David A. Langlois

High temperature properties of poly(styreneco-alkylmaleimide) foams prepared by high internal phase emulsion polymerization

Abstract The influence of three N-substituted maleimides on the processing, thermal, and mechanical performance of high internal phase emulsion (HIPE) polymerized foams are investigated. N-propylmaleimide, N-butylmaleimide, and N-cyclohexylmaleimide were copolymerized with styrene and crosslinked with either divinylbenzene or bis(3-ethyl-5-methyl-4-maleimide-phenyl)methane in a HIPE polymerization process. The foam samples were evaluated by dynamic mechanical analysis, thermogravimetric analysis, compression strength measurements, and scanning electron microscopy. All maleimide modifiers produced increases in the foam glass transition temperature (Tg) as a function of the maleimide concentration. However, the degree of Tg improvement was strongly dependent on the N-substituted maleimide used during processing. Cyclohexylmaleimide produced higher Tg foams than propylmaleimide, which produced higher Tg foams than butylmaleimide. In addition, the N-substituted maleimide solubility in both the oil and water phases played important roles in the processing and final open-celled microstructure. The preferred N-substituted maleimide modifier should possess high solubility in the oil phase and insolubility in the water phase.

Accelerated Testing Validation

The DOE Fuel Cell technical team recommended ASTs were performed on 2 different MEAs (designated P5 and HD6) from Ballard Power Systems. These MEAs were also incorporated into stacks and operated in fuel cell bus modules that were either operated in the field (three P5 buses) in Hamburg, or on an Orange county transit authority drive cycle in the laboratory (HD6 bus module). Qualitative agreement was found in the degradation mechanisms and rates observed in the AST and in the field. The HD6 based MEAs exhibited lower voltage degradation rates (due to catalyst corrosion) and slower membrane degradation rates in the field as reflected by their superior performance in the high potential hold and open-circuit potential AST tests. The quantitative correlation of the degradation rates will have to take into account the various stressors in the field including temperature, relative humidity, start/stops and voltage cycles.

3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports

Understanding the mechanisms associated with Pt/C electrocatalyst degradation in proton exchange membrane fuel cell (PEMFC) cathodes is critical for the future development of higher-performing materials; however, there is a lack of information regarding Pt coarsening under PEMFC operating conditions within the cathode catalyst layer. We report a direct and quantitative 3D study of Pt dispersions on carbon supports (high surface area carbon (HSAC), Vulcan XC-72, and graphitized carbon) with varied surface areas, graphitic character, and Pt loadings ranging from 5 to 40 wt %. This is accomplished both before and after catalyst-cycling accelerated stress tests (ASTs) through observations of the cathode catalyst layer of membrane electrode assemblies. Electron tomography results show Pt nanoparticle agglomeration occurs predominantly at junctions and edges of aggregated graphitized carbon particles, leading to poor Pt dispersion in the as-prepared catalysts and increased coalescence during ASTs. Tomographic reconstructions of Pt/HSAC show much better initial Pt dispersions, less agglomeration, and less coarsening during ASTs in the cathode. However, a large loss of the electrochemically active surface area (ECSA) is still observed and is attributed to accelerated Pt dissolution and nanoparticle coalescence. Furthermore, a strong correlation between Pt particle/agglomerate size and measured ECSA is established and is proposed as a more useful metric than average crystallite size in predicting degradation behavior across different catalyst systems.

Durability Improvements Through Degradation Mechanism Studies

The durability of polymer electrolyte membrane (PEM) fuel cells is a major barrier to the commercialization of these systems for stationary and transportation power applications. By investigating cell component degradation modes and defining the fundamental degradation mechanisms of components and component interactions, new materials can be designed to improve durability. To achieve a deeper understanding of PEM fuel cell durability and component degradation mechanisms, we utilize a multi-institutional and multi-disciplinary team with significant experience investigating these phenomena.

Small-angle neutron scattering study of a thermally aged, segmented poly(ester urethane) binder

Abstract Small-angle neutron scattering (SANS) measurements have been performed on a thermally aged polymeric binder to understand the effects of aging on its microstructure. The binder is a 50%–50% (by weight) mixture of a segmented poly(ester urethane), known as EstaneB 5703, and a nitro-plasticizer (NP). This compound is of interest because it is used in the high-explosive (HE) PBX9501. The addition of the polymeric binder to the crystalline HE imparts both structural integrity and plasticity to the HE, allowing it to be readily machined and pressed to specific densities. In addition, the binder significantly affects the performance and the sensitivity of the HE, due in part to its influence on the propagation of microstresses between crystalline grains under shock or loading conditions. So changes in the binder over time can influence the behavior of the HE system as a whole.

Improving Thermal Performance of High Internal Phase Emulsion Polymerized Foams with Maleimide Based Monomers

The influence of three N-substituted maleimides on the thermal performance of high internal phase emulsion (HIPE) polymerized foams were investigated. N-propylmaleimide, Nbutylmaleimide, and N-cyclohexylmaleimide were copolymerized with styrene and crosslinked with either divinylbenzene or Bis(3-ethyl-5-methyl-4-maleimide-phenyl)methane in a HIPE polymerization process. All maleimide modifiers produced increases in the foam glass transition temperature (Tg) as a function of the maleimide concentration. The degree of Tg improvement was strongly dependent on the N-substituted maleimide used during processing.

Structural Characterization of Segmented Polyurethanes by Small Angle Neutron Scattering

The beneficial mechanical properties of segmented polyurethanes derive from microphase separation of immiscible hard and soft segment-rich domains at room temperature. We are interested in the structure of the domains, how these are affected by hydrolytic aging, and how the structure is modified by low molecular weight plasticizers. To assessed the distribution of the plasticizer in polyurethane, we did small-angle neutron scattering measurements on mixtures of 23% hard segment poly(esterurethane) with different amounts of either non-deuterated or deuterated plasticizer. We analyzed the results using a simple model in which the contrast, Δ=H-, between the hard and soft segment-rich domains is varied by the amount of deuterated or hydrogenated plasticizer, using the fact that I(Q) ∼ Δ 2 . The result demonstrated that the plasticizer is largely associated with the soft segment rich domains. The structure of PESU with the chain extender of the hard segment was assessed after aging under hydrolytic conditions. The results show that the microphase structure coarsens and segregates and that the hard and soft segments segregated as a result of the loss of constraints from hydrolytic soft segment chain scission. The results on plasticizer distribution and the effects of hydrolytic aging give insight on the loss of mechanical properties that occur in each case.

Effect of Platinum Loading on Catalyst Stability under Cyclic Potentials

We have investigated the durability of fuel cells with low platinum loading (<0.2 mg.cm) to identify the main mechanisms of performance degradation on automotive cycles. In this work, we compare the effect of voltage cycling on the degradation in performance of two cells that have different Pt loading on cathode (0.15 vs. 0.4 mg.cm) but are otherwise identical (0.05 mg-cm Pt loading on anode, 15 μm membrane thickness).

Infrared and Raman Spectral Signatures of Aromatic Nitration in Thermoplastic Urethanes

The spectral signatures of nitro attack of the aromatic portion of thermoplastic urethanes (TPU) were determined. Eight fragment molecules were synthesized that represent the nitrated and pristine methylenediphenyl section common to many TPUs. Infrared (IR) and Raman (785 nm illumination) spectra were collected and modeled using the B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistry. Normal mode animations were used to fully assign the vibrational spectra of each fragment. The vibrational assignment was used to develop a diagnostic method for aromatic nitro attack in thermoplastic urethanes. The symmetric NO2 stretch coupled out of phase with the C–NO2 stretch (1330 cm−1) was found to be free from spectral interferences. Spectral reference regions that enable correction for physical differences between samples were determined. The carbonyl stretch at 1700 cm−1 was the best IR reference region, yielding a limit of quantitation (LOQ) of 0.66 ± 0.02 g N/100 g Estane. Secondary IR reference regions were the N–H stretch at 3330 cm−1 or the urethane nitrogen deformation at 1065 cm−1. The reference region in the Raman was a ring stretching mode at 1590 cm−1, giving an LOQ of 0.69 ± 0.02 g N/100 g Estane. Raman spectroscopy displayed a larger calibration sensitivity (slope = 0.110 ± 0.004) than IR spectroscopy (slope = 0.043 ± 0.001) for nitration determination due to the large nitro Raman cross-section. The full spectral assignment of all eight molecules in the infrared and Raman is presented as supplemental material.