Mechanisms of interactive crazing in PC/SAN microlayer composites

Abstract

SYNOPSISMechanisms are proposed for the two types of interactive crazing that have been observed in PC/SAN microlayer composites when the PC layer thickness is on the micron-size scale. It is demonstrated that when the PC layer is thin enough the deformation zone that forms at a craze tip can interact with the next-neighboring SAN layer. By measuring the dimensions of the craze tip in scanning electron micrographs, it was found that the craze-tip opening does not depend on the SAN layer thickness, i.e., the length of the SAN craze. Consequently, the size and shape of the resulting plastic zone in the PC layer are also independent of layer thickness. The zone that forms in the PC layer consists of a colinear plastic zone together with a pair of micro-shearbands that grow at an angle of about 45". When the PC layer is less than 6 pm, the elastic stress concentration from the colinear plastic zone increases the probability of crazing in the neighboring SAN layer with the formation of craze doublets that consist of two aligned crazes in neighboring SAN layers. By taking into consideration the Weibull distribution of crazing, a craze doublet fraction comparable to the 30% observed experimentally was predicted with a stress intensification factor in the range of 1.03-1.05. When the PC layer thickness is less than 1.3 pm, the length of the colinear plastic zone is comparable to the PC layer thickness. Formation of a craze at the point of impingement of the plastic zone on the neighboring SAN layer leads to craze arrays with many aligned crazes in neighboring SAN layers. At higher strains, the micro-shearbands grow through the PC layers and extend into several adjacent SAN and PC layers. This produces a change in deformation mechanism in the SAN layers a t the yield instability, from craze opening to shear yielding.

Comparison of irreversible deformation and yielding in microlayers of polycarbonate with poly(methylmethacrylate) and poly(styrene-co-acrylonitrile)

“…Numerous studies of PC/SAN microlayers revealed the relation between macroscopic stress-strain properties and microdeformation mechanisms in this system. [12][13][14][15][16][17] A dramatic increase in toughness of PC/SAN was found as the number of layers increased. This was attributed to a change in the microdeformation mechanism to cooperative yielding of both PC and SAN as the individual layers became thinner.…”

“…An increase in toughness with increasing number of layers was attributed in past studies of PC/SAN to cooperative shearbanding in the thinner layers. [12][13][14][15][16] It might be expected that PC/ PMMA with 32 layers would be ductile at higher strain rates because cooperative shearbanding was observed. However, isolated surface crazes in the 32-layer specimens opened up into cracks and initiated fracture at low strains.…”

Deformation of rubber‐toughened polycarbonate: Microscale and nanoscale analysis of the damage zone

“…16,17 Here, a cavitational mechanism, crazing of the SAN layer, produces a local stress concentration at the interface with PC. With good adhesion between SAN and PC, a small plastic zone forms in the PC layer at the craze tip.…”

Enhanced fracture energy during deformation through the construction of an alternating multilayered structure for polyolefin blends

“…– and responsible for many of their interesting properties. It is thought to be able to provide many useful properties for different materials, such as excellent mechanical properties, dielectric properties, barrier properties, flame‐resistant materials, electromagnetic shielding materials and so on . Thus, much attention has been paid to the preparation of materials with a multilayered structure.…”

“…Meanwhile, as one of the key properties for multilayered structures, the mechanical performance of polymeric materials with such a structure has been subjected to a considerable amount of attention . The multilayered structure fabricated by HSTWIM is thought to be able to provide interesting mechanical behavior for the overall materials.…”