In this work, the vapor-phase infiltration (VPI) of polyethylene terephthalate (PET) fabrics with trimethylaluminum (TMA) and coreaction with water vapor is explored as a function of limiting TMA reagent conditions versus excess TMA reagent conditions at two infiltration temperatures. TMA is found to sorb rapidly into PET fibers, with a significant pressure drop occurring within seconds of TMA exposure. When large quantities of polymer are placed within the chamber, minimal residual precursor remains at the end of the pressure drop. This rapid and complete sorption facilitates the control of inorganic loading by purposely delivering a limited quantity of the TMA reagent. The inorganic loading for this system scales linearly with a Precursor:C=O molar ratio of up to 0.35 at 140 °C and 0.5 at 80 °C. After this point, inorganic loading is constant irrespective of the amount of additional TMA reagent supplied. The SEM analysis of pyrolyzed hybrids indicates that this is likely due to the formation of an impermeable layer to subsequent infiltration as the core of the fibers remains uninfiltrated. The Precursor:C=O molar ratio in the subsaturation regime is found to tune the hybrid fabric morphology and material properties such as the optical properties of the fabric. Overall, this work demonstrates how a reagent-limited processing route can control the inorganic loading in VPI synthesized hybrid materials in a simpler manner than trying to control kinetics-driven methods.
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This content will become publicly available on May 24, 2024
Interpreting inorganic compositional depth profiles to understand the rate-limiting step in vapor phase infiltration processes
Vapor phase infiltration (VPI) is a post-polymerization modification technique that infuses inorganics into polymers to create organic–inorganic hybrid materials with new properties. Much is yet to be understood about the chemical kinetics underlying the VPI process. The aim of this study is to create a greater understanding of the process kinetics that govern the infiltration of trimethyl aluminum (TMA) and TiCl 4 into PMMA to form inorganic-PMMA hybrid materials. To gain insight, this paper initially examines the predicted results for the spatiotemporal concentrations of inorganics computed from a recently posited reaction–diffusion model for VPI. This model provides insight on how the Damköhler number (reaction versus diffusion rates) and non-Fickian diffusional processes (hindering) that result from the material transforming from a polymer to a hybrid can affect the evolution of inorganic concentration depth profiles with time. Subsequently, experimental XPS depth profiles are collected for TMA and TiCl 4 infiltrated PMMA films at 90 °C and 135 °C. The functional behavior of these depth profiles at varying infiltration times are qualitatively compared to various computed predictions and conclusions are drawn about the mechanisms of each of these processes. TMA infiltration into PMMA appears to transition from a diffusion-limited process at low temperatures (90 °C) to a reaction-limited process at high temperatures (135 °C) for the film thicknesses investigated here (200 nm). While TMA appears to fully infiltrate these 200 nm PMMA films within a few hours, TiCl 4 infiltration into PMMA is considerably slower, with full saturation not occurring even after 2 days of precursor exposure. Infiltration at 90 °C is so slow that no clear conclusions about mechanism can be drawn; however, at 135 °C, the TiCl 4 infiltration into PMMA is clearly a reaction-limited process, with TiCl 4 permeating the entire thickness (at low concentrations) within only a few minutes, but inorganic loading continuously increasing in a uniform manner over a course of 2 days. Near-surface deviations from the uniform-loading expected for a reaction-limited process also suggest that diffusional hindering is high for TiCl 4 infiltration into PMMA. These results demonstrate a new, ex situ analysis approach for investigating the rate-limiting process mechanisms for vapor phase infiltration.
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- Award ID(s):
- 1921873
- NSF-PAR ID:
- 10443715
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 25
- Issue:
- 20
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 14064 to 14073
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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