Optimization of areca leaf sheath nanolignin synthesis by a mechanical method for in situ modification of ultra-low molar ratio urea-formaldehyde adhesives

This study addresses the optimization of the nanolignin preparation method from the areca leaf sheath (ALS) by a mechanical process using a high shear homogenizer at 13,000-16,000 rpm for 1-4 h and its application in enhancing the performance of ultralow molar ratio urea-formaldehyde (UF) adhesive. Response surface methodology (RSM) with a central composite design (CCD) model was used to determine the optimum nanolignin preparation method. The mathematical model obtained was quadratic for the particle size response and linear for the zeta potential response. Under the optimum conditions, a speed of 16,000 rpm for 4 h resulted in a particle size of 227.7 nm and a zeta potential of -18.57 mV with a high desirability value of 0.970. FE-SEM revealed that the characteristic changes of lignin to nanolignin occur from an irregular or nonuniform shape to an oval shape with uniform particles. Nanolignin was introduced during the addition reaction of UF resin synthesis. UF modified with nanolignin (UF-NL) was analyzed for its adhesive characteristics, functional groups, crystallinity, and thermomechanical properties. The UF-NL adhesive had a slightly greater solid content (73.23 %) than the UF adhesive, a gelation time of 4.10 min, and a viscosity of 1066 mPa.s. The UF-NL adhesive had similar functional groups as the UF adhesive, with a lower crystallinity of 59.73 %. Compared with the control plywood which has a tensile shear strength value of 0.79 MPa, the plywood bonded with UF-NL had a greater tensile shear strength of 1.07 MPa, with a lower formaldehyde emission of 0.065 mg/L.

Understanding Water Diffusion Behaviors in Epoxy Resin through Molecular Simulations and Experiments

The unclear understanding of the water diffusion behavior posts a big challenge to the manipulation of water absorption properties in epoxy resins. Herein, we investigated the water diffusion behavior and its relationship with molecule structures inside an epoxy resin mainly by the nonequilibrium molecular dynamics and experiments. It is found that at the initial rapid water absorption stage, bound water and free water both contribute, while at the later slow water absorption stage, free water plays a dominant role. The observed evolution of free water and bound water cannot be explained by the traditional Langmuir model. In addition, molecule polarity, free volume, and segment mobility can all influence the water diffusion process. Hence, the epoxy resin with low polarity and high molecular segment mobility is endowed with higher diffusion coefficients. The saturated water absorption content is almost dependent on the polarity. The understanding of how water diffuses and what decides the diffusion process is critical to the rational design of molecule structures for improving the water resistance in epoxy resin.

Directly Using Paraffin as the Toughening Agent of Epoxy Composites: An Experimental and Molecular Dynamics Simulation Study

It is still a challenge in studying the toughening mechanism by well combining the experimental and atomistic molecular dynamics (MD) simulation study. This article directly introduced eicosane (C20, model compound of paraffin) into the epoxy matrix (DGEBA) by using a special epoxy resin with alkyl side chains (D12) as a compatibilizer, which was synthesized through thiol-ene click chemistry. The toughening mechanism of the ternary DGEBA/D12/C20 (EPDA-X) systems was systematically investigated by experimental and MD simulation methods. Though C20 can be well dispersed in the curing mixture, the huge polarity difference between C20 and DGEBA can be the driving force for C20 to stay away from DGEBA, demonstrating the self-assembly effect of C20 around the alkyl side chains of D12 because of the good compatibility of D12 and C20. The soft alkyl chains of D12 and C20 as well as the self-assembly effect of C20 around the D12 molecules can simultaneously improve the strength, modulus, and toughness of the EPDA-2.5 system. This article not only provides a brand new toughening strategy by directly using nonfunctional alkyl derivatives as the toughening agent of epoxy composites with superior mechanical properties but also provides a systematic MD simulation method to evaluate whether there is the interaction or not and the strength of interaction between different molecular chains so as to provide a theoretical basis for the cause of the microphase separation structure and related toughening mechanism in cross-linking networks on the atomic and molecular levels.

Modeling of a two-stage polymerization considering glass fibre sizing using molecular dynamics

Fibre reinforced polymers are an important class of materials due to their light weight, high strength, and stiffness. However, there is a lack of knowledge about the interaction of fibre surface, sizing (fibre coating), and resin. Often only idealised academic systems are studied, and only rarely realistic systems that are used in an industrial context. Therefore, methods for studying the behaviour of complex sizing are highly desirable, especially as they play a crucial role in the performance of fibre reinforced polymers. Here, a simplified, yet industrially used resin system is extended using molecular dynamics simulations by adding a fibre surface and sizing layers. Furthermore, a common coupling agent was selected, and several additional assumptions were made about the structure of the sizing. Based on this, a systematic procedure for the development of a final cured system is introduced: a condensation reaction to form oligomers from coupling agent monomers is conducted. Subsequently, a two stage reaction, a polyurethane reaction and a radical polymerisation, is modelled based on an established approach. Using the final cured system, evaluations of averaged quantities during the reactions are carried out. Moreover, the system is evaluated along the normal direction of the fibre surface, which proves a spatial analysis of the fibre-sizing-resin interface.

Molecular Dynamic Simulations and Experiments Study on the Mechanical Properties of HTPE Binders

The mechanical properties of HTPE binders have been systemically studied through combining the microstructure molecular simulations with macroscopic experiments. In this study, the crosslinking structures of HTPE binders were established by a computational procedure. Based on the optimized crosslinking models, the mechanical properties and the glass transition temperatures (Tg) of HTPE/N-100, HTPE/HDI, HTPE/TDI, and HTPE/IPDI binder systems were simulated; specifically, the Tg were 245.758 K, 244.573 K, 254.877 K, and 240.588 K, respectively. Then the bond-length distributions, conformation properties, cohesive energy densities, and fraction free volume were investigated to analyze how the microstructures of the crosslinking models influenced the mechanical properties of HTPE binders. Simultaneously, FTIR-ATR spectra analysis of HTPE binders proved that the special peaks, such as -NH and -NCO, could be seen in the crosslinking polyurethane structures synthesized between prepolymers and curing agents. The dynamic mechanical analysis was carried out, and it found that the Tg of HTPE/N-100, HTPE/HDI, HTPE/TDI, and HTPE/IPDI binder systems were -68.18 °C, -68.63 °C, -65.67 °C, and -68.66 °C, respectively. In addition, the uniaxial tension verified that both the ultimate stress and Young's modulus of HTPE binder systems declined with the rising temperatures, while the strains at break presented a fluctuant variation. When it was closer to glass temperatures, especially -40 °C, the mechanical properties of HTPE binders were more prominent. The morphology of the fractured surface revealed that the failure modes of HTPE binders were mainly intermolecular slipping and molecular chain breakage. In a word, the experimental results were prospectively satisfied using the simulations, which confirmed the accuracy of the crosslinking models between prepolymers and curing agents. This study could provide a scientific option for the HTPE binder systems and guide the design of polyurethanes for composite solid propellant applications.

Curing and Molecular Dynamics Simulation of MXene/Phenolic Epoxy Composites with Different Amine Curing Agent Systems

Herein, the curing kinetics and the glass transition temperature (T_g) of MXene/phenolic epoxy composites with two curing agents, i.e., 4,4-diaminodiphenyl sulfone (DDS) and dicyandiamine (DICY), are systematically investigated using experimental characterization, mathematical modeling and molecular dynamics simulations. The effect of MXene content on an epoxy resin/amine curing agent system is also studied. These results reveal that the MXene/epoxy composites with both curing agent systems conform to the SB(m,n) two-parameter autocatalytic model. The addition of MXene accelerated the curing of the epoxy composite and increased the T_g by about 20 K. In addition, molecular dynamics were used to simulate the T_g of the cross-linked MXene/epoxy composites and to analyze microstructural features such as the free volume fraction (FFV). The simulation results show that the introduction of MXene improves the T_g and FFV of the simulated system. This is because the introduction of MXene restricts the movement of the epoxy/curing agent system. The conclusions are in good agreement with the experimental results.

Characterization of Cure Behavior in Epoxy Using Molecular Dynamics Simulation Compared with Dielectric Analysis and DSC

The curing behavior of a thermosetting material that influences the properties of the material is a key issue for predicting the changes in material properties during processing. An empirical equation can describe the reaction kinetics of the curing behavior of an investigated material, which is usually estimated using experimental methods. In this study, the curing process of an epoxy resin, the polymer matrix in an epoxy molding compound, is computed concerning thermal influence using molecular dynamics. Furthermore, the accelerated reaction kinetics, which are influenced by an increased reaction cutoff distance, are investigated. As a result, the simulated crosslink density with various cutoff distances increases to plateau at a crosslink density of approx. 90% for the investigated temperatures during curing time. The reaction kinetics are derived according to the numerical results and compared with the results using experimental methods (dielectric analysis and differential scanning calorimetry), whereby the comparison shows a good agreement between experiment and simulation.

Energy Decomposition Analysis of the Adhesive Interaction between an Epoxy Resin Layer and a Silica Surface

We investigate the adhesive interaction energy (ΔE_int) between an epoxy resin and a silica surface using pair interaction energy decomposition analysis (PIEDA), which decomposes ΔE_int into four components: electrostatic (ΔE_es), exchange repulsion (ΔE_ex), charge-transfer (ΔE_ct), and dispersion (ΔE_disp) energies based on quantum chemistry. Our previous study with PIEDA showed that synergistic effects of ΔE_es and ΔE_disp are critical at the interface between an epoxy resin fragment and a hydrophilic surface. The present study is designed to show in detail that the synergistic effects are significant at the interface between an epoxy layer model consisting of 20 epoxy monomers and a hydrophilic silica surface. The ratio of the dispersion energies to the total interaction energies of the layer model shows good agreement with experimental values, that is, the dispersion ratio of the work of adhesion (W_ad). The 20 epoxy molecules in the layer model are investigated individually to closely correlate the four decomposed energies with their structural features. Our energy-decomposition analyses show that H-bonding and OH-π interactions play important roles at the interface between an epoxy resin and a silica surface. PIEDA calculations for the epoxy layer model also show that the region 3.6 Å from the silica surface accounts for more than 99% of the total interaction energies.

Effect of curing reaction types on the structures and properties of acetylene-containing thermosets: towards optimization of curing procedure

High-temperature thermosets are usually prepared from resins containing alkynyl groups, and their properties depend much upon the curing process containing various types of curing reactions. However, how the curing process affects the properties remains unclear due to the complicated curing reactions. We used molecular dynamics simulations to investigate the effect of curing reaction types, including cyclotrimerization, Diels-Alder reaction, and radical reaction, on the structures and properties of imide oligomers terminated with alkynyl groups. The results show that the cycloadditions such as cyclotrimerization and Diels-Alder reaction endow the thermosets with rigid structures and high moduli. Compared with the cycloadditions, the radical reaction enables the formation of flexible cured structures, which can enhance the toughness of thermosets. The differences in thermal and mechanical properties caused by different curing types were elucidated by the relaxation processes of fragments in these cured systems and were explained by the variation of torsion energy in different curing forms. As this work aims to optimize the curing procedure to obtain high-performance resins with desired properties, different curing procedures were finally designed according to the theoretical studies, and the obtained cured polymers show significant differences in the properties from different curing ways. The results can guide the preparation of desired thermosetting resins by tuning the curing procedure.

MARTINI-based simulation method for step-growth polymerization and its analysis by size exclusion characterization: a case study of cross-linked polyurethane

Simulation studies of step-growth polymerization, e.g., polymerization of polyurethane systems, hold great promise due to having complete control over the reaction conditions and being able to perform an in-depth analysis of network structures. In this work, we developed a (completely automated) simulation method based on a coarse-grained (CG) methodology, i.e., the MARTINI model, to study the cross-linking reaction of a diol, a tri-isocyanate molecule and one-hydroxyl functional molecule to form a polyurethane network without and with dangling chains. This method is capable of simulating the cross-linking reactions not only up to very high conversions, but also under rather complicated reaction conditions, i.e., a non-stoichiometric ratio of the reactants, solvent evaporation and multi-step addition of the reactants. We introduced a novel network analysis, similar to size-exclusion chromatography based on graph theory, to study the growth of the network during the polymerization process. By combining the reaction simulations with these analysis methods, a set of correlations between the reaction conditions, reaction mechanisms and final network structure and properties is revealed. For instance, a two-step addition of materials for the reaction, i.e., first the dangling chain to the tri-isocyanate and then the diol, leads to the highest integrated network structure. We observed that different reaction conditions lead to different glass transition temperatures (T_g) of the network due to the distinct differences in the final network structures obtained. For example, by addition of dangling chains to the network, the T_g decreases as compared to the network without dangling chains, as also is commonly observed experimentally.

Reaction and characterisation of a two-stage thermoset using molecular dynamics

The curing reaction of a two-stage hybrid resin is simulated and different states are evaluated for material properties.