Low Cross-linking Density Research Articles

Designing from biobased to closed-loop circularity: Flexible dynamic polyimine-amide networks

Dynamic polyimine-amide networks with exceptional properties, including high flexibility, excellent thermal stability and dual closed-loop circularity were designed by combining dynamic covalent imine-functionalities with amide-chemistry. The solvent free up-scalable synthesis started from carboxyl-functionalization of lignin-derivable aldehydes followed by melt polycondensation with a triamine to form two dynamic networks (PIAX1 and PIAX2, respectively). While previously reported vanillin-derived polyimine thermosets were typically non-flexible and brittle, our polyimine-amides are flexible with elongation at break 380 % for PIAX1 and 65 % for PIAX2, where the higher flexibility of PIAX1 is deduced to the lower glass transition temperature and crosslinking density. Both materials illustrate fast stress relaxation even at low temperature, down to 50 °C in the case of PIAX1. Retained and even improved mechanical properties were observed after several cycles of thermal reprocessing, e.g., after three reprocessing cycles by hot pressing, PIAX1 recovered 116 % of original tensile stress and 126 % of original elongation at break, while chemical recycling under acidic conditions at room temperature yielded repolymerizable trialdehydes and triamines. Furthermore, rapid self-healing and shape memory behaviour at low temperature were demonstrated for PIAX1. A promising molecular design, further tuneable by choice of aldehyde and triamine, is demonstrated enabling high performance and dual closed-loop circularity.

Soft elasticity enabled adhesion enhancement of liquid crystal elastomers on rough surfaces

The fabrication of pressure-sensitive adhesives (PSA) using liquid crystal elastomers (LCE) that are tolerant to substrate roughness is explored in this work. Traditional soft adhesives are designed by maintaining a balance between their cohesive strength and compliance. However, rough surfaces can significantly affect the adhesion strength of PSAs. Lowering the stiffness of the adhesive by reducing the cross-linking density or using additives can improve contact on rough surfaces. But this also decreases the cohesive strength and affects the overall performance of the adhesive. Additive-free LCE-based adhesives are shown to overcome these challenges due to their unique properties. Soft elasticity of LCE and low cross-link density contribute to their high compliance, while moderate cross-linking provides finite strength. The effect of contact time and substrate roughness on the adhesive performance is evaluated using probe-tack, indentation, lap shear, and static loading experiments. The unique combination of properties offered by LCE can lead to the development of roughness-tolerant adhesives, thereby broadening the application scope of PSAs.

Protocatechualdehyde-based epoxy vitrimer with low dielectric, excellent flame retardancy, and recyclability

Conventional bisphenol A epoxy resins cannot meet the requirements of the new generation of electronic/electrical industries in terms of dielectric properties and flame retardancy and are challenging to reprocess after molding, resulting in waste of resources and environmental hazards. Therefore, a tetrafunctional active ester curing agent (TIE) was successfully synthesized by a one-pot method using biomass protocatechualdehyde as raw materials, followed by curing DGEBA to prepare an epoxy vitrimer (TIE/DGEBA). MHHPA/DGEBA was chosen for comparison since the –OH was also absent after curing. The results showed that TIE/DGEBA had good combinatorial performance, thanks to the unique structure of TIE, including active ester units, siloxane chains, and aromatic Schiff base groups. TIE/DGEBA possessed a lower dielectric constant (2.9 vs. 4.0) compared to MHHPA/DGEBA, which was attributed to the synergistic effect of the less-polar siloxane chain segments, bulky benzene structure, and lower cross-linking density (1971 vs. 733 mol/cm3) of TIE/DGEBA. The aromatic Schiff base not only imparted good thermal stability (T30% = 418.0 vs. 422.2 °C) to TIE/DGEBA but ensured that it allowed for recycling and reprocessing. After solvent recycling, the resin can be remolded without significant change in tensile strength (53.0 vs. 46.3 MPa). TIE/DGEBA displayed better flame retardancy with higher residual weight (28.2 % vs. 5.5 %), lower total heat release (29.6 vs. 43.5 kJ/g), and good resistance to thermal oxidization. The structural and performance characteristics of TIE/DGEBA offer a strategy for designing multifunctional epoxy-based materials in electrical/electronic applications.

FEATURES OF THE INTERACTION AND FORMATION OF NANOSTRUCTURED CHITOSAN SYSTEMS DURING IONOTROPIC GELATION

Bombyx mori chitosan nanoparticles (ChS NPs) were obtained and the kinetic aspects of their size distribution depending on the degree of nonequilibrium of the synthesis by ionotropic gelation using sodium tripolyphosphate (TPP) were studied. The size of ChS NPs was greatly influenced by the ChS concentration, which varied from 0.5 to 10 mg/ml. The size of the NPs increased from 100 to 300 nm when the ChS concentration increased from 0.5 to 10 mg/ml and while maintaining all other parameters of the reaction. An increase in the ChS to-TPP ratio corresponded to a lower cross-linking density and thus ensured slower kinetics for NP formation alongside the formation of large chitosan/TPP aggregates. There was an almost threefold increase in the particle size when the pH of the medium increased from 3.8 to 4.7; thus, the size of ChS NPs depends on the pH of the solution. There was a slight agglomeration of NPs from 363 to 642 nm with an increase in the NP synthesis time from 4 to 24 h. According to calculations, the maximum interaction energy is 50.4 and 69.4 kcal/mol for deprotonated TPP (P3O105-); after adiabatic transfer of the proton, the value decreases to 32.9–33.5 kcal/mol. The maximum interaction energy is –12.8 kcal/mol for the initially protonated TPP (H4P3O10-).

Shape memory and mechanical properties of ESO modified epoxy/polyurethane semi-interpenetrating polymer networks for smart plaster

While numerous studies have explored the potential of shape memory materials in medical applications, the current research output is not enough for significant productivity in industrial products. In this research, a type of shape memory orthopedic plaster was fabricated by using epoxy/polyurethane interpenetrating networks modified with epoxidized soybean oil (ESO) and then compared with some commercial plasters. Polymer networks that were produced could be tailored to have glass transition temperatures (Tgs) within the range of 70–90 °C by changing the percentage composition of polyurethane and soybean oil. Shape recovery and fixity ratios in all samples were above 95 and 97 %. DMA results showed that IPNs with variable amount of soybean oil had lower glass transition temperature (Tg) and cross-link densities compared to IPNs without soybean oil. Based on the tensile test results, most samples exhibited an elastic modulus in the range of 280–400MPa, a level deemed somewhat acceptable when compared to commercial samples. Light microscope images had shown that the increase of polyurethane led to the phase separation of polymers, which was improved by the addition of soybean oil.

Impact of Annealing Chemistry on the Properties and Performance of Microporous Annealed Particle Hydrogels.

Microporous annealed particle (MAP) hydrogels are a promising class of in situ-forming scaffolds for tissue repair and regeneration. While an expansive toolkit of annealing chemistries has been described, the effects of different annealing chemistries on MAP hydrogel properties and performance have not been studied. In this study, we address this gap through a controlled head-to-head comparison of poly(ethylene glycol) (PEG)-based MAP hydrogels that were annealed using tetrazine-norbornene and thiol-norbornene click chemistry. Characterization of material properties revealed that tetrazine click annealing significantly increases MAP hydrogel shear storage modulus and results in slower in vitro degradation kinetics when microgels with a higher cross-link density are used. However, these effects are muted when the MAP hydrogels are fabricated from microgels with a lower cross-link density. In contrast, in vivo testing in murine critical-sized calvarial defects revealed that these differences in physicochemical properties do not translate to differences in bone volume or calvarial defect healing when growth-factor-loaded MAP hydrogel scaffolds are implanted into mouse calvarial defects. Nonetheless, the impact of tetrazine click annealing could be important in other applications and should be investigated further.

Molecular Dynamics Simulations of Gas Transport Properties in Cross-Linked Polyamide Membranes: Tracing the Morphology and Addition of Silicate Nanotubes.

This study employs molecular dynamics (MD) simulations to fundamentally provide insight into the role of cross-link density in the CO2 separation properties of interfacially polymerized polyamide (PA) membranes. For this purpose, two atomistic models of pure polyamide membranes with different cross-link densities are constructed by MD simulations to conceptually determine how the fractional free volume of polyamide affects the gas separation performance of the membrane. The PA membrane with a lower cross-link density (LCPA) shows a higher gas diffusion coefficient, a lower gas solubility coefficient, and a higher gas permeability than the PA membrane with a higher cross-link density (HCPA). Moreover, the pristine and modified silicate nanotubes (SNTs) as the fast gas transport channels are incorporated into the polyamide membranes to assess the effect of the SNT/PA interface chemistry on the CO2 separation properties of the membranes. SNTs are systematically modified by three modifying agents with different CO2-philic groups and different interfacial interaction energies with the polyamide matrix. The results of MD simulations demonstrate that the incorporation of silicate nanotubes into the PA matrix increases the gas diffusivity and permeability and decreases the CO2/gas selectivity. Moreover, the membranes containing modified SNTs possessing high CO2-philicity and high SNTs/PA interfacial interactions show a high CO2 separation performance.

Engineering shrinkage resistance of nano-structured hydrogels in seawater for fast uranium capture

Synthetic hydrogels, which are 3D networks composed of crosslinked hydrophilic polymer chains, have been created using various crosslinked ways for uranium capture due to full exposure of chelate functional groups. However, the conventional honeycomb-like hydrogels exhibit non-connected micropore structure, which locks abundant water molecules and hence is unfavorable for mass transfer. Herein, we demonstrate a simple synthesis of the nano-structured hydrogel with activated nanogels as nano-crosslinkers (denoted as PGP hydrogel), and probe into the relationship between hydrogel structural parameters and uranium adsorption kinetics. It is found that the PGP hydrogel with a low crosslinking density of ∼0.02 mol/m3 possesses interconnected micropores, thereby facilitating diffusion of uranyl ions (UO22+) in 3D polymeric networks. Furthermore, a freeze-assisted soaking strategy is proposed to endow PGP hydrogel (denoted as target hydrogel) with strong shrinkage resistance in natural seawater, resulting in a ∼40 % increase in water permeance to reach 369 Lm−2h−1MPa−1. Correspondingly, the adsorption equilibrium time is shortened to ∼11 days. Although the uranium uptake capacity of PGP hydrogels with and without freeze-assisted soaking treatment remain basically unchanged (∼9.73 mg-U/g-Ads), a high uranium adsorption rate of 0.88 mg/g/d for the target hydrogel surpasses most currently available polymer adsorbents. The presented strategy is generalizable to a broader range of applications involving the adsorption of economically valuable ions in seawater or brine lake.

Exploring the interplay of structure, optical, magnetic and radiation shielding properties in GeO2/Bismuth borate glasses

This article investigates the structural, chemical, optical, and magnetic properties of borate bismuth glasses doped with increasing GeO2 concentrations. X-ray diffraction (XRD), Raman spectroscopy, density measurements, thermal analysis, UV–Vis absorption, and VSM studied the impact of GeO2 on the glass network. The XRD results of the study confirmed that glass is amorphous. The bulk density increased while the molar volume decreased with increasing GeO2 concentration. Glass transition temperature (Tg) slightly decreases due to weaker Ge–O bonds and lower cross-linking density. Crystallization temperature (Tc) increased as GeO2 disrupts the network, hindering crystal nucleation and growth. Optical band gap narrows with increasing GeO2, with direct and indirect gaps decreasing. Urbach energy (EU) rises with GeO2, indicating increased disorder in the glass network. Refractive index (n) and extinction coefficient (k) both increased with GeO2, attributed to non-bridging oxygen formation. VSM measurements reveal an increase in saturation magnetization from 0.1 to 0.3 emu/g with increasing GeO2 content. Mass attenuation coefficient (MAC) and linear attenuation coefficient (LAC) investigations showed the efficiency of the glass system for radiation attenuation at varying energies. The unique properties of these borate bismuth glasses doped with GeO2 show promise for various technological advancements in optoelectronics and radiation shielding.

Increasing the cross-link density in a dual dissociative and associative polythiourethane covalent adaptable network improves both creep resistance and extrudability

There have been significant, recent reports of the effects of cross-link density on the relaxation dynamics and processability of exclusively associative covalent adaptable networks (CANs), also called vitrimers. Similar characterization is lacking for the substantial number of CANs that exhibit dual associative and dissociative dynamic covalent chemistries. Polythiourethane (PTU), which is a thiol-based analogue of polyurethane, is one such polymer class that exhibits both dissociative and associative dynamic chemistries. Here, we tune the cross-link density of PTU CANs by systematic incorporation of long-chain and short-chain thiols. As would be commonly expected, relative to networks with lower cross-link density, PTU networks with higher cross-link density exhibit improved creep resistance at 60 °C and 100 °C. However, we also observe faster elevated-temperature stress relaxation with increasing cross-link density, which is attributable to the dual nature of the PTU dynamic chemistry, with the associative dynamic mechanism being increasingly favored and becoming increasingly more rapid with increasing cross-link density. Because of this shift, our PTU CAN with the highest cross-link density undergoes facile melt extrusion at 150 °C; in contrast, our PTU CAN of lowest cross-link density cannot be melt extruded. Overall, this work highlights the significance of cross-link density in networks exhibiting two or more dynamic chemistries and how that may lead to otherwise unexpected outcomes, e.g., better processability at higher cross-link density.

(Thermo)mechanical and chemical characteristics of photochemically crosslinked acrylates from bio‐ and fossil‐based origin

AbstractThe acrylates with oligomers and monomers from (partially) bio‐based feedstock become available at (semi‐)industrial scale, which can be processed through photochemical crosslinking for applications in coatings, additive manufacturing, electronics, or inks. Although fossil‐ and bio‐based acrylates may have a similar chemical composition, it requires good understanding of processing and structure–property relationships as minor changes in microstructure may strongly alter the performance. A comparative study on mechanical properties and chemical structure of bio‐ and fossil‐based acrylates with different functionalities and backbone structures reveals higher ductility of bio‐based acrylates, in relation with a more complex organization of the intrinsic molecular structure. The latter is confirmed by mechanical testing and visco‐elastic characteristics (dynamic mechanical analysis) yielding lower stiffness and higher dampening of bio‐based acrylates, in parallel with a lower glass transition temperature (differential scanning calorimetry). The complex molecular arrangements include a nanoscale morphology with ordered structure (X‐ray diffraction), conformational changes (infrared spectroscopy), and a residual high‐molecular weight fraction (size exclusion chromatography). The visco‐elastic calculations indicate only 4% to 5% lower crosslinking density and around 10% higher mean molar mass of the polymer chains segments between chemical crosslinks and trapped chain entanglements, which explain the unique structure and performance of bio‐based acrylates.

Crack-Resistant and Tissue-Like Artificial Muscles with Low Temperature Activation and High Power Density.

Constructing soft robotics with safe human-machine interactions requires low-modulus, high-power-density artificial muscles that are sensitive to gentle stimuli. In addition, the ability to resist crack propagation during long-term actuation cycles is essential for a long service life. Herein, a material design is proposed to combine all these desirable attributes in a single artificial muscle platform. The design involves the molecular engineering of a liquid crystalline network with crystallizable segments and an ethylene glycol flexible spacer. A high degree of crystallinity can be afforded by utilizing aza-Michael chemistry to produce a low covalent crosslinking density, resulting in crack-insensitivity with a high fracture energy of 33720 J m-2 and a high fatigue threshold of 2250 J m-2. Such crack-resistant artificial muscle with tissue-matched modulus of 0.7MPa can generate a high power density of 450W kg-1 at a low temperature of 40°C. Notably, because of the presence of crystalline domains in the actuated state, no crack propagation is observed after 500 heating-cooling actuation cycles under a static load of 220kPa. This study points to a pathway for the creation of artificial muscles merging seemingly disparate, but desirable properties, broadening their application potential in smart devices.

Photografted Zwitterionic Hydrogel Coating Durability for Reduced Foreign Body Response to Cochlear Implants.

The durability of photografted zwitterionic hydrogel coatings on cochlear implant biomaterials was examined to determine the viability of these antifouling surfaces during insertion and long-term implant usage. Tribometry was used to determine the effect of zwitterionic coatings on the lubricity of surfaces with varying hydration levels, applied normal force, and time frame. Additionally, flexural resistance was investigated using mandrel bending. Ex vivo durability was assessed by determining the coefficient of friction between tissues and treated surfaces. Furthermore, cochlear implantation force was measured using cadaveric human cochleae. Hydrated zwitterionic hydrogel coatings reduced frictional resistance approximately 20-fold compared to uncoated PDMS, which led to significantly lower mean force experienced by coated cochlear implants during insertion compared to uncoated systems. Under flexural force, zwitterionic films resisted failure for up to 60 min of desiccation. The large increase in lubricity was maintained for 20 h under continual force while hydrated. For loosely cross-linked systems, films remained stable and lubricious even after rehydration following complete drying. All coatings remained hydrated and functional under frictional force for at least 30 min in ambient conditions allowing drying, with lower cross-link densities showing the greatest longevity. Moreover, photografted zwitterionic hydrogel samples showed no evidence of degradation and nearly identical lubricity before and after implantation. This work demonstrates that photografted zwitterionic hydrogel coatings are sufficiently durable to maintain viability before, during, and after implantation. Mechanical properties, including greatly increased lubricity, are preserved after complete drying and rehydration for various applied forces. Additionally, this significantly enhanced lubricity translates to significantly decreased force during insertion of implants which should result in less trauma and scarring.

Effects of curing conditions on adhesive and fatigue properties of hydrogenated epoxy resins in bulk state and single-lap-joint configuration

The adhesive and fatigue properties of single-lap-joint (SLJ) epoxy resins are strongly affected by not only the network structures of an epoxy resin adhesive, but also the adhesive strength between an adhesive and adherend. These properties are closely related to curing conditions, such as curing temperature, and curing time, and their combination. In this study, the effects of curing conditions on an adhesive and fatigue properties of an epoxy resin [hydrogenated diglycidyl ether of bisphenol A and cyclohexanebis(methylamine)] were investigated via a single lap-shear test, a fatigue test, and wide-angle X-ray scattering (WAXS). The tensile and fatigue properties of a bulk epoxy resin were also investigated for comparison. The SLJ and bulk samples were prepared with three different curing conditions to obtain high (H), middle (M), and low (L) cross-linking (CL) density of the network structure. They are denoted as “CL-H″, “CL-M″, and “CL-L″, respectively. SLJ-CL-L and Bulk-CL-L exhibited the lowest degree of cure, adhesive and tensile strengths of the three. Both Bulk-CL-H and Bulk-CL-M yielded in tensile testing and Bulk-CL-M was tougher than Bulk-CL-H. In contrast, neither SLJ-CL-H and SLJ-CL-M yielded, and SLJ-CL-H was tougher than SLJ-CL-M. This can be ascribed that the SLJ sample broke before yielding of epoxy resin adhesive and SLJ-CL-H exhibited a higher adhesive strength between the epoxy resin and adherend than SLJ-CL-M. This was confirmed by X-ray photoelectron spectroscopy. Concerning the fatigue properties, SLJ-CL-H exhibited enhanced properties than SLJ-CL-M, although Bulk-CL-M were better than Bulk-CL-H. This trend corresponds well with difference between single lap-shear and tensile tests. The WAXS measurements revealed that the strain values calculated using distance between molecular chains were almost identical for the bulk and SLJ samples immediately before yielding and breaking, respectively. This is because an increase in stress stopped at yielding of bulk and break of SLJ.

Copper diffusion into epoxy under high temperature

High reliability is critical for semiconductors, especially power semiconductors, which usually work in high temperature conditions. One important reliability issue for power semiconductors is the copper diffusion into polymers. Copper diffusion accelerated polymer decomposition. Also, severe copper diffusion even causes short circuits. Although copper diffusion has been identified, the reason for the initiation of copper diffusion is still not well understood. Here, we encapsulated copper substrate with five different epoxies to investigate the reason for the initiation of copper diffusion. The influence of epoxy impurity elements and epoxy cross-link density was investigated. The results show that the initiation of copper diffusion into epoxy is highly related to the impurity elements, sulfur (S) and chlorine (Cl). In contrast, the low cross-link density of the epoxy cannot initiate copper diffusion. It implies that using S-free and Cl-free epoxy is necessary for semiconductors requiring high reliability.

Effect of CTBN and Nano-SiO2 modified epoxy resin powder on the mechanical properties of cement-based composites

The high brittleness of oil well cement seriously affects the cementing quality. In order to maintain the safe and effective production of oil and gas wells, it is crucial to employ solid flexible fillers to reduce the brittleness of cement. In this study, a new type of flexible resin material (C-ER) was prepared by modifying epoxy resin with liquid nitrile butadiene rubber (CTBN) and nano-SiO2. The chemical composition, hydrophilicity, dispersibility, microstructure, and mechanical properties of C-ER were studied by FT-IR, XPS, contact angle, UV spectroscopy, SEM, DMA, and uniaxial compression. The results show that CTBN reacted with the epoxy group successfully. Due to the presence of SiO2, C-ER has good hydrophilicity and dispersion properties. The introduction of flexible segments makes C-ER have a lower modulus and crosslinking density, and its compressive elastic modulus is reduced by 47.4 % compared with pure resin. In addition, the effect of C-ER on the performance of oil well cement was tested. The results show that the addition of C-ER does not affect the fluidity of cement slurry and can improve the strain capacity of cement stone under load. Compared with blank cement and unmodified resin cement, the flexural strength of cement sample containing 4 % C-ER particles increased by 9.0 % and 25.1 %, respectively, and the elastic modulus of cement stone decreased by 31.2 % and 25.0 %, respectively. SEM images show that C-ER maintains good dispersion and bonding properties in cement matrix. C-ER can reduce the brittleness of cement effectively through its own elastic deformation and debonding to the cement matrix.

Durable anti-icing coating with stability based on self-regulating oil storage layer

In recent years, slippery liquid-infused porous surfaces (SLIPSs) have emerged as a potential alternative to superhydrophobic surfaces (SHSs) due to their exceptional anti-icing properties. However, the current challenge lies in the significant lubricant consumption of SLIPSs, which poses a major obstacle to coating durability. In this study, we developed a durable anti-icing coating with stability (DASS), which combines SHSs and SLIPSs. By incorporating high adsorption particles and low cross-linking density rubber into the oil storage layer, the coating achieves an increased oil storage capacity while effectively inhibiting lubricant loss. Additionally, a superhydrophobic layer is introduced on the surface to ensure surface lubrication and prevent lubricant depletion. The three-layer structure enables the coating to maintain a low τice (<20kPa) during 30 icing/deicing cycles, showcasing superior de-icing durability compared to conventional anti-icing coatings. Interestingly, the original anti-icing properties of the coating can be restored through external oil replenishment. Furthermore, this coating exhibits excellent resistance against fouling, mechanical stress, chemical exposure, temperature variations, and weathering effects, making it highly suitable for practical applications in anti-icing coatings.

Effective viscous lubrication of cartilage with low viscosity microgels

Viscosupplementation is one of the primary treatments for osteoarthritis, with the goal of restoring the viscoelastic properties of native synovial fluid. Recent work shows a strong in vitro in vivo correlation between the lubricating abilities of viscosupplements and improved patient reported outcomes, suggesting new viscosupplement formulations need to achieve sufficient lubrication of cartilage to be clinically relevant. This study describes a library of low viscosity microgel suspensions that lubricate cartilage as if they were highly viscous lubricants. Microgel formulations were characterized by their size, rheological properties, and lubricating abilities. Microgels synthesized with low crosslinking density exhibited lubrication equivalent to bovine synovial fluid (µ = 0.037–0.064), while maintaining low measured viscosities similar to PBS (η(10 s−1) = 2.04–4.71 mPa*s). These results set the foundation for a new era of viscosupplementation using low viscosity lubricants.

Covalent adaptable networks and thermosets of multi-block ethylene/1-octene copolymers made by free-radical processing: Effects of melt flow index and crystallinity on thermomechanical properties and reprocessability

Chain shuttling polymerization has facilitated the rapid development of olefin block copolymers (OBCs) such as ethylene/1-octene multi-block copolymers. The properties of OBCs stem from various parameters such as average block length, number of blocks per chain, etc., which lead to complexity associated with establishing structure–property relationships between precursor OBCs and properties of cross-linked OBCs. Additionally, permanent cross-linking of OBCs sacrifices their recyclability, and cross-linked OBCs capable of being reprocessed have seldom been reported. Here, permanently cross-linked OBCs (OBCXs) and dynamically cross-linked OBCs, or OBC covalent adaptable networks (CANs), were synthesized by melt-state reactive processing of neat OBCs with various crystallinities and melt flow indices (MFIs). The OBC CANs are capable of dialkylamino disulfide dynamic chemistry via the grafting of bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, or BiTEMPS methacrylate, cross-linkers onto OBC backbone chains. Thermomechanical characterizations indicate that increasing crystallinity and decreasing MFI in precursor OBCs lead to higher cross-link densities in OBCXs and OBC CANs. Dynamically cross-linking OBCs into CANs also significantly improves their elevated-temperature creep resistance. Stress relaxation results of OBC CANs indicate an increased average relaxation time with greater cross-link density. Distinct from other CANs capable of dialkylamino disulfide chemistry, the stress relaxation behaviors of OBC CANs of higher cross-link density show evident dependence on their network viscoelasticities. In OBC CANs of lower cross-link density, the dynamic chemistry more strongly governs their stress relaxation behaviors. Finally, the OBC CANs exhibit full cross-link density and thermomechanical property recovery after reprocessing, whereas the OBCXs cannot be reprocessed due to their irreversible cross-links.

Investigating the effect of methacrylic acid on the properties of waterborne epoxy-acrylate core-shell emulsion, film and coating

Waterborne coating has recently become a trend in research, with extensive studies focusing on waterborne epoxy-acrylate (WEA) emulsion. However, the understanding of the role of methacrylic acid (MAA) in the core-shell structure, affecting film and coating performance, is still limited. Here, we investigate the effect of MAA addition on the properties of the WEA core-shell emulsion, film and coating. The WEA core-shell emulsion was synthesized through semi-continuous emulsion polymerization and subsequently cast into WEA film. Two types of crosslinking systems were established by introducing MAA to either the shell layer (interlayer crosslinking) or the core layer (intralayer crosslinking). The interlayer crosslinking provides a high crosslinking density (7.01 × 10−4 mol cm−3), involving the crosslinking of polymer chains between the core and shell layers. Consequently, the film has a compact structure and high modulus (489 MPa) but low elongation at break (3.20 %). Conversely, the intralayer crosslinking has a low crosslinking density (1.59 × 10−4 mol cm−3). Hence, the film has a loose structure and low modulus (0.38 MPa) but high elongation at break (225 %). This occurs because it involves crosslinking with the polymer chains within the same layer, resulting in reduced compatibility between the layers. The inclusion of MAA enhances the clarity in discerning two glass transition temperatures (Tg). Lastly, the stability (shelf-life) of the WEA emulsion remains unaffected by the MAA, with the decisive factor being the presence of the oxirane ring in the Epoxy (EP). We believe that this finding can significantly contribute to a better understanding of the role of MAA in crosslinking systems within WEA core-shell coatings for researchers.