ABSTRACT
I. Introduction ................................................................. 216 II. Micromechanisms of Fracture in Semicrystalline
Polymers....................................................................... 217 III. Crack Tip Microdeformation ...................................... 227 IV. Wedge Testing and Interfacial Failure ...................... 234 V. Conclusions .................................................................. 239 Acknowledgments................................................................. 239 References............................................................................. 239
I. INTRODUCTION
Many polymer-based materials whose successful application depends on their fracture resistance may be considered to be “nanostructured” in that they show some degree of structural heterogeneity at the sub-micron level. These include bulk semicrystalline thermoplastics such as polyethylene (PE) and isotactic polypropylene (iPP), which self-organize to form crystalline lamellae with typical thicknesses of about 10 nm [1]. The lamellae are generally considered to anchor the chains in semicrystalline polymers, so that solid-like behavior (reversibility with respect to small deformations) persists at temperatures,
T
, well above their glass transition temperature,
T
. PE, in particular, shows excellent ductility and fracture resistance under ambient conditions, even though
T
<< 0°C. The term “nano-structured” polymer is also commonly
understood to refer to polymers that contain chemically distinct phases in which the heterogeneity has been deliberately engineered. A particularly high degree of microstructural control in two (or multiple) phase polymers is possible through self-organization of block copolymers with a well-defined molar mass,
M
, which may give rise to a lamellar microstructure on a similar length scale to that of semicrystalline polymers, depending on the ratio of the block lengths [2]. Indeed, the deformation and toughening behavior of lamellar rubbery/rigid block copolymers, which are gaining in importance commercially, may prove to show interesting parallels with the behavior of semicrystalline homopolymers.
