Microphase separation in graphite-adsorbed paraffin solid solutions

Abstract

Using time-resolved small-angle neutron scattering (SANS), the time-dependent microphase separation occurring in metastable, quenched binary paraffin mixtures C30H(D)62/C36D(H)74 doped into porous graphite has been observed. In the presence of graphite, microphase formation is enhanced compared to the bulk mixtures and the isotopic dependence of the demixing process reported for these systems when quenched to 20°C is not apparent. We relate the enhanced microphase separation to an elevation of the eutectic temperature relative to the critical temperature, due to stabilization of the paraffins at the graphite basal plane. For 1:1 mixtures, the microphase forms an alternating lamellar structure, while the 1:4 and 4:1 mixtures exhibit an increase in scattering at lower angles associated with significantly longer repeat-spacings. An increase in quench temperature from 20 to 27°C increases the strength of the microphase scattering over the time period studied, but quenching to 35°C results in a significant reduction in this signal. Parallel small-angle X-ray scattering (SAXS) studies provide complementary information, with the immediate appearance of strong peaks on quenching, indicating the rapid formation of a solid solution with a lamellar structure. On aging, additional weaker peaks are observed, which, for 1:1 mixtures, are consistent with the formation of alternating lamellae. For all mixtures, except 1:4 C30H62/C36D74, there is a constant offset in Q between the strong and weak peaks. The scattering can be understood to arise from a mixed lamellar system in which incommensurate deviations from the mean structure occur. For 1:4 C30H62/C36D74, two sets of peaks are observed, one corresponding to the alternating lamellar structure and the other to pure C36D74. Show more