Humidity-Induced Phase Transitions of Surfactants Embedded in Latex Coatings Can Drastically Alter Their Water Barrier and Mechanical Properties

Water Sorption and Structural Properties of Human Airway Mucus in Health and Muco-Obstructive Diseases

Muco-obstructive diseases change airway mucus properties, impairing mucociliary transport and increasing the likelihood of infections. To investigate the sorption properties and nanostructures of mucus in health and disease, we investigated mucus samples from patients and cell cultures (cc) from healthy, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) airways. Atomic force microscopy (AFM) revealed mucin monomers with typical barbell structures, where the globule to spacer volume ratio was the highest for CF mucin. Accordingly, synchrotron small-angle X-ray scattering (SAXS) revealed more pronounced scattering from CF mucin globules and suggested shorter carbohydrate side chains in CF mucin and longer side chains in COPD mucin. Quartz crystal microbalance with dissipation (QCM-D) analysis presented water sorption isotherms of the three types of human airway mucus, where, at high relative humidity, COPD mucus had the highest water content compared to cc-CF and healthy airway mucus (HAM). The higher hydration of the COPD mucus is consistent with the observation of longer side chains of the COPD mucins. At low humidity, no dehydration-induced glass transition was observed in healthy and diseased mucus, suggesting mucus remained in a rubbery state. However, in dialyzed cc-HAM, a sorption-desorption hysteresis (typically observed in the glassy state) appeared, suggesting that small molecules present in mucus suppress the glass transition.

Effects of Counterion on the Formation and Hydration Behavior of α-Form Hydrated Crystals (α-Gels)

α-Form hydrated crystals form a lamellar gel in which the alkyl chains of the amphiphilic molecules are hexagonally arranged within bilayers below the gel-liquid crystal phase transition temperature. In practice, the lamellar gel network with excess water is called an "α-gel", particularly in the cosmetics industry. In this study, the hydration or water sorption of amphiphilic materials in water vapor was assessed using a humidity-controlled quartz crystal microbalance with dissipation monitoring (QCM-D) technique. The amphiphilic materials used in this study were hexadecyl phosphate salts neutralized with L-arginine (C16P-Arg), CsOH (C16P-Cs), KOH (C16P-K), and NaOH (C16P-Na). Small- and wide-angle X-ray scattering measurements revealed that C16P-Arg and C16P-Cs yielded α-form hydrated crystals. Humidity-controlled QCM-D measurements demonstrated that C16P-Arg and C16P-Cs more readily underwent hydration or water sorption than C16P-K and C16P-Na. The key conclusion is that the significant hydration ability of C16P-Arg and C16P-Cs promotes the formation of the corresponding α-form hydrated crystals.

A water adsorption study on wheat pericarp macrofibrils using atomic force microscopy

Wheat pericarp, which is the most external layer of the wheat kernel, is composed of a polysaccharide matrix, where cellulose macrofibrils, hemicellulose, and lignin are their main components. These polysaccharides modified their structure due to the hydric condition to which they are subjected. This effect is considered as an advantage in the wheat milling process. However, no information about micro and nanostructural changes on wheat pericarp macrofibrils due to their hydric condition, studied by the AFM technique and image analysis, has been reported. On the other hand, cellulose macrofibrils have been extensively studied by AFM but performing the study at constant relative humidity (RH) level. Hence, this study aimed to investigate the water adsorption process on wheat pericarp macrofibrils using AFM and control the RH to which samples were subjected during examinations with a lab equipment specially developed for the AFM experiment. The RH was modified from 10 to 90 %, and peak force error images were acquired, from which macrofibrils' diameter, swelling behavior, and water adsorption isotherms were calculated, using image analysis tools. Also, as an application from the water adsorption isotherms, the specific surface area and the hygroscopic swelling coefficients were determined. Results showed that wheat pericarp macrofibrils presented an unusual swelling behavior, with the most notorious changes after reaching a moisture content in equilibrium to 40 % of RH. The average diameter of the macro-fibrils varied from 45 to 48 nm. The water vapor adsorption isotherm obtained from AFM micrographs image analysis did not resemble the sigmoidal IUPAC Type II, generally obtained by applying gravimetric methods. Results suggest that the macrofibrils swelling controls water accessibility to the internal macrofibrils structures. It was proved with this study the feasibility of using AFM and image analysis to build water vapor isotherms and other mass transport parameters based on the macrofibrils swelling.

Porous Film Coating Enabled by Polyvinyl Pyrrolidone (PVP) for Enhanced Air Permeability of Fabrics: The Effect of PVP Molecule Weight and Dosage

This study developed a versatile and facile method for creating pores and tuning the porous structure in the polymer latex films by selectively etching the added functional polyvinyl pyrrolidone (PVP) molecules. The pore formed in the latex films had a similar morphology to that of PVP aggregation before etching. This observation promotes us to regulate the pore morphology that determines the film's property, such as air permeability through varying the PVP molecule weight and dosage. To this end, the effects of PVP molecule weight and dosage on the pore formation were systematically studied. The results showed that the average pore size of porous film decreased from >10 μm to sub-micron (about 0.4 μm) as the molecular weight or the dosage of PVP increased. This was ascribed to the strong adsorption affinity of PVP molecule onto the latex particle surface, which further hindered the diffusion and self-assembly of PVP molecule. In addition, this interaction became much stronger when the higher molecule weight of PVP or the higher dosage of PVP was employed, leading to the decreased size of PVP aggregation, as well as the formed pores in the latex films. Furthermore, the addition of PVP had little effect on the color of coated fabric based on the results of CIE L*a*b* measurement. The proposed facile method can be used to improve the air permeability of coated fabrics.

Effect of Relative Humidity on the Viscoelasticity of Thin Organic Films Studied by Contact Thermal Noise AFM

Material scientists are in need of experimental techniques that facilitate a quantitative mechanical characterization of mesoscale materials and, therefore, their rational design. An example is that of thin organic films, as their performance often relates to their ability to withstand use without damage. The mechanical characterization of thin films has benefited from the emergence of the atomic force microscope (AFM). In this regard, it is of relevance that most soft materials are not elastic but viscoelastic instead. While most AFM operation modes and analysis procedures are suitable for elasticity studies, the use of AFM for quantitative viscoelastic characterizations is still a challenge. This is now an emerging topic due to recent developments in contact resonance AFM. The aim of this work was to further explore the potential of this technique by investigating its sensitivity to viscoelastic changes induced by environmental parameters, specifically humidity. Here, we show that by means of this experimental approach, it was possible to quantitatively monitor the influence of humidity on the viscoelasticity of two different thin and hydrophobic polyurethane coatings representative of those typically used to protect materials from processes like weathering and wear. The technique was sensitive even to the transition between the antiplasticizing and plasticizing effects of ambient humidity. Moreover, we showed that this was possible without the need of externally exciting the AFM cantilever or the sample, i.e., just by monitoring the Brownian motion of cantilevers, which significantly facilitates the implementation of this technique in any AFM setup.