Dynamic mechanical spectroscopy of plastic crystalline states in n-alkane systems

Dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) were used to study the plastic crystalline region of several model n-alkane systems and a commercial paraffin wax. Results indicate that DMS provides a valuable complement to existing tools for locating plastic or rotator states and characterizing the pre-melting region containing these mesophases. DMS measurements demonstrate that the mechanical properties of plastic crystalline states in n-alkanes are difficult to isolate, and that observed viscoelasticity surrounding the mesophases is in great part due to transitions between phases, not the rotator phases themselves. This is indicated from DMS curves that show the dynamic moduli of rotator phases are inhibited from achieving equilibrium values due to the close proximity of successive transitions. Furthermore, study of a system in which stability of a single rotator phase has been extended through strategic blending of n-alkanes shows that dynamic mechanical properties return to near pre-mesophase values when sufficient opportunity following a transition is provided. These results demonstrate that many n-alkane blends can possess a short temperature interval over which the material oscillates between Hookean and viscous behavior, controlling their performance and possibly providing for new applications requiring changes in viscoelastic properties in a narrow temperature span.