Microphase separation kinetics in n-alkane mixtures

Using time-resolved small-angle neutron scattering, the kinetics of microphase separation at various quench temperatures between 10 and 43°C in metastable, binary paraffin mixtures C30H(D)621C36D(H)74 of 4:1, 1:1 and 1:4 composition, and 1:1 mixtures of CnH2n+21C36D74 for 28⩽n⩽31, both in the bulk and doped into an exfoliated graphite, have been investigated for up to 6000 minutes at 15 min resolution. The data were fitted to a single exponential relaxation function. Graphite adsorption generally has little effect on the rates, with explicable exceptions. The Q-dependent relaxation times, except for 1:4 C30H621C36D74, all peak at Qca. 0.07 A-1, which together with the shape of the structure function, and the relative trends in the relaxation times, indicate a single demixing process to alternating lamellae. The relaxation time decreases with increasing C36 concentration and has a noticeable H/D isotopic dependence. Increasing chain length mismatch strongly decreases the relaxation times. These observations are all explained by a combination of the interplay of C36 conformational defects and screw motion in the individual alkane chains, together with a void mechanism for mobility. Power-law behaviour of the kinetics is shown to occur over usefully large time domains. The range over which this behaviour is observed increases with decreasing C36 concentration. Our analysis suggests that three broad regions of scattering development can be defined corresponding to linear, power-law and logarithmic growth.

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