Molecular motions in bisphenol a polycarbonates as measured by pulsed NMR techniques. I. Homopolymers and copolymers

Molecular motions in homopolymers and copolymers based on bisphenol A polycarbonate (BAP) were investigated over a wide temperature range by means of two pulsed nuclear magnetic resonance methods: the spin-lattice relaxation time T₁ and the spin-lattice relaxation time in rotating frame, T_1ρ. Nuclear magnetic relaxation measurements of bisphenol A polycarbonate indicated the existence of four transitions in the solid amorphous state. This included the glass transition T_α, two transitions associated with localized motions of the main chain, T_β and T_γ, and a transition due to side-group motion, T_δ. The results offer new evidence for the transition T_β, which is ascribed to localized segmental motions of the polycarbonate chain. The physical properties of the polymer (high impact strength and a large fractional free volume in the glassy state) can be explained by the presence of the two secondary transitions T_β and T_γ, which are both associated with main-chain motions. The presence of the bulky bromine on a bisphenol A unit of a polymer chain in a position ortho to the carbonate groups restricts all motions of the main chain, thus increasing T_α, T_β, and T_γ. As a result of an increase in the secondary transition temperatures, impact strength is reduced at least 20-fold as compared with the unsubstituted material. The results for tetrabromobisphenol A polycarbonate and copolymers of bisphenol A with tetrabromobisphenol A and bisphenol A with tetrachlorobisphenol A polycarbonate confirm the idea that the impact strength of the polymer is related to the secondary transitions (T < T_g) arising from the main-chain motions. Copolymers of bisphenol A polycarbonate with tetrabromobisphenol A polycarbonate showed a single glass transition temperature whose value lay between those of the homopolymers. The results indicated multiple secondary transitions which corresponded to the transitions of the homopolymers with magnitudes proportional to the comonomer content.     

Computer simulation of fracture spreading in a visco-elastic solid

The governing equation of motion, which describes the early stages of fracture in a rate-sensitive solid, has been integrated through the use of the analog computer type EAI 380. Three regimes of the tensile pulsating loading, for which there are no closed form solutions, are considered:

1. sinusoidal load,
2. trapezoidal load,
3. randomly oscillating load.

Signals of these shapes superposed on a constant tensile stress produced three different patterns of crack growth and were photographed directly from the screen of a dual-beam oscilloscope. It appears that the rate of fatigue crack spreading in a time-dependent matrix is enhanced by the pulsating component of load in the degree coinciding with the sequence of the above listing.     

Molecular motions in bisphenol a polycarbonates as measured by pulsed NMR techniques. II. Blends,block copolymers,and composites

Molecular motions in several polyblends and composites based on bisphenol A polycarbonate were investigated over a wide temperature range by means of two pulsed nuclear magnetic resonance methods: the spin-lattice relaxation time T₁ and the spin-lattice relaxation time in rotating frame T_1ρ. Characteristic changes in the transitions of the polyblends and composites with respect to the transitions of unmodified homopolymers and copolymers were observed. By selecting different types and quantities of materials to modify the matrix (bisphenol A polycarbonate) these changes were analyzed. It was found that the multiple transitions in the composites and polyblends were not always combinations of the transitions present in the constituent materials, but depended on compatibility of the polymers and the type of molecular motions of the individual components. Unlike other methods of investigating polymers in bulk, nuclear magnetic relaxation methods are sensitive to supramolecular structure or morphology. Supplemented with transmission and scanning electron micrographs, the results from these experiments led to the postulation of interaction domains or zones between the two phases in certain nonhomogeneous polymer systems in which the motions in one phase (usually the continuous phase) were affected by the motions in the other phase (usually the dispersed phase). Information on the nature and extent of this interaction was obtained by the NMR relaxation methods. The experimental results reflect not only the presence of separate phases in the nonhomogeneous materials, but also the complex heterogeneity of such systems. The results suggest correlations between internal molecular motions and physical properties of the materials examined. Based on the above concepts, a mechanism of rubber reinforcement was proposed. The impact strength of a rubber-modified polymer is related to the apparent volume of the rubber phase. This volume consists of the actual volume of the rubber plus the affected portion of the surrounding glassy matrix which, assisted by the segmental motions of the rubber, assumes the same motions.     

High-Tc Oxides 13 Years Since “Woodstock”: From Fundamentals to Nanoengineering

Following introductory remarks on the future history of superconductivity, I discuss several discrepances concerning doping dependence of the pseudogap and open questions across the complex electronic phase diagram. I argue that once we understand the fundamentals, we will eventually be able to fully exploit the richness of the phase diagram of cuprates (and related oxides) and tailor their electronic and magnetic properties and applications within an integrated nanoengineering approach reminiscent of the band-gap engineering of (Al)GaAs.     

Synthesis, characterization and photocatalytic properties of sol-gel TiO2 films

The application of heterogeneous photocatalysis is described as an advanced oxidation process (AOP) for the degradation of the diazo reactive dye using immobilized TiO₂ as a photocatalyst. Starting TiO₂ solutions were prepared with and without the addition of polyethylene glycol (PEG) and TiO₂ films were directly deposited on a borosilicate glass substrate using the sol-gel dip-coating method. The surface morphology and the nanoscale roughness of TiO₂ films were studied by means of atomic force microscopy (AFM). Structural properties of TiO₂ were identified by X-ray diffraction (XRD). The decomposition behaviour of organic compounds from the gels was investigated using thermal gravimetry (TG) and differential scanning calorimetry (DSC). Photocatalytic activities of TiO₂ films in the process of degradation of the commercial diazo textile dye Congo red (CR), used as a model pollutant, were monitored by means of UV/vis spectrophotometry. The kinetics of the degradation of the CR dye was described with the Langmuir-Hinshelwood (L-H) kinetic model.The addition of PEG to the TiO₂ solution resulted in the changes in the film surface morphology, and affected the ratio of anatase-rutile crystal phases and the photocatalytic activity of TiO₂. The TiO₂ film prepared with PEG is characterized by higher roughness parameters (R_a, R_max, R_q, R_z and Z_max), a lower amount of the rutile phase of TiO₂, a higher amount of the anatase phase of TiO₂ and a better photocatalytic activity compared to the TiO₂ film without the addition of PEG.     

Inter-annual variations of planktonic food webs in the northern Adriatic Sea

The temporal dynamics of microphytoplankton, microzooplankton and mesozooplankton were monitored over 37 months in the Adriatic Sea in order to identify alterations in the plankton structures, which can lead to, or enhance the production of macro-aggregates, that affected the entire northern basin in summers 2000 and 2002, and to assess any negative effects of mucilage on plankton temporal patterns. Samples were collected monthly, from June 1999 to July 2002, on three transects at 9 stations across the northern and central Adriatic Sea. Besides the high year-to-year variations in abundances and taxonomical composition, plankton communities only showed a clear seasonal succession during 2001, when since April a grazing food web developed and was able to control large sized phytoplankton increase. In spring-summer 2000 and 2002 consumer abundances remained quite low and the dominant mesozooplankton summer species (Penilia avirostris) did not reach its usual summer maximum. The lack of an efficient top control was more evident on the northernmost transect, where generally grazing food web prevails over the microbial one. A large part of the microphytoplankton blooms, although not particularly intense, was exported to the bottom in the particulate phase, where it was processed by bacteria, enhancing the production of refractory dissolved material.     

Reliability of the bending resistance of welded beam-to-column joints

This paper deals with the reliability of welded beam-to-column joints designed according to EN 1993-1-8. Based on the set of tests of welded beam-to-column joints, the experimental bending resistance has been obtained. The model’s uncertainty has been evaluated based on the comparison of the theoretical and experimental values of the bending resistance. Limit state equations have been defined for each mode of failure. The reliability index for each component of a joint is evaluated by using FORM, SORM and adaptive sampling. The obtained values of the reliability index of the individual components are discussed, as well as the sensitivity factors of basic variables and sensitivities of the reliability index to the mean value and to standard deviation of basic variables. The probabilistic analysis demonstrated that the component method adopted in EN 1993-1-8 has a high reliability level.     

Quantifying nonthermal inactivation of Listeria monocytogenes in European fermented sausages using bacteriocinogenic lactic acid bacteria or their bacteriocins: a case study for risk assessment

Listeria monocytogenes NCTC10527 was examined with respect to its nonthermal inactivation kinetics in fermented sausages from four European countries: Serbia-Montenegro, Hungary, Croatia, and Bosnia-Herzegovina. The goal was to quantify the effect of fermentation and ripening conditions on L. monocytogenes with the simultaneous presence or absence of bacteriocin-producing lactic acid bacteria (i.e., Lactobacillus sakei). Different models were used to fit the experimental data and to calculate the kinetic parameters. The best model was chosen based on statistical comparisons. The Baranyi model was selected because it fitted the data better in most (73%) of the cases. The results from the challenge experiments and the subsequent statistical analysis indicated that relative to the control condition the addition of L. sakei strains reduced the time required for a 4-log reduction of L. monocytogenes (t(4D)). In contrast, the addition of the bacteriocins mesenterocin Y and sakacin P decreased the t(4D) values for only the Serbian product. A case study for risk assessment also was conducted. The data of initial population and t(4D) collected from all countries were described by a single distribution function. Storage temperature, packaging method, pH, and water activity of the final products were used to calculate the inactivation of L. monocytogenes that might occur during storage of the final product (U.S. Department of Agriculture Pathogen Modeling Program version 7.0). Simulation results indicated that the addition of L. sakei strains significantly decreased the simulated L. monocytogenes concentration of ready-to-eat fermented sausages at the time of consumption.     

ZnO micro and nanocrystals with enhanced visible light absorption

In this paper, we investigate the effect of the particle size and morphology on the optical properties of ZnO. A series of ZnO micro and nanocrystals were synthesized by the hydrothermal processing of zinc acetate dihydrate and sodium hydroxide as the starting materials, and polyvinylpyrrolidone (PVP) as the polymer surfactant. The particle size and morphology were tailored by adjusting the reactant molar ratios [Zn²⁺]/[OH⁻], while the reaction temperature and the time remained unchanged. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) have shown that the micro and nanocrystals have a high crystalline pure wurtzite-type hexagonal structure with nanosized crystallites. The size and morphology of the ZnO micro and nanocrystals were investigated by field emission scanning electron microscopy (FE-SEM), which showed a modification from micro-rods via hexagonal-faceted prismatic morphology to nanospheres, caused by simple adjustment of the reactant molar ratio [Zn²⁺]/[OH⁻] from 1:1 to 1:5. The optical properties of the ZnO micro and nanocrystals, as well as their dependence on the particle size and morphology were investigated by Raman and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy (DRS). The UV–vis spectra showed that the modification of the particle size and morphology from nanospheres to micro-rods resulted in increased absorption, and a slight red-shift of the absorption edge (0.06 eV). Besides, the band gap energy of the synthesized ZnO micro and nanocrystals showed the red shift (∼0.20 eV) compared to bulk ZnO. According to the results of a Raman spectroscopy, the enhanced visible light absorption of the ZnO micro and nanocrystals is related to two phenomena: (1) the existence of lattice defects (oxygen vacancies and zinc interstitials), and (2) the particle surface sensitization by PVP.