Morphology Development in Model Polyethylene via Two-Dimensional Correlation Analysis

Two-dimensional (2D) correlation analysis is applied to synchrotron X-ray scattering data to characterize morphological regimes during nonisothermal crystallization of a model ethylene copolymer (hydrogenated polybutadiene, HPBD). The 2D correlation patterns highlight relationships among multiple characteristics of structure evolution, particularly the extent to which separate features change simultaneously versus sequentially. By visualizing these relationships during cooling, evidence is obtained for two separate physical processes occurring in what is known as irreversible crystallization in random ethylene copolymers. Initial growth of primarily lamellae into unconstrained melt (primary-irreversible crystallization) is distinguished from subsequent secondary lamellae formation in the constrained, noncrystalline regions between the primary lamellae (secondary-irreversible crystallization). At successively lower temperatures (reversible crystallization), growth of the crystalline reflections is found to occur simultaneously with the change in shape of the amorphous halo, which is inconsistent with the formation of an additional phase. Rather, the synchronous character supports the view that growth of frustrated crystals distorts the adjacent noncrystalline material. Furthermore, heterocorrelation analysis of small-angle and wide-angle X-ray scattering data from the reversible crystallization regime reveals that the size of new crystals is consistent with fringed-micellar structures (similar to 9 nm). Thus, 2D correlation analysis provides new insights into morphology development in polymeric systems.