Mechanisms of Secondary Crystallization

One might think at first that the formation of the semicrystalline structure is essentially completed when the crystallization at the first chosen temperature is finished. This is not the case. Crystallization continues on cooling to room temperature, proceeding by two different modes of secondary crystallization. 

Detailed insight into the nature of such secondary crystallization processes comes from small angle X-ray scattering experiments. In Fig. 5.14 a series of SAXS curves was presented for a sample of linear polyethylene at the end 

Results like these enable structural interpretations to be made. The observations on the branched polyethylene indicate the formation of additional crystallites during cooling. These become successively inserted into the original stack built up during the initial isothermal crystallization. On the other 

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Mechanisms of Secondary Crystallization 205 Table 5.1. PeCL Thermod5Tiamic data derived from the experiments... 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

A number of authors have developed mechanistic descriptions of the processes causing secondary nucleation in agitated crystallizers (Ottens etal., 1972 Ottens and de Jong, 1973 Bennett etal., 1973 Evans etal., 1974 Garside and Jancic, 1979 Synowiec etal., 1993). The energy and frequency of crystal collisions are determined by the fluid mechanics of the crystallizer and crystal suspension. The numbers of nuclei formed by a given contact and those that proceed to survive can be represented by different functions. 

Contact nucleation is the most common mechanism of secondary nucleation. Crystal-crystal-, crystal-impeller, and crystal-wall collisions are involved. Secondary nuclei arise from microabrasion (crystal surface damage) or ordered cluster removal by fluid shear forces, as noted above. Figure 4-12 shows that for a given substance, impeller speed and material of construction can both play a role. 

Strickland-Constable (1968) described several possible mechanisms of secondary nucleation, such as initial breeding (crystalline dust swept off a newly introduced seed crystal), needle breeding (the detachment of weak outgrowths), polycrystalline breeding (the fragmentation of a weak polycrystalline... 

However, it is well known that the mechanical properties of P(3HB) homopolymer films markedly deteriorate by a process of secondary crystallization. Accordingly, microbial P(3HB) homopolymer has been regarded as a polymer that is required to be copolymerized with other monomer components from the... 

Poly[(/ )-3-hydroxybutyrate], P(3HB), accumulated in various bacteria, is extensively studied as a biodegradable and biocompatible thermoplastic with a melting point of 180°C (Alper et al. 1963 Doi 1990 Anderson and Dawes 1990). However, it is well known that the mechanical properties of P(3HB) materials markedly deteriorate by a process of secondary crystallization, since the glass-transition temperature (T) is about 4°C (Holmes 1988 De Koning and Lemstra 1993 Scandola et al. 1989). Accordingly, P(3HB) is considered as a polymer that is difficult to use in industrial applications because of its stiffness and brittleness. 

Consequently, Chapter 1 written by R. W. Thompson gives a modern account of our present understanding of zeolite synthesis. The fundamental mechanisms of zeolite crystallization (primary and secondary nucleation and growth) in hydrothermal systems are highlighted. 

A basic criterion for this distinction I178J is the presence or absence of a solid phase. While primary nucleation occurs In the absence of solid particles of the crystallized substance, secondary nucleation Is dependent on the presence of crystals. For homogeneous nucleation, no solid phase Is required, while heterogeneous nucleation Is catalytlcally Initiated by any foreign surface. Many details on the mechanisms of secondary nucleation can be found In the literature [152,177,178,214,215,225]. 

When nucleation takes place without any crystal surfaces, we have primary nucleation. Primary nucleation is said to occur by homogeneous nucleation when no dissolved impurities are present. When primary nucleation occurs due to the presence of dissolved impurities, we encounter heterogeneous nucleation. When nuclei are formed due to the presence of existing macroscopic crystals, interaction with the crystallizer wall, rotary impellers, fluid shear, etc., we have secondary nucleation. Mechanisms of secondary nucleation are not sufficiently clear. An introduction to theories on secondary nucleation is provided by Myerson (1993). Here we will focus on homogeneous nucleation. Note that homogeneous nucleation is rarely achieved or desired in practical crystallization (McCabe and Smith, 1976 Myerson, 1993). 

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. 

Nucleation is initiated by secondary mechanisms involving the seed crystals or low super-saturation and high surface area of seed crystals eliminate or minimize nucleation seed crystals grow... 

The significance of this novel attempt lies in the inclusion of both the additional particle co-ordinate and in a mechanism of particle disruption by primary particle attrition in the population balance. This formulation permits prediction of secondary particle characteristics, e.g. specific surface area expressed as surface area per unit volume or mass of crystal solid (i.e. m /m or m /kg). It can also account for the formation of bimodal particle size distributions, as are observed in many precipitation processes, for which special forms of size-dependent aggregation kernels have been proposed previously. 

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