ABC triblocks

At r = 0.5 (Fig. 9b), the most interesting and novel morphology can be observed. This morphology can be described as follows. The P4VP cores of the microspheres form a regular structure, and a P4VP bilayer surrounds each microsphere with a honeycomb-like structure, similar to a cell wall, as the number of the microsphere surrounded by the P4VP wall ( T) was 1.08. Similar structures have been observed for ABC triblock copolymers [39]. Our honeycomb-like novel structure, however, is different from that of the ABC triblock co-... 

Boker A., Muller A.H.E., and Krausch G., Functional ABC triblock copolymers for controlled surface patterns of nanometer scale, Polym. Mater. Sci. Eng., 84, 312, 2001. 

Abstract This review highlights recent (2000-2004) advances and developments regarding the synthesis of block copolymers with both linear [AB diblocks, ABA and ABC triblocks, ABCD tetrablocks, (AB)n multiblocks etc.] and non-linear structures (star-block, graft, miktoarm star, H-shaped, dendrimer-like and cyclic copolymers). Attention is given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Fig. 57 Schematic comparison of chain conformations of the midblock for ABC and ABA triblocks. ABC triblock terpolymers (a) have bridge conformations only, whereas ABA triblock copolymers (b) have bridge and loop conformations. From [159]. Copyright 2002 Wiley...

Addition of the middle B block to an ABC triblock terpolymer has been investigated by Suzuki et al. for the PI- -PS- -P2VP system [ 159]. Starting from the lamellar structure of the unblended triblock (0ps = 0.42) PS homopolymer was subsequently added. At fas 0.50 a morphological transformation into a gryoid structure is observed. Even if the volume fraction of PS is increased up to fas = 0.60, the cell size of the gyroid structure will remain... 

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. 

Although there are very comprehensive studies that explain the behavior of amorphous ABC triblock copolymers, this is not the case when one or more of the components are able to crystallize. In this case, a much more complex behavior is expected because of the interplay of crystalhzation-microphase separation. 

It is clear from the previous discussion that a proportionality between crystallization temperature and the volume of the crystallizing phase has been found for PEO in many cases (Fig. 4), including AB and ABA diblock copolymers however, there have been reports of exceptions to this trend and they have also been included in the data compilation of Fig. 5, in particular the extensive data reported by Xu et al. [96]. In the case of ABC triblock copolymers a different behavior has also been reported and will be analyzed in detail in Sect. 5. 

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]. 

There have been relatively few reports dealing with double-crystalline diblock copolymers [102-110,197-200]. The particular case of ABC triblock copolymers with two semicrystalline blocks will be presented in a separate section. Works pertaining to one of the most studied systems PCL-fo-PEO have already been previously reviewed [43]. Recently, probably the most comprehensive studies on double-crystalline diblock copolymer systems were performed on poly(p-dioxanone)-fc-PCL diblock copolymers, PPDX-fr-PCL, and therefore several important aspects of these works [102,103,107] will be summarized in this section. 

Contributions to the subject of ABC triblock copolymer crystallization are listed in Table 1, where some characteristics of the triblock copolymers involved are reported with the corresponding references. 

Even though the first report about the synthesis of crystallizable ABC triblock copolymers was published in 1978 for PS-fo-PB-fo-PCL copolymers [114], in that work only a preliminary study of the tensile properties was performed, without considering the crystallizability of the materials. It was only 20 years later, when the preparation of these materials was reconsidered and optimized, that triblock copolymers with relatively narrow molecular weight distributions were obtained [115], a requisite which is indispensable for the generation of well-defined morphologies. To illustrate the complexity and richness of semicrystalline ABC triblock copolymers, PS-fc-PB-fc-PCL triblock copolymers have been chosen. These copolymers have been prepared with a wide composition range (with PCL contents from 11 to 77%) and they have been compared with PS-fc-PCL and PB-fo-PCL diblock copolymers [29,98, 115-118]. 

As mentioned in Sect. 3, for PEO it has been found that the crystallization temperature is often a function of the MD volume. The examples quoted in Sect. 3 referred to PEO dispersed in droplets or to PEO that was a component within diblock copolymers. For other block copolymer components like PCL the variation in Tc encountered upon MD size increase is not as pronounced. Nojima et al. [22] found that the variation of Tc for PB-fo-PCL block copolymers with spherical PCL MDs of increasing sizes, ranging from 10.3 to 17.4 nm, was of about 5 °C for crystallization at very large supercoolings (Tc fluctuated between - 50 and - 45 °C approximately). For ABC triblock copolymers, Muller et al. [29], Schmalz et al. [101,119] and Balsamo et al. [118] found, by studying copolymers with minority components of PEO or PCL blocks linked to a rubbery block, that the Tc associated with fractionated... 

Table 4 Peak crystallization temperatures determined by DSC during cooling from the melt of ABC triblock copolymers and selected homopolymers [29,101,118,119]...

A controversy has arisen as to whether the observations by POM and those by transmission electron microscopy reflect the same morphological features or not. In fact, Kim et al. [125] demonstrated that the same block copolymer can exhibit different morphologies depending on sample thickness, this being a possible reason for the sometimes contradictory results found in several works. Nevertheless, before this aspect can be properly treated in this section, we present a review of the morphological investigations carried out in semicrystalline ABC triblock copolymers at a nanoscopic scale. 

ABC Triblock Copolymers with Two Crystal I izable Blocks... 

Only a few publications deal with ABC triblock copolymers where two of the blocks are able to crystallize. The systems that have been investigated include PS-b-PE-b-PCL [94,98], PE-b-PS-6-PCL [94], PS-fc-PEO-fo-PCL [30,134-136] and PE-fo-poly(ethylene-propylene)-fr-PEO [101,119] (see also Table 1). 

Influence of Composition and Crystallizable Block Position within ABC Triblock Copolymers... 

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... 

The study of both star and linear PS-fr-PEO-fr-PCL triblock copolymers demonstrates the complexity of the crystallization behavior of ABC triblock copolymers and also the multiple possibilities of modifying the crystallization behavior of the block components by changing composition and/or molecular architecture. 

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