9 The Transition from Ductile to Brittle Behavior of a Semicrystalline Polyester by Control of Morphology
Downloaded by UNIV LAVAL on April 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch009
CHARLES L. BEATTY and WALTER J. STAUFFER Xerox Corp., Webster Research Center, Webster, N.Y. 14580 Significant variation of the ultimate mechanical properties of poly(hexamethylene sebacate), HMS, is possible by con trol of thermal history without significant variation of per cent crystallinity. Both banded and unhanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of ~0.01 in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior.
he brittle-ductile transition of metals as reported by Orowan (I) is explained on the basis that brittle fracture occurs when the yield stress exceeds a critical value. This is based on the Ludwik-DavidenkovOrowan hypothesis that brittle fracture and plastic flow are independent processes yielding separate curves as a function of temperature and strain rate. Therefore, the operative deformation process is the one occurring at the lower stress. The intersection of the brittle stress and yield stress curves therefore defines the brittle-ductile transition. About the same time Flory (2) proposed that the tensile strength of polymers is related to the number average molecular weight. Vincent followed with an analysis of polymer mechanical property behavior as a A
112 Deanin and Crugnola; Toughness and Brittleness of Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV LAVAL on April 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch009
9.
B E A T T Y
A N D S T A U F F E R
Ductile-to-Brittle Transition
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function of molecular weight similar to that used by Orowan (3). The brittle strength of the polymers studied (polystyrene, polymethyl methacrylate and polyethylene) was determined at —180°C (4). This analysis aided in the classification of polymers as brittle or ductile and suggested factors affecting the brittle-ductile transition (e.g., notching, sidegroup size, crosslinking, plasticizers, orientation, etc.). Recently Moore and Petrie (5) have demonstrated that control of sample thermal history can result in transition from ductile to brittle behavior for polyethylene terephthalate. This transition in behavior was related to volume relaxation of the glassy state. 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. The synthesis (9, 10), dynamic mechanical (11), rheological (12), dielectric (13), electrical (14), NMR (15), and thermal (16) behavior of HMS and its isomeric analog, poly-2-methyl-2-ethyl propylene sebacate (MEPS), and copolymers of HMS and MEPS have been reported. Experimental The poly(hexamethylene sebacate), HMS, usecHor these studies had the following molecular weight characteristics: M = 44,000, M = 20,000, M / M — 2.2. Crystallization at 52° and 60°C was performed between polished steel plates in an air oven following compression molding in a laboratory press. The filament specimen used in the quenching experiments was fabricated by (1) inverting a capillary tube into the HMS powder, (2) evacuating the capillary and sample in a vacuum oven, and (3) admitting air pressure after the sample melted to force the sample into the evacuated capillary tube. Note in Figure 1 that this produces a uniform
