n-Paraffin solid solutions: modification of phase separation with carbon number

Small-angle neutron scattering is used to monitor microphase formation in the solid state in equimolar binary hydrocarbon systems CnH2n+2 : CmD2m+2 for selected n = 20–34 and n = 36. Microphase formation is maximum for n = 28 and negligible for n = 20, 32 and 34. n = 30 os found to be intermediate. These observations are explicable in terms of increasing molar volume differences between the two components forming the mixture. As the chain length difference increases, the formation of a lamellar microphase occurs in preference to a continuous solid solution until this is itself overtaken by two-phase separation.