CRYSTALLIZATION RATE MINIMA IN ULTRA-LONG n-ALKANES. E.Boda, G. Ungar, Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK, A. Keller, H.H.Wills Physics laboratory, University of Bristol, Bristol, BS8 1TL, UK

Crystallisation studies were carried out on monodisperse ultra-long chain n-alkanes which were shown to form chain-folded structures [1]. One of the most intriguing observations is the occurrence of a minimum in crystal growth rate as a function of temperature was found for C₁₉₈H₃₉₈, C₂₄₆H₄₉₄ and C₂₉₄H_590. This effect was observed for solution crystallisation as well as for crystallisation from the melt. Real-time small angle X-ray (SAXS) experiments revealed a transition between an extended-chain and folded-chain crystallisation at the temperature where the minimum occured.

The "self-poisoning" mechanism was proposed as an explanation of the observed retardation in crystallisation rate with increasing supercooling. According to this model, folded chain depositions occur more frequently than extended-chain ones and block the growing face of a polymer crystal.

Isothermal DSC melt-crystallisation studies of C₂₄₆H₄₉₄ and C₂₉₄H₅₉₀ will be presented, together with the results of combined real-time synchrotron SAXS/WAXS experiments.

1. G.Ungar, J.Stejny, A.Keller, I.Bidd, M.C.Whiting, Science, 1985, v.229, pp. 386-389.

2. Ungar, G., "Integration of fundamental Polymer Science and Technology", 1988, 2, 342.