n-Alkane Homogeneous Nucleation: Crossover to Polymer Behavior

Homogeneous nucleation of crystallizing n -alkanes (CH 3 −(CH 2 ) n-2 −CH 3, 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.