The protonated amine functions of these polyurethanes result from the hydrolysis of excess isocyanate functions, at the chain end, followed by alkylation of the primary amine functions formed with alkylating agents containing a hydrophobic group, i.e. compounds of the type RQ or R???Q, in which R and R??? are as defined above and Q denotes a leaving group such as a halide, a sulfate, etc.