I. Mondragon

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.

Cellulose nanocrystals/polyurethane nanocomposites. Study from the viewpoint of microphase separated structure

Cellulose nanocrystals (CNC) successfully obtained from microcrystalline cellulose (MCC) were dispersed in a thermoplastic polyurethane as matrix. Nanocomposites containing 1.5, 5, 10 and 30 wt% CNC were prepared by solvent casting procedure and properties of the resulting films were evaluated from the viewpoint of polyurethane microphase separated structure, soft and hard domains. CNC were effectively dispersed in the segmented thermoplastic elastomeric polyurethane (STPUE) matrix due to the favorable matrix-nanocrystals interactions through hydrogen bonding. Cellulose nanocrystals interacted with both soft and hard segments, enhancing stiffness and stability versus temperature of the nanocomposites. Thermal and mechanical properties of STPUE/CNC nanocomposites have been associated to the generated morphologies investigated by AFM images.

Influence of cure schedule and stoichiometry on the dynamic mechanical behaviour of tetrafunctional epoxy resins cured with anhydrides

Abstract Epoxy networks based on N,N,N′,N′ -tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) prepolymer were prepared with cis -1,2,3,6-tetrahydrophthalic anhydride (THPA) curing agent at anhydride/epoxy group ratios varying from 0.3 to 1.0. For post-cured mixtures, dynamic mechanical tests show that the glass transition temperature reaches the maximum value at stoichiometric ratios between 0.8 and 0.9. This behaviour has been related to the crosslink density of the formed networks, and also to etherification reactions occurring during cure which lower the amount of anhydride needed in order to complete the curing process. The study of cure cycle variations on the viscoelastic properties showed that for epoxy/anhydride mixtures high post-cure temperatures could be needed to reduce the amount of unreacted epoxy groups after curing. Fourier transform infra-red spectroscopy has been used to analyse the residual epoxy groups and also to study the influence of the different cure reactions on the physical properties of these networks.

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.

Design of morphology in PMMA‐modified epoxy resins by control of curing conditions. I. Phase behavior

Rheokinetic and phase separation behavior of diglycidylether of bisphenol-A–4,4′-diaminodiphenyl methane epoxy mixtures, modified with a constant amount (15 wt %) of poly(methyl methacrylate) (PMMA), have been investigated. Stoichiometric epoxy/amine mixtures precured at 80°C several times presented various levels of miscibility. Differential scanning calorimetry (DSC) and dynamic mechanic thermal analysis were used for rheokinetic studies of curing and also for testing the thermal behavior of the fully cured mixtures. Phase separation, through curing, was simultaneously studied by transmission optical microscopy and DSC, showing an excellent correlation between the results obtained with both techniques.

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results are discussed in terms of a proposed model that takes into account the contribution of the filler powder. To this purpose information from positron spectroscopy is important since it allows to correctly evaluate the internal stresses introduced in the composite epoxy lattice by the metal filler.

Characterization of interfacial behaviour in carbon-fibre/cyanate composites

In this study an attempt is made to investigate the influence of surface treatments such as oxidation and sizing on a high-strength carbon fibre in respect of interfacial adhesion in composite materials with a cyanate matrix. For analysing the fibre/resin interfacial adhesion the single-fibre pull-out technique has been used. The interfacial shear strengths obtained by this technique have been compared with results of interlaminar shear strength measurements corresponding to woven composite plates. In order to clarify the nature of this interfacial adhesion, both morphological (by atomic force microscopy, AFM) and chemical characterisation (by Fourier transform infrared, FTIR), have also been carried out.

Influence of the initial formaldehyde to phenol molar ratio (F/P) on the formation of a phenolic resol resin catalyzed with amine

Abstract The influence of the initial formaldehyde/phenol molar ratio (F/P) on the formation kinetics of five resol type phenolic resin prepolymers has been studied. Initial formaldehyde/phenol mixtures were fixed to pH=8.0 by adding triethylamine as alkaline catalyst. The evolution of reactants and first formed addition products were followed by liquid chromatography (HPLC). 13C NMR spectroscopy was applied to final prepolymers. The necessary amount of catalyst to adjust the initial pH decreased with F/P, influencing the rates of consumption and formation of the species. Final formaldehyde and phenol concentrations depend on the initial F/P ratio and on the added amount of triethylamine. The maximum concentrations of first formed addition products decreased with F/P whereas the maximum concentration reached by di- and trisubstituted phenols was independent of this ratio. Higher molecular weight compounds were formed by joining phenolic rings by methylene bridges at para,para and ortho,para positions. No ortho,ortho bonds were detected.

Sustainable optically transparent composites based on epoxidized soy-bean oil (ESO) matrix and high contents of bacterial cellulose (BC)

Production of transparent composites from totally renewable resources with extraordinary potential for different applications can be made possible using cellulose. Composites of epoxidized soybean oil (ESO)/bacterial cellulose (BC) nanofibers have been prepared with high fiber content. Due to the nano-order scale network-like structure of BC nanofibers, composite films present high transparency even at high BC content. Transparency of films has been analyzed by UV–visible spectroscopy observing that only 15% of matrix transmittance is lost in the nanocomposites. ESO/BC composites show better mechanical properties with increasing BC content. Composites combine high stiffness and good ductility due to the incorporation of BC network structure in ESO matrix.

Influence of curing conditions on the morphologies of a PMMA-modified epoxy matrix

The effect of curing conditions such as time and temperature on the morphology developed in a diglycidyl ether of bisphenol-A epoxy resin cured with diamino diphenyl methane, and modified with 15 wt% poly(methyl methacrylate), has been investigated. The reacting mixtures were precured at 80°C for a period of time ranging from 2 to 7 h, afterwards they were cured at 140°C and finally postcured at 200°C. The mixtures were opaque or transparent depending on the precuring time. Dynamic mechanical thermal analysis suggested that all mixtures were heterogeneous. However, phase separation occurred for all precuring times but to a lesser extent for samples precured for 5, 6 or 7 h than for those precured for shorter time intervals at 80°C. Two phases were clearly distinguished by atomic force microscopy, in all mixtures. The phase size was controlled, on one hand, by the time the phases had to grow, i.e. the interval between the cloud and gel points, and on the other hand, by the viscosity of the reacting mixture at the moment of phase separation.