chapter  4
28 Pages

Crazing and Fracture in Amorphous Polymers: Micromechanisms and Effect of Molecular Variables

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.