Phosphonic Acid-Functionalized Polyurethane Dispersions with Improved Adhesion Properties

A facile route to phosphorus-functionalized polyurethane dispersions (P-PUDs) with improved adhesion properties is presented. (Bis)­phosphonic acid moieties serve as adhesion promoting sites that are covalently attached via an end-capping reaction to isocyanate-reactive polyurethane particles under aqueous conditions. The synthetic approach circumvents solubility issues, offers great flexibility in terms of polyurethane composition, and allows for the synthesis of semicrystalline systems with thermomechanical response due to reversible physical cross-linking. Differential scanning calorimetry (DSC) is used to investigate the effect of functionalization on the semicrystallinity. The end-capping conversion was determined via inductively-coupled plasma optical emission spectroscopy (ICP-OES) and was surprisingly found to be almost independent of the stoichiometry of reaction, suggesting an adsorption-dominated process. Particle charge detection (PCD) experiments reveal that a dense surface coverage of phosphonic acid groups can be attained and that, at high functionalization degrees, the phosphonic adhesion moieties are partially dragged inside the colloidal P-PUD particle. Quartz crystal microbalance with dissipation (QCMD) investigations conducted with hydroxyapatite (HAP) and stainless steel sensors as model surfaces show a greatly enhanced affinity of the aqueous P-PUDs and furthermore indicate polymer chain rearrangements and autonomous film formation under wet conditions. Due to their facile synthesis, significantly improved adhesion, and variable film properties, P-PUD systems such as the one described here are believed to be of great interest for multiple applications, e.g., adhesives, paints, anticorrosion, or dentistry

Surfactant-Free Polyurethane Nanocapsules via Inverse Pickering Miniemulsion

We report on a surfactant-free synthesis of Pickering-stabilized submicrometer-sized capsules in inverse miniemulsion. Functionalized silica nanoparticles are able to stabilize water-in-cyclohexane miniemulsions to form stable polyurethane shells via interfacial polyaddition. The effect of the type of silica functionalization on the stabilizing properties is demonstrated by varying the hydrophobicity and, therefore, the contact angle between silica and the two liquid phases. Addition of small amounts of salt leads to a reduction of the capsule size and to a narrow size distribution. The impermeability of the formed capsule shell is proven by encapsulation of an organic fluorescent dye and release studies in aqueous environment. In addition, we show the possibility to encapsulate large amounts of inorganic salts without negative effects concerning the stability of the emulsion, which enables the application for phase-change materials

Synthesis and Thermal Curing of Benzoxazine Functionalized Polyurethanes

Benzoxazine (BOX) functionalized polyurethanes (PU) are introduced to provide a conceptually new thermal curing mechanism for polyurethanes. 3,4-Dihydro-3-methyl-2<i>H</i>-1,3-benzoxazine (P-m) was carefully oligomerized through thermal treatment. In a straightforward synthesis the newly formed hydroxyl groups are used for end-capping reactions with isocyanate-terminated polyurethane prepolymers. The isocyanate reactive hydroxyl content (IRH) of the benzoxazine oligomer was investigated in detail via ¹H NMR spectroscopy, HPLC-MS, indirect potentiometric titration in various solvents, and comparison with model substances and found to be strongly influenced by hydrogen bonding. The corresponding polyurethane/benzoxazine hybrid materials (PU/BOX) can cross-link at elevated temperatures and do not suffer from shelf-life issues or outgassing of blocked isocyanates. The thermally activated curing reaction was investigated via rheology and DSC. Significant improvements over state-of-the-art systems based on phenol-capped PU prepolymers are shorter curing times, increased moduli, and drastically increased glass transition temperatures

A Facile Route toward Structured Hybrid Particles Based on Liquid–Solid Assembly

Structured hybrid particles with strongly improved colloidal stability are synthesized through a facile fabrication method based on the assembly of miniemulsion droplets containing liquid monomeric precursors onto solid nanoparticles. Classical heterocoagulation experiments between solid particles with similar compositions are performed for comparison and result in coagulated samples. A two-step mechanism is proposed which involves polymerization to fixate the final hybrid particle morphology after electrostatically driven self-assembly. Negatively charged polyacrylonitrile (PAN) nanoparticles with a high degree of semicrystallinity are utilized as solid core and combined with positively charged monomer droplets of varying compositions. A simple adjustment of miniemulsion composition enables the tailored synthesis of raspberry or core–shell structured hybrid particles. While the ζ-potential strongly affects the colloidal stability, adjusting the <i>T</i>_g of the polymer and/or the cross-linking degree after polymerization is an efficient tool to determine the final latex morphology. As-prepared hybrid dispersions can form transparent films with embedded PAN domains with an undisturbed high degree of semicrystallinity and thus show potentials in a wide variety of applications, e.g., for coatings and adhesives with reinforced mechanical properties and improved barrier performance

pH-Sensitive Nanocapsules with Barrier Properties: Fragrance Encapsulation and Controlled Release

A facile synthesis method for polymer nanocapsules with high diffusion barrier and stimuli-responsive release properties is presented. The highly volatile fragrance α-pinene was used as hydrophobic model compound for the encapsulation process, which is based on a miniemulsion-analogous free radical polymerization process. The copolymer composition was systematically varied, and increasing contents of methacrylic acid as functional monomer in combination with high glass transition temperatures enabled unusually high encapsulation efficiencies of ≥90% for capsules with <i>z</i>-average diameters of <200 nm. Temperature and pH change can be used as trigger to open the capsules, and the release kinetics can be tailored depending on the polymer shell composition. In contrast to more frequently applied barrier microcapsules the nanocapsules provide drastically improved colloidal stabilities. Furthermore, the barrier nanocapsule approach is principally not restricted to fragrances and is expected to be compatible with other hydrophobic actives

Stimuli-Selective Delivery of two Payloads from Dual Responsive Nanocontainers

Stimuli-Selective Delivery of two Payloads from Dual Responsive Nanocontainer

Molecularly Controlled Coagulation of Carboxyl-Functionalized Nanoparticles Prepared by Surfactant-Free Miniemulsion Polymerization

We present the synthesis of molecularly controlled “CO₂-switchable” polystyrene nanoparticles by surfactant-free miniemulsion polymerization using a carboxyl-functionalized surface-active monomer, which acts as comonomer and stabilizer at the same time. The obtained nanoparticles are about 100 nm in size and show a small size distribution, confirmed by dynamic light scattering (DLS) and electron microscopy. Under ambient conditions, the latex particles form a stable suspension that can be coagulated by bubbling CO₂. The redispersion of the coagulated particles can be easily achieved by ultrasonication. The reversibility of the coagulation is confirmed after several coagulation/redispersion cycles (CO₂ bubbling and ultrasonification) from DLS and zeta potential measurements

Stimulus-Responsive Release from Poly(ferrocenylsilane) Nanocontainers

Redox-responsive poly­(ferrocenylsilane) (PFS) is used to construct nanocontainers that can be loaded with hydrophobic cargo by a miniemulsion approach. The resulting structures comprise a solid shell surrounding a liquid oil core and have diameters of approximately 470 nm with a shell thickness of ca. 29 nm. The electrochemical behavior of the ferrocene group is investigated using cyclic voltammetry. Electrochemical oxidation and the thereby caused change of container morphology are shown. Hydrophobic molecules (Nile Red and 2-propylpyiridine) are loaded into the nanocontainers and can be released upon oxidation of the shell material. The oxidation is achieved chemically by the addition of hydrogen peroxide or by the enzymatic oxidation of glucose to release 2-propylpyridine over a period of time

Polyurethane Dispersions with Peptide Corona: Facile Synthesis of Stimuli-Responsive Dispersions and Films

Peptide–polymer hybrid particles of submicron size yielding stimuli-responsive macroscopic films are presented. A thermoplastic polyurethane (PU) carrying polysiloxane and polyester soft segments serves as core material to obtain flexible, yet semicrystalline films with temperature-sensitivity. The synthesis is based on the high-sheer emulsification of isocyanate-terminated PU prepolymers, which in our model system purposefully lack any ability of colloidal self-stabilization. While emulsification in water leads to immediate coagulation, stable dispersions of polyurethane nanoparticles were formed in aqueous solutions of a hydrolyzed protein from wool. A comparison of dispersion and film properties to nonreactive, otherwise identical dispersions suggests covalent attachment of the peptide to the PU backbone. We show that the colloidal stability of the hybrid particles is completely governed by the peptide corona, and hence pH-triggered coagulation can be employed to induce particle deposition and film formation. Differential scanning calorimetry confirms partial crystallinity in the film and reveals strongly modified crystallization behavior due to the peptide

Triple-Stimuli-Responsive Ferrocene-Containing PEGs in Water and on the Surface

Triple-stimuli-responsive PEG-based materials are prepared by living anionic ring-opening copolymerization of ethylene oxide and vinyl ferrocenyl glycidyl ether and subsequent thiol–ene postpolymerization modification with cysteamine. The hydrophilicity of these materials can be tuned by three stimuli: (i) temperature (depending on the comonomer ratio), (ii) oxidation state of iron centers in the ferrocene moieties, and (iii) pH-value (through amino groups), both in aqueous solution and at the interface after covalent attachment to a glass surface. In such materials, the cloud point temperatures are adjustable in solution by changing oxidation state and/or pH. On the surface, the contact angle increases with increasing pH and temperature and after oxidation, making these smart surfaces interesting for catalytic applications. Also, their redox response can be switched by temperature and pH, making this material useful for catalysis and electrochemistry applications. Exemplarily, the temperature-dependent catalysis of the chemiluminescence of luminol (a typical blood analysis tool in forensics) was investigated with these polymers