A modified particle comprising a particle having attached a group having the formula: ##STR18## wherein Ar represents an aromatic group; R1 represents a bond, an arylene group, an alkylene group, ##STR19## wherein R4 is an alkyl or alkylene group or an aryl or arylene group; R4 ' is an alkyl group or an aryl group; R2 and R3, which can be the same or different, represent hydrogen, an alkyl group,