ESR evidence of the dynamic crossover in the supercooled liquid states of a series of solid n-alkanes.

A study of the rotation dynamics of the spin probe TEMPO by electron spin resonance (ESR) spectroscopy in a series of six short-and medium-sized n-alkanes is presented. In addition to the usual crossover effects connected with solid-liquid phase transitions at temperature T-m and the solid-solid transition at T-ss, we find a strong correlation between the change in reorientation at the characteristic ESR temperature T-X1(fast) < T-ss and/or T-m with the ratio T-X1(fast)/T-m = 0.76 +/- 0.03 and the characteristic dynamic temperature, T-X, obtained from the viscosity data. This indicates the presence of local disordered regions in the crystalline structure which is indeed sensitive to dynamic change at the crossover temperature T-X congruent to T-X1(fast).