The Rotator Phases

Regarding Case 1, it is known that the rotator phases of -paraffins are three dimensionally ordered solids with molecules in aW-trans conformation, their specific volumes, Vh, and specific entropies, Sh, are closer to corresponding values and for the orthorhombic phases than they are to those for the melt and Sm In contrast,... 

Conversely, the high-pressure hexagonal phase is one of high entropy for which the reverse inequality is true. In lower -paraffins, the occurrence of the rotator phase at atmospheric pressure is due to the fact that the orthorhombic-hexagonal (or rotator) transition temperature, is lower than the temperature of fusion of... 

The heat of fusion of the rotator phase in n-paraffms is 38-40 cal/g, while AHo- m for PE irradiated with 1000 Mrad is about 29 cal/g. The corresponding value of AHo->m for hexagonal PE at high pressure (500 MPa) is only around 15 cal/g. Thermograms of constrained PE fibers showed similarly low heats of fusion of the hexagonal phase. 

The cross-sectional area per chain in the hexagonal lattice of irradiated PE varies between 20.6 and 22.0 A. It is, thus, always greater than the cross-sectional area in the rotator phase in paraffins (19.5-20.0 A ), but on average somewhat smaller than that in constrained PE fibers above 7, /, (21.4-22.7 A ). An ethylene-propylene diene copolymer with approximately 64%, 32%, and 4% by weight of each component, respectively, was found to contain hexagonal crystals with a cross-sectional area per chain of 20.3 A". 

The rotational phase shift 5, which cannot exceed a mean angle of a molecular rotation during collisional time (anc), is certainly small in the case of non-adiabatic collisions. This condition is exactly that needed for anisotropic scattering (or IR absorption) spectrum narrowing, just as vibrational dephasing must be weak for an isotropic spectrum to narrow. 

Figure 4.31 Snapshot picture of molecular dynamics of CjsH g, tricosane, viewed along the a axis. A bilayer is shown for (a) the crystalline phase at 31 IK and (b) the rotator phase at 315K. For clarity no H atoms, only the central C atoms along with a few terminal C atoms are shown. Note the disorder in the central C atoms in the rotator phase which is predominantly along the longitudinal axis (after Ryckaert et al, 1989).

Power spectra of the autocorrelation functions of the linear and angular velocities parallel and perpendicular to the C3 symmetrical axes have also been examined by Neusy et al. (32). In the rotator phase, there is good agreement with the Raman data (36). The calculated characteristic time (r4) for reorientation of the C3 axes from one [111] direction to another and also the reorientation time (r3) for rotation of molecules around the C3 axes were similar... 

Solid-solid transitions are observed especially for short alkanes. Odd numbered n-paraffins with more than 9 carbon atoms, and even numbered ones with more than 22 and up to about 45 carbons, undergo the transition to the rotator phase. Solid-state NMR spectra give evidence for this transition at the molecular level. Moller et al. [16] studied the temperature dependences of several n-paraffins and polyethylene systematically. Figure 9.4 shows the... 

Figure 1.3 Fraction of C—C bonds that are gauche plotted as a function of bond position of heneicosane chains for the rotator phase (phase II) at 32 C (305 K) and 39°C (312 K) [31].

A separate control is exerted over the screw cushion. The cushion is the volume of liquid that remains in front of the screw after injection (see Figure 7.37) the cushion affects the pressure transmitted from the system (ram hydraulics and ram) to the cavity during packing. The ram position sensor measures the cushion if it is low the screw is withdrawn funher during the rotation phase of the next cycle. 

In these simple molecules, the dielectric relaxation curves conform to the simple Debye form and hence it is relatively straightforward to determine activation energies from the variation of the relaxation frequency with temperature (Table 4.1). A surprising feature of the rotator phase is that in certain cases the activation for dipolar relaxation is smaller in the solid than it is in the liquid phase, when the solid conforms to a face-centred cubic lattice. When the solid melts, the local force field loses its symmetrical form and the result is that the activation energy for free rotation is observed to increase slightly (Figure 4.5). For most polar liquids, the activation energy for viscosity flow and for dipole... 

Investigations of mixtures of alkanes for C19H40-C20H42 indicate that the temperature range of the rotator phase widens due to chain mixing.Eicosane has also been studied and X-ray scattering measurements indicate the existence of a rotator phase.Studies of the ultrasonic attenuation and DSC measure-... 

The diffusion coefficients in a number of w-alkanes have been investi-gated, which show the influence of the rotator phase assisting the motion of the polymer chains. Rotator phases have been reported in chains with as... 

Rotator phase materials are related to liquid and molecular crystals, discussed in Chapters 2 and 3. The rotator phase is characterized by the following ... 

---