(447f) A Systematic Thermogravimetric Analysis of Catalytic and Noncatalytic Carbonization of Polyetherimide/Graphite Nanocomposites

Polyetherimide (PEI) is an engineering polymer that can be pyrolyzed in a controlled environment to yield a rigid porous carbon structure to be used as a molecular sieve for gas separation or water purification applications. PEI is known to have a high char yield that makes it appropriate for the mentioned use; however, the processing-microstructure-property relationships as a function of PEI chemical composition and pyrolysis conditions are largely unknown. Specially, with respect to the former, the addition of “templating” carbonaceous nanofillers such as graphite or metal catalysts to the PEI matrix can be considered as a facile approach to optimize the final rigid carbon nano- or microstructure. The main objective of this work is to fundamentally investigate the effects of PEI (melt index 18 g/10 min; 337°C/6.6 kg, Sigma-Aldrich) composition (neat PEI vs. PEI/graphite nanocomposite vs. PEI/graphite/metal catalyst nanocomposite) and pyrolysis conditions on the thermal degradation behavior, char yield, and chemical structure of the final pyrolysis products using thermogravimetric analysis (TGA) under an inert nitrogen atmosphere. For this purpose, an I-optimal design of experiments (DOE) was performed with two continuous numeric factors, i.e., Factor A: Graphite (APS 7-11 micron, 99%, Thermo Scientific Chemicals) Content (Low: 0 wt.%; High: 50 wt.%) and Factor B: Heating Rate (Low: 10°C/min; High: 20°C/min), and one nominal categoric factor, i.e., Factor C: Catalyst Type (Level 1: Iron(III) nitrate nonahydrate; Level 2: Cobalt(II) nitrate hexahydrate; Level 3: Nickel(II) nitrate hexahydrate), resulting in 25 experimental runs (a quadratic design model with nine model points, five lack-of-fit points, and five replicate points). The neat uncatalyzed PEI composition was used as control (thermogravimetric data obtained for two heating rates of 10 and 20°C/min). Furthermore, three responses were selected for statistical analysis and multi-response optimization, i.e., Onset Temperature, Degradation Rate, and Percent Residue. The TGA analyses were performed from room temperature to 1,200°C in a Mettler-Toledo® TGA-DSC 3+ Thermal Analysis System. The neat PEI and PEI nanocomposite films were prepared by a solution casing method, where graphite powder was first mixed with n-methyl-2-pyrrolidone (NMP) in the presence of cetyltrimethylammonium bromide (CTAP) and sonicated for 10 minutes to create a stable graphite suspension. Next, PEI was added to the suspension and stirred for 24 h at 60°C. Finally, the solution was cast on glass and the film/glass assembly was submerged in water and kept under water for another 24 h for phase inversion. Samples were then cut from the cast films and TGA analyses were performed under the design-prescribed experimental conditions. Our findings so far suggest that the neat PEI loses only about 40% of its mass during pyrolysis and adding graphite helps stabilizing PEI through a thermal barrier effect. Graphite further prevents rapid PEI volatilization. We are currently performing the experimental runs for the specimens containing graphite and the different metal catalysts and will analyze the data once all 25 runs are complete. We expect to be able to identify the significant factors and interactions that govern the PEI pyrolysis process with the aim of obtaining the optimal PEI composition and heating rate to maximize char yield.