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2019 Volume 50 Issue 5
2019, 50(5):
Abstract:
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Study of Disulfide-exchange Dynamic Cross-linking Mechanism for Controlled Construction of Hydrogels
2019, 50(5): 429-441
doi: 10.11777/j.issn1000-3304.2019.18263
Abstract:
Hydrogels, as a kind of three-dimensional (3D) network of polymer chains constructed by physical or chemical crosslinkers, possess significant potentials in wound healing, drug delivery, and tissue engineering. In recent years, stimuli-responsive hydrogels have received increasing interests on account of their fundamentally architectural features and controllably functional performances under various external stimuli, such as pH, temperature, electricity, redox, and light. Wherein, various hydrogels embedded with dynamic covalent crosslinking networks have been widely developed for high-performance functional materials since dynamic covalent bond could break and reform reversibly under suitable conditions, combining the reversibility of supramolecular non-covalent bond and the robustness of covalent bond. Herein, we demonstrate a facile and universal approach to create "living" controlled in situ gelling systems based on a thiol-disulfide exchange reaction. Thiol-disulfide exchange reaction is reversibly activated or terminated by deprotonating free thiols or protonating thiolates under different pH conditions with an "on/off" function, resulting in dissociation of shells and cross-linking of cores, and thus dynamically optimizing hydrogel structures: from solution to loose and compact hydrogels in macroscopic dimensions. This "living" controlled in situ gelation process can be optionally activated, controllably terminated and interrupted, and reinitiated by external stimuli whenever needed. Associated with an inverse emulsion technique, the controlled cross-linking strategy can be utilized to produce micro/nanoscale hydrogels in a confined space with flexible architectures and designable performances. Under this circumstance, multilayered hydrogel particles with each tailor-made layer are also prepared using the controlled in situ gelation method in association with a seed emulsion technique. By tailoring thiol-disulfide exchange reaction rate in a dilute aqueous solution, a dynamic and programmable morphology and size evolution is well-performed via a hierarchical self-assembly strategy, providing unique advantages to fabricate intelligent drug carriers with high loading efficiency. Furthermore, UV-triggered thiol-disulfide exchange reaction has been developed to prepare the hydrogels with a radical-centered disulfide exchange mechanism, opening up another cross-linking strategy for precise spatiotemporal control on a photochemical gelation process by varying irradiation time. Since these hydrogels are formed through disulfide shuffling of the cores that can be easily cleaved in response to glutathione, these tailor-made hydrogels are biocompatible, biodegradable, and easily fabricated with desired shapes, sizes, and properties in controllable drug delivery systems. In this contribution, we summarize and review this disulfide-exchange-based cross-linking strategy on acquisition of smart hydrogels with adjustable structures and fine-tunable properties in widely biomedical applications.
Hydrogels, as a kind of three-dimensional (3D) network of polymer chains constructed by physical or chemical crosslinkers, possess significant potentials in wound healing, drug delivery, and tissue engineering. In recent years, stimuli-responsive hydrogels have received increasing interests on account of their fundamentally architectural features and controllably functional performances under various external stimuli, such as pH, temperature, electricity, redox, and light. Wherein, various hydrogels embedded with dynamic covalent crosslinking networks have been widely developed for high-performance functional materials since dynamic covalent bond could break and reform reversibly under suitable conditions, combining the reversibility of supramolecular non-covalent bond and the robustness of covalent bond. Herein, we demonstrate a facile and universal approach to create "living" controlled in situ gelling systems based on a thiol-disulfide exchange reaction. Thiol-disulfide exchange reaction is reversibly activated or terminated by deprotonating free thiols or protonating thiolates under different pH conditions with an "on/off" function, resulting in dissociation of shells and cross-linking of cores, and thus dynamically optimizing hydrogel structures: from solution to loose and compact hydrogels in macroscopic dimensions. This "living" controlled in situ gelation process can be optionally activated, controllably terminated and interrupted, and reinitiated by external stimuli whenever needed. Associated with an inverse emulsion technique, the controlled cross-linking strategy can be utilized to produce micro/nanoscale hydrogels in a confined space with flexible architectures and designable performances. Under this circumstance, multilayered hydrogel particles with each tailor-made layer are also prepared using the controlled in situ gelation method in association with a seed emulsion technique. By tailoring thiol-disulfide exchange reaction rate in a dilute aqueous solution, a dynamic and programmable morphology and size evolution is well-performed via a hierarchical self-assembly strategy, providing unique advantages to fabricate intelligent drug carriers with high loading efficiency. Furthermore, UV-triggered thiol-disulfide exchange reaction has been developed to prepare the hydrogels with a radical-centered disulfide exchange mechanism, opening up another cross-linking strategy for precise spatiotemporal control on a photochemical gelation process by varying irradiation time. Since these hydrogels are formed through disulfide shuffling of the cores that can be easily cleaved in response to glutathione, these tailor-made hydrogels are biocompatible, biodegradable, and easily fabricated with desired shapes, sizes, and properties in controllable drug delivery systems. In this contribution, we summarize and review this disulfide-exchange-based cross-linking strategy on acquisition of smart hydrogels with adjustable structures and fine-tunable properties in widely biomedical applications.
2019, 50(5): 485-495
doi: 10.11777/j.issn1000-3304.2019.18280
Abstract:
An emerging and crucial type of high-value-added functional materials, conductive elastomer composites (CEC) have found extensive applications in the fields of military and civil electromagnetic shielding/protection by virtue of their excellent electromagnetic shielding function and environmental sealing performance. However, practical uses of CEC materials can be largely compromised by such disadvantages as difficult rubber recovery, poor interfacial adhesion, and costly conductive fillers. In this study, methyl vinyl silicone rubber (SiR) with high vinyl content (20%, 30% and 50%) was firstly synthesized through an anionic ring-opening reaction, and as-prepared SiR was grafted with furan functional groups via the thiol-ene click chemical reaction to afford furan-grafted SiR (SiR-Fu). SiR-Fu/CNTs composites were then prepared by solution blending of SiR-Fu and carbon nanotubes (CNTs), during which Diels-Alder reaction occurred with SiR-Fu as the diene and CNTs as the dienophiles, giving rise to reversible covalent cross-linking networks throughout the resulting composites. SEM images showed that diameters of most CNTs in SiR-Fu/5wt% CNTs and SiR-Fu/10wt% CNTs composites were significantly larger than those of the raw CNTs due to a SiR layer coated on the nanotube surface, indication of the DA reaction between CNTs and SiR-Fu. However, CNTs tended to agglomerate when being further increased to 20 wt% and some of them showed little change in diameter as compared with the initial values, so that no DA reaction took place in that case. In addition, the gel content of SiR-Fu/5wt% CNTs and SiR-Fu/10wt% CNTs composites was 73% and 90%, respectively, suggesting an enhanced degree of DA reaction at increasing CNTs content within a certain range, while it decreased to 24.5% at 20 wt% CNTs addition for the reduced degree of DA reaction caused by CNTs agglomeration. Therefore, composites with 5 wt% and 10 wt% CNTs showed better interfacial adhesion, higher mechanical strength, greater electrical conductivity, and favorable thermal reversibility. Particularly, the electrical conductivity and tensile strength of SiR-Fu/10wt% CNTs composite reached 0.9 S/cm and 2.3 MPa, respectively, much improved than those of the neat SiR (2.5 × 10−14 S/cm and 0.2 MPa). Moreover, the initial tensile strength, elongation at break, and electrical conductivity could be retained at 77%, 88%, and 86%, respectively, after composite reprocessing.
An emerging and crucial type of high-value-added functional materials, conductive elastomer composites (CEC) have found extensive applications in the fields of military and civil electromagnetic shielding/protection by virtue of their excellent electromagnetic shielding function and environmental sealing performance. However, practical uses of CEC materials can be largely compromised by such disadvantages as difficult rubber recovery, poor interfacial adhesion, and costly conductive fillers. In this study, methyl vinyl silicone rubber (SiR) with high vinyl content (20%, 30% and 50%) was firstly synthesized through an anionic ring-opening reaction, and as-prepared SiR was grafted with furan functional groups via the thiol-ene click chemical reaction to afford furan-grafted SiR (SiR-Fu). SiR-Fu/CNTs composites were then prepared by solution blending of SiR-Fu and carbon nanotubes (CNTs), during which Diels-Alder reaction occurred with SiR-Fu as the diene and CNTs as the dienophiles, giving rise to reversible covalent cross-linking networks throughout the resulting composites. SEM images showed that diameters of most CNTs in SiR-Fu/5wt% CNTs and SiR-Fu/10wt% CNTs composites were significantly larger than those of the raw CNTs due to a SiR layer coated on the nanotube surface, indication of the DA reaction between CNTs and SiR-Fu. However, CNTs tended to agglomerate when being further increased to 20 wt% and some of them showed little change in diameter as compared with the initial values, so that no DA reaction took place in that case. In addition, the gel content of SiR-Fu/5wt% CNTs and SiR-Fu/10wt% CNTs composites was 73% and 90%, respectively, suggesting an enhanced degree of DA reaction at increasing CNTs content within a certain range, while it decreased to 24.5% at 20 wt% CNTs addition for the reduced degree of DA reaction caused by CNTs agglomeration. Therefore, composites with 5 wt% and 10 wt% CNTs showed better interfacial adhesion, higher mechanical strength, greater electrical conductivity, and favorable thermal reversibility. Particularly, the electrical conductivity and tensile strength of SiR-Fu/10wt% CNTs composite reached 0.9 S/cm and 2.3 MPa, respectively, much improved than those of the neat SiR (2.5 × 10−14 S/cm and 0.2 MPa). Moreover, the initial tensile strength, elongation at break, and electrical conductivity could be retained at 77%, 88%, and 86%, respectively, after composite reprocessing.
2019, 50(5): 496-504
doi: 10.11777/j.issn1000-3304.2019.18281
Abstract:
The reversible mechanical deformations of smart hydrogel actuators, such as swelling/shrinking and bending, under various external stimuli have earned them mounting attention in the application arenas of biomimetic actuators, soft robots, etc. Hydrogel actuators were initailly designed with isotropic structures for a simple swelling/shrinking triggered by external stimuli, while the research progress afterwards focuses more on the design of anisotropic structures that aims at complex shape deformation. However, the determined structure of traditional anisotropic hydrogel actuators typically led to fixed shape deformation direction and degree, which limited them from meeting the actual needs. To this end, we got inspired by the assembly of building blocks and integrated boronic acid groups into the hydrogel bulks. Poly(vinyl alcohol) (PVA) promoted the binding process of newly introduced groups by forming PBA-diol ester bonds with them under alkaline conditions, which was further confirmed by microscopic infrared spectroscopy. The dynamic covalent bonds between two hydrogel sheets were so strong that they were adhered firmly with each other without breaking during the tensile test. Then, two kinds of cationic monomers, methacryloxyethyltrimethyl ammonium chloride (METAC) and N-isopropyl acrylamide (NIPAM), were introduced into the hydrogel system, respectively, to afford two types of stimuli-responsive hydrogels, and the smart hydrogel actuators that dually responded to temeperature and ionic strength were successfully fabricated by the sheet combination via PBA-diol ester bonds. Both 2D and 3D architectures could be achieved at elaborate selection of bonding positions. For instance, bonding of a 2D octopus-shaped hydrogel to another planar hydrogel could transfrom the 2D structure into a 3D type along with the swelling of octopus-shaped hydrogel. Finally, integration of METAC and NIPAM into one system could afford a soft gripper with tunable grasping force and dual responsiveness to ion strength and temperature. Our research has provided a new perspective for the design and fabrication of novel hydrogel actuators with complex deformations.
The reversible mechanical deformations of smart hydrogel actuators, such as swelling/shrinking and bending, under various external stimuli have earned them mounting attention in the application arenas of biomimetic actuators, soft robots, etc. Hydrogel actuators were initailly designed with isotropic structures for a simple swelling/shrinking triggered by external stimuli, while the research progress afterwards focuses more on the design of anisotropic structures that aims at complex shape deformation. However, the determined structure of traditional anisotropic hydrogel actuators typically led to fixed shape deformation direction and degree, which limited them from meeting the actual needs. To this end, we got inspired by the assembly of building blocks and integrated boronic acid groups into the hydrogel bulks. Poly(vinyl alcohol) (PVA) promoted the binding process of newly introduced groups by forming PBA-diol ester bonds with them under alkaline conditions, which was further confirmed by microscopic infrared spectroscopy. The dynamic covalent bonds between two hydrogel sheets were so strong that they were adhered firmly with each other without breaking during the tensile test. Then, two kinds of cationic monomers, methacryloxyethyltrimethyl ammonium chloride (METAC) and N-isopropyl acrylamide (NIPAM), were introduced into the hydrogel system, respectively, to afford two types of stimuli-responsive hydrogels, and the smart hydrogel actuators that dually responded to temeperature and ionic strength were successfully fabricated by the sheet combination via PBA-diol ester bonds. Both 2D and 3D architectures could be achieved at elaborate selection of bonding positions. For instance, bonding of a 2D octopus-shaped hydrogel to another planar hydrogel could transfrom the 2D structure into a 3D type along with the swelling of octopus-shaped hydrogel. Finally, integration of METAC and NIPAM into one system could afford a soft gripper with tunable grasping force and dual responsiveness to ion strength and temperature. Our research has provided a new perspective for the design and fabrication of novel hydrogel actuators with complex deformations.
2019, 50(5): 505-515
doi: 10.11777/j.issn1000-3304.2019.19015
Abstract:
Injectable self-healing hydrogels are fancy candidates for biomedical applications, especially in such areas as minimally invasive surgical procedures, interventional therapy, and 3D bio-printing. Herein, a general and robust synthetic route to injectable self-healing hydrogels was developed based on a facile three-component reaction between the primary amine groups in hyperbranched poly(ethylenimine) (PEI), 2-formylphenylboronic acid (2-FPBA), and the cis-diols in sodium alginate (SA). Briefly, 2-FPBA reacted with PEI at first to generate a PEI/2-FPBA conjugate through forming iminoboronate bonds. The residual boronic acid groups in PEI/2-FPBA conjugate further reacted with cis-diols in the sugar unite of SA to generate iminoboronate ester linkages, thereby yielding the target product of hydrogels. The formation of iminoboronate and boronic acid ester bonds in iminoboronate ester linkages was confirmed by 1H- and 11B-NMR spectra. Dynamic rheological measurements revealed that the storage modulus (G′) of hydrogels was dependent on the feeding molar ratios of primary amine groups in PEI, 2-FPBA, and sugar units in SA. Moreover, the resulting hydrogels exhibited excellent self-healing and shear-thinning properties, given that both iminoboronate and boronic acid ester bonds are well known as dynamic covalent bonds. Based on these attributes, the hydrogels prepared were expected to have successful application in 3D printing by serving as a hydrogel " ink”. In addition, their responsiveness towards pH, H2O2, cysteine (Cys), glutathione (GSH), and fructose allowed an accelerated degradation process in acidic medium or in the presence of H2O2, Cys, GSH, or fructose; Scanning electron microscopy (SEM) observation further suggested a significant destruction of their porous structure after a period of degradation. As a result, these hydrogels proved quite applicable for the delivery of protein therapeutics with multi-responsive drug release properties. Their minimal cytotoxicity towards A549, HeLa, and L929 cells was also confirmed by the MTT assay. It is worth mentioning that with 2-FPBA functioning as the cross-linker, many other amine groups-rich polymers, even natural proteins, can be used to fabricate dynamic hydrogels with injectable, seal-healing, and multi-responsive properties. Therefore, hydrogels prepared from the strategy proposed in this study may hold tremendous potentials in tissue engineering, drug delivery, and 3D bio-printing.
Injectable self-healing hydrogels are fancy candidates for biomedical applications, especially in such areas as minimally invasive surgical procedures, interventional therapy, and 3D bio-printing. Herein, a general and robust synthetic route to injectable self-healing hydrogels was developed based on a facile three-component reaction between the primary amine groups in hyperbranched poly(ethylenimine) (PEI), 2-formylphenylboronic acid (2-FPBA), and the cis-diols in sodium alginate (SA). Briefly, 2-FPBA reacted with PEI at first to generate a PEI/2-FPBA conjugate through forming iminoboronate bonds. The residual boronic acid groups in PEI/2-FPBA conjugate further reacted with cis-diols in the sugar unite of SA to generate iminoboronate ester linkages, thereby yielding the target product of hydrogels. The formation of iminoboronate and boronic acid ester bonds in iminoboronate ester linkages was confirmed by 1H- and 11B-NMR spectra. Dynamic rheological measurements revealed that the storage modulus (G′) of hydrogels was dependent on the feeding molar ratios of primary amine groups in PEI, 2-FPBA, and sugar units in SA. Moreover, the resulting hydrogels exhibited excellent self-healing and shear-thinning properties, given that both iminoboronate and boronic acid ester bonds are well known as dynamic covalent bonds. Based on these attributes, the hydrogels prepared were expected to have successful application in 3D printing by serving as a hydrogel " ink”. In addition, their responsiveness towards pH, H2O2, cysteine (Cys), glutathione (GSH), and fructose allowed an accelerated degradation process in acidic medium or in the presence of H2O2, Cys, GSH, or fructose; Scanning electron microscopy (SEM) observation further suggested a significant destruction of their porous structure after a period of degradation. As a result, these hydrogels proved quite applicable for the delivery of protein therapeutics with multi-responsive drug release properties. Their minimal cytotoxicity towards A549, HeLa, and L929 cells was also confirmed by the MTT assay. It is worth mentioning that with 2-FPBA functioning as the cross-linker, many other amine groups-rich polymers, even natural proteins, can be used to fabricate dynamic hydrogels with injectable, seal-healing, and multi-responsive properties. Therefore, hydrogels prepared from the strategy proposed in this study may hold tremendous potentials in tissue engineering, drug delivery, and 3D bio-printing.
2019, 50(5): 516-526
doi: 10.11777/j.issn1000-3304.2019.19017
Abstract:
Boronic acid can form dynamic covalent boronic ester bonds with 1,2-diol or 1,3-diol moieties. This property has been used to prepare malleable and self-healing covalent polymer networks as well as glucose-responsive hydrogels. It remains a challenge to fabricate strong hydrogels with fast glucose-responsivity. Glucose-responsive dynamic covalent hydrogels were prepared by crosslinking poly(vinyl alcohol) (PVA) containing triblock copolymer and poly(ethylene oxide) (PEO) containing phenylboric acid. Specifically, we synthesized a new type of α,ω-phenylboronic acid substituted PEO crosslinker. The feature of the crosslinker is that amino-groups are introduced to the neighboring position of boronic acid, which will accelerate the boronic ester exchange. The gelation of this crosslinker with three PVA-b-PEO-b-PVA triblock copolymers at physiological pH was examined. Time-dependent dynamic storage and loss modulus of the hydrogels were monitored by rheological measurements. Formation of hydrogels turned out be very fast after mixing the crosslinker and triblock copolymer solutions. Stable dynamic covalent hydrogels were obtained after incubating the hydrogels for 12 h. The copolymer composition, PVA chain length, and content of crosslinker were found to greatly affect the hydrogel properties. Higher polymer concentration, longer PVA chains, and more PEO crosslinker endowed hydrogels with higher modulus. Gels with high yield stress were obtained by utilizing block copolymer with longer PVA segments or adding more crosslinker. Both pH and temperature affected the hydrogel properties. The formed hydrogels displayed higher modulus at pH = 7.4 than those at pH = 6.0, and the stability of the hydrogel could still be maintained at 37 °C. In addition, the hydrogels exhibited good structural recovery ability due to the covalent dynamic crosslinking. Finally, the hydrogels could load FITC-BSA, and the release profile of FITC-BSA was accelerated in the presence of glucose.
Boronic acid can form dynamic covalent boronic ester bonds with 1,2-diol or 1,3-diol moieties. This property has been used to prepare malleable and self-healing covalent polymer networks as well as glucose-responsive hydrogels. It remains a challenge to fabricate strong hydrogels with fast glucose-responsivity. Glucose-responsive dynamic covalent hydrogels were prepared by crosslinking poly(vinyl alcohol) (PVA) containing triblock copolymer and poly(ethylene oxide) (PEO) containing phenylboric acid. Specifically, we synthesized a new type of α,ω-phenylboronic acid substituted PEO crosslinker. The feature of the crosslinker is that amino-groups are introduced to the neighboring position of boronic acid, which will accelerate the boronic ester exchange. The gelation of this crosslinker with three PVA-b-PEO-b-PVA triblock copolymers at physiological pH was examined. Time-dependent dynamic storage and loss modulus of the hydrogels were monitored by rheological measurements. Formation of hydrogels turned out be very fast after mixing the crosslinker and triblock copolymer solutions. Stable dynamic covalent hydrogels were obtained after incubating the hydrogels for 12 h. The copolymer composition, PVA chain length, and content of crosslinker were found to greatly affect the hydrogel properties. Higher polymer concentration, longer PVA chains, and more PEO crosslinker endowed hydrogels with higher modulus. Gels with high yield stress were obtained by utilizing block copolymer with longer PVA segments or adding more crosslinker. Both pH and temperature affected the hydrogel properties. The formed hydrogels displayed higher modulus at pH = 7.4 than those at pH = 6.0, and the stability of the hydrogel could still be maintained at 37 °C. In addition, the hydrogels exhibited good structural recovery ability due to the covalent dynamic crosslinking. Finally, the hydrogels could load FITC-BSA, and the release profile of FITC-BSA was accelerated in the presence of glucose.
2019, 50(5): 527-534
doi: 10.11777/j.issn1000-3304.2019.19021
Abstract:
We synthesized a self-healing polyurethane elastomer (PU-SSDA) which displayed excellent healing efficiency at room temperature due to the synergistic contribution of dynamic covalent D-A bond and S–S bond. Mechanical and self-healing properties of the polyurethane elastomers were characterized by tensile test and three-dimensional super depth of field microscope. The cross-linking structure of PU-SSDA is explored by extraction experiments, demonstrating that there are not only crosslinked structures but also linear molecular segments existed in the polyurethane elastomer. Mechanical properties of the PU-SSDA elastomer can be tuned by changing the cross-linking degree. As the crosslinking degree increases from 25% to 45%, the tensile strength increases from 2.9 MPa to 5.5 MPa while the elongation at break decreases from 795% to 304%. After optimization, when the crosslinking degree was 35%, the tensile strength was 3.7 MPa, the elongation at break was 606% and the repair efficiency could be restored to 93% after healing for 60 min at room temperature. Moreover, the healing efficiency still remains above 90% after 4 damage-healing cycles. In addition, the PU-SSDA elastomer can also be reprocessed by hot pressing at 120 °C. This excellent self-healing behavior and reprocessable property were attributed to the reversible fracture recombination reaction of dynamic D-A and S―S bonds and the quick infiltration of the linear polymer chains into damaged surface. The self-healing mechanism can be further confirmed by the dissolution experiments which showed that the PU-SSDA elastomer can be dissolved in DMF at 100 °C while the PU-control can only swell under the same conditions, demonstrating that the reversible cleavage and reformation of D-A and S―S bonds contribute a lot to the self-healing process. Due to the facile and friendly preparation method, fast self-healing behavior at room temperature and fully reprocessable properties, the as-prepared polyurethane elastomers displayed wide potential applications such as protection coatings and wearable electronic devices.
We synthesized a self-healing polyurethane elastomer (PU-SSDA) which displayed excellent healing efficiency at room temperature due to the synergistic contribution of dynamic covalent D-A bond and S–S bond. Mechanical and self-healing properties of the polyurethane elastomers were characterized by tensile test and three-dimensional super depth of field microscope. The cross-linking structure of PU-SSDA is explored by extraction experiments, demonstrating that there are not only crosslinked structures but also linear molecular segments existed in the polyurethane elastomer. Mechanical properties of the PU-SSDA elastomer can be tuned by changing the cross-linking degree. As the crosslinking degree increases from 25% to 45%, the tensile strength increases from 2.9 MPa to 5.5 MPa while the elongation at break decreases from 795% to 304%. After optimization, when the crosslinking degree was 35%, the tensile strength was 3.7 MPa, the elongation at break was 606% and the repair efficiency could be restored to 93% after healing for 60 min at room temperature. Moreover, the healing efficiency still remains above 90% after 4 damage-healing cycles. In addition, the PU-SSDA elastomer can also be reprocessed by hot pressing at 120 °C. This excellent self-healing behavior and reprocessable property were attributed to the reversible fracture recombination reaction of dynamic D-A and S―S bonds and the quick infiltration of the linear polymer chains into damaged surface. The self-healing mechanism can be further confirmed by the dissolution experiments which showed that the PU-SSDA elastomer can be dissolved in DMF at 100 °C while the PU-control can only swell under the same conditions, demonstrating that the reversible cleavage and reformation of D-A and S―S bonds contribute a lot to the self-healing process. Due to the facile and friendly preparation method, fast self-healing behavior at room temperature and fully reprocessable properties, the as-prepared polyurethane elastomers displayed wide potential applications such as protection coatings and wearable electronic devices.
2019, 50(5): 535-542
doi: 10.11777/j.issn1000-3304.2019.19027
Abstract:
Unlike conventional thermoset epoxy resins, epoxy vitrimers with excellent malleability can be recycled, remolded and reshaped. However, most epoxy vitrimers usually shows high fragility and low mechanical properties, which significantly limits their practical applications. To address this issue, we used a carboxyl terminated hyperbranched polymer, Hyper C102, to simultaneously toughen and reinforce a class of vitrimers based on glutaric acid crosslinked bisphenol F epoxy resin (BPF), in which 1-methylimidazole was used as catalyst to endow the system with dynamic exchange properties. Fourier transform (FTIR) and swelling experiments confirmed the formation of covalent crosslinking network in the epoxy vitrimers. DSC and DMA were used to study the dynamic mechanical properties and the rate of transesterification reaction of the materials. The result shows that the crosslink density of the epoxy vitrimers decreases first and then increases with the increasing content of Hyper C102. Such phenomenon can be well explained by the cavitation theory. More intriguingly, the Hyper C102 modified epoxy vitrimers still show high efficiency of transesterification reaction at 180 °C. Their modulus can relax to 1/e of the initial modulus within 30 min, and to 10% of the initial modulus within 1 h. Meanwhile, the tensile strength and strain at break can be simultaneously improved upon the introduction of Hyper C102. Compared with Hyper0 which contains no hyperbranched polymer, the tensile strength and fracture energy of Hyper7.5 that contains 7.5 wt% Hyper C102 is improved by 136% (from 28 MPa to 66 MPa) and 504% (from 280 kJ/m3 to 1410 kJ/m3), respectively. Such significant and simultaneous improvement in both tensile strength and toughness has not been realized in previous studies. Moreover, the epoxy vitrimers manifest decent self-repairing and recyclable properties after mechanical damage. These results fully demonstrate that the addition of the carboxyl terminated hyperbranched polymer can not only maintain the dynamic transesterification, but also significantly improve the mechanical properties of epoxy vitrimers.
Unlike conventional thermoset epoxy resins, epoxy vitrimers with excellent malleability can be recycled, remolded and reshaped. However, most epoxy vitrimers usually shows high fragility and low mechanical properties, which significantly limits their practical applications. To address this issue, we used a carboxyl terminated hyperbranched polymer, Hyper C102, to simultaneously toughen and reinforce a class of vitrimers based on glutaric acid crosslinked bisphenol F epoxy resin (BPF), in which 1-methylimidazole was used as catalyst to endow the system with dynamic exchange properties. Fourier transform (FTIR) and swelling experiments confirmed the formation of covalent crosslinking network in the epoxy vitrimers. DSC and DMA were used to study the dynamic mechanical properties and the rate of transesterification reaction of the materials. The result shows that the crosslink density of the epoxy vitrimers decreases first and then increases with the increasing content of Hyper C102. Such phenomenon can be well explained by the cavitation theory. More intriguingly, the Hyper C102 modified epoxy vitrimers still show high efficiency of transesterification reaction at 180 °C. Their modulus can relax to 1/e of the initial modulus within 30 min, and to 10% of the initial modulus within 1 h. Meanwhile, the tensile strength and strain at break can be simultaneously improved upon the introduction of Hyper C102. Compared with Hyper0 which contains no hyperbranched polymer, the tensile strength and fracture energy of Hyper7.5 that contains 7.5 wt% Hyper C102 is improved by 136% (from 28 MPa to 66 MPa) and 504% (from 280 kJ/m3 to 1410 kJ/m3), respectively. Such significant and simultaneous improvement in both tensile strength and toughness has not been realized in previous studies. Moreover, the epoxy vitrimers manifest decent self-repairing and recyclable properties after mechanical damage. These results fully demonstrate that the addition of the carboxyl terminated hyperbranched polymer can not only maintain the dynamic transesterification, but also significantly improve the mechanical properties of epoxy vitrimers.
