Structure and thermorheological properties of TAG-estolide lipid macromolecules from Orychophragmus violaceus seeds

The rheological behavior of a unique vegetable oil, Orychophragmus violaceus (OV) seed oil, consisting nearly entirely of triglyceride-estolide macromolecules, was systematically characterized across a wide temperature range (-20 °C to 160 °C) and compared with some common oils, that are synthetic polyalphaolefin, castor, and rapeseed oils. Steady-state rotational testing revealed three distinct thermorheological regimes as OV oil transitioned from cryogenic to intermediate and to high temperatures. At cryogenic temperatures (≤ 0 °C), OV oil exhibited the properties of a structured solid, showing a significant yield stress (>100 Pa at -20 °C) and pronounced shear-thinning. To capture this regime, a hybrid "Yield Stress-Cross" model was developed achieving high fitting accuracy. Cyclic shear tests at low temperatures confirmed significant thixotropy, indicating time-dependent breakdown and recovery of the structured fatty acid network. Dynamic oscillatory rheological tests further supported the formation of a strong viscoelastic network upon solidification (Structural Index, S ≈ 0.7), which unexpectedly strengthened during reheating from -20 °C to -10 °C, which was attributed to the thermal annealing of metastable crystal polymorphs. In the intermediate range (20 to 80 °C), OV oil exhibited near-Newtonian behavior, and at high (≥ 100 °C) temperatures it fully transitioned to a simple Newtonian liquid following the Arrhenius model. Shear-sensitive nature of the structured network was further confirmed using the Cox-Merz rule in the liquid and solid states. Ion mobility mass spectrometry revealed that OV oil is composed of diverse oligomeric and isomeric triglyceride estolide macromolecules. This molecular non-homogeneity is responsible for its unique thermal properties and rheological response.