Rheological and dielectric changes during isothermal epoxy-amine cure

Dynamic viscosity and ionic conductivity were measured simultaneously during the cure of a digylcidyl ether of bisphenol-A (DGEBA) epoxy resin with diamino-diphenyl sulfone (DDS) by mounting a microdielectric sensor into the plates of a rheometer. Two different cure temperatures were examined. Periodically, throughout the cure, samples were removed from the plates of the rheometer, quenched, and analyzed for the glass transition temperature and epoxide conversion. The relationship between conductivity and viscosity appeared to be independent of cure temperature. A linear relation with a slope of ?1 was observed between the natural logarithms of conductivity and viscosity during the cure up to approximately 85% cure conversion. It was hypothesized that the reaction rate was hindered by diffusion at this stage in the polymerization. A free volume relationship was used to successfully correlate conductivity with viscosity up to the diffusion limited region. ©1995 John Wiley & Sons, Inc.     

Observations of Gelation and Vitrification of a Thermosetting Resin during the Evolution of Polymerization Shrinkage

Summary: We report a new methodology to determine the gelation and vitrification of a thermosetting material during the polymerization process by detecting the evolution of cure shrinkage through a thermomechanical analyzer (TMA) and a differential scanning calorimeter (DSC). The gelation and vitrification determined by the evolution of cure shrinkage correspond favorably with that measured by conventional rheological techniques. The isoconversional phenomenon at gelation point was further confirmed by monitoring cure shrinkage at temperatures ranging from 90 to 110 °C. Whereas, vitrification was observed to occur at higher degrees of cure with increasing cure temperatures. Inhibited cure shrinkage was also observed in the vitrification region where the reaction transitioned from chemical to diffusion controlled.

Combination of dimension change detected by DMA and heat flow detected by DSC for determining the relationship of cure shrinkage and degree of cure.     

Relaxations in thermosets. XII. Dielectric effects during curing of nonstoichiometric DGEBA-based thermosets

The dielectric permittivity ε′ and loss ε″ of diglycidyl ether of bisphenol-A thermosets cured with nonstoichiometric amounts of diamino-diphenyl methane have been measured during the course of their chemical reactions from the sol to gel to glass-formation regions. ε′ monotonically decreases with time and ε″ initially decreases, increases to a peak value, and finally decreases to extremely low values characteristic of the glassy state. The initial decrease in ε″ is due to the decrease in the dc conductivity, and the peak is due to the dipolar reorientation. The appearance of these features shifts to longer time when the thermoset is stoichiometrically starved by decreasing the amount of the curing agent and, at a molar ratio of 4 : 1 of the epoxide to diamine, the ε″ peak does not appear during the curing process. Complex plane plots of ε′ and ε″ have the shape of an arc in all cases except when the molar ratio of the epoxide to diamine is 4 : 1. The dielectric consequences of the chemical changes with time during the crosslinking of a thermoset are analogous to the frequency dependence of ε″ of a condensed phase. The time dependence of ε″ follows a stretched exponential decay, ?(t = exp ? [(t/τ)^γ], where 0 < γ < 1. The parameter decreases with decreasing amine content. ε″ has been analyzed to obtain the increase in the relaxation time as curing progresses. A representation of ε^* in terms of electrical modulus M^* shows the occurrence of, first a Maxwell relaxation due to dc conductivity, and second a dipolar relaxation, during the period of a typical isothermal cure. Changes in the features of the isothermal cure that occur on changing the amount of the curing agent are discussed in terms of network formation in the thermoset, and the change in the electrical conductivity with curing time has been analyzed in terms of both a power law for gel formation and by a new equation that suggests an approach toward a singularity.     

Relaxations in thermosets. X. Analysis of dipolar relaxations in DGEBA-based thermosets during isothermal cure

Dielectric properties measured during isothermal curing of DGEBA-based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically. It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote γ, in the latter part of the cure. The value of the curing parameter, γ, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch-Williams-Watts parameter β with increasing temperature. The curing time dependence of the dipolar relaxation time ι has been determined and found to have the shape of an elongated S, with a well-defined point of inflexion, as ι increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.     

Kinetic method by using calorimetry to mechanism of epoxy-amine cure reaction

Some specific features of the thermochemistry of epoxy-amine curing at the later stages of the reaction are considered. Possible mechanism of cross-linking and the question about the driving force leading to the infinite network are discussed. The coupling of the reaction kinetics and rearrangement of the chains crosslinked into the rigid supramolecular structure is the essential feature of epoxy-amine vitrified system. It has been proposed that owing to the contribution from the side process, different curing temperatures can result in the structures with different T _g. It was also established that reaction of epoxy ring opening alone is not responsible for the residual curing. The latter is the result of the side processes. As compared with the reaction of epoxy ring opening the side processes are strongly dependent on the geometrical aspects. This revised version was published online in August 2006 with corrections to the Cover Date.     

Relaxations in thermosets. XIII. Effects of post-cure and aging on the sub-Tg relaxations of nonstoichiometric epoxide-based thermosets

The dielectric permittivity and loss of diglycidyl ether of bisphenol-A (DGEBA) cured with greater than and less than the stoichiometric amounts of diaminodiphenyl methane (DDM) have been measured over a temperature range 77–350 K prior to curing and gelation, after curing at about 340 K and further aging for a predetermined period. The height of the γ-relaxation peak monotonically decreases during the post-cure period and becomes masked by the contributions from the β-relaxation peak, whose height, in turn, first increases on postcuring to a same maximum value for both nonstoichiometric thermosets and then decreases. This decrease is attributed to physical aging effects. The β-relaxation peak shifts towards higher temperature on postcuring. Comparison between the changes in the dielectric properties of the saturated and starved thermosets show that while the γ-relaxation process may be attributed to the motion of the epoxide dipolar groups of the unreacted DGEBA, the β-relaxation process is not attributable entirely to the motion of ? OH groups and of the unreacted amines in the thermoset. Explanations involving the chain and network packing in the structure of a thermoset are necessary for the observed behavior of the β-relaxation process in amine saturated and starved thermosets.     

TMDSC and Dynamic Rheometry,Gelation, Vitrification and Autoacceleration in the Cure of an Unsaturated Polyester Resin

The free radical cross-linking copolymerization of an unsaturated polyester resin with styrene is studied in isothermal conditions using temperature modulated differential scanning calorimetry (TMDSC) and dynamic rheometry. The dynamic rheometry measurements show that gelation occurs at a conversion below 5%, while TMDSC measurements show that an important autoacceleration starts near 60% conversion, giving rise to a maximum cure rate closely before the (partial) vitrification of the system near 80%. This indicates that the autoacceleration is not due to the sharp increase in bulk viscosity at gelation, but rather to a change in molecular mobilities at higher conversion.This revised version was published online in November 2005 with corrections to the Cover Date.     

Isobaric cure shrinkage behaviors of epoxy molding compound in isothermal state

The warpage of plastic‐encapsulated IC packages after molding is believed to be induced by thermal and cure shrinkage of epoxy molding compound (EMC). To study the warpage behaviors of EMC, the amount of cure‐induced shrinkage needs to be understood. Volume shrinkage behaviors induced by cure reaction of EMC in isothermal and isobaric states were studied with a differential scanning calorimeter (DSC) and a pressure–temperature‐controlled dilatometer. The results show that higher pressure induce more volume shrinkage under fixed temperature but the difference of volume shrinkage under different pressure levels doesn't obey the principle of linearity. It is observed that the amount of chemical volume shrinkage at 145 °C is higher than those under three other temperatures: 160, 175, and 190 °C. The chemical volume shrinkage of EMC is found to be very process dependent. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2392–2398, 2005     

Determination of the degree of cure under dynamic and isothermal curing conditions with electrical impedance spectroscopy

This article presents a new methodology for the quantitative determination of the progress of the curing reaction of a thermosetting resin, using the results of electrical impedance spectroscopy. The method is an extension of the use of the imaginary impedance maximum as a reaction progress indicator and is based on the demonstration of a close correlation between the reaction rate, as measured by conventional differential scanning calorimetry, and the rate of change of the value of the imaginary impedance spectrum maximum. Tests on a commercial aerospace epoxy resin under both isothermal and dynamic heating conditions with calorimetry and impedance spectroscopy have demonstrated the validity of the method and set the accuracy limits involved. This technique can be used as a real-time online control tool for thermoset composite manufacturing. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 146–154, 2004     

A new class of epoxy thermosets

The reinforcing strategies of epoxy thermosets rely on the control of the phase separation between the additive and the growing thermoset. With standard additives, such as reactive liquid rubbers, the length scale of the resulting domains is the micrometer. Here, we present a route that enable a control of the morphology down to the nanometer scale. This strategy is based upon the self-assembly process of blends of epoxy and SBM triblock copolymers, namely Poly(Styrene-b-1,4 Butadiene-b-Methyl methacrylate). It relies on the respective affinities between the epoxy precursors and each of the three blocks. Liquid epoxy has a strong affinity for PMMA, whilst it is not miscible with polystyrene nor polybutadiene at standard processing temperatures. Thus, within the reactive system, microphase separation leads to a regular network of S-B domains. This nanostructure is governed by thermodynamics. The size and geometry of the dispersed domains are controlled by the concentration and the ratio between blocks lengths. The domain size is of the order of magnitude of the chain length, ranging typically from 10 to 30 nanometers. What controls the blend's morphology throughout the curing process of the thermoset was one topic on which we focused our interest. Nanostructured thermosets have been obtained. These supramolecular architectures yield significant toughness improvements while preserving the transparency of the material. The reinforcing mechanisms are not yet fully understood : it is intriguing to induce significant toughening with elastomer domains smaller than 30 nanometers in diameter. Besides being efficient epoxy tougheners, SBM can broaden the scope of applications of thermosets due to specific rheological behaviors. Thanks to the self assembly process taking place in the blend of the SBM block copolymers with the epoxy thermosets precursors, the reactive solvent can be turned into a reactive gel or solid (before curing). This physical gelation is induced by the microphase separation and is thus thermoreversible. At relatively moderate loadings of block copolymers the reactive blend behaves like a thermoplastic material, with adjustable modulus and tackiness. These results evidence that SBM block copolymers open a broad area for designing new class of thermoset materials.     

Vitrification and devitrification during the non-isothermal cure of a thermoset

The processes of vitrification and devitrification that occur in an epoxy resin when it cures non-isothermally with a hardener are studied in terms of their frequency dependence and as a function of the heating rate. A novel modulated DSC technique, TOPEM, has been used which permits the evaluation of the frequency dependence for a single sample in a scan at constant underlying heating rate, thus avoiding errors arising from the composition of the sample. The effects of both frequency and heating rate on vitrification and devitrification are investigated. Some advantages of this technique are observed and discussed.     

Measuring the Glass Transition of Thermosets by Alternating Differential Scanning Calorimetry

The glass transition temperature of thermosets is determined by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The different values of the glass transition obtained from heat flow measurements (total and reversible) and heat capacity (modulus of the complex heat capacity) are analysed and compared with the values obtained by conventional DSC. The effect of the sample mass on the values of T_g, heat capacity and phase angle has been analysed. The effect of the thermal contact between sample and pan has been studied using samples cured directly inside the pan and disc-shaped samples of different thickness. The results obtained for the thermal properties and the phase angle are compared and analysed. The modulus of the complex heat capacity enables the determination of the dynamic glass transition, T_g, which is frequency dependent. The apparent activation energy ofthe relaxation process associated with the glass transition has been evaluated from the dependence of T_g on the period of the modulation.This revised version was published online in November 2005 with corrections to the Cover Date.     

Control of the block copolymer morphology in templated epoxy thermosets

It has been found that by the addition of low concentrations of an amphiphilic block copolymer to an epoxy resin, novel disordered morphologies can be formed and preserved through curing. This article will focus on characterizing the influence of the block copolymer and casting solvent on the templated morphology achieved in the thermoset sample. The ultimate goal of this work is to determine the parameters that would control the microphase morphology produced. Epoxy resins blended with a series of amphiphilic block copolymers based on hydrogenated polyisoprene (polyethylene-alt-propylene or PEP) and polyethylene oxide (PEO), specifically, were investigated. In this article, the cure-induced order–order phase transition from the spherical to wormlike micelle morphology will also be discussed. It is proposed that the formation of the wormlike micelle structure from the spherical micelle structure is similar to the phase transition behavior that occurs in dilute block copolymer solutions as a function of the influence of the solvent on micelle morphology. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3338–3348, 2007     

Relaxations in thermosets. XVI. Dielectric studies of negative feedback during curing of an epoxide-ethylene-diamine thermoset

The complex dielectric permittivity of thermosets of diglycidly ether of bisphenol-A cured with ethylene diamine has been studied during their isothermal curing at several temperatures. As cross-linking progresses, the dc conductivity decreases. At the beginning of the cure the dc conductivity can be fitted to both the scaling laws with a critical exponent of about 4 and an equation indicating approach toward a singularity. In the later stage of the cure, the change in permittivity corresponds to dipolar relaxation of an infinitely connected network, and the Argand diagram for the complex permittivity measured at a fixed frequency obtained as the curing process proceeds at 305 K is similar to that for the complex permittivity as frequency is varied for a time-invariant system which obeys a stretched exponential relaxation function with the curing parameter or exponent, γ = 0.29. Increase in the temperature of isothermal curing lowers both γ and the net decrease in the equilibrium permittivity on curing. A plot of the calculated relaxation time with curing time is sigmoidal and shifts to shorter times on increasing the curing temperature. Measurement of the dielectric properties during the cure but for different frequencies show that the various parameters for the curing kinetics are independent of the frequency of measurement. These observations confirm the development of our concepts of thermoset curing in terms of a phenomenon of negative feedback between molecular diffusion and chemical reactions.     

Linear Relations of Activation Energy in Non-isothermal Kinetics

Temperature-modulated DSC (TMDSC) measurements at reasonably high frequencies allow for the determination of base-line heat capacity. In this particular case vitrification and devitrification of the rigid amorphous fraction (RAF) can be directly observed. 0.01 Hz seems to be a reasonably high frequency for bisphenol-A polycarbonate (PC). The RAF of PC is established during isothermal crystallization. Devitrification of the RAF seems to be related to the lowest endotherm. For PC the melting of small crystals between the lamellae is expected to yield the lowest endotherm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Time-temperature transformation (TTT) cure diagram of an unsaturated polyester resin

The curing of an unsaturated polyester resin was studied by differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and Fourier-transform infrared spectroscopy (FTIR). The results are presented in the form of a time-temperature-transformation (TTT) diagram. The kinetic analysis was performed by means of the dynamic Ozawa method. This analysis was used to determine the curing times (t) at various conversions (α) and temperatures (T) (isoconversional lines ln t = A + E/RT). The equivalence of the Ozawa method and the isothermal isoconversional adjustment ln t = A + E/RT were demonstrated. The relationship between the glassy transition temperature (T_g) and the conversion α was determined by DSC. It was established that this relationship is one-to-one and independent of mass, initiation system, and curing temperature (T_c). The T_g-α relationship was adjusted using the DiBenedetto equations and heat capacity data. Using the T_g-α relationship and the isoconversional lines, the vitrification curve was determined and it was observed that the vitrification times obtained are consistent with those obtained experimentally when T_c = T_g. Gelation was determined by TMA, the material being considered gelled when it reached sufficient mechanical stability for the TMA measuring probe to become embedded in it. At that moment the conversion reached was determined by DSC. It was seen that the material always gels at constant conversion, regardless of the curing temperature. The gelation line (gel times) were traced from the corresponding isoconversional line. © 1997 John Wiley & Sons, Inc.     

Functional off‐stoichiometry thiol‐ene‐epoxy thermosets featuring temporally controlled curing stages via an UV/UV dual cure process

We present a facile two‐stage UV/UV activation method for the polymerization of off‐stoichiometry thiol‐ene‐epoxy, OSTE+, networks. We show that the handling and processing of these epoxy‐based resins is made easier by introducing a material with a controlled curing technique based on two steps, where the first step offers excellent processing capabilities, and the second step yields a polymer with suitable end‐properties. We investigate the sequential thiol‐ene and thiol‐epoxy reactions during these steps by studying the mechanical properties, functional group conversion, water absorption, hydrolytic stability, and thermal stability in several different thiol‐ene‐epoxy formulations. Finally, we conclude that the curing stages can be separated for up to 24 h, which is promising for the usefulness of this technique in industrial applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2604‐2615     

Miscibility and cure kinetics studies on blends of bisphenol-A polycarbonate and tetraglycidyl-4,4′-diaminodiphenylmethane epoxy cured with an amine

Differential scanning calorimetry (DSC) has been applied to characterize the glass transition behavior of the blends formed by bisphenol-A polycarbonate (PC) with a tetrafunctional epoxy (tetraglycidyl-4,4′-diaminodiphenyl methane, TGDDM) cured with 4,4′-diaminodiphenylsulphone (DDS). A rare miscibility in the complete composition range has been demonstrated in these blends. Additionally, the blend morphology was examined using scanning electron microscopy (SEM) and a homogeneous single-phase PC/epoxy network has been observed in the blends of all compositions. Moreover, polycarbonate incorporation has been found to exert a distinct effect on the cure behavior of the epoxy blends. The cure reaction rates for the epoxy-PC blends were significantly higher due to the presence of PC. In addition, the cure mechanism of the epoxy blends was no longer autocatalytic. An n-th order reaction mechanism with n = 1.2 to 1.5 has been observed for the blends of DDS-cured epoxy with PC of various compositions studied using DSC. The proposed n-th order kinetic model has been found to describe well the cure behavior of the epoxy/PC blends up to the vitrification point. © 1995 John Wiley & Sons, Inc.     

Synthesis,structure, and characterization of nematic liquid‐crystalline thermosets based on bisacrylates

Four bisacrylate mesogenic monomers and the corresponding liquid‐crystalline thermosets were synthesized. The chemical structures of the intermediate compounds and monomers obtained were confirmed by elemental analyses, Fourier transform infrared, and ¹H NMR and ¹³C NMR spectra. The mesomorphic properties and thermal stability were investigated with differential scanning calorimetry, thermogravimetric analysis, polarized optical microscopy, and X‐ray diffraction measurements. The influence of the curing temperatures and time on the phase behavior and thermal stability of the thermosets was discussed. All the monomers and thermosets exhibited a nematic schlieren texture. However, the monomers only showed the melting transition, and the thermosets displayed the glass transition. The experimental results demonstrated that the monomer structures strongly affected the phase behavior and the curing reaction rate, and the glass‐transition temperatures and thermal stability of the thermosets increased with the curing temperature and time. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4478–4485, 2005     

Flame retardant polycyanurate thermosets from the cyanate esters of triphenylphosphine oxide

Three cyanate esters containing phosphorus are synthesized in good overall yields starting from bromoanisoles. Di‐ and tricyanates with meta configuration are most stable while para is less so. The para dicyanate ester isomer is particularly affected by water from the atmosphere. The meta dicyanate ester 2 has good thermal properties with glass transition at 268 °C and char yield of 65% in air at 600 °C. All three phosphorus‐containing cyanate esters are low flammability in an open flame. They make highly combustible cyanate esters resins less flammable simply by blending. Mixing 10 wt% dicyanate ester 2 into bisphenol A or E dicyanate esters makes them rate V‐0. Published 2018.^† J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1100–1110