Block copolymer molecular weight determination via gel permeation chromatography: Choosing a combining rule

We present a method for accurately determining the true molecular weights o f narrow-distribution block copolymers, using only a basic gel permeation c hromatograph (GPC) equipped with a refractive index detector and calibrated with polystyrene standards. Our approach is based on the well-known observ ation that GPC calibration curves for different homopolymers in good solven ts are essentially parallel, allowing the curves for different polymers to be described by simple hydrodynamic equivalence ratios r(B) versus polystyr ene. We present values of r(B), in both toluene and tetrahydrofuran, for va rious polydiene and hydrogenated polydiene homopolymers commonly incorporat ed into commercial styrenic block copolymers. These values of r(B), must be combined to yield the hydrodynamic equivalence ratio of the block copolyme r, from which the block copolymer's true molecular weight can be determined . Three combining rules proposed in the literature are tested against a ser ies of symmetric polystyrene-polybutadiene diblock copolymers of varying mo lecular weight. A simple linear combining rule accurately represents the re sults. (C) 2001 John Wiley & Sons, Inc.