Maria Bruma

Hexafluoroisopropylidene-Containing Polymers for High-Performance Applications

Abstract High-performance polymers are used in applications demanding service at enhanced temperature while maintaining their structural integrity and an excellent combination of chemical, physical, and mechanical properties. Some of the important polymers in this respect are polyimides, polyamides, and polybenzazoles. However, all of these polymers have a common problem of processing difficulty due to their infusibility and poor solubility in organic solvents. The reasons are strong interchain forces, inherent macromolecular rigidity, or semicrystallinity. A great amount of work has been directed to making polymers more tractable, soluble, and processable without sacrificing their high-performance characteristics. Thus, the last two decades have witnessed the spectacular growth of research in a new polymer type, the one containing hexafluoroisopropylidene (6F) groups. Incorporation of 6F units into the macromolecular chain increases the solubility, thermal stability, flame resistance, glass transition te...

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.

Polyamide Copolymers Containing Hexafluoroisopropylene Groups

Abstract New copolymers containing imide and different heterocyclic units such as oxadiazole, benzoxazole, or pyridazine, together with hexafluoroisopropylidene groups, have been synthesized by solution polycondensation of appropriate aromatic diamines with diacid chlorides incorporating preformed imide rings and hexafluoroisopropylidene bridges. In the case of oxadiazole-imide copolymers and pyridazine-imide copolymers, aromatic diamines containing preformed oxadiazole or pyridazine, respectively, have been used, while in the case of benzoxazole-imide copolymers the benzoxazole unit was formed during the polycondensation reaction between bis(o-hydroxy-amine)s and the above-mentioned diacid chlorides. Also, copolyimides containing pendant cyano groups have been prepared from aromatic diamines containing cyano substituents with the same diacid chlorides. Solubility, film-forming ability, thermal stability, and electrical insulating properties of all these compounds are discussed and compared with those of ...

Study of Related Poly(1,3,4-Oxadiazole-Amide)s Containing Silicon or Hexafluoroisopropylidene Groups in the Main Chain

Two series of related poly(oxadiazole-amide)s have been synthesized by solution polycondensation reaction of aromatic diamines containing preformed 1,3,4-oxadiazole rings with diacid chlorides having silicon or hexafluoroisopropylidene groups. All these polymers were soluble in polar amidic solvents, such as N-methylpyrrolidinone and dimethylformamide and gave thin transparent free-standing films on casting their solutions. The polymers exhibited high thermal stability, with the initial decomposition temperature being above 400 °C. Most of these polymers did show a glass transition, being in the temperature range of 238–275 °C. Very thin coatings, in the range of tens of nanometres, which were deposited onto silicon wafers exhibited smooth and pinhole-free surfaces in atomic force microscopy investigations. The study of photoluminescence properties revealed that some of these compounds emit blue light, in the range of 440–475 nm. All these characteristics are discussed and compared with those of conventional aromatic poly(1,3,4-oxadiazole)s.

Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal

The thermotropic and optical characterisations of a novel polymer dispersed liquid crystal (PDLC) system based on polysulfone UDEL P-1700 as polymer matrix and a low molecular weight liquid crystal compound containing a mesogenic azomethine core and a cyano-substituent were investigated. The PDLC samples were prepared by solvent-induced phase separation (SIPS) and thermally induced phase separation (TIPS) methods using various compositions in the two components. Information on the morphology and phase structure was obtained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. PDLC systems with well-defined droplets were obtained for the composite with medium content of liquid crystal compound. The optical characterisation of these materials was performed by analysing their UV–visible absorption and photoluminescence emission as a function of the liquid crystal aggregation degree.

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.

Silicon-Containing Polyesterimides

New silicon-containing polyesterimides have been synthesized by a solution polycondensation reaction, in a high-boiling-point solvent, of a silicon-containing diacid chloride (namely bis(p-chlorocarbonyl-phenylene)-diphenylsilane) with certain dihydroxy-compounds having preformed imide rings. All these polymers were soluble in polar amidic solvents, such as N-methylpyrrolidinone and dimethylformamide, and gave thin transparent films on casting their solutions. The polymers exhibited high thermal stability, with the initial decomposition temperature being above 400 °C for fully aromatic structures. Most of these polymers did show a glass transition: those containing only aromatic rings being in the range of 180–236 °C, while those having some ethylene units along with aromatic ones being in the range of 160–178 °C. The dielectric constants of the free-standing polymer films are in the range of 3.04–3.19. Very thin coatings, in the range of tens of nanometres, which were deposited onto silicon wafers, exhibited smooth and pinhole-free surfaces in atomic force microscopy investigations. All these characteristics are discussed and compared with those of related polyester-imides which do not contain silicon.

SOLID‐STATE PROPERTIES OF MESOMORPHIC COPOLYMERS CONTAINING OXADIAZOLE AND FLUORENE UNITS

A series of copolymers containing oxadiazole and fluorene cromophores was synthesized by polycondensation of a diacid chloride incorporating one diphenylsilane linkage and a mixture of aromatic diamines containing oxadiazole and fluorene moieties. The solubility, thermal behavior, and photoluminescence ability of the thin polymer films were studied and compared with related heterocyclic polymers. These polymers are semicrystalline and form plastic mesophases in the first heating run, which brings about new ordered melted state processing opportunities. They exhibited blue photoluminescence in nanometric films, thus being promising candidates for manufacturing electroluminescent devices.

Synthesis and study of new silicon-containing polyoxadiazoles

A series of new poly-1,3,4-oxadiazoles has been synthesized by polycondensation reaction of hydrazine sulfate with a mixture of a dicarboxylic acid containing unsaturated bonds and a dicarboxylic acid containing silicon, by using methanesulfonic acid/phosphorus pentoxide as a reaction medium. These polymers were highly thermostable but they were only soluble in strong inorganic acids such as sulfuric or methanesulfonic ones. An alternative way was followed by using the corresponding dihydrazides containing unsaturated bonds and the corresponding diacid chloride containing silicon that reacted in N-methylpyrrolidinone (NMP) to give soluble silicon-containing unsaturated polyhydrazides, which were cyclodehydrated either by thermal or chemical treatment to give the corresponding polyoxadiazoles. Very thin coatings of polyhydrazides and polyoxadiazoles were deposited onto silicon wafers and they showed a very smooth surface, free of pinholes, when studied by atomic force microscopy (AFM). Some polyoxadiazole films showed strong blue photoluminescence.

Novel luminescent liquid crystalline polyazomethines. Synthesis and study of thermotropic and photoluminescent properties

Novel semiflexible polyazomethines containing chromophoric units into azomethine mesogenic core have been synthesised by condensation of a flexible dialdehyde with mesogenic diamines containing fluorene, antraquinone, pyrimidine, azobenzene and benzophenone. The thermotropic behaviour of the resulting polyazomethines was assessed by differential scanning calorimetry, polarised light microscopy and investigations of the miscibility relations. While the pure polyazomethines showed a birefringent viscous texture difficult to be ascribed, by mixing with a liquid crystal dimer, clear Schlieren or marbled textures were observed, indicating a nematic mesophase. The polymers exhibited violet, blue or yellow light emission.