We examined the formation of microcellular foam in a thermoplastic polyurethane elastomer (TPU) with supercritical carbon dioxide. Measurements of the permeability and impregnation with carbon dioxide suggested that the impregnation of TPU with carbon dioxide was affected by the soft‐segment chain length and soft‐segment concentration in TPU and that carbon dioxide was mainly present in the soft‐segment area. Thus, it seemed likely that the nucleation and growth of the nuclei in the microcells occurred in the soft‐segment area. In addition, scanning electron microscopy of the microcells indicated that the higher the saturation pressure was, the smaller the mean cell diameter was; that is, the higher the cell number density was, the lower the foaming temperature was, the smaller the mean cell diameter was, and the lower the saturation pressure was, the more conspicuous this phenomenon was. These results showed that the saturation pressure and foaming temperature affected the cell structure. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

ABSTRACT:We have studied the influence of the chemical structures of thermoplastic polyurethane elastomers (TPUs) on their permeability to carbon dioxide and oxygen. The gas permeability of a TPU was dependent on the chemical structures not only of the soft segments but also of the hard segments. A TPU whose soft segment consists of poly(oxytetramethylene) exhibited a higher carbon dioxide permeability than does a TPU with ester bonds in the soft segment. The introduction of poly(dimethylsiloxane) chains into the TPU markedly enhanced the carbon dioxide permeability. In a series of TPUs containing poly(oxytetramethylene)glycol (PTMG), the gas permeability increased with the chain length of the soft segments. In the dissolution-diffusion process of gas permeation, the diffusion process predominated.

Un-ionized polyurethane was obtained by the reaction of an isocyanate-terminated urethane prepolymer, which was synthesized from 4,4Ј-diphenylmethane diisocyanate and poly(oxytetramethylene)-␣,-glycol, with 2,2-bis(hydroxymethyl)propionic acid. A carboxylate-based polyurethane anionomer was then derived from the polyurethane by the use of the sodium, potassium, or magnesium salt of acetic acid as a neutralizer. The ionomerization resulted in the following changes in the characteristics of the polyurethane: (1) an increase in the tensile strength, (2) a decrease in the glass-transition temperature, (3) an increase in the wettability and hygroscopicity with respect to water, and (4) susceptibility to thermal decomposition. A sulfonatebased polyurethane was also synthesized for comparison with the carboxylate-based polyurethane.

In an effort to improve the gas barrier properties of thermoplastic polyurethane (TPU) elastomers, fullerene derivatives were added as fillers, and the resulting O 2 and CO 2 permeabilities were analyzed. The addition of 5 wt % polyhydroxylated fullerene {fullerenol [C 60 (OH) n ] mixture, where n 5 6-12} decreased the gas permeability by approximately 10-20%. According to the hole volumes computed with the results from positron annihilation lifetime spectroscopy, the addition of fullerene derivatives did not produce any changes in the hole volumes of the TPUs. Thus, the reduction in the TPU gas permeability was not caused by changes in the hole volumes. Instead, an inhibited diffusion of gas molecules by fullerene particles was deduced as the cause of the decrease in gas permeability from changes in the diffusion coefficient with temperature. The addition of urethanized fullerene, prepared through the chemical modification of fullerenol, markedly affected the TPU gas barrier properties. As compared to fullerenol addition, the gas barrier properties improved approximately fourfold for O 2 and approximately fivefold for CO 2 . These results suggest that the dispersability of urethanized fullerene in TPU was higher than that of fullerenol. We found that the gas barrier properties were independent of the structure of polyol.

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