Free-standing ultrathin films with universal thickness from nanometer to micrometer by polymer nanosheet assembly

Abstract

Fabrication of smooth and flexible free-standing films with universal thickness and a highly ordered layer structure is described in this paper. N-Dodecylacrylamide polymer forms a well-defined monolayer on a water surface. The acrylamide polymer chains are strongly associated by hydrogen bonding of amide groups in the monolayer, thereby forming a two-dimensional network (polymer nanosheet). The monolayer is transferable onto a substrate using the Langmuir–Blodgett (LB) method with regular deposition, even for more than 700 layers. The deposited polymer multilayers on a substrate where a sacrificial film had been coated in advance were peeled off safely in solution, yielding a free-standing film with a uniform thickness depending on the number of multilayers. Results showed that the film has a highly oriented layer structure in which the alkyl side chain orients vertically and the polymer backbone lies between the hydrophobic layers. The film thickness from nanometer scale to micrometer scale is controllable by various deposited layers maintaining a constant thickness (3.3 nm) per bilayer. A minimum free-standing ultrathin film with bilayer thickness (3.3 nm) was obtained. It resembles a biomembrane. Two-dimensional hydrogen bonding network formation between polymer backbones and molecular interaction between alkyl side chains contribute to the free-standing film formation and to its smooth surface.