Micromorphology and Relaxation Processes of Low Density Polyethylene Probed by Fluorescence Spectroscopys

Fluorescence spectra of molecular guests at several temperatures are useful technique to study several types of polymer properties. In particular, it has been often employed to study polymer relaxation processes either in static or in dynamic conditions. In this work some applications of the steady-state fluorescence spectroscopy of different guests in polyethylene and in particular reports the photophysical behavior of 1,3-di(1-pyrenyl)propane (1Py(3)1Py) sorbed in low density polyethylene are shown. This molecular probe differs of pyrene by its ability to form intramolecular excimer species and we discuss the temperature dependence of this formation. In this study we recorded simultaneously the fluorescence spectra and the differential scanning calorimetry (dsc) traces. The relative dependence of fluorescence intensities on temperature of the higher energy vibronic band at 367 nm, of the isolated choromophore emission and of the excimer emission were discussed and associated with the polyethylene polymer relaxation processes. The influence of the macroscopic stretching upon the polymer relaxation processes and the fluorescence emission was also investigated and compared with previous results.J. B. thanks Brite EuRam/EU (BE97-4472) and to CICYT (MAT2000-0391-P4-02) for financial support. T. D. Z. A. thanks FAPESP and CNPq (Brazil) for financial support and a fellowship. S. B. Y. thanks FAPESP (Brazil) for a fellowship. The authors thank Prof. Carol Collins for useful discussions

Photochemical sensing of semicrystalline morphology in polymers: Pyrene in Polyethylene

Pyrene was inserted into a low-density polyethylene matrix. Fluorescence spectra as a function of temperature and the differential scanning calorimetry (DSC) trace were recorded simultaneously. Along with the usual vibronic bands, a low-intensity band at 365 nm appears at higher energies with respect to the 0−0 transition in the pyrene fluorescence spectra. The fluorescence intensity of this small band increased with temperature, and an isoemissive point was observed to occur at 368 nm. This emission was interpreted as arising from pyrene molecules located in the outer rigid interfacial region of polymer crystallites. Its temperature-dependent fluorescence was interpreted in terms of electron−phonon coupling; two phonons which coincide with fundamental vibrations of polyethylene were necessary to fit experimental data. Coupling with a high-energy phonon was possible at low temperature, whereas, above the β relaxation temperature, phonon coupling occurs with a lower-energy phonon. The α relaxation was detected as a maximum in fluorescence intensity since above its characteristic temperature, nonradiative processes begin to operate.J. Baselga thanks CAM and CICYT (Projects No. 247/92 and MAT 93-0823} for financial support. T.D.Z.A. thanks FAPESP, FINEP, and CNPq (Brazil} for financia} support

Surface Characterization of Silanized Glass Fibers by Labeling with Environmental Sensitive Fluorophores

Glass fibers have been treated with gamma-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 functionality's immobilized on the glass fiber surface. Steady-state and time-resolved 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.Financial support for this work has been provided by Comision Interministerial de Ciencia y Tecnologia (Cl­ CYT) , under grant MAT93-0823, and by Comunidad Autónoma de Madrid (CAM) , under grant 247 / 92. We thank Dr. I. Pierola for her comments and suggestions

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.This research was supported by CICYT MAT93- 0823. This paper is dedicated to the memory of Professor Irmina Hernández-Fuentes

Photophysics of a pyrene probe grafted onto silanized glass fiber surfaces

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).This work has been supported by projects CAM 247/92 and CICYT MAT93-0823