Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 265 Beginning with a very small nucleus (typically around 25 nm diameter) and based on a bias of favourable growth direction with respect to energy, monomer units will begin to form macromolecules over time. It is interesting to note that each plastic displays its own critical weight, at which the growing chain will fold 180 degrees and then continue to grow. Fig. 2 depicts schematic macromolecular growth in a generic melt domain. Figure 2 : Macromolecular growth and chain folding in a generic melt domain. As the chain continues to grow, the folding will occur at repeated positions and begin to form a lamellar structure. The eventually developed lamellae reach thicknesses between 20 to 60 nm. It is helpful to think of the chain as crystalline, and the remaining volume fraction as amorphous. At this stage we are not concerned with orientation effects or density gradients as a result of manufacturing, we simply think of the growth process from a general point of view. This structure creation is first present in the polymerization process 6 , and will be mostly carried over to the manufacturing process to be applied later. The manufacturing process itself will only influence the physical structure. Here the structures we have formed in the polymerization process can be realigned and reshaped as well as new ones formed. Fig. 3 shows the schematic growth of a single lamella in the melt. Figure 3 : Lamellar growth as a result of extended macromolecular growth. The length of the lamellae is finite, and determined mostly by a critical energy level, but also by geometric and structural boundaries. One such boundary is the fact that multiple lamellae begin to grow at different points in time and at different positions in the melt. However, from the nucleus mentioned in Fig. 2, multiple further lamellae will also grow in a radial fashion, such that a geometric structure often akin to a circle becomes apparent. Other possible shapes are maltese crosses, disks and “shish kebabs”, the latter being a stack of the previous geometric structures. The geometric structure arising from this radial growth pattern is dubbed a spherulite [9]. Fig. 4 shows a crossed polar examination of spherulite nucleation in a melt. Though not strictly true, the growth is generally considered to be two-dimensional. The three-dimensional aspect is more being used for simulations of the degree of crystallinity [12-14, 17-22] and not necessarily describes the geometric shape of a structural entity. It is important to note that non-crystallized material exists between lamellae, adding a further layer of complexity to the constitutive behaviour. We refer to this volume as inter-lamellar. 6 Which is the manufacturing process of the plastic after which it is ground into pellets to be later remelted and formed into the final component shape.