THE MECHANISM OF MICROPHASE SEPARATION IN PARAFFIN HYDROCARBON MIXTURES

To understand further the mechanism of microphase separation in paraff in mixtures at room temperature the strong small angle neutron scatter ing from the forming and annealing microphases is reported for a range of concentrations and at a sequence of times after quenching the molt en mixtures. The data are very sensitive to the longitudinal organizat ion of the structures and thus are complementary to recent spectroscop ic studies. The data for solutions of C-36/C-30 aliphatic hydrocarbons identify scattering from a phase rich in the C-36 component (near Q = similar to 0.1 Angstrom(-1)) and 'long period' scattering whose princ ipal wavevector is concentration dependent. Long periods between 80 An gstrom and similar to 210 Angstrom are observed. The rate and extent o f growth of the microphase depend on both composition and quench tempe rature. 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 sys tem enthalpy is minimized by the preferred packing of chains of the sa me length.