Teresa Dib Zambon Atvars

Mapping phases of poly(vinyl alcohol) and poly(vinyl acetate) blends by FTIR microspectroscopy and optical fluorescence microscopy

Fluorescence optical microscopy (FOM) of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) blends in compositions 9/1, 1/1, and 1/9 (w/w) show that these blends present phase separation in the solid state. Each domain of the solid samples was identified by FOM as PVA-richer domains by green fluorescence of fluorescein and PVAc-richer domains by the blue fluorescence of anthracene. The dimensions, shapes, and distributions of these domains were dependent on the initial composition of the polymeric mixtures in the solution. Specific interactions between both homopolymers were studied using FTIR microspectroscopy, which allowed us to obtain spectra for both PVA-richer and PVAc-richer domains. These spectra demonstrated that partial miscibility could occur only for blends with a higher PVAc content and, in these cases, we observed interchain hydrogen-bonded carbonyl groups. Fluorescence microscopy of blends with this partial miscibility exhibited small interconnected domains produced by coalescence of droplets during the polymer phase separation process.

Phase transitions and relaxation processes in ethylene-vinyl acetate copolymers probed by fluorescence spectroscopy

Abstract The steady-state fluorescence of pyrene and anthracene are used to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers, EVA, with defined comonomer compositions (EVA-9, EVA-18, EVA-25, EVA-33 and EVA-40). The temperature of the relaxation processes are compared with those of low-density polyethylene (LDPE) and poly(vinyl acetate) (PVAc). The polymer relaxation processes are assigned to T g =300–310 K (glass transition temperature of the PVAc); T α =270–300 K (relaxation processes of the ethylene units present in LDPE and EVA); T g =220–250 K (glass transition of the LDPE and of the EVA); T γ or T β =160–190 K (relaxation processes of interfacial defects of methylenic chains of LDPE and rotation of the acetate group of the PVAc and the EVA); and T γ =90–130 K (relaxation processes of small sequences of methylene units of LDPE and end groups of PVAc). An Arrhenius-type function was employed as an attempt to represent the experimental data of fluorescence intensity versus temperature above the γ -relaxation temperature. As obtained with other techniques, there is not a simple relationship between the polymer relaxation processes and the vinyl acetate content that can be explained by the morphology in these copolymers.

Electronic absorption and fluorescence spectra of xanthene dyes in polymers

Abstract The electronic absorption and fluorescence spectra of fluorescein, fluorescein disodium, erythrosine B and rose bengal in different polymers (poly(vinyl alcohol), poly(vinyl acetate) and polyethylene glycol 600) were studied. It has been noted that fluorescein displays very complex dissociation and ionization equilibria in polymers, depending on the polymer—dye interactions. Its fluorescence emission may be assigned to the neutral zwitterion form in aprotic polymers and solvents. The photophysical properties of fluorescein disodium, erythrosine B and rose bengal are strongly dependent on the polymer—dye interactions and on the internal heavy atom effect. These results are very similar to those obtained for liquid solvents.

Characterization of static and dynamic properties of a low-density polyethylene. Evidence for translocation of fluorescent probes upon stretching and determination of relaxation processes at the mesoscopic level

Abstract Fluorescence from anthracene guest molecules (AN) has been used to study secondary α, β and γ relaxation processes in unstretched and stretched low density polyethylene (LDPE) in the temperature range 15–400 K. The LDPE was characterized by several different techniques. It contains about 4 vinylidene, vinylene and vinyl type C C bonds and about 320 CH 3 groups per 10 000 CH 2 units. The unstretched polymer is 31% crystalline and has a melting point, T m , of 109°C. The 5x stretched polymer is 51% crystalline and T m = 110°C. Although the degree of crystallinity of the films suffers a large change upon macroscopic stretching, the melting temperature (determined by d.s.c. and fluorescence spectroscopy) and the relaxation temperatures do not. However, plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths vs temperature indicate that dynamic processes involved with the excited singlet states of anthracene are dependent upon polymer stretching. These results, in combination with others obtained previously, suggest that stretching reduces the average amount of free volume at guest sites and induces a translocation of anthracene molecules toward sites at the crystalline-amorphous interfaces.

Dye-polymer interactions controlling the kinetics of fluorescein photobleaching reactions in poly(vinyl alcohol)

Abstract This work presents results for the temperature dependence of the photobleaching reactions of Fluorescein dissolved in poly (vinyl alcohol) (PVA) irradiated by a continuous 150 W Hg high pressure arc lamp. We observed that the photobleaching process efficiency is dependent on: lamp power, dye concentration and temperature. Although the photobleaching processes can only be described by a single-exponential function for lower concentration samples, a biphasic model must be employed for intermediate concentrations, resulting in one slower and one faster processes, while, for higher concentration samples, even multiple-exponential functions are unable to tit the experimental data. This complex behavior is probably due to the co-existence of many different types of energy transfer and photochemical processes with different rate constants. Therefore, a model based on a sequential distribution of site occupation by the dye molecules in the matrix, controlled by dyepolymer interaction strengths, is proposed to explain the complex-exponential kinetic behavior. Moreover, photobleaching process efficiency- is somewhat increased by temperature and, above the PVA glass transition temperature, T g , (350 K), it follows a first order kinetics, fitted by a mono-exponential function.

Translocation of fluorescent probes upon stretching low-density polyethylene films. Comparison between ‘free’ and covalently-attached anthryl groups

Abstract Fluorescence from (9-anthryl)methyl groups (AN-C) attached covalently to polymer chains has been used to study secondary α-, β- and γ-relaxation processes in two types of stretched and unstretched low-density polyethylene (LDPE) with different degrees of crystallinity and frequencies of branches in the temperature range 55–400 K. Plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths versus temperature indicate that the efficiency of radiationless deactivation processes is very sensitive to film stretching (i.e. micromorphological changes induced by macroscopically applied stress) and to secondary relaxation processes of LDPE (i.e. micromorphological changes induced by temperature). Also, since the efficiencies of intermolecular radiative energy transfer processes in LDPE suffered by AN-C and ‘free’ anthracene (ANH), doped into native films, are much different, the mode of ‘reporter’ incorporation into a film and its treatment thereafter can establish differing local concentration gradients. The results suggest that, although stretching reduces, on average, the amount of free volume available at guest sites and partially orients both AN-C and ANH probes, the AN-C (due to their being constrained to their original positions of incorporation) are more influenced by polymer relaxation processes, and the excited singlet states of both probes are more sensitive to temperature in stretched films than in unstretched ones. Overall, comparisons between the behaviour of the covalently-attached and free lumophores provide insights into how external perturbations, such as stress and temperature changes, affect specific aspects of polymer microstructure and mobility and what is the influence exerted by subtly differing environments on the photophysical properties of the probe.

Photostability of xanthene molecules trapped in poly(vinyl alcohol) (PVA) matrices

Photobleaching reactions of fluorescein and Rose Bengal molecules dissolved in PVA matrices were studied as a function of the temperature. We used and compared two theoretical approaches to describe these photobleaching kinetics. Photobleaching quantum yield calculations were done using an adapted model (B-model) for polychromatic excitation that allowed us to estimate the average number of photons absorbed by each dye molecule before its bleaching. The other model uses exponential functions allowing us to calculate rate constants and activation energy. A slower photobleaching process was obtained for Rose Bengal sorbed in PVA matrices compared with fluorescein. We also obtained that the Rose Bengal photobleaching process is always single-exponential kinetic independent of the polymer relaxation process. Nevertheless, fluorescein photobleaching reaction is bi-exponential process at higher concentrations and it is strongly dependent on the polymer relaxation processes. These results were interpreted in terms of the different efficiencies among the processes involving quenching of the lowest triplet state.

Dispositivos poliméricos eletroluminescentes

Here we present an overview of electroluminescent devices that use conjugated polymers as the active media. The principal components of the devices are described and we show some examples of conjugated polymers and copolymers usually employed in polymeric light emitting devices (PLED). Some aspects of the photo and electroluminescence properties as well as of the energy transfer processes are discussed. As an example, we present some of the photophysical properties of poly(fluorene)s, a class of conjugated polymers with blue emission.

Stoichiometry of poly(ethylene glycol) in water and benzene by excess volume

In this work, we studied the excess volume of solutions of low molecular weight poly(ethylene glycol)s (200, 400, and 600) in water and in benzene. We noted that there is a higher contraction of the volume for water solutions in all compositions. Solutions of these polymers in benzene also undergo a volume contraction. However, the magnitude of this effect is lower for benzene solution than for water. Moreover, the stoichiometry of the system at the composition corresponding to the maximum volume contraction is different in each case: one water molecule per two oxyethyelene (EO) units, and one benzene molecule per four EO units. The results are compared with the mixing enthalpy measured by Lakanpal et al. As reported, they observed that while the system PEG/water is exothermic in all compositions, the system PEG/benzene is endothermic, which shows that although in both solvents there is volume contraction, the driving forces for these dissolutions are different.

Effects of temperature and alkyl groups of poly(alkyl methacrylate)s on inter- and intramolecular interactions of excited singlet states of pyrenyl guest molecules.

Temperature-induced changes in the static and dynamic characteristics of the fluorescence from pyrene and N,N-dimethyl-3-(pyren-1-yl)propan-1-amine (PyC3NMe2) have been used to determine the locations and mobilities of these probes in the anisotropic environments provided by films of 5 poly(alkyl methacrylate) (PAMA) polymers in which alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl. Whereas emission from pyrene reports on the polarity of the guest sites and the ability of molecules to diffuse translationally between sites, emission from PyC3NMe2 yields information about the fluidity and the shape of the guest sites. Data have been obtained from 20 to >400 K, a range that spans the onsets of several relaxation processes in the hosts. Those data indicate that the pyrenyl groups reside near to ester functionalities in most of the PAMAs, although the distance from them (and the main chains) depends upon the bulkiness of the alkyl groups. Among the most important conclusions derived from this research is that the rates of segmental relaxation phenomena near the probe molecules--and not free volume, as was concluded previously from fluorescence measurements in polyethylene films--are the dominant contributors to the fluorescence changes. Of practical importance, changes in those rates have permitted the onset temperatures of many of the relaxation phenomena occurring in the vicinity of the probes to be located.