Ultrathin film, process for production thereof, and use thereof for concentrating a specified gas in a gaseous mixture

An ultrathin film composed substantially of silicon-containing polyurea comprising polyaddition bonded units derived from

• (A) at least one polyamine selected from the group consisting of silicon-containing polyamines having in the molecule at least two units of the following formula ##STR98## wherein R.sup.3 represents a monovalent hydrocarbon group or a fluorocarbon group, R.sup.1 and R.sup.2 are identical or different and represent a hydrogen atom or a monovalent hydrocarbon group which may have a primary or secondary amino group, or R.sup.1 and R.sup.2 may be bonded to each other to form an alkylene group interrupted by the nitrogen atom forming the secondary amino group, and n is an integer of 1 to 10, provided that R.sup.1 and R.sup.2 are not simultaneously aliphatic, alicyclic or aromatic groups free from the amino group,
• not all, but at least one, of the units of formula (I) forming a unit represented by the following formula ##STR99## wherein R.sup.1, R.sup.2, R.sup.3 and n are as defined, the free bond from the silicon atom is bonded directly to another carbon atom or another oxygen atom, and the free bond from the oxygen is bonded directly to another siloxane atom,
• and hydrocarbon-type polyamines containing at least two primary or secondary amino groups in the molecule, and
• (B) at least one polyisocyanate compound selected from the group consisting of silicon-containing polyisocyanates having at least 2 isocyanate groups in the molecular chains and hydrocarbon-type polyisocyanates having at least 2 isocyanate groups in the molecular chains;
• provided that when the polyisocyanate compound is the hydrocarbon-type polyisocyanate alone, at least one of the selected polyamines is a silicon-containing polyamine containing in the molecule at least one unit of formula (I)-a in which the free bond from the silicon atom is bonded to a monovalent hydrocarbon group.