MEASUREMENT OF THE GROWTH-KINETICS OF MICRODOMAINS IN BINARY N-ALKANE SOLID-SOLUTIONS BY INFRARED-SPECTROSCOPY

Microphase segregation occurs in certain solid-solution binary mixtures of n-alkanes at room temperature. The kinetics of segregation for the mixtures C28H/C36D, C29H/C36D, and C30H /C36D have been studied at molar concentration ratios of 4:1, 1:4, and 1:1. During the demixing, the mixture maintains a lamellar, highly crystalline orthorhombic subcell structure that is very similar to that of the pure odd-numbered n-alkanes. The average domain sizes of the segregated components of mixtures, in which one component is hydrogenated and the other deuterated, were determined from the band splitting observed for the infrared-active methylene-scissors band. The range of domain sizes that can be monitored extends from single isolated chains to aggregates of the order of 100-150 chains. Curves obtained from plotting log L against log t, where L is the average lateral dimension of the domains and t is time, are nonlinear. However, for mixtures of a given concentration ratio the curves vary in shape with chain length in a systematic way so as to correspond to different segments of a common (S-shape) curve. The relative rates of segregation for the different mixtures are, as expected, very sensitive to the chain-length difference between the two components. The rate increases dramatically as the chain-length difference increases. In the earlier stages of demixing, thr rates for the 1:4 mixtures are generally greater than those for the 4:1 mixtures, suggesting that partial vacancies play an important role in the early-stage kinetics. An asymmetry in the domain sizes of the two components is found in the 1:1 mixtures, especially in the C28H/C36D mixture. This asymmetry indicates that the domains of the longer chains are more compact than those of the shorter chains.