n-alkane homogeneous nucleation: crossover to polymer behavior

Homogeneous nucleation of crystallizing n-alkanes (CH₃-(CH₂)_n-2-CH₃, 17 ≤ n ≤ 60) and some low-MW polyethylene (PE) fractions were studied using calorimetry and compared to reanalyzed PE data of Ross and Frolen. The undercooling and derived surface energy of the n-alkanes starts increasing for n as low as 25, far below the low-n limit for chain-folding. This behavior appears to extrapolate to the high undercooling exhibited by high-MW PE. The behavior is discussed in terms of a possible crossover between full-molecule nuclei of low-n and 'bundle' nuclei of larger n. It is also related to the negentropic model, in that the surface energy increases when the 'cilia' dangling from the bundle nucleus exceed a length where their entropic cost begins to significantly raise the surface energy. At low n, homogeneous nucleation occurs into the metastable rotator phase whose potential importance at high n is discussed.