Quantifying the Thickening Power of High-Molecular-Weight Thermoviscosifying Polymers in Pure Water

Thermoviscosifying polymers (TVPs), which thicken aqueous solutions upon heating, deviate from the conventional thermothinning behavior and hold promise as effective rheology modifiers under harsh conditions. However, their thickening efficiency at low concentrations remains limited by relatively low molecular weights, and the quantitative relationship between thermoviscosifying capacity and thermoassociative contributions has yet to be fully established. In this work, model TVPs were synthesized by grafting amino-terminated poly(N-isopropylacrylamide) (PNIPAM) onto high-molecular-weight partially hydrolyzed polyacrylamide, with systematic variations in backbone molecular weight (Mw) and graft ratio (Gr). A dual-model strategy was adopted to quantify the non-Arrhenius viscosity increase in the thermoviscosifying regime: the Cross model captured logarithmical viscosity growth, while the Gompertz function described sigmoidal viscosity amplification. By integrating molecular parameters into the Gompertz framework, scaling relationships were established between the thickening amplitude (ηmax–ηass) and Mw, Gr, and polymer concentration (Cp), yielding ηmax–ηass ∼ Mw1.75, ∼ Gr1.04 and ∼ Cp2.01, as exemplified by η(T,Mw)=ηass+K·Mw1.75e−e−k(T−Tc). The net thickening power, η–ηass, representing the thermoassociative contribution beyond the nonassociative baseline, scales with PNIPAM content (CPNIPAM) as η–ηass∼CPNIPAMn. Analysis using the time–temperature superposition shift factor revealed distinct scaling exponents (n = 2.78 for Cp vs 1.10 for Gr), highlighting the stronger influence of backbone concentration over grafting density in promoting thermoassociation. These findings provide a quantitative paradigm for assessing thickening performance and establish molecular design principles for TVPs.