Percolation Effects on the Surface of a Polyimide Film Modified with Aqueous Solutions of an Alkali and Acid

Abstract

A cluster structure of the surface of a polypyromellitimide film was studied by the electron microscopy and ATR IR spectroscopy methods at different steps of consecutive treatment with aqueous solutions of an alkali and acid. The effective size and fractal dimension D of polyamidoacid clusters, as well as the degree ρ_s of the filling of the surface with the latter were calculated from the data of the electron microscopy as a function of the degree β of imide group conversion into amidoacid units on the film surface. The ρ_s and D parameters were shown to increase with a rise in β: ρ_s = 0.1–0.3 and D = 1.3–1.4 at β < β_cr and ρ_s ≈ 0.6 and D ≈ 1.7 at β > β_cr, where β_cr is a critical degree of conversion, which corresponds to the formation of a continuous physical network of polyamidoacid macromolecules or a percolation cluster. In a region close to β_cr (at β < β_cr), the correlation length land the concentration C of the clusters vary according to the laws of the percolation theory for two-dimensional lattices: l ∼ (β_cr – β)^–ν and C ∼ (β_cr – β)^α, where ν = 1.3 ± 0.1 and α = 0.67 ± 0.05.