Stress Relaxation in Epoxy Thermosets via a Ferrocene-Based Amine Curing Agent (original) (raw)
Related papers
Effect of Conversion on the Structure-Property Relationships of Amine-Cured Epoxy Thermosets
The effects of conversion beyond the gel point on the structure-property relationships of epoxy thermosets using formulations representative of the most commonly used epoxy resin and amine curing agents at balanced stoichiometry with an emphasis on the thermal, tensile, and fracture properties were studied. The range of T g from just beyond the gel point to full conversion typically is >100 °C. Fracture toughness (as K 1c) of the epoxy thermosets cured with relatively flexible amines such as ethylenediamine (EDA), diethylenetriamine (DETA), and m-xylylenediamine (MXDA) reaches near its full cure value at only ∼65-70% conversion. The maximum in K 1c for these types of epoxy thermosets is at ∼90% conversion for EDA and DETA but just below its full cure for MXDA. Isophoronediamine represents a special case for fracture behavior because of its apparent substantial cyclization during cure. In the 4,4′-diaminodiphenylsulfone series, K 1c generally increases with conversion as the concentrations of their strongly antiplasticizing soluble and pendant fractions decrease. A uniform trend of decreasing tensile modulus with increasing conversion was observed in each formulation and is consistent with the expected decrease in the cohesive energy density as monomer glass is transformed into polymer glass.
European Polymer Journal, 2015
The network build-up process during curing of an epoxy resin using a hyperbranched poly(ethyleneimine) as crosslinking agent has been studied from a theoretical and experimental point of view. A systematic analysis taking into account the stoichiometry of the curing process has been performed. Conversion at gelation has been studied by thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). Crosslinking density has been studied by dynamomechanical analysis (DMA). Gel fraction after extraction in organic solvents has also been determined. The experimental results have been compared with a theoretical network build-up model based on the random recombination of structural fragments, showing good agreement between theory and experimental results, but deviations from the ideal epoxyamine polycondensation appear as a consequence of the dilution of the hyperbranched crosslinker in offstoichiometric formulations.
Effect of chain flexibility in the network structure on the viscoelasticity of epoxy thermosets
Journal of Polymer Research, 2011
In this paper a detailed study in investigating the effect of the chain flexibility in epoxy-amine crosslinked network is done. In order to introduce flexibility into the crosslinked network a homologous series of four aliphatic diamine curing agents varying only in the chain length and having a constant functionality (f=4) is taken and cured stoichiometrically with aromatic epoxy (f=2) resin. For each of the cured mixture the viscoelastic master curve and corresponding shift factors were determined. It is found the introduction of flexibility shifts the viscoelastic curves by 5 decades with respect to frequency scale. This shift in the viscoelastic curve is modeled with a parameterized Havriliak-Negami model for the master curve. The free volume contribution for the changes in the coefficient of thermal expansion at T g is also determined.
Polymer, 1999
This study examines the relaxation process at different temperatures in partially cured samples of an epoxy resin and of the same resin modified with 11.1% carboxyl-terminated butadiene acrylonitrile copolymer (CTBN). A differential scanning calorimeter (DSC) was used, which enabled the relaxation enthalpy, the glass transition temperature (T g ) and the relaxation peak temperature to be found. CTBN modifies the cure reaction and the relaxation process of the epoxy resin. The study of the relaxation process has a practical interest in addition to a theoretical one. It is known that, with these thermosets, a fully cured sample cannot be obtained at the first step. The introduction of an elastomeric modifier is a normal way of increasing the toughness of these materials. ᭧
Molecular dynamics predictions of thermomechanical properties of an epoxy thermosetting polymer
Polymer, 2020
This paper reports the thermomechanical properties of a thermosetting polymer formed by curing a DGEBA resin with a Jeffamine D230 agent predicted by molecular dynamics (MD) simulations. A multistep crosslinking approach is used to form the crosslinked network of the thermosetting polymer. The radial distribution function and X-ray diffraction pattern of the MD predicted crosslinked structure are calculated and compared with experimental results to validate the epoxy network system. Thermomechanical properties such as mass density, gel point, glass transition temperature (g), elastic moduli (Young's modulus and shear modulus), and yield strength in shear and tension are calculated at different temperatures and crosslinking conversions by employing the DREIDING and AMBER force fields. The MD predicted results are in good agreement with theoretical studies and existing experimental data. We find a significant increase in g and yield strength with crosslinking conversion. The elastic modulus is less sensitive to the strain rate, but the yield strength is significantly strain-rate dependent. The high-quality digital epoxy configurations developed in this work are available in LAMMPS data format from the journal website.
Relaxation transitions in mixed epoxy networks based on diane and aliphatic epoxy resins
Russian Journal of Physical Chemistry A, 2015
Spectra of the internal friction of cured epoxy-amine networks based on diane and an aliphatic epoxy oligomers are studied using an aliphatic oligoamine hardener. It is shown that secondary relaxation is observed in the spectra for diane oligomer in the investigated range of temperatures, but not for aliphatic oligomers or cured epoxy amine systems based on them. Using van Krevelen calculations based on group contributions, is confirmed that the secondary relaxation is caused by the motion of aliphatic (glyceryl and oxypropylene) fragments, and the degree of curing does not affect them prior to gelation. It is found that the secondary relaxation of aliphatic fragments is hampered after the gel point, raising the temperature and activation energy of the transition. An original method for calculating the number of structural elements involved in the elementary act of primary relaxation is proposed that allows for the possible movement of aliphatic and diphenylolpropane fragments.
Journal of Polymer Science Part B: Polymer Physics, 2008
Five families of new controlled epoxy thermosets (CENs) using three monophenol chain terminators were prepared to study systematic changes in the structure and amount of the monophenol and the initial molecular weight between crosslinks (M c,i ) on the properties of epoxy thermosets. Glass transition temperature (T g ) decreases with monophenol mole fraction (v) in proportion to both the concentration and flexibility of the chain terminator. Distinct serial relations for T g depression were observed for the three M c,i families. Dynamic mechanical analysis (DMA) shows significant perturbations of the relaxation behavior with added terminator as evidenced by decrease in peak tan d and in post T g damping. The rubbery coefficients of thermal expansion (CTE) increases with monophenol concentration only at v > 0.05 and shows distinct curvature versus temperature, but is largely invariant with monophenol flexibility. The thermal stability of terminated CENs decreases only slightly with v and little difference was found with monophenol structure. Most surprisingly, fracture toughness decreases markedly and discontinuously with v depending on M c,i . The values of the critical monophenol concentration at which fracture toughness markedly decreases (v c ) are inversely proportional to M c,i but are independent of monophenol flexibility. No correlation of v c with any of the calculated network structure parameters was apparent.
Macromolecules, 2012
Molecular dynamics and molecular mechanics simulations have been used to study thermo-mechanical response of highly cross-linked polymers composed of epoxy resin DGEBA and hardener DETDA. The effective cross-linking approach used in this work allowed construction of a set of stress-free molecular models with high conversion degree containing up to 35000 atoms. The generated structures were used to investigate the influence of model size, length of epoxy strands, and degree of cure on thermo-mechanical properties. The calculated densities, coefficients of thermal expansion, and glass transition temperatures of the systems are found to be in good agreement with experimental data. The computationally efficient static deformation approach we used to calculate elastic constants of the systems successfully compensated for the large scattering of the mechanical properties data due to nanoscopically small volume of simulation cells and allowed comparison of properties of similar polymeric networks having minor differences in structure or chemistry. However, some of the elastic constants obtained using this approach were found to be higher than in real macroscopic samples. This can be attributed to both finite-size effect and to the limitations of the static deformation approach to account for dynamic effects. The observed dependence of properties on system size, in this work, can be used to estimate the contribution of large-scale defects and relaxation events into macroscopic properties of the thermosetting materials.
Deformation of a Polydomain, Liquid Crystalline Epoxy-Based Thermoset
Macromolecules, 1998
Liquid crystalline thermosets (LCT's) were prepared by curing a difunctional LC epoxy monomer, diglycidyl ether of 4,4′-dihydroxy-R-methylstilbene, with the tetrafunctional cross-linker 4,4′methylenedianiline (MDA). A commercial, non-LC epoxy monomer of similar starting molecular weight was also cured with MDA to produce an isotropic thermoset for comparison. Dynamic mechanical analysis revealed reduced glassy moduli, increased stiffness in the rubbery state, and broadened and lowered glass transitions for the LCT's compared to the isotropic thermoset based on the non-LC monomer. At room temperature, the true stress versus true strain curves of the LCT's under uniaxial compression showed no strain softening region, substantial plastic deformation ( f ≈ 50%), and increased strain hardening compared to the isotropic thermoset. LCT's with a smectic type of local order exhibited bulk, homogeneous plastic yielding, which led to slow, stable crack propagation and an increased fracture toughness (GIc ) 1.62 kJ/m 2 , KIc ) 1.59 MPa‚m 1/2 . † Current address: