The crystallization of two semicrystalline and water-insoluble polymers with ester moieties, namely poly(epsilon-caprolactone) (PCL) and isotactic poly(methyl methacrylate) (i-PMMA), was studied on the water surface using Langmuir trough measurements and Brewster angle microscopy. Additionally, Langmuir-Blodgett films were prepared and examined by atomic force microscopy. It was demonstrated that PCL forms distinct single crystals on the water surface with polymer chain orientation perpendicular to the water surface. i-PMMA behaved differently and crystallized upon compression on the water surface in a homogeneous layer with polymer chain orientation parallel to the water surface. In contrast, poly(ethylene oxide) (PEO) is a water-soluble polyether and its crystallization at the air/liquid interface could only be studied when the water subphase was replaced with aqueous salt solutions containing kosmotropic salts according to the Hofmeister series.
