The road performance behavior of an asphalt depends upon its susceptibility to changes in its rheological properties with time and temperature. One important fundamental molecular property of an asphalt, which dictates its temperature-dependent performance, is the nature of the molecular motions associated with the asphalt molecular components. Changes in the rotational motions of the methyl carbons and the segmental motions of the methylene carbons in the 1- to 10-kHz frequency range were studied (via changes in spin-spin dipolar-dephasing relaxation times) as a function of temperature for three Strategic Highway Research Program asphalts using carbon-13 dipolar-dephasing cross polarization with magic angle spinning nuclear magnetic resonance. The data indicate that the rotational motion of the terminal methyl carbons of n-alkanes in the amorphous phase is nearly independent of decreasing temperature until the glass-transition region is reached. Below the Tg, the rotational motion of the methyl carbons decreases slowly with decrease in temperature for asphalts AAA-1 and AAB-1. However, the methyl rotation in asphalt AAM-1 decreases more rapidly. The segmental motions of the methylene carbons in the mobile-amorphous phase for the different asphalts decrease rapidly with decreasing temperature from 20 to – 20°C. At temperatures below — 20°C, the segmental motions have essentially ceased. The slow, low-frequency motions of the methylene carbons in the inter-facial (rigid-amorphous) and crystalline phases were found to be independent of temperature above and below Tg. It is suggested that the methyl rotation and segmental motions of the methylene carbons in the amorphous phase extensively influence the low-temperature rheological properties of asphalts.
Rheological properties of rotator and crystalline phases of alkanes
