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n -Alkane Homogeneous Nucleation: Crossover to Polymer Behavior

2000; American Chemical Society; Volume: 33; Issue: 16 Linguagem: Inglês

10.1021/ma000312m

1520-5835

H. Kraack, Moshe Deutsch, E. B. Sirota,

Polymer Nanocomposites and Properties

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.