Stepwise collapse of cyclolinear polysiloxane Langmuir monolayers studied by Brewster angle microscopy and grazing incidence X-ray diffraction
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The surface-pressure-induced layering transition of a 2D nematic-like cyclolinear polyorganosiloxane that consists of six-membered rings of silicon and oxygen joined by oxygen bridges and that has phenyl side groups (CL-PMPhSi) is observed and characterized by film balance measurements, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) at the air/water interface. In the uncompressed state, A ≤ 95 Å2/ monomer and π ≈ 0, BAM images demonstrate partial surface coverage by solidlike birefringent islands, supporting the previously suggested model of extended aligned polymer chains lying parallel to the water surface in a quasi-two-dimensional nematic-like liquid-crystalline arrangement. Further compression, A ≤ 95 Å2/monomer, leads to a complete and uniform monolayer coverage of the surface followed by layer-by layer growth visible as stepwise increasing surface pressure. GIXD reveal one diffraction peak for monolayer as well as multilayer samples of CL-PMPhSi with a d-spacing of d = (10.5 ± 0.1) Å. The maximum Bragg rod intensity is observed at qz ≈ 0 for the monolayer but moved to a well-defined nonzero qz value for the multilayer samples. We infer that there are planes in the structure of the CL-PMPhSi multilayers tilted by ca. 27° from the surface normal. For both mono- and multilayers, the analysis of the observed GIXD peak widths indicates that the extent of the lateral positional correlation between parallel chains ranges from about 12 for the monolayer to about 30 interchain distances in multilayers, implying some mesoscale order similar to a quasi-long-range order. Analysis of the Bragg rods allowed us to estimate the thickness of the monolayer and multilayers. Comparison of the data with independent results of molecular modeling, X-ray reflectivity, and atomic force microscopy gave very good agreement. As a result of the analysis we conclude that both in-plane and out-of-plane GIXD data reveal a high degree of stable structural order of the CL-PMPhSi films.
