Transition from extended-chain to once-folded behaviour in pure n-paraffins crystallized from the melt

Abstract

Nucleation theory is applied to extended-chain crystallization as background to the extended-chain to once-folded transition problem and certain unusual effects found in recent experiments. In the ‘partial stem attachment’ model employed, the activated complex leading to stem addition involves most of the length of the molecule, but only in the form of occasional contacts with the substrate. Here the lateral surface free energy σ is of mostly entropic origin. With a normal σ, the model gives the main features of the striking maximum in the extended-chain growth rate found by Ungar and Keller for n-C 246H 494. Complete register of the chain ends is not attained during nucleation, resulting in a transient layer of cilia on the crystal (‘kinetic ciliation’). This layer leads to an end surface free energy σ′, which is deduced from the growth rate data and used to estimate the initial thickness l a of the unstable ciliated surface layer for n-C 246H 494. Shrinkage of l a through annealing is discussed, with special reference to its effect on increasing the melting point toward its equilibrium value. This leads to an interpretation of the remarkable T′ versus T x plot for extended-chain n-C 192H 386 given by Stack and co-workers. Nucleation theory is employed to predict the temperature T∗ 1 at which once-folding begins in n-C 246H 494. With a normal value of the fold surface free energy (acting in a ‘mean field’ to account for chain end effects) the theory predicts both T∗ 1 and a marked increase in the growth rate of the once-folded species relative to that of the extended-chain at T∗ 1 . This explains the extended-chain to once-folded transition in n-C 246H 494, and also accounts for the previously unexplained minimum in the overall growth rate at T∗ 1 observed by Ungar and Keller. The treatment of the onset of once-folding is supported by data on other systems. The disordered nature of the initially-formed once-folded structure and its fate on annealing are discussed.