Porous Hydrogels: Present Challenges and Future Opportunities.

In this feature article, we critically review the physical properties of porous hydrogels and their production methods. Our main focus is nondense hydrogels that have physical pores besides the space available between adjacent cross-links in the polymer network. After reviewing theories on the kinetics of swelling, equilibrium swelling, the structure-stiffness relationship, and solute diffusion in dense hydrogels, we propose future directions to develop models for porous hydrogels. The aim is to show how porous hydrogels can be designed and produced for studies leading to the modeling of physical properties. Additionally, different methods that are used for making hydrogels with physically incorporated pores are briefly reviewed while discussing the potentials, challenges, and future directions for each method. Among kinetic methods, we discuss bubble generation approaches including reactions, gas injection, phase separation, electrospinning, and freeze-drying. Templating approaches discussed are solid-phase, self-assembled amphiphiles, emulsion, and foam methods.

Vitrimerization of Crosslinked Unsaturated Polyester Resins: A Mechanochemical Approach to Recycle and Reprocess Thermosets.

Unsaturated polyester resins (UPRs) are expansively used in different applications and recycling the significant amounts of UPR waste is still a universal problem. Vitrimerization is a feasible, environmental-friendly, cost effective, and operative method, which can be used for recycling the crosslinked UPRs. In this method, the thermoset permanent network is changed into a dynamic network similar to the vitrimer-type polymers. The results show that the existence of a transesterification catalyst in the system significantly enhances the efficiency of vitrimerization. The vitrimerized UPR thermosets can be reprocessed three times with mechanical properties comparable to the initial UPR. The results show that the excess of external hydroxyl groups in the system can prevent the formation of zinc ligand complexes in the network and consequently reduce the crosslinked density and mechanical properties of vitrimerized samples. The vitrimerized thermoset powder can be reprocessed through injection molding, extrusion, and compression molding which are conventional thermoplastic processing techniques. The unrecyclable UPR thermoset wastes can be recycled and reused through vitrimerization with the least loss in mechanical properties.

Cellulose Nanocrystals: Accelerator and Reinforcing Filler for Epoxy Vitrimerization.

The novel vitrimerization concept of converting permanently cross-linked networks of thermoset polymers into dynamic exchangeable networks often relies on transesterification reactions. Transesterification exchange reactions, for example, in epoxy vitrimers, are usually contingent on equivalent ratios of hydroxyl to ester groups and large amounts of catalysts to achieve proper dynamic exchange capability. In general, postconsumer epoxy cured with anhydrides contains very few hydroxyl groups in the network, and it is challenging to convert it into efficient dynamic networks by vitrimerization. Here, we demonstrate that introducing cellulose nanocrystals as feedstock of external hydroxyl groups in the mechanochemical vitrimerization process could improve the exchange reaction rate as well as the thermomechanical properties of the vitrimerized epoxy. This work offers an effective way to recycle and reprocess postconsumer epoxy/anhydride waste with inherent low ratios of hydroxyl to ester groups into higher value-added vitrimer nanocomposites.

Porous hollow fibers with controllable structures templated from high internal phase emulsions.

A technique to fabricate hollow fibers with porous walls via templating from high internal phase emulsions (HIPEs) has been demonstrated. This technique provides an environmentally friendly process alternative to conventional methods for hollow-fiber productions that typically use organic solvents. HIPEs containing acrylate monomers were extruded into an aqueous curing bath. Osmotic pressure effects, manipulated through differences in salt concentration between the curing bath and the aqueous phase within the HIPE were used to control the hollow structures of polyHIPE fibers. The technique was used to produce porous fibers (with millimeter-scale diameters and micronscale pores) having a hollow core (with a diameter of 50%-75% of the fiber diameter). Two potential applications of the hollow fibers were demonstrated. In vitro drug release studies using these hollow fibers show a controlled release profile that is consistent with the microstructure of the porous fiber wall. In addition, the presence of pores in the walls of polyHIPE fibers also enable size-selective loading and separation of functional materials from an external suspension.

Catalyst-Free Mechanochemical Recycling of Biobased Epoxy with Cellulose Nanocrystals.

Mechanochemical vitrimerization, as a method to recycle cross-linked thermosets by converting the permanent network into a recyclable and reprocessable vitrimer network, inevitably requires a catalyst to accelerate the bond exchange reactions. Here, we demonstrate a catalyst-free approach to achieve the recycling of a cross-linked biobased epoxy into high-performance nanocomposites with cellulose nanocrystals (CNCs). CNCs provide abundant free hydroxyl groups to promote the transesterification exchange reactions while also acting as reinforcing fillers for the resultant nanocomposites. This technique introduces an effective way to fabricate high-performance thermoset nanocomposites based on recycled polymers in an ecofriendly way, promoting the recycle and reuse of thermosets as sustainable nanocomposites for different applications.

Vitrimerization: Converting Thermoset Polymers into Vitrimers.

Thermoset polymers with permanently cross-linked networks have outstanding mechanical properties and solvent resistance, but they cannot be reprocessed or recycled. On the other hand, vitrimers with covalent adaptable networks can be recycled. Here we provide a simple and practical method coined as "vitrimerization" to convert the permanent cross-linked thermosets into vitrimer polymers without depolymerization. The vitrimerized thermosets exhibit comparable mechanical properties and solvent resistance with the original ones. This method allows recycling and reusing the unrecyclable thermoset polymers with minimum loss in mechanical properties and enables closed-loop recycling of thermosets with the least environmental impact.

Design Strategy for Porous Composites Aimed at Pressure Sensor Application.

Flexible and highly sensitive pressure sensors have versatile biomedical engineering applications for disease diagnosis and healthcare. The fabrication of such sensors based on porous structure composites usually requires complex, costly, and nonenvironmentally friendly procedures. As such, it is highly desired to develop facile, economical, and environment-friendly fabrication strategies for highly sensitive lightweight pressure sensors. Herein, a novel design strategy is reported to fabricate porous composite pressure sensors via a simple heat molding of conductive fillers and thermoplastic polyurethane (TPU) powders together with commercially available popcorn salts followed by water-assisted salt removal. The obtained TPU/carbon nanostructure (CNS) foam sensors have a linear resistance response up to 60% compressive strain with a gauge factor (GF ) of 1.5 and show reversible and reproducible piezoresistive properties due to the robust electrically conductive pathways formed on the foam struts. Such foam sensors can be potentially utilized for guiding squatting exercises and respiration rate monitoring in daily physical training.

Vitrimerization: A Novel Concept to Reprocess and Recycle Thermoset Waste via Dynamic Chemistry.

A new approach for reprocessing of existing thermoset waste is presented. This work demonstrates that unrecyclable thermoset materials can be reprocessed using the concept of associative dynamic bonding, vitrimers. The developed recycling methodology relies on swelling the thermoset network into a solution of a catalyst, which enables transesterification reactions allowing dynamic bond exchange between ester and hydroxyl groups within the thermoset network. Thermal and mechanical properties for recycled polyurethane and epoxy networks are studied and a strategy to maintain the properties of recycled materials is discussed. The developed methodology promises recycling and even upcycling and reprocessing of previously thought intractable materials. Moreover, processability of vitrimerized thermosets with common thermoplastic manufacturing methods opens up the possibility of tuning recycled networks by adding nanoparticles. This flexibility keeps the application window of recycled thermosets very broad.

Biomimetic Water-Responsive Self-Healing Epoxy with Tunable Properties.

As dynamic cross-linking networks are intrinsically weaker than permanent covalent networks, it is a big challenge to obtain a stiff self-healing polymer using reversible networks. Inspired by the self-healable and mechanically adaptive nature of sea cucumber, we design a water-responsive self-healing polymer system with reversible and permanent covalent networks by cross-linking poly(propylene glycol) with boroxine and epoxy. This double cross-linked structure is self-healing due to the boroxine reversible network as well as showing a room-temperature tensile modulus of 1059 MPa and a tensile stress of 37 MPa, on a par with classic thermosets. The dynamic boroxine bonds provide the self-healing response and enable up to 80% recovery in modulus and tensile strength upon water contact. The system shows superior adhesion to metal substrates by comparison with the commercial epoxy-based structural adhesive. Besides, this system can change modulus from a stiff thermoset to soft rubber (by a factor of 150) upon water stimulus, enabling potential applications of either direct or transform printing for micro/nanofabrication. Moreover, by incorporating conductive nanofillers, it becomes feasible to fabricate self-healing and versatile strain/stress sensors based on a single thermoset, with potential applications in wearable electronics for human healthcare.

Stretchable elastomer composites with segregated filler networks: effect of carbon nanofiller dimensionality.

Electrically conductive elastomer composites (CECs) have great potential in wearable and stretchable electronic applications. However, it is often challenging to trade off electrical conductivity and mechanical flexibility in melt-processed CECs for wearable electronic applications. Here, we develop CECs with high electrical conductivity and mechanical elasticity by controlling the segregated networks of carbon nanofillers formed at the elastomer interface. The carbon nanofiller dimensionality has a significant influence on the electrical and mechanical properties of thermoplastic polyurethane (TPU) composites. For instance, 3D branched carbon nanotubes (carbon nanostructures, CNSs) have a very low percolation threshold (Φ C = 0.01 wt%), which is about 8-10 times lower than that of 1D carbon nanotubes (CNTs) and 2D graphene nanosheets (GNSs). Besides, the TPU/CNS system has a higher electrical conductivity than other fillers at all filler contents (0.05-2 wt%). On the other hand, TPU/CNT systems can retain high elongation at break, whereas for the TPU/GNS systems elongation at break is severely deteriorated, especially at a high filler content. Different electrical and mechanical properties in the TPU-based CECs enable potential applications in flexible conductors/resistors and stretchable strain sensors, respectively.

Interface Design Strategy for the Fabrication of Highly Stretchable Strain Sensors.

Simultaneously achieving high piezoresistive sensitivity, stretchability, and good electrical conductivity in conductive elastomer composites (CECs) with carbon nanofillers is crucial for stretchable strain sensor and electrode applications. Here, we report a facile and environmentally friendly strategy to realize these three goals at once by using branched carbon nanotubes, also known as the carbon nanostructure (CNS). Inspired by the brick-wall structure, a robust segregated conductive network of a CNS is formed in the thermoplastic polyurethane (TPU) matrix at a very low filler fraction, which renders the composite very good electrical, mechanical, and piezoresistive properties. An extremely low percolation threshold of 0.06 wt %, currently the lowest for TPU-based CECs, is achieved via this strategy. Meanwhile, the electrical conductivity is up to 1 and 40 S/m for the composites with 0.7 and 4 wt % CNS, respectively. Tunable piezoresistive sensitivity dependent on CNS content is obtained, and the composite with 0.7 wt % filler has a gauge factor up to 6861 at strain ε = 660% (elongation at break is 950%). In addition, this strategy also renders the composites' attractive tensile modulus. The composite with 3 wt % CNS shows 450% improvement in Young's modulus versus neat TPU. This work introduces a facile strategy to fabricate highly stretchable strain sensors by designing CNS network structures, advancing understanding of the effects of polymer-filler interfaces on the mechanical and electrical property enhancements for polymer nanocomposites.

Dielectric Properties of Bio-Based Diphenolate Ester Epoxies.

Thermoset bio-based diglycidyl ether of diphenolate esters (DGEDP) exhibit comparable mechanical properties as petroleum-derived diglycidyl ether of bisphenol A (DGEBA), whereas DGEDP is derived from levulinic acid, a safe and readily renewable feedstock. To determine the potential replacement of DGEBA as dielectric materials, a series of DGEDP-esters (i.e., methyl, ethyl, propyl, and butyl esters) were synthesized and studied. Broadband dielectric spectroscopy revealed that DGEDP-propyl has the highest dielectric constant in the series, comparable to DGEBA. Differences in the dielectric properties of DGEDP-esters is attributed to the interplay of segmental, small local, and side-chain motions on one hand and free volume and steric hindrance on the other.

Modeling compressive behavior of open-cell polymerized high internal phase emulsions: effects of density and morphology.

The compressive behavior of poly(HIPE) foams was studied using the developed micromechanics based computational model. The model allowed identifying the morphological parameters governing the foam compressive behavior. These parameters comprise: (i) foam density, (ii) Sauter mean diameter of voids calculated from the morphological analysis of the polydispersed microstructure of poly(HIPE), and (iii) polymer/strut characteristic size identified as the height of the curvilinear triangular cross-section. The model prediction compared closely with the experiments and considered both the linear and plateau regions of the compressive poly(HIPE) behavior. The computational model allows the prediction of structure-property relationships for poly(HIPE) foams with various relative densities and open cell microstructure using the input parameters obtained from the morphology characterization of the poly(HIPE). The simulations provide a pathway for understanding how tuning the manufacturing process can enable the optimal foam morphology for targeted mechanical properties.

Effect of Curing Rate on the Microstructure and Macroscopic Properties of Epoxy Fiberglass Composites.

Curing rates of an epoxy amine system were varied via different curing cycles, and glass-fiber epoxy composites were prepared using the same protocol, with the aim of investigating the correlation between microstructure and composite properties. It was found that the fast curing cycle resulted in a non-homogenous network, with a larger percentage of a softer phase. Homogenized composite properties, namely storage modulus and quasi-static intra-laminar shear strength, remained unaffected by the change in resin microstructure. However, fatigue tests revealed a significant reduction in fatigue life for composites cured at fast curing rates, while composites with curing cycles that allowed a pre-cure until the critical gel point, were unaffected by the rate of reaction. This result was explained by the increased role of epoxy microstructure on damage initiation and propagation in the matrix during fatigue life. Therefore, local non-homogeneities in the epoxy matrix, corresponding to regions with variable crosslink density, can play a significant role in limiting the fatigue life of composites and must be considered in the manufacturing of large scale components, where temperature gradients and significant exotherms are expected.