Even for only 20 cycles in mild conditions, the enhanced stability of the HP-PEI compared to unmodified PEI is evident. The unmodified PEI exhibited approximately 2% capacity loss, likely due to the formation of cyclic urea under repeated dry temperature swing cycles, as reported in past studies. However, after modification with 2-hydroxypropyl, this degradation is suppressed. The steric hindrance around the active primary amine sites provided by 2-hydroxypropyl groups can slow down the formation of intermediates, whereas the hydroxyl group in the adduct provides additional hydrogen bonds that may stabilize the carbamic acid/carbamate species and prevent them from undergoing cyclization reactions. It has been demonstrated that higher extents of functionalization result in further stability enhancement. The gravimetric analysis instrument (TGA) is not airtight, and we hypothesize that the presence of trace oxygen at high temperature desorption conditions is contributing to degradation as well. This is consistent with previous literature demonstrating the impact of oxygen on the degradation of amines. Oxidative cleavage of C-N bonds at the primary amines can lead to a significant loss of CO2 affinity. Guta et al. observed 80% sorbent deactivation after 8 days (17% deactivation in 3 hours) in 400 ppm CO2/air at 70°C, in contrast to 2% sorbent deactivation after 8 days in 400 ppm CO2/N2 at the same temperature. To determine which degradation form best fits the deactivation data, we analyzed cyclic stability data from Choi et al. with harsher cycle conditions -higherconcentration, humid CO2 (15%) for adsorption and pure CO2 at a higher temperature (120 °C) for desorption. The harsher cycle conditions accelerate degradation, providing insight into the degradation profile at longer times. With the accelerated conditions, Choi et al. observed a 40% loss in the working capacity of unmodified PEI over 50 cycles, in contrast to the 2% loss in Fig. with adsorption in 400 ppm, dry CO2 and desorption in N2 at 90 °C. The accelerated degradation data were fit to linear, exponential, and piecewise exponential functions, and the piecewise exponential function was determined to be the best fit for the unmodified and modified polyamine sorbents (Fig. , Fig. ).