Triblock, nucleation

Keywords ABC triblock copolymers Block Copolymers Crystallization Homogeneous nucleation... 

Since excellent reviews on block copolymer crystallization have been published recently [43,44], we have concentrated in this paper on aspects that have not been previously considered in these references. In particular, previous reviews have focused mostly on AB diblock copolymers with one crystal-lizable block, and particular emphasis has been placed in the phase behavior, crystal structure, morphology and chain orientation within MD structures. In this review, we will concentrate on aspects such as thermal properties and their relationship to the block copolymer morphology. Furthermore, the nucleation, crystallization and morphology of more complex materials like double-crystalline AB diblock copolymers and ABC triblock copolymers with one or two crystallizable blocks will be considered in detail. 

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]. 

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... 

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)... 

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)...

The aim of this section, therefore, is to correlate systematically the compatibilization of PPE/SAN 60/40 blends by SBM triblock terpolymers with the foaming behavior of the resulting blend. The reduction of the blend phase size, the improved phase adhesion, a potentially higher nucleation activity of the nanostructured interfaces, and the possibility to adjust the glass transitional behavior between PPE and SAN, they all promise to enhance the foam processing of PPE/SAN blends. 

However, as cell growth proceeds, the physical as well as chemical constraints of the triblock terpolymers inhibit pronounced growth within the PB phase. Instead, the nucleated cells tend to grow into the SAN/PMMA phase. As the PPE/PS phase still stores a significant amount of carbon dioxide, the blowing agent is subsequently transported along the interface towards the foam cells. Apparently the PPE/PS phase still acts similar to a solid phase. 

In summary, compatibilization of PPE/SAN blends via SBM triblock terpolymers allows one to enhance significantly the homogeneity of the foam, while simultaneously reducing the cell size by heterogeneous nucleation activity of the... 

Fig. 21 Nucleation density vs particle density of PPE/SAN blends compatibilized by SMB triblock terpolymers, in comparison to uncompatibilized PPE/SAN blends...

By compatibilizing the immiscible PPE/SAN blend with SBM triblock terpoly-mers, the overall size of the dispersed PPE particles was reduced, increasing the number of potential nucleating sites and easing the incorporation of PPE in the cell walls. Moreover, nanostructured interface between PPE and SAN was formed, which turned out to be highly beneficial for cell nucleation. As a result, a dramatic increase of the cell density and reduction of the cell size was observed, keeping the density reduction at a similar level. 

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