Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.
The prospects for obtaining highly accurate mass assignments in polymer surface studies using state-of-the-art time-of-flight secondary ion mass spectrometry (ToF-SIMS) have been investigated systematically. For a thick film of poly(ethy1ene terephthalate) (PET), the reproducibility of mass measurement in both positive ion (range m/z = 0-600) and negative ion (range m/z 0-350) modes has been studied in statistical fashion over a period of several months. Glyceryl monostearate, representing a typical polymer additive, was studied both on PET and as a very thin film on aluminum in order to compare the behavior of insulating and conducting samples. The effect of different mass calibration strategies (including use of atomic and molecular species), mass interferences and metastable peaks on mass accuracy also have been investigated. Finally, the importance of peak intensity for mass resolution and achievable mass accuracy has been determined. The results show that a useful mass accuracy of better than 20 ppm can be achieved over a mass range m/z = 0-600 for these polymer film surface studies under routine operating conditions.
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