Light Scattering Studies on Dilute Solutions of Semiflexible Polyelectrolytes

Abstract

Abstract Poly(1,4-phenylene-2,6-cis-benzobisoxazole) and poly(2,5-benzoxazole) represent examples of semiflexible chains, forming polyelectrolytes in solution in protic acids. Dilute solutions of these have been studied by static and dynamic light scattering and viscometry. Solutions have been prepared in acidic solvents over a wide range of ionic strength, and hence of Debye electrostatic screening length K −1. The data reveal electrostatic interactions among the protonated chains through a thermodynamic segment diameter dT that depends markedly on K −1. The analysis suggests that the electrostatic component d e to d τ must be large enough to offset a neutral component d n in d τ, providing the mechanism for dissolution of the charged chains. The persistence length ρ ≈ 40 nm for poly(1,4-phenylene-2,6-cis-benzobisoxazole), is essentially independent of K −1. Thus, for this chain the electrostatic component ρe to ρ must be small in comparison with the neutral component ρn, to ρ. This appears to be in reasonable accord with estimates of ρn based on conformational analyses, and ρ, using an electrostatic model. By contrast, ρ is found to depend markedly on K −1 for poly(2,5-benzoxazole), with ρ much larger than ρn for all of the systems studied. The observed Kρ vary from 20 to 50 with K −1 increasing from 0,8 to 8 nm for poly(2,5-benzoxazole). Thus, it may be considered to be a wormlike semiflexible chain in solvents with low K −1 (high ionic strength), and rodlike in solvents with larger K −1. By comparison, values of Kρ calculated with an electrostatic model based on a charged wormlike cylinder are about ten-fold smaller. The discrepancy is attributed to the failure of the model to account for the discrete rotational states available to the poly(2,5-benzoxazole) chain.