The research objective of this award is to elucidate the nature of the instabilities in nano-patterned polymer surfaces. Compared with the elastic or viscous instabilities that have been actively studied in planar polymer films, the viscoelastic instabilities in patterned polymer surfaces remain largely unexplored, and are far more complex due to the multiple variables that can affect their characteristics. The proposed study, combining experimental and theoretical work, will result in a clear understanding on the scientific underpinning of the pattern instabilities, including their boundary conditions, scaling relationships for characteristic wavelength and onset time, the role of nano-fabrication parameters, and materials properties.
If successful, the results of this research will benefit applications of patterned polymer surfaces, which are critical components to many technologies. Thorough understanding of the instabilities in patterned polymer surface will pave the way for high fidelity pattern fabrications and enable post-fabrication processes that utilize solvents and thermal annealing. These instabilities, once understood, will also offer a unique avenue to create novel hierarchical structures that are otherwise challenging to fabricate with parallel lithographic techniques. This award will provide educational benefits to K-12, undergraduate, and graduate students by directly involving them into the research. The grant will also facilitate curriculum for a materials degree program which offers opportunity for students in the front range of rocky mountain area. The results of the research will be incorporated as topics and modules into courses, as well as broadly disseminated through journal publications, national conferences and TeachEngineering digital library.
The goal of this award was to analyze and understand the origins and characteristics of instabilities of lithographically patterned polymer surfaces during post-fabrication processes, particularly thermal treatments. More specifically, we systematically analyzed both experimental and theoretical aspects of pattern instabilities driven by capillary fluctuations (a ubiquitous thermal-noise induced fluctuation present on all liquid surfaces) and/or flow-induced internal stresses (an effect similar to elastic recovery commonly observed in bulk polymers during manufacturing). The intellectual merit of this work is in understanding the origin of two new types of viscoelastic and viscous instabilities in patterned polymer surfaces that occur during thermal annealing. For the first time, we observed a zig-zag instability (Fig.1) in thermally-annealed polymer patterns that are fabricated by nanoimprint lithography. We discovered that these zig-zag instability most often is observed in nanoimprinted polymer patterns with a combination of small feature size, large molecular weight, and high imprinting temperature. The study provided a guidance to avoid or facilitate such pattern instability by properly selecting processing conditions. For the first time, we discovered a simultaneous capillary breakup: An array of lithographically fabricated polymer arrays, suspended on another immiscible polymer, can breakup via an "in phase" (Fig. 2a) or "out of phase" (Fig. 2b) during annealing. The breakup characteristics including periodicity, kinetics, and phase correlation are controlled by the viscosity ratio of the two polymers, pattern-feature sizes and the nanoimprint process. Our studies show that these phase correlations are resulting from the minimization of energy dissipation during the breakup. The broader impact of the work includes the following aspects, Obtained knowledge in relating the occurrence of the two types of newly-observed instabilities in lithographically fabricated polymer nanostructures to the post-fabrication processes, which has been disseminated in eight journal articles in top peer-reviewed journals (Macromolecules 2010, 43, 8191; Soft Matter 2011, 7, 3794; Advanced Materials 2011, 23, 3669; Macromolecules 2012, 45, 1972; Soft Matter 2013, 9, 4455; Langmuir 2013, 29, 3073; Polymer 2013, 54, 5936; Polymer, 2014, 55, 4150) and eight conference presentations (by graduate students). Trained two Ph.D. students and one postdoctoral researcher. The trainings are essential for them to develop the expertise in respective areas (one in experimental study of polymer surfaces and structures, and one in theoretical analysis of polymers with an emphasis on mechanics), which will lay the foundation for their career in academic or industrial organizations. Established a platform for professional development of the graduate students and postdocs. We created a "super group" between our experimental-focused research group with other theoretical-focused groups, in the areas of soft matters. They meet regularly to discuss/share their own work and recent progress in the field of polymer and surface surfaces. Provided research opportunities for several undergraduate and high school students. The project enabled us to recruit and mentor junior researchers from CU Boulder and nearby high school students. These project-focused research experience will encourage them to pursue engineering as their future career.
