On the birefringence of multilayered symmetric diblock copolymer films

The chain extension at lamellar interfaces was studied in thin films of symmetric diblock copolymers on gold substrates. The first copolymer consisted of blocks of polystyrene (PS) and poly(2-vinylpyridine) (P2VP), denoted P(S-b-2VP). The second was a diblock copolymer of PS and poly(methyl methacrylate) (PMMA), denoted P(S-b-MMA), on a gold substrate. Using attenuated total reflectance spectroscopy, the refractive indices parallel, n[sub [parallel]], and perpendicular, n[sub [perpendicular]], to the surface of the films were determined. It was found that the total birefringenece, [Delta][sub T] = n[sub [perpendicular]]-n[sub [parallel]], of the as-cast films was positive, indicative of an orientation of the copolymer chains parallel to the film surface. Upon annealing at 170C, [Delta][sub T] changed sign and attained a limiting value, [approximately][minus]7[times]10[sup [minus]4] for P(S-b-2VP) and [approximately][minus]20[times]10[sup [minus]4] for P(S-b-MMA). It was found that the form birefringence, [Delta][sub F], dominates the intrinsic birefringence, [Delta][sub I], for P(S-b-MMA). On the other hand, [Delta][sub I] was much greater than [Delta][sub F] for P(S-b-2VP). From [Delta][sub I] the extension ratios of the chains at the interface were found to be 1.5-1.6 for both copolymers.