A General Strategy for Sustainable 3D Printing Based on A Multifunctional Photoinitiator.

A multifunctional photoinitiator is presented, offering precise control over light-induced polymerization initiation at 450 nm and material degradation at 365 nm. This is accomplished by covalently linking photoactive bis(acyl)phosphane oxide and photocleavable o-nitrobenzyl ether moieties onto the surface of γ-cyclodextrin. Upon degradation, the resulting linear polymers can be easily re-dissolved in their corresponding monomer and re-cured, exhibiting superior mechanical properties compared to the pristine material. Moreover, this photoinitiator enables the successful 3D printing of intricate and precise structures, representing a promising general strategy for developing recyclable photoresins for 3D printing applications.

Catalyst-Free Thia-Michael Addition to α-Trifluoromethylacrylates for 3D Network Synthesis.

Thia-Michael additions (1,4-additions of a thiol to a Michael acceptor) are generally catalyzed by an external Brønsted or Lewis base. A spontaneous (uncatalyzed) Michael addition of thiols to α-trifluoromethyl acrylates is described, as well as its application to the very efficient preparation of a thermoset. A thorough mechanistic investigation, based on an experimental kinetic study and on DFT calculations, is presented for the addition of arene- and alkanethiols to tert-butyl trifluoromethyl acrylate in polar aprotic solvents, unveiling a probable solvent-assisted proton transfer in the rate-determining step and a considerable lowering of the energy barrier induced by the CF3 group.

Mechanically Strong and Tough Poly(urea-urethane) Thermosets Capable of Being Degraded under Mild Condition.

The development of degradable polymeric materials such as degradable polyurethane or polyurea has been much highlighted for resource conservation and environmental protection. Herein, a facile strategy of constructing mechanically strong and tough poly(urea-urethane) (PUU) thermosets that can be degraded under mild conditions by using triple boron-urethane bonds (TBUB) as cross-linkers is demonstrated. By tailoring the molecular weight of the soft segment of the prepolymers, the mechanical performance can be finely controlled. Based on the cross-linking of TBUB units and hydrogen-binding interactions between TBUB linkages, the as-prepared PUU thermosets have excellent mechanical strength of ≈40.2 MPa and toughness of ≈304.9 MJ m-3 . Typically, the PBUU900 strip can lift a barbell with 60 000 times its own weight, showing excellent load-bearing capacity. Meanwhile, owing to the covalent cross-linking of TBUB units, all the PUU thermosets show initial decomposition temperatures over 290 °C, which are comparable to those of the traditional thermosets. Moreover, the TBUB cross-linked PUU thermosets can be easily degraded in a mild acid solution. The small pieces of the PBUU sample can be fully decomposed in 1 m HCl/THF solution for 3.5 h at room temperature.

Low-Temperature Terpolymerizable Benzoxazine Monomer Bearing Norbornene and Furan Groups: Synthesis, Characterization, Polymerization, and Properties of Its Polymer.

There is an urgency to produce novel high-performance resins to support the rapid development of the aerospace field and the electronic industry. In the present work, we designed and consequently synthesized a benzoxazine monomer (oHPNI-fa) bearing both norbornene and furan groups through the flexible benzoxazine structural design capability. The molecular structure of oHPNI-fa was verified by the combination characterization of nuclear magnetic resonance spectrum, FT-IR technology, and high-resolution mass spectrum. The thermally activated terpolymerization was monitored by in situ FT-IR as well as differential scanning calorimetry (DSC). Moreover, the low-temperature-curing characteristics of oHPNI-fa have also been revealed and discussed in the current study. Furthermore, the curing kinetics of the oHPNI-fa were investigated by the Kissinger and Ozawa methods. The resulting highly cross-linked thermoset based on oHPNI-fa showed excellent thermal stability as well as flame retardancy (Td10 of 425 °C, THR of 4.9 KJg-1). The strategy for molecular design utilized in the current work gives a guide to the development of high-performance resins which can potentially be applied in the aerospace and electronics industries.

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.

Hydrolyzable Poly(ß-thioether ester ketal) Thermosets via Acyclic Ketal Monomers.

Hydrolytically degradable poly(ß-thioether ester ketal) thermosets are synthesized via radical-mediated thiol-ene photopolymerization using three novel dialkene acyclic ketal monomers and a mercaptopropionate based tetrafunctional thiol. For all thermoset compositions investigated, degradation behavior is highly tunable based on the structure of the incorporated ketal and pH. Complete degradation of the thermosets is observed upon exposure to acidic and neutral pH, and under high humidity conditions. Polymer networks composed of cross-link junctions based on acyclic dimethyl ketals degrade the quickest, whereas networks containing acyclic cyclohexyl ketals undergo hydrolytic degradation on a longer timescale. Thermomechanical analysis reveals low glass transition temperatures and moduli typical of thioether-based thermosets.

Designed from Biobased Materials for Recycling: Imine-Based Covalent Adaptable Networks.

Turning thermosets into fully sustainable materials requires utilization of biobased raw materials and design for easy recyclability. Here, dynamic covalent chemistry for fabrication of covalent adaptable networks (CANs) could be an enabling tool. CAN thermosets ideally combine the positive material properties of thermosets with thermal recyclability of linear thermoplastics. Among the dynamic covalent bonds, imine bond, also called Schiff base, can participate in both dissociative and associative pathways. This induces potential for chemical recyclability, thermal reprocessability and self-healing. This review presents an overview of the current research front of biobased thermosets fabricated by Schiff base chemistry. The discussed materials are categorized on the basis of the employed biobased components. The chemical approaches for the synthesis and curing of the resins, as well as the resulting properties and recyclability of the obtained thermosets are described and discussed. Finally, challenges and future perspectives are briefly summarized.

Crystallizable Aliphatic Chains Enhanced Covalent Adaptable Networks: Fast Reprocessing and Improved Performance.

Covalent adaptable networks (CANs) exhibit recyclability such as reprocessing, but it's a challenge to address the contradiction between reprocessing rate and performance. Here, pendent aliphatic chain anhydride monoesters are innovatively introduced into epoxy CANs based on transesterification, which efficiently accelerates the reprocessing without sacrificing thermal and mechanical properties. The transesterification rate is raised on account of the flexible aliphatic chain-promoted segment movement and dynamic transfer auto-catalysis. When the carbon number reflecting the length of the pendent chain is 12, the epoxy CAN exhibits the fastest stress relaxation or reprocessing. Computation via molecular dynamics simulation demonstrates that the increased segmental mobility from the pendent aliphatic chains contributes to enhanced reprocessability. Interestingly, the crystallization of the pendent aliphatic chains maintains or even improves the thermal and mechanical properties. Thus, introducing a flexible and crystallizable aliphatic side chain is an innovative and efficient approach to accelerate dynamic reactions and network arrangement while improving performance.

Self-Evolution of High Mechanical Strength Dry-Network Polythiourethane Thermosets into Neat Macroscopic Hollow Structures.

Organism-inspired hollow structures are attracting increasing interest for the construction of various bionic functional hollow materials. Next-generation self-evolution hollow materials tend to combine simple synthesis, high mechanical strength, and regular shape. In this study, we designed and synthesized a novel dry-network polythiourethane thermoset with excellent mechanical performance. The polymer film could evolve into a neat and well-organized object with a macroscopic hollow interior structure after being immersed in an aqueous NaOH solution. The self-evolution hollow structure originated from a hydrogen-bonded polymer network, which was later transformed into a network bearing both hydrogen bonds and ionic bonds. The swelling and thickness growth of this material could be controlled by the NaOH concentration and the immersion time. This unique self-evolution behavior was further utilized to produce a series of macroscopic 3D hollow-containing molds, which could be potentially applied in the production of smart materials.

Role of Structural Morphology of Commodity Polymers in Microplastics and Nanoplastics Formation: Fragmentation, Effects and Associated Toxicity in the Aquatic Environment.

With the continued growth in plastic production, its ubiquitous use and insufficient waste management and disposal, the increased levels of plastics in the environment have led to growing ecological concerns. The breakdown of these plastic macromolecules to smaller micro and nanosized particles and their detection in the aerial, aquatic, marine and terrestrial environments has been reviewed extensively, especially for thermoplastics. However, the formation of micro and nanoplastics has typically been explained as a physical abrasion process, largely overlooking the underlying chemical structure-morphology correlations to the degradation mechanisms of the plastics. This is particularly true for the common commodity thermosets. This review focuses on the degradation pathways for the most widely produced commodity thermoplastics and thermosets into microplastics (MP)s and nanoplastics (NP)s, as well as their behaviour and associated toxicity. Special emphasis is placed on NPs, which are associated with greater risks for toxicity compared to MPs, due to their higher surface area to volume ratios. This review also assesses the current state of standardized detection and quantification methods as well as comprehensive regulations for these fragments in the aquatic environment.

A Fluorinated Thermocrosslinkable Organosiloxane: A New Low-k Material at High Frequency with Low Water Uptake.

In order to obtain low-k material with good comprehensive properties, a trifluoromethyl-containing organosiloxane with thermocrosslinkable vinyl and benzocyclobutene groups is designed and synthesized through the Piers-Rubinsztajn reaction. After treating at high temperature, the organosiloxane changed to form a cross-linked polysiloxane (called as c-FSi-BCB). c-FSi-BCB exhibits good dielectric properties with dielectric constant (Dk ) of 2.60 and dielectric loss (Df ) of 1.49 × 10-3 at a high frequency of 5 GHz. Importantly, c-FSi-BCB maintains such good dielectric properties and exhibits low water uptake of below 0.076%, even after immersing it in boiling water for 96 h. c-FSi-BCB also displays good thermostability with a 5% weight loss temperature (T5d ) of 453 °C. These data indicate that this fluorinated organosiloxane is suitable as the matrix resin for the fabrication of devices used in 5G communication.

Easily Processable Thermosets with Outstanding Performance via Smart Twisted Small-Molecule Benzoxazines.

High-performance aromatic polymers have excellent thermal, mechanical, and electrical properties and are lightweight, but it is highly challenging to deliver outstanding performances while still maintaining good processability of the precursors. Here, a new family of small-molecule benzoxazine resins with ortho-maleimide functionality is reported, which strikes an exceptional balance between the processability and performance. The excellent processability is attributed to the twisted molecular configurations of ortho-maleimide-substituted benzoxazines, which prevent intermolecular packing in the resin systems. The new benzoxazines can polymerize through multiple routes, which eliminate the twisted structures and create highly cross-linked polymer networks. The resulting new polymers are found to possess fascinating properties such as a high thermal stability (no Tg can be detected before 400 °C), excellent flame retardancy (a heat release capacity of 42.5 J g-1 K-1 ), and low dielectric constants (2.62-2.30 in the frequency range of 1 Hz to 10 MHz). The combined processability and versatility highlight the potential of smart benzoxazines in the preparation of high-performance thermosets, with important new applications that may span aerospace, transportation, and electronic packaging materials.

Rediscovering Surlyn: A Supramolecular Thermoset Capable of Healing and Recycling.

Surlyn consists of ionomers of poly(ethylene-co-methacrylic acid) (PEMA) partially neutralized with metal ions. Considering its huge consumption every year, it is highly desirable to realize efficient healing and recycling of Surlyn through an easily available method. Herein, healable and recyclable Surlyn materials are fabricated by complexation of PEMA with Zn2+ ions followed by a hot-pressing process. The PEMA/Zn composites exhibit a tensile strength of ≈37 MPa, Young's modulus of ≈343 MPa, and toughness of ≈95 MJ m-3 . Structural analysis discloses that the PEMA/Zn composites are dynamically cross-linked with coordination interactions and reinforced with polyethylene nanocrystals, and have the typical structure of supramolecular thermosets. As supramolecular thermosets, the reversibility of coordination interactions endows the PEMA/Zn composites with good healing and recycling capacities. The PEMA/Zn composites can fully heal mechanical damage to restore their original mechanical strength when heated at 90 °C. Under a pressure of 3 MPa at 100 °C, the PEMA/Zn composites can be recycled multiple times to regain their structural integrity and mechanical properties.

Reactive & Efficient: Organic Azides as Cross-Linkers in Material Sciences.

The exceptional reactivity of the azide group makes organic azides a highly versatile family of compounds in chemistry and the material sciences. One of the most prominent reactions employing organic azides is the regioselective copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition with alkynes yielding 1,2,3-triazoles. Other named reactions include the Staudinger reduction, the aza-Wittig reaction, and the Curtius rearrangement. The popularity of organic azides in material sciences is mostly based on their propensity to release nitrogen by thermal activation or photolysis. On the one hand, this scission reaction is accompanied with a considerable output of energy, making them interesting as highly energetic materials. On the other hand, it produces highly reactive nitrenes that show extraordinary efficiency in polymer crosslinking, a process used to alter the physical properties of polymers and to boost efficiencies of polymer-based devices such as membrane fuel cells, organic solar cells (OSCs), light-emitting diodes (LEDs), and organic field-effect transistors (OFETs). Thermosets are also suitable application areas. In most cases, organic azides with multiple azide functions are employed which can either be small molecules or oligo- and polymers. This review focuses on nitrene-based applications of multivalent organic azides in the material and life sciences.

Synthesis of Resins Using Epoxies and Humins as Building Blocks: A Mechanistic Study Based on In-Situ FT-IR and NMR Spectroscopies.

The combination of eco-respectful epoxy compounds with the humins, a by-product of biomass chemical conversion technologies, allow the obtention of materials with high added value. In this work, we propose a chemical connection study of humins with two aliphatic bis-epoxides through copolymerization reactions to synthesize sustainable, bio-based thermosets. The mechanism insights for the crosslinking between the epoxides and humins was proposed considering the different functionalities of the humins structure. Fourier Transform InfraRed (FT-IR), one dimensional (1D) and two-dimensional (2D) Nuclear Magnetic Resonance (NMR) spectroscopy techniques were used to build the proposed mechanism. By these techniques, the principal chain connections and the reactivity of all the components were highlighted in the synthesized networks.

Reprocessable Thermoset Soft Actuators.

Widely used traditional thermosets are good candidates for construction of 3D soft actuators because of their excellent stability; however, it is generally acknowledged that they cannot be reprocessed. The time-temperature equivalence principle enables reprocessing of traditional liquid crystalline epoxy thermosets (LCETs) into 3D soft actuators. Even though the transesterification reaction of LCETs is extremely slow, it is fast enough to induce a topology rearrangement and subsequent reprocessing when prolonging the transesterification time according to aforementioned principle. Therefore, LCETs can be aligned by a simple procedure. The alignment is quite stable at high temperature and remains after more than 1000 heating-cooling actuation cycles. The resulting 3D soft actuators are remouldable, reprogrammable, reconfigurable, weldable, self-healable, recyclable, and stable, which is impossible for any traditional thermosets and is therefore a compelling advance in terms of the applications open to 3D soft actuators.

Photopolymerized Triazole-Based Glassy Polymer Networks with Superior Tensile Toughness.

Photopolymerization is a ubiquitous, indispensable technique widely applied in applications from coatings, inks, and adhesives to thermosetting restorative materials for medical implants, and the fabrication of complex macro-scale, microscale, and nanoscale 3D architectures via additive manufacturing. However, due to the brittleness inherent in the dominant acrylate-based photopolymerized networks, a significant need exists for higher performance resin/oligomer formulations to create tough, defect-free, mechanically ductile, thermally and chemically resistant, high modulus network polymers with rapid photocuring kinetics. This study presents densely cross-linked triazole-based glassy photopolymers capable of achieving preeminent toughness of ≈70 MJ m-3 and 200% strain at ambient temperature, comparable to conventional tough thermoplastics. Formed either via photoinitiated copper(I)-catalyzed cycloaddition of monomers containing azide and alkyne groups (CuAAC) or via photoinitiated thiol-ene reactions from monomers containing triazole rings, these triazole-containing thermosets completely recover their original dimensions and mechanical behavior after repeated deformations of 50% strain in the glassy state over multiple thermal recovery-strain cycles.

Effect of Crosslinker Length and Architecture on the Thermomechanical Properties of CNT-Loaded Elastomeric Polymer Matrix Composites.

An evolving understanding of elastomeric polymer nanocomposites continues to expand commercial, defense, and industrial products and applications. This work explores the thermomechanical properties of elastomeric nanocomposites prepared from bisphenol A diglycidyl ether and three amine-terminated poly(propylene oxides) (Jeffamines). The Jeffamines investigated include difunctional crosslinkers with molecular weights of 2000 and 4000 g mol-1 and a trifunctional crosslinker with a molecular weight of 3000 g mol-1 . Additionally, carbon nanotubes (CNTs) are added, up to 1.25 wt%, to each thermoset. The findings indicate that the T g and storage modulus of the polymer nanocomposites can be controlled independently within narrow concentration windows, and that effects observed following CNT incorporation are dependent on the crosslinker molecular weight. Finally, the impact of crosslinker length and architecture as well as CNT addition on the molecular weight between crosslink points in the glassy and rubbery states are discussed.

A Cocatalytic Effect between Meldrum's Acid and Benzoxazine Compounds in Preparation of High Performance Thermosetting Resins.

In this work, a cocatalytic effect between Meldrum's acid (MA) and benzoxazine (Bz) compounds has been explored to build up a self-promoting curing system. Consequently, the MA/Bz reactive blend exhibits a relatively low reaction temperature compared to the required temperatures for the cross-linking reactions of the pure MA and Bz components. This feature is attractive for energy-saving processing issues. Moreover, the thermosetting resins based on the MA/Bz reactive blends have been prepared. The MA component can generate additional free volume in the resulting resins, so as to trap air in the resin matrix and consequently to bring low dielectric constants to the resins. The MA-containing agent is an effective modifier for benzoxazine resins to reduce their dielectric constants.

From Lignin-derived Aromatic Compounds to Novel Biobased Polymers.

Nowadays, the synthesis of (semi)aromatic polymers from lignin derivatives is of major interest, as aromatic compounds are key intermediates in the manufacture of polymers and lignin is the main source of aromatic biobased substrates. Phenols with a variety of chemical structures can be obtained from lignin deconstruction; among them, vanillin and ferulic acid are the main ones. Depending on the phenol substrates, different chemical modifications and polymerization pathways are developed, leading to (semi)aromatic polymers covering a wide range of thermomechanical properties. This review discusses the synthesis and properties of thermosets (vinyl ester resins, cyanate ester, epoxy, and benzoxazine resins) and thermoplastic polymers (polyesters, polyanhydrides, Schiff base polymers, polyacetals, polyoxalates, polycarbonates, acrylate polymers) prepared from vanillin, ferulic acid, guaiacol, syringaldehyde, or 4-hydroxybenzoic acid.