MICROPHASE SEPARATION IN GRAPHITE-ADSORBED PARAFFIN SOLID-SOLUTIONS

Using time-resolved small-angle neutron scattering (SANS), the time-de pendent 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 i s enhanced compared to the bulk mixtures and the isotopic dependence o f the demixing process reported for these systems when quenched to 20 degrees C is not apparent. We relate the enhanced microphase separatio n to an elevation of the eutectic temperature relative to the critical temperature, due to stabilization of the paraffins at the graphite ba sal plane. For 1:1 mixtures, the microphase forms an alternating lamel lar structure, while the 1:4 and 4:1 mixtures exhibit an increase in s cattering at lower angles associated with significantly longer repeat- spacings. An increase in quench temperature from 20 to 27 degrees C in creases the strength of the microphase scattering over the time period studied, but quenching to 35 degrees C results in a significant reduc tion in this signal. Parallel small-angle X-ray scattering (SAXS) stud ies provide complementary information, with the immediate appearance o f 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 format ion of alternating lamellae. For all mixtures, except 1:4 C30H62/C36D7 4, 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.