Morphological changes Ductile-brittle transition

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

The ductile-to-brittle transition in PA has been studied as a function of (1) materials parameters, including matrix molecular weight (4), type of polyamide (5, 6), and type of rubber (7), and interface (3), and (2) morphological parameters, including rubber concentration (9), particle size (9), ligament thickness (9, 10), use of very small particles (5, 11), and particle distribution (2). When one of these variables is studied, other variables are often changed too. Only in experiments with large series can some idea of the effect of a variable be obtained. 

A TEM study of poly(vinyl chloride)/chlori-nated polyethylene (PVC/CPE) assessed the dispersed phase morphology for correlation with impact properties [342]. Microtomed sections of the blend were stained by a two stage osmium tetroxide method to reveal the CPE phase [343] as the concentration increased, the discrete two phase morphology changed to a continuous network resulting in a transition from brittle to ductile impact fracture (shown... 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

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