. . "Thus, representative examples of suitable copolymerizable crosslinking agents include carboxyl group-containing monomers such as methacrylic acid, acrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid; hydroxyl-group containing monomers such as methallyl alcohol, allyl alcohol, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate," . . .