Self-seeding nucleation

The higher the temperamre at which the blend is kept in the melt prior to crystallization, the less residual crystalline parts (otherwise leading to self-seeded nucleation) remain in the melt. As a result, fewer nuclei are available to nucleate the melt phase, thus leading to the formation of fewer, but larger spherulites. 

The shish-kebab (cylindrite) stractures can be formed by compression as observed in in the UHMWPE/iPP blends. Two different self-seeding nucleation origins can be proposed (a) the classical row-nuclei model (b) thread-like nuclei. ... 

Figure 1 shows the DSC cooling scan of iPP in the bulk after self-nucleation at a self-seeding temperature Ts of 162 °C (in domain II). The self-nucleation process provides a dramatic increase in the number of nuclei, such that bulk iPP now crystallizes at 136.2 °C after the self-nucleation process this means with an increase of 28 °C in its peak crystallization temperature. In order to produce an equivalent self-nucleation of the iPP component in the 80/20 PS/iPP blend a Ts of 161 °C had to be employed. After the treatment at Ts, the cooling from Ts shows clearly in Fig. 1 that almost every iPP droplet can now crystallize at much higher temperatures, i.e., at 134.5 °C. Even though the fractionated crystallization has disappeared after self-nucleation, it should also be noted that the crystallization temperature in the blend case is nearly 2 °C lower than when the iPP is in the bulk this indicates that when the polymer is in droplets the process of self-nucleation is slightly more difficult than when it is in the bulk. In the case of block copolymers when the crystallization is confined in nanoscopic spheres or cylinders it will be shown that self-nucleation is so difficult that domain II disappears. 

Chen et al. [92] also performed self-nucleation experiments by DSC in PB-fr-PEO diblock copolymers and PB/PB-b-PEO blends. The cooling scans presented in their work showed that a classical self-nucleation behavior was obtained for PEO homopolymer and for the PB/PB-b-PEO blend where the weight fraction of PEO was 0.64 and the morphology was lamellar in the melt. For PB/PB-fr-PEO blends with cylinder or sphere morphology, the crystallization temperature remained nearly constant for several self-seeding temperatures evaluated. This observation indicates that domain II or the self-nucleation domain was not observable for these systems, as expected in view of the general trend outlined earlier. 

For athermal nucleation (self-seeding nucleatiOTi as a special case of heterogeneous nucleation), the number of nuclei is fixed. Thus the number of spherulites is fixed, with a density mlV, = Anr 3, r = vt, and then... 

In order to obtain an ideally nucleated polymer, it is heated to just above its melting point so that large number of residual crystal fragments exist in the melt and act as nuclei. This method is referred to as self-seeding or self-nucleation (Blundell 1966). Zhao et al. have evaluated crystallization behavior of propylene/ethylene copolymer by self-seeding approach (Zhao et al. 2001), which was found in good agreement with the earlier published DSC data (Laihonen et al. 1970,1997). 

Table 5 also contains data on a melt-fabricated sample that has been self-seeded to produce a higher nucleation density and consequently a smaller spherulite size. Again the resulting improvement in ductility is apparent. 

Parameter Solution-cast plaque Unnucleated moulding Self-seeded moulding Nucleated moulding... 

It is interesting to note the magnitude of the undercooling necessary to recrystallize PHB in the absence of self-seeding—almost 120°C Similar effects can be seen with other polymers but the maximum undercoolings measured are usually only 60 or 70 C. The massive temperature range over which nucleation and crystallization can be observed in PHB is one of the major reasons for academic interest in the material as a model polymer for morphological and crystallization kinetics studies. 

Five types of nucleation process have been identified for PHB in a recent review.These are homogeneous nucleation, occurring infrequently at random sites when the polymer is cooled from a melt at or above 210°C self-seeding, which has already been described as nucleation by unmelted crystallites when the maximum temperature of the melt does not exceed the melting point by more than a few... 

Figure 13f presents a structure of about 5 pm formed at Cpi. This type of structure is marked by the dotted ellipses in the OM of Fig. 13e. The shape of this structure indicates a tandem-like structure, which is most probably the result of the small fluctuations of Cpi in between Cpimax and Cpimin- The nucleus formed initially at Cpimax and started to grow, but was then partially dissolved when Cpi became Cpimin < Ccriticai, assuming that some seeds could remain. If two such seeds remained close together, a tandem-like structure could grow when the polymer concentration increased again to Cpi > Ccriticai - Such a multipart structure may thus be the result of nucleation by self-seeding.

In comparison to other conventionally produced semicrystalline thermoplastics, PHB and the HB-HV copolymers have remarkably low nucleation densities in the absence of self-seeding or deliberately introduced nucleants (Table 5.6). This low nucleation density, which in practice means that it is possible to grow spherulites several millimetres in diameter on crystallization from the melt, is attributed to the purity of the fermentation-produced polymer, and in particular to the absence of inorganic catalysts residues. This absence of heterogeneous nucleation has led to a great deal of academic interest in the PHBV range of polymers as systems for the study of homogeneous nucleation kinetics, but the poor nucleation of the pure polymer is also of commercial... 

Self-nucleation is a technique to produce self-seeds or self-nuclei within a polymer melt, so that its nucleation can be greatly enhanced. In principle, the best nucleating agent for any polymer should be its own crystallographically ideal crystal fragments or chain segments with a residual crystal memory [30-33]. 

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