The effects of SiO2 nanoparticles on the thermal stability and rheological behavior of hydrolyzed polyacrylamide based polymeric solutions

The primary objective of this study is to investigate the effects of SiO2 nanoparticles on improving the rheological behavior and inhibition of the thermal degradation of hydrolyzed polyacrylamide (HPAM) solutions. The SiO2-HPAM interactions were evaluated through i) Polymer adsorption onto nanoparticles, ii) rheological studies, and iii) evaluation of thermal stability in presence or absence of oxygen. SiO2 nanoparticles and HPAM were characterized through thermogravimetric analyses (TGA), Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS). The nanofluids were prepared by adding a fixed concentration of nanoparticles to an HPAM-containing aqueous solution. The adsorption isotherms of HPAM over the SiO2 nanoparticles were obtained in batch-mode experiments. Results of adsorption experiments showed that isotherms followed a Type III behavior. The adsorption isotherms were modeled using Langmuir, Freundlich and Solid-Liquid Equilibrium (SLE) model. The best fitting was obtained using the SLE model based on the root-mean-square error (RMSE%), which was lower than 9.5. Also, polymer desorption from the surface of nanoparticles was found to be negligible, and thus the sorption process can be considered irreversible under conditions evaluated. Rheological tests in the range of 25 to 70 °C showed a pervasive non-Newtonian behavior for all the SiO2-HPAM combinations tested. The Herschel-Bulkley and Carreau models were used to describe the rheological behavior of the prepared nanofluids with RMSE% values better than 0.3. The thermal stability of polymeric solutions in the absence and presence of nanoparticles was evaluated under inert and oxidative atmospheres at 70 °C for 14 days. It was observed that a lower degree of degradation resulted for polymeric solutions in the presence of nanoparticles and the absence of oxygen, indicating that SiO2 nanoparticles can inhibit HPAM degradation through adsorption, and subsequently improve its thermal stability.