Molecular conformations of monodendron-jacketed polymers by scanning forcemicroscopy

Scanning force microscopy on monodendron-jacketed linear polystyrenes demon strated unusual conformations and ordering depending on the branching densi ty and interplay between intramolecular and surface interactions of the den dritic coat. Single molecules were visualized as wormlike cylinders, which could be twisted to plectonemic coils or aligned in nematic-like order on t he substrate. The attained resolution enabled identification of chain-end s tacking, hairpin folds and crossovers of individual macromolecules. The str ucture of the more densely branched 3,4,5-tris[3',4',5'-tris(n-dodecan-1-yl oxy)benzyloxy]benzyl was predominantly controlled by the intramolecular rep ulsion of the crowded dendron substituents which enforced an extended confo rmation of the molecular backbone. Formation of molecular monolayers was af fected by wetting/dewetting events during casting, and the film structure d id not depend on the substrate type. In contrast, the less crowded 3,4,5-tr is[4-(n-tetradecan-1-yloxy)benzyloxy]benzoate (14-ABG-PS) was demonstrated to undergo conformational transitions upon drying and adsorption, as indica ted by the observed built-up of torsional stress in the backbone. On mica, 14-ABG-PS formed a web of intertwined twisted cylinders. On graphite, latti ce matching between the crystalline surface and the all-trans conformation of the alkyl side chains caused alignment of the macromolecules according t o the S-fold symmetry of highly oriented pyrolytic graphite. Subsequent ann ealing resulted in the formation of large domains of parallel aligned macro molecules.