Copolymer

1. A copolymer, consisting essentially of:

a block chain (A) consisting essentially of at least one repeating unit selected from the group consisting of a repeating unit having a tertiary amino group represented by formula (II):

wherein:

R 3 represents a hydrogen atom or a C1-C3 alkyl group;

R 4 and R 5 each independently represents a C1-C6 alkyl group or a C6-C10 aryl C1-C6 alkyl group; and

X represents a C1-C10 alkylene group or a C1-C10 alkylene-O—C1-C10 alkylene group; and

a repeating unit having a quaternary ammonium salt group represented by formula (VII):

wherein:

R 29 represents a hydrogen atom or a C1-C3 alkyl group;

X 3 represents a C1-C10 alkylene group or a C1-C10 alkylene-O—C1-C10 alkylene group;

R 30 , R 31 , and R 32 each independently represents a C1-C6 alkyl group or a C6-C10 aryl C1-C6 alkyl group; and

Z − represents a counter ion; and

a block chain (B) comprising:

a repeating unit having an acidic group represented by formula (III):

wherein R 9 represents a hydrogen atom or a C1-C3 alkyl group, or

a repeating unit represented by formula (IV):

wherein:

R 10 represents a hydrogen atom or a C1-C3 alkyl group; and

Y represents a C1-C10 alkylene group or a C1-C10 alkylene-O—C1-C10 alkylene group; and

a repeating unit represented by the following formula (I):

wherein:

R 1 represents a hydrogen atom or a C1-C3 alkyl group; and

R 2 represents an aliphatic hydrocarbon group or an alicyclic hydrocarbon group;

wherein:

the repeating unit represented by formula (I) has a copolymerization ratio of 90 wt % or more in the block chain (B); and

a ratio between the block chain (A) and the block chain (B) in the copolymer is 15 to 35:85 to 65 in a wt % ratio.