Ion Sava

Aromatic polyamides with pendent acetoxybenzamide groups and thin films made therefrom

Abstract New aromatic polyamides containing 1,3,4-oxadiazole or benzonitrile units in the main chain and 5-(4-acetoxybenzamido) groups in the side chain have been synthesized and their properties have been characterized and compared with those of related polyamides and polyoxadiazole-amides. These polymers show good thermal stability, with initial decomposition temperature being at about 300 °C and glass transition temperature in the range of 260–280 °C. They are easily soluble in certain solvents such as N-methylpyrrolidinone (NMP), N,N′-dimethylacetamide (DMA) and N,N′-dimethylformamide (DMF) and can be cast from solutions into thin flexible films. The polymer films had tensile strengths in the range of 77–97 MPa, tensile moduli in the range of 2.3–2.6 GPa and elongation at break values ranging from 6% to 24%. One of the polymers containing the 1,3,4-oxadiazole ring exhibited blue fluorescence.

Fluorinated heterocyclic polyamides

Abstract New thermostable fluorinated heterocyclic polyamides have been synthesized by low-temperature solution polycondensation of various aromatic diamines which contain phenylquinoxaline or 1,3,4-oxadiazole units, with a diacid chloride containing imide rings, hexafluoroisopropylidene (6F) group and methylene bridges. These polymers are readily soluble in polar amidic solvents, such as N-methylpyrrolidinone (NMP) and dimethylformamide (DMF) and can be cast into flexible thin films from solutions. They show high thermal stability with decomposition temperature being above 350°C and glass transition temperature in the range of 260–306°C, good electrical insulating properties with dielectric constant being in the range of 3.3–3.6 and tough mechanical properties.

New Phosphorus‐Containing Copolyesters

A series of phosphorus‐containing copolyesters was prepared by polycondensation of 2‐(6‐oxido‐6H‐dibenzo<1,2>oxaphosphorin‐6‐yl)1,4‐benzenediol, 1 , or of an equimoleculare amount of 1 and different bisphenols 2 , such as: hydroquinone, 4,4′‐(hexafluoroisopropylidene)diphenol, 4,4′‐isopropylidenediphenol, 4,4′‐(1,4‐phenylene‐diisopropylidene)bisphenol, 2,7‐dihydroxynaphthalene, 4,4′‐dihydroxybiphenyl, 9,9‐bis‐(p‐hydroxyphenyl)fluorene, with an aromatic diacid chloride containing two preformed ester groups 3 , namely terephthaloyl‐bis‐(4‐oxybenzoyl‐chloride). The copolyesters exhibited thermal stability having the decomposition temperature above 350°C and char yield at 700°C in the range of 19–41%. The glass transition temperature was in the range of 131–164°C. Some of the polymers exhibited thermotropic liquid crystalline behavior.

Synthesis and properties of silicon-containing polyamides

A series of aromatic polyamides incorporating silicon together with phenylquinoxaline or with hexafluoroisopropylidene groups has been synthesized by solution polycondensation of a silicon-containing diacid chloride with aromatic diamines having phenylquinoxaline rings or hexafluoroisopropylidene groups. These polymers are easily soluble in polar aprotic solvents, such as N-methylpyrrolidinone and dimethylformamide, and in tetrahydrofurane, and can be solution-cast into thin, transparent films having low dielectric constant, in the range of 3.26 to 3.68. These polymers show high thermal stability with decomposition temperature being above 400°C and glass transition temperature in the range of 236°C to 275°C.

Preparation and characterization of thermally stable polyimide membranes by electrospinning for protective clothing applications

The aim of the present study was to obtain high-performance materials for heat and flame protective clothing. Therefore, hybrid membranes were prepared using Kevlar as support and aromatic polyimide nanofibers as a protective coating. The exceptional performances of the prepared membranes were highlighted by selected indicators: high thermal stability, fire resistance and improvement in air permeability without modifying drastically the water vapor transmission rate properties. Attenuated total reflectance Fourier transform infrared and 1H nuclear magnetic resonance spectroscopy were employed to confirm polyimide formation. The ability of polyimide to form fibers was investigated by rheological measurements and scanning electron microscopy, respectively. Thermal degradation was studied using thermogravimetric analysis and a microscale combustion calorimeter. The transport properties of the materials were examined by air permeability, water vapor transmission rate and water resistance. It was shown that transport properties of the modified Kevlar membranes could be controlled by varying the spinning time of polyimide solution. Moreover, by annealing the modified Kevlar weavings at 260℃, the structural integrity and transport properties were not affected, whereas a higher resistance to water was found.

Silicon‐containing poly(amide‐ether)s

New aromatic poly(amide-ether)s (II) have been synthesized by solution polycondensation of various aromatic diamines having two ether bridges (I) with a diacid chloride containing silicon, namely bis(chlorocarbonylphenyl)-diphenylsilane. These polymers are easy soluble in polar amidic solvents such as N-methylpyrrolidinone or dimethylformamide and can be cast into thin flexible films or coatings from such solutions. They show high thermal stability with initial decomposition temperature being above 400°C. Their glass transition temperatures lie in the range of 220–250°C, except for polymer IIe which did not show a clear Tg when heated in a differential scanning calorimetry experiment up to 300°C. The large interval between the glass transition and decomposition temperatures of polymers Ia–Id could be advantageous for their processing via compression molding. The polymer coatings deposited by the spin-coating technique onto silicon wafers showed a very smooth, pinhole-free surface in atomic force microscopy investigations. The free-standing films of 20–30 μm thickness show low dielectric constant, in the range of 3.65–3.78, which is promising for future application as high performance dielectrics.

Swelling of Polyheteroarylenes in Supercritical Carbon Dioxide

Two series of polyheteroarylenes have been investigated with regard to their physical properties before and after swelling with supercritical carbon dioxide. The study of the dependence of glass transition temperature and free volume of polymers on their conformational rigidity showed that the process of swelling in supercritical carbon dioxide is influenced by the voluminous side groups and by the high boiling solvent N-methylpyrolidinone used for the preparation of the polymers which facilitates the formation of crosslinks or complexes with the macromolecular chains.

Synthesis and thermotropic properties of polyazomethines–containing side chain azobenzene moieties

Aromatic polyazomethines–containing azobenzene side groups have been synthesized by solution polycondensation reaction of aromatic dialdehydes bearing flexible spacers between the aromatic rings with aromatic diamines having preformed azobenzene side groups. Thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, and wide angle X-ray diffraction have been used to evaluate their thermal and mesomorphic properties. The target polyazomethines showed nematic liquid crystalline phase, large mesophase range, and good thermal stability.

Synthesis and properties of fluorinated poly(ester-imide)s

A series of new, fluorinated poly(ester-imides)s has been synthesized by solution condensation, in a high-boiling-point solvent, of dihydroxy compounds containing imide and hexafluoroisopropylidene units with diacid chlorides containing preformed ester groups. Solutions of these polymers in NMP or in a mixture of trifluoroacetic acid with chloroform were cast into colourless, thin, flexible films having a low dielectric constant. These polymers show high thermal stability, with the initial decomposition temperature being over 400cC. The glass transition temperature is in the range 215-272C for fully aromatic structures and is about 170 TC for those polymers containing some ethylene groups along with aromatic ones. All these characteristics are discussed and compared with those of related poly(esterimide)s which do not contain 6F groups.

Study of the nanostructuration capacity of some azopolymers with rigid or flexible chains

Azobenzene polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light. One of their attractive features is the possibility of changing the orientation of azobenzene chain through trans-cis-trans photoisomerization cycles to photoinduce birefringerence and linear dichroism in thin polymer films. The photochromic behavior and surface structuring capacity of rigid or flexible polymers having side azobenzene units have been investigated. The polyimides with side azobenzene units have rigid structure, while polysiloxane chains modified with azobenzene groups and nucleobases are considered materials with flexible structure. During the photochromic studies of the investigated polymers, a mechanism concerning the possibility to generate a fluid phase under UV/VIS irradiation was proposed. The photoisomerisation kinetic by UV-Vis irradiation in the solid state compared with the solution and the cis-trans azobenzene relaxation phenomena were presented. The surface structuring capacity of these polymers was studied using a Nd:YAG laser at 355 nm, at different incident fluencies and pulse numbers.