Exploring the Vapor Phase Infiltration of Trimethylaluminum into Polyacrylonitrile Fabrics

Abstract

Vapor phase infiltration (VPI) creates a hybrid organic-inorganic material by modifying the bulk of a polymer substrate with a metalorganic vapor phase precursor. In VPI, the metalorganic precursor sorbs to the polymer, then diffuses through the bulk and becomes entrapped—either by a chemical bond between the polymer and the inorganic or by a co-reaction with a second species—yielding a hybrid composed of the polymer and, often, a metal oxide. Some of the polymers that have been studied under VPI include poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), polydimethyl siloxane (PDMS), and poly(3-hexylthiophene) (P3HT). There is a notable exception in this list: polyacrylonitrile (PAN). The high reactivity of the nitrile functional group on the backbone of the PAN polymer chain suggests PAN would perform well in VPI. PAN is used in textiles and filtration, but one of its most common applications is as a precursor to carbon fiber. Previous hybrid materials created via VPI have demonstrated new optical and thermal properties that differ from the neat polymer. If infiltrated successfully, these properties in PAN could be of use in its application spaces. In this work, we expand the VPI material space with the first study of infiltration of PAN. To explore the bounds of this system in terms of achievable inorganic loadings and hybrid material chemical structure, PAN is infiltrated with the metalorganic precursor trimethylaluminum (TMA) and co-reacted with water vapor under variable precursor exposure times, precursor desorption times, and processing temperatures. PAN is found to infiltrate with high quantities of inorganic loading (as measured by thermogravimetric analysis) that can be tuned from 1% to 17% inorganic by weight. Inorganic loading depends strongly on precursor exposure time with a sample processed at 140℃ showing a 25% increase when the exposure time is increased from 5 hours to 25 hours. Infiltration temperature also plays a significant role with hybrid materials demonstrating a 78% increase in inorganic loading when the PAN processing temperature is increased from 80℃ to 140℃. The apparent increase in inorganic loading with temperature may be related to the mass transport and reaction kinetics of the system under VPI conditions. Precursor desorption time, on the other hand, has minimal impact on inorganic loading suggesting most sorbed precursor participates in a chemical interaction with the polymer during infiltration. In exploring the resulting AlOx/PAN hybrid chemical structure with FTIR, this mechanism was supported by an observed decrease of the PAN nitrile peak at ~2240 cm-1 that scaled with inorganic loading. These structural changes in inorganic loading and chemical state are found to directly correspond to the optical (UV-Vis) and thermal (TGA) properties of the hybrid materials. The results of these characterization techniques imply that the VPI process causes a significant chemical change to the PAN fabric and suggests the process could possibly induce PAN cyclization, which may be advantageous for carbon fiber applications. Overall, this work establishes the significant and tunable inorganic loading of AlOx/PAN hybrid fabrics and expands VPI to a new polymer system.