Thuy D. Dang

Synthesis and characterization of fluorinated benzoxazole polymers with high Tg and low dielectric constant

Next generation microelectronic packaging requirements are driving the need to produce increasingly lower dielectric constant materials while maintaining high thermal stability and ease of processing. Efforts have focused on the synthesis and analysis of new polymers with the goals of high thermal stability [degradation temperature (T d ) > 400 °C, low glass-transition temperature (T g ) > 350 °C], low water uptake (< 1%), solubility in selected organic solvents, dielectric constant less than 2.5, and low thermal expansion coefficient. These stringent combined goals have been largely achieved with flexible aromatic benzoxazole polymers. Intramolecular hydrogen bonding between pendant hydroxyl groups and the double-bond nitrogen of the benzoxazole has been exploited to increase the polymer T g , whereas the incorporation of perfluoroisopropyl units effectively decreases the dielectric constant. Out-of-plane impedance measurements on films of materials in this family (38-134 μm thick) have resulted in typical dielectric values of 2.1-2.5 at 1 MHz, depending on copolymer ratios and functionalizations. Results have been correlated with optical waveguide measurements of films 4-μm thick to determine film anisotropy and the high-frequency dielectric constant, and have been corroborated by in-plane interdigitated electrode dielectric measurements on samples 0.75 μm thick. Candidate materials exhibited extremely low water uptake (0.2%) even after submersion in boiling water for several days. Dynamic mechanical analysis of the polymers enabled the determination of the influence of intermolecular hydrogen bonding on the T g and loss tangent magnitude. Finally, the coefficient of thermal expansion has been examined and correlated with copolymer constitution.

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.

Toughening of some high-temperature poly(arylene ether)s and a hydroxypoly(benzoxazole) by sol-gel generated rubbery particles

Abstract Some high-temperature polymers, specifically two poly(arylene ether)s and a hydroxy(benzoxazole) copolymer (HPBO), were toughened using dispersed rubbery phases generated by the sol–gel process. These rubbery phases were introduced using combinations of sol–gel precursors with varying numbers of alkyl groups. Scanning electron micrographs showed uniformly dispersed particles in these composite materials. In poly(arylene ether)s, a significant increase in toughness and ultimate elongation were achieved for samples prepared from relatively large amounts of sol–gel precursor with higher numbers of alkyl groups. In HPBO, samples having low levels of the rubbery phase, both toughness and ultimate elongation were increased with increasing amount of the rubbery phase. Improvements in these properties of the HPBO polymer are rather large compared to the poly(arylene ether)s. The results also demonstrated improved thermal properties and decreased water absorption.

Poly(2,5‐dihydroxy‐1,4‐phenylene benzobisthiazole)/poly(1,4‐phenylene benzobisthiazole) copolymers: Chain packing and properties

Crystal-packing, optical, and electrical properties of poly(2,5-dihydroxy-1,4-phenylene benzobisthiazole) (DiOH-PBZT) and copolymers of DiOH-PBZT/poly(1,4-phenylene-benzobisthiazole) (PBZT) were examined. Intramolecular hydrogen bonds between the hydroxyl units and the neighboring nitrogen atoms, as evidenced by the IR spectra, led to the formation of a pseudoladder chain structure and changed the chain packing. The (200) and (010) planes were both affected by the copolymer composition, with the (200) plane spacing increasing from 5.895 to 6.482 A and the (010) plane spacing decreasing from 3.539 to 3.404 A with the transition from the unsubstituted PBZT homopolymer to the DiOH-PBZT homopolymer. The cell dimensions of the copolymers were simple averages of those of the individual homopolymers, suggesting the isomorphic crystal structure formation of the two units. The c-axis spacing, however, remained unchanged. The increase in the conjugation length of the copolymers as the dihydroxy content increased was confirmed by the bathochromic shift of the absorption band in the ultraviolet-visible spectra. The intrinsic conductivities of the copolymers were 3 orders of magnitude higher than that of the unsubstituted PBZT.

Polyarylenethioethersulfone Membranes for Fuel Cells

High-performance sulfonated polyarylenethioethersulfone (SPTES) polymers have been developed as membranes for fuel cells. These high-molecular-weight polymers synthesized by a polycondensation process have an aromatic backbone along with high sulfonic acid content that provides for their high conductivity and robust mechanical properties. Bulky phenyl-based endcapping agents are incorporated into the system to maintain high water stability and retain high proton conductivity. Films with good mechanical properties were obtained by solvent casting. SPTES polymer systems with a 50% degree of sulfonation (SPTES-50) exhibited high proton conductivity (>100 mS/cm) at 65°C and 85% relative humidity. Membrane electrode assemblies (MEAs) fabricated using SPTES-50 electrolytes that incorporate conventional electrode application techniques have shown high proton mobility. Electrochemical evaluation was performed using nonlinear regression analysis to obtain Tafel parameters. The electrochemical performance of SPTES-50 was comparable to Nafion. Electrochemical impedance spectra were analyzed in terms of a pore-diffusion model. Catalyst utilization for SPTES MEAs using conventional electrode inks with perfluorinated binders was similar to that exhibited by Nafion. Estimates of hydrogen-fuel permeability based upon measured open-circuit voltage indicate that SPTES-50 MEAs exhibit a slightly higher rate of fuel crossover compared to Nafion. Thermogravimetric analysis shows good thermal stability. The high-temperature stability (up to 250°C) and high intrinsic proton conductivities of SPTES-50 qualify it to be a potential candidate for membranes in fuel cells.

Novel Hydrocarbon-Based Electrolytes for Fuel Cells

Novel sulfonated polyarylenethioethersulfone (SPTES) polymers have been developed at the Air Force Research Laboratory (AFRL) which are capable of significantly improving the performance and affordability of polymer electrolyte fuel cells. These high molecular weight polymers have an aromatic backbone with high sulfonic acid content along with bulky phenyl-based end-capping agents. These polymers are strong, chemically inert and thermally stable. SPTES polymer system obtained by solvent casting with 50% degree of sulfonation (SPTES-50) exhibited high proton conductivity (> 100mS/cm) at 65 °C and 85% RH. Single cell analysis using a membrane electrode assembly (MEA) has shown high proton mobility. One of the drawbacks of this system is the high water uptake at operating conditions. High swelling is potentially detrimental and could weaken its structural integrity during thermal cycling by making it prone to developing microcracks or pores and reducing its durability. In order to overcome this problem, AFRL has developed the next generation SPTES polymer system (6F-SPTES) incorporating hydrophobic moieties onto the thermoplastic backbone to enhance its durability by maintaining high water stability and retaining high ionic conductivity. These polymers exhibit good mechanical, chemical and thermal properties. Electrochemical evaluation was performed using non-linear regression analysis to obtain Tafel parameters. Tafel parameters obtained indicate electrochemical performance comparable to Nafion. Catalyst utilization for 6F-SPTES MEAs using conventional electrode inks with perfluorinated binders was similar to that exhibited by Nafion. Overall fuel cell performance was on par with Nafion at both low and high temperatures. The high thermal stability and high intrinsic proton conductivities of 6F-SPTES-50 qualify these polymers to be potential alternatives to Nafion as electrolyte separators for fuel cells.