Transport and solution of gases and vapors in styrene-butadiene block copolymers: Absorption and desorption of n-hexane vapor

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Abstract
The integral absorption and desorption of n-hexane vapor by styrene-butadiene block copolymers were studied as a function of film thickness and vapor pressure at 25°C by use of a weighing method. The morphologies of the samples were (1) polystyrene rods dispersed in a polybutadiene matrix and (2) a series of alternating lamellae of styrene and butadiene components. For both copolymer samples (1) and (2), reduced solvent weight uptake plots for films of differing thicknesses did not give a single curve ½ when plotted against the Fickian variable, t½/X, where t is time and × is film thickness, though the individual absorption and desorption curves appeared to have a Fickian shape. In certain concentration regions, the two-stage type of non-Fickian behavior was observed with copolymer film having a type (2) morphology. As expected, absorption and desorption processes in homopolybutadiene films were purely Fickian. From these results it was concluded that the segmental motions in the polybutadiene phase in the styrene-butadiene copolymers are more restricted than those in the homopolybutadiene. The initial slopes of both the reduced absorption and desorption curves increased with increasing × and appeared to approach limiting values at the limit of infinite X. Mutual diffusion coefficients D of the copoly-mer systems were calculated by assuming that absorption and desorption processes for a hypothetical film of infinite × are purely Fickian. The values of D extrapolated to zero concentration of penetrant for the copolymers were lower than the extrapolated D value for homopolybutadiene. A comparison of the equilibrium solubilities of n-hexane in the copolymers and homopolybutadiene suggests that partial mixing of component block chains occurs at the interface between the polystyrene and polybutadiene domains.