Order–disorder transition in thin films of horizontally-oriented cylinder-forming block copolymers: thermal fluctuations vs. preferential wetting

Abstract

We present experimental and theoretical investigations of the order-disorder transition (ODT) in thin films of cylinder-forming diblock copolymers with asymmetric wetting conditions. Grazing incidence small-angle X-ray scattering (GISAXS) was implemented to determine the ODT temperatures (TODT) for poly(styrene-b-2-vinyl pyridine) (PS-P2VP) block copolymer thin films on a P2VP-preferential silicon substrate. Specifically, films consisting of multilayers of horizontally-oriented cylindrical structures (from 1- to 9-layers) were tested. We find that films with more than 2 cylindrical layers have a TODT comparable to the bulk case. However, TODT decreases as the film becomes thinner and the monolayer system has an ODT 30 °C below the bulk. Using self-consistent field theory (SCFT), we studied the ordering in corresponding thin films with asymmetric (top and bottom surface) wetting conditions. Surprisingly, SCFT is found to predict an opposite trend in TODT with film thickness than observed experimentally. Field-theoretic simulations with complex Langevin sampling were employed to resolve this discrepancy and demonstrate that thermal fluctuations in the PS-P2VP thin-film system dominate its ordering behavior in monolayer and bilayer films.

Order-to-Disorder Transition of Cylinder-Forming Block Copolymer Films Confined within Neutral Interfaces

“…Considering that the film is confined within dual neutral interfaces at the top and bottom, our intuition suggests that the ordering is suppressed for the thinner film, and thus, this result is somewhat counterintuitive. The same trend is already reported in the previous paper performing SCFT, 9,42 and we speculate that this ODT change is due to the enrichment of the minority phase near the interfaces.…”

“…For the multiple-layered cylinder-forming PS-b-P2VP films under such an asymmetric preferential wetting condition, Kim et decreasing film thickness; this is consistent with an increase in (χN) ODT because χ is inversely dependent on temperature, where χ is the Flory−Huggins parameter and N is the polymerization index. 9 In their study, the self-consistent field theory (SCFT) calculations showed an increase in T ODT as the film thickness decreased. For the same case of the BCP films, however, the T ODT decreases when the full fluctuation effect is accounted for by the adoption of complex Langevin fieldtheoretic simulation (CL-FTS).…”

“…It is also notable that even though the ultraviolet (UV) divergence was known to exist in FTS, 1,20 it was not eliminated in the previous CL-FTS study of the BCP film. 9 Recently, Delaney and Fredrickson reported success in conducting the CL-FTS at N̅ ≈ 10 4 by introducing compressibility and density smearing. 13 One disadvantage of this density smearing is the loss of universality of the standard model adopting incompressible Gaussian chains with the contact interaction, which makes it difficult to compare with previous theoretical and experimental results.…”

Dispersity-Driven Stabilization of Coexisting Morphologies in Asymmetric Diblock Copolymer Thin Films

“…Figure e shows that the mixed SCFT model is the best fit obtained from physics-based models, whereas the fitting can be further improved by introducing imperfections in the cylindrical region corresponding to the mixed morphology. One of the origins of such imperfections has been attributed to thermal fluctuations near the substrate . We note that during the NR experiment the reflected intensity corresponds to a signal recorded from the whole macroscopic lateral size of the sample and SCFT modeling does not take into account the experimental imperfectness of the samples.…”

“…One of the origins of such imperfections has been attributed to thermal fluctuations near the substrate. 72 We note that during the NR experiment the reflected intensity corresponds to a signal recorded from the whole macroscopic lateral size of the sample and SCFT modeling does not take into account the experimental imperfectness of the samples. Although a fully consistent reflectivity profile may not be directly obtained from SCFT, the obtained results demonstrate a realization of the unprecedented approach to data analysis.…”