Microphase separation kinetics in n-alkane mixtures

Abstract

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 C_nH_2n+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 Å^-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.