Atomistic molecular modelling of crosslinked epoxy resin

In the present study, a new method was developed to construct atomistic molecular models of crosslinked polymers based on commercially important epoxy resin. This method employed molecular dynamics/molecular mechanics schemes and assumed close proximity. The generic Dreiding2.21 force-field and advanced compass force-field were used for the construction of models and prediction of properties, respectively. A polymer network with conversion up to 93.7% was successfully generated by this method. Density and elastic constants of the system were calculated from the equilibrated structure for the validation of the generated models. The simulated results compared reasonably with experimental data available. The developed method would hold great promise in further molecular simulations for structure and properties of epoxy resin or other cured systems.     

Atomistic simulation study of absorbed water influence on structure and properties of crosslinked epoxy resin

Absorbed water can generally degrade most polymer materials with many desired thermal and mechanical properties. In this work, a series of two-step molecular dynamics simulations have been carried out to study the moisture effects on an epoxy-amine polymer network. Copolymer of diglycidyl ether bisphenol A (DGEBA) and isophorone diamine (IPD) was investigated together with its four moist networks with concentrations of water 1.30, 2.54, 5.02 and 11.5 wt%. Simulated results clearly indicated that with addition of water molecules the density of the polymer system decreases whereas the mobility of the network chains increases at higher water concentration. However, at lower water concentration the results present the contrast trends. These differences resulting from water presence can be attributed to the interplay between the hydrogen bonding interactions and the free volumes, the former is responsible for antiplasticization whereas the latter leads to plasticization of the polymer materials. Further analysis confirms that the networks form hydrogen bonds with absorbed water molecules, which preferred to locate in the vicinity of polar groups on the polymer network, and water at higher concentration can cluster with each other. The diffusion coefficient increased with increasing water concentration; the same trend as the change in fractional free volume but contrary to that in density. These results from our simulations were in good agreement with the general experimental observations and the free volume theory.     

Proposed solution for the Flory-Charlesby equation for crosslinked polymers and application for 1,2-polybutadiene crosslinked with AIBN and aryl diazide

The derivation and properties of a tractable function that approximates well the solution of the Flory-Charlesby equation for crosslinked polymers with logarithmic normal primary molecular size distribution is presented. This function gives the possibility to determine the crosslinking density, ρ, from measurements of insoluble fraction, what is the only quantitative method that can be applied for postsynthesis crosslinked polymers. Applications to 1,2-polybutadiene crosslinked with AIBN and 2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone by heating allowed to obtain information about the gel point as well as about the crosslinking yield and mechanism. In addition, it was found that the temperature where the damping factor in DMA curves is maximum increases with ρ following a second order dependence.     

The thermal characteristics of epoxy resin: Design and predict by using molecular simulation method

Trial-and-error approaches for experimentally designing and optimizing the polymer matrix for advanced composites are time-consuming and expensive. The simulation for curing behavior and structure–property relationships of epoxy resins can provide a guideline for designing resin matrix which will possess desirable properties. So far there are few reports in which the accuracy of the molecular simulation for the different amine-epoxy systems are addressed. In this paper, an atomistic modeling technique was used to theoretically investigate the curing and thermal transition behavior of two epoxy resin matrices containing amine curing agent with different chemical structures i.e. diaminodiphenyl methane (DDM)/diglycidyl-4,5-epoxycyclohexane-1,2-dicarboxylate (TDE85) and diaminodiphenyl sulfone (DDS)/TDE85 to give help for designing high heat-resistant epoxy matrix. The simulated results successfully predicted that the reaction process was catalyzed in the early stage of the curing and the slight modification in the diamine structure resulted in significant change in the curing and glass transition behavior of epoxy resin. As the bridging group of diamine changed from methylene to sulphone, the reactivity of diamine toward epoxy declined and the glass transition temperature increased from about 190 °C to about 230 °C. This simulated method presented a good agreement with experimental data, and can be used to design and predict high performance resin matrix for advanced composites.     

UV generation of a multifunctional hyperbranched thermal crosslinker to cure epoxy resins

A new hyperbranched polymer (HBP) was obtained via an iterative synthetic procedure that consists of esterification and thiol-ene click reaction. This polymer was used as a latent multifunctional macroinitiator for the dual curing of a commercially available cycloaliphatic epoxy resin and this process was studied by photo-DSC and FTIR. The presence of thioether groups in the HBP structure leads, by photoirradiation, to the formation of sulfonium salts, which are thermal cationic initiating species. The materials obtained were characterized by DMTA, TGA, gel content and FESEM. By means of the last technique domains of HBP as a second phase within the epoxy matrix were observed.     

聚氨酯和聚醚改性环氧树脂分子结构和性能的探讨

分子复合结构设计是将一种树脂的特性赋予其他种类树脂，来达到改性和提高固化物性能的目的。本文对聚氨酯和聚醚改性环氧树脂的分子结构及性能作了简要的论述。     

核壳聚合物增韧环氧树脂的进展

核壳聚合物（CSP）现已用于增韧环境树脂，它具有许多优点：预先设计的CSP在环氧基体中的形态、大小和分散状态与固化规范无关；在提高环氧树脂韧性的同时不降低玻璃化温度。本文综述了CSP／环氧共混物的性能和增韧机理。主要的增韧机理是CSP粒子空穴化，释放裂缝附近的三轴度，继而产生膨胀形变和剪切屈服。     

Atomistic molecular simulations of structure and dynamics of crosslinked epoxy resin

Many excellent thermal and mechanical performances of cured epoxy resin products can be related to their specific network structure. In this work, a typical crosslinked epoxy resin was investigated using detailed molecular dynamics (MD) simulations, in a wide temperature range from 250 K to 600 K. A general constant-NPT MD procedure widely used for linear polymers failed to identify the glass transition temperature (T_g) of this crosslinked polymer. This can be attributed to the bigger difference in the time scales and cooling rates between the experiments and simulations, and specially to the highly crosslinked infinite network feature. However, by adopting experimental densities appropriate for the corresponding temperatures, some important structural and dynamic features both below and above T_g were revealed using constant-NVT MD simulations. The polymer system exhibited more local structural features in case of below T_g than above T_g, as suggested by some typical radial distribution functions and torsion angle distributions. Non-bond energy, not any other energy components in the used COMPASS forcefield, played the most important role in glass transition. An abrupt change occurring in the vicinity of T_g was also observed in the plots of the mean squared displacements (MSDs) of the crosslinks against the temperature, indicating the great importance of crosslinks to glass transition. Rotational dynamics of some bonds in epoxy segments were also investigated, which exhibited great diversity along the chains between crosslinks. The reorientation functions of these bond vectors at higher temperatures can be well fitted by Kohlrausch-Williams-Watts (KWW) function.     

基体种类对CTBN改性环氧树脂结构和性能的影响

研究了ＣＴＢＮ对不同种类的环氧树脂（Ｅ－５１、Ｆ－５１）力学性能的影响,通过扫描电镜观察了２种增韧体系的微观结构形态,并探讨了２种增韧体系的微观结构形态与力学性能间的关系。     

环氧树脂改性超支化聚酯的合成及性能

以偏苯三酸酐、环氧氯丙烷及甲基丙烯酸缩水甘油酯为原料合成了超支化聚酯(HBP),再通过超支化聚合物的羧基与环氧树脂环氧基的反应得到环氧改性超支化聚合物;用GPC1、H-NMR、DSC、TGA表征了环氧改性超支化聚合物的结构和热性能;比较了不同环氧树脂用量改性前后树脂的光反应活性以及光固化涂层的耐擦洗性和硬度,测定了凝胶率-曝光时间曲线;以环氧改性超支化聚合物配制了光刻胶,在混合光源以及接触曝光的条件下,分辨率达到2~3μm,且图像十分清晰,断面整齐。环氧树脂用量为HBP羧基物质量的70%左右时,改性的超支化聚酯的光固化活性有明显提高,力学性能得到明显改进。     

Cavitation in crosslinked polymers: Molecular dynamics simulations of network formation

Crosslinked organic polymers are used in a wide variety of coatings and composites to distribute stress, increase toughness and protect the substrate by limiting the passage of aggressive chemicals. Enhancing performance of crosslinked polymers requires understanding how precursor chemistry and geometry, as well as crosslinking protocol, determine the structure and performance of the resulting network. Previous molecular dynamics studies have indicated that cavitation produces pores in simulated liquids, even metals (and the resultant solids), when there is only a single type of force, usually van der Waals, between particles. Here we show that nano-sized cavitation voids also occur in a system bound by van der Waals (Lennard–Jones) forces that is additionally crosslinked with strong covalent (FENE) bonds to form 3 or 6 functional solid networks. Cavitation was observed in both systems. These voids are not a consolidation of “free volume”, nor due to a loss of volatiles, but happen as the solidification/cooling stresses exceed the local tensile strength of the material. At temperatures well above the glass transition temperature, “free volume” is distributed evenly throughout the sample in very small pores. As the system cools through its rubbery phase, a few larger voids form via cavitation. Although the loci of these larger voids is associated with crosslinked nodes, cavitation involves the rupturing of weak van der Waals (Lennard–Jones) bonds between molecular chains in regions not constrained by the strong intramolecular bonds. Voids were observed to form during rapid quenches, as well as during much slower cooling at fixed volume, which emulates adhesion of the network to a more rigid body. The voids are large compared to the dimensions of aggressive ionic species and water molecules, and may potentially reduce the barrier properties of a crosslinked coating or composite. Such pore formation, via cavitation, during network formation and curing is not incorporated in current theories of the crosslinking process.     

Fabrication and characterization of the mechanical properties of multi-walled carbon nanotube-reinforced epoxy resins by e-beam irradiation

In this study, the multi-walled carbon nanotubes (MWNT) were reinforced to epoxy resin as fabrication of epoxy/MWNT nanocomposites by electron beam (e-beam) curing. An attempt is made to disperse MWNT into diglycidyl ether of bisphenol A (DGEBA) as epoxy resins, using triarylsulfonium hexafluoroantimonate (TASHFA) as an initiator. E-beam irradiation effect on the curing of the epoxy resin was investigated in oxygen and nitrogen atmospheres at room temperature. The flexural modulus was measured by a universal testing machine (UTM). Here, the flexural modulus factor exhibits an upper limit at 0.3 wt% MWNT. The dynamic mechanical and thermal properties of the irradiated epoxy resins were characterized using DMA, DSC and TGA machines. DMA curves of the storage modulus revealed an increase with an increasing MWNT content and radiation dose. However, the T_g curve decreased as a function of the increasing MWNT content and radiation dose. The thermal properties of the TGA and DSC data were improved by increasing the content of the MWNT and the radiation dose. Likewise, the thermal properties were stabilized by increasing the amount of initiator and irradiating the resins in a nitrogen atmosphere.     

Material properties of the cross-linked epoxy resin compound predicted by molecular dynamics simulation

Molecular dynamics (MD) simulations were conducted to estimate the material properties of the cross-linked epoxy resin compound. A periodic amorphous structure of the cross-linked epoxy resin compound was constructed and it was simulated by continuous accumulation of structure configurations at various temperatures. Based on the simulation results, glass transition temperature (T_g), linear thermal expansion coefficients and Young's modulus of the cross-linked epoxy resin compound were predicted. The predicted values of these material properties are in good agreement with the experimental values in the literature.     

Molecular modeling of crosslinked epoxy polymers: The effect of crosslink density on thermomechanical properties

Molecular dynamics and molecular mechanics simulations are used to establish well-equilibrated, validated molecular models of the EPON 862-DETDA epoxy system with a range of crosslink densities using a united atom force field. Molecular dynamics simulations are subsequently used to predict the glass transition temperature, thermal expansion coefficients, and elastic properties of each of the crosslinked systems. The results indicate that glass transition temperature and elastic properties increase with increasing levels of crosslink density and the thermal expansion coefficient decreases with crosslink density, both above and below the glass transition temperature. The results demonstrate reasonable agreement with thermomechanical properties in the literature. The results also indicate that there may be a range of crosslink densities in epoxy systems beyond which there are limited changes in thermomechanical properties.     

New epoxy thermosets modified with amphiphilic multiarm star polymers as toughness enhancer

The synthesis and characterization of two novel amphiphilic multiarm star polymers with linear polyethylene glycol (PEG) and poly(ε-caprolactone) (PCL) arms and their use as toughening modifiers of epoxy anhydride thermosets are reported. The new star polymers were obtained by partial pegylation of a hyperbranched polyester and subsequent growth of PCL arms. The curing process was studied by calorimetry and thermomechanical analysis, demonstrating the accelerating effect and the influence on gelation of the hydroxyl terminal groups. The curing kinetics was analyzed by model-free and model-fitting methods. The final properties of the resulting materials were determined by thermal and mechanical tests. The addition of the star-like modifiers led only to notable improvement on impact strength in the material containing a 10% of the star with PCL and PEG arms, without compromising glass transition temperature and thermal stability. The morphology of the resulting materials depended on the structure of the toughness modifier used, as demonstrated by electron microscopy, but all modified thermosets obtained showed phase-separated morphologies with nanosized particles.     

Structure and adhesive properties of epoxy resins modified with acrylic particles

The morphology and physical properties of dicyandiamide (DICY)-cured epoxy resin modified with acrylic particles were studied. We used one homopolymer of methyl methacrylate (MMA) and three copolymers of MMA and glycidyl methacrylate (GMA) [P(MMA-GMA)] containing different amounts of GMA as the acrylic particles. When a mixture of the acrylic particles and the epoxy resin was heated, the particles were swollen with the epoxy resin and thus a soft gel with no fluidity was formed. Further heating to the reaction temperature of the DICY cured the soft gel. The structure of the acrylic particles strongly affected the physical properties of the soft gel and the cured epoxy resin. The cured system containing 5 mol% of GMA showed the best physical properties (impact strength and adhesive property), but there was a tendency for the physical properties to decline with a higher GMA content. We have determined that the GMA content of the acrylic particles affects the concentration of network chains in the system.     

Phosphorus-containing diamine for flame retardancy of high functionality epoxy resins. Part II. The thermal and mechanical properties of mixed amine systems

Bis(4-aminophenoxy)phenyl phosphonate (BAPP) was used in various ratios in combination with diethyltoluenediamine (DETDA) to investigate the effect of increasing phosphorous addition upon the thermal and mechanical properties of diglycidyl ether of bisphenol A (DGEBA) and tetra glycidyl dimethylenedianiline (TGDDM) based epoxy networks. Increasing phosphorous content was shown to decrease the temperature of the initial onset of degradation, while the char yield at 540 °C increased in both the nitrogen and air atmosphere. At 700 °C, however, the char formation was shown to be dependent upon the atmosphere with an oxidative environment promoting further chain scission, while a non-oxidative environment promoted char formation through crosslinking reactions. Increasing phosphorous content was found to produce only modest decreases in the glass transition temperatures, while no significant deleterious effects upon the mechanical and physical properties such as flexural properties and CTE were observed.     

Effect of structure on coating performance properties of novel alkylenedioxydiphenol based epoxy resins

Although epoxy resins exhibit a wide range of useful properties for protective coatings on metal substrates, an increasing trend in the coatings industry toward more formable coatings places a severe burden on the traditional bisphenol. A based epoxy resins. Presented in this work are the results of experiments with a new series of epoxy resins based on alkylenedioxy diphenols demonstrating the relationship between coating performance properties (formability, chemical resistance, and adhesion) and structural parameters such as resin backbone flexibility, degree of phenol/formaldehyde crosslinkmg, and concentration of OH groups in the backbone of epoxy resins. Optimizing the three structural parameters for the new resins results in substantial improvements in coating performance properties over the traditional bisphenol A based resins.     

Maleimide-epoxy resins: preparation, thermal properties, and flame retardance

Maleimide modified epoxy compounds were prepared through reacting N-(4-hydroxylphenyl)maleimide (HPM) with diglycidylether of bisphenol-A. Triphenylphosphine and methylethylketone were utilized in the reactions as a catalyst and a solvent, respectively. The resulting compounds possessed both oxirane ring and maleimide group. The kinetics of the curing reactions of the maleimide-epoxy compounds and amine curing agents, 4,4-diaminodipheylmethane (DDM) and dicyandiamide (DICY), were studied. Incorporation of maleimide groups into epoxy resins provided cyclic imide structure and high cross-linking density to the cured resins, to bring high glass transition temperatures (179 °C) and good thermal stability (above 380 °C) to the cured resins. High char yields in the thermogravimetric analysis and high limited oxygen index values (25.5-29.5) were also observed for the cured resins to impy their good flame retardance.