Block Formation during Polymer Crystallization

Abstract

The lamellae which are formed when a polymer crystallizes have a granular substructure. Evidence is provided by the widths of the Bragg reflections in wideangle X-ray scattering patterns. For polymers they are much broader than in the case of low molar mass crystals and generally indicate lateral coherence lengths of the order of several to some tens of nanometers. This is the same order of magnitude as the lamellar thickness. There are cases in which the block structure also shows up directly in images, for example, in TEM images, when the staining agent can enter into the boundaries within the lamellae1 or in favorable cases also in AFM tapping mode images, for both flat-on and edge-on oriented lamellae.2,3 In a recent work of Goderis et al.4 block sizes in AFM images and coherence lengths derived from reflections half-widths were compared, and the agreement was satisfactory. The blocky substructure is fundamental for the deformation properties of semicrystalline polymers.5,6 A main yielding mechanism is block sliding, and this sets in cooperatively at the yield point. The strain-controlled, comparatively simple deformation properties of semicrystalline polymers are mainly based on the many degrees of freedoms offered by block slidingsinternally stiff crystal layers would cause a quite different, much more complex deformation behavior. The granular substructure thus represents a basic structural property of semicrystalline polymers, even if it remains unconsidered in many discussions. However, knowledge about it is limited. There exist, of course, many measurements of reflection line widths, carried out for different reasons, but we are not aware of studies that would have systematically analyzed the variation of the lateral block size with the crystallization temperature or, in chemically nonregular polymers, the effect of counits or stereodefects. In recent years we had carried out comprehensive SAXS investigations to determine the temperature dependence of crystal thicknesses. As it was generally found, crystal thicknesses vary inversely with the supercooling below a temperature Tc , which is always located many degrees above the equilibrium melting point.7,8 We now selected three of the investigated systems, polyethylene with octene copolymers, s-polypropylene with octene copolymers, and i-polystyrene, and determined in addition in WAXS studies the variations in the lateral coherence lengths. The results are briefly reported in this paper.