Abstract: A copolymer of acrylamide or of an acrylamide derivative and of trialkylammonium salts which is a radical polymerization product, in the presence of an initiator, of at least a first monomer chosen from acrylamide and its derivatives of formula H2C═CH—(CH2)n″—CONH2  in which n″ is an integer which can vary from 1 to 5, and of a second monomer chosen from trialkylammonium salts corresponding to the following formula (I) (H2C═C(R1)—(CH2)n—A—(CH2)n′—N+(R2)(R3)(R4))X−  (I)  in which, R1 represents H or an alkyl group having from 1 to 3 carbon atoms, R2, R3 and R4, which are identical or different, each represents an alkyl group having from 1 to 3 carbon atoms, n and n′ each represents, independently of one another, a non-zero integer at most equal to 15, A represents a divalent carboxyl or amide functional group, and X− represents a counterion.