orientation) rather than directional eutectic solidification. In fact, the microstructure orientation of crystalline block copolymers is the consequence of both block copolymer microphase separation and crystallization of the crystallizable block interacting with the substrate. The objective of this study is to clarify the origins of the induced microstructure orientation and better differentiate the mechanisms of directional eutectic solidification and crystallization-induced orientation. Weakly and strongly segregated samples of polystyrene-b-poly(L-lactide) (PS-PLLA) diblock copolymers with different molecular weights were used. PS-PLLAs in the presence of crystallizable solvent were isothermally treated and quenched (see Experimental section for details). Crystallizable solvents including BA and HMB (that is crystalline substrates with and without lattice matching to crystalline PLLA, respectively) were used to examine the role of lattice matching in microstructure orientation.