Aromatic Cross-linkers Research Articles

Synthesis and Porous Structure Characteristics of Allyl Methacrylate/Divinylbenzene Polymers

A set of six porous copolymers of allyl methacrylate (AllMeth) and divinylbenzene (DVB), containing 5 to 50 wt% of the latter crosslinker, are synthesized by suspension polymerization method in the presence of inert diluents. Obtained polymers have various specific surface areas—those with 20 to 50 wt% of DVB have a high surface area in the range of 410–480 m2/g, depending only slightly on the amount of the aromatic crosslinker. Specific surface area decreases strongly only when DVB content is 15 and 5 wt%. That is the feature that is unusual for crosslinked polymers and indicates the participation of allyl groups in making a novel polymeric network of high porosity. All polymers contain polar carbonyl groups, with an electron pair able to interact with polar sorbates. Polymers are characterized using elemental analysis, FTIR, and their porous structure is characterized by nitrogen adsorption at 77 K. All polymers contain residual allyl groups, which possibly can serve as convenient points for chemical modification, allowing for the future synthesis of specialty polymers (such as ion-exchangers and coordinating resins).

One-Pot Preparation of Double-Network Epoxy Vitrimers with High Performance, Recyclability, and Two-Stage Stress Relaxation.

Although considerable progress has been made in developing different types of vitrimers, ongoing challenges remain in tuning their mechanical and rheological properties, self-healing, and adhesion. Here, we demonstrate a one-pot method to produce a novel double-network epoxy vitrimer using an aliphatic amine cross-linker with a siloxane covalent bond and an aromatic amine cross-linker with a disulfide covalent bond. When a controlled two-stage curing process is employed, the markedly different reactivities of aliphatic amine and aromatic amine with epoxy allow for sequential cross-linked network formation, leading to the development of a double network that incorporates two types of dynamic covalent bonds. As a result, the produced vitrimers exhibit controllable mechanical, thermal, and rheological properties, as well as recyclability. This is evidenced by a tensile strength as high as 72 MPa, while maintaining ∼10% elongation at break, a wide glass-transition temperature range from 91 to 171 °C, and an adjustable two-stage stress relaxation. These characteristics suggest opportunities to develop high-performance cross-linked polymers with specific responses to time and temperatures.

Fluorescent Labeling of a Target Protein with an Alkyl Diazirine Photocrosslinker Bearing a Cinnamate Moiety.

A novel fluorogenic alkyl diazirine photocrosslinker bearing an o-hydroxycinnamate moiety has been developed for identification of the targets of bioactive molecules. The o-hydroxycinnamate moiety can be converted to the corresponding 7-hydroxycoumarin derivative, which should be created on the interacting site within the photocaptured target protein. The label yield and fluorescence intensity have been immensely improved in comparison with our previous aromatic crosslinkers to facilitate target identification in small quantities.

Aromatic Disulfide Cross-Linkers for Self-Healable and Recyclable Acrylic Polymer Networks

Aromatic Disulfide Cross-Linkers for Self-Healable and Recyclable Acrylic Polymer Networks

Comprehensive use of fossil derivative: High value-added conversion of waste plastics to thermal energy storage materials

As one of the most common fossil derivatives, plastic is indispensable in daily life because of its promising mechanical properties, chemical stability, and easy accessibility. However, plastics still present a huge burden on the global environment due to their high production volume, difficulty with degradation and low recycling rate. At the same time, global primary energy reserves are decreasing, thus achieving highly efficient use of energy has become a hot topic of research. Herein, the waste plastics can be utilized as a supporting material and encapsulation the phase change materials through intermolecular hyper-crosslinking reaction with the assistance of Lewis acids. To illustrate the effect of porous structure on the thermophysical properties and energy storage characteristics of the obtained materials, three aromatic crosslinkers, triphenylphosphine, triphenylbenzene, and benzene were examined regarding their performance on the Fridel-Crafts reactions. It is found that the obtained materials bearing a thermal conductivity and latent heat of 0.22 W/mK and 145.7 J/g, respectively, without liquid leakage.

Internally bridged nanosilica for loadings and release of sparsely soluble compounds

The engineering of a new monodisperse colloid with a sea urchin-like structure with a large complex internal structure is reported, in which silica surfaces are bridged by an aromatic organic cross-linker to serve as a nanocarrier host for drugs such as doxorubicin (DOX) against breast cancer cells. While dendritic fibrous nanosilica (DFNS) was employed and we do not observe a dendritic structure, these particles are referred to as sea urchin-like nanostructured silica (SNS). Since the structure of SNS consists of many silica fibrils protruding from the core, similar to the hairs of a sea urchin. For the aromatic structured cross-linker, bis(propyliminomethyl)benzene (b(PIM)B-S or silanated terephtaldehyde) were employed, which are prepared with terephtaldehyde and 3-aminopropyltriethoxy-silane (APTES) through a simple Schiff base reaction. b(PIM)B-S bridges were introduced into SNS under open vessel reflux conditions. SPS refers to the product obtained by incorporating the cross-linker b(PIM)B-S in ultra-small colloidal SNS particles. In-situ incorporation of DOX molecules resulted in SPS-DOX. The pH-responsive SPS nanocomposites were tested as biocompatible nanocarriers for controllable doxorubicin (DOX) delivery. We conclude that SPS is a unique colloid which has promising potential for technological applications such as advanced drug delivery systems, wastewater remediation and as a catalyst for green organic reactions in water.

Graphene Oxide-Based Membranes Intercalated with an Aromatic Crosslinker for Low-Pressure Nanofiltration.

Graphene oxide (GO), a carbonaceous 2D nanomaterial, has received significant interest as a next-generation membrane building block. To fabricate high-performance membranes, an effective strategy involves stacking GO nanosheets in laminated structures, thereby creating unique nanochannel galleries. One outstanding merit of laminar GO membranes is that their permselectivity is readily tunable by tailoring the size of the nanochannels. Here, a high-performance GO-based nanofiltration membrane was developed by intercalating an aromatic crosslinker, α,α/-dichloro-p-xylene (DCX), between the layers in laminated GO nanosheets. Owing to the formation of strong covalent bonds between the crosslinker and the GO, the resulting GO laminate membrane exhibited outstanding structural stability. Furthermore, due to the precisely controlled and enlarged interlayer spacing distance of the developed DCX-intercalated GO membrane, it achieved an over two-fold enhancement in water permeability (11 ± 2 LMH bar−1) without sacrificing the rejection performance for divalent ions, contrary to the case with a pristine GO membrane.

Chemical and mechanical reprocessed resins and bio-composites based on five epoxidized vegetable oils thermosets reinforced with flax fibers or PLA woven

Bio-epoxy resins based on five epoxidized vegetable oils were formulated with an aromatic disulfide crosslinker with diacid functionality, 2,2′-dithiodibenzoic acid (DTBA), to obtain recyclable epoxy thermosets. Flax fibres (FF) and PLA woven were used as bio-based reinforcements for these matrices. Different percentages of reinforcement were tested and the effect of the natural fibres on the matrix's crosslinking reaction was studied by DSC analysis. Then, the obtained materials were analyzed by Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), water absorption and Scanning Electron Microscopy (SEM). The reinforcements improved the thermal and mechanical properties of the neat resins, with tan δ values varying from 91 to 148 °C showing a good compatibility matrix-FF but a reduced one in case of matrix-PLA woven. The dynamic nature of the networks crosslinking allowed the chemical and mechanical recycling of both resins and bio-composites. Moreover, the obtained results show the possibility to recuperate the natural FF filler for the preparation of second-life generation bio-composites.

Iron-Montmorillonite-Cyclodextrin Composites as Recyclable Sorbent Catalysts for the Adsorption and Surface Oxidation of Organic Pollutants.

Iron-clay-cyclodextrin composites were designed as sorbent catalysts to adsorb and oxidize pollutants from water. The clay-iron backbone served as a mechanical support and as a heterogeneous Fenton catalyst, and the cyclodextrin monomers or polymers cross-linked with polyfluorinated aromatic molecules were used to accommodate adsorption of the pollutants. The composite based on iron-clay-cyclodextrin-polymers (Fe-MMT-βCD-DFB) exhibited superior adsorption and degradation of the model pollutants, bisphenol A (BPA), carbamazepine (CBZ), and perfluorooctanoic acid (PFOA), compared to the monomer-based composite and the native iron clay. The variety of adsorption sites, such as the polyfluorinated aromatic cross-linker, cyclodextrin toroid, and iron-clay surface, resulted in high adsorption affinity toward all pollutants; BPA was primarily adsorbed to the cyclodextrin functional groups, CBZ showed high affinity toward the Fe-MMT surface and the Fe-MMT-βCD-DFB composite, whereas PFOA was adsorbed mainly to the βCD-DFB polymer. Degradation, using H2O2, was highly efficient, reaching over 90% degradation in 1 h for BPA and CBZ and ∼80% for PFOA. The composite also showed excellent degradation efficiency in a multicomponent system with all three model pollutants. Furthermore, the composite's activity remained steady for five consecutive cycles of adsorption and degradation. The ability to remediate a broad range of pollutants, and the high overall removal exhibited by this novel material, demonstrates the potential for future application in water remediation technologies.

Biocomposites from bamboo fibers and palm oil-based thermosets: Effects of natural phenolic cross-linkers

The development of highly biobased and robust thermosetting matrices using renewable feedstocks has become an interesting topic in the field of composites. A series of palm oil (PO)-based thermosets were prepared from a PO vinyl monomer and natural phenolic (NP) cross-linkers. The PO fatty acid-ethyl acrylamide (POFA-EA) was synthesized via a facile transesterification of PO with N-(2-hydroxyethyl)acrylamide (HEAA). NPs including eugenol, methyl gallate, and tannic acid were used to generate aromatic rings-containing cross-linkers via a solvent-free and ultrasonic-assisted esterification. The mechanical properties and processibility of the developed PO resins were closely associated with the chemical structures and unsaturation degrees of the incorporated NP cross-linkers. Besides, the thermosets were further used as the matrices for bamboo fibers-reinforced biocomposites. The resulting biocomposites showed comparable mechanical properties and Tgs toward those of petroleum-based counterparts. The biocomposites are anticipated to be fully decomposed because the PO matrices could be degraded in a mild alkali condition. This study provides insights to develop high performance and recyclable PO-based thermosets and their biocomposites.

High-Throughput Synthesis and Characterization of Aryl Silicones by Using the Piers-Rubinsztajn Reaction.

Arylsilicones are widely exploited for their thermal and optical properties. The creation of phenylsilicone elastomers with specific physical properties is typically done by a "one-off" formulation and test process. Herein, it is demonstrated that high-throughput synthesis methods can be used to rapidly prepare a series of arylsilicone elastomers and then the relative impact of different aryl groups on their physical properties is assessed. Aromatic groups were incorporated into polydimethylsiloxane (PDMS) elastomers by exploiting the relative reactivity of different functional groups in the Piers-Rubinsztajn reaction. To analyze trends in the silicone mechanical properties as a function of increasing aryl concentration-structure/property relationships-libraries of elastomers were both quickly synthesized and characterized by using high-throughput suites starting from low viscosity silicone oils/monomers in 96-well plates. Liquid handling parameters were optimized to effectively work with the silicones. Incorporating aryl instead of alkyl crosslinkers into the PDMS backbone increased the silicone elastomer modulus by approximately 50 % (at a crosslink density of 6 %); elastomers prepared with an aromatic crosslinker with three contact points led to much higher moduli compared with those with one contact point at the same crosslink density. When located at precise rather than random points on the silicone chains, diphenylsilicones had lower moduli than analogous monophenylsilicones.

Improving mass spectrometry analysis of protein structures with arginine-selective chemical cross-linkers

Chemical cross-linking of proteins coupled with mass spectrometry analysis (CXMS) is widely used to study protein-protein interactions (PPI), protein structures, and even protein dynamics. However, structural information provided by CXMS is still limited, partly because most CXMS experiments use lysine-lysine (K-K) cross-linkers. Although superb in selectivity and reactivity, they are ineffective for lysine deficient regions. Herein, we develop aromatic glyoxal cross-linkers (ArGOs) for arginine-arginine (R-R) cross-linking and the lysine-arginine (K-R) cross-linker KArGO. The R-R or K-R cross-links generated by ArGO or KArGO fit well with protein crystal structures and provide information not attainable by K-K cross-links. KArGO, in particular, is highly valuable for CXMS, with robust performance on a variety of samples including a kinase and two multi-protein complexes. In the case of the CNGP complex, KArGO cross-links covered as much of the PPI interface as R-R and K-K cross-links combined and improved the accuracy of Rosetta docking substantially.

β-Cyclodextrin Polymers on Microcrystalline Cellulose as a Granular Media for Organic Micropollutant Removal from Water.

Organic contaminants at low concentrations, known as micropollutants, are a growing threat to water resources. Implementing novel adsorbents capable of removing micropollutants during packed-bed adsorption is desirable for rapid water purification and other efficient separations. We previously developed porous polymers based on cyclodextrins that demonstrated rapid uptake and high affinity for dozens of micropollutants (MPs) in batch experiments. However, these polymers are typically produced as powders with irregular particle size distributions in the range of tens of micrometers. In this powdered form, cyclodextrin polymers cannot be implemented in packed-bed adsorption processes because the variable particle sizes yield insufficient porosity packing and consequently generate high back-pressure. Here we demonstrate a facile approach to remove micropollutants from water in a continuous manner by polymerizing cyclodextrin polymer networks onto cellulose microcrystals to provide a core/shell structure. Batch adsorption experiments demonstrate rapid pollutant uptake and high accessibility of the cyclodextrins on the adsorbent. Similarly, column experiments demonstrate rapid uptake of a model pollutant with minimal back-pressure, demonstrating potential for use in packed-bed adsorption processes. Furthermore, the pollutant-saturated columns were regenerated using methanol and reused three times with almost no change in performance. Column experiments conducted with a mixture of 15 micropollutants at environmentally relevant concentrations demonstrated that removal was determined by the affinity of each micropollutant for cyclodextrin polymers. The cyclodextrin polymer grafted onto cellulose microcrystals is more resistant to both anaerobic and aerobic biodegradation as compared to cyclodextrins and unmodified cellulose crystals, presumably due to the aromatic cross-linkers, demonstrating persistence. Collectively, the findings from this study demonstrate a general strategy to incorporate novel cyclodextrin adsorbents onto cellulose substrates to enable rapid and efficient removal of micropollutants during packed-bed adsorption as well as their promising long-term stability and regeneration capabilities.

An exploration of the Knoevenagel condensation to create ambient curable coating materials based on acetoacetylated castor oil

Using a Knoevenagel condensation cross-linking technique, we developed new ambient curable coatings based on acetoacetylated castor oil. The acetoacetylated castor oil was synthesized via the reaction of castor oil with t-butyl acetoacetate, and the films were cured at ambient temperatures (25°C) using aromatic dicarboxaldehydes as crosslinkers. In this article, we optimized the curing conditions of the coating and found that a ratio of acetoacetylated castor oil to dicarboxaldehyde groups of 1:0.75 and a catalyst loading of 7.5wt % provided an appropriate curing time and the coating with the best performance. Moreover, we prepared films with different aromatic dicarboxaldehyde cross-linkers and characterized the mechanical and thermal properties of these films.

Charge Matters: Modulating Secondary Interactions in Hyaluronan Hydrogels

Hyaluronic acid (HA) is a vital, functional component of the extracellular matrix (ECM). Following a synthetic biology approach, we designed polyelectrolyte hydrogels composed of HA as a polymeric backbone interconnected by short covalent crosslinkers. Utilizing a thiol-Michael addition, defined network structures are created, which can be modulated by the aromatic core and charge of the crosslinkers. With increasing negative network charge, varied both through the charge of the crosslinker and the degree of functionalization of the HA chain, the Young's modulus of the hydrogels decreases linearly as swelling ratios increase. Interestingly, secondary interactions with aromatic crosslinkers are stronger for elastin inspired crosslinkers with a pyridinium core compared to analogous triazolium crosslinkers. Based on the defined covalent interconnectivity of these HA-hydrogels, we were able to demonstrate the specific electrostatic and aromatic interactions induced by different short crosslinkers.

Aromatic diselenide crosslinkers to enhance the reprocessability and self-healing of polyurethane thermosets

We explore the utilization of aromatic diselenides as new low temperature exchangeable monomers for their utilization in thermosets.

Pyrolytic conversion of organopolysiloxanes

This study extends previous work on in-situ generation of nanographene domains in silicon oxycarbide polymer-derived ceramic nanocomposites. The thermal conversion of cross-linked polyhydridomethylsiloxanes (PHMS) has been studied as a function of its initial organic structure and its content. Two cross-linking additives containing vinyl groups, tetramethyl-tetravinyl cyclotetrasiloxane (TMTVS) and divinylbenzene (DVB), have been reacted with the SiH groups of the PHMS via Pt catalyzed hydrosilylation. These polymers gave a high ceramic yield of 78–85wt% upon pyrolysis at 1000°C–1400°C in inert atmosphere. The use of the aromatic cross-linker (DVB) induces significantly higher carbon content in a controlled fashion into the derived ceramics compared with the TMTVS. The chemistry involved in the in-situ evolution of low-dimensional graphene architectures embedded into silicon oxycarbide network has been studied in detail using thermal gravimetric, mass spectroscopy analyses (TG-MS) and Raman spectroscopy. The presence of aromatic functional groups (DVB) at the polymeric stage leads to graphene generation at lower temperatures than in the case of TMTVS cross-linker. Raman spectroscopy analysis of DVB cross-linked PHMS showed evidence of the beginning of formation of free sp2 carbon domains from 800°C. Raman mapping allows obtaining the chemical and the structural distribution of the resulting phases.

Influence of the chemical structure of cross-linking agents on properties of thermally reversible networks

Abstract It is well-known that the properties of cross-linked rubbers are strongly affected by the cross-link density. In this work it is shown that for thermoreversibly cross-linked elastomers, the type and length of the cross-linker also have a significant effect. A homologous series of diamine and bismaleimide cross-linkers was used to cross-link maleic-anhydride-grafted EPM irreversibly and furan-modified EPM thermoreversibly, respectively. Bismaleimide cross-linkers with a polarity close to that of EPM and a relatively low melting point have a better solubility in the rubber matrix, which results in higher chemical conversion and, thus, higher cross-link densities at the same molar amount of cross-linker. Samples cross-linked with different spacers (aromatic and aliphatic spacers of different lengths) were compared at the same cross-link density to interpret the effects on the material properties. The rigid character of the short aliphatic and the aromatic cross-linkers accounts for the observed increase in hardness, Young´s modulus and tensile strength with respect to the longer, more flexible aliphatic cross-linkers. In conclusion, the structure of the cross-linking agent can be considered as an alternative variable in tuning the rubber properties, especially for thermoreversibly cross-linked rubber.

Factors influencing the preparation of hollow polymer-graphene oxide microcapsules via Pickering miniemulsion polymerization

The synthesis of hollow, cross-linked polymer particles (‘capsules’) via Pickering miniemulsion polymerization using graphene oxide (GO) nanosheets as sole surfactant is reported. The influence of monomer, cross-linker and initiator type was studied, in addition to hydrophobe loading and initiator concentration. The desired hollow capsule morphology was shown to be strongly dependent on the choice of cross-linker; an aromatic crosslinker (divinylbenzene) consistently yielded hollow structures as determined by transmission electron microscopy, whereas ethylene glycol dimethacrylate typically resulted in polymer particles with a solid core. The use of an aromatic monomer with high propagation rate coefficient (benzyl methacrylate) and a strongly oil-soluble initiator, lauroyl peroxide, resulted in capsule synthesis with very high conversion (>85%) after 6 h. Surface area and pore analysis of the capsules established that while the capsules possessed a hollow interior, the shell was essentially non-porous. The potential of these materials towards novel nanocarbon-based materials was demonstrated via the preservation of colloidal stability and particle morphology after chemical reduction of GO, in addition to successful encapsulation of hydrophobic nanoparticles within the capsule core.

Thin-film composite crosslinked polythiosemicarbazide membranes for organic solvent nanofiltration (OSN)

In this work we report a new class of solvent stable thin-film composite (TFC) membrane fabricated on crosslinked polythiosemicarbazide (PTSC) as substrate that exhibits superior stability compared with other solvent stable polymeric membranes reported up to now. Integrally skinned asymmetric PTSC membranes were prepared by the phase inversion process and crosslinked with an aromatic bifunctional crosslinker to improve the solvent stability. TFC membranes were obtained via interfacial polymerization using trimesoyl chloride (TMC) and diaminopiperazine (DAP) monomers. The membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and contact angle measurement.The membranes exhibited high fluxes toward solvents like tetrahydrofuran (THF), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) ranging around 20L/m2h at 5bar with a molecular weight cut off (MWCO) of around 1000g/mol. The PTSC-based thin-film composite membranes are very stable toward polar aprotic solvents and they have potential applications in the petrochemical and pharmaceutical industry.