Regioselective palladium-catalysed aerobic oxidation of dextran and its use as a bio-based binder in paperboard coatings.

The coatings industry is aiming to replace petrochemical-based binders in products such as paints and lacquers with bio-based alternatives. Native polysaccharide additives are already used, especially as adhesives, and here we show the use of oxidised dextran as a bio-based binder additive. Linear dextran with a molecular weight of 6 kDa was aerobically oxidised in water at the C3-position of its glucose units, catalysed by [(neocuproine)PdOAc]2(OTf)2. The resulting keto-dextran with different oxidation degrees was studied using adipic dihydrazide as a crosslinker in combination with the commercial petrochemical-based binder Joncryl®. Coating experiments show that part of the Joncryl® can be replaced by keto-dextran while maintaining the desired performance.

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.

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

Polyelectrolyte complex (PEC) films such as polyelectrolyte multilayers have demonstrated excellent oxygen barrier properties, but unfortunately, the established layer-by-layer approaches are laborious and difficult to scale up. Here, we demonstrate a novel single-step approach to produce a PEC film, based on the use of a volatile base. Ammonia was used to adjust the pH of poly(acrylic acid) (PAA) so that direct complexation was avoided when it was mixed with polyethylenimine (PEI). Different charge ratios of homogeneous PEI/PAA solutions were successfully prepared and phase diagrams varying the concentration of ammonia or polyelectrolyte were made to study the phase behavior of PEI, PAA, and ammonia in water. Transparent ∼1 µm thick films were successfully deposited on biaxially orientated polypropylene (BOPP) sheets using a Meyer rod. After casting the films, the decrease in pH, caused by the evaporation of ammonia, triggered the complexation during drying. The oxygen permeation properties of films with different ratios and single polyelectrolytes were tested. All films displayed excellent oxygen barrier properties, with an oxygen permeation below 4 cm3·m-2·day-1·atm-1 (<0.002 barrer) at the optimum ratio of 2:1 PEI/PAA. This ammonia evaporation-induced complexation approach creates a new pathway to prepare PEC films in one simple step while allowing the possibility of recycling.

End-capping of amphiphilic nanotubes with phospholipid vesicles: impact of the phospholipid on the cap formation and vesicle loading under osmotic conditions.

Soft amphiphilic nanotubes are capped with vesicles comprised of either overall neutral, zwitterionic phospholipids, or those that carry a net charge. The phase transition temperature of the zwitterionic phospholipids plays a crucial role in the phase separation that leads to the end-capped nanotubes. The cationic vesicle caps can be loaded into the nanotubes via osmosis whereas the anionic vesicle caps are stable under hyper-osmotic conditions. Furthermore, no additional salt needs to be added for the cationic vesicle caps to induce the loading of the vesicles into the nanotubes due to the presence of counterions.

Amphiphilic Molecular Motors for Responsive Aggregation in Water.

The novel concept of amphiphilic molecular motors that self-assemble into responsive supramolecular nanotubes in water is presented. The dynamic function of the molecular motor units inside the supramolecular assemblies was studied using UV-vis absorption spectroscopy and cryo-transmission electron microscopy (cryo-TEM) microscopy. Reorganization between distinct, well-defined nanotubes and vesicles can be reversibly induced by light, going through the rotation cycle of the motor, i.e. driven by alternate photochemical and thermal isomerization steps in the system. This is the first example in which a molecular rotary motor shows self-assembly in an aqueous medium with full retention of its functionality, paving the way to increasingly complex, highly dynamic artificial nanosystems in water.

Acylhydrazones as Widely Tunable Photoswitches.

Molecular photoswitches have attracted much attention in biological and materials contexts. Despite the fact that existing classes of these highly interesting functional molecules have been heavily investigated and optimized, distinct obstacles and inherent limitations remain. Considerable synthetic efforts and complex structure-property relationships render the development and exploitation of new photoswitch families difficult. Here, we focus our attention on acylhydrazones: a novel, yet underexploited class of photochromic molecules based on the imine structural motif. We optimized the synthesis of these potent photoswitches and prepared a library of over 40 compounds, bearing different substituents in all four crucial positions of the backbone fragment, and conducted a systematic study of their photochromic properties as a function of structural variation. This modular family of organic photoswitches offers a unique combination of properties and the compounds are easily prepared on large scales within hours, through an atom-economic synthesis, from commercially available starting materials. During our thorough spectroscopic investigations, we identified photoswitches covering a wide range of thermal half-lives of their (Z)-isomers, from short-lived T-type to thermally stable P-type derivatives. By proper substitution, excellent band separation between the absorbance maxima of (E)- and (Z)-isomers in the UV or visible region could be achieved. Our library furthermore includes notable examples of rare negative photochromic systems, and we show that acylhydrazones are highly fatigue resistant and exhibit good quantum yields.

Loading of Vesicles into Soft Amphiphilic Nanotubes using Osmosis.

The facile assembly of higher-order nanoarchitectures from simple building blocks is demonstrated by the loading of vesicles into soft amphiphilic nanotubes using osmosis. The nanotubes are constructed from rigid interdigitated bilayers which are capped with vesicles comprising phospholipid-based flexible bilayers. When a hyperosmotic gradient is applied to these vesicle-capped nanotubes, the closed system loses water and the more flexible vesicle bilayer is pulled inwards. This leads to inclusion of vesicles inside the nanotubes without affecting the tube structure, showing controlled reorganization of the self-assembled multicomponent system upon a simple osmotic stimulus.

Autoamplification of molecular chirality through the induction of supramolecular chirality.

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring-open diarylethenes is doped with a small amount of their chiral, ring-closed counterpart. The molecules co-assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring-closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring-closed product, which is enriched in the template enantiomer.

Light-induced disassembly of self-assembled vesicle-capped nanotubes observed in real time.

Molecular self-assembly is the basis for the formation of numerous artificial nanostructures. The self-organization of peptides, amphiphilic molecules composed of fused benzene rings and other functional molecules into nanotubes is of particular interest. However, the design of dynamic, complex self-organized systems that are responsive to external stimuli remains a significant challenge. Here, we report self-assembled, vesicle-capped nanotubes that can be selectively disassembled by irradiation. The walls of the nanotubes are 3-nm-thick bilayers and are made from amphiphilic molecules with two hydrophobic legs that interdigitate when the molecules self-assemble into bilayers. In the presence of phospholipids, a phase separation between the phospholipids and the amphiphilic molecules creates nanotubes, which are end-capped by vesicles that can be chemically altered or removed and reattached without affecting the nanotubes. The presence of a photoswitchable and fluorescent core in the amphiphilic molecules allows fast and highly controlled disassembly of the nanotubes on irradiation, and distinct disassembly processes can be observed in real time using fluorescence microscopy.