Single-Step Application of Polyelectrolyte Complex Films as Oxygen Barrier Coatings

Hybrid organic-inorganic coating with enhanced oxygen- and UV-barrier performance: Polyelectrolyte complex based on sodium alginate, poly (vinyl alcohol), and reconstructed layered double hydroxide

Organic-inorganic hybrid materials with high oxygen- and UV-barrier properties were developed using a polyelectrolyte complex comprising sodium alginate (SA), poly (vinyl alcohol) (PVA), and reconstructed layered double hydroxide (RLDH). These materials were applied to poly (ethylene terephthalate) (PET) as a barrier coating layer at a harsh drying temperature of 120 °C, similar to environments for the industrial coating process. The RLDH nanoplatelets within the coating matrix restricted the polymer chain mobility, elevating the glass transition temperature to 105.222-159.114 °C. Below RLDH 0.2 %, the apparent coating density significantly increased to 0.93-0.94 g/cm3. The embedded RLDH gave a tortuosity within the matrix, as evidenced by an intensified (003) diffraction peak in the XRD analysis. These structural alterations contributed to high oxygen- and UV-barrier performance. Notably, the PET/SA1.0PVA0.5RLDH0.2 film exhibited an extremely low oxygen transmission rate of <0.005 cm3/m2·day, with effectively blocking UV-A (62.41 %), -B (92.45 %), and -C light (100 %). Moreover, the susceptibility of the coated film to water vapor was mitigated by laminating cast polypropylene, achieving a water vapor transmission rate of 1.17 g/m2·day. Overall, the packaging materials with advanced oxygen-, water vapor-, and UV-barrier properties show great potential for practical applications in various sectors, including food packaging and medical/electrical devices.

Evaporation-Induced Polyelectrolyte Complexation: The Role of Base Volatility and Cosolvents

Film formation is a vital step for coating applications where a homogeneous, defect-free solid phase should be obtained, starting from a liquid casting formulation. Recently, an alternative waterborne-coating approach was proposed, based on the formation of a polyelectrolyte complex film. In this approach, an evaporating base induces a pH change during drying that initiates the complexation of oppositely charged polyelectrolytes, followed by further densification. In previous studies, ammonia was used as the evaporative base, leading to relatively fast evaporation and resulting in films showing significant brittleness, which tended to crack at low relative humidity or larger thicknesses. We hypothesize that slower complexation and/or evaporation can reduce the problematic stress build-up in the prepared polyelectrolyte complex coatings. For this reason, we studied the changes in the film formation process when there are different bases and cosolvents. We found that reducing the evaporation rate by changing ammonia to the slower evaporating dimethylamine or by adding DMSO as a cosolvent, led to less internal stress build-up during film formation, which could be beneficial for film application. Indeed, films prepared with ammonia showed cracking after 1 h, while films prepared with dimethylamine only showed cracking after one month. The fast evaporation of ammonia was also found to cause a temporary turbid phase, indicating phase separation, while for the slower evaporating bases, this did not occur. All prepared films remained sensitive to humidity, which poses the next challenge for these promising coatings.

Self-healing polymers for surface scratch regeneration

Recently, there has been a significant increase in academic and industrial interest in self-healing polymers (SHPs) due to their remarkable ability to regenerate scratched surfaces and materials of astronomical significance. Scientists have been inspired by the magical repairing mechanism of the living world. They transformed the fiction of self-healing into reality by designing engrossing polymeric materials that could self-repair mechanical abrasions repeatedly. As a result, the durability of the materials is remarkably improved. Thus, the idea of studying SHPs passively upholds economic and environmental sustainability. However, the critical areas of self-healing (including healing efficiency, healing mechanism, and thermo-mechanical property changes during healing) are under continuous scientific improvisation. This review highlights recent notable advances of SHPs for application in regenerating scratched surfaces with various distinctive underlying mechanisms. The primary focus of the work is aimed at discussing the impact of SHPs on scratch-healing technology. Beyond that, insights regarding scratch testing, methods of investigating polymer surfaces, wound depths, the addition of healing fillers, and the environmental conditions maintained during the healing process are reviewed thoroughly. Finally, broader future perspectives on the challenges and prospects of SHPs in healing surface scratches are discussed.

Flocculation of Cellulose Microfiber and Nanofiber Induced by Chitosan-Xylan Complexes

This study aims to provide a comprehensive understanding of the key factors influencing the rheological behavior and the mechanisms of natural polyelectrolyte complexes (PECs) as flocculation agents for cellulose microfibers (CMFs) and nanofibers (CNFs). PECs were formed by combining two polyelectrolytes: xylan (Xyl) and chitosan (Ch), at different Xyl/Ch mass ratios: 60/40, 70/30, and 80/20. First, Xyl, Ch, and PEC solutions were characterized by measuring viscosity, critical concentration (c*), rheological parameter, ζ-potential, and hydrodynamic size. Then, the flocculation mechanisms of CMF and CNF suspensions with PECs under dynamic conditions were studied by measuring viscosity, while the flocculation under static conditions was examined through gel point measurements, floc average size determination, and ζ-potential analysis. The findings reveal that PEC solutions formed with a lower xylan mass ratio showed higher intrinsic viscosity, higher hydrodynamic size, higher z-potential, and a lower c*. This is due to the high molecular weight, charge, and gel-forming ability. All the analyzed solutions behave as a typical non-Newtonian shear-thinning fluid. The flocculation mechanisms under dynamic conditions showed that a very low dosage of PEC (between 2 and 6 mg PEC/g of fiber) was sufficient to produce flocculation. Under dynamic conditions, an increase in viscosity indicates flocculation at this low PEC dosage. Finally, under static conditions, maximum floc sizes were observed at the same PEC dosage where minimum gel points were reached. Higher PEC doses were required for CNF suspensions than for CMF suspensions.

Nonaqueous Interfacial Polymerization-Derived Polyphosphazene Films for Sieving or Blocking Hydrogen Gas

A series of cyclomatrix polyphosphazene films have been prepared by nonaqueous interfacial polymerization (IP) of small aromatic hydroxyl compounds in a potassium hydroxide dimethylsulfoxide solution and hexachlorocyclotriphosphazene in cyclohexane on top of ceramic supports. Via the amount of dissolved potassium hydroxide, the extent of deprotonation of the aromatic hydroxyl compounds can be changed, in turn affecting the molecular structure and permselective properties of the thin polymer networks ranging from hydrogen/oxygen barriers to membranes with persisting hydrogen permselectivities at high temperatures. Barrier films are obtained with a high potassium hydroxide concentration, revealing permeabilities as low as 9.4 × 10^-17 cm³ cm cm^-2 s^-1 Pa^-1 for hydrogen and 1.1 × 10^-16 cm³ cm cm^-2 s^-1 Pa^-1 for oxygen. For films obtained with a lower concentration of potassium hydroxide, single gas permeation experiments reveal a molecular sieving behavior, with a hydrogen permeance of around 10^-8 mol m^-2 s^-1 Pa^-1 and permselectivities of H₂/N₂ (52.8), H₂/CH₄ (100), and H₂/CO₂ (10.1) at 200 °C.

Electrostatically Complexed Natural Polysaccharides as Aqueous Barrier Coatings for Sustainable and Recyclable Fiber-Based Packaging

Driven by the ever-growing awareness of sustainability and circular economy, renewable, biodegradable, and recyclable fiber-based packaging materials are emerging as alternatives to fossil-derived, nonbiodegradable single-use plastics for the packaging industry. However, without functional barrier coatings, the water/moisture vulnerability and high permeability of fiber-based packaging significantly restrain its broader application as primary packaging for food, beverages, and drugs. Herein, we develop waterborne complex dispersion barrier coatings consisting of natural, biodegradable polysaccharides (i.e., chitosan and carboxymethyl cellulose) through a scalable, one-pot mechanochemical pathway. By tailoring the electrostatic complexation, the key element to form a highly crosslinked and interpenetrated polymer network structure, we formulate complex dispersion barrier coatings with excellent film-forming property and adaptable solid-viscosity profiles suitable for paperboard and molded pulp substrates. Our complex dispersions enable the formation of a uniform, defect-free, and integrated coating layer, leading to a remarkable oil and grease barrier and efficient water/moisture sensitivity reduction while still exhibiting excellent recyclability profile of the resulting fiber-based substrates. This natural, biorenewable, and repulpable barrier coating is a promising candidate to serve as a sustainable option for fiber-based packaging intended for the food and food service packaging industry.

Quantification of Water-Ion Pair Interactions in Polyelectrolyte Multilayers Using a Quartz Crystal Microbalance Method

Water existing within thin polyelectrolyte multilayer (PEM) films has significant influence on their physical, chemical, and thermal properties, having implications for applications including energy storage, smart coatings, and biomedical systems. Ionic strength, salt type, and terminating layer are known to influence PEM swelling. However, knowledge of water’s microenvironment within a PEM, whether that water is affiliated with intrinsic or extrinsic ion pairs, remains lacking. Here, we examine the influence of both assembly and post-assembly conditions on the water–ion pair interactions of poly(styrene sulfonate)/poly(diallyldimethylammonium) (PSS/PDADMA) PEMs in NaCl and KBr. This is accomplished by developing a methodology in which quartz crystal microbalance with dissipation monitoring is applied to estimate the number of water molecules affiliated with an ion pair (i), as well as the hydration coefficient, π_salt^H₂O. PSS/PDADMA PEMs are assembled in varying ionic strengths of either NaCl and KBr and then exposed post-assembly to increasing ionic strengths of matching salt type. A linear relationship between the total amount of water per intrinsic ion pair and the post-assembly salt concentration was obtained at post-assembly salt concentrations >0.5 M, yielding estimates for both i and π_salt^H₂O. We observe higher values of i and π_salt^H₂O in KBr-assembled PEMs due to KBr being more effective in doping the assembly because of KBr’s more chaotropic nature as compared to NaCl. Lastly, when PSS is the terminating layer, i decreases in value due to PSS’s hydrophobic nature. Classical and ab initio molecular dynamics provide a microstructural view as to how NaCl and KBr interact with individual polyelectrolytes and the involved water shells. Put together, this study provides further insight into the understanding of existing water microenvironments in PEMs and the effects of both assembly and post-assembly conditions.

Spontaneous water-on-water spreading of polyelectrolyte membranes inspired by skin formation

Stable interfaces between immiscible solvents are crucial for chemical synthesis and assembly, but interfaces between miscible solvents have been less explored. Here the authors report the spontaneous water-on-water spreading and self-assembly of polyelectrolyte membranes. An aqueous mixture solution containing poly(ethyleneimine) and poly(sodium 4-styrenesulfonate) spreads efficiently on acidic water, leading to the formation of hierarchically porous membranes. The reduced surface tension of the polyelectrolyte mixture solution drives the surface spreading, while the interfacial polyelectrolytes complexation triggered by the low pH of water mitigates water-in-water mixing. The synergy of surface tension and pH-dependent complexation represents a generic mechanism governing interfaces between miscible solvents for materials engineering, without the need for surfactants or sophisticated equipment. As a proof-of-concept, porous polyelectrolyte hybrid membranes are prepared by surface spreading, exhibiting exceptional solar thermal evaporation performance (2.8 kg/m²h) under 1-sun irradiation.

Stable interfaces between immiscible solvents are crucial for chemical synthesis and assembly, but interfaces between miscible solvents have been less explored. Here the authors report the spontaneous water-on-water spreading and self-assembly of polyelectrolytes.

Protected Poly(3-sulfopropyl methacrylate) Copolymers: Synthesis, Stability, and Orthogonal Deprotection

Because of their permanent charge, strong polyelectrolytes remain challenging to characterize, in particular, when they are combined with hydrophobic features. For this reason, they are typically prepared through a postmodification of a fully hydrophobic precursor. Unfortunately, these routes often result in an incomplete functionalization or otherwise require harsh reaction conditions, thus limiting their applicability. To overcome these problems, in this work a strategy is presented that facilitates the preparation of well-defined strong polyanions by starting from protected 3-sulfopropyl methacrylate monomers. Depending on the chemistry of the protecting group, the hydrophobic precursor could be quantitatively converted into a strong polyanion under nucleophilic, acidic, or basic conditions. As a proof of concept, orthogonally protected diblock copolymers were synthesized, selectively deprotected, and allowed to self-assemble in aqueous solution. Further conversion into a fully water-soluble polyanion was achieved by deprotecting the second block as well.