ON THE BIREFRINGENCE OF MULTILAYERED SYMMETRICAL DIBLOCK COPOLYMER FILMS

The chain extension at lamellar interfaces was studied in thin films o f symmetric diblock copolymers on gold substrates. The first copolymer consisted of blocks of polystyrene (PS) and poly(2-vinylpyridine) (P2 VP), 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 substr ate. Using attenuated total reflectance spectroscopy, the refractive i ndices parallel, n(parallel-to), and perpendicular, n(perpendicular-to ), to the surface of the films were determined. It was found that the total birefringence, DELTA(T) = n(perpendicular-to), of the as-cast fi lms was positive, indicative of an orientation of the copolymer chains parallel to the film surface. Upon annealing at 170-degrees-C, DELTA( T) changed sign and attained a limiting value, approximately -7 X 10(- 4) for P(S-b-2VP) and approximately -20 X 10(-4) for P(S-b-MMA). It wa s found that the form birefringence, DELTA(F), dominates the intrinsic birefringence, DELTA(I), for P(S-b-MMA). On the other hand, DELTA(I) was much greater than DELTA(F) for P(S-b-2VP). From DELTA(I) the exten sion ratios of the chains at the interface were found to be 1.5-1.6 fo r both copolymers.