Physical aging studies in epoxy resins. I. Kinetics of the enthalpy relaxation process in a fully cured epoxy resin

The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of T_g-10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τ_eff. The rate of relaxation of the system given by 1/τ_eff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = ?333, E_H = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub-T_g peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.     

Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Contributions of side groups to the water diffusion in cured epoxy resins (2) study on physical aging

Novolac epoxy resins cured with novolac resin, novolac acetate resin, novolac butyrate resin, and novolac phenylacetate resin named as EP, EPA, EPB, and EPP, respectively, were prepared. Their physical aging behavior at a T_g‐30 °C (30 °C below glass‐transition temperature) was examined by positron annihilation lifetime spectroscopy and differential scanning calorimetry. The ortho‐positronium annihilation lifetime τ₃ variation extent of EP is less apparent than that of the other three esterified samples during physical aging. The time dependence of ops intensity I₃ agreed with the Kohlrausch‐Williams‐Watts (KWW) equation. The relaxation time (τ₀) and nonexponential parameter were calculated. The free volume and enthalpy relaxation rate characterized by the reciprocal of τ₀ and ?ΔH/?logt, respectively, exhibit the same order—EPP > EPB > EPA > EP. These results suggest that the extend and rate of relaxation are not only related to the frozen free volume produced by quenching but also significantly influenced by segmental mobility of the network that attributed to the side‐group flexibility and their interaction with networks. This work also supports the fact that side‐group flexibility and the free‐volume fraction and distribution act in concert to control the water‐diffusion behavior in epoxy networks. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1135–1142, 2003     

Physical Aging of Maltose Glasses as Measured by Standard and Modulated Differential Scanning Calorimetry

The physical aging characteristics of maltose glasses aged at two temperatures below the glass transition temperature, T_g, (T_g-10°C and T_g-20°C) from 5 to 10 000 min were measured by standard differential scanning calorimetry (SDSC) and modulated differential scanning calorimetry (MDSC). The experimentally measured instrumental T_g, the calculated T_g, and the excess enthalpy values were obtained for aged glasses using both DSC methods. The development of excess enthalpy as a function of aging time, as measured by both SDSC and MDSC, was fit using the Cowie and Ferguson and Tool-Narayanswamy-Moynihan models. The change in the T_g values and the development of the excess enthalpy resulting from physical aging measured by the two DSC methods are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The physical properties of bisphenol-a-based epoxy resins during and after curing. II. Creep behavior above and below the glass transition temperature

The creep behavior of a series of fully cured epoxy resins with different crosslink densities was determined from the glassy compliance level to the equilibrium compliance J_e at temperatures above T_g and at the glassy level below T_g during spontaneous densification at four aging temperatures, 4,4-diamino diphenyl sulfone DDS was used to crosslink the epoxy resins. The shear creep compliance curves J(t) obtained with materials at equilibrium densities near and above T_g were compared at their respective T_gs. T_gs from 101 to 205°C were observed for the epoxies which were based on the diglycidyl ether of bisphenol A. Creep rates were found to be the same at short times, and equilibrium compliances J_e were close to the predictions of the kinetic theory of rubberlike elasticity. Time scale shift factors determined during physical aging were reduced to T_g. At compliances below 2 × 10^?10 cm²/dyn, Andrade creep, where J(t) is a linear function of the cube root of creep time, was observed. The time to reach an equilibrium volume at T_g was found to be longer for the epoxy resin with lower crosslink densities. The increase of density during curing is illustrated for the epoxy resin with the highest crosslink density.     

Synergism and multiple mechanical relaxations observed in ternary systems based on benzoxazine,epoxy, and phenolic resins

The synergism in the glass‐transition temperature (T_g) of ternary systems based on benzoxazine (B), epoxy (E), and phenolic (P) resins is reported. The systems show the maximum T_g up to about 180 °C in BEP541 (B/E/P = 5/4/1). Adding a small fraction of phenolic resin enhances the crosslink density and, therefore, the T_g in the copolymers of benzoxazine and epoxy resins. To obtain the ultimate T_g in the ternary systems, 6–10 wt % phenolic resin is needed. The molecular rigidity from benzoxazine and the improved crosslink density from epoxy contribute to the synergistic behavior. The mechanical relaxation spectra of the fully cured ternary systems in a temperature range of −140 to 350 °C show four types of relaxation transitions: γ transition at −80 to −60 °C, β transition at 60–80 °C, α₁ transition at 135–190 °C, and α₂ transition at 290–300 °C. The partially cured specimens show an additional loss peak that is frequency‐independent as a result of the further curing process of the materials. The ternary systems have a potential use as electronic packaging molding compounds as well as other highly filled systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1687–1698, 2000     

Acceleration of the cationic polymerization of an epoxy with hexanediol

Thin films of 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexane carboxylate were UV irradiated (1.1 J cm^-2) under isothermal conditions ranging from 0 to 50°C. Under these conditions the polymerization advanced quickly but only to a conversion level of less than 10% before the reaction rate slowed by more than an order of magnitude. This drop off in rate was not caused by the glass transition temperature, T _g, reaching or exceeding the reaction temperature, T _rxn, since the epoxide's T _g remained at least 40°C below T _rxn. Raising the sample temperature above 60°C caused a sharp increase in the conversion level. At 100°C conversion exceeds 80% and the ultimate T _g approaches 190°C. The addition of 10 mass% 1,6-hexanediol, HD, to the epoxy caused the conversion at room temperature to quintuple over the level obtained without the alcohol present. The heat liberated from this alcohol epoxy blend during cure on a UV conveyor belt system caused the sample's temperature to increase by about 100°C above ambient whereas the epoxy alone under these conditions only experienced a modest temperature rise of about 26°C. If the amount of HD in the blend is increased above 10% the heat of reaction at 23°C decreases due to HD being trapped in a nonreactive crystalline phase. Boosting reaction temperatures above 50°C melts the HD crystals and yields significantly improved conversion ratios. As the level of alcohol blended with the epoxy is raised its ultimate T _g is lowered and when the concentration of alcohol in the blend nears 30 mass%T _g drops below room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Epoxy resins (DGEBA): The curing and physical aging process

Differential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm^?l. The fictive temperature T_{f, g} was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature T_c of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (T_t,g ? T_c) increased. Although an endothermic peak was evident after only 1 h of cure (T_{f, g} = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature T_{f, a}, reflecting physical aging, increased with curing time. In contrast, the T_{f, a}, for fully cured samples decreased with sub-T_g aging time. The characteristic jump in the heat capacity ΔC_p which occurred at the T_{f, g} decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature T_{f, a}, of partially and fully cured epoxy material from measured endothermic peak areas was developed.     

Modulus and yield resistance of glassy blends containing diluents manifesting varying degrees of mobility: Polyphenylene ether/polystyrene/diluent mixtures

The glass temperatures, moduli, and yield stresses for shear failure have been characterized for homogeneous blends of poly(2,6-dimethyl-1,4-phenylene ether), homopolystyrene (1/1), and a number of diluents of T_gs from ?134 to +32°C. In a blend series based of a given diluent, the mechanical properties vary with blend T_g at rates that are characteristics of the particular diluent. These characteristic rates are found to depend strongly on the T_g of the neat diluent. Thus, for blends all with T_g = 90°C, for example, modulus and yield stress differ by as much as 50% over the range of diluents. Low-temperature relaxation measurements together with a number of previous dynamic spectroscopy studies of polymer/diluent systems at low temperatures suggest that many single-phase polymer/diluent blends may exhibit two primary relaxations—the depressed alpha relaxation of the resin and a somewhat elevated alpha relaxation of the diluent—at quite different temperatures. Both of these relaxations influence stiffness and strength properties at intermediate temperatures. The often-studied antiplasticization phenomena are viewed as a aberrations from a much more general influence of plasticizers on properties at temperatures below the alpha relaxation of the resin.     

Study of the physical aging of an epoxy/cycloaliphatic amine resin modified with abs

Using differential scanning calorimetry (DSC) we have studied the physical aging of an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) modified by two different contents of an acrylonitrile-butadiene-styrene (ABS) and cured with 1,3-bisaminomethylcyclohexane (1,3-BAC). Samples fully cured were annealed at temperature of 125°C for periods of time of 72 and 120 h, to determine the process of physical aging. The apparent activation energy for the enthalpy relaxation, Dh*, is determined as the sample is heated at 10°C min^-1 following cooling at various rates through the glass transition region. DSC studies suggested that the presence of thermoplastic inhibits the process of relaxation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpy relaxation in a partially cured epoxy resin

The enthalpy relaxation of a partially cured (70%) epoxy resin, derived from diglycidyl ether of bisphenol-A cured by methyl-tetrahydrophthalic anhydride with accelerator, has been investigated. The key parameters of the structural relaxation (the apparent activation energy Δh*, the nonlinearity parameter x, and the nonexponentiality parameter β) are compared with those of the fully cured epoxy resin. The aging rates, characterized by the dependences of the enthalpy loss and peak temperature on log(annealing time), are greater in the partially cured epoxy than they are in the fully cured resin at an equivalent aging temperature (T_a = T_g − 20°C). There is a significant reduction in Δh*, from 1100 kJ mol⁻¹ for the fully cured system to 615 kJ mol⁻¹, as the degree of cure is reduced. The parameter x determined by the peak-shift method appears essentially independent of the degree of cure (x = 0.41 ± 0.03 for the partially cured resin compared with 0.42 ± 0.03 obtained previously for the fully cured resin), and does not follow the usually observed correlation of increasing x as Δh* decreases. This invariability of the parameter x seems to indicate that it is determined essentially by the local chemical structure of the backbone chain, and rather little by the supramolecular structure. On the other hand, the estimated nonexponentiality parameter β lies between 0.3 and 0.456, which is significantly lower than in the fully cured epoxy (β ≅ 0.5), indicative of a broadening of the distribution of relaxation times as the degree of cross-linking is reduced. Like the parameter x, this also does not follow the usual correlation with Δh*. These results are discussed in the framework of strong and fragile behavior of glass-forming systems, but it is difficult to reconcile these results in any simple way with the concept of strength and fragility. © 1996 John Wiley & Sons, Inc.     

Physical aging of epoxy polymers and their composites

Exposure to extended periods of sub‐T_g temperatures causes physical changes in the molecular structure of epoxy resins and epoxy‐based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamical, and physical properties are affected in ways that can compromise the reliability of epoxy‐based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk‐level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability is reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/copolymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The effect of masked isocyanates on the moisture absorption of MY 720/DDS epoxy resin

Several masked isocyanates were prepared with variations in both the type of isocyanate and masking group. They were characterized by elemental analysis and NMR spectroscopy, and their unblocking temperatures were determined. In general, higher unblocking temperatures were obtained using acyclic and cyclic aliphatic isocyanates and fluorinated phenols. Those with unblocking temperatures in the range of 120–180°C were incorporated into MY 720/DDS epoxy resin prior to cure. Highly fluorinated variations were incompatible with the resin. IR and DSC analyses showed that residual functional groups in the epoxy resin reacted with the masked isocyanates. Reductions in moisture absorption as high as 65% were obtained depending on the masked isocyanate. DMA studies showed that the T_g of the epoxy resin is lowered by incorporation of the masked isocyanate but the elastic modulus (E′) is relatively unchanged at temperatures below T_g.     

Further Study of Sub‐Tg Heat Flow Transition of a Cured Epoxy Resin

To further study the sub‐T_g heat flow transition of a cured epoxy resin, cured samples with different thermal history were investigated using torsion pendulum analysis (TPA) and thermal mechanical analysis (TMA). The results indicate that sub‐T_g heat flow transition could be related to the molecular relaxation from 20°C to the α‐peak, and that frozen‐in extra free volume is necessary for the appearance of sub‐T_g heat flow transition.     

Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

The properties of diglycidyl ether of bisphenol-A epoxy resin toughened with poly(ether sulfone ether ketone) (PESEK) and poly(ether sulfone) (PES) polymers were investigated. PESEK was synthesised by the nucleophilic substitution reaction of 4,4’-difluorobenzophenone with dihydroxydiphenylsulfone using sulfolane as solvent and potassium carbonate as catalyst at 230 °C. The T _g–composition behaviour of the homogeneous epoxy resin/PESEK blend was modelled using Fox, Gordon–Taylor and Kelley–Bueche equations. A single relaxation near the glass transition of epoxy resin was observed in all the blend systems. From dynamic mechanical analysis, the crosslink density of the blends was found to decrease with increase in the thermoplastic concentration. The storage modulus of the epoxy/PESEK blends was lower than that of neat resin, whilst it is higher for epoxy/PES blends up to glass transition temperature, thereafter it decreases. Scanning electron microscopic studies of the blends revealed a homogeneous morphology. The homogeneity of the blends was attributed to the similarity in chemical structure of the modifier and the cured epoxy network and due to the H-bonding interactions between the blend components. The fracture toughness of epoxy resin increased on blending with PESEK and PES. The increase in fracture toughness was due to the increase in ductility of the matrix. The thermal stability of the blends was comparable to that of neat epoxy resin.     

Physical aging and glass transition of hairy nanoparticle assemblies

Matrix free assemblies of polymer-grafted, “hairy” nanoparticles (aHNP) exhibit novel morphology, dielectric, and mechanical properties, as well as providing means to overcome dispersion challenges ubiquitous to conventional polymer-inorganic nanocomposite blends. Physical aging of the amorphous polymer glass between the close-packed nanoparticles (NPs) will dominate long-term stability; however, the energetics of volume recovery within the aHNPs is unknown. Herein, we compare glass transition temperature (T_g) and enthalpy recovery of aHNPs to NP-polymer blends, across different nano-silica loadings (0–50 v/v%) and canopy architecture of polystyrene (PS) grafted silica. For aHNPs, the grafting of PS to silica imposes an additional design constraint between silica volume fraction, graft density, and graft molecular weight. At low and intermediate silica volume fraction, the T_g of blended nanocomposites is independent of silica content, reflecting a neutral polymer-NP interface. For aHNPs, the T_g decreases with silica content, implying that chain tethering decreases local segment density more than the effect of molecular weight or polymer-NP interactions. Additionally, the T_g of the aHNPs is higher than a linear matrix of comparable molecular weight, implying a complementary effect to local segment density that constrains cooperativity. In contrast, enthalpy recovery rate in the blend or aHNP glass is retarded comparably. In addition, a cross-over temperature, T_x, emerges deep within the glass where the enthalpy recovery process of all nanocomposites becomes similar to linear unfilled matrices. Differences between structural recovery in aHNP and blended nanocomposites occur only at the highest silica loadings (∼ 50 v/v%), where enthalpy recovery for aHNPs is substantially suppressed relative to the blended counterparts. The absence of physical aging at these loadings is independent of brush architecture (graft density or molecular weight of tethered chains) and indicates that the impact of chain tethering on effective bulk structural relaxation starts to appear at particle-particle surface separations on the order of the Kuhn length. Overall, these observations can be understood within the context of how three separate structural characteristics impact local segment density and relaxation processes: the dimension and architecture of the tethered polymer chains, the separation between NP surfaces, and the confinement imposed by chain tethering and space filling within the aHNP. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 319–330     

Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

Effect of thermoplastic toughening agent on glass transition temperature and cure kinetics of an epoxy prepreg

The isothermal time–temperature-transformation (TTT) cure diagram is developed in this article to investigate the effect of thermoplastic toughening agent on glass transition temperature (T _g) and cure kinetics of an epoxy carbon fiber prepreg, Cycom 977-2 unidirectional (UD) tape. The glass transition temperature was measured using differential scanning calorimetry (DSC) over a wide range of isothermal cure temperatures from 140 to 200 °C. Times to gelation and vitrification were measured using shear rheometry. The glass transition temperature master curve was obtained from the experimental data and the corresponding shift factors were used to calculate the activation energy. The kinetic rate model was utilized to construct iso-T _g contours using the calculated activation energy. It was observed that the iso-T _g contours did not follow the behavior of the neat epoxy resin, since they deviated from the gel time curve. This deviation was believed to be the effect of the thermoplastic toughening agent. The behavior of the neat epoxy resin in 977-2 was shown by constructing the iso-T _g contours using the activation energy obtained from gel time modeling.     

Comparison of enthalpy relaxation between two different molecular masses of a bisphenol-A polycarbonate

Summary The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔH_a) and fictive temperature (T_f) as a function of aging temperature (T_a) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔH_a and T_f changes were only appreciable for (T_g -70 K)<T_a<T_g . While the influence of molecular mass was somewhat discernible, enthalpy measurements were not as sensitive as stress relaxation tests in differentiating molecular mass effects. In a second investigation, the kinetics of enthalpy relaxation upon isothermal aging at 130°C was evaluated using the peak shift method and found to be comparable to literature values. The plot of ΔH_a as a function of log (aging time) showed two distinct regions: a brief non-linear portion (less than 1 h aging) which is followed by a linear relationship as typically reported in the literature. In contrast to the linear region, the non-linear relaxation behaviour of the poorly aged state does not appear to be dependent on molecular mass.