Interplay of Hydrogen Bonding and Hydrophobic Interactions to Control the Mechanical Properties of Polymer Multilayers at the Oil-Water Interface

Fabricating Robust Constructs with Internal Phase Nanostructures via Liquid-in-Liquid 3D Printing

The ability to print soft materials into predefined architectures with programmable nanostructures and mechanical properties is a necessary requirement for creating synthetic biomaterials that mimic living tissues. However, the low viscosity of common materials and lack of required mechanical properties in the final product present an obstacle to the use of traditional additive manufacturing approaches. Here, a new liquid-in-liquid 3D printing approach is used to successfully fabricate constructs with internal nanostructures using in situ self-assembly during the extrusion of an aqueous solution containing surfactant and photocurable polymer into a stabilizing polar oil bath. Subsequent photopolymerization preserves the nanostructures created due to surfactant self-assembly at the immiscible liquid-liquid interface, which is confirmed by small-angle X-ray scattering. Mechanical properties of the photopolymerized prints are shown to be tunable based on constituent components of the aqueous solution. The reported 3D printing approach expands the range of low-viscosity materials that can be used in 3D printing, and enables robust constructs production with internal nanostructures and spatially defined features. The reported approach has broad applications in regenerative medicine by providing a platform to print self-assembling biomaterials into complex tissue mimics where internal supramolecular structures and their functionality control biological processes, similar to natural extracellular matrices.

Growth Mechanism of Polymer Membranes Obtained by H-Bonding Across Immiscible Liquid Interfaces

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process, but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA (polypropylene oxide-polymetacrylic acid) membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h ∼ t^1/2, which is reminiscent of a diffusion-limited process. However, counterintuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties.

Synthesis of poly(amide-thioether) with tunable hydrophilicity via thiolactone chemistry and its application in oil-in-oil emulsions

Oil-in-oil emulsions are ideal systems for water-sensitive reactions such as polymerizations and catalytic reactions, which has received extensive attention in recent years. The application of oil-in-oil emulsions has been developed slowly due to the limited types of surfactants and complicated synthesis process. Herein, we proposed a simple method to prepare poly(amide-thioether)-based surfactant for oil-in-oil emulsions via taking advantage of single-pot multicomponent and click characters of thiolactone chemistry. Using a combination of alkyl amine and acrylamide thiolactone, the aminolysis of thiolctone occurred first, generating thiol group in-situ, and then the generated thiol group would sequentially react with the double bonds of acrylamide to form polythioether in the presence of amine. The hydrophobicity of the surfactant could be effectively adjusted by the chain length of the alkyl amine and thus this polymer could serve as a promising surfactant for oil-in-oil emulsion. Notably, the emulsion types could be switched by changing the chain length of the alkyl amine. In addition, the effects of surfactant loading, volume ratio of oil phases, oil types on the size and stability of oil-in-oil emulsions were further investigated. It was demonstrated that the oil-in-oil emulsion stabilized by poly(amide-thioether)s kept stable after more than five months. Besides, we preliminarily explored the application of the oil-in-oil emulsion to prepare closed cell foam and porous particles via photo-initiated thiol-ene polymerization. It is believed that this super-stable oil-in-oil emulsion could offer more possibilities for highly potential water-sensitive systems.

Microfluidic probing of the complex interfacial rheology of multilayer capsules

Encapsulation of chemicals using polymer membranes enables control of their transport and delivery for applications such as agrochemistry or detergency. To rationalize the design of polymer capsules, it is necessary to understand how the membranes' mechanical properties control the transport and release of the cargo. In this article, we use microfluidics to produce model polymer capsules and study in situ their behavior in controlled divergent flows. Our model capsules are obtained by assembling polymer mono and hydrogen-bonded bilayers at the surface of an oil droplet in water. We also use microfluidics to probe in situ the mechanical properties of the membranes in a controlled divergent flow generated by introducing the capsules through a constriction and then in a larger chamber. The deformation and relaxation of the capsules depend on their composition and especially on the molecular interactions between the polymer chains that form the membranes and the anchoring energy of the first layer. We develop a model and perform numerical simulations to extract the main interfacial properties of the capsules from the measurement of their deformations in the microchannels.

Encapsulation of lipophilic fragrance by polymerization of the intermediate aqueous phase of an oil-in-water-in-oil (O/W/O) double emulsion

This paper deals with the encapsulation of lipophilic compounds in an oil-in-water-in-oil (O/W/O) double emulsion.

Responsive Emulsions Stabilized by Amphiphilic Supramolecular Graft Copolymers Formed in Situ at the Oil-Water Interface

Amphiphilic supramolecular graft copolymers which can stabilize oil-in-water (o/w) emulsions and enable responsive demulsification were demonstrated in this study. Linear poly[( N, N-dimethylacrylmide)- stat-(3-acrylamidophenylboronic acid)] (PDMA- stat-PAPBA) copolymers with phenylboronic acid (PBA) groups and linear polystyrene homopolymers with cis-diol terminals (PS(OH)₂) were synthesized by reversible addition-fragmentation chain transfer polymerization. By the homogenization of the biphasic mixtures of an alkaline water solution of PDMA- stat-PAPBA copolymer and a toluene solution of PS(OH)₂ homopolymer, stable o/w emulsions could be generated, although neither PDMA- stat-PAPBA nor PS(OH)₂ alone was able to stabilize the emulsion. It was verified that the dispersed oil droplets in the emulsions were stabilized by the amphiphilic PDMA- stat-PAPBA- g-PS supramolecular graft copolymers, which were formed in situ at the oil-water interface by the complexation between the lateral PBA groups of PDMA- stat-PAPBA and the diol terminals of PS(OH)₂ during homogenization. These emulsions showed pH- and glucose-responsive demulsification because of the reversible B-O bonds between the PDMA- stat-PAPBA backbones and the PS side chains. The effects of polymer concentrations on emulsion formation were also investigated. The current study provides an alternative method for the facile preparation of responsive polymeric emulsifiers, which potentially may be extended to other polymer pairs containing PBA and cis-diol groups.

Polymerizations in oil-in-oil emulsions using 2D nanoparticle surfactants

Oil-in-oil emulsions are especially attractive for compartmentalized reactions with water-sensitive monomers which cannot be used with traditional oil/water emulsions.

Versatile Encapsulation Technology Based on Tailored pH-Responsive Amphiphilic Polymers: Emulsion Gels and Capsules

We present a multipurpose technology to encapsulate hydrophobic substances in micron-sized emulsion droplets and capsules. The encapsulating agent is a comblike stimuli-responsive copolymer comprising side-chain surfactants attached to a methacrylic acid/ethyl acrylate polyelectrolyte backbone. The composition and structure of the hydrophobic moieties of the side chains are customized to tune the particle morphology and the processing conditions. The technology exploits the synergy of properties provided by the copolymer: interfacial activity, pH responsiveness, and viscoelasticity. A one-pot process produces emulsion gels or capsule dispersions consisting of a hydrophobic liquid core surrounded by a polymer shell. The dispersions resist high ionic strengths and exhibit long-term stability. The versatility of the method is demonstrated by encapsulating various hydrophobic substances covering a broad range of viscosities and polarities-conventional and technical oils, perfumes, and alkyd paints-with a high degree of morphological and rheological control.

Controlling Oil-in-Oil Pickering-Type Emulsions Using 2D Materials as Surfactant

Emulsions are important in numerous fields, including cosmetics, coatings, and biomedical applications. A subset of these structures, oil-in-oil emulsions, are especially intriguing for water sensitive reactions such as polymerizations and catalysis. Widespread use and application of oil-in-oil emulsions is currently limited by the lack of facile and simple methods for preparing suitable surfactants. Herein, we report the ready preparation of oil-in-oil emulsions using 2D nanomaterials as surfactants at the interface of polar and nonpolar organic solvents. Both the edges and basal plane of graphene oxide nanosheets were functionalized with primary alkyl amines and we demonstrated that the length of the alkyl chain dictates the continuous phase of the oil-in-oil emulsions (i.e., nonpolar-in-polar or polar-in-nonpolar). The prepared emulsions are stable at least 5 weeks and we demonstrate they can be used to compartmentalize reagents such that reaction occurs only upon physical agitation. The simplicity and scalability of these oil-in-oil emulsions render them ideal for applications impossible with traditional oil-in-water emulsions, and provide a new interfacial area to explore and exploit.

Carboxylated Fullerene at the Oil/Water Interface

The self-assembly of carboxylated fullerene with poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) with different molecular weights, poly-2-vinylpyridine, and amine-terminated polystyrene, at the interface between toluene and water was investigated. For all values of the pH, the functionalized fullerene interacted with the polymers at the water/toluene interface, forming a nanoparticle network, reducing the interfacial tension. At pH values of 4.84 and 7.8, robust, elastic films were formed at the interface, such that hollow tubules could be formed in situ when an aqueous solution of the functionalized fullerene was jetted into a toluene solution of PS-b-P2VP at a pH of 4.84. With variation of the pH, the mechanical properties of the fullerene/polymer assemblies can be varied by tuning the strength of the interactions between the functionalized fullerenes and the PS-b-P2VP.

Interfacial rheological properties of self-assembling biopolymer microcapsules

Tuning the mechanical properties of microcapsules through a cost-efficient route of fabrication is still a challenge. The traditional method of layer-by-layer assembly of microcapsules allows building a tailored composite multi-layer membrane but is technically complex as it requires numerous steps. The objective of this article is to characterize the interfacial rheological properties of self-assembling biopolymer microcapsules that were obtained in one single facile step. This thorough study provides new insights into the mechanics of these weakly cohesive membranes. Firstly, suspensions of water-in-oil microcapsules were formed in microfluidic junctions by self-assembly of two oppositely charged polyelectrolytes, namely chitosan (water soluble) and phosphatidic fatty acid (oil soluble). In this way, composite membranes of tunable thickness (between 40 and 900 nm measured by AFM) were formed at water/oil interfaces in a single step by changing the composition. Secondly, microcapsules were mechanically characterized by stretching them up to break-up in an extensional flow chamber which extends the relevance and convenience of the hydrodynamic method to weakly cohesive membranes. Finally, we show that the design of microcapsules can be 'engineered' in an extensive way since they present a wealth of interfacial rheological properties in terms of elasticity, plasticity and yield stress whose magnitudes can be controlled by the composition. These behaviors are explained by the variation of the membrane thickness with the physico-chemical parameters of the process.

One-Step Fabrication of pH-Responsive Membranes and Microcapsules through Interfacial H-Bond Polymer Complexation

Biocompatible microencapsulation is of widespread interest for the targeted delivery of active species in fields such as pharmaceuticals, cosmetics and agro-chemistry. Capsules obtained by the self-assembly of polymers at interfaces enable the combination of responsiveness to stimuli, biocompatibility and scaled up production. Here, we present a one-step method to produce in situ membranes at oil-water interfaces, based on the hydrogen bond complexation of polymers between H-bond acceptor and donor in the oil and aqueous phases, respectively. This robust process is realized through different methods, to obtain capsules of various sizes, from the micrometer scale using microfluidics or rotor-stator emulsification up to the centimeter scale using drop dripping. The polymer layer exhibits unique self-healing and pH-responsive properties. The membrane is viscoelastic at pH = 3, softens as pH is progressively raised, and eventually dissolves above pH = 6 to release the oil phase. This one-step method of preparation paves the way to the production of large quantities of functional capsules.

Highly stiff yet elastic microcapsules incorporating cellulose nanofibrils

Microcapsules with high mechanical stability and elasticity are desirable in a variety of contexts. We report a single-step method to fabricate such microcapsules by microfluidic interfacial complexation between high stiffness cellulose nanofibrils (CNF) and an oil-soluble cationic random copolymer. Single-capsule compression measurements reveal an elastic modulus of 53 MPa for the CNF-based capsule shell with complete recovery of deformation from strains as large as 19%. We demonstrate the ability to manipulate the shell modulus by the use of polyacrylic acid (PAA) as a binder material, and observe a direct relationship between the shell modulus and the PAA concentration, with moduli as large as 0.5 GPa attained. These results demonstrate that CNF incorporation provides a facile route for producing strong yet flexible microcapsule shells.

Interfacial Rheology of Hydrogen-Bonded Polymer Multilayers Assembled at Liquid Interfaces: Influence of Anchoring Energy and Hydrophobic Interactions

We study the 2D rheological properties of hydrogen-bonded polymer multilayers assembled directly at dodecane-water and air-water interfaces using pendant drop/bubble dilation and the double-wall ring method for interfacial shear. We use poly(vinylpyrrolidone) (PVP) as a proton acceptor and a series of polyacrylic acids as proton donors. The PAA series of chains with varying hydrophobicity was fashioned from poly(acrylic acid), (PAA), polymethacrylic acid (PMAA), and a homemade hydrophobically modified polymer. The latter consisted of a PAA backbone covalently grafted with C12 moieties at 1% mol (referred to as PAA-1C12). Replacing PAA with the more hydrophobic PMAA provides a route for combining hydrogen bonding and hydrophobic interactions to increase the strength and/or the number of links connecting the polyacid chains to PVP. This systematic replacement allows for control of the ability of the monomer units inside the absorbed polymer layer to reorganize as the interface is sheared or compressed. Consequently, the interplay of hydrogen bonding and hydrophobic interactions leads to control of the resistance of the polymer multilayers to both shear and dilation. Using PAA-1C12 as the first layer improves the anchoring energy of a few monomers of the chain without changing the strength of the monomer-monomer contact in the complex layer. In this way, the layer does not resist shear but resists compression. This strategy provides the means for using hydrophobicity to control the interfacial dynamics of the complexes adsorbed at the interface of the bubbles and droplets that either elongate or buckle upon compression. Moreover, we demonstrate the pH responsiveness of these interfacial multilayers by adding aliquots of NaOH to the acidic water subphase surrounding the bubbles and droplets. Subsequent pH changes can eventually break the polymer complex, providing opportunities for encapsulation/release applications.

Soft microcapsules with highly plastic shells formed by interfacial polyelectrolyte-nanoparticle complexation

Composite microcapsules have been aggressively pursued as designed chemical entities for biomedical and other applications. Common preparations rely on multi-step, time consuming processes. Here, we present a single-step approach to fabricate such microcapsules with shells composed of nanoparticle-polyelectrolyte and protein-polyelectrolyte complexes, and demonstrate control of the mechanical and release properties of these constructs. Interfacial polyelectrolyte-nanoparticle and polyelectrolyte-protein complexation across a water-oil droplet interface results in the formation of capsules with shell thicknesses of a few μm. Silica shell microcapsules exhibited a significant plastic response at small deformations, whereas lysozyme incorporated shells displayed a more elastic response. We exploit the plasticity of nanoparticle incorporated shells to produce microcapsules with high aspect ratio protrusions by micropipette aspiration.

One-Step Generation of Multifunctional Polyelectrolyte Microcapsules via Nanoscale Interfacial Complexation in Emulsion (NICE)

Polyelectrolyte microcapsules represent versatile stimuli-responsive structures that enable the encapsulation, protection, and release of active agents. Their conventional preparation methods, however, tend to be time-consuming, yield low encapsulation efficiency, and seldom allow for the dual incorporation of hydrophilic and hydrophobic materials, limiting their widespread utilization. In this work, we present a method to fabricate stimuli-responsive polyelectrolyte microcapsules in one step based on nanoscale interfacial complexation in emulsions (NICE) followed by spontaneous droplet hatching. NICE microcapsules can incorporate both hydrophilic and hydrophobic materials and also can be induced to trigger the release of encapsulated materials by changes in the solution pH or ionic strength. We also show that NICE microcapsules can be functionalized with nanomaterials to exhibit useful functionality, such as response to a magnetic field and disassembly in response to light. NICE represents a potentially transformative method to prepare multifunctional nanoengineered polyelectrolyte microcapsules for various applications such as drug delivery and cell mimicry.

Interface-enforced complexation between copolymer blocks

Binary diblock copolymers and corresponding ternary miktoarm stars are studied at oil-water interfaces. All polymers contain oil-soluble poly(propylene oxide) PPO, water-soluble poly(dimethylaminoethyl methacrylate) PDMAEMA and/or poly(ethylene oxide) PEO. The features of their Langmuir compression isotherms are well related to the ones of the corresponding homopolymers. Within the Langmuir-trough, PEO-b-PPO acts as the most effective amphiphile compared to the other PPO-containing copolymers. In contrast, the compression isotherms show a complexation of PPO and PDMAEMA for PPO-b-PDMAEMA and the star, reducing their overall amphiphilicity. Such complex formation between the blocks of PPO-b-PDMAEMA is prevented in bulk water but facilitated at the interface. The weakly-interacting blocks of PPO-b-PDMAEMA form a complex due to their enhanced proximity in such confined environments. Scanning force microscopy and Monte Carlo simulations with varying confinement support our results, which are regarded as compliant with the mathematical random walk theorem by Pólya. Finally, the results are expected to be of relevance for e.g. emulsion formulation and macromolecular engineering.