Polymer/inorganic particle nanocomposites (or nanodielectrics) have attracted pronounced attention for electric energy storage applications, based on a hypothesis that polymer nanodielectrics could combine the high permittivity of nanoparticles and the high electrical breakdown strength of the polymer matrix for enhanced dielectric performance. Although higher discharged energy densities have been reported for numerous polymer nanodielectrics, the dielectric loss mechanisms, which are extremely important for ultimate applications, are rarely discussed. In this work, we intend to address the intrinsic dielectric loss mechanisms associated with polymer nanodielectrics using a model system comprised of 70 nm BaTiO3 nanoparticles (BT NPs) in an isotactic polypropylene (PP) matrix. The effect of space charge-induced interfacial polarization on dielectric losses was investigated using bipolar and unipolar electric displacement -electric field (D-E) loop tests. Since the bipolar D-E loops always exhibited greater nonlinearity than the unipolar loops, the dielectric loss was attributed to the internal AC conduction loss from space charges (e.g., electrons) in the BT NPs, including boundary layer and bulk conductions. To mitigate the internal conduction along the PP/BT interface, atomic layer deposition of a nanolayer (5 nm) of amorphous TiO2 was applied to the BT NPs. Due to a higher resistivity, the coated amorphous TiO2 effectively reduced the boundary layer conduction loss. Nonetheless, the bulk conduction loss in BT NPs still needed to be reduced. This study suggests that more insulating high permittivity NPs are demanded for polymer nanodielectrics to enhance the dielectric performance.

Biphenyl was grafted on rigid-rod aromatic poly(p-phenylenesulfonic acids) (IEC ∼ 8 mequiv g −1 ) to generate cross-linkable polyelectrolytes. Cross-linking conditions and film properties before and after cross-linking were studied. Films equilibrated between 20 and 30% relative humidity (RH) had tensile moduli of 2 to 1 GPa and broke at 5 to 9% elongation. The moduli decreased as RH increased and dropped drastically at high humidity if the films were not cross-linked. Grafted films had conductivities at 80 °C 4−5 times that of Nafion NR-212 over the whole relative humidity range even after cross-linking. The sample reported in detail here had a conductivity of 0.10 S cm −1 at 120 °C and 30% RH. As is usual for this class of materials, even after the loss of 24% of starting sulfonic acid groups by grafting and cross-linking, ionic conductivity was high at low humidity. A membrane electrode assembly (MEA) prepared with this rigid-rod poly(phenylenesulfonic acid) proton exchange membrane and tested in a fuel cell exhibited performance and properties similar to those of Nafion NR-212 films. The power density was ∼95% of that of the Nafion MEA over the operating range in spite of relatively high hydrogen crossover due to the presence of nanocracks in the membrane.

Water insoluble poly(para-phenylene disulfonic acid) and its copolymers were synthesized by direct polymerization of 1,4-dibromobenzene-2,5-disulfonic acid and 4,4 0 -dibromobiphenyl-3,3 0 -disulfonic acid lithium salts using Ullmann coupling and subsequent grafting of long-tail alkylbenzene groups onto the polymer backbones. Copolymers with ion exchange capacities of 4.3 to 7.7 meq. g À1 were obtained. Polymers and copolymers prepared under optimized polymerization conditions were characterized by NMR, TGA, DSC and viscometry. The physicochemical characteristics of the copolymers were tailored by adjusting monomer compositions and by varying the grafting reaction temperature and time. These polymers could hold eight or more strongly bound water molecules per acid group, which facilitated their high conductivity. In the dry state, the polymers were very brittle. Membranes prepared from these polymers exhibited proton conductivity as much as ten times higher than that of NafionÒ 212, at elevated temperature and low relative humidity.

A novel sulfonic acid-containing benzoxazine monomer, 3-[4-(4-(6-sulfo-3,4-dihydro-2H-1,3-benzoxazine-3yl)benzyl)phenyl]-3,4-dihydro-2H-1,3-benzoxazine-6-sulfonic acid (abbreviated as SHS-ddm), was synthesized via Mannich reaction and acidizing reaction using sodium 4-hydroxybenzenesulfonate, 4,4′-diaminodiphenylmethane, and paraformaldehyde as raw materials. The structure of SHS-ddm was characterized by Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance ( 1 H NMR) spectroscopy, and electrospray ionization mass spectrometry (ESI−MS). The crosslinked sulfonic acid-containing polybenzoxazine [poly(SHS-ddm)] membrane was prepared through high temperature solutioncasting method. Membrane properties including proton conductivity, methanol permeability, mechanical property, hydrolytic and oxidative stabilities were evaluated in detail. Our results showed that poly(SHS-ddm) membrane exhibited a high proton conductivity of 0.154 S cm −1 at 80 °C and a very low methanol permeability of 5.8 × 10 −8 cm 2 s −1 . The calculated selectivity parameter was 2.655 × 10 6 S s cm −3 , which was much higher than that of Nafion 117 (8.081 × 10 4 S s cm −3 ). Employing this membrane in the membrane-electrode assembly (MEA), high methanol concentration up to 7 M was used. The cell exhibited a stable open circuit voltage (V OC ) of 0.76 V and a maximum power density of 66.5 mW cm −2 . Accordingly, only relatively low voltage drop of 1.415 mV h −1 was observed after a continuous operation for 12 h. Overall, this novel polybenzoxazine exhibited special potential for proton exchange membrane with low methanol permeability and low cost in direct methanol fuel cells.