Jeffrey P. Youngblood

Anisotropic wetting on tunable micro-wrinkled surfaces

We examine the wettability of rough surfaces through a measurement approach that harnesses a wrinkling instability to produce model substrate topographies. Specifically, we probe the wetting of liquids on anisotropic micro-wrinkled features that exhibit well-defined aspect ratios (amplitude wavelength of the wrinkles) that can be actively tuned. Our study provides new insight into the wetting behavior on rough surfaces and into the interpretation of related liquid contact-angle measurements. In particular, we find that droplet wetting anisotropy is governed primarily by the roughness aspect ratio. In addition, comparison of our measurements to theoretical models demonstrates that droplet distortions and observed contact angles on surfaces with a strongly anisotropic texture can be quantitatively attributed to the difference in the energetic barriers to wetting along and perpendicular to substrate features.

Recyclable organic solar cells on cellulose nanocrystal substrates

Solar energy is potentially the largest source of renewable energy at our disposal, but significant advances are required to make photovoltaic technologies economically viable and, from a life-cycle perspective, environmentally friendly, and consequently scalable. Cellulose nanomaterials are emerging high-value nanoparticles extracted from plants that are abundant, renewable, and sustainable. Here, we report on the first demonstration of efficient polymer solar cells fabricated on optically transparent cellulose nanocrystal (CNC) substrates. The solar cells fabricated on the CNC substrates display good rectification in the dark and reach a power conversion efficiency of 2.7%. In addition, we demonstrate that these solar cells can be easily separated and recycled into their major components using low-energy processes at room temperature, opening the door for a truly recyclable solar cell technology. Efficient and easily recyclable organic solar cells on CNC substrates are expected to be an attractive technology for sustainable, scalable, and environmentally-friendly energy production.

Coatings based on side-chain ether-linked poly(ethylene glycol) and fluorocarbon polymers for the control of marine biofouling.

The preparation of side group modified polystyrene-based surface-active block copolymers (SABC) for use as marine fouling resistance/release applications is described. Modifying moieties such as poly(ethylene glycol) (PEG) and semifluorinated segments were used. A novel bilayer methodology has been employed that provides both suitable mechanical properties through the use of an elastomeric primer layer of styrene-ethylene/butylene-styrene (SEBS) and control of surface-chemistry through use of the SABCs. This approach has potential as a cost-effective technology for environmentally benign coatings that resist and release marine biofouling. Initial testing of these materials included determination of captive bubble contact angles and protein adsorption. Testing against marine fouling organisms was performed using settlement and adhesion bioassays with zoospores of the green alga Enteromorpha . The results showed that all surfaces had markedly reduced levels of zoospore settlement compared with glass controls and that adhesion strength was strongly affected by the semifluorinated SABC. The results are discussed in terms of surface properties.

Amphiphile grafted membranes for the separation of oil-in-water dispersions

Perfluorinated end-capped polyethylene glycol surfactants were covalently attached to fritted glass membranes as a means to improve the separation of oil-in-water emulsions. Hexadecane was used as representative oil for the oil-in-water emulsions; membrane pore size was varied between 10 and 174 microm. Membranes were characterized with respect to contact angle, permeability of bulk fluids, and separation efficiency. Performance was compared to similar metrics applied to unmodified membranes. Modified membranes demonstrated static hexadecane contact angles which were higher than static water contact angles converse to their unmodified counterparts. The relative hydrophilicity and corresponding oleophobicity of the modified membranes resulted in greater water permeability as compared to hexadecane permeability. The presence of the perfluorinated constituent of the amphiphile retarded the flow of hexadecane. For modified membranes, suspended hexadecane coalesced at the membrane surface, was undercut by water, and floated to the surface such that only trace amounts of oil were present in the permeate. Therefore, modified membranes resisted fouling from oil due to the self-cleaning properties of the attached amphiphile.

Thermal Expansion of Self-Organized and Shear-Oriented Cellulose Nanocrystal Films

The coefficient of thermal expansion (CTE) of cellulose nanocrystal (CNC) films was characterized using novel experimental techniques complemented by molecular simulations. The characteristic birefringence exhibited by CNC films was utilized to calculate the in-plane CTE of self-organized and shear-oriented self-standing CNC films from room temperature to 100 °C using polarized light image correlation. CNC alignment was estimated via Hermans order parameter (S) from 2D X-ray diffraction measurements. We found that films with no preferential CNC orientation through the thickness (S: ∼ 0.0) exhibited an isotropic CTE (∼25 ppm/K). In contrast, films with aligned CNC orientations (S: ∼0.4 to 0.8) had an anisotropic CTE response: For the highest CNC alignment (S: 0.8), the CTE parallel to CNC alignment was ∼9 ppm/K, while that perpendicular to CNC alignment was ∼158 ppm/K. CNC film thermal expansion was proposed to be due primarily to single crystal expansion and CNC-CNC interfacial motion. The relative contributions of inter- and intracrystal responses to heating were explored using molecular dynamics simulations.

In Vitro Biocompatibility Studies of Antibacterial Quaternary Polymers

Quaternized copolymers of 4-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate are known to have antibacterial properties and have displayed biocompatibility in red blood cell hemolysis assays. The results from hemolysis assays have shown substantial promise, but the technique is rudimentary and only a first step toward the determination of biocompatibility. The present paper further explores the biocompatibility of these copolymers through comprehensive cell viability assays performed on Caco-2 human epithelial cells cultivated in vitro. We have shown that these copolymers are biocompatible at concentrations above their minimum bactericidal concentrations, leading to selectivity values that compare well with other microbicidal products.

Structure-activity relationships of antibacterial and biocompatible copolymers.

The development of polymers that are both bactericidal and biocompatible would have many applications and are currently of research interest. Following the development of strongly bactericidal copolymers of 4-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate, biocompatibility assays have been completed on these materials to measure their potential biocompatibility. In this article, a new methodology for measuring protein interaction was developed for water-soluble polymers by coupling proteins to surfaces and then measuring the adsorption of copolymers onto these surfaces. Ellipsometry was then used to measure the thickness of adsorbed polymers as a measurement of biocompatibility. These results were then compared and correlated with the results of other biocompatibility assays previously conducted on these polymers, affording a greater understanding of the biocompatibility of the copolymers as well as improving the understanding of the effect of hydrophilic and hydrophobic groups that is vital for the development of these materials.

Thermal Conductivity in Nanostructured Films: From Single Cellulose Nanocrystals to Bulk Films

We achieved a multiscale description of the thermal conductivity of cellulose nanocrystals (CNCs) from single CNCs (∼0.72-5.7 W m(-1) K(-1)) to their organized nanostructured films (∼0.22-0.53 W m(-1) K(-1)) using experimental evidence and molecular dynamics (MD) simulation. The ratio of the approximate phonon mean free path (∼1.7-5.3 nm) to the lateral dimension of a single CNC (∼5-20 nm) suggested a contribution of crystal-crystal interfaces to polydisperse CNC films heat transport. Based on this, we modeled the thermal conductivity of CNC films using MD-predicted single crystal and interface properties along with the degree of CNC alignment in the bulk films using Hermans order parameter. Film thermal conductivities were strongly correlated to the degree of CNC alignment and the direction of heat flow relative to the CNC chain axis. The low interfacial barrier to heat transport found for CNCs (∼9.4 to 12.6 m(2) K GW(-1)), and their versatile alignment capabilities offer unique opportunities in thermal conductivity control.

Stable low-voltage operation top-gate organic field-effect transistors on cellulose nanocrystal substrates.

We report on the performance and the characterization of top-gate organic field-effect transistors (OFETs), comprising a bilayer gate dielectric of CYTOP/Al2O3 and a solution-processed semiconductor layer made of a blend of TIPS-pentacene:PTAA, fabricated on recyclable cellulose nanocrystal-glycerol (CNC/glycerol) substrates. These OFETs exhibit low operating voltage, low threshold voltage, an average field-effect mobility of 0.11 cm(2)/(V s), and good shelf and operational stability in ambient conditions. To improve the operational stability in ambient a passivation layer of Al2O3 is grown by atomic layer deposition (ALD) directly onto the CNC/glycerol substrates. This layer protects the organic semiconductor layer from moisture and other chemicals that can either permeate through or diffuse out of the substrate.

A comparative guide to controlled hydrophobization of cellulose nanocrystals via surface esterification

Surface esterification methods of cellulose nanocrystals (CNC) using acid anhydrides, acid chlorides, acid catalyzed carboxylic acids, and 1′1-carbonyldiimidazole (CDI) activated carboxylic acids were evaluated with acetyl-, hexanoyl-, dodecanoyl-, oleoyl-, and methacryloyl-functionalization. Their grafting efficiency was investigated using Fourier-transform infrared spectroscopy and 13C solid state NMR spectroscopy. Acid anhydride and CDI were found to be the most applicable reagents to graft short and long chain aliphatic carbons, respectively. The preservation of structural morphology and crystallinity of grafted CNCs were confirmed using transmission electron microscopy and X-ray diffraction. The hydrophobicity of grafted CNCs was evaluated by dispersing them in organic solvents with different Hansen’s solubility parameters. The dispersibility of grafted CNCs in organic solvents was improved by using never-dried CNCs as source materials and keep CNCs wet in their washing solvents after grafting, thus increasing the solvency range to disperse CNCs.