Polyethylene unit cell

Fig. 4. Structure of orthorhombic polyethylene unit cell projected on the ab plane. The right-hand pictures define the setting angle .

Figure 7.3 Nine polyethylene unit cells arranged to show how they are packed in a crystallite...

Figure 1.65 (a) Crystal structure of polyethylene unit cell shown in relation to chains, (b) View of unit cell perpendicular to chain axis. Reprinted, by permission, from Heimenz, P., Polymer Chemistry The Basic Concepts, p. 236. Copyright 1984 by Marcel Dekker, Inc. 

Swann, P. R. Polyethylene unit cell variations with branching. J. Polymer Sci., 56, 409 (1962). 

Assuming that the backbone of the polyethylene molecule in the crystal has the CCC angle 112° and that the HCH angle is tetrahedral, calculate the polarisability of each molecule per translational repeat length in the three symmetry directions. Use the result in problem 9.6 to deduce the refractive indices for light polarised parallel and perpendicular to the chain axis in the crystal. (Data for the polyethylene unit cell are given in section 4.4.1.)... 

Figure 8.3. Section through the ab plane in the orthorhombic polyethylene unit cell, a and b are the unit cell dimensions.

THERMAL EXPANSION OF THE POLYETHYLENE UNIT CELL. SHENM HANSEN WN ROMO P C... 

POLYETHYLENE UNIT CELL VARIATIONS WITH TEMPERATURE. 

THERMAL EXPANSION OF POLYETHYLENE UNIT CELL EFFECT OF LAMELLA THICKNESS. 

Fig. 14.1. Illustration of semicrystalline polyethylene on the left, consisting of crystalline lamellae and non-crystalline material (not necessarily indicative of actual morphology). The simulation box in the middle shows two crystalline lamellae and the interlameUar phase in between, which consists of loops thin solid lines), bridge chains thick solid lines), and tails dashed lines). The polyethylene unit cell grey box, middle figure) with coordinate axes (a, b, c) is tilted by an angle Qtnt with respect to the surface normal and the coordinate system x, y, z). The bars with label I indicate the extended interface between crystalline and non-crystalline material, while B denotes regions with bulk-like properties, as explained in the text. On the right, snapshots are shown, viewing along the x- and j/-direction, respectively...

In order to obtain an idea of the scale of this volume note (see Fig. 2.1) that the volume of the polyethylene unit cell (which contains 4 CH2 units) is 0.254 x 0.736 x 0.492 = 0.1 nm thus F is a volume equivalent to approximately 260 polymer carbon atoms. 

Partial inclusion of small branches in the polyethylene unit cell was confirmed by solid-state nuclear magnetic resonance ( C NMR) [38-40]. It was observed that while methyl groups partitioned into the polyethylene unit cell on an equilibrium basis, the composition of the crystalline phase in copolymers containing longer side groups was dependent on crystallization history. Slow-cooled copolymer samples showed lower degrees of defect incorporation because the crystallite was allowed more time to perfect its order [38]. 

The dimensions of the a, b, and c axes of an unperturbed polyethylene unit cell are reported to be 7.417,4.945, and 2.547 A respectively [7-9]. These values were measured for high density polyethylene at room temperature. The density of a unit cell with these dimensions is 1.00 g/cmh This value is widely accepted and is commonly used in the calculation of the degree of crystallinity from sample density. 

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