Session Q04: Polymer Thin Films and Interfaces

Novel Ice-Shedding Surfaces

Ice accretion has a negative impact on critical infrastructure, as well as a range of commercial and residential activities. Icephobic surfaces are defined by an ice adhesion strength t_ice < 100 kPa. However, the passive removal of ice requires much lower values of t_ice, such as on airplane wings or power lines (t_ice < 20 kPa). Such low t_ice values are scarcely reported, and robust coatings that maintain these low values have not been reported previously. In the first part of the talk, I will discuss how, irrespective of material chemistry, by tailoring the crosslink density of different elastomeric coatings, and by enabling interfacial slippage, it is possible to systematically design coatings with extremely low ice-adhesion (t_ice< 0.2 kPa). By utilizing these mechanisms, we fabricate extremely durable coatings that maintain t_ice < 10 kPa after severe mechanical abrasion, acid/base exposure, 100 icing/de-icing cycles, thermal cycling, accelerated corrosion, and exposure to Michigan wintery conditions over several months. 

 

Next, we will discuss how the force required to remove ice from a surface is typically considered to scale with the iced area. This imparts a scalability limit to the use of icephobic coatings for structures with large surface areas, such as power lines or ship hulls. I will then describe a class of materials that exhibit a low interfacial toughness with ice, resulting in systems for which the forces required to remove large areas of ice (few cm² or greater) are both low and independent of the iced area. Coatings made of such materials allow ice to be shed readily from large areas (~1m²) merely by self-weight.   

Durable icephobic coatings exhibiting very low interfacial toughness

The accretion of ice on different surfaces such as power lines, buildings, wind turbines, car windshields, ships and airplanes can have a significant detrimental impact on human society, both in terms of economic impact and safety issues. Thus, over the last few decades, sizable efforts have been put towards the development of passive ice shedding surfaces i.e. surfaces that can shed any accreted ice without the application of external force. Ice shedding is controlled by the ice adhesion strength for small interfacial contact area, and by interfacial toughness at a larger interfacial contact area. Traditionally, the material design parameters that enable low interfacial toughness, and low ice-adhesion strength have been orthogonal to one another. In this study, we describe a novel coating composed of a lubricant-reacted porous structure that simultaneously enables both low ice adhesion strength (=32kPa) and the lowest interfacial toughness (=0.035 J/m²) ever reported. The coating also possesses excellent durability and abrasion resistance which makes it promising for different commercial applications.

    

Development of functional surfaces with controllable wettability and water adhesion by utilizing appropriate chemistry and hierarchical roughness

The design of multifunctional surfaces based on biomimetic structures has gained the interest of the scientific community. Such biomimetic structures can be achieved by using suitable coatings onto appropriately micro/nano-structured substrates. Hierarchically roughened surfaces can be prepared by irradiating a metallic or semiconductor surface using ultrafast (femtosecond) laser or by utilizing polymer nanocomposite coatings attached onto the surfaces; suitable chemistry of the coatings provides the desired functionality. We use polymer nanocomposite coatings onto polymeric substrates, where the inorganic particulate additives create the appropriate roughness and the polymer matrix provides the proper functionality, which can produce superhydrophobic and water repellent surfaces as well as superoleophobic ones. Second, we will show that allowing a laser irradiated metallic surface to remain under low temperature heating or under moderate vacuum can lead to the development of superhydrophobic and, even, water repellent metal alloy surfaces; the ones that remained under vacuum were water-repellent whereas the ones that underwent thermal processing exhibited superhydrophobicity with high water adhesion.   

Adhesion of nanocarriers to endothelial cells under flow

Design of nanocarriers (NCs) for targeted drug delivery to endothelial cells is a biomedical and pharmacological challenge. Different surface chemistry of functional NCs, like homogenous (Homo) and Janus nanoparticles, provide different opportunities for loading multiple drugs and accurately control targeting and cargo release. Herein, using Dissipative Particle Dynamics (DPD) simulations we study flow past the chains of the endothelial glycocalyx (EG) layer and the receptors in a microchannel. We also investigate the effect of flow on morphology and dynamics of the EG layer and adhesion of NCs to the receptors. Using a energy calculation method, we systematically explore the adhesion of Homo and Janus NCs to the endothelial cell through calculation of the energy gains and losses as a function of a series of effects, such as the initial orientation, ligand density, shape, and size of NCs. Finally, we compare the state under flow with the equilibrium state.

    

Author not Attending

Polymer brushes are nanostructures characterised by dense polymer brushes tethered onto a surface, resembling real-life brushes. In the past, polymer brushes have been investigated theoretically either in the monodisperse limit or in flat geometries. These circumstances however aren’t as experimentally relevant, as monodisperse brushes are difficult to construct and brushes are found in curved geometries most of the times.

Here, we attempt to resolve this issue by extending the already established mean-field theory describing polymer brushes developed by Cates et.al. .We develop a theory that allows us to investigate for arbitrary geometries and polydispersities. We then explore the uniform, Schulz-Zimm and monodisperse polymer distributions, and note the emergence of an End Exclusion Zone in convex geometries and we note the conditions that give rise to it.

    

Suppressed Transition and Dynamic self-asembly of ionic superdiscs in cylindrical nanochannels

 Liquid crystalline mesophases in nanoconfinement exhibit intriguing phase transition behaviors and relaxation dynamics. Here, we investigate the molecular mobility and electrical conductivity of a columnar ionic liquid crystal confined in self-ordered nanoporous alumina oxide membranes of pore size ranging from 180 nm down to 25 nm. We use nano-broadband dielectric spectroscopy (BDS) and calorimetry to study the dynamics and phase behavior. Calorimetric investigation reveals a complete suppression of the columnar – isotropic transition, while the plastic crystalline – columnar transition temperature decreases with inverse pore size and deviates from the Gibbs – Thomson equation.

For the bulk case, BDS detects two relaxation modes in the crystalline phase, the γ relaxation and the α₁ relaxation, and two relaxation modes in the columnar phase, the α₂ and α₃ relaxation. All relaxation modes slow down for the confined case compared to the bulk. However, a new relaxation mode reflecting the interfacial layer emerges for the 80 and 25 nm. We discuss the possible molecular origins of the different relaxation modes observed. For the bulk ILC, a clear jump of 4 orders of magnitude in the absolute values of DC conductivity occurs at the transition from the plastic crystalline to hexagonal columnar phase, for the confined ILC, this transition is smooth. DC conductivity is reduced for the confined case, except for the 25nm, where the values are similar to the bulk.

    

Effects of charge fraction on the swelling behavior of weak polybasic brushes

Weakly charged polymer brushes attached to planar surfaces have found applications as highly tunable pH responsive coatings in separations and biological industries. The effects of pH on the conformation of brushes have been widely studied, however the chemically modified charge fraction (f) is left mostly unexplored. Here we investigate the influence of charge fraction on the swelling behavior of weak polybasic brushes by changing the pH in the 3 – 10 range using a combination of in-situ ellipsometry and streaming zeta potential measurements. Brushes are synthesized on silicon using surface-initiated copper(0) controlled radical polymerization. The brushes consist of random copolymers of DMAEA – a weak base – and neutrally charged HEA. The various charge fractions enable a thorough understanding of the effects of charged monomer over the whole f range. We couple ellipsometry results with titrations curves obtained from zeta potential measurements to better understand the purely electrostatic effects of f on swelling behavior. We observe swollen brush thicknesses decrease as f decreases for the entire pH range. We see hysteresis in the intermediate pH range, between the swelling and de-swelling cycles, where the extent of hysteresis decreases by decreasing f. The hysteresis behavior is important as it could possibly hinder the effects of brushes in tuning adsorption of chemicals and biologics due to the difference between the expected conformation and the actual conformation of the brush.   

Study of Swelling Behavior of Sustainable Chitosan Nanocomposite Thin Films

Humidity sensors have attracted significant attention in recent years in both the academic and industrial field. Recently, sensors made of synthetic thermoplastic polymers and inorganic semiconductors have raised environmental concerns. Biodegradable chitosan-based nanocomposite films provide a more sustainable alternative. The protonation of NH₂ groups of chitosan increase moisture affinity. Films of 120-350nm thickness were coated on Si substrate, and rapid swelling of the thin films in humid environment was identified with visible changes in color. Film thicknesses over the relative humidity range of 45-90% increased about 50% compared to dry state; confirmed by in-situ interferometry. Addition of graphene oxide (GO) at 0.5-2wt% increased film stability due to polarity. The affinity of blended nanocomposite films towards moisture followed same trend because of the interaction of oxygen-rich GO groups and NH₂–rich groups of chitosan. This highly responsive, humidity-dependent colorimetric property of chitosan nanocomposite thin films enables potential applications as a biopolymer-based sensor.

Funding: AFOSR W911NF2010281   

Eliminating the Tg- and Fragility-Confinement Effects in Polystyrene Films with Very Low Levels of 2-Ethylhexyl Acrylate Comonomer

With nanoconfinement below 50 nm thickness, polystyrene (PS) films exhibit glass transition temperature (T_g) values reduced relative to T_g(bulk). Such effects are eliminated down to 15 nm thickness with copolymers of styrene (S) with low levels of 2-ethylhexyl acrylate (EHA). Confinement effects in 98/2 and 94/6 mol% S/EHA copolymers were studied by ellipsometry on piranha-treated and dichlorodimethylsilane-treated silicon wafers. The T_g-confinement effect was eliminated in both cases. Thus, interfacial hydrogen bonds play no significant role in eliminating the effect. Dynamic fragility (m) of bulk films decreases strongly with EHA content; m decreases from 166 to 139 to 103 as EHA increases from 0 to 2 to 6 mol%. Nanoconfinement effects on m are very strong for PS, relatively weak for 98/2 mol% S/EHA copolymers and absent for 94/6 mol% S/EHA. This suggests that the elimination of confinement effect with 2-6 mol% EHA originates from EHA improving the bulk chain packing efficiency and reducing the susceptibility to free-surface perturbations. Notably, these results are not generalizable to other acrylate monomers, e.g., n-butyl acrylate.

    

Statistical field theory for the free energy of an electro-mechanical polymer chain: non-local dipole-dipole interactions in the fixed applied field ensemble

Existing theoretical approaches for polarizable polymers subject to a combined applied electric field and stretch are based on discrete monomer models. It is challenging to account for the non-local dipole-dipole interaction in this framework. The prior work typically considers only the interaction between the applied field and dipoles. To go beyond this approximation, we apply the statistical field theoretic framework that is based on a continuous description of the polymer chain in terms of density fields. We introduce a self-consistent formulation that enables us to address the setting of constant applied electric field ensembles that transforms the nonlocal interactions into a PDE constraint corresponding to Gauss’ equation. We implement the model in a finite element method to compute the free energy, average density, and average polarization distribution at equilibrium. We find that the presence of dipole-dipole interactions leads to qualitative changes in the dipole distributions, total polarization, and equilibrium electric field in the domain. We further notice a sharp instability leading to a collapse of the chain under the strong electric fields as a consequence of dipole-dipole interactions.

    

Diffusion of Brønsted Acidic Dopants in Conjugated Polymers

Depth-dependent doping levels in electrically doped organic semiconductor films are controlled by the relative rates of reaction and diffusion of the dopant. Many semiconductor devices (e.g., light emitters and photovoltaics) utilize heterojunctions that depend on dopant gradients to control charge transport. To understand the formation and stability of gradients requires understanding of dopant diffusion and the complex changes in polymer properties that arise during doping. Here, the doping of thin films of poly(3-hexylthiophene) (P3HT) using bistriflimide acid (HTFSI) from solution in methanol is evaluated. The contributions of the reaction rate and diffusion to doping were deconvoluted by examining the evolution in films of P3HT with varying thickness using a combination of spectroscopic techniques. The rate dependency of doping by hydrogenated and deuterated acids shows a significant kinetic isotope effect, indicating that doping is limited by proton transfer to the polymer. Dynamic secondary ion mass spectrometry (DSIMS) of doped films show that H/D is retained in doped films after the doping process in contrast to propositions in literature of hydrogen evolution. To assess diffusion limitations to the doping process, dopant concentration profiles were measured and quantified using complementary XPS and DSIMS depth profiling. The dopant concentration profiles show evidence of enrichment at the P3HT top surface. These limited concentration profiles suggest that dopant diffusivity varies inversely with dopant concentration due to doping-induced changes to the semiconducting polymer.   

Sequential Infiltration Synthesis of Silicon Dioxide for Nanopatterning – In Situ FTIR study

Silicon dioxide (SiO₂) is one of the most abundant and well-studied semiconductor materials in because of its electrical resistivity, and stability against oxidation and moisture. With the dimensions of the devices scaling down to sub-100 nanometer, nanopatterns of SiO₂ are becoming crucial for semiconductor device applications and emerging technologies. The majority of the works on SiO₂ nanopatterning for different applications are performed with conventional and expensive lithography processes such as e-beam and optical lithography. In this work, we are presenting SiO₂ material deposition using sequential infiltration method (SIS), a process that has been demonstrated to make inorganic nanopatterns using a polymer as a template. SIS is a two-step gas-phase molecular assembly reaction and enables localized inorganic material growth in the targeted domains of polymers with interactive functional groups. We have used polymethylmethacrylate (PMMA) polymer film for our study. We have performed in-situ Fourier Transform Infrared Spectroscopy (FTIR) study during the half and full cycles of the SIS process inside PMMA to understand the reaction mechanism of the precursor and polymer. The selective infiltration is advantageous for assuring large-scale uniformity of organized nanoscale materials. A better understanding of the SiO₂ growth mechanism using in-situ FTIR will open up new avenues for nanopatterning this material for many low-dimensional dielectric-based applications.   

A chiral luminophore guided self-assembly of achiral block copolymer for ultra-amplification of circularly polarized luminescence

Circularly polarized luminescence (CPL) materials have been gaining significant attention due to their promising applications in 3D-displays, data-storage, optical switches, and biosensors. The CPL-active materials generally exhibit moderate signal of CPL with dissymmetry factors for photoluminescence (g_lum) in the range of 10^-5~10^-3, that may not serve the purpose for engineering applications. Herein, this work aims to demonstrate the feasibility to greatly amplify the chiroptical signals by self-assembly of achiral block copolymer (BCPs) polystyrene-b-poly(ethylene oxide) (PS-b-PEO) associated with chiral luminophores, R/S-1,1’-Bi-2-naphthol (R/S-BINOL), forming helix phase (H*) driven by chiral communication via host-guest interaction at various stages from self-assembly. Astonishingly, self-assembled H* exhibits ultra-amplification of the g_lum up to ~100 folds (g_lum ±0.3) as compared to the disordered phase resulting from the well-ordered helical arrangement of luminophores. Evidently, the CPL-activity is highly dependent on the self-assembled textures; note that for an achiral DG network phase with equal population of left-handed and right-handed domains, the symmetry restoration will nullify the effective chiroptical signals. This work provides a conceptual insight to utilize the mesochiral co-assembly of block copolymers with chiral luminophores, giving self-assembled phases in thin-film state for significant enhancement of dissymmetry factor.

    

Effects of the lamellar microstructure on the conformations of chiral block copolymers

Recent advances in polymer synthesis have led to the development of chiral polymer molecules with sequence-specific chirality. When incorporated into a block copolymer architecture, these systems are capable of exhibiting chirality transfer (formation of chiral conformations and chiral structures from chiral building blocks), or the thermodynamics is affected in unexpected ways. In this work, we employ particle-based simulations to study compute thermodynamic and conformational metrics to study the behavior of chiral molecules in a lamellar microstructure. Surprisingly, we find that the morphological constraints of the microstructure result in a change in the helical nature of the conformations, when compared to an isolated single chain. This suggests a tradeoff occurs, where enthalpic contacts between unlike monomers are reduced, but that an internal strain occurs in chain conformations.   

Design and Growth of Bundlemer Brushes

Bundlemers are discrete, rigid 4nm x 2nm cylindrical nanoparticles assembled in water from computationally designed peptides.  More specifically, bundlemers are homotetrameric, anti-parallel coiled coils with a hydrophobic core buried in the center of the coiled coil cylinder nanoparticle and hydrophilic and polar groups populating the surface. These hydrophilic and polar surface amino acid side chains are highly tunable. We have previously used this tunability to design bundlemers that self-assemble into different preprogrammed 2-D lattices as well as covalently link bundlemers in a step growth like reaction into high aspect ratio rigid rods with similar dimensions and rigidity to single wall carbon nanotubes. In this talk I will discuss how we can leverage this tunability to design and control bundlemer binding to inorganic colloids. By balancing the surface charge of bundlemer nanoparticles with substrate binding terminal residues, we can design bundlemers to bind to nanoparticles with a preferred orientation. We can then leverage this interfacial ordering to design complex nanostructured coatings called bundlemer brushes. We then characterize these coating through a combination of dynamic light scattering, transmission electron microscopy, UV/Vis and FTIR spectroscopy.