Prediction of polyurethane behaviour via time-temperature superposition: Meanings and limitations

Two main issues are essential nowadays for practitioners in the field of polymeric materials: how a polymer will behave under dynamic loading conditions and for how long a polymer is reliable. In this sense, the timetemperature superposition principle was applied to the main viscoelastic properties (E', E '' and tan delta) of a series of polyurethane coatings (PU-DEG-TMP) tested for mechatronic devices. Polyurethanes are derived from an ester glycol (poly(ethylene adipate) glycol), an aromatic diisocyanate (4,4'-dibenzyldiisocyanate) and di/trifunctional chain extenders - diethylene glycol (DEG) and trimethylol propane (TMP). Despite polyurethane intrinsic rheologic complexity, the moduli/loss factor curves superimpose well over several decades of reduced frequency at the glass transition temperature (T-g), 0 degrees C and 15 degrees C, the last temperature being considered the midpoint of the practical testing range. Three criteria were for checking the applicability of the time-temperature superposition: the Cole-Cole plot, the similarity between the a T calculated from both moduli (E', E '') and the visual appearance of the final master curve. The presence of both hydrogen bonding and chemical joint points, along with some dangling chains put in a broader context the discussion of the microstructural features resulted from the application of the William-Landell-Ferry (WLF) equation.