Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 266 Figure 4 : Spherulites of i-PolyPropylene growing from the melt, notice the spherical nature and Maltese crosses [16]. Introducing yet another simplification (referring back to Fig. 3), we will assume that the radial lamella growth is concurrent for each individual spherulite. Since the spherulites began to nucleate at different points in time, this in turn means that their boundaries will start colliding at certain locations in the melt. Areas between spherulites that have not crystallized, and where nucleation has ceased, are referred to as inter-spherulitical volume, which will to a major extent be amorphous in nature. These events are illustrated schematically in Fig. 4. Figure 5 : Spherulite growth and volumetric designation of amorphous and crystalline states. On the whole, the crystalline volume fraction of the general melt domain is responsible for stiffness, strength, temperature performance and has a high density. (This explains the difference in behaviour between LDPE, having 45-55% crystallinity and HDPE having 70-80% crystallinity). The amorphous volume governs toughness, impact performance and damping, its density being lower than that of the crystalline volume. As a closing piece to this section we have added a classical scale depiction of the structural entities discussed in the preceding paragraphs. It serves to illustrate the big picture in terms of morphological complexity for plastics, as well as indicate the level of detail necessary to capture in analytical or numerical assessment schemes.