Influence of ion-complexation on the alignment of polystyrene-<i>block</i>-poly(methyl methacrylate) copolymer thin films under an electric field
With the miniaturization of devices, block copolymers (BCPs) are emerging as promising candidates for scaffolds and templates in the fabrication of nanoscopic structures. Key to the use of BCPs is control of the orientation and lateral ordering of their microdomains in thin films. An external electric field was previously shown to effectively orient BCP microdomains in a desired direction. However, complete alignment of microdomains in BCP thin films remains a challenge due to achieve due to the preferential interactions of blocks at the interfaces with the electrodes. Here, we demonstrate that lithium-PMMA complexes, formed by adding lithium salts to polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer, markedly enhanced alignment of BCP microdomains in thin films under an electric field, mainly by the increased dielectric contrast and mediated surface interactions, which significantly reduced the critical field strength. In addition, formation of lithium-PMMA complexes induces a transition in the reorientation process of lamellar microdomains, from disruption/re-formation to grain rotation. The transition arises in an increase in the segmental interaction parameter, χeffc., of the ion-complexed copolymer, which drives stronger phase segregation. This feature is evidenced by increase of microdomain spacing and ordering, the induced disorder-to-order transition (DOT), and order-to-order (OOT) transitions (sphere-to-cylinder, S-to-C). The increase in χeffc is further confirmed by small-angle neutron scattering (SANS) from the complexed copolymers in the disordered state. Moreover, the temperature dependence of χ effc for the complexed copolymer becomes weaker than that of the neat copolymer.
The electric-field-induced sphere-to-cylinder transition in thin films depends on the strength of the interfacial interactions. As a result, a strong preferential interfacial interaction suppresses the transition. However, the formation of ion-complexes enhances the ability of an applied electric field to induce S-to-C transition even on a surface with unbalanced interfacial interactions, providing a simple route to ordered arrays of high aspect ratio cylinders oriented normal to the surface.