Impact of trimethylaluminum exposure time on the mechanical properties of single-cycle atomic layer deposition modified cellulosic nanopaper

Abstract

Cellulosic nanomaterials can improve the performance of various products and can be renewably sourced. In this study, nanocellulosic paper (nanopapers) is chemically and physically altered with simple gas-phase processing to achieve enhanced mechanical performance. Cellulosic nanofibril paper is exposed to single cycles of trimethylaluminum (TMA) and water to modify the surface and subsurface chemistry with small quantities of aluminum oxide. Precursor exposure times are found to significantly influence the amount of inorganic deposited within the cellulosic structure and its crystallinity. This result differs from the common assumption that exposing cellulose to TMA will lead to an “atomic layer deposition (ALD)” type of process in which self-limited surface saturation is quickly achieved. These results suggest that with extended exposure times, the TMA precursor finds new pathways to chemically or physically alter the cellulosic material. Through the x-ray photoelectron spectroscopy analysis, we find that cellulose undergoes a decomposition process during the TMA exposure and/or subsequent reaction with H2O, creating at least one additional pathway to inorganic uptake. Interestingly, uniaxial tensile strength measurements reveal that longer TMA exposure times significantly increase the nanopaper's elongation at break and ultimate tensile strength, with only a modest loss in Young's modulus. While similar inorganic loading can be achieved with multiple ALD cycles, mechanical toughness exhibits significantly less change than for the increased TMA exposure times. X-ray diffraction suggests that the TMA exposures are transforming crystalline portions of the nanocellulose into amorphous structures. These amorphous regions lead to crazing, which increases the strain to break and toughness of the nanopaper.