J. Bravo

Fluorescence monitoring of polymerization reaction. A new method for treating fluorescence experimental data

Abstract A new method has been found for monitoring polymerization reactions in situ and in real time. The first moment of fluorescent emission, 〈ν〉=∑IF(ν)ν/∑IF(ν) is calculated from fluorescence spectra as a function of polymerization time and can be successfully correlated with the conversion of functional groups, obtained by an independent technique, with a very low level of experimental scatter. The statistical analysis of the method has been performed; some simple computer experiments allowed to study the influence of the most important experimental variables yielding the confidence interval of 〈ν〉 as a function of the noise to signal ratio. This method was applied with stepwise polyaddition (epoxide curing) and polymerization by free radical mechanisms. 5-Dimethylaminonaphthalene-1-sulfonamide derivatives, 4-dialkylamino-4′-nitrostilbene and pyrene were used as probes and/or labels. Other methods reported in the literature have been applied also. Comparison with them reveals that the first moment method is more reliable for monitoring polyaddition reactions.

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.

Chemical Imaging of Phase-Separated Polymer Blends by Fluorescence Microscopy

Blends of poly(vinylacetate) (PVAc) and poly(cyclohexylmethacrylate) (PCHMA) labeled by copolymerization with 4-methacryloylamine-4′-nitrostilbene (Sb), with (1-pyrenylmethyl)methacrylate (Py), or with 3-(methacryloylamine)propyl-N-carbazole (Cbz) were prepared by casting dilute solutions in tetrahydrofurane (THF) or chloroform. Films about 10 μm thick were formed. Phase separation in two types of domains is observed by transmission optical microscopy (TOM) and epifluorescence microscopy (EFM): small craters of 1 to 10 μm placed at the polymer-air interface and larger domains, on the scale of 100 μm. The morphology of samples depends on the composition of the polymer blend and on solvent. The green fluorescence of Sb, the violet of Py, or the blue of Cbz provides imaging of the distribution of PCHMA in the different domains and in the matrix. It is thus observed that (i) superficial craters and large domains are formed mainly by PCHMA and (ii) the matrix is composed of PVAc in films cast from THF and it is a blend of the two polymers, homogeneous at the submicrometric scale, for chloroform. The emission intensity of Py, recorded by microfluorescence spectroscopy (MFS), yields a mapping similar to imaging detection. It is remarkable that in films cast from chloroform, the smaller domains are distributed with a 2D hexatic order disrupted by dislocations and disclinations, whereas in films cast from THF, a larger heterogeneity is found, denoting different mechanisms of solvent evaporation.

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.

FLUORESCENCE MONITORING OF CURING PROCESS AND WATER ACCESSIBILITY AT GLASS FIBER/EPOXY INTERPHASE ON COMPOSITE MATERIALS

Modified glass fibers with three different amino silanes—(1) 3-aminopropyltriethoxysilane (APTES), (2) 3-aminopropylmethyldiethoxysilane (APDES), and (3) 3-aminopropyldimethylethoxysilane (APMES)—were labeled with 5-dimethylaminonaphthalene-1-sulfonylchloride (DNS), and its fluorescence was followed as a function of curing time for two epoxide formulations (TRIEPOX-LM, Gairesa, Spain, and diglycidyl ester bisphenol A–ethylenediamine), as well as a function of water uptake. It was demonstrated that the fluorescence response from fluorophores directly attached to the coupling region in glass fibers/epoxy composites is a very sensible method for nondestructive and “in situ” monitoring of any changes that appear at the interphase (rigidity and solvent accessibility). In addition, fluorescence from a dansyl label shows more rigidity when APMES is used as the coupling agent. This suggests that the greater the cross-linking degree at the coupling region, the smaller the epoxy chain interdiffusion. Finally, the water uptake process in glass fiber/epoxy composites can be divided into two steps; the first step corresponds to the diffusion of water to the coupling region, and the second step corresponds to the hydrolysis of siloxane bonds in the interphase. Both steps are dependent on the surface treatment of the glass fibers.

Photophysics of a pyrene probe grafted onto silanized glass fiber surfaces

Abstract Glass fibers have been silanized with γ-aminopropyltriethoxysilane. 1-pyrenesulphonyl chloride (PSC) has been grafted onto the fibers via adduct formation with the amine functionality. Absorption spectra from SPC stock solutions shows aggregation. Emission spectra from the fibers also reveal aggregation of the cromophore at extremely low concentration (10 −9 mol/g).

Pyrene Fluorescence as a Probe for the Monitoring of Polymerization Processes: Simultaneous DSC and Fluorescence Study

The polymerization of cyclohexylmethacrylate was monitored, over a wide temperature range, by simultaneous differential scanning calorimetry (DSC) and fluorimetry (FL); the equipment for both being optically coupled. Pyrene was used as a fluorescent probe. There was excellent agreement between the exothermic peak time obtained by DSC and the time at which the maximum gradient was obtained in the fluorescence intensity–time curves, but only for temperatures above 60°C, Activation energies for the gel effect onset were obtained for both, the conversion–time and intensity–time curves being concordant except for data at low temperatures, below 60°C. It was concluded that vitrification occurs at temperatures below 60°C and its presence was demonstrated by fluorimetry, by means of the conversion change associated with the fluorescence intensity jumpΔα(off-on).

Intramolecular energy transfer in compounds with two 1-pyrenoate groups separated by methylene spacers.

Steady-state fluorescence anisotropy (r) and fluorescence lifetime (τ) measurements have been used to study the efficiency of nonradiative singlet energy transfer as a function of alkane size in 1-pyrenecarboxylic acid alkanediyl esters (as a function ofm in Py-COO-(CH2)m-OOC-Py, where Py denotes pyrene substituted in the 1-position, andm=2–6). Experiments were performed in media of different viscosity, η, obtained by changing the temperature (from −20 to 40°C) of dilute solutions in ethylene glycol and by examination of the compounds in a solid matrix of poly(methyl methacrylate) (PMMA) at ambient temperature. The Py-COO-(CH2)m-OOC-Py exhibit intramolecular excimer emission in ethylene glycol at these temperatures, but the intensity of this emission is much lower than when these compounds are placed in common solvents of lower η. The values of τ indicate that excitation hopping or intramolecular energy transfer takes place between the chromophores attached to the ends of the alkane bridges. Values ofr obtained by the extrapolationsT/η→0 orτT/η→0 in ethylene glycol, as well as the values obtained in the rigid matrix of PMMA, show very little dependence onm. A theoretical conformational analysis, using the rotational isomeric state (RIS) model, was also performed. The combination of the experimental results forr in the media of high η with the theoretical (RIS) analysis produces an estimated value of 21 ±2 Å for the Förster radius in Py-COO-(CH2)m-OOC-Py.