Design of Abrasion-Resistant, Long-Lasting Antifog Coatings.

Fog formation is a common challenge for numerous applications, such as food packaging, mirrors, building windows, and freezer/refrigerator doors. Most notably, fog forms on the inner surfaces of prescription glasses and safety eyewear (particularly when used with a mask), face shields, and helmet lenses. Fogging is caused by the distortion of light from condensed water droplets present on a surface and can typically be prevented if the surface static water contact angle (θ) is less than ∼40°. Such a low contact angle can be readily achieved by either increasing the substrate surface energy or by engineering surface nanotexture. Unfortunately, such nanotexture can be readily damaged with use, while high surface energy substrates get covered with low surface energy foulants over time. Consequently, even with numerous ephemeral antifog coatings, currently there are no commercially available, durable, and permanent antifog coatings. Here we discuss the development of a new class of high-performance antifog coatings that are abrasion-resistant and long-lasting. These polyvinylpyrrolidone-based coatings, designed based on the classical Ratner-Lancaster wear model, dramatically outperform the base polymer, as well as all tested commercially available antifog coatings. Specifically, these coatings exhibit a > 400% increase in fogging time compared to base polymer, a > 50,000% increase in wear resistance, and excellent long-term antifog performance. The developed coatings also significantly outperformed all tested commercially available antifog coatings in terms of their antifog performance, wear resistance, and long-term cyclical performance. Additionally, the key design strategies employed here─incorporation of toughening agents and hydrophilic slip additives─offer a new approach to developing high-performance, durable antifog coatings based on other well-known antifog polymers.

Probing the Stoichiometry Dependence of Enzyme-Catalyzed Junction Zone Network Formation in Aiyu Pectin Gel via a Reaction Kinetics Model.

We investigate the enzymatic self-catalyzed gelation process in aiyu gel, a natural ion crosslinked polysaccharide gel. The gelation process depends on the concentration ratio (Rmax) of the crosslinking calcium ions and all galacturonic acid binding sites. The physical gel network formation relies on the assembly of calcium-polysaccharide crosslink bonds. The crosslinks are initially transient and through break-up/rebinding gradually re-organizing into long, stable junction zones. Our previous study formulated a reaction kinetics model to describe enzymatic activation, crosslinker binding, and crosslink microstructural reorganization, in order to model the complex growth of elasticity. In this study, we extend the theory for the time-dependent profile of complex moduli and examine the interplay of enzyme conversion, crosslink formation, and crosslink re-organization. The adjusted model captures how the gelation and structural rearrangement characteristic times vary with the polymer and calcium concentrations. Furthermore, we find that calcium ions act as both crosslinkers and dopants in the excess calcium ion scenario and the binding dynamics is determined by Rmax. This study provides perspectives on the dynamic binding behaviors of aiyu pectin gel system and the theoretical approach can be generalized to enzyme-catalyzed ionic gel systems.

Numerical Design and Experimental Realization of a PNIPAM-Based Micro Thermosensor.

Stimuli-responsive polymers are capable of responding to external stimuli and therefore have been widely used for sensing. However, such applications are often based on naïve designs and cannot achieve the desired performance. In this study, we created a micro thermosensor with temperature-sensitive poly( N-isopropylacrylamide) (PNIPAM) hydrogel and temperature-insensitive poly(ethylene glycol) diacrylate (PEGDA) hydrogel using stop-flow lithography. The thermosensor is a bihydrogel microparticle consisting of a NIPAM-rich section and a NIPAM-poor section. Since the sensor is similar to a bimetallic strip in structure, its deformation can be easily identified to indicate temperature. To gain better control over the sensor performance, a numerical model capable of predicting the thermal behavior of the sensor was also developed. The model simulated the mass transfer and polymerization reaction during the fabrication process to determine the distributions of PNIPAM and PEGDA in the sensor. The information was then applied to predict the sensor deformation at various temperatures. We have used the model to access the effects of sensor geometry and fabrication temperature on the performance of the sensor. The sensor made under the guidelines from the numerical model has a working range between 16 and 55 °C. Except at very large deformation, the thermal response of the microsensor measured in experiments follows closely the numerical prediction. We believe such a numerical model can also be used for developing other applications involving stimuli-responsive polymers such as shape-evolving microparticles and origami-based microstructures. With the small size, ease of use, low manufacturing cost, good biocompatibility, and broad sensing range near physiological condition, the PNIPAM-based micro thermosensor should have strong potential to be used for bio-related applications and in a confined environment.