Conformational disorder in crystalline n-alkanes prior to melting

By means of infrared spectroscopy, the concentration of gauche bonds has been monitored as a function of increasing temperature for the crystalline n-alkanes, C{sub n}, n = 17, 25, 36, 50, and 60. For each n-alkane, the number of gauche bonds, and hence the degree of conformational disorder, is found to increase as the melting point is approached. The temperature at which the increase in disorder becomes measurable is 10-40 K below the melting point. For C{sub 17}, C{sub 25}, and C{sub 36}, the increase begins abruptly and occurs in association with a solid-solid phase transition. However, for the C{sub 50} and C{sub 60}, the onset of disorder is not associated with a solid-solid transition and occurs gradually. Therefore, C{sub 50} and C{sub 60} can be said to undergo true premelting. Although the onset of disordering for the shorter chains (C{sub 17}, C{sub 25}, C{sub 36}) occurs abruptly and that for the longer chains (C{sub 50}, C{sub 60}) gradually, there are, otherwise, close similarities. The similarities are in the width of the temperature range over which disordering occurs, in the total concentration of the disorder, and in its distribution. These similarities suggest a common disordering mechanism. Our earlier studies on conformational disorder for crystalline n-alkanes in the hexagonal phase suggest that the disordering mechanism involves longitudinal diffusion. In addition to premelting disorder, we have observed an inherent or background disorder for melt crystallized samples. The concentration of this disorder is temperature independent up to the temperature of premelting and, on a gauche bond per chain basis, has essentially the same value for all the n-alkanes studied here. The origin of this disorder is not clear, but it is probably associated with the lamellar surface.