A porous article which is formed from a polymeric material, in which characteristics of the surface of the article provided by the polymeric material are modified by having grafted to it chains of polymerized vinyl monomer units, in which the chains each comprise a number of units of a first vinyl monomer and include a unit of a second vinyl monomer which is different from the first vinyl monomer. Preferably, the ratio of the number of units of the first vinyl monomer in each chain to the number of units of the second vinyl monomer is not more than about 60. When the number of second vinyl monomer units in each chain is one, the mean number of first vinyl monomer units in each chain will be not more than 60.