On the Differences in Trimethylaluminum Infiltration into PMMA and PLA Polymers for Sequential Infiltration Synthesis: Insights from Experiments and First-Principles Simulations

Sequential infiltration synthesis (SIS) is a powerful approach for templated growth of solid materials, such as oxides or metals, that exploits the difference in interaction of a precursor molecule with a polymer or block copolymer. While there have been studies showing that infiltration of trimethyl-aluminum (TMA) in polymers can be used to grow Al₂O₃, there are still many atomic level details of the SIS process that require more investigation, including the origin of the differences in infiltration of TMA into different polymers. In this paper, we investigated in detail the infiltration of Al₂O₃ into poly(methyl methacrylate) (PMMA) and poly(lactic acid) (PLA) experimentally and theoretically. SIS was performed in a standard ALD reactor, operating at 70 °C in quasi-static mode, using TMA and water as the metal and oxygen precursors, respectively. Operando spectroscopic ellipsometry and ex-situ X-ray photoelectron spectroscopy (XPS) evidenced that Al₂O₃ incorporation in PLA is significantly higher than in PMMA even if, in both cases, TMA incorporation occurs through the formation of an Al–O covalent bond at the C–O–C group. The extent of swelling of the polymers upon TMA infiltration is assessed and is clearly larger for TMA in PLA than in PMMA. First-principles density functional theory (DFT) calculations highlighted that both polymers display swelling upon TMA infiltration, saturating with increasing TMA, consistent with operando ellipsometry observations. The DFT results also show the origin of the larger swelling in PLA compared to TMA. Changes in vibrational modes of carbonyl backbone groups in the polymers are used to demonstrate TMA-polymer interactions from both experiment and simulation. The differences in TMA infiltration and swelling arise from differences in the TMA-polymer C–O–C group interaction, which is more exothermic in PLA than in PMMA, in agreement with experimental results. The combination of experimental and theoretical studies herein reported provides a toolkit to disclose the complexities of SIS at the molecular level.