Silicon-containing fluorochemical surface-treating agent

A surface treatment agent comprising a fluoropolymer which has repeating units derived from a fluorine-containing monomer of the formula:

• wherein X is a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, a CFX1X2 group (in which X1 and X2 are each a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom or an iodine atom), a cyano group, a linear or branched fluoroalkyl group having 1 to 21 carbon atoms, a substituted or unsubstituted benzyl group, or a substituted or unsubstituted phenyl group;
• Y is an aliphatic group having 1 to 10 carbon atoms, an aromatic or cycloaliphatic group having 6 to 10 carbon atoms, a —CH2CH2N(R1)SO2— group (in which R1 is an alkyl group having 1 to 4 carbon atoms) or a —CH2CH(OY1)CH2— group (in which Y1 is a hydrogen atom or an acetyl group);
• and
• Rf is a linear or branched fluoroalkyl or fluroalkenyl group having 1 to 21 carbon atoms, or a fluoroether group having totally 1 to 200 repeating units selected from the group consisting of the repeating units:
• —C3F6O—, —C2F4O— and —CF2O—, and
• wherein (a) the fluoropolymer contains a silicon atom, and/or (b) the surface treatment agent comprises the fluoropolymer (a first polymer) and a second polymer different from the first polymer, the second polymer being a silicon-containing polymer which contains a silicon atom.