ABSTRACT
I. Introduction ................................................................. 246 II. Polyethylene: The Influence of Molecular
Parameters on Morphology and Micromechanical Mechanisms ................................................................. 248 A. Influence of Chain Architecture:
Branching.......................................................... 248 B. Influence of Molecular Weight......................... 249
III. Isotactic Polypropylene: Experimental ...................... 250 A. Materials and Sample Characteristics ........... 250 B. Techniques for the Analysis of Deformation
Structures.......................................................... 250 IV. Isotactic Polypropylene: Morphology and
Micromechanical Deformation Mechanisms ............. 253 A. Morphology: General Features ........................ 253 B. Influence of the Molecular Weight on the
Micromechanical Mechanisms......................... 256 C. Influence of Crystal Polymorphism on
Micromechanical Mechanisms......................... 259 D. Influence of the Deformation
Temperature...................................................... 268 V. Mechanical Behavior and Micromechanical
Deformation Mechanisms in Polyolefins: Comparison of Results................................................. 272
VI. Summary and Conclusions ......................................... 273 Acknowledgments................................................................. 275 References............................................................................. 276
I. INTRODUCTION
Polyolefins are a class of thermoplastic polymers that include the polymerization products of alkenes (polyethylene, polypropylene, etc), the copolymers of ethylene and propylene and the copolymers of alkenes with vinyl monomers. The aim of this chapter is to discuss the technically most relevant micromechanical deformation mechanisms in some of these materials: low-density polyethylene (LDPE), linear-low-density polyethylene (LLDPE), high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) and the two main crystalline modifications of isotactic polypropylene. The common feature of these systems is the semicrystalline morphology, which is controlled by the architecture of the macromolecules (chemical structure, configuration, conformation) and the processing history [1]. The basic morphological units, i.e., the highly ordered crystalline lamellae that are alternated by amorphous regions, are formed by
self-organization during the crystallization process (see Chapter 1). Their typical dimensions lie within the range of 5 to 25 nm. Hence, these polymers, showing a typical coexistence of two different phases within one chemically homogeneous polymer can be considered as nanostructured materials. Under normal conditions (atmospheric pressure, 23°C, moderate deformation rate), the coexisting phases are a combination of a soft (amorphous) and a relatively stiff (crystalline) component connected by covalently bonded links (tie molecules and entanglements within the amorphous regions) that give rise to a good balance of stiffness and toughness to the polymer material. Such a nanostructure of alternating hard and soft regions seems to be characteristic for “high-end” materials such as lamellar styrene-butadiene block copolymers [2], toughened amorphous polymers, nanocomposites, bone tissue, etc.
