A. James Clark School of Engineering

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Author: search.filters.author.Etha, Sai Ankit

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    Direct visualization of nanoparticle morphology in thermally sintered nanoparticle ink traces and the relationship among nanoparticle morphology, incomplete polymer removal, and trace conductivity
    (Institute of Physics, 2023-06-19) Chandel, Ghansham Rajendrasingh; Sun, Jiayue; Etha, Sai Ankit; Zhao, Beihan; Sivasankar, Vishal Sankar; Nikfarjam, Shakiba; Wang, Mei; Hines, Daniel R.; Dasgupta, Abhijit; Woehl, Taylor; Das, Siddartha
    A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%–40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.
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    MOLECULAR-SCALE EXPLORATION OF INTERACTIONS BETWEEN DROPS AND PARTICLES WITH A POLYMERIC LAYER
    (2023) Etha, Sai Ankit; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Surface-grafted polymer molecules have been extensively employed for surface modifications as they ensure changes to the inherent physical/chemical properties of surface. Bottom-up surface processing with well-defined polymeric structures becomes increasingly important in many current technologies. Polymer brushes, which are polymer molecules grafted to a substrate by its one end at close enough proximity (thereby ensuring that they stretch out like the “bristles” of a toothbrush), provide an exemplary system of materials capable of achieving such a goal. In particular, producing functional polymer brushes with well-defined chemical configurations, densities, architectures, and thicknesses on a material surface has become increasingly important in many fields. In my dissertation, I employ Molecular Dynamics (MD) simulations to study the interplay of interactions between nanoparticles (NPs), solvent drops and polymer grafted surfaces under various system conditions. This study will help us to understand (1) the wetting dynamics of brush grafted surfaces and the associated brush conformational changes, (2) polymer-insoluble solvophilic NP assembly in brush grafted surfaces and the steric interactions driven establishment of direct contacts between a NP and a polymer layer (highly phobic to the NP), and (3) microphase separation and distillation-like behavior of grafted polymer bilayers interacting with a binary liquid mixture, and the resulting nanofluidic valving behavior of swollen polymer bilayers in a weak interpenetration regime. In Chapter 1, I provide the background and motivation of the research presented in this thesis. In Chapter 2, I study the spreading and imbibition of a liquid drop on a porous, soft, solvophilic, and responsive surface represented by a layer of polymer molecules grafted on a solvophilic solid. These polymer molecules are in a crumpled and collapsed globule-like state before the interaction with the drop, but transition to a “brush”-like state as they get wetted by the liquid drop. We hypothesize that for a wide range of densities of polymer grafting (σg), the drop spreading is dictated by the balance of the driving inertial pressure and balancing viscoelastic dissipation, associated with the spreading of the liquid drop on the polymer layer that undergoes globule-to-brush transition and serves as the viscoelastic solid. Finally, I argue that these simulations raise the possibility of designing soft and “responsive” and widely deployable liquid-infused surfaces where the polymer grafted solid, with the polymer undergoing a globule-to-brush transition, serving as the responsive “surface”. In Chapter 3, I employ coarse-grained molecular dynamics (MD) simulations and establish that under appropriate conditions, it is possible to develop numerous stable direct contacts between a polymer-insoluble NP and a solvated polymer layer (the polymer layer is phobic to the NP, while the solvent/liquid is philic to both the NP and the polymer). The NP is driven inside a layer of collapsed and phobic (to the NP) polymer molecules by a drop of this liquid (which is philic to both the NP and the polymer layer). The liquid molecules imbibe and diffuse inside the polymer layer, but the NP remains localized within the polymer layer, due to large Steric effects, ensuring the establishment of highly stable numerous direct contacts between the NP and the highly phobic polymer molecules. Finally, I argue that our finding will open up avenues for leveraging NP-polymer interactions for a myriad of applications even for cases where the polymer molecules are phobic to the NPs. In Chapter 4, I study the interaction of a binary mixture drop, containing two-miscible-liquids, with a polymer functionalized nanochannel that is philic to one of the liquids and phobic to the other. Liquid-liquid phase separation is achieved due to the asymmetry of interaction of the liquid species and we observe distillation like behavior wherein the drop becomes progressively concentrated with the phobic liquid with each ‘“pass” with the polymer bilayer absorbing an increasing fraction of the philic liquid molecules and transitioning into the polymer brush regime. Depending on the nanochannel height, the number of allowed passes varies, as the polymer chains stretch out until the oppositely grafted layers overlap and create a dense region of liquid infused polymer layers that act as a valve. Any further passage of drops through this nano-confined interpenetrating brush bilayer requires a much greater magnitude of applied force on the drop. I finally propose a design of nanovalves based on this mechanism of creating partially porous interpenetrating polymer brush layers.