A minimal model for solvent evaporation and absorption in thin films

Formation of glassy skins in drying polymer solutions: approximate analytical solutions

We study the formation of a glassy skin at the air interface of drying polymer solutions. We introduce a simple approximation, which is valid for most diffusion problems, and which allows us to derive analytical relationships for the polymer concentration as a function of time. We show that the approximate results differ by less than 15% from those obtained by numerically solving the diffusion equation. We use the approximation to study skin formation in evaporating solutions. We focus on the influence of variations of the mutual diffusion coefficient with concentration, when the latter decreases sharply at high concentrations, as observed in the vicinity of the glass transition. We show that the skin thickness depends very strongly on the exponent characterising the decrease of the diffusion coefficient, in contrast to the polymer volume fraction at the interface, which varies only slightly with the exponent.

Fast Evaporation Enabled Ultrathin Polymer Coatings on Nanoporous Substrates for Highly Permeable Membranes

Thin polymer coatings covering on porous substrates are a common composite structure required in numerous applications, including membrane separation, and there is a strong need to push the coating thicknesses down to the nanometer scale to maximize the performances. However, producing such ultrathin polymer coatings in a facile and efficient way remains a big challenge. Here, uniform ultrathin polymer covering films (UPCFs) are realized by a facile and general approach based on rapid solvent evaporation. By fast evaporating dilute polymer solutions spread on the surface of porous substrates, we obtain ultrathin coatings (down to ∼30 nm) exclusively on the top surface of porous substrates, forming UPCFs with a block copolymer of polystyrene-block-poly(2-vinyl pyridine) at room temperature or a homopolymer of poly(vinyl alcohol) (PVA) at elevated temperatures. Upon selective swelling of the block copolymer and crosslinking of PVA, we obtain highly permeable membranes delivering ∼2–10 times higher permeance in ultrafiltration and pervaporation than state-of-the-art membranes with comparable selectivities. We have invented a very convenient but highly efficient process for the direct preparation of defective-free ultrathin coatings on porous substrates, which is extremely desired in different fields in addition to membrane separation.

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Fast solvent evaporation is developed to produce UPCFs on porous substrates

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Selective swelling to cavitate block copolymers to form interconnected mesopores

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UPCFs enable the preparation of highly permeable membranes

Sustainable Fully Printed UV Sensors on Cork Using Zinc Oxide/Ethylcellulose Inks

Low-cost and large-scale production techniques for flexible electronics have evolved greatly in recent years, having great impact in applications such as wearable technology and the internet of things. In this work, we demonstrate fully screen-printed UV photodetectors, successfully fabricated at a low temperature on a cork substrate, using as the active layer a mixture of zinc oxide nanoparticles and ethylcellulose. The photoresponse under irradiation with a UV lamp with peak emission at 302 nm exhibited a quasi-quadratic behavior directly proportional to the applied voltage, with a photocurrent of about 5.5 and 20 μA when applying 1.5 V and 5 V, respectively. The dark current stayed below 150 nA, while the rise and falling times were, respectively, below 5 and 2 s for both applied voltages. The performance was stable over continuous operation and showed a degradation of only 9% after 100 bending cycles in a 45 mm radius test cylinder. These are promising results regarding the use of this type of sensor in wearable applications such as cork hats, bracelets, or bags.

Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation

Frontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.

Solvent quality influences surface structure of glassy polymer thin films after evaporation

Molecular dynamic simulations are used to investigate the structural effects of treating a glassy polymer thin film with solvents of varying quality and subsequently evaporating the solvent. Both a monodisperse film and a polydisperse film are studied for poor to good solvent conditions, including the limit in which the polymer film is fully dissolved. In agreement with previous studies, the dissolved polymer-solvent mixtures form a polymer-rich skin on top of the forming film during evaporation. In the case of the polydisperse films, a segregation of the lower molecular weight polymer to the film interface is observed. We provide a detailed, systematic analysis of the interface structure and properties during and after evaporation. We find that for non-dissolved films, the surface width of the film after solvent evaporation is enhanced compared to the case without solvent. Our results show that due to the kinetic arrest of the surface structure, the increased surface width is preserved after solvent evaporation for both mono- and polydisperse films. We conclude that it is important to take poor solvent effects into account for the surface morphology of already formed thin glassy films, an effect which is often neglected.

Effect of including a gas layer on the gel formation process during the drying of a polymer solution

In this paper, we study the influence of the upper gas layer on the drying and gelation of a polymer solution. The gel is formed due to the evaporation of the binary solution into (inert) air. A one-dimensional model is proposed, where the evaporation flux is more realistically described than in previous studies. The approach is based on general thermodynamic principles. A composition-dependent diffusion coefficient is used in the liquid phase and the local equilibrium hypothesis is introduced at the interface to describe the evaporation process. The results show that the high thickness of the gas layer reduces evaporation, thus leading to longer drying times. Our model is also compared with more phenomenological descriptions of evaporation, for which the mass flux through the interface is described by the introduction of a Peclet number. A global agreement is found for appropriate values of the Peclet numbers and our model can thus be considered as a tool allowing to link the value of the empirical Peclet number to the physics of the gas phase. Finally, in contrast with other models, our approach emphasizes the possibility of very fast gelation at the interface, which could prevent all Marangoni convection during the drying process.