The ordering of a poly(styrene-b-butadiene) block copolymer solution into hexagonally packed cylinders during the extraction of a neutral solvent from a solution-cast film was studied in real time using in situ small-angle X-ray scattering (SAXS). By tracking changes in the scattering intensity as solvent is continually removed from the film, information on the ordering rate was obtained as the film progresses from the dilute to the concentrated regime. The ordering rate was found to be controlled by thermodynamics below a critical concentration and by limited chain mobility at higher concentrations. The effect of drying temperature on the ordering kinetics was investigated by extracting solvent at three different temperatures: 30, 40, and 60 °C. The net effect of drying temperature on the ordering rate was found to depend on a competition between the thermodynamics of the drying film and the mobility of the copolymer chains. An intermediate temperature, 40 °C, provided a balance between these competing phenomena, resulting in improved overall ordering in the films. The effect of drying rate was considered by drying films in the presence of a sweep gas. The introduction of a sweep gas to remove solvent resulted in appreciable concentration gradients that hindered ordering in the films.
Changes in the spacing of hexagonally packed cylinders
in a poly(styrene-b-butadiene) copolymer film during
the continual extraction of toluene under different solvent removal
conditions were studied by in situ small-angle X-ray
scattering (SAXS) measurements. As the solvent is removed from the
film, the structure spacing increases due to increasing segregation
power and then decreases as the polymer chains become kinetically
trapped while microdomains continue to deswell. The structure spacing
in the vertical film direction decreases more drastically due to compression
of the domains as the film thickness decreases. The degree of compression
increased with increased polymer concentration because of an increase
in the chain relaxation time. Increasing the drying temperature resulted
in more severe domain compression, which is ascribed to faster solvent
removal. Removing solvent in the presence of a sweep gas further increased
the domain compression as a result of the formation of a skin at the
film surface. When the solvent removal rate was decreased and skinning
was avoided by saturating the sample chamber with the casting solvent,
domain compression was impeded.
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