E. El Shafee

Electrical properties of multi walled carbon nanotubes/ poly(vinylidene fluoride/trifluoroethylene) nanocomposites

Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)/multi-walled carbon nanotube (MWCNT) nanocomposites were prepared by the method of solution mixing/casting. The dispersity of the MWCNTs in the PVDF-TrFE matrix was investigated using transmission electron microscopy (TEM), revealing that MWCNT are well distributed in the PVDF matrix. Both individual and agglomerations of MWCNT’s were evident. The electrical properties were characterized by ac conductivity measurements. The conductivity was found to obey a percolation-like power law with a percolation threshold below 0.30 wt. %. The electrical conductivity of the neat PVDF-TrFE could be enhanced by seven orders of magnitude, with the addition of only 0.3 wt. % MWCNTs, suggesting the formation of a well-conducting network by the MWCNT’s throughout the insulating polymer matrix. The intercluster polarization and anomalous diffusion models were used to explain the dielectric behaviors of the composites near the percolation threshold, and the analyses of ac conductivity and dielectric constant imply that the intercluster polarization is more applicable to our systems.

The influence of semicrystalline morphology on the dielectric relaxation properties of poly(3-hydroxybutyrate)

Abstract The dielectric relaxation characteristics of poly(3-hydroxybutyrate) (PHB) in the glass–rubber (α) relaxation region has been investigated. A series of cold-crystallized samples were examined, with emphasis on the influence of semicrystalline morphology on relaxation properties. The presence of crystallinity had a marked impact on the α-relaxation characteristics of the various cold-crystallized specimens as compared to the wholly amorphous material. The constraining influence of the crystallites produced a progressive relaxation broadening and a positive offset in relaxation temperature. With regard to the dielectric relaxation strength, Δ e , we found that the amorphous phase relaxation in the semicrystalline sample had a completely different temperature dependence compared to the wholly amorphous sample, leading to an increase in relaxation strength as the temperature increases above the glass transition temperature ( T g ). This was explained by the existence of a rigid amorphous phase interface which relaxes gradually above the T g of the mobile amorphous material. We suggest that the mobile material is essentially located in the amorphous gaps between lamellar stacks.

Effect of aging on the mechanical properties of cold-crystallized poly(trimethylene terephthalate)

Abstract The changes in the mechanical and thermal properties of cold-crystallized poly(trimethylene terephthalate) during aging at 60 and 80 °C were investigated. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of the aged sample showed an endothermic annealing peak 10–20 °C above the previous aging temperature, the maximum temperature and enthalpic content of these peaks increased with aging time. Dynamic mechanical measurements indicated a relaxation process starting at about 20 °C above the aging temperature and correlate with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigation revealed that neither the crystallinity increased significantly nor did the crystal structure changed. These results were explained by the existence of a third phase besides the crystalline and the ‘classical amorphous’ which involves oriented and constrained ‘non-crystalline’ polymer chain sequences close to the crystalline lamellae.

Dielectric and conductivity relaxation in sodium carboxymethyl cellulose and its acid form

Abstract We report a study on the dielectric relaxation behaviour of samples of sodium carboxymethyl cellulose and its acid form covering the frequency range 20-10 5 Hz and the temperature range 40–90 °C. Treating the data in terms of an impedance formalism using the electrical modulus rather than admittance formalism using the complex dielectric constant revealed that two relaxation processes in NaCMC while its acid form, HCMC, shows only one. The high frequency process in both NaCMC and HCMC is attributed to the reorientation of the carboxymethyl groups. These results suggest that the previous assumption of lacton formatiom in HCMC seems to be untenable. The lower frequency in NaCMC is due to a conductivity relaxation. The shorter relaxation time of the latter indicates that the transport of charge carrier requries the motion of the chain segment. The effect of the degree of substitution on the dipolar relaxation in NaCMC is also discussed.

Crystallization and melting behavior of poly(ethylene oxide)/poly(n-butyl methacrylate) blends

Abstract The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly(n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ12=0, indicating a relatively low interaction strength.

Investigation of the phase structure of poly(3-hydroxybutyrate)/poly(vinyl acetate) blends by dielectric relaxation spectroscopy

Abstract Blends of poly(3-hydroxybutyrate) (PHB) and poly(vinyl acetate) (PVAc) were isothermally cold crystallized from homogeneous samples obtained by quenching. Dielectric relaxation spectroscopy has been used to investigate the glass–rubber relaxation characteristics and corresponding phase separation in cold-crystallized PHB/PVAc blends as a function of blend composition and crystallization conditions. All isothermally cold-crystallized blends displayed two glass–rubber relaxations corresponding to the coexisting of a mixed amorphous interlamellar phase, and a pure PVAc phase residing in interfibrillar/interspherulitic regions. No PHB relaxation was observed, indicating the absence of a pure crystal–amorphous interphase. The composition of the interlamellar phase was enriched in PVAc with increasing overall PVAc content in the blend. Lowering the crystallization temperature led to an increase in the amount of PVAc trapped in the interlamellar regions, which was consistent with kinetic control of the evolving morphology: as crystallization temperature decreased, the crystallization rate increased and the diffusion of PVAc molecules decreased. Comparison of the relaxation characteristics of interfibrillar/interspherulitic phase with those of pure PVAc indicated a much broader spectrum of local relaxation environments of PVAc in the blend, consistent with PVAc segregation across a wide range of size scales.

Studies on the miscibility and phase structure in blends of poly(epichlorohydrin) and poly(vinyl acetate)

Abstract The miscibility of atactic poly(epichlorohydrin) (aPECH) with poly(vinyl acetate) (PVAc) was examined under two different conditions: (i) in dilute solution, using vicometeric measurements and (ii) as cast films, using differential scanning calorimetric (DSC) and FT-infrared spectroscopy. Phase separation on heating, i.e. lower critical solution temperature (LCST) behavior of the aPECH/PVAc blends was examined by the measurement of transmitted light intensity against temperature. From viscosity measurements, the Krigbaum–Wall polymer–polymer interaction (Δ B ) was evaluated. The DSC results show that the aPECH/PVAc blends are miscible as evidenced by the observation of a single composition-dependent glass-transition temperature ( T g ) which is well described by the Couchman and Gordon Taylor models. The Flory–Huggins interaction parameter ( χ 12 ) calculated from the T g -method was negative and equal to −0.01, indicating a relatively low interaction strength. The FT-IR results match very well with those of DSC. The cloud point phenomenon is thermodynamically driven but phase separation, once taken place, is diffusion controlled in normal accessible time.

Effect of photodegradation on the β-relaxation in poly(methylmethacrylate)

Abstract The effect of photodegradation using UV radiation on the β-relaxation in poly(methylmethacrylate), PMMA, was investigated dielectrically within the temperature range 60–90 °C and a 20–10 5 Hz frequency band. It is shown that the irradiated samples exhibit a β-relaxation process similar to that of unirradiated PMMA but with broader distribution of the relaxation times. Increasing the time of UV exposure increases the magnitude of the relaxation and displaces the peak toward lower temperature. The apparent activation energy decreases with irradiation. It is suggested that these observations can be interpreted in terms of the chain scissioning of the polymer chains without extensive depolymerization.

Dielectric relaxation study of atactic poly(epichlorohydrin)/poly(3-hydroxybutyrate) blends

Abstract Binary blends of atactic poly(epichlorohydrin) (aPECH) and poly(3-hydroxybutyrate) (PHB) were investigated as a function of blend composition and crystallization conditions by dielectric relaxation spectroscopy. The quenched samples were found to be miscible in the whole composition range by detecting only one glass transition relaxation, for each composition, which could be closely described by the Gorden–Taylor equation. The cold-crystallized blends displayed two glass transition relaxations at all blend ratios indicating the coexisting of two amorphous populations: a pure aPECH phase dispersed mainly in the interfibrillar zones and a mixed amorphous phase held between crystal lamellae. The interlamellar trapping of aPECH was small and decreases with increasing the overall PHB content in the blend. At high crystallization temperatures the aPECH molecules was found to reside mainly in the interfibrillar regions due to its high mobility relative to the crystal growth rate of PHB. Our results suggest that because the intersegmental interaction in aPECH/PHB blends is weak, the mobility of the amorphous component at a given crystallization temperature decides diluent segregation.

Effects of solvent treatment on glass transition characteristics of treated poly(p-phenylene sulphide)

Abstract The relationship between solvent-crystallized morphology and glass transition characteristics of poly(p-phenylene sulphide) that had been previously processed with 1-chloronaphthalene is reported. Dielectric relaxation studies revealed a strong positive offset in glass transition temperature (Tg) of the solvent-crystallized samples compared to the corresponding amorphous sample, as well as thermally crystallized samples of similar bulk crystallinity. Additionally, the apparent activation energy for the α-relaxation was higher for the solvent-crystallized case. The dielectric relaxation strength (Δe=e0−e∞) determined for the solvent-crystallized PPS is small and showing weak temperature dependence, leading to an increase in Δe as the temperature increased above Tg. This was explained by the existence of a large amount of tightly bound, or rigid-amorphous materials in the solvent-treated sample. We suggested that the large amount of immobile amorphous material, and its ability to relax at temperatures above Tg, is related to strong secondary crystallization process in solvent-treated PPS.