IN-SITU OPTICAL OBSERVATION OF THE CHAIN DIFFUSION IN AN N-ALKANE CRYSTAL - TEMPERATURE-DEPENDENCE OF THE CHAIN DIFFUSIVITY

The interdiffusion process of n-alkane molecules in the crystal and th e temperature dependence of the diffusivity are investigated in situ b y optical microscopy. The optical observation of the molecular interdi ffusion is possible owing to the mixing-induced solid state transition to the rotator phase; the molecularly mixed region and the low-temper ature phase matrix form a sharp interphase boundary which is visible u nder the polarized microscope. We make a diffusion couple by a mechani cal junction of single-crystalline C23H48 and polycrystalline C21H44. The chain diffusivity at each temperature is determined from the advan ce of the interphase boundary. The diffusivity, measured at various te mperatures from 31 to 39 degrees C, shows a marked temperature depende nce; the interdiffusion is rather active above 33 degrees C but it bec omes remarkably inactive below 33 degrees C. An apparent activation en ergy for the interdiffusion above 33 degrees C is estimated to be abou t 300 kJ mol(-1). The mechanism of the interphase-boundary motion is d iscussed on the basis of the formulation devised for the moving bounda ry problem (the Stefan problem). (C) 1997 Elsevier Science Ltd.