N. Venkatasubramanian

Solvent cast thermoplastic and thermoset rigid-rod molecular composites

Acid–base interaction-mediated compatibilization between rigid-rod and matrix polymer components facilitated the formation and processing of solvent cast aromatic heterocyclic rigid-rod thermoplastic as well as thermoset molecular composites above the critical concentration (Ccr) of the rigid-rod polymer in solution, without phase separation. The blends were solvent cast by the mechanism of ionic interchange between a sulfonic acid-pendent poly(p-phenylenebenzobisimidazole) (SPBI), solubilized in alcohol as its triethylammonium salt and basic thermoplastics such as poly(vinylpyridine)s or secondary or tertiary amines with thermosettable phenylethynyl, nadimide and bisbenzoxazine functionalities. Morphological characterization, utilizing SEM, WAXS and SAXS of as cast as well as annealed/thermally cured optically clear film composites of a broad range of compositions revealed homogeneous microstructures with no observable phase-separated domains, indicating high miscibility, ascribable to the favorable negative enthalpy of the ionic association between the rod-matrix components. A preliminary dynamic mechanical study of compression molded rigid-rod thermoplastic blends with relatively low rod contents showed significant enhancement in thermomechanical properties vis-a-vis the pristine matrix.

Synthesis and thermal properties of thermosetting bis‐benzocyclobutene–terminated arylene ether monomers

A series of new bis-benzocyclobutene-endcapped arylene ether monomers was prepared and characterized. Whereas 2,6-bis(4-benzocyclobutenyloxy)benzonitrile (BCB-EBN) could be prepared in good yield using the standard procedure (K2CO3/NMP/toluene/Dean–Stark trap/120°C), other bis(benzocyclobutene) (BCB)-terminated monomers containing ether-benzophenone (BCB-EK), ether-phenylsulfone (BCB-ES), and ether-6F-benzoxazole (BCB-EBO) moieties were invariably contaminated by mono-endcapped products under similar reaction conditions. This can be attributed to a much greater activating effect of the nitrile group on the ortho-fluorides in the aromatic nucleophilic displacement reaction than the carbonyl, sulfonyl, and benzoxazolyl groups. However, the latter monomers could be synthesized (70–80%) from 4-trimethylsiloxybenzocyclobutene and respective aromatic fluorides in the presence of CsF at 140°C. Similar curing behaviors under N2 (DSC: extrapolated onset and peak temperatures at 227–230° and 260–262°C, respectively) characterized all four monomers. BCB-EK, BCB-ES, and BCB-EBN showed melting transitions at 108, 119, and 146°C, in that order. As BCB-EBO contained more rigid benzoxazole segments, it only exhibited a glass transition (Tg) at 85°C prior to curing exotherm, after it had been previously heated to 125°C. The following Tgs were observed for the cured materials: BCB-EK (201°C), BCB-EBN (224°C), BCB-ES (264°C), and BCB-EBO (282°C). The relative thermal stability according to TGA (He) results is: BCB-ES < BCB-EBN < BCB-EK < BCB-EBO. Finally, the results from thermal analysis, infrared spectroscopic, and variable temperature microscopic studies indicated that the nitrile group plays an important role in the cure chemistry, thermal, and microstructural properties of BCB-EBN.

Aromatic polyamides containing keto-benzocyclobutene pendants

A crosslinkable polymer having repeating units of the formula: ##STR1## wherein Ar is selected from the group consisting of ##STR2## These polymers are useful in the preparation of molecular composites with rigid-rod polymers, including para-oriented benzobisazole polymers, such as benzobisthiazole, benzobisoxazole and benzobisimidazole polymers.

Synthesis, Properties and Potential Applications of Sulpho-Pendent Poly(Arylene Ether Ketone)s

High molecular weight sulpho-pendent poly(arylene ether ketone) homopolymers and copolymers were synthesized with inherent viscosities ranging from 0.94 dl g−1 to 1.20 dl g−1 and glass transition temperatures (T gs) in the range 190–200 °C. Their potential use as transparent matrix hosts for second-order NLO chromophores was explored from the point of view of obtaining monodisperse guest–host systems mediated by specific interaction between the sulphonic acid pendant of the polymer host and the basic functionality of the chromophore structure. Homogeneously dispersed, optically clear thin film composites were obtained for aromatic heterocyclic chromophores with electron-rich thienyl, N,N-dialkylamino or N,N-diphenylamino donors and a pyridyl acceptor in their molecular structures.

Influence Of Structure On The Dielectric Properties Of PECVD Polymer Films

Polymer dielectric films fabricated by plasma enhanced chemical vapor deposition (PECVD) have inherent superiority due to their smooth surface, pin-hole free morphology, and dense crosslinked bulk structure. These spatially uniform films also exhibit good adhesion to a variety of substrates, excellent chemical inertness, high thermal resistance, and are formed from a rapid, inexpensive, solvent-free, room temperature process. In this work, we describe PECVD polymer dielectric films prepared from three precursors including benzene, octafluorocyclobutane (OFCB) and hexamethyldisiloxane (HMDS) using two different feed locations including in the plasma zone center and in the downstream region. The chemical structure of the PECVD films was determined by XPS, FTIR and ESR. The dielectric constant and dissipation of the films were studied over a range of frequencies up to 1 MHz, and the dielectric strength was characterized by the current-voltage method. Spectroscopic ellipsometry was performed to determine the thickness and refractive index of the resultant films. The PP-benzene films showed strong aromatic characteristics, PP-OFCB films maintained a high F/C ratio with a variety of fluorine moieties, and PP-HMDS retained a large fraction of Si-O bonds. All the PECVD films showed higher dielectric constants than those of corresponding conventional polymers. There is a small sharp drop in dielectric constant at low frequencies for all the PECVD films, attributed to the orientational polarization caused by trapped free radicals, oxygenated groups (C=O), and unsaturated moieties. The largest drop occurred for the PP-HMDS films. All films exhibit a continuous increase in dielectric loss as a function of frequency. Among all the PECVD films, the PP-benzene exhibited high breakdown strength. These variations in the dielectric properties are closely associated with the unique structure features of PECVD films.