Subsidiary lamellae

Much effort has been devoted to investigating the detailed architectures and the construction of spherulites. Early investigations of the crystallization of polymers through optical microscopy (OM) [7,8] posited that polymer spherulites consisted of radiating fibrous crystals with dense branches to fill space. Later, when electron microscopy (EM) became available, spherulites were shown to be comprised of layer-like crystallites [9,10], which were named lamellae. The lamellae are separated by disordered materials. In the center of the spherulites, the lamellae are stacked almost in parallel [5,6,11-15]. Away from the center, the stacked lamellae splay apart and branch, forming a sheaf-like structure [11,13-15]. It was also found that the thicknesses of lamellae are different [5,6,11,12]. The thicker ones are believed to be dominant lamellae while the thinner ones are subsidiary lamellae. 

Fig. 6 A series of AFM phase images obtained on a BA-C8 film at room temperature, a An embryo b a short lamella (founding lamella) developed from the embryo shown in a c-f. The growth of the founding lamella g-p branching and splaying apart of the subsidiary lamellae [61]...

In the initial stage of the crystallization, the formation of a skeleton of dominant lamellae of equal widths separated by the melt is clearly visible as shown in Fig. 12a. The onset of branching is also visible in Fig. 12a. As the crystal grows, the hedrite becomes more asymmetrical with respect to the central dominant lamellae because it is tilted with respect to the surface (c.f., Fig. 12b-d). The dynamics of this space filling can clearly be observed in Fig. 12c,d. The subsidiary lamellae originating from the edge of the skeleton eventually develop a dominant character. 

However, it is difficult to measure L0, while in most cases, Li and L, as defined in Fig. 20a, could be obtained easily when a subsidiary lamella grew to... 

Fig. 20 a A schematic illustrating the branching of a founding lamella and b a plot of the reciprocal of induction time for subsidiary lamellae as a function of crystallization temperature [64]... 

Fig. 25 Plots of the length of founding and subsidiary lamellae of BA-C8 as a function of time at 22 °C [62]...

If we take the radiation-growing axialites at high temperatures as the dominant lamellae, and the empty spacing is filled with the subsidiary lamellae grown at low temperatures, we can obtain the sphere-like crystals with dense filling, often called the type-I spherulites. Besides this kind of spherulites obtained by sequential formation of dominant and subsidiary lamellae during cooling, there exists another... 

Crystallization of most polymers is accompanied by the separation of different molecular species, a process referred to as molecular fractionation. In linear polyethylene, fractionation occurs due to differences in molar mass. The low molar mass material crystallizes at low temperatures in subsidiary lamellae located between the dominant lamellae and in the spherulite boundaries (see Chapter 7). 

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