Tetraglycidyl Diaminodiphenyl Methane Research Articles

Rigid and crosslinkable polyimide curing epoxy resin with enhanced comprehensive performances

The flammability and hydrophilicity of epoxy resins hinder their applications in the high-tech fields of aerospace and electronics. To break these limitations, a highly rigid and hydrophobic polyimide structure has been designed as the harder for epoxy resins. This diamine monomer polyimide (PI–OH) featured a new spiral fluorene-xanthene core doubly grafted phenolic hydroxyl units, to match the commercial bi-trifluoromethyl dianhydride monomer. The curing performance of the polyimide PI-OH was studied in a concentration gradient fashion with the epoxy resin N, N, N′, N′-tetraglycidyl-4, 4′-diaminodiphenylmethane (TGDDM), and compared to the benchmark curer 4, 4′-diaminodiphenyl sulfone (DDS). Due to the unique structural rigidity of spiral crosslinks and the compatibility of the polyimide, the cured PI-OH/TGDDM exhibited Young's modulus of 6.18 GPa and a hardness of 480 MPa, 27.7% and 42.3% higher than that of DDS/TGDDM, demonstrating excellent comprehensive performance. The introduction of PI-OH also reduced the curing temperature with higher curing activity. Resin with 40% loading of the polyimide reached a residual carbon rate of 49.0% at 700 °C (under nitrogen), presenting flame retardancy superior to DDS/TGDDM. Furthermore, the involvement of trifluoromethyl groups reduced the surface free energy and hydrophilicity, lowering the water absorption rate for the modified epoxy resin. This study opens an avenue for comprehensively improving the performance of epoxy resins at lower curing temperatures, which has broad application prospects in aerospace and motor vehicle fields.

Constructing and characterizing various multi-component crosslinked epoxy resins based on molecular dynamics simulations with a curing reaction model

Epoxy resins are extensively used as the matrix for carbon-fiber-reinforced plastic (CFRP) in commercial airplanes due to their outstanding thermomechanical properties. Until now, few researchers have simulated multicomponent epoxy resins. In this work, the impact of stoichiometric variations in the base resins in the Di Glycidyl Ether of Bisphenol A (DGEBA)/Tetra Glycidyl Diamino Diphenyl Methane (TGDDM)/4,4′-Diamino Diphenyl Sulfone (4,4′-DDS) system on the curing process and thermomechanical properties is investigated. When the proportions of each component are comparable, the curing process is primarily determined initially by the component with a lower activation energy and subsequently by the component with a relatively greater quantity during the high conversion rate stage. Furthermore, a higher proportion of TGDDM in the DGEBA/TGDDM/4,4′-DDS system results in an increased number of ring structures, which in turn leads to enhanced thermomechanical properties. Our Python-LAMMPS algorithm can be further developed for designing new multi-component epoxy resin materials.

Fully aminated rigid‐rod aromatic polyamide efficiently reinforces tetraglycidyl diamino diphenyl methane epoxy resin and its carbon fiber composite

AbstractTetraglycidyl diamino diphenyl methane (TGDDM) and its carbon fiber composite are widely studied and used because of their heat resistance and high performance. In order to obtain efficient reinforcing effects in a simple manner, we adopt the idea of molecular composite—uniformly dispersing the rigid‐rod macromolecules in the TGDDM matrix. Herein, we prepared fully aminated‐poly(p‐phenylene terephthalamide) by direct polycondensation and hydrogenation reduction as the reinforcing agent. The fully aminated‐PPTA is distributed uniformly in the cross‐linked epoxy resin (EP) network by chemical bonds, and achieves efficient reinforcement. With the addition of only 0.5 wt% fully aminated‐PPTA, the tensile strength and Young's modulus of EP are improved to 127.7 ± 7.4 MPa (+28%) and 3.36 ± 0.15 GPa (+44%); the flexural strength and modulus are improved to 209.6 ± 9.4 MPa (+42%) and 3.70 ± 0.14 GPa (+37%); the notch impact strength is improved by 32%. Crucially, the heat resistance of EP almost remains unchanged. Furthermore, the flexural properties and interlaminar shear strength of the carbon fiber composite are also comprehensively increased. Compared with other modified fillers, the fully aminated‐PPTA is simple to prepare and blend, and is an efficient reinforcing agent for TGDDM and its carbon fiber composite benefitting from its rigid‐rod main chain and amino side groups.Highlights Molecular composite can achieve excellent strengthen and toughen effects. Fully aminated‐PPTA is embedded uniformly in EP network by chemical bonds. TGDDM is effectively reinforced without deteriorated heat resistance. Fully aminated‐PPTA is also a comprehensive reinforcement for EP/CF composite.

Fracture behavior of high‐performance carbon fiber composites toughened by reactive polyetherimide

AbstractCarbon fiber‐reinforced epoxy composites (CFECs) continue to grow in demand for aerospace, mass transportation, electric vehicle, and energy efficient architecture due to their lightweight, high strength, and high modulus. However, CFECs are susceptible to delamination upon impact which can lead to premature structural failure. This paper investigates the interlaminar fracture toughness of unidirectional CFECs based on reactive polyetherimide (rPEI) modified multifunctional epoxy matrix system. The matrices were cured formulations of tetraglycidyl diamino diphenyl methane, triglycidyl para‐amino phenol, rPEI having different molecular weights (MW) and diaminodiphenyl sulfone. Multi‐phase morphologies were observed from the resin castings and CFECs containing different MW rPEI modifiers. Results show that the mode I fracture toughness (GIC) of the rPEI‐toughened CFECs can be increased by as much as 150% over the unmodified CFECs based on the MW and type of phase morphology that promotes crack‐bridging mechanism. In addition, rPEI‐modified CFECs exhibit better property retention (~80% of GIC) after prolonged solvent exposure.Highlights A processing‐structure–property relationship based on CFECs is established. The amine‐terminated rPEI facilitates the formation of a strong bonding. The molecular weight effect of rPEI on the phase morphology was investigated. The rPEI enhances the interlaminar fracture toughness of the composites. The rPEI particle bridging is found to be responsible for the toughening effect.

Synthesis and characterization of epoxy resin composites modified by reactive polyimide containing hydroxyl groups

Reactive polyimide (PI) containing phenolic hydroxyl groups was prepared by the polymerization of 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane and mixed thiodiphthalic anhydride. A series of epoxy resin composites were obtained by adding reactive PI to N,N,N′,N′-tetraglycidyl-4,4′-diamino-diphenylmethane (TGDDM) epoxy resin in different proportions. The effect of PI content on curing behavior, thermal properties, and mechanical properties was carefully investigated. Even a small amount of PI addition improved the mechanical properties and heat resistance of the epoxy resin, particularly the fracture toughness. Among them, epoxy resin composites EP-PI-1.5 with the addition of 1.5 wt% PI showed the best mechanical properties, whose impact strength, fracture toughness, and tensile strength were increased by 114%, 59%, and 34%, respectively. The phase morphology was also investigated to illustrate the fracture mechanism by scanning electron microscope (SEM) characterization.

Enhancing the mechanical properties and heat resistance of epoxy resin via blending with polythioetherimide

AbstractIn this work, poly(thioetherimide) (PTEI) was used to enhance the mechanical properties and thermal stability of an epoxy resin (EP),N,N,N′,N′‐tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM). The effect of PTEI on curing behavior, dynamic mechanical, thermal properties, mechanical properties and phase morphology were carefully investigated. The addition of PTEI could improve the performances of EP, and even if only low content of PTEI was added, the heat resistance and mechanical properties were improved, especially for the fracture toughness. When the addition of PTEI was 2 wt%, the impact strength, fracture toughness, elongation at break, tensile strength, and flexural strength of the modified epoxy increased by 78%, 50%, 25%, 30%, and 20%, respectively. In addition, phase morphology was also discussed to validate the results of thermal and mechanical analysis. All these results proved the superior performance of the modified EP.

Mechanical properties of reactive polyetherimide-modified tetrafunctional epoxy systems

Highly crosslinked multifunctional epoxy resins are commonly brittle. Thermoplastic toughening agents have been used to improve the fracture toughness of crosslinked epoxy resins. However, toughness improvement is limited due to poor interfacial adhesion between epoxy matrix and thermoplastic toughener. Furthermore, it is desired that toughness improvements should not come at the expense of other properties, including modulus, thermal properties and processability. In this work, amine functionalized reactive polyetherimide (rPEI) having two different molecular weights and loading levels were incorporated in tetraglycidyl-4, 4′-diaminodiphenyl methane (TGDDM) epoxy resin to study the structure-property relationship, including effects of reaction-induced phase separation on fracture toughness. Results demonstrate that the phase morphologies of rPEI in an epoxy resin can be drastically affected by the functionality, molecular weight and loading levels of rPEI. The mode-I fracture toughness (KIC) of rPEI-toughened epoxy is increased by up to 140% versus untoughened epoxy control. An investigation into the fracture mechanisms of rPEI-toughened TGDDM epoxy showed evidence of rPEI particle bridging as one of the key mechanisms of toughening. Our work further demonstrated that the rPEI-toughened TGDDM epoxy maintained both high modulus and high Tg, rendering it suitable for a wide range of applications.

Fast Upcycling of Poly(ethylene terephthalate) into Catalyst-Free Vitrimers

We reported a catalyst-free technique to fabricate transesterification-based poly(ethylene terephthalate) (PET) vitrimers via melt reacting PET with N,N′-tetraglycidyldiaminodiphenyl methane (TGDDM) in several minutes. We investigated the effect of TGDDM content on the reaction mechanism and chemical structure of the final products in detail. At low TGDDM content (≤0.5 wt %), branched thermoplastic PET was obtained by the chain-extension reaction between carboxyl groups of PET and epoxy groups of TGDDM. At medium TGDDM content (0.7–1.0 wt %), PET vitrimers with relatively low cross-link density were achieved through the cross-linking reaction between original carboxyl plus hydroxyl groups of PET and epoxy groups of TGDDM. At high TGDDM content (1.2–2.0 wt %), soluble chain-extended products were first formed through reacting original carboxyl and hydroxyl groups of PET with part of epoxy groups of TGDDM, and PET vitrimers were then obtained after the excess epoxy groups were consumed by the newly formed functional groups, which were generated by thermal decomposition of ester groups of PET under catalyzation of tertiary amines of TGDDM moieties. The cross-linked network structure endowed the PET vitrimers with enhanced heat resistance and creep resistance. The obtained PET vitrimers could relax stress rapidly and showed excellent processability and weldability, owing to the fast transesterification between hydroxyl and ester groups under catalyzation of internal tertiary amines. We believe that the facile fabrication technique showed great potentiality for upcycling of PET into vitrimers in large-scale production.

High-performance carbon nanofiber reinforced epoxy-based nanocomposite adhesive materials modified with novel functionalization method and triblock copolymer

New high-performance epoxy-based nanocomposite adhesive materials were developed with the innovative use of nanomaterials to enhance the bond strength, especially at elevated temperatures. A tetraglycidyl diaminodiphenylmethane (TGDDM) based aerospace-grade epoxy adhesive (EA9396) with a high glass transition temperature and viscosity was combined with functionalized CNFs and a phase-separated poly(styrene)-poly(butadiene)-poly(methyl methacrylate) triblock copolymer (SBM). The ozone-treated CNFs (OZ-CNFs) were functionalized with polyethyleneimine dendrimer (PEI + OZ-CNFs), or a polyamine hardener (H + OZ-CNFs) and characterized using X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to confirm the chemical reaction of the amine functional groups. Room temperature mechanical testing revealed the optimized nanocomposite adhesives containing SBM and CNFs functionalized with either PEI or hardener resulted in a 40% increase in lap shear strength (50 MPa) when compared to the unmodified epoxy (35.6 MPa). At the elevated temperature of 90 °C, a strength improvement of 35% and 38% (29.5 MPa and 30 MPa) was observed for the adhesives containing SBM and CNFs functionalized using PEI or hardener, respectively. SEM images showed that CNF functionalization provided a more uniform distribution in the adhesive and enhanced the SBM plastic deformation during crack propagation. The PEI and hardener functionalization also significantly reduced the level of CNF pull-out, which was believed to enhance the energy dissipation mechanisms responsible for the significant increase in the lap shear strength.

Evaluation of augmented thermal, thermo-mechanical, mechanical properties of nano alumina reinforced TGDDM epoxy nanocomposites

N,N′-Tetraglycidyldiaminodiphenyl methane (TGDDM) was reinforced with various weight fractions (0.5, 1, and 1.5 wt%) of amine functionalized nano alumina (F-Al) were cured with diaminodiphenyl-methane (DDM). FT-IR analysis revealed that formation of functionalized nano alumina (F-Al) structure, was brought about via coupling agent APTES. Furthermore, the morphology of TGDDM epoxy nanocomposites was studied using X-ray Diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), microscopic analysis and an atomic force microscope (AFM). We found a bonding relationship between TGDDM epoxy and F-Al in TGDDM/F-Al nanocomposites It was interesting to note that the values of tensile, flexural and impact strength of 1 wt% F-Al reinforced TGDDM epoxy nanocomposites were found to be 141.5, 192.5 MPa, and 92.4 J/m2, respectively., which resulted in a substantial improvement in the dynamic mechanical analysis (DMA) to 4.3 and 5.5 for 0.5 and 1 wt% F-Al reinforced TGDDM epoxy nanocomposites and the glass transition temperature (Tg) increased from 210°C to 225°C as the F-Al content increased. The initial degradation temperature (IDT) of 0.5, 1, and 1.5 wt% F-Al reinforced TGDDM epoxy nanocomposites were significantly enriched to 328°C, 345°C, and 335°C respectively from 290°C of neat (TGDDM) epoxy matrix. Likewise, the char yield for the neat (TGDDM) epoxy matrix was 13% and that for 0.5, 1, and 1.5 wt% F-Al reinforced TGDDM epoxy nanocomposites were 17%, 25%, and 20% respectively. It is feasible to state unequivocally that considerable F-Al diffusion within the TGDDM epoxy can only occur at low weight percentages. The results clearly showed that F-Al reinforced TGDDM epoxy nanocomposites may be investigated for advanced high performance industrial engineering applications.

Molecular-scale investigation on relationship between thermal conductivity and the structure of crosslinked epoxy resin

Epoxy resins are widely used as matrix resins in many fields due to their excellent physical and mechanical properties. However, their low thermal conductivities restrict their applications, especially in electronic packaging. In this work, nonequilibrium molecular dynamics (NEMD) simulations were used to investigate the relationship between thermal conductivity and epoxy resin structure. Three systems, diglycidyl ether of bisphenol A (DGEBA)/4,4’-diamino diphenyl sulfone (4,4’-DDS), DGEBA/diethylene triamine (DETA), and tetraglycidyl diamino diphenyl methane (TGDDM)/4,4’-DDS, were simulated based on our previous crosslinking algorithm. The thermal conductivity of DGEBA/4,4’-DDS increased with the degree of crosslinking, which is due to increases in new thermal pathways through crosslinking bonds. The sequence of thermal conductivities was found to be TGDDM/4,4’-DDS > DGEBA/DETA > DGEBA/4,4’-DDS which was quantitatively validated with our experimental measurements, consistent with the sequence of crosslink density. To explore the microscopic mechanism governing thermal conductivity, separate heat transfer modes were investigated, revealing that the thermal conductivity contribution of bonded interactions dominates over that of non-bonded interactions for all three systems. This finding verifies that introducing crosslinking bonds dominantly contributes to higher thermal conductivity in epoxy materials. Our research provides suggestions for large scale manufacture of higher thermal conductive epoxy resins.

Inherent cohesive failure of epoxy adhesive in carbon-fiber-reinforced plastic composites revealed by micro-tensile testing and finite element analysis

Cohesive failure in the epoxy adhesive layers of carbon-fiber-reinforced plastic (CFRP) joints is incomprehensible because the adhesion interface with almost no covalent bonds is supposed to be the weakest in the joined structure. It is a long-standing unsolved issue that adhesive failure at the interface is a rare case. Cohesive failure in CFRP joints may be caused by micrometer-scale defects in their adhesive layers. In the aviation industry, one of the typical thermosetting molecular systems is a combination of the diglycidyl ether of bisphenol A (DGEBA) adhesive and the tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) adherend, both of which are hardened with the 4,4′-diaminodiphenyl sulfone (DDS) curing agent. Eliminating the influence of microdefects, the fracture mechanism was studied using micro-tensile test specimens with gauge dimensions of 10 × 10 × 30 μm3, which were locally extracted from macroscale adhesion blocks. Two cohesive failure modes were predominantly observed with equal probabilities: 0.1–1 μm away from the interface and at the middle of the DGEBA section, except infrequent adhesive failure because of weak bonds. Cohesive failure is demonstrated to be an inherent behavior, which is consistent with the finite-element-method (FEM)-derived stress profiles, provided that the adhesion interface sustains the load stress. A phenomenological model for cohesive failure is proposed, which helps explain the fact that the elastically deformable adhesion interface with the physical bonding due to van der Waals forces is stronger than the adhesive and adherend hardened by both physical and chemical bonding. These findings lead to improvements in the reliability of CFRP components.

Internal topology and water transport in tetrafunctional epoxy resins

AbstractWater sorption into epoxy resins limits the performance lifetime of many composites, adhesives, and coatings. One hypothesis for the well‐known hygroscopicity of epoxies is that the well‐known internal nodular topology of epoxy networks enhances permeability. This theory has however remained untested to date, since previous attempts to manipulate the internal topology have been accompanied by changes in the hydrogen bonding capability of resins, which dominates water uptake. Here, the nanostructure of highly cross‐linked network polymers based on tetra glycidyl diaminodiphenyl methane is varied in the absence of significant polarity effects by careful optimization of the cure schedule. The internal morphology of resins cured at 100, 150, and 200°C are characterized using Peakforce tapping mode atomic force microscopy, whilst the cure progression at each temperature is analyzed using in‐situ transmission mode infrared spectroscopy. Distinct nodular morphologies are found to form at high cure temperatures (150 and 200°C), where spectroscopic analysis demonstrates that excess epoxy consumption occurs prior to the gel point, as a result of intra‐cluster cross‐linking and reduced reaction selectivity. Surprisingly, the establishment of an internal nanostructure is, however, found to have negligible effect on the extent and kinetics of moisture sorption.

Uncovering the Mechanism of Size Effect on the Thermomechanical Properties of Highly Cross-Linked Epoxy Resins.

Epoxy resins are widely used as matrix resins, especially for carbon-fiber-reinforced plastic, due to their outstanding physical and mechanical properties. To date, most research into cross-linking processes using simulation has considered only a distance-based criterion to judge the probability of reaction. In this work, a new algorithm was developed for use with the large-scale atomic/molecular massively parallel simulator (LAMMPS) simulation package to study the cross-linking process; this new approach combines both a distance-based criterion and several kinetic criteria to identify whether the reaction has occurred. Using this simulation framework, we investigated the effect of model size on predicted thermomechanical properties of three different structural systems: diglycidyl ether of bisphenol A (DGEBA)/4,4'-diaminodiphenyl sulfone (4,4'-DDS), DGEBA/diethylenetriamine (DETA), and tetraglycidyl diaminodiphenylmethane (TGDDM)/4,4'-DDS. Derived values of gel point, volume shrinkage, and cross-linked resin density were found to be insensitive to model size in these three systems. Other thermomechanical properties, i.e., glass-transition temperature, Young's modulus, and yield stress, were found to reach stable values for systems larger than ∼40 000 atoms for both DGEBA/4,4'-DDS and DGEBA/DETA. However, these same properties modeled for TGDDM/4,4'-DDS did not stabilize until the system size reached 50 000 atoms. Our results provide general guidelines for simulation system size and procedures to more accurately predict the thermomechanical properties of epoxy resins.

Aromatic tetra-glycidyl ether versus tetra-glycidyl amine epoxy networks: Influence of monomer structure and epoxide conversion

In this work, two epoxy resins, bis(2,7 glycidyl ether naphthalenediol) methane (NNE), tetraglycidyl diaminodiphenylmethane (TGDDM) and a miscible blend of the two are cured with 4,4′-diaminodiphenylsulfone (DDS) to investigate the differences between glycidyl ether and glycidyl amine crosslinked networks. This work explores in depth, and directly compares for the first time, the structure/property relationships of a highly aromatic and thermally stable glycidyl ether epoxy network with a well-known highly aromatic and thermally stable glycidyl amine epoxy network. The highly aromatic naphthalene motif of NNE imparts improved thermal stability and a significantly higher glass transition temperature, above 340 °C depending on the measurement technique. However, it also produces comparatively lower epoxide conversion, higher moisture ingress and lower flexural properties. These properties are attributed to the NNE having higher free volume and glassy state mobility as suggested by the sub-ambient β relaxations. The glycidyl ether groups react primarily via epoxide amino addition displaying only very minor evidence of homopolymerisation or other side reactions even at 240 °C. This contrasts with the glycidyl amine groups of TGDDM which exhibit a much higher degree of non-epoxy amine addition. Increasing cure conversion also increases the glass transition temperature, fracture toughness and reduced coefficient of thermal expansion. Overall, the results show that despite some reduced mechanical and thermal properties the NNE epoxy resin system is a very thermally stable high temperature epoxy resin based upon glycidyl ether functional groups suitable for a wide range of new applications.

Synergetic effect between curing reaction and CO2 diffusion for microcellular epoxy foam preparation in supercritical CO2

Directly rapid depressurization foaming process has been successfully applied to prepare microcellular epoxy foams using supercritical CO2 as blowing agent. It is found that both curing reaction and CO2 diffusion are crucial for controlling cell morphology. For epoxy of tetraglycidyl-4,4′-diaminodiphenyl-methane (TGDDM) and 4,4-diaminodiphenylsulfone (4,4-DDS) system, the experimental and molecular simulation results displayed that higher CO2 pressure always promotes curing reaction, and CO2 diffusion is more sensitive to pressure at low curing degree, while limited by high curing degree. The foaming experimental results showed that there existed a suitable curing degree range, in which both unsaturated CO2 concentration and curing degree contribute to good cell structure. Higher CO2 pressure can broaden this foamable window. The microcellular epoxy foam with an average cell diameter of 14.6 µm and cell density over 109 cells/cm3 as well as volume expansion ratio of 14.5 has been obtained at 85% curing degree under 22 MPa CO2 pressure.

Catalyst-free curing and closed-loop recycling of carboxylated functionalized rubber by a green crosslinking strategy

Current adopted rubber curing methods undergo large amounts of environmental issues, such as the release of toxic volatile organic compounds (VOCs) during curing process and difficulties in recycling of end-of-life rubbers. Many new curing reactions and cross-linking structures have been developed to solve these problems. A green curing strategy based on epoxy-acid reactions and ester linkages could be a potential alternative to the current curing ones. However, the curing reactions between epoxy and carboxyl groups generally require a catalysis to improve curing rate, and the recycling process requires toxic organic solution or high temperature. It is of great desired to develop a real environmentally friendly crosslinking strategy for rubber materials. In this paper, we report a facile and high-efficiency green crosslinking strategy for the carboxyl-functionalized rubber. N,N,N′,N'-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) was selected as a multifunctional amine-type high-efficiency crosslinker for carboxylated nitrile rubber (XNBR). TGDDM-cured XNBR exhibit extremely fast crosslinking rate and high crosslinking efficiency due to the four epoxy terminated units and tertiary amine structure acting as an internal catalyst. Besides, the mechanical properties and crosslinking density of TGDDM-cured XNBR can be regulated by TGDDM content. Furthermore, the ester linkage formed by curing reaction could be selectively and efficiently cleaved in a mixed solution of ethanol and water under mild condition. The closed-loop recovered XNBR maintains chemical structure and molecular weight and exhibit good mechanical performance. This strategy offers a promising way for high-efficient crosslinking of diene rubber as well as a real cost-effective and environmentally friendly recovery of end-of-life rubber materials.

Simulation Study of the Effects of Interfacial Bonds on Adhesion and Fracture Behavior of Epoxy Resin Layers.

The adhesion and fracture behavior of tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM)/4,4'-diaminodiphenyl sulfone (44DDS)-bisphenol A diglycidyl ether (DGEBA)/44DDS layer interfaces were investigated by molecular dynamics (MD) simulation, mainly focusing on the role of covalent and noncovalent interactions. To accurately investigate the bond dissociation processes, the force field parameters of several bond potentials of the epoxy resin polymers were optimized by density functional theory calculations. In the MD simulations under a tensile load, small voids gradually developed without covalent bond dissociation in the plateau region. In the final large strain region, the stress rapidly increased with bond breaking, leading to failure. When the chemical bonds across the interface between the two layers were removed, the stress-strain curve in the initial elastic region was almost the same as that with interfacial bonds. This showed that the nonbonded interactions governed adhesion strength in the initial elastic region. In contrast, the bonded interactions at interfaces played important roles in the hardening regions because the bonded interactions made the major contribution to the fracture energies. We also investigated the effect of the etherification reaction in cross-linking. It was found that the etherification reaction mainly contributed to the behavior in the late region with large strain. These simulation results revealed that the nonbonded interactions, especially, van der Waals interactions, are important factors for adhesion of the different polymer layers in the small strain region up to the yield point.

The Curing Kinetics Analysis of Four Epoxy Resins Using a Diamine Terminated Polyether as Curing Agent

Diamine terminated polyether (DAPE) is used as a novel curing agent for ductile epoxy resins. To investigate the curing kinetics, non-isothermal differential scanning calorimetry (DSC) tests were conducted to investigate four ductile epoxy/DAPE systems, including tetraglycidyl-4,4′-diaminodiphenyl methane (TGDDM)/DAPE, diglycidyl 4,5-epoxycyclohexane-1,2-dicarboxylate (TDE-85)/DAPE, triglycidyl-p-aminophenol (TGPAP)/DAPE and hydantoin epoxy resin (HER)/DAPE systems. According to the Malek method, the four epoxy systems suit the Sestak-Berggren kinetic model (SB (m,n)). The SB (m,n) model can well fit the curing reaction of all epoxy systems. By applying the Friedman method, the activation energies (Ea) at different conversion (α) were calculated, which provide an insight of curing mechanism of the flexible curing agent for ductile epoxy resins.

Preparation Characterization of Modified Poly(Lactic Acid) Using Dicumyl Peroxide and Tetraglycidyl Diamino Diphenyl Methane

Poly(lactic acid) (PLA) was modified using dicumyl peroxide (DCP) and tetraglycidyl diamino diphenyl methane (TGDDM) to improve its melt viscosity and molecular weight for a variety of industrial applications. The PLA was branched or crosslinked with DCP at 180 °C, and gel content was detected as the DCP concentration was over 0.6 phr. However, for TGDDM, it was observed that only chain extension or branching occurred; the number- and weight-average molecular weights and polydispersity index (PDI) increased. The melt flow index decreased as the concentration levels of DCP and TGDDM increased. However, when the TGDDM was over 0.5 phr, the opposite results were observed. The variations in the mechanical properties of modified PLA were marginal; however, the elongation at break slightly decreased. The onset value of the decomposition temperature of the DCP-modified PLA slightly decreased. In contrast, with TGDDM, the onset value slightly increased. As the branching and crosslinking of the modified PLA increasingly occurred, relatively low relaxation enthalpy was detected. However, the glass transition temperature (Tg) was not influenced. An increase in the storage modulus (G′), loss modulus (G″), and complex viscosity (η*), and a decrease in tan δ at low frequencies showed that the rheological behavior of the modified PLA turned from liquid-like to solid-like because of the modifications achieved using DCP and TGDDM.