Effect of crosslink length on the enthalpy relaxation of fully cured epoxy–diamine resins

Abstract

Enthalpy relaxation of epoxy-diamine thermosets of different crosslink lengths (CLL) has been studied by DSC. The epoxy resins based on diglycidyl ether of bisphenol A were cured with ethylenediamine (FEDA), and diamines of polyoxypropylene of 2.6 and 5.6 oxypropylene units, named FJ230 and FJ400, respectively. As was expected, increasing the CLL decreases the glass transition temperature T g from 121°C (FEDA) to 47°C (FJ400). Aging experiments at T g Ϫ 20 K for each resin permit the determination of the enthalpy loss, the relaxation rate per decade (␤ H ), and the nonlinearity parameter, x. The apparent activation energy, ⌬h*, and the nonexponentiality parameter ␤ are found for each resin from intrinsic cycles in which the sample is heated at 10 K min Ϫ1 following cooling at various rates through the glass transition region. An increase of CLL is related to an increase of ␤ H , and of the nonlinearity parameter. In agreement with the general trend for thermoplastic polymers, the increase of the parameter x is correlated with a decrease of ⌬h* and with an increase in the nonexponentiality parameter. Application of the AdamGibbs (AG) theory reveals that the parameters B and T f /T 2 increase with CLL, corresponding to a decrease of the nonlinear behavior of the glassy epoxies. However, the T 2 values calculated in this way appear unrealistic, and the alternative assumption that T 2 ϭ T g Ϫ51.6 K, making use of the "universal" WLF constant, leads to a much smaller variation of B, which nevertheless still increases with CLL. From a consideration of the minimum number of configurations required for a cooperative rearrangement, it is argued that the elementary activation energy ⌬ increases, and the minimum size of the cooperatively rearranging region decreases as CLL increases. This is consistent with the relaxation process becoming more cooperative as the CLL decreases, as is suggested by the decrease in the value of ␤.

Physical aging of epoxy polymers and their composites

“…As the aging time increases, it is clear that the endothermic peak temperature also increases, which is consistent with the trends shown in Figure 5. This trend has been observed in other studies as well 16–18, 82, 85, 87, 89, 92, 93. From Figure 17, it also appears that the endothermic peak temperature generally increases as the isothermal aging temperature increases.…”

Physical aging of epoxy polymers and their composites

“…As the aging time increases, it is clear that the endothermic peak temperature also increases, which is consistent with the trends shown in Figure 5. This trend has been observed in other studies as well 16–18, 82, 85, 87, 89, 92, 93. From Figure 17, it also appears that the endothermic peak temperature generally increases as the isothermal aging temperature increases.…”

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

“…The calculated activation energy is Δh ⁎ = 520 ± 20 kJ • mol −1 (the uncertainty is associated with the linear regression). This value is lower than the one measured for most epoxy networks [6], however it is consistent with partially cured epoxy networks [12,34]. The adhesive studied has a longer aliphatic sequence (18 methylene groups) and it is partially cured [35]; it can explain the low value of the activation enthalpy.…”

Very long physical ageing in inorganic polymers exemplified by the GexSe1−x vitreous system

“…Indeed it is also well known that ageing leads in addition to the increase of the relaxation peak and to an increase in the apparent glass transition temperature. The simultaneous variations of the peak surface and the peak temperature are at the origin of the peak shift method [19] used by several researchers to describe quantitatively the relaxation kinetics in glassy polymers [20][21][22][23]. From the second series of measurements, we have determined the values of the glass transition temperatures taken at the Table 1.…”

Physical aging of epoxy polymers and their composites

“…As the aging time increases, it is clear that the endothermic peak temperature also increases, which is consistent with the trends shown in Figure 5. This trend has been observed in other studies as well 16–18, 82, 85, 87, 89, 92, 93. From Figure 17, it also appears that the endothermic peak temperature generally increases as the isothermal aging temperature increases.…”

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

“…The calculated activation energy is Δh ⁎ = 520 ± 20 kJ • mol −1 (the uncertainty is associated with the linear regression). This value is lower than the one measured for most epoxy networks [6], however it is consistent with partially cured epoxy networks [12,34]. The adhesive studied has a longer aliphatic sequence (18 methylene groups) and it is partially cured [35]; it can explain the low value of the activation enthalpy.…”

Very long physical ageing in inorganic polymers exemplified by the GexSe1−x vitreous system

“…Indeed it is also well known that ageing leads in addition to the increase of the relaxation peak and to an increase in the apparent glass transition temperature. The simultaneous variations of the peak surface and the peak temperature are at the origin of the peak shift method [19] used by several researchers to describe quantitatively the relaxation kinetics in glassy polymers [20][21][22][23]. From the second series of measurements, we have determined the values of the glass transition temperatures taken at the Table 1.…”

Physical aging of epoxy polymers and their composites

“…As the aging time increases, it is clear that the endothermic peak temperature also increases, which is consistent with the trends shown in Figure 5. This trend has been observed in other studies as well 16–18, 82, 85, 87, 89, 92, 93. From Figure 17, it also appears that the endothermic peak temperature generally increases as the isothermal aging temperature increases.…”

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

“…The calculated activation energy is Δh ⁎ = 520 ± 20 kJ • mol −1 (the uncertainty is associated with the linear regression). This value is lower than the one measured for most epoxy networks [6], however it is consistent with partially cured epoxy networks [12,34]. The adhesive studied has a longer aliphatic sequence (18 methylene groups) and it is partially cured [35]; it can explain the low value of the activation enthalpy.…”

Very long physical ageing in inorganic polymers exemplified by the GexSe1−x vitreous system

“…Indeed it is also well known that ageing leads in addition to the increase of the relaxation peak and to an increase in the apparent glass transition temperature. The simultaneous variations of the peak surface and the peak temperature are at the origin of the peak shift method [19] used by several researchers to describe quantitatively the relaxation kinetics in glassy polymers [20][21][22][23]. From the second series of measurements, we have determined the values of the glass transition temperatures taken at the Table 1.…”