Physical Cross-linking Network Research Articles

Tough and body-temperature self-healing polysiloxane elastomers through building a double physical crosslinking network via competing non-covalent interactions

Polydimethylsiloxane (PDMS)-based elastomers with superior mechanical and body-temperature self-healing properties might find attractive applications in wearable electronic devices.

Fabrication of microcellular epoxidized natural rubber foam with superior ductility by designable chemical and physical crosslinking networks

Though the introduction of microcells improves the impact strength of polymer, it weakens the tensile properties since microcells act as defects, leading to stress concentration and crack initiation during stretching. Reducing the cell size to micro/nano-scale is a facile strategy to eliminate the stress concentration. Herein, we propose supercritical CO2 assisted post-vulcanization approach to fabricate microcellular epoxidized natural rubber-SiO2 foam (f-ENR-SiO2) with high ductility. The designable chemical and physical crosslinking networks facilitated by vulcanization and SiO2 increase the complex viscosity and storage modulus of ENR, while did not induce second nucleation. These effects reduced the cell size to 1 µm and endowed f-ENR-SiO2 with 600% elongation at break and high cell structure stability (the cell structure stays intact without deformation after 250 stretching cycles), far superior to those of the reported microcellular foams. Overall, this study adopted superimposed crosslinking networks to fabricate flexible microcellular rubber foam with excellent ductility.

Vinyl‐functionalized polyborosiloxane for improving mechanical and flame‐retardancy performances of silicone rubber foam composites

AbstractIn this work, we first synthesized vinyl‐terminated polyborosiloxane (pBS) filler, and then incorporated it into silicone rubber foam (SRF) matrix to prepare composite materials through a simple chemical dehydrogenative foaming method under ambient conditions. The pBS filler with reactive groups could form physical and chemical crosslinking networks in SRF, leading to an excellent dispersion level of pBS in the SRF matrix. Moreover, the inserted pBS could remarkably improve the mechanical and flame‐retardancy properties of SRF‐pBS composites. Intriguingly, the SRF‐pBS6 foam containing 6 wt% of pBS possessed a uniform porous structure and balanced mechanical properties (σb = 65.4 kPa, εb = 56.7%, compression stress 97.1 kPa), thermal conductivity (0.102 W (m K)−1), limiting oxygen index (29.8%) and UL‐94 rating (V‐0) among the prepared foams. In addition, the incorporated pBS (6 wt%) could synergistically catalyze the formation of a silicon–boron strengthened ceramic‐like protective layer under fire and was capable of suppressing heat and smoke production in the SRF‐pBS6 foam. The present work provides a promising way for developing high mechanical and flame‐retardant polymeric foam materials with good thermal insulation. © 2021 Society of Industrial Chemistry.

Enhancement of EPDM Crosslinked Elastic Properties by Association of Both Covalent and Ionic Networks.

The objective of this study was to replace elastomer crosslinking based on chemical covalent bonds by reversible systems under processing. One way is based on ionic bonds creation, which allows a physical crosslinking while keeping the process reversibility. However, due to the weak elasticity recovery of such a physical network after a long period of compression, the combination of both physical and chemical networks was studied. In that frame, an ethylene-propylene-diene terpolymer grafted with maleic anhydride (EPDM-g-MA) was crosslinked with metal salts and/or dicumyl peroxide (DCP). Thus, the influence of these two types of crosslinking networks and their combination were studied in detail in terms of compression set. The second part of this work was focused on the influence of different metallic salts (KOH, ZnAc2) and the sensitivity to the water of the physical crosslinking network. Finally, the combination of ionic and covalent network allowed combining the processability and better mechanical properties in terms of recovery elasticity. KAc proved to be the best ionic candidate to avoid water degradation of the ionic network and then to preserve the elasticity recovery properties under aging.

Highly stretchable porous composite hydrogels with stable conductivity for strain sensing

Stretchable hydrogels have undergone a rapid development in recent years particularly for wearable strain sensors. However, most of hydrogel-based sensors exhibit poor mechanical properties. Here, we report a highly stretchable porous composite hydrogel that has a tensile strain of 400% and a compressive strain of 80%. The porous hydrogel is composed of polyvinyl alcohol (PVA) and copolymer of N-(3-sulfopropyl)-N-methacroyloxyethyl-N,N-dimethylammonium betaine (SBMA) and acrylamide (AM) prepared via graphene oxide (GO)-stabilized Pickering aqueous foam templates. The pore morphology of the composite hydrogels was observed by field emission scanning electron microscope (FE-SEM). The tensile and compressive curves of porous hydrogels with different GO concentrations were characterized. GO can act as Pickering stabilizer to produce aqueous foams and also construct the physical cross-linking network in porous hydrogels to improve their mechanical strength. In addition, when GO is reduced by L-ascorbic acid, it provides effective electric pathways in porous hydrogels with the conductive percolation of less than 0.40 wt%. The composite hydrogels thus exhibit excellent strain sensitivity and can be used to detect various human motions. The facile preparation process of the porous composite hydrogels with outstanding performance properties facilitates the development of flexible wearable strain sensing materials.

Robust Conductive Hydrogels with Ultrafast Self-Recovery and Nearly Zero Response Hysteresis for Epidermal Sensors.

Robust conductive hydrogels are in great demand for the practical applications of smart soft robots, epidermal electronics, and human–machine interactions. We successfully prepared nanoparticles enhanced polyacrylamide/hydroxypropyl guar gum/acryloyl-grafted chitosan quaternary ammonium salt/calcium ions/SiO2 nanoparticles (PHC/Ca2+/SiO2 NPs) conductive hydrogels. Owing to the stable chemical and physical hybrid crosslinking networks and reversible non-covalent interactions, the PHC/Ca2+/SiO2 NPs conductive hydrogel showed good conductivity (~3.39 S/m), excellent toughness (6.71 MJ/m3), high stretchability (2256%), fast self-recovery (80% within 10 s, and 100% within 30 s), and good fatigue resistance. The maximum gauge factor as high as 66.99 was obtained, with a wide detectable strain range (from 0.25% to 500% strain), the fast response (25.00 ms) and recovery time (86.12 ms), excellent negligible response hysteresis, and good response stability. The applications of monitoring the human’s body movements were demonstrated, such as wrist bending and pulse tracking.

High-performance green electronic substrate employing flexible and transparent cellulose films

Today's widely used and rapidly updated electronic substrates are composed of petroleum-based polymers, but the resulting electronic waste (such as Dioxin, oxole, PCBs, etc.) will cause massive harm to the environment and human body. Therefore, we report an effective approach for fabricating recyclable and high-performance cellulose films as green electronic substrates by calendering. The crosslinking between CH and CHCH in cellulose modified by maleic anhydride led to the in-situ formation of a chemical crosslinking network, and hydrogen bonds acted as a sacrificial physical crosslinking network. The dual crosslinked cellulose film exhibits high strength (120.56 MPa), improved elongation (increased by 263%), and outstanding thermal stability (thermal decomposition temperature is 311 °C). Further, the film has been successfully used as a substrate for biomass sensor and realized apparent responses to changes. The scientific strategy paves the way for the large-scale fabrication of high-performance cellulose films and simultaneously promotes green electronic substrates' industrialization.

Fabrication of porous tubular carbon fibers from the fruits of Platanus orientalis and their high oil adsorption properties

Faced with the stark challenge of global oil spills, development of clean absorbent materials is crucial. In this paper, the plant fibers extracted from the waste Platanus orientalis are transformed by vacuum carbonization method, and the fibers with micron-size cavities and porous pipe walls are obtained simply and cheaply. The fibers exhibit high oil adsorption capacity. The effects of carbonization conditions, adsorption temperature and time on adsorption capacity are systematically investigated. Specific surface area of F900–10 obtained by vacuum carbonization at 900 °C for 10 h is 702.44 m2/g. It has the best adsorption performance and can absorb 71.72–172.37 times of its own weight. Meanwhile, oil can be removed quickly by simple extrusion, and the oil removal rate reaches 63.18–95.67%. The adsorption sites of the tubular carbon fibers to the oil include micron-scale cavities, the physical cross-linking network that overlaps between the fibers, and the nano-scale pores on the tube wall. Further, the practicability of F900–10 is tested by simulating oil leakage in diverse environments, and its removal efficiency ranged from high to low in seawater, glass, soil and sand. High adsorption capacity and significant oil removal rate make F900–10 a promising oil adsorption material with high potential value.

The preparation and characterization of biodegradable PCL/PLA shape memory blends

The biodegradable thermoresponsive shape memory blends with polycaprolactone (PCL) as reversible phase and polylactide (PLA) as fixed phase were prepared by the melt blending of PCL and PLA. The scanning electron microscopy (SEM) shows that the PCL/PLA blends are partially miscible blends. According to XRD analysis, a diffraction peak appears at 16.7° (200/110), it may be the crystal diffraction peak of PLA. The results of differential scanning calorimetry (DSC) shows that the blends have two melting temperature (around 56 °C and 155 °C). According to thermal and shear stability analysis, as the addition of PLA, the balance torque increases from 0.7 to 1.5, it indicates that the macromolecular chains are entangled with each other. Thus, the physical crosslinking network is formed by the entanglement of the microcrystals and macromolecules. Furthermore, we used the melting point of PCL as the switching temperature (T s) of the blends for the shape memory test. The results show that the PLA has a positive effect on shape memory property and the recovery rate first increases and then decreases. It is observed in thermogravimetric analysis (TGA) that PLA has little effect on the high temperature stability of material.

Study on waterborne self-healing polyurethane with dual dynamic units of quadruple hydrogen bonding and disulfide bonds

The novel self-healable waterborne polyurethanes (WPU) with the synergistic reversible units of aliphatic disulfide bonds and quadruple hydrogen bonding have been successfully prepared. The ureidopyrimidone (UPy) functional motifs were introduced into polymer backbones of polyurethane, where also placed the reversible disulfide bonds. The quadruple hydrogen bonds of UPy groups endowed the material a strong physical cross-linking network, which provided a superior Young's moduli. The features of dual dynamic structure could also produce a relatively low active energy of 36.1 kJ/mol, resulting in a good self-repairing efficiency of 94% under a mild condition. Furthermore, the influence of self-healing time and different repairing temperature on the healed performance was also systematically investigated. In order to describe the self-healing progress of multiple dynamic units, a physical model was used to study the evolution of self-repairing efficiency with variable healing time, indicating that quadruple H-bonding and disulfide bonds predominantly supported time dependent reconstruction of physically crosslinking network to recover the original mechanical property.

A novel, facile and straightforward approach to achieve high-performance and efficient utilization of sustainable tyrosine cyclic peptide

The preparation of high-performance thermosets from sustainable raw resources is a vital research topic in current society faced with the pressure of resource shortage and environmental deterioration. In this work, the cyclic peptide dominated hydrogen bonding physical cross-linking network (HB–PCN) and promoted nitrile reaction produced chemical cross-linking network (NC–CCN) were utilized to achieve the target. A novel bio-based thermoset was prepared by co-polymerization of 70% of tyrosine cyclic peptide (TCP) and 30% of 1,3-bis(3,4-dicyanophenoxy) (3BOCN). Thermal analysis was applied to determine the unique curing behavior of this system. The cured resin exhibits excellent thermal properties and relatively high glass transition temperature (Tg), which embodies the characteristics of typical high-performance polymers and reaches a trade-off between biomass content and performance. The curing reaction pathway of this system was reasonably proposed by Fourier-Transform Infrared Spectroscopy (FT-IR) analysis. Moreover, scanning electron microscope (SEM) confirmed that TCP-3BOCN blend could form porous material via ambient-molding and hot-sintering process. Ultraviolet–visible (UV–Vis) spectrometer was employed to quantitatively describe the adsorption capability of the porous materials to methyl violet.

Toughening of Polylactide with High Tensile Strength via Constructing an Integrative Physical Crosslinking Network Based on Ionic Interactions

For polylactide (PLA) toughening modification by rubbery materials, the toughness generally improved at the expense of lowered mechanical strength even after interfacial compatibilization. In this ...

Synthesis and properties of poly(ethylene terephthalate) modified with a small amount of 1,10-decanediamine and hydrogen bonds

Linear 1,10-decanediamine was chosen as a modified monomer to prepare modified poly(ethylene terephthalate) (PET) with a low content of amide bonds through two different synthetic methods. To evaluate the effect of amide bonds on the properties of poly(ester amide) analogs (PETAs), the performance of this new type of PET was compared with 1,10-decanediol-modified PET as a control group. The formation of hydrogen bonds in the PETAs was confirmed, and the stability of hydrogen-bonding interactions based on the low content of amide bonds at high temperature was discussed. Interestingly, physical cross-linking networks formed by hydrogen-bonding interactions improved the thermal stability, rheology, and mechanical properties of the PETAs. The crystallization properties were studied in depth by a series of tests, the results of which illustrated that the low content of amide bonds acted as a critical crystallization accelerator in the PETAs. A feasible strategy to prepare high-performance PET, which may be used as a packaging material, was introduced in this work.

Formation of Stereocomplex Crystal and Its Effect on the Morphology and Property of PDLA/PLLA Blends.

Stereocomplex-polylactic acid (SC-PLA) is obtained in poly(d-lactic) acid/poly(l-lactic) acid (PDLA/PLLA) blends under adjusting processing conditions. It is found that the degree of crystallinity of overall SC-PLA is up to 43.7% in PDLA/PLLA blends of 1:1 mass ratio. Formation of stereocomplex (SC) crystals forces molecular chains in the blends to be more closely arranged and further enhances interaction between molecular chains, thus forming a physical cross-linking network in the SC crystals, resulting in the blends having a special microstructure. The mechanism of formation of the SC crystal physical cross-linking network is elucidated by dielectric spectroscopy, and the relationships between homocomplex (HC) crystals, SC crystals, and amorphous regions in the blends are also analyzed. Interestingly, mechanical properties of the blends are significantly improved due to formation of an SC crystal cross-linking network.

Highly compressible and durable superhydrophobic cellulose aerogels for oil/water emulsion separation with high flux

Nowadays high porosity aerogels are widely applied in wastewater purification, while the fragility and unversatility of nascent cellulose aerogels limited the application in oil/water filtering separation. Here, highly compressible, elastic, superhydrophobic cellulose/poly(vinyl alcohol) (PVA) composite aerogels were fabricated via combination of chemical cross-linking, freeze drying and silanization. The obtained cellulose/PVA aerogels exhibited durable superhydrophobicity with WCA of 156.6 °, even maintaining 155.0 ° after 30 days. Synergistic effects of physical entanglement, chemical cross-linking and H-bond network between cellulose and PVA molecular chains endowed the aerogels with excellent compressibility and recoverability. The aerogels exhibited maximum compressive stress of 490.7 kPa at 90% strain, even without reduction in mechanical strength under 70% compressive strain for 10 cycles. More importantly, the strengthened aerogels exhibited remarkable separation performances for both immiscible oil/water mixtures and surfactant-stabilized water-in-oil emulsions. Fascinating permeation flux of up to 7176.3 L m−2 h−1 and total separation amount as high as 4550.6 L m−2 with separation efficiency of 98.5% for water-in-cyclohexane emulsion were harvested. Thus, this work provides a green, sustainable and mass-producible cellulose-based aerogel for high-efficiency oily wastewater remediation.

New methods for flame-retarding PET without melt dripping

Polyethylene terephthalate (PET), as the largest synthetic polymer with an annual output of more than 90 million tons, has found widespread applications in our daily life and industry, especially its fiber accounts for about 90% of all synthetic fibers. However, PET is highly flammable and melt-dripping during burning, which causes serious casualties and property loss in case of fire. Although many phosphorus-based flame-retardant PETs are already commercially available, their flame retardancy is usually achieved by promoting the melt dripping of PET. The issue on flame retardation of PET without melt dripping was a big challenge both in the academia and industry, and has not been addressed for a long time because the anti-dripping behavior conflicts with the “dripping-promotion” flame-retardant mechanism of phosphorus-containing PET. Therefore, how to flame-retard PET without melt dripping has become an urgent difficult problem needed to solve. The key to designing flame-retardant PET with anti-dripping performance is to increase both the melt viscosity and char formation ability during burning. Herein, we reviewed our original works about a series of flame-retardant and anti-dripping PET copolyesters designed and synthesized via chemically incorporated functional monomers into the PET chains based on “physical interaction”, “high-temperature chemical self-crosslinking” and “high-temperature rearrangement”. Those monomers exhibit high polymerization activity with PET monomers, and those special functional groups do not affect the polymerization or processing process. Once at a higher temperature or during burning, under the actions of “ionic aggregation”, “hydrogen bond”, “chemical self-crosslinking” and “high-temperature rearrangement”, the melt viscosity and melt strength of those PET-based copolyesters would be greatly increased, upgrading excellent anti- or non-dripping performance. What is more, the dense cyclic aromatic structures, enabled by these functional groups, can further evolve into stable intumescent char layers on the surface of copolyesters, and inhibit the heat or oxygen transfer. Thus, the copolyesters exhibit good flame retardancy at the same time. In detail, the physical crosslinking networks, formed by thermally-reversible ionic aggregation, can limit the movement of molecular chains to some extent, but such effects are not enough to make corresponding copolyesters achieve non-dripping performance during the vertical burning test (UL-94). Compared with physical crosslinking, irreversible chemical crosslinking networks are more stable and stronger. Based on this idea, we have proposed a series of novel “intelligent chemical self-crosslinking” copolyesters containing crosslinkable phenylacetylene, phenylacetylene-phenylimide, phenylmaleimide, and aromatic schiff base or azobenzene groups. Those copolyesters can rapidly crosslink during burning or at the temperature higher than polymerization/processing to form stable aromatic cross-linked networks, resulting in excellent flame retardancy and non-dripping properties. In addition, the special aryl ether, bisphenol F and o-hydroxy phenylimide groups can also be used to improve the flame retardancy and anti-dripping of copolyesters via the rearrangement reactions and charring promotion at high temperatures. Among all the above, the self-crosslinkable or rearrangeable PET copolyesters exhibited higher flame-retardant efficiency, which can attain the non-dripping standard at a lower amount of third monomers. Without the use of traditional flame retardant elements such as halogen, these novel mechanisms and approaches have not only opened up a way of high temperature carbonization for the flame-retardant and anti-dripping polyester, but also provided new strategies for the green development of flame-retardant technology. Although the understanding of the design and synthesis of flame-retardant and anti-dripping PET-based copolyesters has been deepened with some breakthroughs in recent years, we still have faced the unavoidable challenges of reducing monomer contents, decreasing cost, optimizing mechanical properties, improving crystallinity and keeping spinnability in the process of practical application and industrialization of these copolyesters in the future. Actually, we have designed some more efficient and cheaper self-crosslinkable monomers recently. The much lower content of monomers can endow corresponding copolyesters with excellent comprehensive performances such as flame retardance, non-dripping, crystallization, spinnability, dyeability and mechanical properties. These state-of-art results will be presented in the near future.

Multifunctional Poly(vinyl alcohol) Nanocomposite Organohydrogel for Flexible Strain and Temperature Sensor.

Hydrogels are important for stretchable and wearable multifunctional sensors, but their application is limited by their low mechanical strength and poor long-term stability. Herein, a conductive organohydrogel with a 3D honeycomb structure was prepared by integrating carbon nanotubes (CNTs) and carbon black (CB) into a poly(vinyl alcohol)/glycerol (PVA/Gly) organohydrogel. Such a nanocomposite organohydrogel is built on a physical cross-linking network formed by the hydrogen bonds among PVA, glycerol, and water. CNTs and CB had an add-in synergistic impact on the mechanical and electrical performances of the PVA/Gly organohydrogel because of the distinct aspect ratios and geometric shapes. The prepared organohydrogel integrated with a tensile strength of 4.8 MPa, a toughness of 15.93 MJ m-3, and flexibility with an elongation at break up to 640%. The organohydrogels also showed good antifreezing feature, long-term moisture retention, self-healing, and thermoplasticity. Sensors designed from these organohydrogels displayed high stretching sensitivity to tensile strain and temperature, with a gauge factor of 2.1 within a relatively broad strain range (up to ∼600% strain), a temperature coefficient of resistance of -0.935%·°C-1, and long-term durability. The sensors could detect full-range human physiological signals and respond to the change in temperature, which are highly desired for multifunctional wearable electronic devices.

Near-infrared triggered on-demand local anesthesia using a jammed microgels system

To conveniently modulate the degree of local analgesia in response to changes in patients’ needs and level of activity, a NIR-activated drug delivery system based on jammed microgels was introduced in the present study to realize on-demand local anesthesia. Chemically cross-linked gelatin microgels (5–15 μm) containing N-isopropylacrylamide (NIPAM), methylallyl polyethylene glycol (APEG) and graphene oxide (GOs) were fabricated through emulsion. After the in situ free radical polymerization, the physical network was formed, producing microgels with double networks (DN microgels). The DN microgels exhibited thermosensitive properties. The copolymerization of APEG resulted in the increase of lower critical solution temperature (LCST) of microgels. The maximum volume shrinkage ratio of DN microgels (NIPAM40 + APEG60) increased with the increase of the content of physical cross-linking network. The DN microgels also exhibited NIR-responsive ability. Under the NIR irradiance of 272 mW/cm2, the temperature of DN microgels with 3 mg/mL GOs reached 40 °C within 60 s, resulting in the volume shrinkage of 14%. Ropivacaine release from DN microgels could be effectively triggered by NIR irradiation in vitro. After centrifugation, a jammed microgels system was produced where microgels packed densely, displaying shear-thinning behavior for achieving injection. The jammed DN microgels carrying ropivacaine were injected subcutaneously into rat footpad. NIR irradiation produced on-demand and repeated infiltration anesthesia in the rat footpad. The jammed DN microgels system thus was beneficial in the management of pain.

Co-precipitated poly(vinyl alcohol)/chitosan composites with excellent mechanical properties and tunable water-induced shape memory

The preparation of polymeric materials with high performance and functional performance remains a focus of extensive research. In this paper, a good dispersion of regenerated chitosan (RCS) in the poly(vinyl alcohol) (PVA) and a physical cross-linking network of molecular chain entanglements and hydrogen bonding were realized by unique co-precipitation to prepare composites. Dynamic mechanical analysis and tensile tests demonstrated that RCS achieved a significant increase in the mechanical properties. The storage modulus and tensile strength of composites containing 5% RCS were 5205 MPa and 105.7 MPa, which were significantly higher than the 3416 MPa and 60.2 MPa of the pure PVA. Intriguingly, the fabricated PVA/RCS composites exhibited good pH-sensitivity due to internal hydrogen bonding. Moreover, this achieved a tunable water-induced shape memory behavior at different pH environments. This study provides a promising strategy for the use of renewable polysaccharide to achieve high performance and functionality of polymer materials.

Tough, high-strength PDAAM-co-PAAM hydrogels synthesized without a crosslinking agent

Hydrogels find a variety of uses across various fields, but their development might be limited by their high-cost, toxic chemical crosslinking and complicated reactions. To address these issues, we developed a tough, high-strength hydrogel of polydiacetone acrylamide-co-poly(acrylamide), prepared using acrylamide, diacetone acrylamide (DAAM) and ammonium persulfate in a one-step reaction. The multiple physical crosslinking networks endow hydrogel with excellent overall properties given appropriate DAAM levels. The tensile modulus, fractured strain, fractured stress and toughness of the developed D1A9 hydrogel are, respectively, as high as 0.15 MPa, 21 mm/mm, 0.71 MPa and 7 MJ/m2. Its compressive modulus under a strain of 80% is 0.088 MPa, while its shear modulus at a shear frequency of 100 Hz is 0.071 MPa. At the same time, D1A9 hydrogel exhibits a high self-recovery efficiency of 50% during two continuous cyclic tensile tests, with the efficiency increasing to 65% for hydrogel incubated at 50 °C for 2 h. Finally, cytocompatibility and excellent drug-releasing behavior of the hydrogel make it a candidate for biomedical use.