Insertion of Supramolecular Segments into Covalently Crosslinked Polyurethane Networks towards the Fabrication of Recyclable Elastomers

Thermoset plastics have become one of the most important chemical products in the world. The consequent problem is that although the thermosets possess excellent performance in mechanical strength, they cannot be reprocessed because of the internal permanent network structures. Optimizing the molecular design of thermosets is one of the most feasible ways to improve their recyclability. Here we present a facile and robust strategy to engineer the reprocessability of thermoset polyurethanes without compromising their mechanical toughness and chemical resistance via adding supramolecular additives during the polymer synthesis process. By using a multiple hydrogen bonding moiety as the model supramolecular additive, we demonstrate that the mechanical properties, recyclability, and chemical resistance of the crosslinked polyurethanes can be precisely controlled by adjusting the contents of the supramolecular additive. Systematic studies on the relations between molecular design and material properties are performed, and the optimized polyurethane network with a moderate amount of the supramolecular additive achieves the right balance between the robustness and recyclability. This work provides a cost-effective and practical way to chemically engineer thermoset plastics, aiming to enable the recycling of mechanically tough and chemically stable polymer materials.     

A Cleavable Crosslinking Strategy for Commodity Polymer Functionalization and Generation of Reprocessable Thermosets

Covalently crosslinked polymeric materials, known as thermosets, possess enhanced mechanical strength and thermal stability relative to the corresponding uncrosslinked thermoplastics. However, the presence of covalent inter-chain crosslinks that makes thermosets so attractive is precisely what makes them so difficult to reprocess and recycle. Here, we demonstrate the introduction of chemically cleavable groups into a bis-diazirine crosslinker. Application of this cleavable crosslinker reagent to commercial low-functionality polyolefins (or to a small-molecule model) results in the rapid, efficient introduction of molecular crosslinks that can be uncoupled by specific chemical inputs. These proof-of-concept findings provide one potential strategy for circularization of the thermoplastic/thermoset plastics economy, and may allow crosslinked polyolefins to be manufactured, used, reprocessed, and re-used without losing value. As an added benefit, the method allows the ready introduction of functionality into non-functionalized commodity polymers.     

A furan-derived epoxy thermoset with inherent anti-flammability,degradability, and raw material recycling

The production of multifunctional thermosets with flammability, degradability and raw material recycling from epoxy thermosets made from renewable resources is one of the hottest topics in the context of sustainable development. In this work, we fabricated a fully bio-based epoxy thermoset by curing an as-synthesized furan-derived epoxy monomer (HMF-DDDS-EP) with a furan-based hardener (DFA). Owing to its unique structure containing a Schiff base and disulfide bonds, the cured HMF-DDDS-EP/DFA thermoset integrates a high glass transition temperature, high tensile strength, inherent anti-flammability, degradability, and recyclability. Specifically, a glass transition temperature as high as 171 °C, tensile strength of 62.9 MPa, a storage modulus of 2,356 MPa and outstanding anti-flammability (UL-94 V-0 rating and high LOI of 36.0%) were observed for this fully bio-based epoxy thermoset. Additionally, it was capable of degrading under mildly acidic conditions because of the cleavage of the Schiff base into the original aldehyde monomer. This fully bio-based epoxy thermoset can be considered a representative for fostering the synthesis of advanced thermosetting materials derived from renewable resources.     

Thermal stability and dynamic mechanical thermal analysis of epoxy thermosets possessing N,N′-disubstituted pyromellitimide units

Abstract

In this work, three epoxy resins including diglycidyl ethers of N,N′-bis(2-hydroxyethyl)pyromellitimide (DIDGE), bisphenol-A (BADGE), and polyethylene glycol (PEDGE) were isothermally cured by an amine curing agent possessing N,N′-disubstituted pyromellitimide units (denoted by DIDAM). DIDGE resin was synthesized from the reaction of N,N′-bis(2-hydroxyethyl)pyromellitimide with an excess of epichlorohydrin. Also, DIDAM curing agent was prepared from the reaction of pyromellitic dianhydride with an excess of ethylene diamine. Completion of the isothermal curing processes was approved by both Fourier transform-infrared spectroscopy and non-isothermal differential scanning calorimetry (DSC). The DSC traces showed only the phase transitions related to the thermal degradation of the resulting thermosets. According to the thermogravimetric analyses, the DIDGE/DIDAM thermoset showed higher thermal stability at temperatures above 425?°C than the other two thermosets. While BADGE/DIDAM and PEDGE/DIDAM thermosets showed about 70% weight loss in the thermal range of 400–850?°C, DIDGE/DIDAM thermoset was encountered with only about 40% weight loss. The glass transition temperatures (T_g ) of the resulting thermosets were determined using tan δ vs temperature plots obtained from dynamic mechanical thermal analysis. The T_g values of BADGE/DIDAM, DIDGE/DIDAM, and PEDGE/DIDAM thermosets were found to be 211?°C, 189?°C, and 81?°C, respectively.     

Dynamic elastomers based on bio-derived crosslinker

Petroleum-derived monomers are the most common building blocks for ester-based thermosets. Bio-derived thermoset elastomers are becoming viable alternatives to conventional thermosets. Herein, we developed a biobased vitrimer-type thermoset elastomers using abundant and sustainable raspberry ketone as feedstock. We utilize raspberry ketone to create building blocks for dynamic oxime chemistry and crosslinked these through free radical polymerization with poly(ethylene glycol) methyl ether methacrylate as a comonomer. In contrast to other dynamic networks based on ester bonds, which need catalysts, this is undesirable since catalyst deactivation or leaching lowers its effect over time and may impair reuse. This network incorporates catalyst-free bond exchange reactions in catalyst-dependent polyester networks by substituting oxime-esters for typical ester linkages. The elastomer exhibits stress relaxation, a low glass transition temperature (T_g) (−55 to −40.2°C) and tensile strength up to 5.2 ± 3.0 kPa. Furthermore, the dynamic oxime transesterification exchange mechanism allows elastomers to be reprocessed using a hot press at 160°C and 8 × 10³ kPa pressure. After reprocessing, the tensile strength of elastomers can be recovered up to 78.1 ± 10.9%. This work integrates the principles of catalyst-free dynamic exchange process and mechanical recycling coupled with biobased components to provide a rational solution towards conventional elastomers. In the future, these elastomers can be exploited for the development of hydrogels, recyclable elastomers, and commodity plastics.     

Toughening rigid thermoset films via molecular enforced integration of covalent crosslinking and multiple supramolecular interactions

Rigid thermosets show great potential application value in several fields; however, enhancing their extensibility without losing the required mechanical strength is always a huge challenge. In this work, a new class of rigid thermosets, poly(triethylenetetramine bisphenol A epoxy phenylimidazole)s, with both multiple supramolecular interactions, including “face–face” π–π stacking, “point–point” hydrogen bond, and ion-pair electrostatic interaction, and covalent crosslinks is developed by introducing the special functional group 2-phenylimidazole into the crosslinked thermoset. Owing to the synergistic effect of the strong covalent crosslinks and the multiple supramolecular interactions, synchronously improved tensile strengths and elongation at break have been successfully obtained with these rigid thermosets.     

Hybrid inorganic–organic poly(carborane‐siloxane‐arylacetylene) structural isomers with in‐chain aromatics: Synthesis and properties

Structural isomers of thermo‐oxidatively stable poly(carborane‐siloxane‐arylacetylene) (PCSAA), namely, m‐PCSAA and p‐PCSAA, were synthesized by the reaction of the dimagnesium salts of m‐diethynylbenzene or p‐diethynylbenzene with 1,7‐bis(chlorotetramethyldisiloxyl)‐m‐carborane. The developed polymers have exceptional thermo‐oxidative properties similar to their diacetylene counterpart poly(carborane‐siloxane‐acetylene), PCSA. Thermal treatment of either of the PCSAAs results in a fully crosslinked thermoset by 500 °C resulting from the cycloaddition reactions involving the acetylene and aryl functionalities and subsequent formation of bridging disilylmethylene entities as discerned from Fourier transform infrared, ¹³C and ²⁹Si solid‐state NMR, and XPS studies. X‐ray diffraction analysis revealed that the thermosets obtained from p‐PCSAA possess enhanced crystallinity when compared to that obtained from m‐PCSAA possibly due to more efficient packing interactions of the p‐diethynylbenzene groups during thermoset formation. The presence of the aryl groups in the backbone of the PCSAAs' chains appeared to have enhanced the storage and bulk moduli of their thermosets when compared to the thermoset of PCSA. Dielectric studies of m‐PCSAA and p‐PCSAA revealed segmental relaxation peaks, α, above their glass transition temperatures with p‐PCSAA exhibiting a broader peak with a slower relaxation rate than m‐PCSAA. © 2013 Wiley Periodicals, Inc.^? J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2638–2650     

A flexible approach to the preparation of polymer scaffolds for tissue engineering

Porous polyester thermoset xerogels have been produced via sol-gel chemistry as a first step in the development of sol-gel derived tissue engineering scaffolds templated by replica molding and/or salt leaching. The pore structure of these untemplated thermosets is tunable and can be altered independent of or in tandem with alterations in composition. Cytocompatibility studies on these xerogels imply the effects of both pore size and materials chemistry, with fully aliphatic polyesters with large pore structures allowing the growth of mammalian cells. To the best of our knowledge, this represents the first report examining the preparation and potential of sol-gel derived porous polymer xerogels as tissue engineering scaffolds.     

The synthesis and characterization of nadimidized aromatic bisaspartimides and their polymerization to high-temperature-resistant laminating resins

4,4′-Bis(N²)-{4-[4-(2,3-bicyclo-[2.2.1]hept-5-ene-dicarboximido)phenoxy]-phenyl(-aspartimido)-di-phenyl ether (2NAD/1020) and various similar polymer percursors were synthesized from bismaleimides by the Michael addition of two moles of aromatic diamine followed by end capping of the resultant amino-terminated bisaspartimide groups with nadic anhydride. These precursors were characterized by using Fourier-transform infrared (FT-IR), proton, and carbon-13 nuclear magnetic resonance spectroscopies (¹H- and ¹³C-NMR). Thermal polymerization of 2NAD/1020 by heating it above its melting point gave a reddish brown thermoset polymer. The curing behavior and thermal stability of these nadimides were evaluated by differential scanning calorimetry and thermogravimetric analysis studies. Graphite fiber laminates were prepared from these polymer precursors and their mechanical properties evaluated. Gas Chromatography/Mass Spectra of the precursors and thermosets have given an understanding of their decomposition behavior. The structures of the thermosets were examined by FT-IR spectroscopy.     

Facile recycling of anhydride-cured epoxy thermoset under mild conditions with multifunctional hydrazine hydrate

Environmental economics is accelerating the urgency to develop recycling technologies for the ever-growing quantity of discarded thermoset polymers. Herein, we developed a mild and energy-saving process for high-efficiency degradation and reuse of anhydride-cured epoxy thermoset with the aid of hydrazine hydrate. The degradation degree of the epoxy resin reached 99.6% at 120 °C within a short time of 60 min. During the reaction, the ester bonds in the cross-linked network were selectively cleaved by the amination of hydrazine hydrate, and the epoxy resin was fully converted to new monomers that contained hydrazide and hydroxyl groups, respectively. Moreover, the degradation mechanism of the epoxy resin in hydrazine hydrate was studied and a nucleation model was utilized to predict the actual degradation behavior of the system. Finally, the degradation products can be directly mixed with epoxy precursor to prepare a new waterborne epoxy coating with good comprehensive properties. This work not only demonstrates a new way to realize the efficient degradation of epoxy resins, but also provides a facile and efficient recycling protocol for thermosets.     

Benzoxazine-epoxy thermosets with smectic phase structures for high thermal conductive materials

ABSTRACT

To further increase the intrinsic thermal conductivity (TC) of polybenzoxazine, a series of benzoxazine-epoxy thermosets (s-PBEI) were obtained through the sequential curing of a smectic phase epoxy monomer (s-EP) and a bifunctional benzoxazine monomer (BZ) in the presence of imidazole. The results show that s-PBEI exhibits a smectic mesophase. The formation mechanism of the smectic phase is reaction-induced phase separation caused by the preferential curing of s-EP. Owing to the increment of the liquid crystalline structure content, the TC of s-PBEI increases with increasing s-EP content. The TC of s-PBEI55 containing equal weight of BZ and s-EP reaches 0.30 W m^?1 K^?1, which is higher than that of n-PBEI55, a benzoxazine-epoxy thermoset with nematic phase structures. Additionally, the TC, glass transition temperature, and 10% weight loss temperature of s-PBEI64 containing 60 wt% BZ and 40 wt% s-EP are 0.28 W m^?1 K^?1, 216°C, and 334°C, respectively, indicating its potential applications in electronic packaging, LED lighting, and other fields requiring a high TC resin matrix.     

Towards Recycling of LLZO Solid Electrolyte Exemplarily Performed on LFP/LLZO/LTO Cells**

All-solid-state lithium ion batteries (ASS-LIBs) are promising due to their safety and higher energy density as compared to that of conventional LIBs. Over the next few decades, tremendous amounts of spent ASS-LIBs will reach the end of their cycle life and would require recycling in order to address the waste management issue along with reduced exploitation of rare elements. So far, only very limited studies have been conducted on recycling of ASS-LIBS. Herein, we investigate the recycling of the Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolyte in a LiFePO₄/LLZO/Li₄Ti₅O₁₂ system using a hydrometallurgical approach. Our results show that different concentration of the leaching solutions can significantly influence the final product of the recycling process. However, it was possible to recover relatively pure La₂O₃ and ZrO₂ to re-synthesize the cubic LLZO phase, whose high purity was confirmed by XRD measurements.     

Flame‐retardant performance and mechanism of epoxy thermosets modified with a novel reactive flame retardant containing phosphorus,nitrogen, and sulfur

A novel phosphorus‐containing, nitrogen‐containing, and sulfur‐containing reactive flame retardant (BPD) was successfully synthesized by 1‐pot reaction. The intrinsic flame‐retardant epoxy resins were prepared by blending different content of BPD with diglycidyl ether of bisphenol‐A (DGEBA). Thermal stability, flame‐retardant properties, and combustion behaviors of EP/BPD thermosets were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limited oxygen index (LOI) measurement, UL94 vertical burning test, and cone calorimeter test. The flame‐retardant mechanism of BPD was studied by TGA/infrared spectrometry (TGA‐FTIR), pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS), morphology, and chemical component analysis of the char residues. The results demonstrated that EP/BPD thermosets not only exhibited outstanding flame retardancy but also kept high glass transition temperature. EP/BPD‐1.0 thermoset achieved LOI value of 39.1% and UL94 V‐0 rating. In comparison to pure epoxy thermoset, the average of heat release rate (av‐HRR), total heat release (THR), and total smoke release (TSR) of EP/BPD‐1.0 thermoset were decreased by 35.8%, 36.5% and 16.5%, respectively. Although the phosphorus content of EP/BPD‐0.75 thermoset was lower than that of EP/DOPO thermoset, EP/BPD‐0.75 thermoset exhibited better flame retardancy than EP/DOPO thermoset. The significant improvement of flame retardancy of EP/BPD thermosets was ascribed to the blocking effect of phosphorus‐rich intumescent char in condensed phase, and the quenching and diluting effects of abundant phosphorus‐containing free radicals and nitrogen/sulfur‐containing inert gases in gaseous phase. There was flame‐retardant synergism between phosphorus, nitrogen, and sulfur of BPD.     

Flame retardant and toughening behaviors of bio‐based DOPO‐containing curing agent in epoxy thermoset

Based on bio‐based furfural, a phosphorus‐containing curing agent (FPD) was successfully synthesized, via the addition reaction between 9,10‐dihydro‐9‐oxa‐10 phosphaphenanthrene‐10‐oxide (DOPO) and furfural‐derived Schiff base. Then, as co‐curing agent, FPD was used to prepare flame retardant epoxy thermosets (EP) cured by 4, 4′‐diaminodiphenyl methane. The incorporated FPD improved the flame retardancy and toughness of epoxy thermoset, simultaneously. When 5 wt% FPD was added into EP, the FPD/EP achieved 35.7% limited oxygen index (LOI) value and passed UL94 V‐0 rating, meanwhile. In FPD/EP thermoset, the incorporated FPD reduced the thermal decomposition rate, increased the charring capacity, and inhibited the combustion intensity of epoxy thermoset. Through gas‐phase and condensed‐phase actions in weakening fuel supply, suppressing volatile combustion, and enhancing charring barrier effect, FPD decreased the heat release of burning epoxy thermoset, significantly. For the outstanding effectiveness on both flame retardancy and toughness, the study on FPD provides a promising way to manufacture high‐performance epoxy thermoset.     

A new class of epoxy thermosets

The reinforcing strategies of epoxy thermosets rely on the control of the phase separation between the additive and the growing thermoset. With standard additives, such as reactive liquid rubbers, the length scale of the resulting domains is the micrometer. Here, we present a route that enable a control of the morphology down to the nanometer scale. This strategy is based upon the self-assembly process of blends of epoxy and SBM triblock copolymers, namely Poly(Styrene-b-1,4 Butadiene-b-Methyl methacrylate). It relies on the respective affinities between the epoxy precursors and each of the three blocks. Liquid epoxy has a strong affinity for PMMA, whilst it is not miscible with polystyrene nor polybutadiene at standard processing temperatures. Thus, within the reactive system, microphase separation leads to a regular network of S-B domains. This nanostructure is governed by thermodynamics. The size and geometry of the dispersed domains are controlled by the concentration and the ratio between blocks lengths. The domain size is of the order of magnitude of the chain length, ranging typically from 10 to 30 nanometers. What controls the blend's morphology throughout the curing process of the thermoset was one topic on which we focused our interest. Nanostructured thermosets have been obtained. These supramolecular architectures yield significant toughness improvements while preserving the transparency of the material. The reinforcing mechanisms are not yet fully understood : it is intriguing to induce significant toughening with elastomer domains smaller than 30 nanometers in diameter. Besides being efficient epoxy tougheners, SBM can broaden the scope of applications of thermosets due to specific rheological behaviors. Thanks to the self assembly process taking place in the blend of the SBM block copolymers with the epoxy thermosets precursors, the reactive solvent can be turned into a reactive gel or solid (before curing). This physical gelation is induced by the microphase separation and is thus thermoreversible. At relatively moderate loadings of block copolymers the reactive blend behaves like a thermoplastic material, with adjustable modulus and tackiness. These results evidence that SBM block copolymers open a broad area for designing new class of thermoset materials.     

Multifunctionality in Epoxy Resins

Abstract

Epoxy resin will continue to be in the forefront of many thermoset applications due to its versatile properties. However, with advancement in manufacturing, changing societal outlook for the chemical industries and emerging technologies that disrupt conventional approaches to thermoset fabrication, there is a need for a multifunctional epoxy resin that is able to adapt to newer and robust requirements. Epoxy resins that behave both like a thermoplastic and a thermoset resin with better properties are now the norm in research and development. In this paper, we viewed multifunctionality in epoxy resins in terms of other desirable properties such as its toughness and flexibility, rapid curing potential, self-healing ability, reprocessability and recyclability, high temperature stability and conductivity, which other authors failed to recognize. These aspects, when considered in the synthesis and formulation of epoxy resins will be a radical advance for thermosetting polymers, with a lot of applications. Therefore, we present an overview of the recent finding as to pave the way for varied approaches towards multifunctional epoxy resins.     

Tailor‐made and chemically designed synthesis of coumarin‐containing benzoxazines and their reactivity study toward their thermosets

Coumarins are used as a natural renewable resource to synthesize coumarin‐containing benzoxazine resins. The coumarin‐containing benzoxazines are fully characterized in terms of their chemical structure by Fourier‐transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. The influence of electronic effects caused by the substituents on the polymerization temperature is also evaluated. Thermal properties of the resulting thermosets are characterized by differential scanning calorimetry and thermogravimetric analysis, showing good stability and char yields higher than 50%. The coumarin‐containing polybenzoxazine thermosets show T_g values in the range between 160 and 190 °C. Thus, the herein presented coumarin‐containing benzoxazine resins are proven to be competitive monomers when compared with other petroleum‐based benzoxazine resins toward the generation of high‐performance thermoset. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1428‐1435     

Co‐immobilized Phosphorylated Cofactors and Enzymes as Self‐Sufficient Heterogeneous Biocatalysts for Chemical Processes

Enzyme cofactors play a major role in biocatalysis, as many enzymes require them to catalyze highly valuable reactions in organic synthesis. However, the cofactor recycling is often a hurdle to implement enzymes at the industrial level. The fabrication of heterogeneous biocatalysts co‐immobilizing phosphorylated cofactors (PLP, FAD⁺, and NAD⁺) and enzymes onto the same solid material is reported to perform chemical reactions without exogeneous addition of cofactors in aqueous media. In these self‐sufficient heterogeneous biocatalysts, the immobilized enzymes are catalytically active and the immobilized cofactors catalytically available and retained into the solid phase for several reaction cycles. Finally, we have applied a NAD⁺‐dependent heterogeneous biocatalyst to continuous flow asymmetric reduction of prochiral ketones, thus demonstrating the robustness of this approach for large scale biotransformations.     

Thermally and oxidatively stable thermosets derived from preceramic monomers

Monomers 1,3-bis(4-phenylethynylphenyl)tetramethyldisiloxane and 1,7-bis(4-phenylethynylphenyltetramethyldisiloxyl)-m-carborane were synthesized and compared with bis(4-phenylethynylphenyl)dimethylsilane as potential preceramic precursors. These monomers were heated to free flowing liquids above 100°C and thermally polymerized above 300°C to form heat-resistant thermosets or ceramic residues. The ceramic yields for the silane (13%) and siloxane (30%) were much lower than that for the carborane (64%) monomer. The thermoset and ceramic made from the carborane monomer were the best thermally and oxidatively stable materials. After curing, the thermoset had a weight loss of only 6% and after pyrolysis, the ceramic residue had no additional weight loss up to 1000°C in air. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1033–1038, 1997     

Co-immobilized Phosphorylated Cofactors and Enzymes as Self-Sufficient Heterogeneous Biocatalysts for Chemical Processes

Enzyme cofactors play a major role in biocatalysis, as many enzymes require them to catalyze highly valuable reactions in organic synthesis. However, the cofactor recycling is often a hurdle to implement enzymes at the industrial level. The fabrication of heterogeneous biocatalysts co-immobilizing phosphorylated cofactors (PLP, FAD⁺, and NAD⁺) and enzymes onto the same solid material is reported to perform chemical reactions without exogeneous addition of cofactors in aqueous media. In these self-sufficient heterogeneous biocatalysts, the immobilized enzymes are catalytically active and the immobilized cofactors catalytically available and retained into the solid phase for several reaction cycles. Finally, we have applied a NAD⁺-dependent heterogeneous biocatalyst to continuous flow asymmetric reduction of prochiral ketones, thus demonstrating the robustness of this approach for large scale biotransformations.