Annealing Secondary Crystallization

Before the crystallized material can be processed at higher SSP temperatures, the melting point of the crystals formed in the crystallization stage needs to 

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Annealing effects due to this heating on exposure also occur, and can be seen In LDPE as a form of secondary crystallization when examined by thermal analysis. Figure 2 shows the presence of a large shoulder on the melting endotherm, representing material that has recrystalllzed during outdoor exposure. 

Secondary crystallization, which has a time constant of approximately 5 min for the LLDPE. The conuibutions 4 and 5 contribute to the annealing peak, usually characterized by an endotherm at 5-15 K higher temperature (see Fig. 7.42). 

Figure 4.69 illustrates the very complicated case of crystallisation of PEcoO also known as linear-low-density polyethylene. More details about the annealing and the fraction of reversible melting and crystallizing of this polymer are discussed in Sections 4.4 and 5 [24,29]. An initial analysis by standard DSC shows a sharp, largely irreversible crystallisation peak, followed by a broad, even larger secondary crystallization, which reaches... 

A consequence of post or secondary crystallization is a thickening in the long period of polymer crystals. As already noted in Section D.lc this effect can be studied over relatively short periods of time (Le., several hundred hours) by annealing semicrystalline material. In Fig. 44 this effect is demonstrated for solution grown polyethylene single crystals. 

It is well known that the mechanical properties of P(3HB-co-8%-3HV) films markedly deteriorate to stifhiess and brittleness by a process of secondary crystallization. The cold-drawn and annealed films of P(3HB-co-8%-3HV) were stored for 6 months at room temperature to study the time dependent change of the mechanical properties, and the stress-strain test was performed. The tensile strength and elongation to break of cold-drawn and annealed films remained unchanged for 6 months as summarized in Table 1. It is of importance to note that the mechanical properties of the cold-drawn and annealed film did not deteriorate during 6 months. It is concluded that a highly oriented and crystallized P(3HB) film keeps superior mechanical properties for long periods. 

Fig. 25 Annealing time evolution of the difference SAXS intensity in the induction period (a) and the crystallization period (b) for the melt crystallization of PET at 244 °C [18]. This system corresponds to crystallization from the metastable state where a nucle-ation and growth type of primary phase separation first occurs followed by the spinodal decomposition type secondary phase separation...

Polymer crystal growth is predominantly in the lateral direction, because folds and surface entanglements inhibit crystalliza- 4 don in the thickness direction. Neverthe-1 less, there is a considerable increase in the fold period behind the lamellar front during crystallization from the melt and, as we have j seen, polymers annealed above their crys-tallization temperature but below Tm also irreversibly thicken. Nevertheless, in most theories of secondary nucleation, the most i widely used being the theory of Lauritzen and Hoffman,28 it is assumed that once a part of a chain is added to the growing crystal, its. fold period remains unchanged. 

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