  An experimental study was carried out to investigate the existence of a
critical layer thickness in nanolayer coextrusion, under which no continuous
layer is observed. Polymer films containing thousands of layers of alternating
polymers with individual layer thicknesses below 100 nm have been prepared by
coextrusion through a series of layer multiplying elements. Different films
composed of alternating layers of poly(methyl methacrylate) (PMMA) and
polystyrene (PS) were fabricated with the aim to reach individual layer
thicknesses as small as possible, varying the number of layers, the mass
composition of both components and the final total thickness of the film. Films
were characterized by atomic force microscopy (AFM) and a statistical analysis
was used to determine the distribution in layer thicknesses and the continuity
of layers. For the PS/PMMA nanolayered systems, results point out the existence
of 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 on
process parameters. We propose this breakup phenomenon is due to small
interfacial perturbations that are amplified by (van der Waals) disjoining
forces.
