Patricia A. Oyanguren

Phase separation in polysulfone-modified epoxy mixtures. Relationships between curing conditions, morphology and ultimate behavior

Chemorheology of curing as well as the phase separation behavior of polysulfone (PSU)-modified diaminodiphenylmethane-cured diglycidylether of bisphenol-A epoxy mixtures have been studied using several techniques. The delay in polymerization for the modified mixtures with respect to that of the neat epoxy can be explained by dilution and viscosity effects. The immiscibility of these mixtures has been proved for various PSU contents and as a function of the precuring conditions used as well. The control of the generated morphologies can be performed only by varying the processing temperature. The thermal and dynamic viscoelastic behavior of the modified matrices has been examined and compared to the parent epoxy matrix. Finally, the mechanical properties, including fracture toughness, have been discussed in terms of the morphological behavior for an epoxy matrix modified with various amounts of PSU and for a 15 wt.% PSU-containing matrix precured at different temperatures.

Development of bicontinuous morphologies in polysulfone-epoxy blends

The development of bicontinuous morphologies in 10 wt% polysulfone (PSu)‐epoxy (DGEBA)/anhydride (MTHPA) blends, was followed by optical and scanning electron microscopy. Blends cured at 808C revealed the formation of large epoxy-rich domains surrounded by a PSu-rich matrix, soon after the cloud point. Advancing the cure led to an increase in the volume fraction and the coalescence of epoxyrich domains. A bicontinuous primary morphology was thus generated. A secondary phase separation was observed in both primary phases from the very beginning of the phase-separation process. While spinodal demixing was clearly the mechanism by which the primary morphology was generated, nucleation-growth could be responsible of the secondary phase separation. Postcure steps produced a change in the composition of phases as revealed by DMA, and in the secondary morphology as observed by SEM. A postcure at 1208C led to a single Tg at 1158C with a small shoulder at higher temperatures. A postcure at 2008C led to a Tg at 1088C for the epoxy-rich phase and a Tg at 1378C for the PSu-rich phase. The partial purification of the thermoplastic phase produced a significant enhancement of toughness.KIC was increased from 0.65 MPa m 1/2 for the neat thermoset to 1.10 MPa m 1/2 for the blend postcured at 2008C. q 1999 Elsevier Science Ltd.

Phase separation induced by a chain polymerization : Polysulfone-modified epoxy/anhydride systems

The reaction-induced phase separation in a blend of a commercial polysulfone (PSu) with diepoxide-cyclic anhydride monomers, was studied. The diepoxide was based on diglycidylether of bisphenol A (DGEBA) and the hardener was methyl tetrahydrophthalic anhydride (MTHPA), used in stoichiometric proportion. Benzyldimethylamine (BDMA) was used as initiator. PSu had no influence on the polymerization kinetics, the gel conversion, and the overall heat of reaction per epoxy equivalent. A kinetic model including initiation, propagation, and termination steps was used to estimate the distribution of linear and branched species in the first stages of the chain-wise copolymerization. This distribution, together with the PSu distribution, were taken into account in a thermodynamic model of the blend. The interaction parameter was fitted from experimental determinations of conversions at the start of phase separation, obtained under different conditions. The thermodynamic model was used to explain the complex morphologies developed in materials containing different PSu concentrations as well as their dynamic mechanical response. The shift in glass transition temperatures was explained by the fractionation of different species during the phase separation process. Phase inversion produced a significant decrease of the elastic modulus in the glassy state and a thermoplastic-like behavior of the material in the rubbery region.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems. 2. Morphologies generated and resulting properties

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out in situ , during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.

Role of intrinsic flaws upon flexural behaviour of a thermoplastic modified epoxy resin

Abstract A bisphenol A-based epoxy resin (DGEBA) was modified with 15 weight percent polysulphone (PSU) and thermally cured using 4-4′diaminodiphenylsulphone (DDS). Starting from a homogeneous DGEBA/DDS/PSU mixture, the system developed a two-phase morphology upon network formation. Dynamic mechanical analysis (DMA), transmission optical microscopy (TOM) and scanning electron microscopy (SEM) studies showed that the system developed a co-continuous morphology consisting of two distinct domains. One of the domains was an epoxy rich matrix containing PSU particles while the other consisted of a dispersion of epoxy particles within a PSU rich phase. Flexural strength distributions of unmodified and thermoplastic modified epoxy resin were obtained by testing the materials in three-point bending according to the ASTM D790 protocol. The flexural behaviour of the epoxy resin was not improved by the presence of thermoplastic. In addition, the thermoplastic modified epoxy resin displayed a higher data scatter compared with the neat resin. The fracture mechanism of unmodified and thermoplastic modified epoxy resins was demonstrated to be sensitive to the intrinsic flaw distribution. The two-parameter Weibull model, which was used to analyse the experimental data, gave a good representation of the fracture loads distribution with regression coefficients of 0.99.

Blends of epoxy/anhydride thermosets with a high-molar-mass poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA, M n =232 000) was used to modify a stoichiometric epoxy (diglycidyl ether of bisphenol A; DGEBA)-anhydride (methyl tetrahydrophthalic anhydride; MTHPA) thermoset. PMMA concentrations in the range 3-7 wt% led to morphologies consisting of a continuous PMMA-rich region that appeared rough and striated in scanning electron micrographs and large domains of the thermoset exhibiting a dispersion of PMMA-rich particles in the micrometre range. These morphologies are the result of the critical point location, estimated at 2. 1 wt% PMMA as a result of the high molar mass of the additive. A 5 wt% PMMA led to an increase of the stress intensity factor K IC from 0.65 MPa m 1/2 , for the neat thermoset, to 0.94MPa m 1/2 . However, T g was reduced from 117 °C for the neat thermoset to about 105 °C for the PMMA-modified material. The T g decrease is ascribed to the differential segregation of both monomers to the PMMA-rich phase. No influence of PMMA addition on the cure kinetics was observed. An upper critical solution temperature was observed, meaning that cloud-point conversions increased with cure temperature. Phase separation took place before gelation in the temperature range investigated in this study.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems: 1. Conversion-temperature transformation diagrams

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) or 4,4′-diaminodiphenyl-sulfone (DDS). PBT was more miscible in DGEBA-MCDEA than in DGEBA-DDS formulations, as revealed by the melting point depression observed in binary mixtures. Melting temperatures as a function of conversion were obtained for both systems using differential scanning calorimetry together with size exclusion chromatography. In the case of the PBT-DGEBA-DDS system, a cloud-point curve was also obtained, showing an upper-critical-solution-temperature behaviour. On the basis of melting, cloud-point, vitrification and gelation curves, conversion-temperature transformation diagrams were generated for both systems. These diagrams can be used to design particular cure cycles to generate different morphologies in the phase separation process. In the case of PBT-DGEBA-MCDEA systems, PBT could be either kept in solution in the matrix or separated by crystallization (initially or in the course of polymerization). For PBT-DGEBA-DDS systems, PBT was always segregated from the matrix, either initially through crystallization or by attainment of the cloud-point curve in the course of reaction. Morphologies generated and resulting mechanical properties will be discussed in the second part of the series.

Rejuvenation of epoxy glasses subjected to uniaxial compression

Abstract Two epoxy networks, differing significantly in their glass transition temperatures (Tgs), were subjected to uniaxial compression tests in the glassy state (20°C). One of the systems was based on the diglycidyl ether of bisphenol-A (DGEBA) cured with ethylenediamine (EDA). The other one was based on an epoxidized novolac (EPN) cured with 4,4′-diaminodiphenyl sulfone (DDS). Both systems were physically aged by specific thermal treatments. The endothermic enthalpy relaxation peak, characteristic of aged glasses, could be erased at temperatures well below Tg by large mechanical deformations, i.e. close to the incipient strain hardening level in uniaxial compression tests. This phenomenon, which was accompanied by an increase in specific volume, constitutes a clear manifestation of rejuvenation produced by large mechanical stimuli. The validity of statements presenting an opposite point of view is also discussed.

Light responsive thin films of micelles of PS-b-PVP complexed with diazophenol chromophore

We have incorporated push-pull azobenzene units into diblock-copolymer micelles by supramolecular assembly. Specifically, we encapsulated a phenol-functionalized chromophore, DO13, within PS-b-P4VP micelles in toluene by means of H-bond interactions developed between DO13 molecules and pyridine groups of P4VP block. The solutions were spin-coated onto glass substrates resulting in multi- or mono-layered thin films of micelles with P4VP(DO13) core and PS corona. We show that the use of DO13 as a building block of micellar aggregates allowed us to manipulate the developed nanostructures. Spherical to cylindrical micellar transition was found when we increased the degree of chromophore complexation. Also, it was found that the polymer concentration in the solution plays an important role in determining the micellar nanostructures. The chain extension and change in composition of the P4VP core in the presence of the chromophore may be responsible for the structural changes observed in the micelles. The optical properties of the thin films have been investigated focusing on the effect of the micellar morphology over the photoinduced birefringence. The optical anisotropy (Δn) increased with respect to the analogous homogeneous system P4VP(DO13), indicating that the protective micelle environment can enhance the optical properties of the embedded chromophores significantly. Furthermore, we show very interesting new results in which we have related changes in optical properties to the film morphology (spheres to cylinders). This can be exploited for producing optical devices having improved optoelectronic properties and stability.

Analysis of the epoxidation of bisphenol A and phenolic Novolacs with epichlorohydrin

Abstract Theoretical equations based on a kinetic scheme were developed to predict the number average molecular weight, M n , the weight per epoxy equivalent, WPE, and the number average functionality, f , of resins arising from the expoxidation of bisphenol A or phenol-formaldehyde Novolacs with epichlorohydrin. Good agreement with experimental results was obtained by assuming a reactivity ratio of chain extension over epoxidation close to 0.5. Significant departures were shown, however, for the epoxidation of high molecular weight Novolacs. This was ascribed to the formation of intramolecular rings by reaction of terminal epoxides and phenolic hydroxyls, thus reducing the epoxidation efficiency, i.e. transformation of phenolic hydroxyls into aryl glycidyl ethers. The characterization of a commercial epoxidized Novolac showed that the epoxidation efficiency was 84.3%, most of the remaining fraction being composed of intra- and intermolecular CH2CHOHCH2 bridges.