weight, and the above-mentioned polymerizable monomer mixtures each contain a monomer of the formula (I): ##STR7## wherein R1 and R2 may be the same or different and each is a hydrogen atom or a methyl, and p is an integer of 1-4, and a monomer of the formula (2): ##STR8## wherein R3 and R4 may be the same or different and each is a hydrogen atom or a methyl, m and n are each an integer of 1-5, a