Logo image
Back
The molecular mechanism of slow crack growth in polyethylene
Dissertation

The molecular mechanism of slow crack growth in polyethylene

Arnold Lustiger
Doctor of Philosophy (Ph.D.), Drexel University
1985
DOI:
https://doi.org/10.17918/00009448
pdf
Lustiger_Arnold_19853.92 MB
PDF Access upon request, Email title, URL, or DOI to archives@drexel.edu

Abstract

Materials Polyethylene--Testing Polymerization--Fracture
Slow crack growth in polyethylene is characterized by a crack which propagates over a period of few minutes to many years after application of a load. The molecular mechanism of slow crack growth in polyethylene is posited to be interlamellar separation. The lamellae tend to separate as a response to the applied stress due to the slipping of tie molecule entanglements rather than reorganize into the fiber morphology typical of ductile creep-type deformation. Environmental stress cracking in polyethylene is an acceleration of this same process, since the environmental stress cracking agent acts as a plasticizer which accelerates the slipping of the tie molecule entanglements. Fracture surfaces of failed specimens display a short nonuniform fiber morphology due to craze formation at the crack tip. This suggests an interruption in the uniform fiber formation found in ductile creep occurring due to interlamellar separation. Cracks which are made to initiate on the surface of these specimens occur preferentially either on spherulite boundaries or radially down the center of spherulites, again suggesting interlamellar separation. Using transmission electron microscopy, interlamellar separation is directly observed. When applied strain is varied environmental stress cracking is not found in the regions of the specimens strained beyond yield (i.e. at the point beyond which lamellae have broke up and reorganized into fibers). On the basis of this understanding the slow crack resistance of polyethylene can be optimized by increasing the interlamellar strength. This can be done by increasing tie molecule concentration and enhancing the effectiveness of tie molecule entanglements. On the basis of a graphic model for failure it is proposed that this can be accomplished through increasing molecular weight, minimizing crystallinity, and lamellar thickness, maximizing comonomer content and using longer comonomer chains. Observed differences in slow crack growth resistance between seven lots of polyethylene piping materials tested can be rationalized by comparing these parameters among them.

Metrics

22 Record Views

Details

Logo image