Rheological characterization at the gel point of poly (ethylene oxide) and poly(ε-caprolactone): Influence of temperature on viscoelastic moduli

Abstract

The gel point represents a critical rheological transition in polymeric systems, characterized by the equality between the elastic (G′) and viscous (G″) viscoelastic moduli, and plays a key role in defining material processability and structural stability. In this study, a comparative analysis of the gel point rheology of poly (ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) was conducted, focusing on the influence of temperature on viscoelastic moduli and complex viscosity. Rheological measurements were performed at 80, 100, and 120 °C, and the gel point was identified under the condition G′ ≈ G″ for each polymer. The results indicate that PEO reaches the gel point at angular frequencies that increase with temperature, from 1.80 rad s⁻¹ at 80 °C to 2.70 rad s⁻¹ at 120 °C. Across this temperature range, G′ and G″ remain nearly constant, with values between 4.6 and 5.0 Pa, while the complex viscosity decreases from 2.92 Pa·s to 2.32 Pa·s as temperature increases. In contrast, PCL exhibits gel point behavior at slightly lower angular frequencies, ranging from 1.60 rad s⁻¹ to 2.10 rad s⁻¹, but with consistently higher viscoelastic moduli. G′ and G″ values increase from approximately 5.1 Pa at 80 °C to 5.6 Pa at 120 °C, accompanied by higher complex viscosity values in the range of 3.38–3.65 Pa·s, indicating a more pronounced elastic contribution at the gel point. These findings reveal significant differences in the viscoelastic response and molecular reorganization mechanisms of PEO and PCL under equivalent thermal conditions. The obtained rheological parameters at the gel point provide valuable guidelines for defining processing windows, enabling improved control over melt processing, filament extrusion, and temperature-dependent additive manufacturing processes such as fused filament fabrication (FFF).