Polyethylene orientation effects

In [332] it was noted that the strength of samples cut out at different locations of an article made from filled thermoplastics by pressure molding may differ widely — which is due to the non uniform orientation of the polymer at different locations of the mold. The very high strength parameters of composites with PMF in molded specimens are obviously also due to orientation effects, while for standard mixed samples of similar composition (that is, a matrix which, apart from the filler, contains some superhigh molecular polyethylene imitating the PMF coats) the... 

Typically, WPC based on polypropylene and polyethylene show deviation from the Cox-Merz rule. This is due to the different nature of flow. Capillary flow is a pressure-driven flow, including entrance and exit effects, wall slip, friction in the barrel, and orientation effects. Parallel-plate flow is pure drag shear flow, in which particle-particle and matrix-particle interactions result in higher viscosities for filled polymers. In other words, a straightforward question is a 100-fold increase in shear rate and 100-fold increase in frequency result in the same effects the answer would be yes for neat polymers, and no for wood-filled composites. 

Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

Hay, I. L. and Keller, A. (1967) A study on orientation effects in polyethylene in the light of crystalline texture part 2, correlation of the molecular orientation with that of the textural elements, J. Mater. Sci., 2, 538-558. 

Smith P, Lemstra PJ, Pijpers JPL. Tensile strength of highly oriented polyethylene. II Effect of molecular weight distribution. J Polymer Sci Polym Phys Ed 1982 20 (12) 2229-2241. Stevens PS. Polymer Chemistry An Introduction. 3rd ed. New York Oxford Press 1999. p 234—235. Strobl GR. The Physics of Polymers Concepts for Understanding Their Structures and Behavior. Heidelberg Springer 2007. 

Smith P, Lemstra PJ, Pijpers JPL. Tensile strength of highly oriented polyethylene. B effect of molecular weight distribution. J Polym Sci Polym Phys Ed 1982 20 2229-2241. 

Joseph K, Thomas K, Pavithran C (1992) Viscoelastic properties of short sisal fiber filled low-density polyethylene composites. Effect of fiber length and orientation. Mater Lett... 

Figures 4.1 la and b, respectively, are examples of dark-field and direct transmission electron micrographs of polyethylene crystals. The ability of dark-field imaging to distinguish between features of the object which differ in orientation is apparent in Fig. 4.11a. The effect of shadowing is evident in Fig. 4.11b, where those edges of the crystal which cast the shadows display sharper contrast.

---