Radu-Dan Rusu

Six-member polyimides incorporating redox chromophores

This study focuses on the thermal, photo-optical and electrochemical characterization of some copoly(oxadiazole-naphthylimide)s containing tetramethyldisiloxane units. The thermal stability and glass transition temperatures of these copolymers were measured and compared with those of related polymers. The polymers were processed in very thin films having smooth surfaces, without pinholes or cracks. Upon irradiation with UV light the polymers showed photoluminescence maxima in the blue spectral range. Cyclic voltammetry was performed to obtain information about the electrochemical stability and reversibility of the redox processes of these polyimides. The HOMO and LUMO energy levels, and electrochemical and optical band gap values were calculated by using cyclic voltammetry and UV–Vis spectroscopy data, showing very good electron-injection and transport characteristics. These properties make the present polymers suitable for application in electroluminescent devices.

Dielectric Behavior of Thin Films made from poly(oxadiazole-naphthylimide)s

Oxadiazole-containing poly(naphthylimide)s were prepared by polycondensation reaction and their solid-state properties were investigated. Flexible films having good mechanical properties were made therefrom. The tougher polymer films were the subject of electrical insulating properties measurements. The dielectric constant values, measured at room temperature and in the frequency domain of 1 Hz–1 MHz, are in the range of 2.63–3.24, being lower or comparable with those of Kapton HN measured under the same conditions. The dielectric loss values are low, in the same range with those of Kapton HN. The dielectric spectroscopy data showed distinct subglass transitions for these polymers: γ relaxations with activation energies of 35 and 46 kJ/mol, and a β relaxation with the activation energy of 86 kJ/mol.

Study of thin films made from aromatic polymers containing six-member imide rings

This study was concerned with the synthesis and study of thin films made from polyimides containing naphthalene and oxadiazole rings. These polymers were prepared by polycondensation reaction in solution at high temperature of aromatic diamines containing preformed oxadiazole ring with a mixture 1: 1 of a naphthalene-containing dianhydride with hexafluoroisopropylidene-dianhydride. The resulting aromatic polymers containing six-member imide rings were soluble in polar amidic solvents and in less polar solvents, and their solutions gave flexible films having tough mechanical properties. The thermal stability and glass transition temperature of these polymers were measured and compared. The quality of the spin-coated films of these polyimides was investigated by atomic force microscopy. The photoluminescence properties of the polymers in solution and in thin films were studied to survey the color of emission. The UV absorption was also investigated to determine the Stokes shift and hence possible reabsorption effects, and the photo-optical stability of the polymers at high temperatures.

Copolyimides containing perylene and hexafluoroisopropylidene moieties

Three poly(ether-peryleneimide)s were prepared by one-step solution polycondensation reaction at high temperature of an aromatic diamine containing an oxadiazole ring and/or diphenylether units with a mixture of perylenetetracarboxylic dianhydride and hexafluoroisopropylidene-diphthalic dianhydride. The structure of the polymers was confirmed by Fourier transform-infrared analysis. These polymers possess good solubility in N-methylpyrrolidinone and can be processed into thin and tough flexible films. They are highly thermostable, up to 465°C, and display glass transition in the range of 227–266°C. The dielectric constant values of these polyimides at 25°C and in the frequency domain of 1 Hz to 100 kHz are in the range of 2.29–2.96. The photoluminescence spectra of those poly(ether-peryleneimide)s consist of the emission of perylenediimide chromophore and conjugated segments containing five-imide rings, without the emission from oxadiazole chromophore.

Copoly(1,3,4-oxadiazole-naphthylimide)s containing siloxane units in the main chain: synthesis and properties

This study was concerned with the synthesis and properties of new soluble copolyimides containing naphthalene, oxadiazole and tetramethyldisiloxane units. These copolyimides were prepared by two-step polycondensation reaction in solution of aromatic diamines containing preformed oxadiazole ring with a mixture 1 : 1 of naphthalene tetracarboxylic dianhydride and tetramethyldisiloxane- [1,3-bis(4′-phthalic anhydride)]. The structure of these polymers was confirmed by Fourier transform infrared and 1 H-NMR spectroscopy. The solubility, thermal stability and glass transition temperature of these copolyimides were measured and compared with those of related polyimides which do not contain siloxane groups. All these polymers emitted an intense blue color light upon irradiation with UV light. These copoly(oxadiazole-naphthylimide)s could be considered promising candidates for organo-soluble high-performance polymers for use in novel technological applications.

Dielectric properties of thin polyimide films

Heterocyclic polyimides based on bis (ketonaphthalic) dianhydrides and aromatic diamines containing oxadiazole rings were prepared and their dielectric properties were investigated. The dielectric constant values, measured at room temperature and in the frequency domain of 1 Hz-1MHz, were in the range of 2.7–3.1, being lower or comparable with those of commercial polyimide film Kapton which is one of the most used dielectric materials in microelectronic applications. The dielectric loss values are also low, in the same range with those of Kapton.

Polynaphthylimides based on isomeric 2,5-bis[(aminophenoxy)phenylen]-1,3,4-oxadiazoles

Polynaphthylimides containing 1,3,4-oxadiazole cycles in main chains of macromolecules are prepared via the interaction of isomeric 2,5-bis[(aminophenoxy)phenyl]-1,3,4-oxadiazoles with dianhydrides of naphthalene-1,4,5,8-tetracarboxylic acid and 1,3-bis(1,8-dicarboxynaphthoyl-4)benzene. These polymers are synthesized through high-temperature polycyclocondensation in N-methylpyrrolidone and molten phenol. The relationships between the solubility and thermal characteristics of polynaphthylimides and their structure are studied, and the optical properties of the polymers are estimated.

Polyperyleneimide — Based materials for optoelectronic devices

Here we present a study of two copolyimides containing oxadiazole and perylene units in the main chain with emphasis on their photo-optical properties and electrochemical behavior. Upon irradiation with light of different wavelengths these polymers showed photoluminescence maxima in the UV, blue or green-yellow spectral range. Cyclic voltammetry was performed in order to obtain information about the electrochemical behaviour of these polymers that proved their n-type behaviour.

KrF pulsed laser ablation of thin films made from fluorinated heterocyclic poly(naphthyl-imide)s.

Among the many aspects of laser ablation, development of conical structures induced by excimer laser radiation on polyimide surfaces has been thoroughly investigated. Because the mechanisms that produce these surface textures are not fully understood, two theories, photochemical bond breaking and thermal reaction, have been introduced. Here we present the first study of ultraviolet laser ablation behavior of thin films made from fluorinated poly(naphthyl-imide)s containing oxadiazole rings and the investigation of the mechanism of cone-like structure formation at two laser fluences, 57 and 240 mJ/cm(2). The morphology of thin films before and after laser ablation was studied by using various spectroscopy techniques such as Fourier transform infrared spectroscopy, time-resolved emission and X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. All of the data suggest impurities shielded at low fluence radiation (57 mJ/cm(2)) and a radiation hardening process at high value fluence (240 mJ/cm(2)), which are proposed as the main mechanisms for laser ablation of our polyimide films, and we bring evidence to support them.

Dielectric behaviour of polyperyleneimide films

Thermostable polyimides based on perylenetetracarboxylic dianhydride and aromatic diamines containing oxadiazole rings were prepared and thin films were made therefrom. Dark-red, opaque coatings resulted having strong adhesion to the glass support. The composition of these polyimide films was analysed using X-ray photoelectron spectroscopy (XPS). The dynamo-mechanical and dielectric behaviour of the polyimide films were investigated, as well.