Patent ID: 8431664

Claim:
A copolymer comprising the following polymerized units (a), polymerized units (b), polymerized units (c2) and 1 to 6 mol % based on all monomers of polymerized units (c1): polymerized units (a): polymerized units based on a monomer represented by the following formula (1): Z—Y—X (1) in the formula (1), Z is a C 4-6 polyfluoroalkyl group or a group represented by the following formula (2), Y is a bivalent organic group or a single bond, and X is any one of groups represented by the following formulae (3-1) to (3-5): C i F 2i+1 O(CFX 1 CF 2 O) j CFX 2 — (2) wherein i is an integer of from 1 to 6, j is an integer of from 0 to 10, and each of X 1 and X 2 is a fluorine atom or a trifluoromethyl group, —CR═CH 2 (3-1) —C(O)OCR═CH 2 (3-2) —OC(O)CR═CH 2 (3-3) —OCH 2 -φ-CR═CH 2 (3-4) —OCH═CH 2 (3-5) wherein R is a hydrogen atom, a methyl group or a halogen atom, and φ is a phenylene group, polymerized units (b): polymerized units based on a linear or branched alkyl (meth)acrylate wherein the alkyl moiety has from 6 to 18 carbon atoms, provided that when in the polymerized units (a), X in the formula (1) is a group represented by any one of the formulae (3-1) to (3-4) wherein R is a group being a methyl group or a halogen atom, the polymerized units (b) are polymerized units based on an alkyl acrylate, and when in the polymerized units (a), X in the formula (1) is a group represented by any one of the formulae (3-1) to (3-4) wherein R is a group being a hydrogen atom, or a group represented by the formula (3-5), the polymerized units (b) are polymerized units based on an alkyl methacrylate, polymerized units (c1): polymerized units based on a monomer having a hydroxy group without having a polyfluoroalkyl group, and polymerized units (c2): polymerized units based on a monomer having a functional group which is reactive with the hydroxy group in the polymerized units (c1) to form a crosslinked structure.