Phase transition in poly(vinylidene fluoride) investigated with micro-Raman spectroscopy

The phase transition from the non-polar a-phase to the polar beta-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous because it is a nondestructive technique. Films of alpha-PVDF were subjected to stretching under controlled rates at 80 degrees C, while the transition to P-PVDF was monitored by the decrease in the Raman band at 794 cm(-1) characteristic of the a-phase, along with the concomitant increase in the 839 cm-1 band characteristic of the P-phase. The alpha ->beta transition in our PVDF samples could be achieved even for the sample stretched to twice (2 X -stretched) the initial length and it did not depend on the stretching rate in the range between 2.0 and 7.0 mm/min. These conclusions were corroborated by differential scanning calorimetry (DSC) and X-ray diffraction experiments for PVDF samples processed under the same conditions as in the Raman scattering measurements. Poling with negative corona discharge was found to affect the a-PVDF morphology, improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, i.e., X-ray diffraction and infrared spectroscopy.

Morphologic changes in the vesical transition epithelium of alcoholic rats

Few studies are available about the effect of alcohol on the epithelium of the urinary bladder. In the present investigation we studied the ultrastructure of the vesical transition epithelium of normal rats and of rats submitted to experimental chronic alcoholism. Adult rats were submitted to experimental chronic alcoholism by the ingestion of sugar cane liquor. The vesical epithelium was examined after 60, 120, 180 and 240 days of alcohol treatment by transmission electron microscopy. Surface cells presented nuclear and cytoplasmic changes and marked cellular desquamation. There was an increase in multivesicular bodies and lysosomes suggesting cell degeneration. Mast cell infiltration was observed, possibly related to increased epithelial sensitivity. Intercellular spaces were frequently observed. The transition epithelium of the urinary bladder was found to be sensitive to the action of alcohol, as demonstrated by the changes in the components of the blood-urine barrier, the greater sensitivity to inflammation, the increase in cell desquamation and the greater recycling of the apical membrane and of the fusiform vesicles of surface cells observed in alcoholic rats.

Soliton clusters inducing insulator-to-metal transition in polyacetylene: a Su-Schrieffer-Heeger model study

The Su-Schrieffer-Heeger (SSH) Hamiltonian has been one of most used models to study the electronic structure of polyacetylene (PA) chains. It has been reported in the literature that in the SSH framework a disordered soliton distribution can not produce a metallic regime. However, in this work (using the same SSH model and parameters) we show that this is possible. The necessary conditions for true metals (non-vanishing density of states and extended wavefunctions around the Fermi level) are obtained for soliton concentration higher than 6% through soliton segregation (clustering). These results are consistent with recent experimental data supporting disorder as an essential mechanism behind the high conductivity of conducting polymers. (C) 2001 Elsevier B.V. B.V. All rights reserved.

INSULATOR-TO-METAL TRANSITION IN POLYTHIOPHENE

In the present work, the electronic structure of polythiophene at several doping levels is investigated by the use of the Huckel Hamiltonian with sigma-bond compressibility. Excess charges are assumed to be stored in conformational defects of the bipolaron type. The Hamiltonian matrix elements representative of a bipolaron are obtained from a previous thiophene oligomer calculation, and then transferred to very long chains. Negative factor counting and inverse iteration techniques have been used to evaluate densities of states and wave functions, respectively. Several types of defect distributions were analyzed. Our results are consistent with the following: (i) the bipolaron lattice does not present a finite density of states at the Fermi energy at any doping level; (ii) bipolaron clusters show an insulator-to-metal transition at 8 mol% doping level; (iii) segregation disorder shows an insulator-to-metal transition for doping levels in the range 20-30 mor %.

The investigation of alpha -> beta phase transition in poly(vinylidene fluoride) (PVDF)

The phase transition from the non-polar a-phase to the polar beta-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous for being a non-destructive technique. Films of alpha-PVDF were subjected to stretching under controlled rates and at 80 degrees C, the transition to beta-PVDF being monitored by the decrease in the Raman band at 794 cm(-1) characteristic of the a-phase, with the concomitant increase in the 839 cm(-1) band characteristic of the beta-phase. Poling with negative corona discharge was found to affect the alpha-PVDF morphology improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, viz. X-ray diffraction and infrared spectroscopy.

Diversity of fish species in the River Paranapanema Jurumirim Reservoir transition region (São Paulo, Brazil)

The diversity of fish species in an transition zone between the River Paranapanema and the head mouth of the Jurumirim Reservoir was evaluated. Fish samples were collected from October 1995 to September 1996, by experimental fishing (gill-nets and sieve), in three marginal shallow lagoons and two channel river zones, Gill-net collections resulted in 21 taxons of native fishes. Detritivorous Characiformes such as Steindachnerina insculpta were predominant Sieve collections resulted in 31 taxa and the Tetragonopterinae subfamily was the most representative group, which included small species as Cheirodon stenodon. The equitability index characterized the high heterogeneity of fish numeric abundance, reflecting the dominance of a few groups. A significant relationship between biotic and abiotic variables was indicated by the first canonical function. In comparison with other regions of the drainage basin, the ecotone zone studied indicated a higher species richness and, in the case of some species, fishes in all stages of development. This study demonstrated the necessity to preserve the ecological zone for the fishing and fishery management in the reservoir.

Myosin heavy chain expression and atrophy in rat skeletal muscle during transition from cardiac hypertrophy to heart failure

The purpose of this investigation was to determine whether changes in myosin heavy chain (MHC) expression and atrophy in rat skeletal muscle are observed during transition from cardiac hypertrophy to chronic heart failure (CHF) induced by aortic stenosis (AS). AS and control animals were studied 12 and 18 weeks after surgery and when overt CHF had developed in AS animals, 28 weeks after the surgery. The following parameters were studied in the soleus muscle: muscle atrophy index (soleus weight/body weight), muscle fibre diameter and frequency and MHC expression. AS animals presented decreases in both MHC1 and type I fibres and increases in both MHC2a and type IIa fibres during late cardiac hypertrophy and CHF. Type IIa fibre atrophy occurred during CHF. In conclusion, our data demonstrate that skeletal muscle phenotype changes occur in both late cardiac hypertrophy and heart failure; this suggests that attention should be given to the fact that skeletal muscle phenotype changes occur prior to overt heart failure symptoms.

Electrical conduction mechanism and phase transition studies using dielectric properties and Raman spectroscopy in ferroelectric Pb0.76Ca0.24TiO3 thin films

We have studied the phase transition behavior of Pb0.76Ca0.24TiO3 thin films using Raman scattering and dielectric measurement techniques. We also have studied the leakage current conduction mechanism as a function of temperature for these thin films on platinized silicon substrates. A Pb0.76Ca0.24TiO3 thin film was prepared using a soft chemical process, called the polymeric precursor method. The results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. However, a diffuse character-type phase transition was observed upon transformation from a cubic paraelectric phase to a tetragonal ferroelectric phase. The temperature dependency of Raman scattering spectra was investigated through the ferroelectric phase transition. The soft mode showed a marked dependence on temperature and its disappearance at about 598 K. on the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive above the phase transition temperature. The origin of these modes must be interpreted in terms of a local breakdown of cubic symmetry by some kind of disorder. The lack of a well-defined transition temperature suggested a diffuse-type phase transition. This result corroborate the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in the thin film. The leakage current density of the PCT24 thin film was studied at elevated temperatures, and the data were well fitted by the Schottky emission model. The Schottky barrier height of the PCT24 thin film was estimated to be 1.49 eV. (C) 2003 American Institute of Physics.

Thermostimulated sol-gel transition in suspensions of sulfate zirconium oxychloride

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Role of the surface state and structural feature in the thermoreversible sol-gel transition of a zirconyl aqueous precursor modified by sulfuric acid

The sols produced by admixture of ZrOCl2 acidified solutions to hot H2SO4 aqueous solutions were studied to clarify the effects of Cl- and SO42- ions on the kinetic stability of nanoparticles and to obtain some new evidence concerning the mechanism of a thermoreversible sol-gel transition observed in this system. The study of suspensions prepared with different molar ratios R-S = [Zr]/[SO42-] and R-Cl = [Zr]/[Cl-] revealed domains of composition of formation of thermoreversible gels, thermostable sols, and powder precipitation. The effects of R-S and R-Cl on the structural features of nanoparticles and on the particle solution interface were systematically analyzed for samples of thermoreversible and thermostable sol domains. Small-angle X-ray scattering measurements revealed the presence of small fractal aggregates in all samples of thermoreversible domains, while compact packing aggregates of primary particles are present in the thermostable sol. Extended X-ray absorption fine structure and elemental chemical analysis revealed that irrespective of the nominal value of R-S and R-Cl all studied samples of the thermoreversible domain are constituted by a well-defined compound possessing an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded on the surface by complexing sulfate ligands. zeta potentials of powders extracted by freeze-drying from the thermoreversible gel revealed a point of surface charge inversion attributed to the specific adsorption of SO42- ion. Thermoreversible gel formation is rationalized by considering the effect of the specific adsorption on the electrical double-layer repulsion together with the temperature dependency of the physical chemical properties of ions in solution.

Sol-gel transition in SnO2 colloidal suspensions: viscoelastic properties

This paper describes particle aggregation process during gelation of SnO2 hydrosols. The effect of the concentration of SnO2 colloidal particles on the kinetics of gelation of hydrosols containing PVA (poly(vinyl alcohol)) was analysed by dynamic rheological measurements. The complex viscosity and the storage and loss moduli have been measured during the sol-gel transition and the results correlated to mass fractal growth, nearly linear growth models, and scalar percolation theory. The analysis of the experimental results shows that a linear aggregation occurs in the initial step of the gelation followed by a fractal growth to form a three-dimensional network. Near the gel point this physical gel exhibits the typical scaling expected from an electrical percolation analogy. (C) 1999 Elsevier B.V. B.V. All rights reserved.

THERMAL-HISTORY-DEPENDENT TRANSITION IN PRESSED PELLETS OF CLO4(-)-DOPED POLY(3-METHYLTHIOPHENE)

Electron-spin-resonance and dc conductivity data show a thermal-history-dependent transition at 240 K in pressed pellets of ClO4--doped poly(3-methylthiophene) (P3MT). We discuss the possibility of this transition to be a Peierls transition from a room-temperature-metallic to a charge-density-wave state driven by anions ordering at this temperature. Below 100 K, dc conductivity shows a change from linear to exponential decay. Nonlinear conductivity has also been observed in this system for very low electric fields.

A Raman and dielectric study of a diffuse phase transition in (Pb1-xCax)TiO3 thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb1-xCaxTiO3 thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca2+ content. on the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE-PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films.

Thermodynamic aspects of ion intercalation in KhFek[Fe(CN)(6)](l)center dot mH(2)O compounds: Application to the Everit's salt/Prussian Blue transition

The K+ reversible processes for ion exchange in KhFek[Fe(CN)(6)](l)center dot mH(2)O host compounds (Prussian Blue) were thermodynamically analyzed. A thermodynamic approach was established and developed based on the consideration of a lattice-gas model where the electronic contribution to the chemical potential is neglected and the ion-host interaction is not considered. The occupation fraction of the intercalation process was calculated from the kinetic parameters obtained through ac-electrogravimetry in a previous paper. In this way, the mass potential transfer function introduces a new way to evaluate the thermodynamic aspect of intercalation. Finally, based on the thermodynamic approach, the energy used to put each K+ ion into the host material was calculated. The values were shown to be in good agreement with the values obtained through transient techniques, for example, cyclic voltammetry. As a result, this agreement between theory and experimental data validates the thermodynamic approach considered here, and for the first time, the thermodynamic aspects of insertion were considered for mixed valence materials.