Mechanically Stable Thermally Crosslinked Poly(acrylic acid)/Reduced Graphene Oxide Aerogels

Graphene oxide (GO) aerogels, high porosity (>99%) low density (∼3–10 mg cm^–3) porous materials with GO pore walls, are particularly attractive due to their lightweight, high surface area, and potential use in environmental remediation, superhydrophobic and superoleophilic materials, energy storage, etc. However, pure GO aerogels are generally weak and delicate which complicates their handling and potentially limits their commercial implementation. The focus of this work was to synthesize highly elastic, mechanically stable aerogels that are robust and easy to handle without substantially sacrificing their high porosity or low density. To overcome this challenge, a small amount of readily available and thermally cross-linkable poly­(acrylic acid) (PAA) was intermixed with GO to enhance the mechanical integrity of the aerogel without disrupting other desirable characteristic properties. This method is a simple straightforward procedure that does not include multistep or complicated chemical reactions, and it produces aerogels with mass densities of about 4–6 mg cm^–3 and >99.6% porosity that can reversibly support up to 10 000 times their weight with full recovery of their original volume. Finally, pressure sensing capabilities were demonstrated and their oil absorption capacities were measured to be around 120 g oil per g aerogel^–1 which highlights their potential use in practical applications

Soybean Oil-Based Thermoset Films and Fibers with High Biobased Carbon Content via Thiol–Ene Photopolymerization

While a number of vegetable oil derivatives have been integrated with petroleum-based materials to prepare thermosetting polymers, existing examples usually incorporate low total biorenewable content into the final product. With the goal of generating thermosets with high biorenewable content, two different soybean oil derivatives with multifunctional thiol and acrylate groups were photocured via thiol–acrylate photopolymerization. For this purpose, l-cysteine, a nonhazardous amino acid, was coupled with epoxidized soybean oil to synthesize a mercaptanized soybean oil derivative containing multiple thiol groups. After being mixed with acrylate counterparts suitable for performing thiol–ene photopolymerizations, these monomer mixtures were processed into thermoset films (via monomer mixture film casting followed by photopolymerization) and fibers (via simultaneous electrospinning of the monomer mixture and photopolymerization in flight). The resulting materials possessed high biobased carbon content (BCC; over 90%) and higher elasticity than cross-linked acrylated epoxidized soybean oil without the thiol-containing component. This can be attributed to a change in the cross-link density that is controlled by different photopolymerization mechanisms (e.g., step-growth polymerization vs chain-growth homopolymerization). We anticipate that the approaches outlined in this study could be generalized to other bioderived triglyceride oils for increasing the BCC and imparting biodegradability in a number of materials applications

Polyhedral Oligomeric Silsesquioxane-Containing Thiol–ene Fibers with Tunable Thermal and Mechanical Properties

Polyhedral oligomeric silsesquioxanes (POSS) are versatile inorganic–organic hybrid building blocks that have potential applications as reinforcement nanofillers, thermal stabilizers, and catalyst supports for metal nanoparticles. However, fabrication of fibrous materials with high POSS content has been a challenge because of the aggregation and solubility limits of POSS units. In this paper, we describe a robust and environmentally friendly fabrication approach of inorganic–organic hybrid POSS fibers by integrating UV initiated thiol–ene polymerization and centrifugal fiber spinning. The use of monomeric liquids in this approach not only reduces the consumption of heat energy and solvent, but it also promotes homogeneous mixing of organic and inorganic components that allows integration of large amount of POSS (up to 80 wt %) into the polymer network. The POSS containing thiol–ene fibers exhibited enhanced thermomechanical properties compared to purely organic analogs as revealed by substantial increases in residual weight and a factor of 4 increase in modulus after thermal treatment at 1000 °C. This simple fabrication approach combined with the tunability in fiber properties afforded by tailoring monomer composition make POSS containing thiol–ene fibers attractive candidates for catalyst supports and filtration media, particularly in high-temperature and harsh environments

Conflicting Confinement Effects on the <i>T</i>_g, Diffusivity, and Effective Viscosity of Polymer Films: A Case Study with Poly(isobutyl methacrylate) on Silica and Possible Resolution

The glass transition temperature (<i>T</i>_g), in-plane diffusivity (<i>D</i>), and effective viscosity (η_eff) were measured for the same thin film system of poly­(isobutyl methacrylate) supported by silica (PiBMA/SiOx). We found that both the <i>T</i>_g and <i>D</i> were independent of the film thickness (<i>h</i>₀), but η_eff decreased with decreasing <i>h</i>₀. We envisage the different <i>h</i>₀ dependencies to be caused by <i>T</i>_g, <i>D</i>, and η_eff being different functions of the local <i>T</i>_g’s (<i>T</i>_g,<i>i</i>) or viscosities (η_<i>i</i>), which vary with the film depth. By assuming a three-layer model and that <i>T</i>_g(<i>h</i>₀) = ⟨<i>T</i>_g,<i>i</i>⟩, <i>D</i>(<i>h</i>₀) ∼ <i>k</i>_B<i>T</i>/⟨η_<i>i</i>⟩, and η_eff(<i>h</i>₀) = <i>h</i>₀³/3<i>M</i>_tot(η_<i>i</i>), where ⟨...⟩ denotes spatial averaging and <i>M</i>_tot is the mobility of the films, we were able to account for the experimental data. By extending these ideas to the analogous data of polystyrene supported by silica (PS/SiOx), a resolution was found for the long-standing inconsistency regarding the effects of confinement on the dynamics of polymer films

Generating Large Thermally Stable Marangoni-Driven Topography in Polymer Films by Stabilizing the Surface Energy Gradient

Marangoni forces drive a fluid to flow in response to positional differences in surface energy. In thin polymer films, a difference in surface energy between two coincident liquid polymers could offer a useful route to manufacture topographically patterned surfaces via the Marangoni effect. Previously, we have demonstrated a photochemical method using the Marangoni effect for patterning thin polystyrene films. To generalize the approach, a theoretical model that gives the underlying physics of this process was also developed, which further revealed that low viscosities, low diffusivities, and large surface energy gradients favor rapid evolution of large film thickness variations. However, as described by the Stokes−Einstein equation or the Rouse model, low viscosity is generally correlated with high diffusivity in a single-component system. Herein, we report a strategy to decouple film viscosity and diffusivity by co-casting a high molecular weight surface energy gradient creating copolymer (low diffusivity) with a low molecular weight majority homopolymer (high diffusivity and low viscosity), which are miscible with each other. Patterned light exposure through a photomask imposes a patterned surface energy gradient between light-exposed and unexposed regions due to photochemical reactions involving only the low diffusivity component. Upon heating the film to the liquid state, the film materials (primarily the low viscosity homopolymer component) flow from the low to high surface energy regions. This strategy either eliminates or greatly slows dissipation of the prepatterned surface energy gradient while maintaining rapid feature formation, resulting in formation of ca. 500 nm high features within only 30 min of thermal annealing. Furthermore, the formed features are stable upon extended thermal annealing for up to one month. It is found that a ratio of Marangoni forces to capillary forces can provide a predictive metric that distinguishes which scenarios produce features that dissipate or persist

Marangoni Instability Driven Surface Relief Grating in an Azobenzene-Containing Polymer Film

The Marangoni effect describes fluid flow near an interface in response to a surface tension gradient. Here, we demonstrate that the Marangoni effect is the underlying mechanism for flow driven feature formation in an azobenzene-containing polymer film; features formed in azobenzene-containing polymers are often referred to as surface relief gratings or SRGs. An amorphous poly­(4-(acryloyl­oxyhexyl­oxy)-4′-pentyl­azobenzene) was synthesized and studied as a model polymer. To isolate the surface tension driven flow from the surface tension pattern inscription step, the surface tension gradient was preprogrammed via photoisomerization of azobenzene in a glassy polymer film without forming topographical features. Subsequently, the latent image was developed in the absence of light by annealing above the glass transition temperature where the polymer is a liquid. The polymer flow direction was controlled with precision by inducing different surface tension changes in the exposed regions, in accordance with expectation based on the Marangoni effect. Finally, the height of the formed features decreased upon extensive thermal annealing due to capillary leveling with two distinct rates. A scaling analysis revealed that those rates originated from dissimilar capillary velocities associated with different azobenzene isomers

Reduced-Graphene Oxide/Poly(acrylic acid) Aerogels as a Three-Dimensional Replacement for Metal-Foil Current Collectors in Lithium-Ion Batteries

We report the synthesis and properties of a low-density (∼5 mg/cm³) and highly porous (99.6% void space) three-dimensional reduced graphene oxide (rGO)/poly­(acrylic acid) (PAA) nanocomposite aerogel as the scaffold for cathode materials in lithium-ion batteries (LIBs). The rGO-PAA is both simple and starts from readily available graphite and PAA, thereby providing a scalable fabrication procedure. The scaffold can support as much as a 75 mg/cm² loading of LiFePO₄ (LFP) in a ∼430 μm thick layer, and the porosity of the aerogel is tunable by compression; the flexible aerogel can be compressed 30-fold (i.e., to as little as 3.3% of its initial volume) while retaining its mechanical integrity. Replacement of the Al foil by the rGO-PAA current collector of the slurry-cast LFP (1.45 ± 0.2 g/cm³ tap density) provides for exemplary mass loadings of 9 mg_LFP/cm² at 70 μm thickness and 1.4 g/cm³ density or 16 mg_LFP/cm² at 100 μm thickness and ∼1.6 g/cm³ density. When compared to Al foil, the distribution of LFP throughout the three-dimensional rGO-PAA framework doubles the effective LFP solution-contacted area at 9 mg/cm² loading and increases it 2.5-fold at 16 mg/cm² loading. Overall, the rGO-PAA current collector increases the volumetric capacity by increasing the effective electrode area without compromising the electrode density, which was compromised in past research where the effective electrode area has been increased by reducing the particle size