A hydrocarbon mixture containing modified polycyclic aromatic compounds or modified PAC having a chemical structure of mean molecular weight ranging between 300 and 10,000, the chemical structure being represented by formula (I): (A).sub.x(PAC).sub.y, wherein x and y are integers corresponding to the numbers of A or PAC units, selected such that x/y is equal to at least 1, preferably ranging between 1 and 50 and wherein A is a unit corresponding to a polymerizable monomer containing between 2 and 60 carbon atoms.