Submicron films prepared from aqueous dispersions of nanoscale polymer crystals

Abstract

Thin and ultrathin films of polyethylene of variable thickness are obtained from aqueous dispersions of prefabricated nanoscale crystals by spin-coating. Continuous films with a thickness of only 15 nm, up to 220 nm, homogeneous over hundreds of μm , or assembled discontinuous monolayers of flat-on lamella particles were prepared, depending on the solids content of the dispersion employed, as revealed by AFM and TEM. The morphology of melt-recrystallized films was not affected by the surfactant present. Homogeneous continuous films without undesirable dewetting were retained upon melting and recrystallization of the films upon cooling, composed of polygonal spherulites for a thicker film (220 nm), randomly grown edge-on lamella for a 40 nm film, and dominant flat-on lamella for a 15 nm thick film. Annealing below Tm resulted in lamella thickening, without changes of crystaJ orientation or structure of the particle assemblies for discontinuous monolayers. Surfactant adsorbed to the nanocrystals in the aqueous dispersion desorbs at least partially during formation of the nascent films, and upon annealing below the melting point surfactant migrates to the film-air interface to form aggregates, which can be removed by rinsing, during which the film stays intact and structurally unaltered as revealed, amongst others, by water contact angles.