Reprocessable and Recyclable Crosslinked Polyethylene with Triple Shape Memory Effect

Thermosetting polymers, such as epoxy resins, have found widespread applications because of their excellent thermomechanical and solvent resistance properties. However, the recycling of thermosets remains a challenge, due to the irreversible covalent crosslinking bonds in the network. The exchangeable covalent bonds in vitrimers open opportunities in the way that materials can be dynamically repaired. However, in the case of polyolefins, the strategy of exchangeable vitrimers is still underexplored. Herein, a simple way is reported to prepare a high‐density polyethylene (HDPE) vitrimer with a triple shape memory effect, where exchangeable esters are introduced into the network of covalently crosslinked HDPE. With the incorporation of polyethers or polyesters into the crosslinked network, the two obtained vitrimers show outstanding recyclability and a good triple shape memory effect. This new strategy of preparing an HDPE vitrimer might allow the high‐value utilization of crosslinked polyolefins.     

Carbon fiber/epoxy vitrimer composite patch cured with bio-based curing agents for one-step repair metallic sheet and its recyclability

Vitrimers have gained a great deal of attention from researchers, yet research on its application is still lacking. This study, a novel bio-based vitrimer was developed from epoxy (EP) and bio-based curing agents, that is, cashew nut shell liquid (CNSL) and citric acid (CA), and then reinforced by carbon fiber. The vitrimers with different ratios of acid to epoxy (R ratio) at 0.30–0.40 contained ester and ether linkages. All EP/CA/CNSL vitrimers showed the stress relaxation over 70–100°C due to transesterification. The vitrimers were applied as polymer matrices for the carbon fiber composites and then used as repair patches. By using the carbon fiber-reinforced vitrimer with the R ratio of 0.30, patch repair on a damaged alloy sheet revealed that approximately 98% of the tensile strength of the damaged alloy sheet was recovered. The vitrimer can be dissolved from carbon fiber composite to recover carbon fiber. The recovered carbon fiber retained good tensile strength compared to the pristine composite. Based on this study, the EP/CA/CNSL vitrimers showed the comparable thermomechanical properties with the epoxy vitrimer cured by the petroleum-based curing agent. The vitrimer composite patch could therefore be an alternative new repair method to extend the service life of damaged structures.     

Dynamic bond exchangeable thermoset vitrimers in 3D-printing

Dynamic bond exchanging vitrimers have emerged recently due to their malleability, self-heal ability, recyclability, and mechanical stability. Likewise, 3D printing is consciously introduced at different platforms for ease of fabrication, high throughput, cost-effectiveness, and waste reduction. These two distinctive techniques have recently made their consensus performance, resulting from a phenomenal change in the printing field. Conventionally, thermoplastic inks have been primarily used in 3D printing, owing to their effortless processability. At the same time, thermosets were utilized for their superior mechanical strength. However, these two essential properties have been required to be presented in the printed material. In that scenario, thermoset vitrimer materials have been introduced in 3D printing, where malleability and mechanical stability have been observed in the same material. Thus, this article details the recent vitrimer material included with the different 3D printing system systems with their reported results to understand and make them widespread. Eventually, the outlook and perspectives could be helpful to understand and enhance this specific field.     

生物基vitrimer材料研究进展

介绍了类玻璃高分子（vitrimer）材料的发展史、拓扑冻结转变温度,对基于酯交换、烷基转移和转氨基化的vitrimer动态机理和基于木质素、植物油、纤维素和香草醛的生物基vitrimer的研究进行了概述,并对生物基vitrimer材料进行了展望。     

Shape memory epoxy vitrimers based on waste frying sunflower oil

Thermoset polyesters are prepared from epoxidized waste frying sunflower oil (ESO), commercially available epoxy compounds and glutaric acid. Influence of the nature and concentration of bi- and trifunctionnal epoxy compounds on mechanical properties is studied. Static and dynamic mechanical tests are performed. The molar amount of commercial epoxy compounds used ranges from 20% to 80% regarding the molar amount of ESO. It enables to obtain thermosetting polyesters with glass transition temperatures ranging from 6°C to 102°C, as well as storage modulus ranging from 8 GPa to 14 GPa. 40% of trifunctional epoxy compound and 60% of commercial epoxy compound are found to be the best compromises between bio-based content and mechanical properties. Furthermore, shape memory and vitrimer behavior of those epoxy-acid based networks with 60% of commercial epoxy compounds are evaluated both qualitatively and quantitatively by cyclic thermo-mechanical and stress relaxation tests. Excellent shape memory behavior with fixity ratios above 94% and recovery ratios above 98% is demonstrated. A transesterification catalyst is needed to obtain good vitrimer behavior. Overall, thanks to the previously mentioned properties of those partially biobased thermosets polyesters, industrial applications such as protective coatings, foams and temperature-memory polymer actuators might be considered.     

High-Performance Vitrimeric Benzoxazines for Sustainable Advanced Materials: Design,Synthesis, and Applications

Polybenzoxazines are high-performance materials capable of replacing conventional thermosets such as phenolics, epoxies, and bismaleimides in composites manufacturing due to their excellent thermomechanical and chemical behavior. Their versatility and compatibility with biobased precursors make them an attractive option as composite matrices. Like other thermosets, polybenzoxazines are not recyclable and cannot be reprocessed. Incorporating dynamic bonds in benzoxazine monomers can produce vitrimeric polybenzoxazines, which can potentially overcome this limitation and can be tuned to exhibit smart functionalities such as self-healing and shape memory. Dynamic bond exchange mechanisms for vitrimer development such as transesterification, imine bond, disulfide exchange, transamination, transcarbamoylation, transalkylation, olefin metathesis, transcarbonation, siloxane-silanol exchange, boronic ester, silyl ether exchange, and dioxaborolane metathesis are potentially applicable to benzoxazine chemistry, with disulfide bond and transesterification having successfully vitrimerized benzoxazines with topological transitions at −8.5 and 88 °C, respectively. Benzoxazine vitrimers featuring glass transitions of 193, 224, and 222–236 °C are now known. These place polybenzoxazines at the forefront of the development of reprocessable and recyclable thermosetting polymers and composite matrices.     

Room temperature-responsive poly(vinyl hydroxyalkyl ether-alt-dialkyl maleate)-based vitrimers with dynamic boronic ester bonds

Dynamic B O bonds are often applied to fabricate reversible-cross-linking and reprocessable polymer networks due to their high thermodynamic stability and kinetic tunability. However, it is still difficult to tailor boronic ester polymers, which have both excellent mechanical and self-healing properties in the absence of external stimuli. To address the above challenge, a range of room-temperature self-healable vitrimers are prepared by the reactions of various poly(vinyl hydroxyalkyl ether-alt-dialkyl maleate) copolymers and 1,4-phenyldiboronic acid. The dynamic cross-linking networks offer the as-prepared vitrimers superior mechanical properties and thermostability. Because of the presence of dynamic boronic ester bonds, the networks also demonstrated self-healing (the tensile strength recovered to 83% after 3 days at room temperature) and reprocessing capabilities (no significant change in tensile strength after thrice process recycles). Moreover, the vitrimers have potential as promising damping materials with wide temperature windows around room temperature. For example, the vitrimer of poly(ethylene glycol monovinyl ether-alt-dibutyl maleate) and 1,4-phenyldiboronic acid demonstrates its tanδ_max 1.18, T_g = 6.2°C and effective damping temperature range is from −16.4 to 48.5°C.     

Novel amphiphilic block copolymer modifiers based on chain‐extended polyester for improved toughness of epoxy resins

Novel amphiphilic block copolymer modifiers based on chain‐extended polyester for improved toughness of epoxy resins were synthesized by attaching polyols having different structures and compatibilities with a dianhydride chain‐extender. Used polyols in this research were polytetrahydrofuran as miscible and hydroxyl‐terminated polybutadiene as immiscible segment. Generated carboxyl groups will be prepolymerized with an excess of epoxy resin to exclusively form epoxy groups between the polyol spacer. Resulting morphologies in the prepared diglycidyl ether of bisphenol‐A‐based epoxy thermosets cured with dicyandiamide and urea were strongly dependent on the initial parameters of the block copolymerization process. Changing the dianhydride concentration as well as relation between miscible and immiscible polyol spacer showed various particle sizes including unimodal and bimodal distributions. The best mechanical performance in terms of fracture toughness (K_1C) could be achieved with bimodal particle size distributions indicating synergistic effects between the different particle sizes in the range of 0.1–7 μm. POLYM. ENG. SCI., 59:E216–E223, 2019. © 2018 Society of Plastics Engineers     

Poly(methyl methacrylate)–epoxy vitrimer composites

In this study, we synthesized poly(methyl methacrylate) (PMMA) epoxy vitrimer composites by doping methyl methacrylate (MMA) and benzoyl peroxide into a curing system of epoxy resin and citric acid. The vitrimer composites were characterized with dynamic mechanical thermal analysis, scanning electron microscopy, and stress‐relaxation and lap‐shear testing. The test results show that with increasing amount of MMA, the existence of PMMA in the epoxy vitrimer matrix in the form of intermiscible, slightly soluble, and phase separation became more evident. When the doping amount of PMMA reached 10–25 wt %, the bonding strength of the PMMA–epoxy vitrimer composites was about two times that of the epoxy vitrimer (from 2.3 to 4.3 MPa). This showed that the self‐healing strength of the vitrimer composites was better than that of the pure vitrimer. When the PMMA in the epoxy matrix was in a slightly soluble form, the linear PMMA improved the mechanical properties of the epoxy vitrimer by physical winding. At the same time, the doping of PMMA promoted the transesterification rate of the epoxy vitrimer and enhanced the bonding strength of the composites without lowering the epoxy vitrimer glass‐transition temperature. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46307.     

New Epoxy-Anhydride Thermosets Modified with Multiarm Stars with Hyperbranched Polyester Cores and Poly(ϵ-caprolactone) Arms

Multiarm star polymers with hyperbranched aromatic or aromatic-aliphatic cores and poly(?-caprolactone) arms have been used as toughness modifiers in epoxy-anhydride formulations catalyzed by benzyldimethylamine. The curing process has been studied by dynamic scanning calorimetry, demonstrating little influence of the mobility of the reactive species and the hydroxyl content on the kinetics of this process. The obtained materials were characterized by thermal and mechanical tests and the microstructure by electron microscopy. Homogeneous thermosets have been obtained with a remarkable increase in impact strength without compromising glass transition temperature, thermal stability or hardness.     

Enhanced mechanical properties of epoxy composites using cellulose micro- and nano-crystals

Epoxy polymers are commonly utilized in structural applications due to their high bearing capacity and excellent chemical resistance. However, their inherent brittleness poses a significant challenge for their use in high shock and fracture strength products. To address this shortcoming, fillers can be incorporated into the polymer during preparation. In this study, we aimed to investigate the effect of incorporating cellulose-based fillers, namely cellulose nanocrystals (CNCs) and microcrystalline cellulose (MCC), on the mechanical properties of epoxy polymer composites. The study evaluated the impact of various factors, including filler concentration, particle size, and moisture content, on the mechanical properties of the composites. The results demonstrated that the incorporation of CNC or MCC powders at concentrations below 5% could enhance the mechanical properties of the resulting epoxy composites without adversely affecting their surface and thermal properties. The maximum tensile strength and fracture toughness of the filler-based epoxy composites were achieved at 2 and 4 wt% for CNCs and MCC, respectively. CNCs with a smaller particle size distribution were found to be much more effective than MCC in improving the mechanical properties of the epoxy composites. Furthermore, utilizing dried fillers resulted in a higher improvement in tensile strength, which was achieved at lower filler concentrations.     

“In Water” Fabricated and Recycled Covalent Adaptable Acylhydrazone Thermosets

Thermosetting resins play an increasingly important role in daily life due to their good mechanical properties. However, they can hardly be recycled and reused, leading to severe environmental pollution. A very promising solution to this dilemma is to develop sustainable thermosets from biomass that can be recycled and reprocessed on demand under environmentally friendly conditions. In this study, sustainable thermosets based on dynamic acylhydrazone bonds made from biomass acid derivatives is reported. The sustainability of the presented acylhydrazone biomass covalent adaptable networks (CANs) thermosetting resins is summarized as follows: 1) The raw materials are renewable resources; 2) the resins can be recycled to maintain their mechanical properties, which can considerably extend their service life; and 3) the material preparation and recycling can be conducted in the mixed solvent of water/ dimethyl sulfoxide (7/3), which excellent reduced the use of organic solvents. In addition, these acylhydrazone CANs thermosets have excellent mechanical performance and outstanding heat resistance. The acylhydrazone bonds in the thermoset networks can enhance the hydrophobicity and water resistance of the acylhydrazone CANs thermosets. Taken together, these acylhydrazone CANs thermosets which are fabricated and recycled in the mixed solvent provide a solution to developing sustainable materials.     

Reuse of cured epoxy as a reinforcement in an epoxy composite

This article discusses the reuse of a thermoset‐based epoxy polymer. In this method, cured epoxy polymer is ground to powder of particle size ranging from 1 to 30 μm. The ground epoxy is then filled in an epoxy polymer to form an epoxy–epoxy composite system using both room and high temperature processing. The amount of filler material was varied from 1 to 10 wt% in the epoxy matrix. Rheology and tensile properties test were then performed. The result shows that the room temperature‐processed epoxy composites (above 5 wt% of powders) resulted in the formation of voids, agglomeration of particles, and reduced degree of cure leading to a decrease in tensile properties. These drawbacks (voids, agglomeration, and low degree of cure) were correspondingly absent in composites processed at high temperature. Results from this work suggest that the thermoset polymer can be reused effectively with minimal changes to the unfilled resin properties. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Epoxy Doped,Nano-scale Phase-separated Poly-Acrylates with Potential in 3D Printing

An efficient method to improve the mechanical performance of a commercially available photocure resin is described wherein the resin is modified with a mixture of a cycloaliphatic epoxy and an anhydride curing agent. Photocured samples are thermally treated in a subsequent step to cure the epoxy to obtain an interpenetrated polymer network (IPN) and also complete reaction of the acrylate monomers remaining from the photocure. The latter is accomplished by a thermal radical initiator added earlier into the formulation together with the epoxy-anhydride. The thermal properties and microstructure of the resulting IPN are analyzed. Uniform and quantitative conversions are obtained, with glass transition temperatures comparable to conventional epoxies. The liquid, uncured samples containing different amounts of epoxy are stable at 30 °C for several weeks. In the fully cured epoxy-rich materials, nano-scale phase separation is observed by atomic force microscopy. This is corroborated by the existence of multiple relaxations determined by dynamic mechanical analysis analysis. Specimens from a formulation containing 50% by weight of epoxy-anhydride are 3D printed in a customized masked image processing stereolithography, thermally treated, and are subjected to compression tests. Results show that Young's modulus increases by 900% over the neat resin.     

Characterization of thermally reworkable thermosets: materials for environmentally friendly processing and reuse

In recent years, several research groups have created reworkable thermoset systems. A prominent use of such materials is in microelectronics packaging areas to enable the repair or reprocessing of electronic components. A wider implication of such an application is that it may facilitate the future recycling or reuse of older computer systems. Recent studies indicate millions of computers are discarded each year due to obsolescence or other factors. The research presented here involves studies of thermosets incorporating a cycloaliphatic epoxy monomer that contains a tertiary ester linkage. When part of a fully crosslinked network, the reworkable epoxy unit will disconnect the network under predetermined thermal conditions. We studied the chemical and thermo-mechanical breakdown mechanisms of the monomer and resulting polymer networks as a function of their rework conditions. Via analytical chemistry techniques, the materials were found to degrade in a controlled fashion consistent with prior polyester degradation studies. Monitoring the change in glass transition temperature of the materials under rework conditions yielded both kinetic and mechanistic data of the degradation process, as well as providing insight into the materials' mechanical strength.     

Up‐cycling end‐of‐use materials: Highly filled thermoplastic composites obtained by loading waste carbon fiber composite into fluidified recycled polystyrene

Carbon fibers reinforced epoxy resins are used in a wide range of applications, such as automotive and aerospace industry. Because of their thermosetting nature, recycling at the end of the life cycle is a difficult issue. However, lack of recyclability poses environmental concerns to the use of these composite materials. In this article, a sustainable, cost‐effective technological approach aiming at recycling postconsumer carbon fibers reinforced thermosets (CFRT) is proposed. Composites containing 50 and 70 wt% of CFRT particles were prepared by incorporating the filler fraction into a fluidified postconsumer expanded polystyrene matrix. A cold mixing approach consisting in the use of a low boiling solvent as a binder to guarantee the dispersion homogeneity on micro‐ and macroscopic level was set up. For comparison, analog composites were also prepared through melt mixing process. Morphological, mechanical, and thermal analyses allowed to prove the effectiveness of the cold mixing approach and to evaluate the influence of particle size on the performance of new recycled composites. Thermogravimetric analysis and thermal conductivity tests of samples highlighted further peculiarities of the cold mixing process. The approach proposed is an effective recycling technology for CFRT and could be extended to other postconsumer materials. POLYM. COMPOS., 35:1621–1628, 2014. © 2013 Society of Plastics Engineers     

An Effective Approach to Improve the Thermal Conductivity,Strength, and Stress Relaxation of Carbon Nanotubes/Epoxy Composites Based on Vitrimer Chemistry

An effective method was developed to improve the interfacial interaction between Mutiwalled carbon nanotubes (MWCNTs) and epoxy matrix. The performance of thermal conductivity and strength of the epoxy vitrimer were enhanced by polydopamine (PDA) coating. Polydopamine is a commonly used photothermal agent, which of course, was effective in modifying MWCNTs used in photoresponsive epoxy resin. The surface temperature of the epoxy composite with 3% MWCNTs@PDA fillers added increased from room temperature to 215 °C in 48 s. The metal–catechol coordination interactions formed between the catechol groups of PDA and Zn²⁺ accelerated the stress relaxation of epoxy vitrimer. Moreover, the shape memory, repairing, and recycling of epoxy vitrimer were investigated. Therefore, dopamine coating is a multifunctional approach to enhance the performance of epoxy vitrimer.     

Mechanical and thermal studies on epoxy toluene oligomer‐modified epoxy resin/marble waste composites

In this research, marble dust waste was recycled as raw material for the preparation of composite materials. Epoxy toluene oligomer (ETO) was synthesized from toluene and epichlorohydrin, which was used as a comatrix in 50 wt% with commercial epoxy resin (ER). Its chemical structure was characterized with Fourier transform infrared spectroscopy and chemical analyses. The rigid filler used in epoxy polymer matrix was the marble processing waste obtained from wastewater using different coagulants, such as sepiolite, zeolite, or pumice. The thermal and mechanical properties of the composites were evaluated with thermogravimetric and mechanical analyses. The results showed that the marble wastes with all coagulants can significantly improve the thermal stability of an ER–ETO matrix at temperatures above 350°C. Composites exhibited a higher thermal degradation temperature with a much higher char yield. Surface hardness and tensile strength of the composites were higher than those of pure epoxy polymer matrix, as well. Scanning electron microscopy was used for the characterization of surface and cross‐sections of the composites to verify the results. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Structure-property relationships of controlled epoxy networks with quantified levels of excess epoxy etherification

Epoxy thermosets are commonly formulated with an excess of epoxy resin to ensure complete reaction of co-reactive hardeners and to optimize performance. However, the degree to which the reaction of the excess epoxy resin contributes to the thermal and mechanical properties of the thermoset is incompletely understood. In this report, the preparation of controlled epoxy thermosets containing varying amounts of excess epoxy resin having essentially complete excess epoxy conversion is described. The extent of conversion was determined using a solid-state ¹³C NMR method with enhanced resolution due to solvent swelling of the thermosets. This etherification reaction increases the crosslink density of the epoxy thermosets but uniquely affects the thermal and mechanical properties of the materials. Significant property differences observed with respect to analogous thermosets made by varying crosslink density using different extender/hardener ratios are the sensitivity of T_g to the crosslink density and enhanced fracture toughness and tensile yielding with reduced bulk density.     

双亲环氧树脂基荧光形状记忆薄膜的构筑

以聚乙二醇单甲醚为亲水段，双酚A型环氧树脂为疏水段，异氟尔酮二异氰酸酯为链接剂制备了含环氧基的双亲性大分子WEG，该双亲性大分子与荧光剂(3-(9-咔唑基)苯甲酸)、双酚A型环氧树脂E51在选择性溶剂(水)中共组装形成水性乳液。该乳液与改性胺类固化剂以一定比例混合并于室温固化后，制备了具光致发光(荧光)功能的形状记忆高分子薄膜，采用FTIR、NMR、GPC、纳米粒度仪、荧光光谱仪和DSC等对WEG、乳液和薄膜的结构和性能进行了表征。结果表明，乳液具有较好的粒径稳定性。荧光形状记忆薄膜具有较好的光致发光功能，可在刺激响应条件下（如热）快速（7 s）恢复其原始形状，形状回复率可达94.3%。