Surface and sub-surface reactions during low temperature aluminium oxide atomic layer deposition on fiber-forming polymers

Fundamental reaction processes between vapor-phase chemical precursors and high molecular weight polymers are important for polymer coating, encapsulation and surface modification. Using trimethylaluminium and water in an atomic layer deposition (ALD) exposure sequence, reactions between vapor-phase trimethylaluminium and common polymers with different substituents are quantified using in situ infrared transmission analysis. Exposing polypropylene to trimethylaluminium results in reactant uptake with minimal precursor/polymer reaction, but the precursor/water ALD sequence leads to subsurface alumina nucleation. A similar treatment to polyvinyl alcohol and polyamide-6 results in rapid precursor diffusion and significant reaction observed by IR, and the extent of reaction is consistent with results from in situ quartz crystal microgravimetry and transmission electron microscopy. Reacting trimethylaluminium with polyamide-6 leads to methyl group insertion into the amide carbonyl group and interaction with the hydrogen-bonded amine units. Multiple ALD reaction cycles produce film coatings on all polymers studied, but the coating structure depends strongly on the starting polymer composition. For the weakly interacting polypropylene, cross-sectional transmission electron microscopy demonstrates enhanced sub-surface growth at 90 degrees C as compared to that at 60 degrees C, while images of coated polyamide-6 fibers showed that growth is not strongly temperature dependent in that range. Micrograph images of polyamide-6 samples exposed to extended TMA doses revealed significant modification of the fiber surface region, demonstrating that the precursor could diffuse and react to depths in excess of 100 nm into the surface of the polymer at 90 degrees C. Improved understanding of specific precursor/polymer reaction pathways can be important to optimize the performance of conformal inorganic thin film coatings on polymers.