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Journal of Structural Chemistry

2012 year, number 5


N. V. Platonova, E. N. Kotel'nikova, S. K. Filatov, G. A. Puchkovskaya, I. I. Gnatyuk, J. Baran, M. Drozd
Keywords: monoclinic even long chain n-paraffins, phase transformations, crystalline and rotationally crystalline states, thermal X-ray diffraction, IR spectroscopy, differential scanning calorimetry
Pages: 991-1005


Thermal deformations and polymorphic transformations of even long chain monoclinic normal paraffins С30Н62 and С32Н66 (homological purity 97-99%) are studied. Thermal deformations and temperature limits of the existence of crystalline (1Mcryst and 2Mcryst), low-temperature rotationally crystalline triclinic (Tcrot.1), monoclinic (Mrot.1), and orthorhombic (Orrot.1) and high-temperature rotationally crystalline hexagonal (Hrot.2) phases of these n-paraffins are estimated by the character of changes in their diffraction patterns and unit cell parameters (thermal X-ray diffraction). The molecular structure and conformational composition of these n-paraffins in different phase states are determined (IR spectroscopy). In order to determine the temperatures of phase transitions differential scanning calorimetry is also applied. The data of all three methods are in good agreement with each other. The initial sample of С30Н62 n-paraffin at room temperature is characterized by a two-layer monoclinic 2M crystalline modification. When the melt is rapidly cooled, it crystallizes in monolayer monoclinic 1M and orthorhombic Or modifications (two-phase mixture 1M+Or). The initial sample of С32Н66 n-paraffin at room temperature is characterized by the monolayer monoclinic modification 1M. When the melt is rapidly cooled, as well as С30Н62 n-paraffin, it crystallizes in monolayer monoclinic 1M and orthorhombic Or modifications (two-phase mixture 1M+Or). It is found that both n-paraffin undergo irreversible polymorphic transformations from the crystalline state cryst to the low-temperature rotationally crystalline state rot.1. For С30Н62 n-paraffin the transition is performed by the scheme 2McrystМrot.1Orrot.1, and for С32Н66 n-paraffin it is performed by the scheme 1McrystTcrot.1. Both paraffins do not undergo the transition to the high-temperature rotationally crystalline state rot.2 (hexagonal phase Hrot.2), similarly to monoclinic С28Н58 n-paraffin studied previously. These and previously obtained data allow us to consider that as the molecular chain length increases, differences in the polymorphic modification of initial crystalline phases cease to be significant and the sequence of polymorphic transformations is mainly determined by the length of the molecule.