Ionela-Daniela Carja

A straightforward, eco-friendly and cost-effective approach towards flame retardant epoxy resins

Modification of epoxy resins with organophosphorus compounds, either as reactive co-reactants or additives, is the key to achieving non-flammable advanced epoxy materials. Herein, through a straightforward and cost-effective approach, epoxy thermosets were prepared by simply mixing a new phosphorus flame retardant (13.5 wt% phosphorus) with a bifunctional bisphenol-A based epoxy polymer, followed by thermal curing in the presence of an aromatic aminic hardener. It was proved that a very low content of phosphorus led to composites exhibiting remarkable improved flame retardancy. 1 wt% phosphorus was enough to increase the limiting oxygen index value to about 30%. Furthermore, the peak of the heat release rate was reduced up to 45%, depending on the content of the flame retardant additive introduced into the epoxy matrix. UL-94 V-0 materials were obtained when 2 and 3 wt% phosphorus were added into the epoxy matrix. Thermogravimetric data showed that the incorporation of the flame retardant additive significantly increased the char yield and thermal stability of the gradually forming phosphorus-rich carbonaceous layer at elevated temperatures.

Environmentally friendly fire-resistant epoxy resins based on a new oligophosphonate with high flame retardant efficiency

Advanced flame retardant epoxy resins, environmentally friendly, with different contents of a new oligophosphonate (PFR), were prepared using dicyandiamide as a hardener and 1,1-dimethyl-3-phenylurea as an accelerator. PFR, with a high phosphorus content, was synthesized by polycondensation reaction of phenylphosphonic dichloride with a phosphorus-containing bisphenol, namely bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)-(4-hydroxyaniline)methylene)-1,4-phenylene. The bisphenol was prepared by reacting 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide with an imine bisphenol resulting from the condensation of 4-aminophenol with terephthalaldehyde. The structure and morphology of cured epoxy resins were evaluated by Fourier transform infrared spectroscopy and scanning electron microscopy (SEM) analysis, respectively. Differential scanning calorimetry analysis revealed that cured epoxy resins containing PFR possessed slightly higher glass transition temperatures than phosphorus-free cured epoxy resin. Thermogravimetric analysis and limiting oxygen index values indicated that the incorporation of PFR into epoxy resin substantially enhanced the thermal stability and flame retardancy of the char layer at high temperature. The surface morphology of the char residue was studied by SEM measurements. The kinetic processing of thermogravimetric data was carried out using Friedman and Vyazovkin methods. The lifetime prediction analysis established that the fire-resistant phosphorus-containing epoxy resins could be used at a constant temperature of 200 °C up to 620–630 minutes. The new PFR can be successfully used as a very efficient flame retardant for improving the fire-resistance properties of epoxy resins.

New highly thermostable aromatic polyamides with pendant phthalonitrile groups

AbstractA phthalonitrile-containing diamine monomer, 4,4′-diamino-4″-(3,4-dicyanophenoxy)-triphenylmethane, 1, was prepared, and its cure behavior was investigated. The diamine 1 was thermally polymerized at 250 °C even in absence of curing additives. The resulting resins exhibited exceptional high thermal and thermo-oxidative stability, with the 5% weight loss being above 500 °C and the char yield at 700 °C being over 62%, in air or inert atmosphere. New soluble aromatic polyamides derived from diamine 1, 2,2-bis[4-(4-aminophenoxy)phenyl]propane and terephthaloyl chloride were synthesized. They showed good solubility in organic solvents, film forming properties, and high thermal stability, with the decomposition temperature being above 400 °C. The kinetic processing of thermogravimetric data was carried out using differential and integral methods. Electrical properties of the polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The influence of the phthalonitrile content on the polymer properties was studied.

Effect of conformational parameters on physical properties of polymers containing pendant phenoxyphthalonitrile substituents

Heteroaromatic polymers are considered to be high performance organic materials due to their unique and highly attractive properties, including outstanding thermal and mechanical resistance, that arise from their aromatic structure and strong interactions between macromolecular chains. Modification or designing new molecular architectures with tailored physico-chemical characteristics allows expanding the applications of these materials in various advanced technologies. Herein, a series of polymers containing bulky phenoxyphthalonitrile pendant units was synthesized and their physical properties were studied and correlated with their conformational parameters, as well as free and van der Waals volumes. For comparison, the related polymers without lateral moieties were also investigated to highlight the effect of bulky substituent on the polymer rigidity. Thus, it is shown that conformational rigidity determines the packing of macromolecules in solid state, and, therefore, the free volume, glass transition, and decomposition temperatures. The values found experimentally for Tg correlate well with those obtained using the conformational rigidity parameters. The dependence of Tg of these polymers on Kuhn segment is described by linear equations, with very good factors of convergence. The correlations established by Monte Carlo method allow obtaining the Tg values for related polymers where the experimental measurement of this parameter is difficult.

Thermal Degradation and Kinetic Studies of New Flame-Retardant Phosphorus-Containing Polymers with Liquid Crystalline Properties

In this study, two new thermotropic liquid crystalline phosphorus-containing polymers were prepared by polycondensation reaction of 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)carbinol) with two different phosphonic dichlorides, namely ethyl dichlorophosphate and phenyl dichlorophosphate. The polymers were characterized by means of FT-IR, 1H NMR, differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction, and thermogravimetric analysis. Thermogravimetric analysis was performed in nitrogen atmosphere by heating the samples over a range of temperatures from 25° to 700°C using five different heating rates. The kinetic processing of data was achieved using the Freeman-Carroll, ASTM E1641, and Kissinger methods.

Synthesis and photophysical study of new blue fluorescent poly(azomethine-1,3,4-oxadiazole)s containing dimethylamino groups:

New aromatic polyazomethines were synthesized by polycondensation reaction of a diamine containing 1,3,4-oxadiazole ring, namely, 4,4′-diamino-4″-[2-(4-phenoxy)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole)]triphenylmethane, with terephthalic aldehyde or bis(4-formylphenoxyphenyl)fluorene, by using N-methyl-2-pyrrolidinone (NMP) as solvent. The polymers were easily soluble in polar solvents, such as NMP, N,N-dimethylacetamide, or chloroform, and showed high thermal stability with the initial decomposition temperature above 415°C and the temperature of 10% weight loss in the range of 450–460°C. They exhibited high char yield at 800°C in the range of 52–56%. Optical properties were studied by absorption and photoluminescence spectra. In solution, the polymers presented two absorption maxima in the ranges 273–278 and 330–346 nm and emitted violet-blue light in the range of 413–468 nm, depending on the solvent polarity. The Stokes shift and emission quantum yield values depend on the polymer structure and solvent polarity. The emission intensity in NMP solution was enhanced upon increasing the HCl concentration, while the absorption spectral profile was slightly influenced.

Influence of conformational parameters on physical properties of copolyimides containing pendant diphenylphosphine oxide units

Modification or designing new molecular architectures are the key strategies to obtain materials with improved/tunable properties able to address environmental restrictions and public needs. Herein, a series of copolyimides containing bulky diphenylphosphine oxide pendant units was synthesized and the physical properties were studied and correlated with the conformational parameters, such as Kuhn segments, Van der Waals and free volumes. It is shown that conformational rigidity determines the packing of macromolecules in glass state, and therefore the free volume, glass transition and dielectric permittivity. The Kuhn segment of the synthesized random copolyimides calculated by Monte Carlo method exhibited a nonlinear dependence on chemical composition. The use of correlations between the chemical structure and conformational parameters is the key to achieve polymers with tailored physicochemical characteristics by varying the comonomers’ ratio.

Synthesis and characterization of new dual stimuli-responsive bisazobenzene derivatives

New organophosphonates containing bisazobenzene moieties in the main chain were synthesized by solution polycondensation reaction of 4,4′-(4,4′-(biphenyl-4,4′-diylbis(oxy))bis(4,1-phenylene))bis(diazene-2,1-diyl)diphenol and phenylphosphonic dichloride/phenyl dichlorophosphate. FTIR and NMR spectroscopy were used to confirm the chemical structure of the new compounds, while thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The optical properties of these compounds were evaluated in different solvents and in the presence or absence of several external stimuli by UV-vis and fluorescence spectroscopy. The photoresponsive trans–cis photoisomerization and cis–trans relaxation process of these compounds were investigated. The absorption and fluorescence properties were sensitive to chemical stimuli (acid/base treatments). In the presence of NaOH a strong bathochromic shift of the absorption band of the bisazobenzene derivatives was observed (deprotonation). The fluorescence signal was noticeably improved upon deprotonation of the polymer solution.

Preparation and characterization of new hydrogels based on poly(vinyl alcohol)/phosphoester — chondroitin sulphate

New polymer hydrogels based on partially phosphorylated poly(vinyl alcohol) (PVA/Phosphoester) have been prepared. Fourier transform infrared spectroscopy (FT-IR) was employed to confirm PVA/Phosphoester formation. Contact angle measurements were performed to evaluate the surface characteristics of the hydrogels. The PVA/Phosphoester hydrogels were co-networked with Chondroitin sulfate (CS) in various ratios by chemical crosslinking. The synthetic-natural mixed resulted semi-IPN hydrogels were structurally and morphologically investigated by ATR — FT-IR spectroscopy and scanning electron microscopy, respectively. The swelling behavior and dynamic moisture sorption capacity of the PVA/Phosphoester (p-methyl-phenyl phosphonic dichloride) (P3)-CS semi-IPN hydrogels were followed. It was found that the performance of the semi-IPN hydrogels was influenced by the CS. By kinetic studies, it has been shown that the swelling processes occurred by an anomalous transport mechanism.Graphical abstract

Synthesis and characterization of novel main chain bisazobenzene polyphosphonates

Preparation and experimental studies on the phase transition and morphology of novel phosphorus-containing copolymers, having bisazobenzene units and variable molar ratio of aliphatic segments in the main chain, were performed. The chemical structure of the novel compounds has been confirmed by Fourier transform infrared and 1H-NMR spectroscopy. The thermal stability was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) while the mesomorphic behavior was closely observed with polarized light microscope (PLM) and by X-ray diffraction measurements, structure-properties relationships being established and discussed.