Self-nucleate

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

Once the oligomers have formed, two mechanisms, self-nucleation and aggregate nucleation, are used to describe particle nucleation. 

In self-nucleation, the extended oligomer chain collapses upon itself to nucleate a particle. 

The aggregation of oligomers requires a lower average degree of polymerization for nuclei formation than the self-nucleation model where a larger individual chain is required. 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

Fig.l Differential scanning calorimetry (DSC) cooling scans from the melt, at 10°Cmin 1, of the following materials (from top to bottom) Isotactic polypropylene (iPP) iPP after self-nucleation treatment at TS = 162°C 80/20 polystyrene (PS)/iPP melt mixed blend 80/20 PS/iPP melt mixed blend after self nucleation treatment at Ts = 161 °C 80/20 PS/iPP unmixed blend (UB), see text and atactic PS homopolymer. (From [68] with permission)... 

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. 

The technique of self-nucleation can be very useful to study the nucleation and crystallization of block copolymers that are able to crystallize [29,97-103]. Previous works have shown that domain II or the exclusive self-nucleation domain disappears for systems where the crystallizable block [PE, PEO or poly(e-caprolactone), PCL] was strongly confined into small isolated MDs [29,97-101]. The need for a very large number of nuclei in order to nucleate crystals in every confined MD (e.g., of the order of 1016 nuclei cm 3 in the case of confined spheres) implies that the amount of material that needs to be left unmolten is so large that domain II disappears and annealing will always occur to a fraction of the polymer when self-nucleation is finally attained at lower Ts. This is a direct result of the extremely high number density of MDs that need to be self-nucleated when the crystallizable block is confined within small isolated MDs. Although this effect has been mainly studied in ABC triblock copolymers and will be discussed in Sect. 6.3, it has also been reported in PS-fc-PEO diblock copolymers [29,99]. 

The technique of self-nucleation [75] can be very useful to study the nucleation and crystallization of block copolymer components, as already mentioned in previous sections. In block copolymers, factors like the volumetric fraction and the degree of segregation affect the type of confinement and therefore modify the self-nucleation behavior. In the case of semicrystalline block copolymers, several works have reported the self-nucleation of either one or both crystallizable components in PS-fc-PCL, PS-b-PB-b-PCL, PS-b-PE-b-PCL, PB-fr-PIB-fr-PEO, PE-fr-PEP-fr-PEO, PS-fc-PEO, PS-h-PEO-h-PCL, PB-b-PEO, PB/PB-fc-PEO and PPDX-fc-PCL [29,92,98,99,101-103,134] and three different kinds of behavior have been observed. Specific examples of these three cases are given in the following and in Table 5 ... 

Several block copolymer systems have shown only domains I and III upon self-nucleation. This behavior is observed in confined crystallizable blocks as PEO in purified E24EP57EO1969 [29]. Crystallization takes place for the PEO block at - 27 °C after some weak nucleating effect of the interphase. Domain II is absent and self-nucleation clearly starts at Ts = 56 °C when annealed crystals are already present, i.e., in domain III (Fig. 17b). The absence of domain II is a direct consequence of the extremely high... 

Table 5 Self-nucleation behavior for diblock and triblock copolymers ...

Fig. 17 a Classical self-nucleation behavior for polyethylene (PE) within E18EP57EO25133 triblock copolymer [101]. b Self-nucleation behavior for PEO within purified E24EP57EO1964 triblock copolymer [29], c Self-nucleation behavior for PE within S35E15C50219 triblock copolymer [29,98]. (a, c from [98,101] with permission, b Reprinted with permission from [29], Copyright 2002 American Chemical Society)... 

In those cases where the injection of self-nuclei in every MD is most difficult in view of the very large number of MDs, domain III is split into two domains. Evaluation of the self-nucleation of the PE block within S35E15C50219 shows that not only domain II is absent, but upon decreasing Ts, the PE block is annealed before any detectable self-nucleation occurs (Fig. 17c). Therefore two subdomains were defined [98] domain IIIa, where annealing without previous self-nucleation occurs and domain IIIsa> where self-nucleation and annealing are simultaneously observed for Ts < 88 °C. Domain IIIsa would be the exact equivalent to the standard domain III established by Fillon et al. [75]. 

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. 

Fig. 18 Peak crystallization temperature as a function of self-nucleation temperature for hydrogenated and nonhydrogenated S27B15C58 triblock copolymers. (Reprinted with permission from [97]. Copyright 1998 American Chemical Society)...

Bergen and Borisy (1980) used the axoneme-hased approach to explore the commitment to treadmilling, and they also found that the efficiency was quite low. An 5 value of 0.07 0.04 was obtained, corresponding to the net addition of 1.6 0.8 tubulin protomers/second. Interestingly, their estimates of the dissociation constants for the two ends were 2.2 0.6 and 3.2 0.6 iiM for the assembly and disassembly ends, respectively. We calculate that this corresponds to only about 0.2 kcal/mol. In a more recent investigation from the same laboratory (Cote and Borisy, 1981), porcine tubulin was found to exhibit an s value of about 0.26, and the rate of the tubulin flux was about 28 protomers/ second. The authors of the latter study suggested that the discrepancy might be accounted for in terms of the need to use MAP-depleted tubulin with the axoneme system to prevent self-nucleation, or in terms of the temperature differences in the two studies. 

Note Nucleation may be classified as primary or secondary. Primary nucleation can be homogeneous or heterogeneous if heterogeneous nucleation is initiated by entities having the same composition as the crystallizing polymer, it is called self-nucleation. Secondary nucleation is also known as surface nucleation. 

This boundary condition is the diffusion analog of Newton s law of cooling in heat conduction theory. A noteworthy conclusion is that a polydisperse self-nucleating sol tends to become monodisperse, i.e., the initial size distribution becomes more peaked as growth progresses. Waite (Wl)... 

Oxidation of terpenes creates low-vapor pressure species that self-nucleate to form small aerosols, or condense on and increase the mass of existing aerosols. These secondary organic aerosols (SOAs) are of concern because they can signifi-... 

This assumes that only individual radicals nucleate ("self-nucleation") primary particles. If aggregation of radicals is involved, then the first term on the right of Equation 3 should be modified to bR., where b is the reciprocal of the aggregation number in a primary particle. The absolute number of particles is obtained by integration ... 

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