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To understand the flow improvement of waxy oils by comb-type polymer additives, the crystallization of long-chain n-paraffins from solutions of decane was studied by X-ray diffraction. The effects of poly(ethylene-butene) (PEB) and poly(maleic anhydride amide-co-α-olefin) (MAC) polymers on the crystallization of C28, C32, and C36 n-paraffins and their mixtures were studied. In the absence of the polymer additives, paraffin platelet crystals give well-resolved low-angle spectra. PEB and MAC reduce the low-angle scattering from the layered structure, which is expected since the polymers cocrystallize with the paraffin and the noncrystallizable portion of the chain provides a steric barrier against platelet stacking. This is consistent with observations by optical microscopy and differential scanning calorimetry and can account for the significant reduction of yield stresses of model waxy oils upon addition of these polymers. The assembly of PEB and MAC with paraffins during crystallization results in improved flow of waxy oils on cooling. X-ray diffraction also reveals that the cooling rate has significant impact on the crystal structure of long-chain n-paraffins. [ABSTRACT FROM AUTHOR]