Convenient synthesis of microspheres by self-assembly of random copolymers in supercritical carbon dioxide

A novel and convenient synthesis of microspheres was attained by molecular self-assembly of random copolymers in supercritical carbon dioxide. Poly{2-(perfluorooctyl)ethyl acrylate-ran-2-(dimethylamino)ethyl acrylate} random copolymers (P[POA-r-DAA]), with 7:3, 8:2, and 9:1 as the molar ratios of 2-(perfluorooctyl)ethyl acrylate (POA)/2-(dimethylamino)ethyl acrylate (DAA), were soluble in supercritical CO₂ and had their cloud points at about 75 bar higher than the critical pressure of CO₂. The CO₂ density at the cloud points decreased with an increase in temperature. Although the copolymers made a slight difference in the cloud point in the absence of perfluoroazelaic acid (PA), they made a marked difference in the presence of PA. The copolymer with a lower DAA content had a cloud point at lower CO₂ density. The scanning electron microscopy observation demonstrated that the copolymers produced microspheres in the presence of PA at 0.5 as PA/DAA in the heterogeneous state below the cloud points. The microspheres were unstable to pressure and changed to unspecific forms as a result of increasing the CO₂ pressure, although the microspheres were stable to temperature. The POA/DAA ratio in the copolymer had effect not only on the cloud point but also on the size of the microspheres. The copolymer with a lower DAA content formed smaller microspheres.     

Superhydrophobic surfaces of microspheres obtained by self-assembly of poly[2-(perfluorooctyl)ethyl acrylate-<Emphasis Type="Italic">ran</Emphasis>-2-(dimethylamino)ethyl acrylate] in supercritical carbon dioxide

Superhydrophobic surfaces were obtained by coating with microspheres formed by the self-assembly of poly[2-(perfluorooctyl)ethyl acrylate-ran-2-(dimethylamino)ethyl acrylate] (P[POA-r-DAA]) in the presence of dicarboxylic acids in supercritical carbon dioxide. The P[POA-r-DAA] random copolymer aggregated into micellar microspheres through the hydrogen bond cross-linking of the amino groups via the carboxylic acids. The size of the microspheres and the amount of the acids needed to produce them were dependent on the kinds of acids. Glutaric acid (Glu) and perfluorosuccinic acid (Psuc) provided microspheres at a 0.5 molar ratio of the acid/DAA. Psuc produced smaller microspheres than Glu. Maleic acid (Ma), succinic acid (Suc), and azelaic acid (Az) required a higher molar ratio to produce the microspheres. These acids provided spherical particles at the ratio of 1.0. The microspheres produced by Suc and Az contained particles with a several hundred nanometer size. The surface coated with the microspheres showed high water contact angles of 164°–172°.

Stabilization of organic field-effect transistors by tert-butyl groups in dibenzotetrathiafulvalene derivatives

For a material for organic thin-film transistors, not only high mobility but also low threshold voltage and long-term stability are important requirements. In order to realize these properties, materials with relatively large oxidation potentials, namely weak donors, have been designed as p-channel organic semiconductors. Here we propose a different strategy; transistor properties of dibenzotetrathiafulvalene (DBTTF) are significantly improved by the introduction of tert-butyl groups. Although this chemical modification does not much change the ionization potential, small threshold voltage and stability over several months are attained together with the improved mobility, probably due to some kind of passivation effect of the bulky tert-butyl groups. In contrast, the systematic fluorine substitution rapidly diminishes the transistor performance. There are two kinds of herringbone structures with much different dihedral angles of about 50° and 130°, and the tert-butyl compound falls into the former category.