The Rotator Phases of the Normal Alkanes
- PDF / 303,211 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 71 Downloads / 195 Views
T=D
RTATOR FRASES 01?
=
N0JIM6LL A!MAIAN
E. B. Sirota, H.E. King Jr., D.M. Singer and H. Shao Corporate Research Science Laboratories, Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801
ABSTRACT We present results of an x-ray scattering study on the rotator phases of normal alkanes: CH2-(CH3)n-CH2 (20 5 n 5 33). We have characterized a new tilted rotator phase and measured the distortion and tilt order parameters in all 5 rotator phases. We have shown that there is no strong even-odd chain effect within the rotator phases.
Between the fully ordered crystalline phases of the n-alkanes and the isotropic liquid phase, occurs a series of phases known as rotator phases, which are layered, and usually crystals with long range positional order of the molecules in 3 dimensions but where there is no long range order in the rotational degree of freedom of the molecule about its long axis. This gives rise to finite but weak crystalline ordering and such phases are also known as plastic crystals or highly ordered smectics. Since the normal alkanes are one of the most basic organic series and are building blocks for surfactants, lipids and liquid crystals, it is important to understand their behavior in as much detail as possible. This will enable us to sort out the contributions to the phase behavior of the various parts of the more complicated molecules of which alkanes are a component. The rotator phases of alkanes and the other weakly ordered phases of related systems are also of particular interest due to the fact that the interaction energies are weak (compared to most solid state systems) and a large number of phases can occur related to subtle entropic effects which would not be observed if the interaction terms in the free energy were strong. The rotator phases have been known for a long time due to the early work of Mueller [11, however, it was only more recently that distinctions between the rotator phases were made. The most detailed work to date has been that Doucet and collaborators [21, Ungar 13] and Dorset [4]. We have built on this work, clarifying some aspects of the phase diagram and measuring the temperature and chain length dependence of various structural parameters.
Mat. Res. Soc. Symp. Proc. Vol. 248. @1992 Materials Research Society
78
We summarize here an x-ray scattering study [51 of the rotator phases of the normal alkanes CnH2n+2 for n between 20 and 33 using well aligned samples. We have identified and fully characterized the structure of a tilted rotator phase, the Rv which occurs in C23, C24, C25, C26 and C27 above the crystal phase. We have found that the tilted RIV phase appears in C26 prior to melting which leads to re-entrant tilt in that material. In C27 the Rrv to Rill to Rv sequence results in reentrant monoclinic structures with an intervening triclinic. In this work we are primarily interested in the rotator phases and transitions between them, not on the relative stability of the crystalline phases, thus, the rotator to crystal transition and its kineti
Data Loading...
