Effect of comonomers on the rate of crystallization of PET: U‐turn comonomers

The effect of incorporating phthalate, 1,8‐naphthalenedicarboxylate, and 1,8‐anthracenedicarboxylate structural units on the crystallization rate of PET are evaluated by isothermal and dynamic calorimetry. Although all of the comonomers retard crystallization, the 1,8‐naphthalene unit shows no concentration dependence between 2.5 and 10% incorporation, in contrast to the smaller phthalate and larger anthracene units. The greater rate at which the 1,8‐naphthalene copolymer crystallizes, relative to that of the other copolymers, is consistent with the notion that the U‐turn geometry induces chain folding and nucleates crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1675–1682, 2001     

Oxygen barrier properties of PET copolymers containing bis(2‐hydroxyethyl)hydroquinones

A series of poly[ethylene‐co‐bis(2‐ethoxy)hydroquinone terephthalate], PET‐co‐BEHQ copolymers were prepared by polymerization of various substituted bis(2‐hydroxyethyl)hydroquinones (BEHQs), dimethyl terephthalate (DMT), and ethylene glycol (EG). In addition to copolymers containing 6–16.5 mol % BEHQ, the homopolymer of BEHQ with dimethyl terephthalate, p(BEHQ‐T), was also prepared. The thermal and barrier properties of amorphous materials were studied. As the amount of comonomer was increased, the T_g and T_m of the materials decreased relative to those of PET. Oxygen permeability also decreased with increasing comonomer content. This improvement in barrier‐to‐oxygen permeability was primarily due to a decrease in solubility of oxygen in the polymer. All of the copolymers tested displayed similar oxygen diffusion coefficients. The decrease in solubility correlates with the decrease in T_g. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 934–942, 2003     

Influence of carbon support microstructure on the polarization behavior of a polymer electrolyte membrane fuel cell membrane electrode assemblies

The influence of carbon support morphology on the polarization behavior of a PEM fuel cell membrane electrode assembly has been investigated in this communication. Nanometer sized platinum electrocatalyst particles were deposited on lower surface area fibrous (carbon nanofibers) and particulate carbon supports (carbon blacks) by the well-documented ethylene glycol route for supported electrocatalyst synthesis. These supported catalyst systems were subsequently utilized to prepare catalyst inks and membrane electrode assemblies (MEA) in conjunction with a perflurosulfonated ionomeric membrane-Nafion^®. Level of liquid Nafion binder in the supported catalyst inks was varied and the ramifications of such a variation on polarization behavior of the MEA determined. The trend in polarization performance was found to be independent of the carbon support morphology in the various ink compositions. The two varieties of carbon supports were also mixed together in various weight ratios and platinum was deposited by the glycol method. Key parameters such as the platinum content on carbon and platinum particle size were determined to be independent of the nature of the supports on which the particles had been deposited. The results indicate that lower surface area carbon supports of vastly contrasting morphologies can be interchangeably employed as catalyst support materials in a PEM fuel cell MEA.     

Evaluation of electrochemical performance for surface-modified carbons as catalyst support in polymer electrolyte membrane (PEM) fuel cells

The electrochemical performance of platinum (Pt) catalyst deposited on various functionalized carbon supports was investigated and compared with that of a commercial catalyst, Pt on Vulcan XC-72 carbon. The supports employed were graphitic or amorphous with a wide range of surface areas. Cyclic voltammetry (CV) and rotating disk electrode (RDE) studies on the supported catalysts indicated equivalent platinum catalyst activities. Fuel cell performance was determined for membrane electrode assemblies (MEA) fabricated from the supported catalysts. The use of high surface area supports did not necessarily translate into a higher electrochemical utilization of platinum. Electrochemical impedance spectroscopy (EIS) measurements indicated lower ohmic losses for low surface area carbon MEAs. This is explained by the supported catalyst electrode microstructures and their intrinsic resistivities. Correlation of all data indicates that for low surface carbons, nature of the support does not significantly affect the Pt catalytic activity. The influence of the support is more critical when high surface area carbons are used because of the vastly different electrode morphology and resistivity.     

Structure–property evaluation of trisilanolphenyl POSS®/polysulfone composites as a guide to POSS melt blending

A series of polysulfone/phenyl trisilanol POSS nanocomposites were produced by melt blending by twin screw batch mixing. These materials were then injection molded, and their thermal, mechanical, and morphological properties were tested. The tensile properties of polysulfone were moderately compromised by the addition of phenyl TPOSS, because of the formation of large (∼ 1 μm) voided POSS aggregates. These domains however did cause the improvement of the impact resistance of the composites as described by the mechanism of crack pinning and bowing. Flexural properties remained essentially unchanged, which is attributed to the formation of an aggregate free-skin layer, which formed in the injection molded parts. Thermal behavior of the composites also remained largely unchanged due to the lack of POSS-polymer interactions on the molecular/chain segment scale. Initially, it was hypothesized that a high degree of POSS-polymer interactions would be present in these composited based on examination of their chemical structures. This however, was not the case as phase separation was clearly present. This work highlights the need for a better understanding of the prediction of POSS-polymer interaction. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The effects of physical and chemical interactions in the formation of cellulose aerogels

Aerogels are low density materials which are produced from wet gels, and find a variety of potential uses. The relative importance of shape/geometry and self-association of the starting materials for the production of aerogels is studied herein. Aerogels were produced from microcrystalline cellulose (MCC) and its functionalized analog, carboxymethyl cellulose (CMC). With increasing functionalization, CMC gains the potential for self-association, differentiating itself from MCC. The present study explores the preparation of aerogels from MCC and CMC, comparing performance with and without significant self-association potential, and more broadly evaluating the production of low density structural materials from renewable cellulose. It was observed that the self-association present in CMC substantially increases aerogel mechanical properties when compared those of non-interactive MCC. Aspect ratio is proposed to also be an import parameter in the structure–property relationship for these materials.