ELSEVIER PII: S0032-3861 (96)00796-3 Polymer Vot. 38 No. I 1, pp. 2643-2647, 1997 ;Q) 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/97/$17.00 + 0.00 In situ optical observation of the chain diffusion in an n-alkane crystal: temperature dependence of the chain diffusivity Takashi Yamamoto*, Hironao Aoki, Sadanobu Miyaji and Koji Nozaki Department of Physics, Facu/ty of Science, Yamaguchi University, Yamaguchi 753, Japan (Received 17 June 1996; revised 22 July 1996) The interdiffusion process of n-alkane molecules in the crystal and the temperature dependence of the diffusivity are investigated in situ by optical microscopy. The optical observation of the molecular interdiffusion is possible owing to the mixing-induced solid state transition to the rotator phase; the molecularly mixed region and the low-temperature phase matrix form a sharp interphase boundary which is visible under the polarized microscope. We make a diffusion couple by a mechanical junction of single- crystalline C23H48 and polycrystalline C21 H44. The chain diffusivity at each temperature is determined from the advance of the interphase boundary. The diffusivity, measured at various temperatures from 31 to 39°C, shows a marked temperature dependence; the interdiffusion is rather active above 33°C but it becomes remarkably inactive below 33°C. An apparent activation energy for the interdiffusion above 33°C is estimated 1 to be about 300 kJ tool- . The mechanism of the interphase-boundary motion is discussed on the basis of the formulation devised for the moving boundary problem (the Stefan problem). © 1997 Elsevier Science Ltd. (Keywords: in situ observation;chain diffusion; n-alkane; crystal; temperaturedependence) INTRODUCTION Diffusion is a long-range migration of atoms or molecules in space. It is of great significance in polymer science as well as in the science of low molecular mass materials. Polymer diffusion plays an important role in deformation, annealing, phase separation, crystallization, etc. Numer- ous investigations have been made using various experi- mental 1'2, theoretical 3 and computational methods4, 5. Investigations of polymer diffusion have hitherto been exclusively on polymer solutions, melts or other loosely aggregated states such as gels. Polymer diffusion in crystals is very slow and hard to study experimentally6. The inhomogeneous two-phase structure of crystalline polymers makes interpretation of experimental results even more difficult. Relatively short n-alkanes, on the other hand, are known to show long-range diffusion in crystals 7 l l, both along the chain axis and perpendicular to it. The molecular diffusion process of such anisotropic mole- cules is expected to have very interesting features, and it also gives insight into the molecular motion in polymer crystalsl2 14. Furthermore, molecular diffusion in the crystalline state also has a close connection with molecular diffusion in thin membranes 15. For detailed investigation of the chain diffusion in a crystal, we need a perfect single crystal since molecular diffusion is very sensitive to crystalline defects. Unfortunately, the preparation of a high-quality single crystal that can be used for macro- scopic diffusion measurements such as the tracer experi- ment is very difficult; only very small and thin crystallites are available. * To whom correspondence should be addressed In our previous study, we examined the interface of the diffusion couple of the n-alkanes heneicosane (Czl) and tricosane (C23) by optical microscopy. We found it possible to observe in situ the interdiffusion of C21 molecules into a single crystal of C23 taking advantage of the solid state transition to the rotator phase induced by the molecular mixing of C21 and C2316; we could observe the movement of the boundary between the mixing- induced rotator phase and the matrix low-temperature phase. We also found that the boundary moves a distance which is proportional to the square root of time; it was clearly indicated that the boundary move- ment is a diffusion-controlled process. In this paper we study the interdiffusion at various temperatures by our in situ optical observation method. We also discuss the detailed mechanism underlying the method. PRINCIPLE AND METHOD OF MEASUREMENTS A binary system of n-alkanes of similar chain length has the pronounced feature that the mixed crystals exhibit transition to the rotator phase at remarkably low temperatures (Figure 1) 17,18. This feature, if a single crystal is available, enables high-resolution monitoring of the molecular mixing process in situ by the optical microscope. Figure 2 shows the basic principle of our method. We consider a mechanical junction of two alkanes C21 and C23. If the molecular interdiffusion occurs at a suitable temperature, the mixed region at the junction should transform to the rotator phase. The rotator phase has high optical isotropy around the POLYMER Volume 38 Number 11 1997 2643