Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

Summary: Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems, depending on monomer concentration. In addition to pure hydrogels, the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. N,N′-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization, thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent β obeyed the percolation result.     

One‐Component Thioxanthone Acetic Acid Derivative Photoinitiator for Free Radical Polymerization

Acetic acid–based thioxanthone (TXCH₂COOH) was synthesized and characterized and used as a photoinitiator for free radical photopolymerization of methyl methacrylate (MMA) in the absence and presence of a tertiary amine (MDEA) in different solvents. Different absorption properties were observed depending on the solvent. Fluorescence and phosphorescence experiments were also carried out successfully. The fluorescence quantum yield was found to be 0.09 and the phosphorescence lifetime was calculated as 138 ms at 77 K. The photoinitiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state possesses π–π* configuration. Laser flash photolysis experiments show that transient absorption of TXCH₂COOH is similar to the parent thioxanthone and the triplet lifetime was calculated as 2.3 μs at 630 nm.     

Spectral Characterization and Antimicrobial Activity of Some Schiff Bases Derived from 4‐Methyl‐2‐aminophenol

A series of N‐(5‐methyl‐2‐hydroxyphenyl)‐(2/3/4/5‐substituted)‐benzaldimines ( I – XIII ) were synthesized using appropriate synthetic route. Their structures were characterized by FT‐IR, UV‐Visible, ESI‐MS, ¹H‐ and ¹³C‐NMR spectroscopic techniques and analytical methods. The crystal structure of N‐(5‐methyl‐2‐hydroxyphenyl)‐3,4‐dimethoxybenzaldimine ( XIII ) was determined by X‐ray diffraction at room temperature. Relationship between the melting points and the structures of the compounds were examined. Antibacterial activities of the compounds were evaluated against Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Proteus mirabilis. Antifungal activities were reported for Candida albicans. Some of the Schiff bases showed considerable antimicrobial activity against S. aureus and C. albicans.     

Syntheses, crystal structures, spectral and thermal analysis and biological activities of copper(II)-pyridine-2,5-dicarboxylate complexes with 4-methylimidazole, imidazole, and 3,4-dimethylpyridine

In this study, three novel Cu(II)-pyridine-2,5-dicarboxylate (pydc) complexes with 4-methylimidazole (4-Meim), [Cu(pydc)(H₂O)(4-Meim)₂]·H₂O (1), imidazole (im), {[Cu(μ-pydc)(im)₂]·2H₂O}_n (2), and 3,4-dimethylpyridine (dmpy), [Cu(μ-pydc)(H₂O)(dmpy)]_n (3) have been synthesized. Elemental and thermal analyses, magnetic susceptibilities, IR and UV/vis spectroscopic studies have been performed to characterize the complexes. The molecular structures of mononuclear (1) and polynuclear (2 and 3) complexes have been determined by the single crystal X-ray diffraction technique. In 1 and 2, Cu(II) ions have distorted square planer geometry, while 3 has distorted octahedral coordination. The pyridine-2,5-dicarboxylate exhibits three different coordination modes namely bidentate (1), tridentate (2) and tetradentate (3). The complex 1 is further constructed to form three-dimensional framework by hydrogen bonding, C–Hπ and ππ stacking interactions. The adjacent chains of 2 and 3 are then mutually linked via hydrogen bonding, ππ and C–Hπ interactions, which are further assembled to form three-dimensional framework. 1 exhibits the magnetic moment value of 1.70 BM, which corresponds to one of the unpaired electron, while the polynuclear complexes 2 and 3 exhibit 1.58 and 1.46 BM, which is lower than the spin only value for one unpaired electron, indicates to antiferromagnetic effect. The first thermal decomposition process of all the complexes is endothermic dehydration. This stage is followed by partial (or complete) decomposition of the neutral and pydc ligands. In the later stage, the remained organic residue exothermically burns. The final decomposition products which identified by IR spectroscopy were the CuO.     

Liquid phase oxidation of benzyl alcohol to benzaldehyde over chromium borophosphate catalyst synthesized by solution combustion method using different types of fuel

In this paper, chromium borophosphates have been prepared by the solution combustion method using different types of fuel and enhanced as a catalyst for liquid-phase oxidation of benzyl alcohol to benzaldehyde for the first time. Carbohydrazide, citric acid, hexamethylenetetramine, oxalyldihydrazide and urea used as an organic fuel and the obtained chromium borophosphates were systematically characterized by infrared spectroscopy, X-ray powder diffraction, thermogravimetry, scanning electron microscopy, and surface analysis techniques. They are structurally identical and thermally stable up to 1000 °C. According to fuel, their surface properties differ from each other and the long and short rod-like particles, which have a surface area between 7 and 19 m² g^?1, are formed. The effects of solvent type, reaction temperature, reaction time, peroxide amount and catalyst amount were also examined for catalytic studies, and the reaction conditions were optimized. As compared with the other fuels, hexamethylenetetramine-assisted synthesized chromium borophosphate catalyst provides high benzaldehyde conversion (52.09%) and selectivity (100%) under the optimized condition and has potential as a heterogeneous catalyst for oxidation reactions.