The mechanism of microphase separation in paraffin hydrocarbon mixtures

To understand further the mechanism of microphase separation in paraffin mixtures at room temperature the strong small angle neutron scattering from the forming and annealing microphases is reported for a range of concentrations and at a sequence of times after quenching the molten mixtures. The data are very sensitive to the longitudinal organization of the structures and thus are complementary to recent spectroscopic studies. The data for solutions of C36/C30 aliphatic hydrocarbons identify scattering from a phase rich in the C36 component (near Q = ∼ 0·1 A-1) and ‘long period’ scattering whose principal wavevector is concentration dependent. Long periods between 80A and ∼ 210A are observed. The rate and extent of growth of the microphase depend on both composition and quench temperature. The rate of microphase growth at room temperature increases by a factor of about seven times in changing composition from 4 : 1 C30H62:C36D74 to 1:1 C30H62:C36D74. The data are consistent with a mechanism in which the free energy is minimized by crystal vacancy (even void) elimination and molecular conformational ordering with time as the system enthalpy is minimized by the preferred packing of chains of the same length.