Molecular Layer Deposition of Poly(p-phenylene terephthalamide) Films Using Terephthaloyl Chloride and p-Phenylenediamine

Abstract

Ultrathin polymer films can be fabricated using the gas-phase method known as molecular layer deposition. This process typically uses bifunctional monomers in a sequential, self-limiting reaction sequence to grow conformal polymer films with molecular layer control. In this study, terephthaloyl chloride (TC) and p-phenylenediamine (PD) were used as the bifunctional monomers to deposit poly(p-phenylene terephthalamide) (PPTA) thin films. 3-Aminopropyl trimethoxysilane or ethanolamine was used to prepare amine-terminated surfaces prior to the PPTA MLD. The surface chemistry and growth rate during PPTA MLD at 145 degrees C were studied using in situ transmission Fourier transform infrared (FTIR) spectroscopy experiments on high surface area powders of SiO2 particles. PPTA MLD thin film growth at 145 degrees C was also examined using in situ transmission FTIR experiments on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbances of the N-H and amide I stretching vibrations were measured and used to estimate the thin film thickness. X-ray reflectivity (XRR) experiments were also employed to measure the film thickness after PPTA MLD at 145 degrees C and 180 degrees C. The experiments revealed that the TC and PD reactions displayed self-limiting surface chemistry. The surface species alternated with sequential TC and PD exposures and the PPTA MLD films grew continuously. However, the growth rates per MLD cycle at 145 degrees C were less than expectations based on the size of the molecules involved in the reaction chemistry and were variable between 0.5 and 4.0 A per TC/PD reaction cycle. The lower growth rates are explained by the growth of a limited number of polymer chains on the substrate. The variability in the growth rate is attributed to the difficulties with the bifunctional monomer precursors. Alternative surface chemistries for polymer MLD are proposed that would avoid the use of bifunctional monomers.