Abstract
Craze initiation stress was measured in three-point bending isochronal creep tests on a series of entangled bimodal blends of polystyrenes of narrow dispersity, on three polystyrenes of broad dispersity, and on four blends of polystyrenes of broad dispersity. Crazing stress was found to increase rapidly with small additions of the higher molar mass component, quickly reaching a plateau. A simple model based on the weighted addition of the crazing stress contributions of the individual weight fractions obtained from an established piecewise linear crazing law was able to predict the crazing stress accurately in the bimodal blends using a power law exponent of 2.59 (90% CI [1.75–17.34]). In broad dispersity systems, in particular where short unentangled chains dilute the polymer, it was found necessary to modify the model using dynamic tube dilution theory. Dilution leads to a change in the entanglement length and hence in the molar mass at which transitions to disentanglement and chain scission crazing occur. With the improved model, crazing stress could be predicted even for the broad dispersity polymers with wide and bimodal distributions. This represents an opportunity for the molecular design of polymers by blending to achieve improved resistance to craze initiation.
Highlights
Over the past few decades considerable effort has been devoted to understanding the role of microstructural characteristics on the macroscopic mechanical response of entangled polymers.Most of the early studies in this field employed polymers of broad molar mass distributions but differing molar mass averages to conclude that several properties of practical engineering interest such as tensile strength,[1,2] fracture toughness,[3] and fatigue life[4,5] can be linked to a measure of the molar mass distribution
This work has investigated the relationship between craze initiation and molar mass distribution in a series of bimodal blends of narrow distributions of polystyrene, a second series of broad distribution polystyrenes, and a third series of blends of broad distribution polystyrenes
Isochronal experiments carried out in three-point bending creep showed that in both the blends and the broad distributions systems the crazing stress was found to be heavily dominated by the presence of a high molar mass component
Summary
Most of the early studies in this field employed polymers of broad molar mass distributions but differing molar mass averages to conclude that several properties of practical engineering interest such as tensile strength,[1,2] fracture toughness,[3] and fatigue life[4,5] can be linked to a measure of the molar mass distribution. In all of these cases, properties generally appear to depend on molar mass, up to a critical molar mass before saturating. The brittle-to-ductile transition of PS was observed to change as a function of molar mass and blending,[10] and it was shown that inherently brittle PS of narrow dispersity could be rendered tough, and more useful, by the addition of longer chains
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