A novel concept in describing elastic and plastic properties of semicrystalline polymers: polyethylene

Abstract

An analysis of micro-hardness and elastic modulus data for different iamellar systems in the light of both eutectoid copolymer and chain folded Iamellar microphases is presented. A novel thermodynamically derived expression offering a fair description of hardness (stress required to plastically deform a crystal) of autonomous non-homogeneous microphases in terms of the average crystal thickness, including a defective surface boundary is developed. The present results characterize the mechansim of plastic deformation as primarily governed by the initial mosaic-block structure controlling the "solid state" mechanism underlined. The average dimensions of the remaining blocks after crystal destruction are thus related to the original block dimensions before plastic deformation. Within this context it is shown that the dissipated energy for crystal destruction increases very rapidly with the molar mass-function of crystalline material.The elastic deformation of these lamellar systems at small strains is correlated to the rubber-like behaviour of the cluster-network. Finally, the role of the average thickness of the non-homogeneous microcrystallites is stressed as describing concurrently the elastic and plastics properties of the polymer allowing a quantitative description of the correlation found between micro-hardness and elastic modulus.

Tensile yield of isotactic polypropylene in terms of a lamellar-cluster model

“…In addition, microindentation hardness was demonstrated to be governed by the cooperative shearing of mosaic blocks within clusters. 19 Separately from Kilian's work, the direct observation of stacked lamellar units was confirmed by Tagawa and Ogura 20 from high-resolution scanning electron micrographs of blown PE films. They emphasized that the piles of lamellae containing 3-10 lamellae act as one unit and do not separate into single lamellae during deformation.…”

New aspects of yielding in semicrystalline polymers related to microstructure: Branched polyethylene

“…The measurement of microhardness defined in this way is bound up primarily with plastic properties. In a preceding work [8,9], a model of local plastic deformation under the indenter was proposed in terms of the energy dissipated by the plastically deformed lamellar crystals and volume of modified crystalline material under the indenter. According to this model, the crystalhardness (H ~) of a polymer material can be predicted as a function of the average lamellar thickness, (I~) including the longitu-*) Dedicated to Professor Dr. Dr. h. c. Armin Weiss on the occasion of his 60th birthday.…”

Monoaxial drawing behaviour of isotactic polypropylene quenched at different temperatures

“…On the other hand, a relationship between crystal size and drawing behaviour has been recently suggested, using a theoretical approach [21].…”