MICROPHASE SEPARATION IN COMB COPOLYMERS

Microphase separation in comblike copolymers, in which the backbone is of one monomeric species and the teeth are of another species, is dis cussed. Within the random phase approximation of Leibler, we calculate the spinodal transition curve (chi(N))s as a function of compositiona l fraction f of backbone monomers. The comb is assumed to have either evenly spaced grafting points or randomly placed grafting points. The evenly spaced copolymers have teeth symmetrically or asymmetrically pl aced on the backbone. In the evenly spaced copolymer melt, as the numb er of teeth (n(t)) is increased in the comb, the spinodal curve change s toward a fixed form. The spinodal curves approach the large-n(t) for m as a function of n(t) from significantly different directions depend ing on the symmetry. This difference led us to consider the consequenc e of random placement of the teeth. The copolymer melt with randomly p laced teeth approaches a significantly different spinodal curve in the limit of large n(t). More importantly, the scaling of the instability wave vector in the limit of large n(t) changes from the expected q a pproximately (n(t)/N)1/2 to q approximately (n(t)1/4/N1/2). These res ults call into question the assumption of well-ordered comb copolymers as a basis for computing detailed properties of melts.