(316b) Patterning Of Functional Polymer Thin Films Using Rod-Coil Block Copolymers

Functional polymer thin films for applications in organic electronics and biotechnology often require patterning of the polymers on the 10-100 nm length scale to control electronic properties or cell interactions. Block copolymers provide an elegant route to self-assemble such nanostructured films; however, the rodlike nature of functional polymers such as helical proteins or conjugated semiconducting polymers complicates self-assembly. Changes in chain topology in functional rod-coil block copolymers containing these materials causes interplay between liquid crystalline interactions and microphase separation that results in novel phase behavior. To understand how these complex effects impact block copolymer patterning, we have designed a model weakly-segregated rod-coil diblock system with accessible thermodynamic transitions and used this system to study nanostructured rod-coil block copolymer thin films.

Self-assembly in the thin film differs significantly from that of the bulk due to the impact of surface energy and geometric confinement. Using a combination of atomic force microscopy (AFM), grazing-incidence X-ray scattering (GISAXS), cross-sectional transmission electron microscopy (TEM), and dynamic secondary ion mass spectrometry (DSIMS), we are able to obtain a picture of the three-dimensional structure of our films. In the case of these rod-coil block copolymers, segregation of the coil block to the supported interface causes the lamellae to take on a parallel orientation in films less than a few lamellar spacings thick. Perpendicularly oriented lamellae form defects between the parallel grains. The top surface of the film is less preferential, and in thick films a layer of perpendicular lamellae covers the upper surface.

In addition, the liquid crystalline nature of the rod nanodomains has a significant impact on the grain shape and defect structures formed in these films. The high bending modulus of the perpendicularly oriented defect structures enforces straight edges on the grains, resulting in irregular polygon shapes for nearly symmetric block copolymers. This is a striking contrast to the curved interfaces normally observed in soft materials. Increasing the coil fraction results in increased mobility in the polymers, allowing the tetragonal crystalline structure of the rods to template grain growth. Unlike traditional block copolymers, the high bending modulus also results in defect accommodation through dilation of the lamellar domain spacing.