Structural analysis of the curing of epoxy thermosets crosslinked with hyperbranched poly(ethyleneimine)s

Novel Crosslinked Anion Exchange Membranes Based on Thermally Cured Epoxy Resin: Synthesis, Structure and Mechanical and Ion Transport Properties

Limitations in existing anion exchange membranes deter their use in the efficient treatment of industrial wastewater effluent. This work presents an approach to fabricating novel anion-conducting membranes using epoxy resin monomers like hydrophobic or hydrophilic diglycidyl ether and quaternized polyethyleneimine (PEI). Manipulating the diglycidyl ether nature, the quantitative composition of the copolymer and the conditions of quaternization allows control of the physicochemical properties of the membranes, including water uptake (20.0-330%), ion exchange capacity (1.5-3.7 mmol/g), ionic conductivity (0.2-17 mS/cm in the Cl form at 20 °C), potentiostatic transport numbers (75-97%), as well as mechanical properties. A relationship was established between copolymer structure and conductivity/selectivity trade-off. The higher the quaternized polyethyleneimine, diluent fraction, and hydrophilicity of diglycidyl ether, the higher the conductivity and the lower the permselectivity. Hydrophobic diglycidyl ether gives a much better conductivity/selectivity ratio since it provides a lower degree of hydration than hydrophilic diglycidyl ether. Different mesh and non-woven reinforcing materials were also examined. The developed membranes demonstrate good stability in both neutral and acidic environments, and their benchmark characteristics in laboratory electrodialysis cells and batch-mode dialysis experiments are similar to or superior to, commercial membranes such as Neosepta© AMX, FujiFilm© Type1, and Fumasep FAD-PET.

Noncovalent Interactions in Mechanical Response of Thermoset Epoxy Resin

Free volume in polymers is known to influence the mechanical response of the polymers. Noncovalent interactions such as hydrogen bonds, Coulombic electrostatic interactions, and van der Waals interactions are present within these free volume regions. The manuscript presents a comprehensive identification, characterization, and evolution of noncovalent interactions as a thermoset epoxy resin (typically used as an interfacial adhesive material) is subjected to uniaxial tension, shear, and shock loading. Even though noncovalent interactions dominate uniaxial tension and shear response (up to strain levels of 50% wherein covalent bond dissociation is not observed), both covalent and noncovalent interactions define response for shock loading. Van der Waals interactions dominate the response as the samples are subjected to strain levels of 50% in tension and shear. In contrast, hydrogen bonds influence shock response.

Room-Temperature Self-Healable and Mechanically Robust Thermoset Polymers for Healing Delamination and Recycling Carbon Fibers

The advocacy of carbon neutrality and circular economy encourages people to pursue self-healing and recycling of glassy thermoset polymers in a more realistic and energy-saving manner, the best being intrinsic healing under room temperature. However, the high mechanical robustness and healing ability are mutually exclusive because of their completely opposite requirements for the mobility of the polymer networks. Here, we report a dual-cross-linked network by slightly coupling the low-molecular-weight branched polyethylenimine with an ester-containing epoxy monomer in a nonstoichiometric proportion. The highly mobile and dense noncovalent hydrogen bonds at the chain branches and ends can not only complement the mechanical robustness (tensile strength of 61.6 MPa, elastic modulus of 1.6 GPa, and toughness of 19.2 MJ/m3) but also endow the glassy thermoset polymer (Tg > 40 °C) with intrinsic self-healing ability (healing efficiency > 84%) at 20 °C. Moreover, the resultant covalent adaptive network makes the thermoset polymer stable to high temperatures and solvents, yet it is readily dissolved in ethylene glycol through internal catalyzed transesterification. The application to room temperature delamination healing and carbon fiber recycling was demonstrated as a proof-of-concept.

The Use of Click-Type Reactions in the Preparation of Thermosets

Click chemistry has emerged as an effective polymerization method to obtain thermosets with enhanced properties for advanced applications. In this article, commonly used click reactions have been reviewed, highlighting their advantages in obtaining homogeneous polymer networks. The basic concepts necessary to understand network formation via click reactions, together with their main characteristics, are explained comprehensively. Some of the advanced applications of thermosets obtained by this methodology are also reviewed.

Multiscale Polymer Dynamics in Hierarchical Carbon Nanotube Grafted Glass Fiber Reinforced Composites

Carbon nanotube (CNT) grafted glass fiber reinforced epoxy nanocomposites (GFRP) present a range of stiffnesses (MPa to GPa) and length scales (μm to nm) at the fiber-matrix interface. The contribution of functionalized CNT networks to the local and bulk polymer dynamics is studied here by using a combination of torsion dynamical mechanical thermal analysis (DMTA), positron annihilation lifetime spectroscopy (PALS), and neutron scattering (NS) measurements. DMTA measurements highlight a reduction in the storage modulus (G') in the rubbery region and an asymmetric broadening of the loss modulus (G″) peak in the α-transition region. NS measurements show a suppressed hydrogen mean-square displacement (MSD) in the presence of glass fibers but a higher hydrogen MSD after grafting functionalized CNTs onto fiber surfaces. PALS measurements show greater free volume characteristics in the presence of the functionalized CNT modified composites, supporting the view that these interface layers increase polymer mobility. While NS and DMTA are sensitive to different modes of chain dynamics, the localization of functionalized nanotubes at the fiber interface is found to affect the distribution of polymer relaxation modes without significantly altering the thermally activated relaxation processes.

High Mechanical and Thermal Properties of Epoxy Composites with Liquid Crystalline Polyurethane Modified Graphene

Graphene nanosheets (GNs) often result in incompatibility with the hydrophobic polymer matrix, and the tendency to form aggregates during processing. Herein, liquid crystalline polyurethane modified GNs (GPLP) were obtained by π⁻π stacking interactions between GNs and perylene bisimide derivatives, and then in-situ polymerization of liquid-crystalline polyurethane. Spectroscopic studies, elemental analysis, and thermal properties confirmed the successful π⁻π stacking and the integrated structure of GPLP. The good dispersion state of GPLP in the epoxy matrix (EP), and the strong interactions between GPLP and EP, lead to the significant improvement of the thermal and mechanical performance of the GPLP/EP composites. The impact strength, Young's modulus, tensile strength, and toughness of the GPLP/EP composites with 1.47 wt % GNs reached the highest values of 54.31 kJ/m², 530.8 MPa, 112.33 MPa and 863 J/m³, which significantly increased by 210%, 57%, 143%, and 122% compared to that of neat epoxy, respectively. As well, the glass transition temperature increased by a notable 33 °C. It is hoped that this work can be used to exploit more efficient methods to overcome the poor adhesion between GNs and polymers.

Epoxy Vitrimers: The Effect of Transesterification Reactions on the Network Structure

Vitrimers are covalently crosslinked polymers that behave as conventional thermosets below the glass transition temperature (Tg) but can flow above a particular temperature, Tv > Tg, by bond exchange reactions. In epoxy vitrimers, transesterification reactions are responsible for their behavior at T > Tv that enables flow, thermoforming, recycling, self-healing and stress relaxation. A statistical analysis based on the fragment approach was performed to analyze the evolution of the network structure of epoxy vitrimers during transesterification reactions. An analytical solution was obtained for a formulation based on a diepoxide and a dicarboxylic acid. A numerical solution was derived for the reaction of a diepoxide with a tricarboxylic acid, as an example of the way to apply the model to polyfunctional monomers. As transesterification acts as a disproportionation reaction that converts two linear fragments (monoesters) into a terminal fragment (glycol) and a branching fragment (diester), its effect on network structure is to increase the concentration of crosslinks and pendant chains while leaving a sol fraction. Changes in the network structure of the epoxy vitrimer can take place after their synthesis, during their use at high temperatures, a fact that has to be considered in their technological applications.

Sequential curing of off-stoichiometric thiol–epoxy thermosets with a custom-tailored structure

A novel sequential dual-curable thermosetting system with a custom-tailorable structure based on a click thiol–epoxy reaction and excess epoxy is presented.

Environmentally-friendly processing of thermosets by two-stage sequential aza-Michael addition and free-radical polymerization of amine–acrylate mixtures

A new dual-curing, solvent-free process is described for the preparation of tailor-made materials from offstoichiometric amine–acrylate formulations.