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An experimental study was carried out to investigate the existence of acritical layer thickness in nanolayer coextrusion, under which no continuouslayer is observed. Polymer films containing thousands of layers of alternatingpolymers with individual layer thicknesses below 100 nm have been prepared bycoextrusion through a series of layer multiplying elements. Different filmscomposed of alternating layers of poly(methyl methacrylate) (PMMA) andpolystyrene (PS) were fabricated with the aim to reach individual layerthicknesses as small as possible, varying the number of layers, the masscomposition of both components and the final total thickness of the film. Filmswere characterized by atomic force microscopy (AFM) and a statistical analysiswas used to determine the distribution in layer thicknesses and the continuityof layers. For the PS/PMMA nanolayered systems, results point out the existenceof a critical layer thickness around 10 nm, below which the layers break up.This critical layer thickness is reached regardless of the processing route,suggesting it might be dependent only on material characteristics but not onprocess parameters. We propose this breakup phenomenon is due to smallinterfacial perturbations that are amplified by (van der Waals) disjoiningforces.

Existence of a critical layer thickness in PS/PMMA nanolayered films