Order-to-Disorder Transition of Lamella-Forming PS-b-P2VP Films Confined between the Preferential Surface and Neutral Substrate

Ultrathin random copolymer mats with cross-linkable glycidyl methacrylate units, consisting of styrene (S) and 2-vinylpyridine (2VP), are prepared on bare substrates to tune substrate interactions. Lamella-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films supported by a balanced (or neutral) interfacial interaction, opposite to those supported on a preferentially attracting substrate, exhibit the sensitivity of the order-to-disorder transition (ODT) to slight variations in thin thickness. A neutral composition of the random copolymers is determined to be S = 48.9 mol % for 81 kg/mol PS-b-P2VP films, favoring lamellae oriented perpendicular to the substrate. On the contrary, the lamellae of 15 kg/mol PS-b-P2VP films supported on a neutral mat prefer a parallel orientation to the surface, presumably because such low-molecular-weight PS-b-P2VP films (close to the weak segregation regime) are under the influence of the surface energy difference between the two blocks and a possible chain-end effect. Intriguingly, this effect of neutral mat interaction leads to thickness-dependent ODTs of the 15 kg/mol PS-b-P2VP films. The ODT temperature (TODT) decreases with decreasing film thickness when the film thickness is less than 13L0 (where L0 is the lamellar spacing) owing to the substrate compatibility effect of a neutral bottom mat toward the PS-b-P2VP films. Especially for the PS-b-P2VP films confined in the one-neutral and dual-neutral conditions, theoretical calculation based on the discrete chain self-consistent field theory supports that the TODT of the films decreases with decreasing film thickness below a certain thickness