Orientation of polyethylene

Structures Generated from Solidification of Oriented Melts 

The elastic modulus of samples crystallized from the melt increases with melt orientation, but even in the most favorable circumstances it is an order of magnitude lower than that predicted for a perfectly aligned sample. This is indicative of the large number of defects (principally entanglements and chain ends) trapped within the sample, which limit the overall level of ordering that can be developed. 

As melt orientation prior to crystallization increases, the entropy of the sample decreases due to better alignment of the molecules. One result of this improved alignment is a general increase in the degree of crystallinity as a function of increased orientation, which reflects the lower energy barrier to crystallization. This effect is most noticeable for linear polyethylene samples, especially those with high molecular weights. 

When highly branched polyethylene samples, either (dendritic) low density polyethylene or (comblike) very low density polyethylene, crystallize from oriented melts they do not form cylindrites because they contain insufficient linear chain segments to generate microfibrillar nuclei. In such cases the relatively slow crystallization kinetics and low crystallization temperature permit the molecules a relatively long time to relax prior to solidification. The lamellae that form under these circumstances are well separated from one another and do not share a common axis. The resultant semicrystalline morphology is similar to that of low density samples crystallized from an isotropic melt. 

In the context of this discussion, solid-state deformation will encompass any orientation that takes place at temperatures below the final melting temperature of the polymer. Such deformation may be imposed on samples that are initially isotropic or anisotropic. During commercial forming processes, such deformations are usually taken to the point at which a stable morphology is formed, i.e., beyond the yield point. For a general description of the macroscopic phenomena associated with solid-state deformation, the reader s attention is directed to the section on mechanical properties in Chapter 5. 

There has been some comparison of the amorphous orientation of linear polyethylene as measured by the X-ray-birefringence combination with that obtained from a Congo Red dyed sample with good agreement.  

Samuels has extensively studied the orientation of pol) ropylene by a number of techniques.  

Typical orientation diagram plots obtained by X-ray diffraction for hot drawn film melt spun fibres, and cold drawn films of iso tactic polypropylene are shown in Fig. 50. It is noted that the crystal orientation change is 

The orientation of the crystals was also followed by infra-red dichroism using the 1220 cm crystal band. The absolute orientation could not be independently established from these measurements because of the uncertainty of the angle that the transition moment makes with respect to the crystal axes. This angle may, however, be determined by calibrating the infra-red dichroism measurements against X-ray diffraction measurements of crystal orientation. These measurements by Samuels S established that the transition moment for this band makes an angle of 12° with respect to the crystal c-axis. 

This procedure leads to values of AJ, = 33Txl0 and A m = 46-8 X 10 . It is noted that the intrinsic birefringence of the amorphous phase is greater than that of the crystalline. The difference is probably due to the anisotropic internal field of the crystal and is smaller than the corresponding difference believed to exist with polyethylene. This value of A°, is in reasonable agreement with the values of 26 x 10 obtained from studies by Wilchinsky, and of 30 x 10 by Padden and Keith.  

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Zachariades, A. E., Mead, W. T., Porter, R. S. Recent developments in ultramolecular orientation of polyethylene by soldid-state extrusion, in Ultra-High Modulus Polymers (eds.) Ciferri, A., Ward, I. M., p. 77, London, Applied Science 1979... 

One example is the toughening of HDPE by rubber or calcite particles. To address this issue, the morphology and orientation of polyethylene in thin films... 

Bartczak, Z., Argon, A. S., Cohen, R. E., and Kowalewski, T. (1999c) The morphology and orientation of polyethylene in films of sub-micron thickness crystallized in contact with calcite and rubber substrates, Polymer, 40, 2367-2380. 

Some of the generalized orientation factors defined by equations (15) and (16) have been independently reported by several authors. One of the second order orientation factors F200 has been proposed by Hermanns and Platzek to relate the birefringence of fibrous materials to the uniaxial orientation of cylindrical-symmetric structural units, and F q has been defined by Stein and Norris to characterize the uniaxial orientation of polyethylene crystals. F220 F 2 Iso the... 

Orientation of polyethylene introduces anisotropy with respect to virtually every physical property. At extreme levels of orientation, the degree of anisotropy developed surpasses that attainable by any other polyolefin and is unmatched by other organic polymers with the exception of carbon fibers. Most deliberate attempts to orient polyethylene to high degrees are made with the intent of improving mechanical properties, especially tensile modulus. It is therefore no surprise that the majority of literature references to highly drawn polyethylene detail the effects of orientation on such mechanical properties as elastic modulus, tensile strength, and draw ratio at break. 

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