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 discussed. Within the random phase approximation of Leibler, we calculate the spinodal transition curve (chi(N))s as a function of compositional 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 placed on the backbone. In the evenly spaced copolymer melt, as the number of teeth (n(t)) is increased in the comb, the spinodal curve changes toward a fixed form. The spinodal curves approach the large-n(t) form as a function of n(t) from significantly different directions depending on the symmetry. This difference led us to consider the consequence of random placement of the teeth. The copolymer melt with randomly placed 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* approximately (n(t)/N)1/2 to q* approximately (n(t)1/4/N1/2). These results call into question the assumption of well-ordered comb copolymers as a basis for computing detailed properties of melts.