Reversible Physical Crosslinks Research Articles

Salt-inactive hydrophobic association hydrogels with fatigue resistant and self-healing properties

Traditional hydrophobic association (HA) hydrogels can obtain excellent mechanical properties in the presence of anionic surfactants. However, such surfactants are sensitive to salt solutions, resulting in large fluctuation of mechanical properties for hydrogels. Here, a combined surfactant consisting of anionic surfactants (sodium dodecyl sulfate, SDS) and amphoteric surfactants (dodecyl dimethyl betaine, BS-12) was successfully introduced into HA hydrogels as stable physical crosslinking points. The resulting SDS/BS-12-HA hydrogels exhibited high tensile stress up to about 700 kPa. Moreover, the hydrogels demonstrated salt-inactive, self-healing and fatigue resistance properties owing to their unique dynamic and reversible physical crosslinking structure. Therefore, it's envisioned that this work may open a new method to develop a soft salt-inactive material with stable and excellent mechanical properties for extending the application range of hydrogels.

Hybrid double‐network hydrogel based on poly(aspartic acid) and poly(acryl amide) with improved mechanical properties

ABSTRACTHydrogels usually have a smaller mechanical strength and toughness than generic polymeric materials. Therefore, many studies report improvements for mechanical properties of hydrogels by preparing double‐network hydrogels, nanocomposite hydrogels, and nanostructured hydrogels. In this study, interpenetrating‐type dually‐crosslinked hydrogels were prepared via free radical crosslinking polymerization of acrylamide monomers in the presence of poly(aspartic acid) and subsequent immersion in a metal ion containing aqueous solution to induce extra physical crosslinking through ionic or coordination bonding. Using this approach, the mechanical properties of inherently weak and brittle homopolymer gels could be improved via interpenetrating the double network formed by both covalent bonding and metal coordination‐assisted reversible physical crosslinks. The preparation, swelling behavior, morphology, and mechanical properties of these hydrogels are presented. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45925.

Effect of hard segments on the thermal and mechanical behaviour of a novel hybrid silicon thermoplastic elastomer

A novel hybrid organic silicon thermoplastic elastomer (Si-TPE) was copolymerized by adopting branched organic polysiloxane and diphenyl methane diisocyanate, and the thermal processing mechanism and the effect of hard segments on the thermal processability and mechanical properties of Si-TPE were studied. The results showed that Si-TPE presents excellent thermal processability ascribed to its reversible physical crosslinking, i.e. hydrogen bonding between NH–C=O of the hard chains, and could be stably thermally processed into different products including films, tubes, tapes and sheets at 160–180 °C. With increasing hard segment content, the hydrogen bonding in the system was enhanced, leading to increases in both the melt viscosity and the softening temperature of Si-TPE. Simultaneously, the mechanical properties of Si-TPE were improved. When the hard segments increased to 20.3%, the tensile strength and Young’s modulus of Si-TPE reached 13.4 MPa and 43.8 MPa, respectively.

Fabrication of Surface Protein-Imprinted Nanoparticles Using a Metal Chelating Monomer via Aqueous Precipitation Polymerization.

Molecular imprinting is a promising way for constructing artificial protein recognition materials, but it has been challenged by difficulties such as restricted biomacromolecule transfer in the cross-linked polymer networks, and reduced template-monomer interactions that are due to the required aqueous media. Herein, we propose a strategy for imprinting of histidine (His)-exposed proteins by combining previous approaches such as surface imprinting over nanostructures, utilization of metal coordination interactions, and adoption of aqueous precipitation polymerization capable of forming reversible physical crosslinks. With lysozyme as a model template bearing His residues, imprinted polymer nanoshells were grafted over vinyl-modified nanoparticles by aqueous precipitation copolymerization of a Cu(2+) chelating monomer with a temperature-responsive monomer carried out at 37 °C, above the volume phase-transition temperature (VPTT) of the final copolymer. The imprinted nanoshells showed significant temperature sensitivity and the template removal could be facilitated by swelling of the imprinted layers at 4 °C, below the VPTT. The resultant core-shell imprinted nanoparticles exhibited strikingly high rebinding selectivity against a variety of nontemplate proteins. An imprinting factor up to 22.7 was achieved, which is among the best values reported for protein imprinting, and a rather high specific binding capacity of 67.3 mg/g was obtained. Moreover, this approach was successfully extended to preliminary imprinting of hemoglobin, another protein with accessible His. Therefore, it may be a versatile method for fabrication of high-performance surface-imprinted nanoparticles toward His-exposed proteins.

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.

Self-Healing and Thermoresponsive Dual-Cross-Linked Alginate Hydrogels Based on Supramolecular Inclusion Complexes.

β-Cyclodextrin (β-CD), with a lipophilic inner cavity and hydrophilic outer surface, interacts with a large variety of nonpolar guest molecules to form noncovalent inclusion complexes. Conjugation of β-CD onto biomacromolecules can form physically cross-linked hydrogel networks upon mixing with a guest molecule. Herein, the development and characterization of self-healing, thermoresponsive hydrogels, based on host-guest inclusion complexes between alginate-graft-β-CD and Pluronic F108 (poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol)), are described. The mechanics, flow characteristics, and thermal response were contingent on the polymer concentration and the host-guest molar ratio. Transient and reversible physical cross-linking between host and guest polymers governed self-assembly, allowing flow to occur under shear stress and facilitating complete recovery of the material's properties within a few seconds of unloading. The mechanical properties of the dual-cross-linked, multi-stimuli-responsive hydrogels were tuned as high as 30 kPa at body temperature and are advantageous for biomedical applications such as drug delivery and cell transplantation.

Synthesis of surface protein-imprinted nanoparticles endowed with reversible physical cross-links.

Researches on protein molecularly imprinted polymers have been challenged by the difficulties in facilitating biomacromolecular transfer, in particular upon the template removal step, and enhancing their recognition performance. Addressing these issues, herein we report synthesis of core–shell structured surface protein-imprinted nanoparticles with reversible physical cross-links formed in the imprinted nanoshells. The imprinted layers over nanoparticle supports are fabricated via aqueous precipitation polymerization (PP) of di(ethylene glycol) methyl ether methacrylate (MEO2MA), a thermo-responsive monomer bearing no strong H-bond donor, and other functional and cross-linking monomers. During polymerization, physical cross-links together with chemical cross-links are in site produced within the imprinted shells based on hydrophobic association among the PMEO2MA, favoring formation of high-quality imprints. While cooled appropriately below the polymerization temperature, these physical cross-links can be dissociated rapidly, thus facilitating removal of the embedded template. For proof of this concept, lysozyme-imprinted nanoparticles were synthesized at 37 °C over the nanoparticles functionalized with carboxylic and vinyl groups. The template removal from the imprinted nanoparticles was readily achieved by washing with a dilute acidic detergent solution at 4 °C. As-prepared imprinted nanoparticles showed greatly higher imprinting factor and specific rebinding than obtained with the same recipe but by solution polymerization (SP). Moreover, such imprinted nanomaterials exhibited satisfactory rebinding selectivity, kinetics and reusability.

Shape Memory Hydrogel based on a Hydrophobically-Modified Polyacrylamide (HMPAM)/α-CD Mixture via a Host-Guest Approach.

A novel thermally sensitive shape memory (SM) hydrogel is prepared by block copoly-merization of a cationic surfactant monomer, dimethylhexadecyl[2-(dimethylamino)ethylmethacrylate]ammoniumbromide (C(16)DMAEMA), and acrylamide (AM) in the presence of α-cyclodextrin (α-CD) using N,N'-methylenebisacrylamide (MBA) as a crosslinker. XRD, solid state (13)C NMR, and DSC measurements show that the crystalline domains, induced by the hydrogen bonds between α-CDs threaded on the hydrophobic units of the polymer chains through the host-guest approach, can reversibly melt and crystallize at different temperatures. Rheological measurements show that both the elastic modulus G' and viscous modulus G'' drastically change due to the formation and dissolution of the crystalline domains. These thermo-sensitive crystalline domains serve as reversible physical crosslinks, endowing the hydrogel with excellent SM properties. Cyclic experiments show that the hydrogel can recover to almost 100% of the deformation in each cycle and can be reused several times.

Synthesis of polyborosiloxane and its reversible physical crosslinks

A hybrid polyborosiloxane was synthesized and the effect of boron concentration on the Si–O : B weak bonding of polyborosiloxane was investigated mainly using FTIR and dynamic rheology.

Effect of the Super Network on Rubber Reinforcement

The influence of carbon black (CB) on rubber reinforcement was studied. A new reinforcement model, the super network structure model, was proposed. The super network is composed of irreversible chemical crosslinks and reversible physical crosslinks due to CB–rubber interaction. The two crosslink systems are not isolated but interlaced with each other. With increased interaction strength of the reversible physical crosslinks, the CB reinforcement became more effective.

Marine Adhesive Containing Nanocomposite Hydrogel with Enhanced Materials and Bioadhesive Properties

ABSTRACT4-arm poly(ethylene glycol) end-capped with mimics of adhesive moiety found in mussel adhesive protein, dopamine, was combined with a biocompatible nano-silicate, Laponite, in creating a nanocomposite hydrogel with improved materials and adhesive properties. Dopamine’s ability to form both irreversible covalent (cohesive and interfacial) and reversible physical (with Laponite) crosslinks was exploited in creating an injectable tissue adhesive. Incorporation of Laponite did not interfere with the curing of the adhesive. In some instances, increasing Laponite content reduced gelation time as dopamine-Laponite bond reduced the required number of covalent bonds needed for network formation. Incorporation of Laponite also increased compressive materials properties (e.g., max strength, energy to failure, etc.) of the nanocomposite without compromising its compliance as strain at failure was also increased. From lap shear adhesion test using wetted pericardium as the substrate, incorporating Laponite increased work of adhesion by 5 fold over that of control. Strong, physical bonds formed between dopamine and Laponite increased bulk materials properties, which contributed to the enhanced adhesive properties.

Gold Nanoparticles Incarcerated in Nanoporous Syndiotactic Polystyrene Matrices as New and Efficient Catalysts for Alcohol Oxidations

The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully tested in the oxidation of primary and secondary alcohols, in which we used dioxygen as the oxidant under mild conditions. Accordingly, (±)-1-phenylethanol was oxidised to acetophenone in high yields (96%) in 1 h, at 35 °C, whereas benzyl alcohol was quantitatively oxidised to benzaldehyde with a selectivity of 96% in 6 h. The specific rate constants calculated from the corresponding kinetic plots were among the highest found for polymer-incarcerated AuNPs. Similar values in terms of reactivity and selectivity were found in the oxidation of primary alcohols, such as cinnamyl alcohol and 2-thiophenemethanol, and secondary alcohols, such as indanol and α-tetralol. The remarkable catalytic properties of this system were attributed to the formation, under these reaction conditions, of the nanoporous ε crystalline form of syndiotactic polystyrene, which ensures facile and selective accessibility for the substrates to the gold catalyst incarcerated in the polymer matrix. Moreover, the polymeric crystalline domains produced reversible physical cross-links that resulted in reduced gold leaching and also allowed the recovery and reuse of the catalyst. A comparison of catalytic performance between AuNPs and annealed AuNPs suggested that multiple twinned defective nanoparticles of about 9 nm in diameter constituted the active catalyst in these oxidation reactions.

Shape‐Memory Polymers: Novel Biodegradable Shape‐Memory Elastomers with Drug‐Releasing Capabilities (Adv. Mater. 19/2011)

Shape-memory poly(diol-co-citrates) are poly-esters with thermosensitive optical and mechanical behaviour. Below their transition temperature (Ttrans) these materials become opaque and hard, whereas above Ttrans they are transparent elastomers. The mechanism for these phenomena includes the formation of reversible physical crosslinks within the covalently cross-linked polymer network, report Guillermo Ameer and co-workers on p. 2211.

The Influence of Ion Pair Formation on the Phase Behavior of Polyelectrolyte Solutions

In this paper we calculate the phase diagrams of polyelectrolyte solutions taking into account the possibility of ion pair formation between counterions and ions on polymer chains with their subsequent aggregation in multiplets. Multiplets play a role of reversible physical crosslinks between different polymer chains. Furthermore, we assume that the dielectric constant, and hence the binding energy of ion pairs and multiplets, depends on the volume fraction of polymer in the solution. We have shown that ion pairing and multiplet formation leads to instability of the homogeneous phase, so that at some conditions the phase separation can proceed even in doog solvent. Another qualitatively new feature of the behavior of polyelectrolyte solutions is the emergence of a triple point on the phase diagram. The effect of the solvent polarity and the polarity of the dry polymer on the general form of the phase diagram is studied in detail.