ABSTRACT:The general principle for synthesizing higher n-alkanes of the highest purity is described briefly. Thermal behavior of the synthesized n-alkanes from n-C32H66 to n-C80H162 were examined by means ofDSC, small-and wide-angle X-ray scattering and optical microscopy. All the crystalline n-alkanes except for n-C80H162 , when crystallized from solution, showed transitions to high-temperature monoclinic modifications (M. 01 , h =I or 2) before the rotator transition or the melting, irrespective of the number of carbon atoms (n) being odd or even and the difference in room-temperature modifications of even n-alkane. One feature of the transition was its bility. The odd n-alkanes of n-C33H68, n-C37H76, and n-C45 H92 showed also another type of solidsolid transition below the transition mentioned above. Both types of the transitions were thermally proved to be of the first-order. Although no simple functional forms were generally given to the relationships between the transition temperature and n, the temperatures for the transition of orthorhombic higher n-alkanes to the M.0. 1 phase increased with n, reaching about 90°C for n-C69H140. The transition to M•01 was accompanied by a morphological change on the crystal surface, in the form of characteristic striations parallel to the b, subcell axis. This suggests that the solid-solid transition proceeds successively from a nucleating site with a staggering of molecules in the chain direction. If the slow rate of the process and complicated behavior of the M•01 phase are taken into account, the kinetical view point of the phase transition leads to the working hypothesis that the molecular motion of the long chains in the n-alkane crystals, which consists of rotational and longitudinal motions or a flip-flop motion, takes place only in this transformation process.KEY WORDS Higher n-Alkanes I Synthesis I DSC I Small-Angle X-Ray Diffraction I Thermal Behaviors I Solid-Solid Phase Transition I Crystalline n-alkane homologues have been studied as a simple model system to obtain insight into the thermal behavior of the polyethylene crystal in particular and polymeric crystals in general. Of particular interest is the first-order solid-solid transition observed for solution-grown crystals of heptacosane (n-C27 H 56 ), 1 tritriacontane (n-C33H68),2 hexatriacontane (n-C36 H74)1.3· 4 and tetratetracontane (n-C 44 H 90 ), 3 besides (if present) a transition to a phase generally called as the rotator phase. The heat of this transition is much smaller than the heat of fusion and even than that of the rotator transition. Another feature of the transition is its irreversibility.Transformations in the crystalline forms may be schematically described in terms of the model structures proposed by Keller 5 for crystalline n-alkanes. For then-alkanes discussed below, four methylene groups in the two chains are considered to occupy an identical orthorhombic subcell, irrespective of the . differences in modification and chain length. End methyl groups form a terminal plane indexable in te...

Polyethylene single crystals were prepared from isoamyl acetate solutions, whose thickness was 200 A. They melted without significant reorganization during the heating process

No abstract

No abstract

No abstract