Very long n-alkanes, oligomers of polyethylene, exhibit unusual crystallization behaviour. As crystallization temperature is lowered below the melting point Tm, crystallization rate increases, as expected. However it soon reaches a maximum and with furthes reduction in temperature the rate starts to decrease until it reaches a sharp minimum- see figure on the right. On further cooling the rate again increases steeply.
The interpretation is as follows: Between Tm and the temperature of the minimum only extended-chain crystals are stable and can grow. Below the temperature of the minimum once-folded ("hairpin") chain crystals also become stable and grow much faster than the extended-chain crystals. The reason that crystallization of extended chains slows down in the vicinity of the hairpin melting point is attributed to the effect of surface "self-poisoning". Folded chains deposit on the growth face much more frequently than extended chains. Not far above their melting point, folded chain depositions are almost stable, hence have a long lifetime. However, they melt away eventually. In the meantime they block the growth surface and inhibit the only productive process, the growth of extended-chain crystals.
The work now in progress aims to link crystallization kinetics and changes in crystal shape and establish how "self-poisoning" affects crystallization of conventional commercial polymers.