Thermogravimetric analysis coupled with infrared spectroscopy (TGA-FTIR; under nitrogen atmosphere) was conducted on PEI samples post-acid treatment and desalination experiments to further elucidate the protonation degree and the structural stability. We observed that PEI-C16 and PEI-C16* have comparable TGA profiles with complete decomposition at 420 °C (Figure ). TGA and the first derivative of TGA curves (dTGA) showed >95% mass loss occurred between 240 and 350 °C, thus suggesting that >90% of PEI amines were protonated in the aq. HCl solution (Figure ). The proportional correlation of the coherent length of the C16 polymeric chain and the thermal stability can be concluded as follows: acid treatment and protonation reduce the thermal stability of PEI-C16 following the smaller coherent length and crystalline index of PEI-C16 as confirmed by PXRD analysis.