Brian C. Benicewicz

Polybenzimidazole/Acid Complexes as High-Temperature Membranes

This chapter reviews the progress towards applying acid-doped polybenzimidazoles (PBIs) as polymer electrolyte membrane (PEM) fuel cell membranes over approximately the last ten years. The major focus of the first part of the chapter is on three main systems: (1) the well-developed meta-PBI (poly(2,2′-m-phenylene-5,5′-bibenzimidazole)); (2) the various derivatives and filled systems based on meta-PBI; and (3) poly(2,5-benzimidazole) (AB-PBI). The polymer membrane properties, such as thermal and chemical stability, ionic conductivity, mechanical properties, and ability to be manufactured into a membrane and electrode assembly (MEA), are discussed in detail. Preliminary fuel cell performance is reported for a number of PBI chemistries. The second section of the chapter highlights recent work on developing a novel process to produce phosphoric acid (PA)-doped PBI membranes for use in high-temperature PEMFCs. This novel sol-gel process, termed the polyphosphoric acid (PPA) process, allows production of a gel membrane that exhibits properties not observed with the “traditionally” prepared PBIs, such as improved ionic conductivity, mechanical properties, fuel cell performance, and long-term stability. The final section of the chapter focuses on the possible degradation modes of the commercially available products from BASF Fuel Cells.

TiO2 nanocomposites with high refractive index and transparency

Transparent polymer nanocomposites with high refractive index were prepared by grafting polymer chains onto anatase TiO2 nanoparticlesvia a combination of phosphate ligand engineering and alkyne-azide “click” chemistry. Highly crystalline TiO2 nanoparticles with 5 nm diameter were synthesized by a solvothermal method and used as high refractive index filler. The synthesized phosphate-azide ligand anchors strongly onto the TiO2 nanoparticle surface and the azide end group allows for attachment of poly(glycidyl methacrylate) (PGMA) polymer chains through an alkyne-azide “click” reaction. The refractive index of the composite material increased linearly from 1.5 up to 1.8 by increasing the loading of TiO2 particles to 30 vol % (60 wt %). UV-vis spectra show that the nanocomposites exhibited a transparency around 90% throughout the visible light range. It was also found that the PGMA-grafted TiO2 nanoparticles can be well dispersed into a commercial epoxy resin, forming transparent high refractive index TiO2-epxoy nanocomposites.

Bimodal Surface Ligand Engineering: The Key to Tunable Nanocomposites

Tuning the dispersion of inorganic nanoparticles within organic matrices is critical to optimizing polymer nanocomposite properties and is intrinsically difficult due to their strong enthalpic incompatibility. Conventional attempts to use polymer brushes to control nanoparticle dispersion are challenged by the need for high graft density to reduce particle core-core attractions and the need for low graft density to reduce the entropic penalty for matrix penetration into the brush. We validated a parametric phase diagram previously reported by Pryamtisyn et al. (Pryamtisyn, V.; Ganesan, V.; Panagiotopoulos, A. Z.; Liu, H.; Kumar, S. K. Modeling the Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. J. Chem. Phys.2009, 131, 221102) for predicting dispersion of monomodal-polymer-brush-modified nanoparticles in polymer matrices. The theoretical calculation successfully predicted the experimental observation that the monomodal-poly(dimethyl siloxane) (PDMS)-brush-grafted TiO(2) nanoparticles can only be well dispersed within a small molecular weight silicone matrix. We further extended the parametric phase diagram to analyze the dispersion behavior of bimodal-PDMS-brush-grafted particles, which is also in good agreement with experimental results. Utilizing a bimodal grafted polymer brush design, with densely grafted short brushes to shield particle surfaces and sparsely grafted long brushes that favor the entanglement with matrix chains, we dispersed TiO(2) nanoparticles in high molecular weight commercial silicone matrices and successfully prepared thick (about 5 mm) transparent high-refractive-index TiO(2)/silicone nanocomposites.

Segmental Dynamics in PMMA-Grafted Nanoparticle Composites

We have recently shown that silica nanoparticles grafted with polystyrene chains behave akin to block copolymers due to the “dislike” between the nanoparticles and the grafts. These decorated nanoparticles, thus, self-assemble into various morphologies, from well-dispersed nanoparticles to anisotropic superstructures, when they are placed in homopolystyrene matrices of different molecular masses. Here, we consider a slightly different case, where the grafted chains and thematrix (both PMMA) are strongly attracted to the silica nanoparticle surface. We then conjecture that these systems show phase mixing or demixing depending on the miscibility between the brush andmatrix chains (“autophobic dewetting”). At 15 mass % particle loading, composites created using the same grafted nanoparticle, but with two different matrices, yield well dispersed nanoparticles or nanoparticle “agglomerates”, respectively. Rheology experiments show that the composites display solid-like behavior only when the particles are aggregated. As deduced in previous work, this difference in behavior is attributed to the presence of percolating particle clusters in the agglomerated samples which allows for stress propagation through the system. Going further, we compare the local mobility of matrix and grafted segments of both composites using quasi-elastic neutron scattering experiments. For the liquid-like system, the mean square displacements of the grafted chains and matrix chains, the particle structuring and mechanical response are all unaffected by annealing time. In contrast, in the reinforced case, only the localmatrixmotion is unaffected by time. Since the particle clustering and solid-like mechanical reinforcement increase with increasing time, we conclude that mechanical reinforcement in polymer nanocomposites is purely based on the nanoparticles, with essentially no “interference” from the matrix. In conjunction with other results in the literature, we then surmise that mechanical reinforcement is caused by the bridging of particles by the grafted polymer layers and not due to the formation of “glassy” polymer layers on the nanoparticles.

Montmorillonite K 10-catalyzed regioselective addition of thiols and thiobenzoic acids onto olefins: an efficient synthesis of dithiocarboxylic esters

Abstract The addition of thiols and thiobenzoic acids onto olefins proceeded regioselectively in a Markovnikov manner in the presence of Montmorillonite K 10 (Mont K 10) clay as the catalyst to afford thioethers and thiocarboxylic S -esters, while high selectivity to anti-Markovnikov products was realized in the absence of any catalyst. Treatment of the esters with Lawessons reagent provided the corresponding dithiocarboxylic esters in high yields.

Review : Polymers for Absorbable Surgical Sutures—Part II

f) oly(glycolic acid) (PGA) sutures are routinely used for general and surgical specialties. They are more predictable with regard to tissue response and absorption profile than catgut sutures [27,36]. PGA sutures are also significantly stronger for a longer period of time than catgut sutures, which experience a greater rate of strength loss. However, PGA is a relatively rigid polymer and is supplied as a multifilament braid in sizes larger than 7-0. For reasons discussed earlier, strong, absorbable yet flexible monofilament sutures are more desirable. In 1985, Davis and Geck introduced a new synthetic absorbable monofilament suture with the trade name of MaxonTM in an effort to provide a more desirable suture. The material is a copolymer produced by the ring opening polymerization of glycolide and trimethylene carbonate. The general structure is shown in Figure 3, and was reported to contain approximately 32.5 percent of trimethylene carbonate by weight [45]. Diethylene glycol was used as an initiator and stannous

Ligand Engineering of Polymer Nanocomposites: From the Simple to the Complex

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

Mechanical properties of thin glassy polymer films filled with spherical polymer-grafted nanoparticles

It is commonly accepted that the addition of spherical nanoparticles (NPs) cannot simultaneously improve the elastic modulus, the yield stress, and the ductility of an amorphous glassy polymer matrix. In contrast to this conventional wisdom, we show that ductility can be substantially increased, while maintaining gains in the elastic modulus and yield stress, in glassy nanocomposite films composed of spherical silica NPs grafted with polystyrene (PS) chains in a PS matrix. The key to these improvements are (i) uniform NP spatial dispersion and (ii) strong interfacial binding between NPs and the matrix, by making the grafted chains sufficiently long relative to the matrix. Strikingly, the optimal conditions for the mechanical reinforcement of the same nanocomposite material in the melt state is completely different, requiring the presence of spatially extended NP clusters. Evidently, NP spatial dispersions that optimize material properties are crucially sensitive to the state (melt versus glass) of the polymeric material.

Dielectric breakdown strength of epoxy bimodal-polymer-brush-grafted core functionalized silica nanocomposites

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

Open-celled polymeric foam monoliths for heavy metal separations study

Open-celled polymeric foam monoliths prepared by high internal phase emulsion polymerization (HIPE) are being investigated as improved materials for separation of heavy metals. In column flow studies, the foam monoliths have high flow rates and are durable up to at least 40 psi. A 4-vinylpyridine functionality has been incorporated into vinylbenzylchloride/styrene copolymer foams by graft-polymerization of vinylpyridine. The open structure of the foam and the flexible graft-polymerized ion-exchange chains result in improved kinetics in metal uptake. Iron uptake kinetics were greatly increased in the grafted foams over resin beads of similar structure. Plutonium uptake kinetics were moderately increased in the foams.