Facile synthesis and properties of a new generation of soluble and thermally stable polyimides

A new diamine, 1,4-phenylene bis((E)-1-(4-chloro-3-aminobenzylidene) thiourea) (PCABT), containing phenylthiourea and azomethine groups was prepared from the reduction of dinitro compound, 1,4-phenylene bis((E)-1-(4-chloro-3-nitrobenzylidene)thiourea), PCNBT. The structures of resulting monomers were characterized by elemental analysis, FTIR, ¹H and ¹³C NMR techniques. Afterwards, this diamine was reacted with various aromatic dianhydrides (ODPA, BTDA and 6FDA) in glacial acetic acid to afford poly(phenylthiourea azomethine imide)s (PPTAIs) with η_inh of 1.59-1.66 dL/g, depending on the dianhydride used. The ensuing PPTAIs exhibited ample solubility in organic solvents (DMAc, DMF, DMSO and NMP) and were obtained in quantitative yields. Also, all polyimides were amorphous according to wide-angle X-ray determination. GPC measurements of polymers revealed M_w around 69,000-72,000. Moreover, thermogravimetric analyses indicated that PPTAIs were fairly stable up to 550 °C, and 10% weight loss temperatures were recorded in the range of 563-578 °C (N₂ atmosphere). Ultimately, these polyimides own high glass transition temperatures about 281-285 °C.     

Design of Nonlinear Autoregressive Exogenous Model Based Intelligence Computing for Efficient State Estimation of Underwater Passive Target

In this study, an intelligent computing paradigm built on a nonlinear autoregressive exogenous (NARX) feedback neural network model with the strength of deep learning is presented for accurate state estimation of an underwater passive target. In underwater scenarios, real-time motion parameters of passive objects are usually extracted with nonlinear filtering techniques. In filtering algorithms, nonlinear passive measurements are associated with linear kinetics of the target, governing by state space methodology. To improve tracking accuracy, effective feature estimation and minimizing position error of dynamic passive objects, the strength of NARX based supervised learning is exploited. Dynamic artificial neural networks, which contain tapped delay lines, are suitable for predicting the future state of the underwater passive object. Neural networks-based intelligence computing is effectively applied for estimating the real-time actual state of a passive moving object, which follows a semi-curved path. Performance analysis of NARX based neural networks is evaluated for six different scenarios of standard deviation of white Gaussian measurement noise by following bearings only tracking phenomena. Root mean square error between estimated and real position of the passive target in rectangular coordinates is computed for evaluating the worth of the proposed NARX feedback neural network scheme. The Monte Carlo simulations are conducted and the results certify the capability of the intelligence computing over conventional nonlinear filtering algorithms such as spherical radial cubature Kalman filter and unscented Kalman filter for given state estimation model.     

Polyimide-silica Hybrids: Structural and Morphological Investigations

Polyimide (PI) containing pendant hydroxyl functional groups have been employed for preparation of PI-silica hybrids through the sol-gel process. A stoichiometric amount of pyromellitic dianhydride (PMDA) was reacted with a mixture of oxydianiline (ODA) and 3,3′-diamino-4,4′-dihydroxybiphenyl (DAHP) in dimethylacetamide (DMAc) solvent to prepare the precursor poly(amic acid) (PAA) solution for the PI. Various proportions of tetraethoxysilane (TEOS) were mixed with PAA to prepare PI-silica hybrids through sol-gel process. The structure and morphology of these hybrids were investigated with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), optical surface profilometery, and Fourier transform infrared (FTIR) spectroscopy and compared with the one in which the matrix was prepared from PMDA and ODA with no pendant hydroxyl functionalities. Formation of silica lean and silica rich phases (SLP and SRP) as a result of formation of nano-sized silica clusters with diffused boundaries, dispersed in the matrix and their agglomerates, respectively, along with totally different morphology suggest a strong influence of hydroxyl groups in controlling the morphology of PI-silica hybrids. A model namely “Retain and React” has been introduced to explain observed structure.     

Thermodynamics of Adsorption and Micellization of Triblock Copolymers of Oxyethylene and Oxybutylene in Aqueous Medium Using Surface Tensiometry

The aqueous solutions of three triblock copolymers based on polyoxyethylene and polyoxybutylene of the type E_mB₁₀E_m have been analyzed by surface tension measurements. Surface activity of these triblock copolymers was studied by measuring surface parameters, like surface excess concentration, Γ₂, area per molecule of polymer and standard Gibb's free energy of adsorption, Δ^oG_ads, at various temperatures in the range of 20 to 50°C. The Effect of block length of hydrophilic portion of triblock copolymers on surface activity was investigated in this work. Miceller behavior of these triblock copolymers was also investigated using above technique. Critical micelle concentration (CMC) was determined from the plot of surface tension versus log of concentration in the range of temperature of 20–50°C. Thermodynamic parameters, standard free energy of micellization, Δ^oG_mic, standard enthalpy of micellization, Δ^oH_mic, and standard entropy of micellization, Δ^oS_mic were calculated from CMC value using closed association model in this range of temperature. Self assembly behavior of triblock copolymer E₂₀B₁₀E₂₀ was compared with E₃₀B₁₀E₃₀ and E₄₈B₁₀E₄₈ triblock copolymer. Effect of temperature on surface and miceller properties of the triblock copolymers was also studied.     

Design of fractional-order variants of complex LMS and NLMS algorithms for adaptive channel equalization

Equalization filtering is an effective technique applied to minimize the inter-symbol interference (ISI) in multipath fading channels; the problem gets worse for higher-order constellations which are required for high data rates in today’s communication systems. The least mean square (LMS) filter is a computationally efficient and easily implementable algorithm but suffers from slow convergence; highly complex filters are required to nullify the effects of ISI. In this paper, we develop complex modified fractional-order (FO) nonlinear variants of the LMS and the NLMS algorithms and apply in adaptive channel equalization, in both feed-forward and decision feedback configurations. In addition to the standard first-order derivative, the update in the modified LMS also depends on the FO derivative of the mean square error, the final update is formed using a combination of conventional update term and a nonlinear term obtained through Riemann–Liouville fractional derivative. The step size of the FNLMS scheme in fractional part is not only a function of the input energy but also the FO. The differintegral operator working as differentiator helps improve the convergence rate because the algorithm becomes nonlinear; the fractional algorithms provide more parameters to control the rate of convergence and have simple implementation with almost similar complexity. The performances of the schemes are validated through extensive simulation results for block fading channels (frequency flat and selective) to evaluate the symbol error rate for higher-order quadrature amplitude modulation schemes, mean square error and combined channel and equalizer responses to show the improved inverse modeling of the channel. Simulation experiments confirm the superiority of the proposed algorithm over the traditional counterparts.     

2-D PAGE analysis of pesticide-induced stress proteins of E. coli

Logarithmically growing batch cultures of Escherichia coli were exposed to sublethal concentrations of pyrethroid and carbamate pesticides of four different technical grades. This induced 17–20 stress proteins, as observed using two-dimensional polyacrylamide gel electrophoresis. An E. coli culture growing exponentially in Luria Bertani medium (cell density ~2.3×10⁹ cells/ml) was exposed to predetermined sublethal doses of individual pesticides. The cells were harvested after 30 minutes of induction and the stress response was developed in fresh LB medium for three hours under the same growth conditions. Cell pellets were obtained and stored in sonication buffer. Two-dimensional polyacrylamide gel electrophoresis was performed to resolve the proteins. Visualization of the protein spots by rapid silver staining showed 17–20 stress proteins which were absent in the standard protein profile of E. coli. On average 29% of these stress proteins were unique to the pollutant, while the remaining stress proteins overlapped with those of other pesticides. The iso-electric points (PIs) and molecular weights of the proteins were determined by comparing with protein markers with known PIs and molecular weights. Furthermore, upon comparing the pesticide-induced proteins within the same class and between the two different classes (pyrethroid and carbamate), it was apparent that the general nature of the stress remained the same throughout, which indirectly proved that the gene or set of genes responsible for stress expression are also the same, irrespective of the chemical nature of the substituents of the pesticides.     

Design,Synthesis, Antimicrobial Evaluation,and Laccase Catalysis Effect of Novel Benzofuran–Oxadiazole and Benzofuran–Triazole Hybrids

Novel structural hybrids of benzofuran–oxadiazole and benzofuran–triazole have been synthesized and evaluated for their potential against Staphylococcus aureus, Bacillus subtilis, and Escherichia coli. The excellent antibiotic activity was shown by compounds 5c and 9c against S. aureus with minimum inhibitory concentration values in 1.74–5.16 mg/mL range. The estimation of in vitro antifungal activity of synthetic compounds was performed against Trichoderma harzianum, Aspergillus niger, and Metarhizium anisopliae. Among compounds 5a – 5j , only 5h and 5i showed promising antifungal potential against T. harzianum and A. niger, whereas compound 5j showed enhanced antifungal effect only against A. niger when their activity values were compared with standard drug amphotericin. No pronounced antifungal activity was shown by synthesized compounds 9a–j , except for compound 9g , which was active against all fungal strains having minimum inhibitory concentration values in 1.90–2.03 mg/mL range. In addition to antimicrobial evaluation, the synthesized compounds were also analyzed to study their effects on the catalytic potential of laccase, and it was found that among all, compound 9b showed very strong activity with maximum relative reactivity of 145% at 0.03‐mM concentration.     

Nucleophilic ring opening reactions of aziridines

Aziridine ring opening reactions have gained tremendous importance in the synthesis of nitrogen containing biologically active molecules. During recent years, a great effort has been put forward by scientists toward unique bond construction methodologies via ring opening of aziridines. In this regard, a wide range of chiral metal- and organo-catalyzed desymmetrization reactions of aziridines have been reported with carbon, sulfur, oxygen, nitrogen, halogen, and other nucleophiles. In this review, an outline of methodologies adopted by a number of scientists during 2013–2017 for aziridine ring opening reactions as well as their synthetic applications is described.     

Reduction of water-soluble colloidal manganese dioxide by thiourea: a kinetic and mechanistic study

Kinetic data for the colloidal MnO₂–thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO₂ by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log (absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO₄] and the kinetics reveals complex order dependence in [HClO₄]. Addition of P₂O ₇ ⁴⁻ and F⁻ in the form of Na₄P₂O₇ and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO₂. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. From the observed kinetic data, colloidal MnO₂–thiourea adsorption constant (K _ad1) and rate constant (k ₁) were calculated to be 1.25×10¹⁰ mol⁻¹ dm³ and 3.1×10⁻⁴ s⁻¹, respectively. The variation of temperature does not have any effect on the reaction rate.     

Structural Refolding and Thermal Stability of Myoglobin in the Presence of Mixture of Crowders: Importance of Various Interactions for Protein Stabilization in Crowded Conditions

The intracellular environment is overcrowded with a range of molecules (small and large), all of which influence protein conformation. As a result, understanding how proteins fold and stay functional in such crowded conditions is essential. Several in vitro experiments have looked into the effects of macromolecular crowding on different proteins. However, there are hardly any reports regarding small molecular crowders used alone and in mixtures to observe their effects on the structure and stability of the proteins, which mimics of the cellular conditions. Here we investigate the effect of different mixtures of crowders, ethylene glycol (EG) and its polymer polyethylene glycol (PEG 400 Da) on the structural and thermal stability of myoglobin (Mb). Our results show that monomer (EG) has no significant effect on the structure of Mb, while the polymer disrupts its structure and decreases its stability. Conversely, the additive effect of crowders showed structural refolding of the protein to some extent. Moreover, the calorimetric binding studies of the protein showed very weak interactions with the mixture of crowders. Usually, we can assume that soft interactions induce structural perturbations while exclusion volume effects stabilize the protein structure; therefore, we hypothesize that under in vivo crowded conditions, both phenomena occur and maintain the stability and function of proteins.