Flexible aromatic disulfide monomers for high-performance self-healable linear and cross-linked poly(urethane-urea) coatings

Implementation of the self-healing concept in coatings is challenging because they have to combine mechanical strength and chain mobility. This challenge is addressed in this work by studying the effect of the polymer microstructure on the mechanical properties and self-healing ability of waterborne poly(urethane-urea) coatings containing aromatic disulfide dynamic bonds. The structural modifications studied are the concentration and flexibility of the aromatic disulfide units and the effect of cross-linking. The effects and limits of these structural changes on the mechanical properties of the polymers and their healability were determined via a combination of DMA measurements, tensile tests, and rheological and scratch closure experiments. It was found that the flexibility of the disulfide unit was key to develop more efficient self-healing materials which offer the necessary molecular mobility for self-healing while simultaneously maintaining a level of mechanical strength that are essential for coating applications.The European Union’s Horizon 2020 research and innovation programme is accredited for the financial support through Project TRACKWAY-ITN 642514 under the Marie Sklodowska-Curie grant agreement. N.B. acknowledges the financial support obtained through the Post-Doctoral fellowship Juan de la Cierva - Incorporación (IJCI-2016-28442), from the Ministry of Economy and Competitiveness of Spai

Synthesis of mechanically strong waterborne poly(urethane-urea)s capable of self-healing at elevated temperatures

Although various chemistries have been introduced into polyurethanes in order to obtain self-healing abilities, implementing these materials in applications requiring high strength is challenging as strong materials imply a limited molecular motion, but without movement of polymer chains self-healing is not possible. Here, waterborne poly(urethane-urea)s (PU(U)s) based on aromatic disulfide compounds are developed which balance these contradictory requirements by presenting good mechanical properties at room temperature, while showing the mobility necessary for healing when moderately heated. The influence of hard monomers on the stability and mobility of the materials is investigated by scratch closure, cut healing and rheological measurements, so that the limits of the readily available aromatic disulfide compounds, bis(4-aminophenyl)- and bis(4-hydroxyphenyl)disulfide, can be determined. Subsequently, a modified aromatic disulfide compound, bis[4-(3'-hydroxypropoxy)phenyl]disulfide, with increased reactivity, solubility and flexibility is synthesized and incorporated into the PU backbone, so that materials with more attractive mechanical properties, reaching ultimate tensile strengths up to 23 MPa, and self-healing abilities at elevated temperatures could be obtained.The European Union’s Horizon 2020 research and innovation programme is accredited for the financial support through Project TRACKWAY-ITN 642514 under the Marie Sklodowska-Curie grant agreement. N.B. acknowledges the financial support obtained through the Post-Doctoral fellowship Juan de la Cierva - Incorporación (IJCI-2016-28442), from the Ministry of Economy and Competitiveness of Spai

Stabilization of polymer colloid dispersions with pH-sensitive poly-acrylic acid brushes

Polyelectrolyte brushes are widely used for surface modification of nano-and colloidal particles because of their ability to dramatically change their conformation, hydrophobicity, polarity, charge, etc., as a response to smooth variations in environmental conditions. In this work, we have studied experimentally the stability behavior of polymer colloids with grafted poly-acrylic acid (PAA) surface brushes. We have measured the Fuchs stability ratio (W) as a function of electrolyte concentrations at different pH. It is observed that at pH  8), since most of the carboxylic groups are ionized, the colloidal stability is much higher than that at pH ~ 5. However, the W values are basically the same with 1% and 2% PAA, implying that the contribution of the ionized AA in the two cases is practically the same. This experimental evidence indicates that only the ionized AA groups in the outer region of long brushes contribute to colloidal stability, thus supporting the hypothesis of local electroneutrality in the inner region of long brushes (LEA

Amphiphilic block copolymers as stabilizers in emulsion polymerization: Effects of molecular weight dispersity and evidence of self-folding behavior

Emulsion polymerizations, used to produce many commodity materials, require stabilizing agents to prevent phase separation. Incorporation of these stabilizers in the final polymer may have negative effects on product properties, so the design of new stabilizers is being actively pursued. Amphiphilic diblock copolymers are a promising type of emulsion polymerization stabilizer and are the focus of this work (Fig. 1). First, the tolerance of an amphiphilic diblock copolymer stabilizer’s performance to high molecular weight dispersity and homopolymer impurity has been investigated. Polystyrene-b-poly(acrylic acid) block copolymers were studied due to their previously demonstrated efficacy as stabilizers in emulsion polymerization, and their similarity to commercially important polystyrene-r-poly(acrylic acid) stabilizers. Neither greater molecular weight dispersity nor homopolymer impurity was found to negatively impact the stabilization performance of these block copolymers, suggesting that the economically unfavorable conditions required to achieve low molecular weight dispersity and homopolymer impurity may be avoided. We then examined novel polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers which were shown to stabilize emulsion polymerizations with up to 50 weight percent solids content, exceeding what was possible using the polystyrene-b-poly(acrylic acid) block copolymers. Of even greater significance and scientific value is that the block-random copolymers were also observed to have unusual solution behavior, self-folding rather than self-assembling, to give single chain nanoparticles. Emulsion polymerizations stabilized by these block-random copolymers had a total particle surface area which was directly proportional to the stabilizer concentration and was unaffected by polymerization kinetics. A novel “seeded-coagulative” emulsion polymerization mechanism has been proposed to explain these results, which were unexplainable by any known emulsion polymerization mechanism. Please click Additional Files below to see the full abstrac

Phase Separation Driven On-Demand Debondable Waterborne Pressure-Sensitive Adhesives

A waterborne pressure-sensitive adhesive (PSA) that shows high adhesive performance and easy debondability on demand without leaving residues on the substrate (adhesive failure) has been developed. A key component of the PSA is a semicrystalline phase that is beneficial for the adhesive properties and that becomes fluid when heated above the melting temperature. Migration of this liquid-like polymer to the substrate-adhesive interface and hardening upon cooling results in a hard non-tacky interface that facilitates debonding. The effect of the particle morphology on the debonding ability is discussed.This research was funded by AkzoNobe

Synthesis of mechanically strong waterborne poly(urethane-urea)s capable of self-healing at elevated temperatures

Although various chemistries have been introduced into polyurethanes in order to obtain self-healing abilities, implementing these materials in applications requiring high strength is challenging as strong materials imply a limited molecular motion, but without movement of polymer chains self-healing is not possible. Here, waterborne poly(urethane-urea)s (PU(U)s) based on aromatic disulfide compounds are developed which balance these contradictory requirements by presenting good mechanical properties at room temperature, while showing the mobility necessary for healing when moderately heated. The influence of hard monomers on the stability and mobility of the materials is investigated by scratch closure, cut healing and rheological measurements, so that the limits of the readily available aromatic disulfide compounds, bis(4-aminophenyl)- and bis(4-hydroxyphenyl)disulfide, can be determined. Subsequently, a modified aromatic disulfide compound, bis[4-(3'-hydroxypropoxy)phenyl]disulfide, with increased reactivity, solubility and flexibility is synthesized and incorporated into the PU backbone, so that materials with more attractive mechanical properties, reaching ultimate tensile strengths up to 23 MPa, and self-healing abilities at elevated temperatures could be obtained.The European Union’s Horizon 2020 research and innovation programme is accredited for the financial support through Project TRACKWAY-ITN 642514 under the Marie Sklodowska-Curie grant agreement. N.B. acknowledges the financial support obtained through the Post-Doctoral fellowship Juan de la Cierva - Incorporación (IJCI-2016-28442), from the Ministry of Economy and Competitiveness of Spai

Amphiphilic Block Copolymers as Stabilizers in Emulsion Polymerization: Effects of the Stabilizing Block Molecular Weight Dispersity on Stabilization Performance

Molecular weight dispersity is not typically studied as a design parameter of block copolymer stabilizers but is often assumed to impact stabilization performance; low molecular weight dispersity is generally assumed to be associated with best performance. This is the first quantitative investigation of the effects of block copolymer molecular weight dispersity with regards to stabilization performance in an emulsion polymerization. Poly­(styrene)-<i>b</i>-poly­(acrylic acid) block copolymers were synthesized by nitroxide-mediated radical polymerization and employed as stabilizers in the emulsion polymerization of styrene. The effect of the stabilizing poly­(acrylic acid) block molecular weight dispersity on stabilization behavior was studied, independent of molecular weight and composition. Block copolymer stabilizers were evaluated in terms of critical aggregation concentration, dispersed phase particle size, distribution, and zeta potential. The molecular weight dispersity of the stabilizing block affected the aggregation number and final number of particles but displayed no negative effects on stability or size distribution

Ultrathin monomolecular films and robust assemblies based on cyclic catechols

We introduce a newly designed catechol-based compound and its application for the preparation of homogeneous monomolecular layers as well as for robust assemblies on various substrates. The precisely defined cyclic catechol material (CyCat) was prepared from ortho-dimethoxybenzene in a phenolic resin-like synthesis and subsequent deprotection, featuring molecules with up to 32 catechol units. The CyCat’s chemical structure was carefully assessed via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF), proton nuclear magnetic resonance (1H NMR), diffusion ordered spectroscopy (2D DOSY) and high resolution electrospray ionization mass spectrometry (ESI MS) experiments. The formation of colloidal aggregates of the CyCat material in alkaline solution was followed by dynamic light scattering (DLS) and further verified by dropcasting CyCat from solution on highly oriented pyrolytic graphite (HOPG), which was examined by Kelvin probe force microscopy (KPFM). The adsorption behavior of the CyCat to form monomolecular layers was investigated in real time by surface plasmon resonance (SPR). Formation of these thin CyCat layers (1.6–2.1 nm) on Au, SiO2 and TiO2 substrates was corroborated by spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). The prepared coating perfectly reflects the surface structure of the underlying substrate and does not exhibit CyCat colloidal aggregates as verified by atomic force microscopy (AFM). The functional nature of the prepared catechol monolayers was evidenced by reaction with 4-bromophenethylamine and bis(3-aminopropyl)-terminated poly(ethylene oxide) (PEO). Multilayer assemblies were prepared by a simple procedure of iterative immersion in solutions of CyCat and a multifunctional amine on Au, SiO2 and TiO2 substrates forming thicker coatings (up to 12 nm). Postmodification with small organic molecules was performed to covalently attach trifluoroacetyl, tetrazole and 2-bromo-2-methylpropanoyl moieties to the amine groups of the multilayer assembly coating. Furthermore, the versatility of the novel multilayer coating was underpinned by “grafting-to” of phenacyl sulfide–terminated PEO and “grafting-from” of poly(methyl methacrylate) via surface-initiated atom transfer radical polymerization (ATRP)