J. González-Benito

The nature of the structural gradient in epoxy curing at a glass fiber/epoxy matrix interface using FTIR imaging

The curing process of an epoxide system was studied at the interface formed between a silane-coated glass fiber and an epoxy matrix. The gradient in the structure of the epoxy resin as a result of the cure process at the fiber/matrix interfacial region was monitored by FTIR imaging. For comparison, the epoxy curing at the interface formed between the epoxy resin and (a) an uncoated glass fiber and (b) a polyorganosiloxane (obtained from the silane used for the glass-fiber coating) were also monitored. Chemically specific images of the OH and the H-N-H groups near the interface region were obtained. These images suggest that there is a chemical gradient in the structure of the matrix from the fiber surface to the polymer bulk due to different conversions. The basis of the different kinetics of the curing reactions is a result of amino group inactivation at the interface. This deactivation translates into an off-stoichiometry of the reaction mixture, which is a function of the distance from the surface of the glass fiber.

Microstructural and wettability study of surface pretreated glass fibres

Abstract Commercial glass fibres were calcinated for the removal of organic matter, activated for surface silanol regeneration and silanizated with γ -aminopropyltri-ethoxysilane. Two different activation methods were used: reflux with neutral water and reflux with 10% HCl aqueous solution. Acid treatments hydrolize Si–O bonds, greatly changing the composition of the glass and regenerating silanol groups, some of them being of intraglobular nature. Water treatment does not change appreciably the glass composition, but its surface free energy value is the lowest. The degree of silanization is the greatest for the acid activated samples and the lowest for the water activated one. The polar contribution to the total surface free energy, once the samples are silanizated, is the highest for the fibres activated with boiling water whilst the dispersion component is about the same for all of them. SEM observations indicate that for the greatest degree of coating, debonding of the filaments may induce a peeling-out fractural mechanism.

Kinetic study of epoxy curing in the glass fiber/epoxy interface using dansyl fluorescence

The fluorescence response of the dansyl chromophore has been used to study the kinetic of epoxy curing processes. With this new method, comparison between the curing at the interface of a glass fiber/epoxy and in the epoxy bulk of a composite material was studied. The effect of two glass fiber surface treatments was investigated. Commercial E-glass fibers were surface coated with 3-aminopropyltriethoxysilane (APTES) and 3-aminopropylmethyldiethoxysilane (APDES). Fluorimetry (using fluorescent labels) and FT-NIR (Fourier transformed infrared spectroscopy in the near range) techniques were used to monitor the curing process in these composite materials. From the analysis of the data obtained, different simple kinetic models were discussed and apparent activation energies were obtained. Furthermore, from those techniques the respective results were compared to obtain complementary information. Independently of the sample and the technique used for the kinetic analysis, no variation of the activation energy of the epoxy curing reaction was found, which suggests that there are no changes in the mechanism of the reaction along the process. Fluorescence from dansyl located at the glass fiber/epoxy interface reflected that the kind of reinforcement treatment clearly affects the epoxy curing process exactly in that region. However, when analytical response comes from the whole system the mechanism of the reaction does not seem to change with the silane coating used although is quite different in comparison with the process at the interface.

Surface characterization of silanized glass fibers by labeling with environmentally sensitive fluorophores

SYNOPSIS Glass fibers have been treated with y-aminopropyl-triethoxysilane (APES) through different silanizating procedures, which include APES aqueous solutions and APES vapor adsorption. Transmission Fourier transform IR (FTIR) measurements have been performed on the silanized samples to characterize the silanization reaction. Dansyl-sulfonamide conjugates have then been formed by reaction of dansyl chloride in dimethylformamide solution with the amine functionalitys immobilized on the glass fiber surface. Steady-state and timeresolved fluorescence measurements have been performed on dansylated samples. A dependence of the fluorescence intensity and the wavelength of the maximum emission on the silanization procedure has been observed. Good fits of the fluorescence decays of dansyl labels are found when biexponential functions are used for deconvolution, whereas the decay of dansylamides in fluid solution is single exponential. A two-state model for the solid solvent relaxation seems to apply for this samples. Several surface structural changes produced by the different silanization methods have been proposed. FTIR results support the conclusions drawn from fluorescence measurements. C3 1996 .John Wiley 6; Sons, Inc.

Morphology of Epoxy/Polyorganosiloxane Reactive Blends

The morphology of the diglycidyl ether of bisphenol-A/poly(3-aminopropylmethylsiloxane) (DGEBA/PAMS) reactive blends was studied by fluorescence techniques as a function of the initial composition Some fluorescence results were compared with those from optical and electron microscopy investigations, Several morphological aspects were studied including the distribution of PAMS in the blend. The microsegre gation of PAMS was discussed in terms of diffusion restriction of DGEB A through the PAMS dispersed phase.

Fluorescence labels to monitor water absorption in epoxy resins

Water uptake phenomena was studied in a group of commercial epoxy based thermosets using gravimetric and fluorimetric analysis. The different epoxy formulations were labeled with two dansyl derivatives differing in the spacer length between the chain and the fluorophore moiety. The fluorimetric method consisted of monitoring the changes in the first moment of the dansyl emission band as a function of water immersion time. Using the fluorescence, it was possible to obtain the parameters that govern the water diffusion process and there was a good concordance with gravimetric results. Furthermore, the fluorescence response of the dansyl moieties was used to study the effect of the molecular structure of the polymers in the water absorption process.

Hydrolytic damage study of the silane coupling region in coated silica microfibres: pH and coating type effects

Aqueous solutions of three silanes to cover silica microfibres were used, being the 3-aminopropyltriethoxysilane (APTES) and the 3-aminopropylmethyldiethoxysilane (APDES) the reagents for preparing them at the proportion of (APTES/APDES): 1/0, 1/1 and 0/1. The 1-pyrene sulphonyl chloride (PSC) fluorescent moiety was chemically attached to the silanised substrate via the sulphonamide formation. The hydrolytic degradation phenomenon of the silane coupling layer was studied as a function of: (i) temperature, (ii) coating layer type and (iii) pH (4, 7 and 10). The hydrolytic damage in the coupling region of the silica microfibres composite materials occurs under an equilibrium process. It was obtained the activation energies ( Ea) for the hydrolytic damage considering the rate to reach the equilibrium. The values of Ea depended on the type of coating and on the pH. As a consequence, the rate of hydrolytic damage could be related to the proportion of Sisilane–O–Sisilane, while the OH − groups were thought to catalyse the reaction.

Solvent and Temperature Effects on Polymer-Coated Glass Fibers. Fluorescence of the Dansyl Moiety

E-type glass fibers were coated with poly(γ-aminopropyltriethoxysilane), treating them with a 1% (v/v) monomer aqueous solution. The fibers were labeled with a dansyl-sulfonamide conjugate by reaction of acetonitrile solutions of dansyl chloride with the amine groups immobilized on the glass fiber surface. Interactions of the labeled coating polymer with solvents of different polarities were estimated by measurements of the fluorescence band shifts of the label. It was found that for aprotic solvents, the solvent dipolar coupling relaxation mechanism is dominated by thermodynamic interactions of the solvent with the polymer matrix, while for protic solvents this mechanism is dominated by specific interactions between solvent molecules and the excited state of the chromophore. Different experimental excited-state dipole moments were obtained for nonpolar and polar solvents (μ*NP = 7.2 ± 1.6 D, μ*P = 11.9 ± 1.5 D). Using the AM1 method, excited-state dipole moments for the first and second singlets were calculated and it was concluded that μ*NP ≃ 〈μ*21〉1/2 and μ*P ≃ 〈μ*22〉1/2. Accordingly, neither the glass support nor the coating polar influence the excited-state properties of dansyl. The temperature dependence of dansyl emission allows the determination of the relaxation temperature of the coating polymer, which was estimated as 175 K for the coating used.

Morphology and interphase formation in epoxy/PMMA/glass fiber composites: Effect of the molecular weight of the PMMA

In this work ternary composites based on an epoxy thermoset modified with a thermoplastic polymer and reinforced with glass fibers were prepared. The aim of this study is to analyze the influence of the molecular weight of the thermoplastic polymer on the final morphologies. To obtain tailor made interphases four poly(methylmethacrylate), PMMA, which differ in their molecular weight (34,000, 65,000, 76,000 and 360,000 g/mol) were chosen to modify the epoxy resin. The amount of PMMA in the composites was fixed to 5 wt.%. Neat polymer matrices (epoxy-PMMA without fibers) were also prepared for comparison. To study all systems dynamic mechanical analysis (DMA), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used. Although all the systems showed the typical phase separation in the epoxy/PMMA blend, DMA experiments revealed a new phase with more restricted mobility when the glass fibers are present. The amount of this phase increases as molecular weight of PMMA does. The morphologies as well as the fracture surface in the immediate surroundings of the fibers were found to be different from those observed further away from the surface of the fiber, suggesting therefore that, in this case, different fracture mechanism operates. These observations allow us to conclude that an interphase with specific properties is formed. This interphase is based on a polymer or a polymer blend (epoxy-PMMA) enriched in the component with lower mobility.

Pyrene-Doped Polyorganosiloxane Layers over Commercial Glass Fibers

Commercial glass fibers have been subjected to different activation treatments under neutral and acidic conditions to achieve different coating degrees when silanized with γ-aminopropyltriethoxisi lane (APES). A fluorescent sulfonamide (PSA) was formed between the amine residue and a fluorescent probe, pyrenesulfonyl chloride (PSC). Reflectance UV–Vis spectra of the pyrene-doped fibres show that pyrene is present in the form of preassociated dimers when the coating degree is low. Emission and excitation fluorescence spectra reveal the existence of a charge transfer ground-state complex with exciplex emission at 460–515 nm and absorption red-shifted with respect to the S0 → S1 transition. Lifetime measurements yield three lifetimes, which are assigned to dimer, exciplex, and monomer emission. From the photophysical data it is concluded that the fibers with the highest silane content have an open structure with the highest fraction of isolated fluorescent moieties.