Existence of a Critical Layer Thickness in PS/PMMA Nanolayered Films

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.