Low Cross-linking Density Research Articles

Green preparation of robust hydrophobic bio-based epoxy resin with low dielectric constant

Epoxidized soybean oil (ESO) thermosets show poor performance owing to the low cross-linking density and flexibility of their backbone structure. Herein, fully bio-derived benzoxazine (GD) cured ESO thermosets (PGEs) with high-performance were prepared through a green process. As a curing agent, GD improved the cross-linking density and rigidity of ESO resins. Moreover, different types of hydrogen bonds formed in the PGEs, further strengthening the intermolecular interactions. Therefore, the glass transition temperature (Tg) and tensile strength of PGEs attained 93 °C and 34 MPa, respectively, which are higher than those reported for conventional ESO resins. Additionally, PGEs exhibited substantial hydrophobicity and a low dielectric constant, which can be associated with complex intermolecular and intramolecular hydrogen bonds within the thermoset network. Furthermore, as the GD content in PGE increased, the dielectric constant and water absorption of the polymer reduced to 2.72 % and 0.52 %, respectively. Herein, the preparation of novel all-biological ESO thermosets using GD as the curing agent is reported, providing a green and convenient strategy for the design of high-performance ecological materials.

Effects of Primary Structure of Reactive Polymers on Network Structure and Mechanical Properties of Gels

Front Cover: The effects of the primary structure of reactive polymers on the structure and mechanical properties of gels are studied by a molecular dynamics simulation. The figure illustrates the improvement of structure uniformity by changing the arrangement of functional groups from a random one (top) to a periodic one (bottom), where red surfaces show low crosslinking density regions. This is reported by Tsutomu Furuya and Tsuyoshi Koga in article number 2200044.

The role of molecular architecture on the viscoelastic properties of thermoreversible polyurethane adhesives

New polyurethane (PUR) adhesives containing covalent Diels-Alder (DA) bonds that can break and form with temperature have been recently developed for sustainable multilayer packaging. The understanding of the mechanical properties and in particular of creep in terms of molecular architecture appears of outmost importance in view of their applications and potential reutilization. Oscillatory shear rheometry shows a modulus decrease with temperature associated to bond breakage and the enhancement found upon cooling can be associated with network reconstruction. Analysis of storage modulus values above the glass transition and below the retro-DA reaction suggests a lower network crosslinking density with increasing adduct content. Room temperature indentation and shear rheometry tests reveal that DA bonds improve the modulus and reduce the resistance to linear viscoelastic flow. The modulus improvement can be associated to the enhanced chain rigidity introduced by the DA moieties and the reduced creep resistance seems to be related to the decrease of crosslinking density.

(Digital Presentation) Design of High Barrier Property and Flexible Inorganic/Polymer Hybrid Film for Encapsulation of Flexible Electronics

Designing encapsulation film with both high barrier property and excellent flexibility has been challenging for the current brittle inorganic films owing to the strain-induced cracking. Inorganic/organic multilayer has been proposed due to the organic layer can function as buffers of the inorganic layer and defect-decoupling layer, realizing the improvement of flexibility and maintaining barrier property. Polydimethylsiloxane (PDMS) was widely used as an organic encapsulation layer due to its high stability, elasticity, and transparency. The introduction of PDMS helps the barrier film to resist distorting under the bending motions. However, the delamination and cracking occurred under bending motion in a small curvature radius due to the mismatched strain and coefficient of thermal expansion (CTE) between PDMS and inorganic films such as Al2O3. Realization of improving their strain and CTE mismatch is possible by doping nanoparticles into PDMS. The Al2O3/PDMS hybrid film with the doping of SiO2 fabricated via atomic layer infiltration (ALI) method showed mechanical stability and endure 1000 repetitive stretch tests with 1%, but the water vapor transmission rate (WVTR) was only 10-3 g/m2/day, which was attributed to the lower crosslinking density of PDMS result in higher free volume and chain mobility. It’s a challenge to develop a strategy for improving the crosslink density of PDMS. In this work, the PDMS-based composite substrate with engineered stiffness was developed by employing the addition of silane coupling agent and SiO2. Atomic layer infiltration (ALI) coupled with UV-curing was introduced for sub-surface cross-linking of PDMS in Al2O3/SiO2/PDMS hybrid film, and the resulted Al2O3/SiO2/PDMS hybrid film showed highly stretchable and barrier properties. The result indicates that Al2O3/SiO2/PDMS hybrid film is a promising encapsulation film applicable to flexible electronics.Keywords: atomic layer infiltration (ALI), flexible film, encapsulation, UV-curing method, water vapor transmission rate (WVTR)Refs. Y. Li, D. Wen, Y. Zhang, Y. Lin, K. Cao, F. Yang, R. Chen, Highly-stable PEN as a gas-barrier substrate for flexible displays via atomic layer infiltration, Dalt. Trans. (2021) 16166–16175. https://doi.org/10.1039/d1dt02764f.Zhang, D. Wen, M. Liu, Y. Li, Y. Lin, K. Cao, F. Yang, R. Chen, Stretchable PDMS Encapsulation via SiO2 Doping and Atomic Layer Infiltration for Flexible Displays, Adv. Mater. Interfaces. 9 (2022) 1–9. https://doi.org/10.1002/admi.202101857.

A molecular dynamics simulation on tunable and self-healing epoxy-polyimine network based on imine bond exchange reactions

ABSTRACT In this work, a post-crosslinking method is used to construct crosslinked epoxy-polyimine networks (C-PIs) based on the imine bond. Due to the dynamic nature of the imine bond, bond exchange reactions (BERs) and rearrangements would take place in polyimine segments under heat or water external stimuli. Molecular dynamic (MD) simulations are performed to investigate imine polycondensation and post-crosslinking procedures. The results indicate that C-PIs exhibit different glass transition temperatures (Tg s) and crosslinking densities by tuning the ratio of epoxy and imine groups, and the Tg values of C-PIs increase linearly with the crosslinking density. In addition, the interface welding behaviour of different C-PIs is investigated, and our results show that BERs can improve the diffusion rate and the final penetration depth. By tracking the microstructures and examining the mechanical properties of samples varying with healing time, it is revealed that the C-PI with lower crosslinking density is easy to self-heal. Furthermore, MD results also imply that the internal stress of the C-PI can be eliminated under the coupling effect of chain relaxation and BERs. This study proves the reliability of a new crosslinking method and provides some useful guidance for designing new materials.

Mucus-penetrating nanoparticles based on chitosan grafted with various non-ionic polymers: Synthesis, structural characterisation and diffusion studies

Transmucosal administration offers numerous advantages for drug delivery as it usually helps to avoid first pass metabolism, provides rapid onset of action, and is a non-invasive route. Mucosal surfaces are covered by a viscoelastic mucus gel layer which acts as a protective barrier preventing the entrance of harmful substances into the human tissues. This function of mucus also inhibits the diffusion of drugs and nano-formulations and can result in a significant reduction of their efficacy. The design of mucus-penetrating nanoparticles can overcome the barrier function of mucus which may lead to better therapeutic outcomes. In this study, chitosan was chemically modified by grafting short chains of poly(ethylene glycol), poly(2-hydroxyethyl acrylate), poly(2-ethyl-2-oxazoline), or poly(N-vinyl pyrrolidone) and the resulting chitosan derivatives were used to prepare nanoparticles using an ionic gelation method with sodium tripolyphosphate. These nanoparticles were characterised using dynamic light scattering, transmission electron microscopy, small-angle neutron scattering and nanoparticle tracking analysis. Small-angle neutron scattering data revealed the presence of a large amount of water inside these nanoparticles and lack of a heterogeneous internal structure. The nanogel model with low crosslinking density is suggested as the most feasible model to describe the structure of these nanoparticles. The studies of the behaviour of these nanoparticles in bovine submaxillary mucin solutions and their penetration into sheep nasal mucosa indicated greater diffusivity of modified chitosan nanoparticles compared to unmodified chitosan nanoparticles with the best results achieved for the chitosan grafted with poly(N-vinyl pyrrolidone).

Sustainable bio-based dialdehyde cellulose for transforming crystalline urea–formaldehyde resins into amorphous ones to improve their performance

Sustainable bio-based dialdehyde cellulose (DAC) was employed to transform crystalline urea–formaldehyde (UF) resins into amorphous ones for simultaneouly improving their adhesion strength and formaldehyde emission. Serial samples of the UF resins modified with DAC during the resin synthesis were extracted to understand the chemical reactions between the DAC and UF species. Fourier transform infrared, 1H-nuclear magnetic resonance (NMR), and 13C NMR spectroscopies confirmed the occurrence of reactions between the DAC and UF species. As the synthesis proceeded, the crystallinity of the modified UF resins decreased from 51.7% to 17.4%, transforming the crystalline domains into amorphous ones. Thermograms showed that the DAC in the modified UF resins was decomposed at temperatures over 200 °C as degraded form, resulting in a lower cross-linking density than that of the neat UF resins. The adhesion strength of the modified UF resins was statistically similar to that of the neat UF resins, and the formaldehyde emission of the modified UF resins dramatically decreased to ∼ 64.6%. These results evidence the significant application potential of bio-based DAC in improving the sustainability and performance of UF resins.

Intra- vs Intermolecular Cross-Links in Poly(methyl methacrylate) Networks Containing Enamine Bonds

The molecular dynamics of a copolymer composed of methyl methacrylate (MMA) and (2-acetoacetoxy)ethyl methacrylate (AEMA) monomers and the influence on it of intra- to intermolecular cross-links of AEMA units with ethylenediamine (EDA) was studied by combining dielectric relaxation experiments and thermal investigations. The dielectric spectra of the non-cross-linked copolymer show three dynamical processes: a slow relaxation (α) and a faster (β), both dominated by the MMA dynamics, and an even faster secondary relaxation (γ) reflecting the AEMA dynamics. Already for low cross-linking densities, the γ process is very much affected and eventually disappears, increasing the cross-linking density. The secondary β relaxation however was nearly unaffected by cross-linking. The effect of cross-linking on the α relaxation was very pronounced with an important increasing of the glass transition temperature Tg. There was also an increase of the dynamic heterogeneity and the relaxation intensity when increasing the cross-linking density (up to the maximum explored, 9 mol % EDA). The quality of the average time scale and Tg value have similarities in behavior for intra- and intermolecular cross-linking, but clear differences in the dynamic heterogeneities where observed. These differences can be interpreted in connection with the sparse internal structure of the collapsed single chains obtained by intramolecular cross-linking.

Flame retardation of vinyl ester resins and their glass fiber reinforced composites via liquid DOPO-containing 1-vinylimidazole salts

Aiming for obtaining flame-retardant vinyl ester resins (VER) and the composite, a kind of liquid 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-containing 1-vinylimidazole salt (called VIDOP) was conveniently synthesized through the neutralization of the DOPO-derived acid and 1-vinylimidazole to be applied as flame-retardant crosslinking agents of VER. Curing behaviors, flame retardancy, thermal decomposition behaviors, thermal-mechanical properties, and mechanical performance of VER materials were investigated. With 25 wt% addition of VIDOP, the modified resin (VIDOP25/VER) and their glass fiber reinforced composites (VIDOP25/VER/GF) achieved UL 94 V-0 rating with a limiting oxygen index value of 32.1% and 38.3%, respectively. Both heat release and smoke release of VIDOP25/VER and VIDOP25/VER/GF decreased compared to those of the reference sample in the cone calorimetry test. VIDOP showed flame-retardant activities in both gaseous and condensed phases for the resin, while they didn't work well in the condensed phase for the composite. VIDOP/VER presented a little lower a glass transition temperature than VER with a relatively low crosslinking density, in which 1-vinylimidazoles covalently linked with the VER chain and the DOPO-derived group attached to the resin via ionic bonds. VIDOP/VER and the composite had better mechanical properties than the unmodified sample and outperform other systems in well-balanced performance.

Highly Cross-linked Epoxy Coating for Barring Organophosphate Chemical Warfare Agent Permeation.

Chemical warfare agents (CWAs) can be absorbed in polymeric coatings through absorption and permeation, thus presenting a lethal touch and vapor hazards to people. Developing a highly impermeable polymer coating against CWAs, especially against organophosphate CWAs (OPs), is challenging and desirable. Herein, fluorinated epoxy (F-EP) and epoxy (EP) coatings with different cross-link densities were prepared to resist OPs. The effects of the polymer coating structure, including cross-link density, chemical composition and free volume, on the chemical resistance to dimethyl methylphosphonate (DMMP, Soman simulant) were investigated in detail. Meanwhile, the chemical resistance to Soman and VX was examined. The results reveal that the cross-link density is a critical factor in determining the chemical resistance of the coatings. Highly cross-linked EP and F-EP coatings with dense and solid cross-linked networks can fully bar DMMP and OPs permeation during the test time. At low or medium cross-link densities, the EP coating with a lower retention of DMMP exhibited a higher resistance than the F-EP coating due to the lower interaction with DMMP and smaller free-volume holes and lower relative fractional free volume. These results suggest that increasing the cross-link density is a reasonable approach to control the chemical resistance of polymer networks against OPs.

Effects of Secondary Porosity on Microstructure and Mechanical Properties of SAP-Containing Lime-Based Plasters.

Despite the many benefits associated with the utilization of superabsorbent polymers (SAPs), several drawbacks have been reported. In particular, the effect of SAPs on microstructure, together with its consequences for mechanical properties, is not fully understood yet for some composite materials. This study analyzes the role of SAPs in the formation of the microstructure of lime composites, taking into account their chemical composition. The obtained experimental results show that the particle size and cross-linking density of used SAPs are crucial parameters affecting both the microstructure and mechanical performance of the analyzed composites. Coarser SAPs with low cross-linking density in the dosage of 0.5 and 1 wt.% are found as the most suitable solution, leading even to a slight improvement of mechanical parameters. The secondary porosity formed by swelled hydrogels is identified as a very significant factor since hydrogel-filled voids do not contribute to the strength parameters. The formation of the affected zone around SAP cores depends on the chemical composition of SAPs considerably as the higher cross-linking density influences the desorption rate. Based on achieved results, utilization of SAPs in building materials should be studied at a more detailed level with particular importance on the definition of SAP-related voids and affected zone around SAP particles.

Multivalent Allylammonium-Based Cross-Linkers for the Synthesis of Homogeneous, Highly Swelling Diallyldimethylammonium Chloride Hydrogels.

N,N’-methylenebisacrylamide (BIS) is a very popular cross-linker for the radical polymerisation in water. It is highly reactive but prone to alkaline hydrolysis and suffers from a low solubility. This study shows that with slow polymerising systems such as N,N-diallyldimethylammonium chloride, only inhomogeneous networks are formed. As a consequence, gels with very low cross-linking densities, i.e., high swelling capacities, disintegrate during the swelling test and firm, coherent gels are not accessible due to the solubility limit. A promising alternative are multivalent tetraallyl-based compounds, of which tetraallylammonium bromide (TAAB), N,N,N’,N’-tetraallylpiperazinium dibromide (TAPB) and N,N,N’,N’-tetraallyltrimethylene dipiperidine dibromide (TAMPB) are the subject of this study. With these, the cross-linking polymerisation appears to be statistical, as gels formed at low monomer conversion have essentially the same swelling properties as those formed at high conversions. This is not observed with BIS. However, gelation with the tetraallyl cross-linkers is much slower than with BIS and follows the order TAPB < TAMPB < TAAB, but the differences become significantly smaller with increasing content. At low contents, all three allow the preparation of gels with high swelling capacities of up to 360 g/g.

Interfacial self-assembly of SiO2-PNIPAM core-shell particles with varied crosslinking density.

Spherical particles confined to liquid interfaces generally self-assemble into hexagonal patterns. It was theoretically predicted by Jagla two decades ago that such particles interacting via a soft repulsive potential are able to form complex, anisotropic assembly phases. Depending on the shape and range of the potential, the predicted minimum energy configurations include chains, rhomboid and square phases. We recently demonstrated that deformable core-shell particles consisting of a hard silica core and a soft poly(N-isopropylacrylamide) shell adsorbed at an air/water interface can form chain phases if the crosslinker is primarily incorporated around the silica core. Here, we systematically investigate the interfacial self-assembly behavior of such SiO2-PNIPAM core-shell particles as a function of crosslinker content and core size. We observe chain networks predominantly at low crosslinking densities and smaller core sizes, whereas higher crosslinking densities lead to the formation of rhomboid packing. We correlate these results with the interfacial morphologies of the different particle systems, where the ability to expand at the interface and form a thin corona at the periphery depends on the degree of crosslinking close to the core. We perform minimum energy calculations based on Jagla-type pair potentials with different shapes of the soft repulsive shoulder. We compare the theoretical phase diagram with experimental findings to infer to which extent the interfacial interactions of the experimental system may be captured by Jagla pair-wise interaction potentials.

Dynamic Pressure Sensitive Adhesion in Nematic Phase of Liquid Crystal Elastomers

AbstractThe correlation between the anomalous mechanical dissipation and the pressure‐sensitive adhesion in nematic liquid crystalline elastomers (LCEs) is explored, comparing hard glassy and soft materials (high and low crosslinking density, respectively) and nematic versus isotropic genesis. Two significant factors are identified contributing to strong adhesion: the intrinsic damping, which is high in the nematic phase, and the surface roughness, which is affected by the elastomer genesis. At the same time, it is found that surface energy plays only a minor role in pressure‐sensitive adhesion. Theoretical calculations of dynamic adhesion are carried out by summing over the full range of frequencies in the dynamic‐mechanical master curves data, and by averaging over surface roughness in soft and hard regimes, the results fitting the measured data with quantitative accuracy. Finally, the adhesion strength in LCEs altered by controlling the nematic phase temperature range is demonstrated.

Guar Gel Binders for Silicon Nanoparticle Anodes: Relating Binder Rheology to Electrode Performance.

Binding agents are a critical component of Si-based anodes for lithium-ion batteries. Herein, we introduce a composite hydrogel binder consisting of carbon black (CB) and guar, which is chemically cross-linked with glutaraldehyde as a means to reinforce the electrode structure during lithiation and improve electronic conductivity. Dynamic rheological measurements are used to monitor the cross-linking reaction and show that rheology plays a significant role in binder performance. The cross-linking reaction occurs at a faster rate and produces stronger networks in the presence of CB, as evidenced from higher gel elastic modulus in guar + CB gels than guar gels alone. Silicon nanoparticle (SiNP) electrodes that use binders with low cross-link densities (trxn < 2 days) demonstrate discharge capacities ∼1200 mAh g-1 and Coulombic efficiencies >99.8% after 300 cycles at 1-C rate. Low cross-link densities likely increase the capacity of SiNP anodes because of binder-Si hydrogen-bonding interactions that accommodate volume expansions. In addition, the cross-linked binder demonstrates the potential for self-healing, as evidenced by an increased elastic modulus after the gel was mechanically fragmented, which may preserve the electrode microstructure during lithiation and increase capacity retention. The composite hydrogel with integrated conductive additives gives promise to a new type of binder for next-generation lithium-ion batteries.

Elucidation of the accelerated sulfur vulcanization of bio oil‐extended natural rubber compounds

AbstractThe elucidation of the role of bio‐oils on the accelerated sulfur vulcanization of natural rubber (NR) compounds is discussed in this study. Two types of bio‐oil, palm oil and soybean oil, were studied in direct comparison with a distillate aromatic extract oil (DAE) as a reference. The scorch and cure times of the bio‐oil‐extended NR compounds were shorter than those containing DAE. The use of bio‐oils gave a higher cure reaction rate constant along with a lower activation energy than the use of DAE. The attenuated total reflectance‐Fourier transform infrared spectroscopy analysis revealed that the fatty acid segment of the bio‐oils can react with zinc oxide to give zinc carboxylate, which is then involved in and promotes the vulcanization reaction. The use of bio‐oils to increase the rate of vulcanization strongly influenced the crosslink density of the obtained NR vulcanizates, yielding NR vulcanizates with a lower crosslinking density. It is proposed here that the bio‐oils might consume the curing agent via the reaction between their own unsaturated fatty acid and sulfur. This was supported by the increased viscosity of the oils after exposure to sulfur at a high temperature. The tensile strength and elongation at break of the bio‐oil‐extended NR compounds were lower and higher, respectively, than the NR extended with DAE oil due to the lower crosslink density of the bio oil‐extended NR vulcanizates.

Printability during projection-based 3D bioprinting

Since projection-based 3D bioprinting (PBP) could provide high resolution, it is well suited for printing delicate structures for tissue regeneration. However, the low crosslinking density and low photo-crosslinking rate of photocurable bioink make it difficult to print fine structures. Currently, an in-depth understanding of the is lacking. Here, a research framework is established for the analysis of printability during PBP. The gelatin methacryloyl (GelMA)-based bioink is used as an example, and the printability is systematically investigated. We analyze the photo-crosslinking reactions during the PBP process and summarize the specific requirements of bioinks for PBP. Two standard quantized models are established to evaluate 2D and 3D printing errors. Finally, the better strategies for bioprinting five typical structures, including solid organs, vascular structures, nerve conduits, thin-wall scaffolds, and micro needles, are presented.

A Dynamically Hybrid Crosslinked Elastomer for Room‐Temperature Recyclable Flexible Electronic Devices

AbstractThe rapid development of flexible electronics has resulted in serious pollution in the form of electronic waste. Accordingly, recyclability is highly desirable for these devices, but this remains a significant challenge. A dynamically hybrid crosslinked polyurethane (FPU) elastomer is designed in this study to address this challenge. Distinctive Diels–Alder adducts with suitable dissociation and reassociation dynamics are designed as crosslinking units to provide an efficient time frame for recycling. FPU is maintained in a state with a low crosslinking density after heating at 120 °C for 5 min. FPU‐based electronics can therefore be dissolved in chloroform under ambient conditions to separate the electronic components and polymers for the refabrication of new electronic devices. This is the first reported thermoset elastomer that can be completely recycled at room temperature without chemical treatment to decompose the polymer chain. The design concept is applied by demonstrating the fabrication by recycling of different FPU‐based flexible electronic devices: position sensor, flexible keyboard, and motion sensor. Furthermore, the FPU has many advantages as a material for flexible electronics in terms of its biomimetic mechanical properties, room‐temperature self‐healing, and facile processability. This study provides promising new design principles to develop materials for promoting sustainable flexible electronics.

Understanding indentation, scratch and wear behavior of UV-cured wood finishing products

In the wood furniture and flooring industry, the protective and aesthetic properties of the final product often rely, at least partially, on the coatings applied to the wood surface. For flat surfaces, UV-cured coatings are often preferred due to their many advantages, such as fast curing times, low volatile organic compound (VOC) emissions, low energy consumption and high crosslinking densities. To increase the durability of interior wood products, the behavior of the protective coatings while subjected to wear and deterioration has to be understood in order to be enhanced. Depending on the type of solicitation, mar (i.e. shallow defects), scratches or indentations can be formed, which impact the coating's appearance and can shatter the perception of the whole product. In this work, formulations based on different monomer-oligomer couples were prepared and photo-polymerized to investigate their performances. First, the glass transition temperature and the crosslinking density were determined to understand the contribution of both components in the polymeric network formed upon UV-curing. Then, hardness, and abrasion, scratch and wear resistances were studied. The results showcased the importance of the monomer and oligomer structure, functionality and main physical properties. In scratch experiments, the hard and brittle coatings tend to display failures at lower loads than soft and ductile ones. The friction experiments also caused subsurface tearing and fissuring in the soft coatings and generated several fractures in the harder ones. Furthermore, the best overall mechanical resistance was obtained for the couples with high crosslinking density, and a correlation was found between the crosslinking density and the hardness of UV-cured coatings. Interestingly, the investigation of tracks after scratch and wear experiments revealed a significant amount of information about the coatings' behavior under various mechanical loads.

Nanoscale Mechanical Properties of Core–Shell-like Poly-NIPAm Microgel Particles: Effect of Temperature and Cross-Linking Density

Poly-NIPAm microgel particles with two different cross-linking densities were prepared with the classical batch polymerization process. These particles were adsorbed onto modified silica surfaces, and their nanomechanical properties were measured by means of atomic force microscopy. It was found that these particles have a hard core-soft shell structure both below and above the volume transition temperature. The core-shell-like structure appears due to a higher reaction rate of the cross-linker compared to that of the monomer, leading to depletion of cross-linker in the shell region. The microgel beads with lower average cross-linking density were found to be less stiff below the volume transition temperature than the microgel with higher cross-linking density. Increasing the temperature further to just above the volume transition temperature led to lower stiffness of the more highly cross-linked microgel compared to its less cross-linked counterpart. This effect is explained with the more gradual deswelling with temperature for the more cross-linked microgel particles. This phenomenon was confirmed by dynamic light scattering measurements in the bulk phase, which showed that the larger cross-linking density microgel showed a more gradual collapse in aqueous solution as the temperature was increased.