Instabilities of Polymer Nanostructures
Nanostructured polymers are of significant interests from lithography to biomedical applications. However, polymer based micro- and nanostructures are subject to a variety of ever-present surface/interface stresses that are detrimental to the stability and reliability of the polymer nanostructures. Furthermore, post-fabrication processes such as thermal and solvent annealing become increasingly important for improving the pattern quality and increasing throughput. Under these circumstances, polymer patterns are often shown to collapse, dewet, and fracture. Thus, the ability to produce reliable devices requires a clear understanding of the nature of the pattern instabilities as a result of the surface and interfacial stresses during both fabrication and post-fabrication processes. In this project, we aim to explore and understand a range of viscous and viscoelastic instabilities in patterned polymer nanostructures. Shown in the above representative images, correlated capillary breakup of polymer pattern stripes can be controlled to achieve either in-phase or out-of-phase modes. This leads to a controlled composite-type polymer thin film with desired dispersion of other polymers.
Surface-patterning of Polymeric Membranes for Enhanced Membrane Performance in Water Treatment and Bioseperations
Polymer-based membranes are the critical components for industrial separations ranging from chemical purification, water desalination, pharmaceutical separation, to a range of biomedical applications such as kidney dialysis. All these liquid-based separation processes suffer from fouling, which decreases the separtaion efficiency and increase the opertion cost. Approches to enhance the anti-fouling aspects of membranes have been extensively researched, with efforts span process design, materials engineering, and surface treatment of membranes. Recently, we show that sub-micron surface patterns, when imparted atop the membranes, can improve fouling resistance during microfiltration (MF), ultrafiltation (UF), nanofiltration (NF) and reverse osmosis (RO) processes. In this project, we study the surface-patterning process and the effects of the surface patterns on the overall filtration productivity and regeneration characteristics in bioseparations and desalination processes.
Reconfigurable, Smart Polymer Particles and Surfaces
Elastic energy in crosslinked polymer particles (from sub-micrometer diameters to macroscopic length scale) and nanostructured polymer surface can be controllably stored and released through controlled prgramming and recovery. Under compression, the microparticles and nanostructured surfaces can undergo trememdous amount of deformation. The recovery of the particles and surfaces upon external stimuli (heat or solvent vapor) highly depends on and thus is tunable by the substrate adhesion and size (of the particles or surface features). The degree of elastic energy storage can be further tuned by incorporating colvant adaptive network (CAN) polymer chemistry. Such shape-memoryand shape-reconfiguration capability can be harnessed to generate smart particles for adhesion, preparation of metal/dielectric Janus particles (SEM images below), smart composites, and smart optics.
Creating Hiearchical Porous Polymer Films via Photocrosslinking during Forced Evaporation of Solvents
Photocrosslinking precursors during forced convection of solvent is a complex process, which nontheless can be used to create a range of hiearchical polymer films. For example, fast evaporation of solvent would leads to a concentration gradient of monomers, which yields a dense skin layer atop a porous layer upon polymerization. This process, can be further integrated with other physical process such as breath figure formation by incoporating moisture into the N2 flow to achieve more uniform pores on the skin layer (right).