Diffusion and Interaction Effects On Molecular Release Kinetics From Collapsed Microgels

The efficient transport of small molecules through dense hydrogel networks is crucial for various applications, including drug delivery, biosensing, catalysis, nanofiltration, water purification, and desalination. In dense polymer matrices, such as collapsed microgels, molecular transport follows the solution-diffusion principle: Molecules dissolve in the polymeric matrix and subsequently diffuse due to a concentration gradient. Employing dynamical density functional theory (DDFT), we investigate the nonequilibrium release kinetics of nonionic subnanometer-sized molecules from a microgel particle, using parameters derived from prior molecular simulations of a thermoresponsive hydrogel. The kinetics is primarily governed by the microgel radius and two intensive parameters: the diffusion coefficient and solvation free energy of the molecule. Our results reveal two limiting regimes: a diffusion-limited regime for large, slowly diffusing, and poorly soluble molecules within the hydrogel; and a reaction-limited regime for small, rapidly diffusing, and highly soluble molecules. These principles allow us to derive an analytical equation for release time, demonstrating excellent quantitative agreement with the DDFT results-a valuable and straightforward tool for predicting release kinetics from microgels.

Improving the Efficiency, Stability, and Adhesion of Perovskite Solar Cells Using Nanogel Additive Engineering

Additive engineering has been applied widely to improve the efficiency and/or stability of perovskite solar cells (PSCs). Most additives used to date are difficult to locate within PSCs as they are small molecules or linear polymers. In this work, we introduce, for the first time, carboxylic acid-functionalized nanogels (NGs) as additives for PSCs. NGs are swellable sub-100 nm gel particles. The NGs consist of poly(2-(2-methoxyethoxy) ethyl methacrylate)-co-methacrylic acid-co-ethylenegylcol dimethacrylate (PMEO₂MA-MAA-EGD) particles prepared by a scalable synthesis, which have a diameter of 40 nm. They are visualized in the perovskite films using SEM and are located at the grain boundaries. X-ray photoelectron and FTIR spectroscopy reveal that the NGs coordinate with Pb²⁺ via the -COOH groups. Including the NGs within the PSCs increased the grain size, decreased nonradiative recombination, and increased the power conversion efficiency (PCE) to 20.20%. The NGs also greatly increase perovskite stability to ambient storage, elevated temperature, and humidity. The best system maintained more than 80% of its original PCE after 180 days of storage under ambient conditions. Tensile cross-cut tape adhesion tests are used to assess perovskite film mechanical integrity. The NGs increased both the adhesion of the perovskite to the substrate and the mechanical stability. This study demonstrates that NGs are an attractive alternative to molecularly dispersed additives for providing performance benefits to PSCs. Our study indicates that the NGs act as a passivator, stabilizer, cross-linker, and adhesion promoter.

Scaling Laws in the Diffusive Release of Neutral Cargo from Hollow Hydrogel Nanoparticles: Paclitaxel-Loaded Poly(4-vinylpyridine)

We study the nonequilibrium diffusive release of electroneutral molecular cargo encapsulated inside hollow hydrogel nanoparticles. We propose a theoretical model that includes osmotic, steric, and short-range polymer-cargo attractions to determine the effective cargo-hydrogel interaction, u_eff*, and the effective diffusion coefficient of the cargo inside the polymer network, D_eff*. Using dynamical density functional theory (DDFT), we investigate the scaling of the characteristic release time, τ_1/2, with the key parameters involved in the process, namely, u_eff*, D_eff*, and the swelling ratio. This effort represents a full study of the problem, covering a broad range of cargo sizes and providing predictions for repulsive and attractive polymer shells. Our calculations show that the release time through repulsive polymer networks scales with q²e^βu_eff*/D_eff* for βu_eff* ≫ 1. In this case, the cargo molecules are excluded from the shell of the hydrogel. For attractive shells, the polymer retains the cargo molecules on its internal surface and its interior, and the release time grows exponentially with the attraction strength. The DDFT calculations are compared to an analytical model for the mean first passage time, which provides an excellent quantitative description of the kinetics for both repulsive and attractive shells without fitting parameters. Finally, we apply the method to reproduce experimental results on the release of paclitaxel from hollow poly(4-vinylpyridine) nanoparticles and find that the slow release of the drug can be explained in terms of the strong binding attraction between the drug and the polymer.

Using Soft Polymer Template Engineering of Mesoporous TiO2 Scaffolds to Increase Perovskite Grain Size and Solar Cell Efficiency

The mesoporous (meso)-TiO₂ layer is a key component of high-efficiency perovskite solar cells (PSCs). Herein, pore size controllable meso-TiO₂ layers are prepared using spin coating of commercial TiO₂ nanoparticle (NP) paste with added soft polymer templates (SPT) followed by removal of the SPT at 500 °C. The SPTs consist of swollen crosslinked polymer colloids (microgels, MGs) or a commercial linear polymer (denoted as LIN). The MGs and LIN were comprised of the same polymer, which was poly(N-isopropylacrylamide) (PNIPAm). Large (L-MG) and small (S-MG) MG SPTs were employed to study the effect of the template size. The SPT approach enabled pore size engineering in one deposition step. The SPT/TiO₂ nanoparticle films had pore sizes > 100 nm, whereas the average pore size was 37 nm for the control meso-TiO₂ scaffold. The largest pore sizes were obtained using L-MG. SPT engineering increased the perovskite grain size in the same order as the SPT sizes: LIN < S-MG < L-MG and these grain sizes were larger than those obtained using the control. The power conversion efficiencies (PCEs) of the SPT/TiO₂ devices were ∼20% higher than that for the control meso-TiO₂ device and the PCE of the champion S-MG device was 18.8%. The PCE improvement is due to the increased grain size and more effective light harvesting of the SPT devices. The increased grain size was also responsible for the improved stability of the SPT/TiO₂ devices. The SPT method used here is simple, scalable, and versatile and should also apply to other PSCs.