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Instabilities of Polymer Nanostructures

AFM images of two modes of correlated capillary instabilities observed after annealing PS/PMMA bilayer patterns (dark regions - PS droplets; bright regions - PMMA matrix.). Left: PS stripes on PMMA patterns break up via an in-phase mode. Right: Bi-mode distribution of the PS droplets formed after the capillary breakup. The larger PS threads were largely out-of-phase. (Ahn et al. Soft Matter 2010, 6, 4900-4907; Soft Matter 2011, 7, 3794 – 3800; Zhang et al. Macromolecules 2012, 45, 1972-1981; Langmuir 2013, 29, 3073-3079.)

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.

Left: SEM image of a surface-patterned ultrafiltration (UF) membrane. Middle: Schematic illustration of crossflow filtration of colloidal suspensions over patterned membranes. Right: Critical flux, the flux at which irreversible fouling occurs, as a function of angle between pattern lines and flow direction for two different pattern heights, and two different sized silica particles. (Maruf et al. Journal of Membrane Science 2013, 428, 598-607; Journal of Membrane Science 2013, 444, 420-428; Journal of Membrane Science 2014, 452, 11-19; Journal of Membrane Science 2014, 471, 65-71; Journal of Membrane Science 2016, 512, 50-60; Journal of Membrane Science 2017, 527, 102-110; Separation Science and Technology 2017, 52, 240-257; Current Opinion in Chemical Engineering 2018, 20, 1-12.)

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.   

A: Schematic illustration of "programming" a lightly crosslinked PS particles using nanoimprint lithography process, and the subsequent recovery. B, C, and D are the SEM images of the PS particles in permanent, temporary, and recovered shapes. (Cox et al. Advanced Materials 2014, 26, 899-994; Polymer 2014, 55, 5933-5937; Langmuir 2016, 32, 3691-3698; ACS Applied Materials & Interfaces 2017, 9, 14422-14428; Wang et al. Advanced Materials 2011, 23, 3669-3673; )   Right: SEM images of Cr-capped PS particle after complete recovery. The difference in the surface morphology is related to the initial compression ratio of the PS particles, metal film thickness, and the recovery conditions.

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).

   

Left: A crosslinked PNIPAm film formed by UV-crosslinking during forced convection of CS2, featuring a dense layer atop a particulate layer. The latter is caused by the phase separation of polymerizing (crosslinking) NIPAm with remaining CS2. (Wang et al. European Polymer Journal 2015, 66, 99-107; Soft Matter 2013, 9, 4455-4463) Right: Porous films with regular micron-sized pores on crosslinked methacrylate films formed by Breath-Figure Temlated Assembly (BFTA) during uv-induced crosslinking. (Wang et al. Polymer 2012, 53, 3749-3755; Maniglio et al. Polymer 2011, 52, 5102-5106.)