Confined crystallization in block copolymers

It is now firmly established that confinement of crystalline stems has a profound influence on crystallization in block copolymers. Confinement can resnlt from the presence of glassy domains or simply strong segregation between domains. In contrast crystallization can overwhelm microphase separation when a sample is cooled from a weakly segregated or homogeneons melt (152-154). The lamellar crystallites can then nncleate and grow heterogeneonsly to produce spherulites (152,155), whereas these are not observed when crystallization is confined to spheres or cylinders. 

In the following sections, we will discuss the principal aspects and advances of the crystallization within confined geometries. In Table 12.1, we have listed recent works dealing with confined crystallization within block copolymers, polymer droplets, and polymer blends however, previous references are also discussed in the text below. 

Loo Y.-L., Register R. A., and Ryan A. J. (2002) Modes of crystallization in block copolymer microdomains Breakout, templated, and confined. Macromolecules 35 2365-2374. 

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 ... 

Many papers have been published in the last 20 years that deal with fractionated crystallization and confined crystallization in polymer blends or in block copolymers blended with homopolymers. Some of these studies are briefly mentioned here while more recent works are treated in more detail. 

Strength needed to confine crystallization within spheres). Given the substantial chemical and physical differences between the EO/BO, E/SEB, and E/MB systems, the classification map in Figure 6.10 should be a useful guide for determining the conditions needed to confine crystallization in any semicrystalline-rubbery block copolymer. 

Quiram D. J., Register R. A., Marchand G. R., and Adamson D. H. (1998) Chain orientation in block copolymers exhibiting cylindrically confined crystallization. Macromolecules 31 4891-4898. 

Equally important is the description of the stmcture-properties relationship in block copolymers, which has been predominantly focused on the analysis of thermal and mechanical properties for various morphologies. The properties of block copolymers in different application tidds were explored thdr behavior in polymer blends and as compaobitizets, thdr improved toughness and use as TPEs, and the crystallization studies imder the confinement within microdomains. A section has been dedicated to block copolymer thin films, where these systems find multiple applications. Alignment techniques have been introduced for both bulk and thin film materials, since the control over the sdf-assembly has various relevance according to the application. 

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. 

From this section we can summarize the general behavior of confined crystallizable MDs. These generalizations apply to block copolymers that are in the strong segregation regime and that can crystallize within their specific MD without breakout. When a block copolymer component crystallizes within isolated MD structures like spheres, cylinders or lamellae it may nucleate homogeneously. For homogeneous nucleation to take place, several requirements should be met ... 

Several of the ABC triblock copolymers with two crystallizable blocks that have been studied include PE as one of the crystallizable components. The PE block can be found either at the end (PE-fo-PS-b-PCL [94], PE-fr-PEP-fo-PEO [101,119]) or at the center (PS-fr-PE-fr-PCL [98]). When the PE block is located at the center of the copolymer, as is the case in PS-fo-PE-fr-PCL triblock copolymers [94], there are higher constraints on the PE block owing to the absence of free ends. If the PE block is a minor component, confined crystallization with possible homogeneous nucleation is usually encountered. It may be possible that when the PE block does not have free ends, it may be... 

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... 

Floudas et al. [135] also studied the isothermal crystallization of PEO and PCL blocks within PS-b-PEO-h-PCL star triblock copolymers. In these systems the crystallization occurs from a homogeneous melt Avrami indexes higher than 1 are always observed since the crystallization drives structure formation and does not occur under confined conditions. A reduction in the equilibrium melting temperature in the star block copolymers was also observed. 

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