Dumitru Mircea Vuluga

Synthesis and Characterization of Novel Azobenzene Methacrylate Monomers

New methacrylic monomers containing pendant azobenzene structures have been prepared by reacting azobenzene assemblies possessing phenolic functions with methacryloyl chloride. The most favorable synthetic routes are described together with the yields obtained. The new monomers have been purified and completely characterized by elemental analysis and melting points, as well as by recording their UV, FT-IR and NMR spectra.

Synthesis and Copolymerization Study of New Polyimide Precursors with Potential Application in Optical and Photonic Field

New polyimide precursors were synthesized by simple one step procedure. (Z)-3-(4-Nitro-phenylcarbamoyl)-acrylic acid (A2), (Z)-3-(4-Ciano-phenylcarbamoyl)-acrylic acid (A3), (Z)-3-(4-Diethyl-phenylcarbamoyl)-acrylic acid (A4) and (Z)-3-[N′-(2,4-Dinitro-phenyl)-hydrazinocarbonyl]-acrylic acid (A6) were characterized by FT-IR, FT-NMR and UV-VIS spectroscopy. Test copolymerization successfully yielded copolymers that were characterized by FT-IR and FT-NMR spectroscopy. Improved solubility of these copolymers make them candidates for potential NLO applications.

Morphology and thermal stability of bacterial cellulose/collagen composites

The aim of this paper was to prepare composites of bacterial cellulose (BC) and collagen to evaluate both the effect of collagen on the morphological, mechanical and thermal properties of BC and the effect of BC on the thermal stability of collagen for designing composites with increased potential biomedical applications. Two series of composites were prepared, the first series by immersing BC pellicle in solutions of collagen obtained in three forms, collagen gel (CG), collagen solution (CS) and hydrolysed collagen (HC), followed by freeze drying; and the second series of composites by mixing BC powder in solutions of collagen (CG, CS and HC), also followed by freeze drying. The properties of obtained composites were evaluated by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), mechanical tests, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results revealed that BC acts as a thermal stabilizer for CS matrix, while with CG matrix it interacts synergistically leading to composites with improved properties. On the other hand, the BC sheet impregnated with collagen has a significantly improved thermal stability. Collagen (as HC, CS or CG) has also a positive influence on the mechanical properties of lyophilized BC sheet. A four times increase of modulus was observed in BC/HC and BC/CG composites. and an increase of 60 times for BC/CS. The spectacular increase of elastic modulus and tensile strength in the case of BC/CS composite was explained by the easier penetration of collagen solution in the BC network and impregnation of BC fibrils as revealed by SEM and AFM analyzes.

Thermal behavior of silicophosphate gels obtained from different precursors

Phosphosilicate glasses are of great interest in important fields, such as optical active systems, energy generating systems, humidity sensors, and as materials for biomedical applications. Many studies were accomplished to establish the influence of different reaction parameters on the evolution and final structure of sol–gel prepared phosphosilicate gels. In the present work, we studied the thermal behavior of the silicophosphate gels obtained starting with different phosphorous precursors, the influence of these precursors on the composition and structure of the resultant gels, and their evolution with thermal treatment. By Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and thermodifferential analysis (DTA/TG), and Differential Scanning calorimetry coupled with Mass Spectrometry (DSC-MS), it was established that the type of the precursors essentially influences the composition and structure and consequently the thermal behavior of the obtained gels. In the case of triethylphosphate precursor, all used methods of investigation have shown that the ester is trapped in the silica matrix and it is eliminated during the thermal treatment. Triethylphosphite partially hydrolizes and reacts with the silica network during post-preparation thermal treatment. Only in the case of H3PO4, an interaction with TEOS takes place and leads to Si–O–P bond formation. By thermal treatment, the gels with different composition and structure lead to materials with different properties.

Titania Modified Layered Silicate for Polymer/Inorganic Nanocomposites

Sodium montmorillonite, from purified Romanian bentonite, and quasiamorphous TiO2 aerogel were used for obtaining binary hybrids by solution intercalation method. In the nanohybrid, TiO2 results in crystalline-anatase form. The morphostructural properties of the TiO2 modified layered silicate were studied by X-Ray Diffraction, FTIR Spectroscopy, High Resolution Transmission Electron Microscopy and Thermogravimetric Analysis. By uniform dispersing of binary nanohybrid in a collagen matrix, nanocomposite with intercalated lamellar structure was obtained. The basal spacing (d001) increases from 14.8 A˚ in the binary hybrid to 18 Å in the nanocomposite with collagen. Applications in regenerative medicine of hard tissue are foreseen.

Poly(2-isopropenyl-2-oxazoline) as a versatile platform towards thermoresponsive copolymers

The present study describes the synthesis of thermoresponsive polymers with tailorable properties using a simple, straightforward and robust polymer modification approach. Herein, we first report a simple strategy to obtain poly(2-isopropenyl-2-oxazoline) (PiPOx) polymers with narrow distribution by controlled living anionic polymerization initiated with commercially available n-butyllithium. Further on, the well-defined PiPOx polymer is reacted with four aliphatic acids by a partial ring-opening reaction of the pendant 2-oxazoline side chains, thus enabling access to a large structural diversity of statistical (co)polymers with LCST behavior. Depending on the final PiPOx copolymer composition and the hydrophobic character of the side-chain functionalities, the cloud point temperatures (TCP) of the resulting PiPOx copolymers span the entire liquid water temperature scale (5 °C to 97 °C). As such, this work opens up the use of PiPOx as a highly versatile platform for advanced smart materials as the retained 2-oxazoline side chains can be further modified to introduce functionalities.

New side-chain azo-polymers for optical applications: synthesis and characterization

In this paper we report a series of side-chain polymers bearing a push-pull azo-moiety, from pseudo-stylbene type molecules. The azobenzene monomer is chemically incorporated through radical copolymerization in: poly(methacrylate), poly(styrene); and poly(oxazoline) matrices. The different classes of azo-polymers are discussed comparatively to one another from the point of view of: solubility, molecular weights, glass transition temperatures and thermal stability, chromophore contents, and as well as from the point of view of their spectral features.

Polymer Architectures Based on Oxazoline Monomers

Nonaqueous dispersion copolymerization of 2-isopropenyl 2-oxazoline (IPRO) with methyl methacrylate (MMA) and chloromethylstyrene (CMS) was studied. The resulting copolymers were analyzed with FT-IR and FT-NMR techniques. The morphologies and average diameter were investigated for the characterization of the resulting particles. When the comonomer was MMA the particle size was dependent upon the molar ratio MMA/IPRO in copolymer. MMA content in the resulting copolymers is influenced by the type of stabilizing agent. Narrow size distribution was obtain for copolymer rich in IPRO. When CMS was the commoner, polymerization occurred via two mechanisms: radicalic and cationic with preference for the last one.

Cellulose-Based Composites for Membranes by “In Situ” Radical Polymerization

Cellulose-containing composites based on copolymers of acrylic acid with styrene, 4-chloro-methyl-styrene and maleic anhydride were prepared for proton conducting membranes. The influence of the substrate on the copolymer composition as well as the copolymer/cellulose ratio on the basic properties of composite materials is discussed using the FT-IR, optical microscopy, SEM, TGA-DSC and conductivity measurements. Moderate water swelling (2–10%) of the synthesized membranes provided volume resistivities down to 1 × 105 Ω · cm, adequate for further processing as proton exchange membrane.

Photoresponsive behavior of azobenzene hybrid materials

The trans-cis-trans isomerization process for a series of azobenzene hybrid materials has been studied by UV-Vis spectroscopy. The photoisomerization process is mainly independent of the position and chemical nature of the substituents. The kinetics of the thermal back reaction for the azo-hybrids depend strongly on the chemical nature of the substituent and substitution pattern, with thermal relaxation rate constant ranging from 3.29×10-5 to 6.49×10-2 s-1. The photoresponsive properties can be tune up by changing substituent nature and position on the azo-chromophore. These photoresponsive hybrid materials are optimal molecules for storage data applications and for designing fast optical switching devices.