ABSTRACT
I. Introduction ................................................................. 132 II. Competition between Elastic Extension, Shear and
Cavitation..................................................................... 133
III. Nucleation of Voids and Crazes in Entanglement Networks ...................................................................... 138
IV. Craze Growth and Toughness .................................... 141 V. The Effect of Temperature on Craze Initiation
Mechanisms ................................................................. 144 VI. Molecular Characteristics of the Investigated
Semi-Aromatic Polyamides......................................... 148 VII. Effect of Molecular Variables on Toughness ............. 150
A. Toughness Testing ............................................ 150 B. Effect of Chain Length and Molecular
Composition ...................................................... 151 VIII. Conclusions .................................................................. 155 Acknowledgments................................................................. 155 References............................................................................. 156
I. INTRODUCTION
Good ultimate properties are among the most important prerequisites for the successful use of a polymer material, no matter whether the mechanical, the optical or some specific functional properties are to be exploited. For this reason, the deformation and fracture behavior and the means for their improvement have always been studied intensively [1-4]. The mechanical strength of an isotropic thermoplastic polymer derives primarily from the van der Waals attraction between chain segments. Nevertheless strength and toughness evidently depend on the molecular properties of the chosen material, on molecular packing (density, phase structure, micro-morphology), on the way stresses are transmitted between them (through cohesive forces, cross-links or entanglements) and on the nature and intensity of relaxation (and/or damage) mechanisms.
