Preparation and characterization of glassy plasma polymer membranes

Abstract

Plasma chemical vapor deposition of hexamethyldisilazane was investigated for its applicability as solubility controlled gas separation membranes. Substrates were porous inorganic membranes (Anotec: 0.02 µ) for permeation measurements and silicon wafers for deposition rate, density, and infrared measurements. To ensure a uniform membrane on the porous support a deposit of five times the pore diameter was required. Membranes deposited at 2.45 GHz from HMDSN/O₂ mixtures exhibit a high deposition rate (1.8 nm · s⁻¹) and a wide density range (1.15–1.60 g · cm⁻³). — Typical nitrogen permeation rate of a 100 nm film was 2 · 10⁻⁸ m³ (STP) s⁻¹ m⁻² Pa⁻¹. Permeation rates of CO₂ and C₄H₁₀ related to N₂ and the thermal dependence indicate that the permeation is viscosity controlled. These films exhibit a microgel-like structure with an estimated pore diameter of 2 nm. Anodic films prepared in a 13.56 MHz parallel plate reactor at low deposition rate (0.3 nm · s⁻¹) have densities between 1.3 and 1.5 g · cm⁻³. They exhibit an infrared absorption of the ·:CH₃ vibration at ∼1260 cm⁻¹, which is an easy accessible indication of crosslinking and oxidation of more than twice than in microwave plasma films. For an 0.6-µm anodic film an ideal separation factor for CO₂/N₂ and C₄H₁₀/N₂ of ∼6 is obtained, which is comparable to that of 1-µm conventional polydimethylsiloxane.