Meaning and measurement of crystallinity in polymers: A Review

Abstract

A useful concept in polymer science is the degree of crystallinity-the fraction of the polymer that exists in a relatively ordered state. Methods of determination of the degree of crystallinity using density, infrared, thermal, N.M.R. and X-ray measurements are examined in light of modern notions of the structure of semi-crystalline polymers. POLYMER STRUCTURE AND CONCEPT OFDEGREE OF CRYSTALLINITY Investigators early recognized that polymers were intermediate in order between ideally crystalline solids and liquid-like amorphous materials. X-ray diffraction patterns of polymers showed broad but distinct crystal-like Bragg reflections superimposed on a background of diffuse liquid-like scattering. In order to explain such observations the micellar theory of polymer crystallization was devised. The long polymer chains were visualized as being randomly tangled in the solid state. In areas where a few chains came together in approximately parallel orientation, a crystallite or micelle was believed formed ( F i g . 1 ).According to this view then, polymeric material consisted of numerous small crystallites randomly distributed through the solid and linked by the intervening amorphous areas. Investigations of the width of the Bragg reflections, Bunn and Alcock (1) for example, showed polyethylene "crystallites" to be only 100-300 A in size, in good accord with the concepts of the model. It was, therefore, entirely reasonable to inquire in this context about the degree of crystallinity of the polymer; to question what fraction of the solid was composed of crystallites.In more recent times, of course, the power of the electron microscope has shown the polymeric solid to consist largely of folded chain lamellae. Linear polyethylene and polyoxymethylene, for example, are literally filled with these regularly stacked crystal structures. The breadth of the diffraction lines must now be interpreted as a result of either a mosaic structure or disorder within the lamellae. Furthermore, much evidence has suggested that the surface of the lamellae are composed of regular folds of minimum length so that effectively an interlamellar zone does not exist ( F i g . 2). On this basis then there is no amorphous phase, the degree of crystallinity is approximately 100% and observations which might be attributed to an amorphous phase are explained on the basis of disorder within the lamellae ( 2 ) . However, not a11 evidence is in accord with such a viewpoint. Some of the evidence to the contrary has previously been reviewed by Mandelkern ( 3 ) .In 1963, in a notable contribution, Ruland (4) presented a clear and rigorous criterion for distinguishing between X-ray patterns caused by one phase disordered systems and two phase crystalline and amorphous systems. Ruland began his analysis with the premise that the material in question is a one phase disordered paracrystal and on this basis, he developed expressions for both the sharpness and intensity of diffraction. First, the breadth of a diffraction peak was attributed solely to disorder so...

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