Polymer Thin Films
Many applications and devices require controlled distribution of material functionality in multiple dimensions. At the nanometer length scale, attempts to meet this challenge have included template-mediated materials chemistry. Interest in block copolymers has evolved because of their potential use in numerous nanotechnologies including nanotemplating, filtration membranes, and organic optoelectronics (LEDs and photovoltaics). Self-assembly of block copolymers in thin films is a complex phenomenon. A large parameter space, including film thickness, annealing conditions (thermal or solvent), molecular mass, and surface energy, governs the film morphology. Surface energetics and interface interactions also direct morphology orientation.
The behavior of thermally-responsive block copolymers compounds this complexity. When a thermally-responsive block copolymer undergoes a thermal transition resulting in a mass loss, the parameter space expands to include volume fraction shift, thickness decrease, surface energetic shifts of the relative blocks, and a change in substrate and free surface energetics. The resulting phenomenon is impacted by the complexity of multiple and often co-dependent variables. Control in chemically amplified transformations such as in thermal deprotection reactions can prove extremely useful especially when the self-assembly of the block copolymer is affected. Current investigations include controlling the final self-assembled morphology and orientation of thermally-responsive block copolymers using different surface chemistries and fabrication techniques as well as high-throughput methods for rapid characterization and identification of critical parameters.
An important aspect of exploiting high-throughput methods has been the development of novel gradient fabrication devices to efficiently probe the effects of substrate surface energy/chemistry and annealing conditions on block copolymer thin film morphology. These gradient approaches are becoming increasingly important for mapping the phase behavior of new materials for specific applications. In the following example, we used controlled vapor deposition to generate a gradient in substrate surface energy/chemistry and we show how the orientation of a cylinder-forming PS-b-PMMA thin film evolves with changes in substrate surface chemistry from a pure benzyl silane monolayer on silicon (left) to a pure methacryl silane monolayer on silicon (right), with gradient compositions and morphologies shown in between.
We have also designed a solvent resistant microfluidic mixing device that produces discrete gradients in solvent vapor composition and/or concentration to quickly and easily examine the use of solvent mixtures (versus a single solvent) for controlling thin film self-assembly. The image below shows a schematic of our solvent vapor annealing setup with the microfluidic device and its use as a screening tool to locate phase transformations in a poly(styrene-b-isoprene-b-styrene) triblock copolymer as a function of solvent composition and swollen film thickness.
