Preferential Wetting Effects on Order-to-Disorder Transition in Polystyrene-b-poly(2-vinylpyridine) Films: A Reconsideration on Thickness Dependence

We present a compelling evidence for thickness dependence on the order-to-disorder transition (ODT) behavior in cylinder- and lamella-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films. Such an asymmetric wetting condition that confines the films with selective interactions of the PS/air and P2VP/substrate interfaces generates a parallel orientation of cylindrical and lamellar microdomains. We evaluated thickness-dependent phase transition as a function of interlattice distance (L-0) using ex situ grazing incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM). Below an onset thickness (t(0)) above which the ODT temperatures (T(ODT)s) of the films are independent of film thickness, the T(ODT)s of cylinder- and lamella-forming PS-b-P2VP films remarkably increase as the film thickness decreases. Our results confirmed that preferential wetting at the PS/air and P2VP/substrate interfaces in very thin films substantially leads to an ordered state over accessible temperature range up to similar to 260 degrees C. More interestingly, the t(0) of lamellar morphology (similar to 22L(0)) is thicker than that of cylindrical morphology (similar to 10L(0)), indicating that the interfacial interactions are more influential to a 1D multilayer structure of lamellar microdomains than a 2D hexagonally packed order of parallel cylinders. Our theoretical calculation utilizing the self-consistent field theory (SCFT) of a discrete bead-spring model with finite-range interactions exhibited the similar thickness dependence of ODTs for cylinder- and lamella-forming PS-b-P2VP films confined in such an asymmetric wetting condition.