Sub-Micrometer Phase Change Materials as Thermal Stabilizers for Partially Hydrolyzed Polyacrylamide Solutions

The thermal stability of partially hydrolyzed polyacrylamide (HPAM) under reservoir conditions remains a critical challenge for polymer-based enhanced oil recovery (EOR). This work presents the synthesis, characterization, and application of submicrometric phase change materials (PCMs) as thermal stabilizers for HPAM solutions. PCMs were synthesized via the sol–gel method, targeting the encapsulation of KNO3 within a SiO2 shell. The synthesized particles exhibited a spherical morphology with a median hydrodynamic diameter of 530 nm, a phase transition temperature of 133 °C, and high thermal conductivity and heat capacity. The effect of PCM on HPAM thermal stability was systematically evaluated through rheological measurements at 70 °C over 21 days, using a 3k full factorial design of experiments (DOE) comprising 80 systems. The results demonstrate that PCM effectively inhibits HPAM viscosity loss through a synergistic mechanism involving hydrogen bonding, electrostatic shielding, and dipole–ion interactions between the SiO2 shell and the functional groups of HPAM, combined with sensible heat absorption by the KNO3 core. At 1000 mg/L HPAM, the optimal PCM concentration was 100 mg/L, achieving a viscosity loss of only 24% after 21 days, compared to 72% for the PCM-free system. Furthermore, the system containing 5000 mg/L HPAM and 1000 mg/L PCM exhibited a viscosity increase of 13%, confirming the existence of a critical HPAM:PCM concentration ratio that governs thermal stabilization performance. ANOVA confirmed that PCM concentration, HPAM concentration, and their interaction are the dominant factors controlling polymer degradation, highlighting the importance of considering synergistic effects in PCM-based EOR applications.