Microphase separation kinetics in n-alkane mixtures

Using time-resolved small-angle neutron scattering, the kinetics of microph ase separation at various quench temperatures between 10 and 43 degrees C i n metastable, binary paraffin mixtures C30H(D)(62)-C36D(H)(74) of 4 : 1, 1 : 1 and 1 : 4 composition, and 1 : 1 mixtures of CnH2n+2-C36D74 for 28 less than or equal to n less than or equal to 31, both in the bulk and doped in to 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, wi th explicable exceptions. The Q-dependent relaxation times, except for 1 :4 C30H62-C36D74, all peak at Q ca. 0.07 Angstrom(-1), which together with th e shape of the structure function, and the relative trends in the relaxatio n times, indicate a single demixing process to alternating lamellae. The re laxation time decreases with increasing C-36 concentration and has a notice able H/D isotopic dependence. Increasing chain length mismatch strongly dec reases the relaxation times. These observations are all explained by a comb ination of the interplay of C-36 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 wit h decreasing C-36 concentration. Our analysis suggests that three broad reg ions of scattering development can be defined corresponding to linear, powe r-law and logarithmic growth.