Effect of the chemical structure of aromatic polyimides on their thermal aging,relaxation behavior and mechanical properties

This article examines the effects of structural changes and thermal aging treatments on the relaxation processes and mechanical properties of three polyimides differing for their molecular structure i.e. PMDA-ODA, 6FDA-ODA, and 6FDA-6FpDA. These polyimides were obtained by thermal imidization of their polyamic acid precursors, which were synthesized from the respective dianhydrides [pyromellitic anhydride (PMDA), hexahydrofluoroisopropylidene diphthalic anhydride (6FDA)], and diamines [4,4′-diaminodiphenyl ether (ODA), 4,4′-(hexafluoroisopropylidene) dianiline (6FpDA)]. After the curing process, the polyimides were thermally aged at a fixed temperature for various times Dynamic mechanical measurements performed in a multi-frequency mode, were used to determine the glass-rubber and sub-glass transitions, as well as the activation energy of the β transition. It was found that the T _g decreased in the order PMDA-ODA > 6FDA-6FpDA > 6FDA-ODA as a result of an increased chain rigidity and molecular packing induced by charge transfer interactions during the thermal imidization process. The β sub-glass transition showed two relaxation processes identified as β′ and β′′. The β′ process was attributed to the local motion of the diamine constituents while the β′′ process was caused by the local motion of the dianhydride moiety. The cooperativity of these molecular motions was also assessed via the Starkweather method. The thermal aging enhanced the state of aggregation of polyimide chains and thus the T _g and the sub-glass transition properties. This effect was particularly marked for the PMDA-ODA polyimide. Also the mechanical properties were significantly affected by chemical structure and aging treatments. For non-aged samples the more influenced parameter was the elongation at break, which decreased in the order PMDA-ODA > 6FDA-ODA > 6FDA-6FpDA. The aging enhanced the elastic modulus and the tensile strength and reduced the elongation at break.     

Probabilistic topic models for sequence data

Probabilistic topic models are widely used in different contexts to uncover the hidden structure in large text corpora. One of the main (and perhaps strong) assumption of these models is that generative process follows a bag-of-words assumption, i.e. each token is independent from the previous one. We extend the popular Latent Dirichlet Allocation model by exploiting three different conditional Markovian assumptions: (i) the token generation depends on the current topic and on the previous token; (ii) the topic associated with each observation depends on topic associated with the previous one; (iii) the token generation depends on the current and previous topic. For each of these modeling assumptions we present a Gibbs Sampling procedure for parameter estimation. Experimental evaluation over real-word data shows the performance advantages, in terms of recall and precision, of the sequence-modeling approaches.     

Energy storage and control optimization for an electric vehicle

Two big issues involving electric vehicles are energy supply and power management control. To deal with the energy supply problem, this paper proposes the application of a hybrid energy source system, composed of battery pack and ultracapacitor bank. The power management control between the energy supplies was defined by a fuzzy logic with inference rules optimized through genetic algorithm. The genetic algorithm optimizes lower and upper limits of membership functions aiming to reduce the hybrid energy source system total mass while maximizing the electric vehicle drive range and performance. Through the Pareto frontier, we found the best trade‐off solution.     

Novel quasi-autothermal hydrogen production process in a fixed-bed using a chemical looping approach: A numerical study

This paper presents a novel quasi-autothermal hydrogen production process. The proposed layout couples a Chemical Looping Combustion (CLC) section and a Steam Methane Reforming (SMR) one. In CLC section, four packed-beds are operated using Ni as oxygen carrier and CH₄ as fuel to continuously produce a hot gaseous mixture of H₂O and CO₂. In SMR section, two fixed-beds filled with Ni-based catalyst convert CH₄ and H₂O into a H₂-rich syngas. Four heat exchangers were employed to recover residual heat content of all the exhaust gas currents. By means of a previously developed 1D numerical model, a dynamic simulation study was carried out to evaluate feasibility of the proposed system in terms of methane conversion (100% circa), hydrogen yield (about 0.65 mol_H2/mol_CH4) and selectivity (about 70%), and syngas ratio (about 2.3 mol_H2/mol_CO). Energetic and environmental analyses of the system performed with respect to conventional steam methane reforming, highlights an energy saving of about 98% and avoided CO₂ emission of about 99%.     

Effects of saline stress on mineral composition,phenolic acids and flavonoids in leaves of artichoke and cardoon genotypes grown in floating system

BACKGROUND: Cynara cardunculus is a species native to the Mediterranean basin. It includes globe artichoke and cultivated cardoon as well as their progenitor wild cardoon. The species is a source of biophenols, and its leaf extracts have been widely used in herbal medicine as hepatoprotectors and choleretics since ancient times. The aim of this study was to determine the effect of increasing the level of salinity in the nutrient solution (1 or 30 mmol L^?1 NaCl) on biomass production, mineral composition, radical‐scavenging activity, caffeoylquinic acids and flavonoids in three artichoke (‘Romolo’, ‘Violetto di Provenza’ and ‘Violetto di Romagna’) and three cultivated cardoon (‘Bianco Avorio’, ‘Bianco Gigante Inerme’ and ‘Gigante di Romagna’) cultivars grown in a floating system. RESULTS: Increased salinity in the nutrient solution decreased the leaf dry biomass and leaf number of artichoke and cultivated cardoon cultivars. Salinity reduced macro‐ and microelement accumulation in leaves (e.g. N, K, Ca, Mg, Fe, Mn and B) but improved their antioxidant activity, total polyphenols, chlorogenic acid, cynarin and luteolin. The cultivated cardoons, especially ‘Bianco Avorio’ and ‘Gigante di Romagna’, showed higher biomass and leaf number than those observed in artichoke genotypes. ‘Violetto di Provenza’ exhibited the highest content of chlorogenic acid, closely followed by ‘Violetto di Romagna’, whereas for cynarin content the highest values were recorded in ‘Violetto di Provenza’, ‘Bianco Avorio’ and ‘Gigante di Romagna’. The highest content of luteolin was recorded in ‘Gigante di Romagna’ and ‘Bianco Avorio’, while the highest content of apigenin was observed in ‘Gigante di Romagna’. CONCLUSION: The results showed that the floating system could be considered an effective tool to improve quality aspects through proper management of the salt concentration in the nutrient solution. They also suggest that specific cultivars should be selected to obtain the desired profile of bioactive compounds. © 2012 Society of Chemical Industry     

Optical and electrical characterization of a gold nanoparticle dispersion in a chiral liquid crystal matrix

We report on the effect of gold nanoparticle (Au NP) dispersion in a chiral nematic liquid crystal (LC). Polarized optical microscopy and X-ray diffraction measurements evidence the insurgence of an order change in the LC host. Moreover, a comparative analysis based on dielectric and voltammetric spectroscopies performed on pure LC and on Au NP-doped LC shows that Au NP’s presence besides affecting LC order influences its electric properties: ion conductivity results importantly reduced, and beyond a threshold value of the applied field electrophoresis phenomena are induced.     

Decomposing regional business change at plant level in Italy: A novel spatial shift‐share approach

In this paper, spatial shift‐share decomposition is analysed when applied to Italian data on regional business change at plant level, over the period 2004–2009. A new type of spatial decomposition, which looks more effectively at neighbourhood influence, is introduced here. Notable results emerge from the empirical investigation. First, it can be seen that the spatial level of aggregation greatly affects results. Second, evidence of neighbourhood advantage in the Southern NUTS 3 regions is found, together with opposite results for the Central‐Northern NUTS 3 regions. Finally, evidence of positive industrial mix effects is only found in Central‐Northern Italy.     

Application of chemical and physical agents in model systems to controlling phenoloxidase enzymes

Several chemical and physical anti-browning agents are studied in different model systems in which caffeic acid (as substrate) and laccase from Trametes versicolor (LAC) and polyphenoloxidase from sunflower seeds (PPO) (as enzymes) are used to emulate the browning reaction. Temperature and low electric current were the tested physical agents, while acetic acid, sodium acetate, sodium chloride and, finally, sodium bisulfite were the chemical agents. Sunflower PPO was observed to be less heat sensitive than LAC that was fully inactivated after 1 and 3 min of exposure to 100 and 80 °C, respectively. Conversely, PPO required more than 3 min at 100 °C to be fully inactivated, and it still showed a significant activity (ca. 17%) after an exposure to 80 °C for 15 min. Both LAC and PPO were found to be active at frozen (−18 °C) and cool (+4 °C) temperature, and their activities were strengthened at 40 and 60 °C. As concerning chemical agents, inhibitory power of acetic acid on LAC was observed to be very weak. In the sodium acetate solution at the concentrations of 0.5, 1.0, 2.0 and 4.0%, LAC residual activity was equal to 81.5, 63.9, 61.1 and 35.2%, respectively. PPO is shown to be more sensitive to the NaCl than LAC and indifferent to the presence of NaHSO₃. A 28% residual activity of LAC was found in the solution with 200 mg L⁻¹ NaHSO₃. Finally, LAC activity was decreased to 72.3, 60.0, 16.7 and 8.4% after a low electric current (LEC) treatment of 30 s and 1, 3 and 6 min, respectively. Conversely, PPO activity was not affected under these conditions.     

Fly ash addition in clayey materials to improve the quality of solid bricks

The technical quality of two compositionally different groups of solid bricks fired between 800 and 1000 °C was evaluated. Five weight percentage of fly ash was added to both groups and they were compared with similar bricks with no added fly ash.The textures of the bricks with fly ash were very similar to the textures of those without it, except that the samples with the additive contained spherical fly ash particles with diameters ranging from 0.1 to 10 μm. These particles led to a reduction in the density of the bricks and a substantial improvement in their durability, with less decay being caused by salt crystallization in the pores. This is because fly ash causes a reduction in the number of micropores, the pores that make porous materials most vulnerable to salt-induced decay. Use of this additive could have practical implications as a means of recycling and for achieving cost savings in brick production.