Charge density gradients of polyelectrolyte thin films generated by diffusion and reaction in the vapor phase

We present fabrication and characterization of charge density gradients on polymeric thin films. Tertiary amines in poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thin films were quaternized with methyl iodide (MI) through vapor phase diffusion and reaction. The quaternized PDMAEMA (qPDMAEMA), a strong polyelectrolyte, bears permanent charges. The degree of quaternization (DQ) was characterized by FT-IR spectra as well as a refractive index from ellipsometry at elevated temperature to minimize moisture uptake. The resulting films have position-dependent gradients of charge density. The gradient location on the sample depends on process time and concentration of MI. We discovered that the diffusion of MI through air is the rate-limiting step for the entire process. The vapor phase reaction allows for creating charge density gradients in both grafted PDMAEMA brushes and non-grafted PDMAEMA films. In addition, we also evaluated the coefficient of thermal expansion (CTE) and thermo-optic coefficient (TOC) as a function of DQ and showed their linear relationship.