Isotope effects on the microphase separation of paraffin mixtures

Isotopic sensitivity is reported in both the formation and the above ambient temperature annealing behaviour of lamellar microphases in C₃₀:C₃₆ paraffin solid solutions. The time resolved small angle neutron scattering from a 1:1 C₃₀H₆₂:C₃₆D₇₄ mixture and its complementary pair 1:1 C D :C H has been studied. Upon quenching such mixtures from the melt; the solid solution so formed nucleates spontaneously to a ‘long period’ microphase. The two systems differ in that for 1:1 C₃₀D₆₂:C₃₆H₇₄ there is also strong diffraction at Q = 0·136 A^-1 associated with C₃₆H₇₄ enrichment of the solid solution or a separate phase. Both processes are sensitive to the isotopic composition of the mixture. For both systems the long period scattering from a quenched liquid mixture develops in periods of hours at ambient temperature. Heating the aged microphase structures to temperatures above about 43°C causes them to disappear, but again isotropic sensitivity is noted. It is concluded that the effects are largely kinetic in origin, in agreement with earlier IR studies on intralamellar microaggregation in this system.