Copolymers ABC triblock

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. 

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. 

Recent progress in novel micellar structures, including micelles containing exotic blocks such as natural or synthetic polypeptides and metal-containing segments, micelles from ABC triblock copolymers, Janus micelles and other noncentrosymmetric micelles, micelles based on interpolyelectrolyte or other noncovalent complexes, and metallosupramolecular micelles, will be discussed in Sect. 7. 

In the following discussion, block copolymers will be simply designated by the acronym A-B for a diblock copolymer, A-B-A for a triblock copolymer with two identical outer blocks, A-B-C for an ABC triblock copolymer, etc. A complete list of abbreviations for the A, B, and C polymer blocks is given in the Abbreviations and Symbols section. 

Fig. 3 TEM (top) and AFM phase contrast images (bottom) of aqueous micelles formed by a PS200-P2VP140-PEO590 ABC triblock copolymer at pH > 5 (left) and pH< 5 (right). For TEM pictures, the PS and P2VP blocks have been stained by Ru04. AFM images have been recorded with tapping mode (contrast scale black 0°, white 45°). Adapted from [47]...

The vast majority of block copolymer micelles has been constructed from AB diblock copolymers. However, ABC triblock copolymers have attracted a great deal of interest due to the huge number of different morphologies that have been observed so far in bulk and because the introduction of a third block may introduce interesting new functionalities. Although many investigations have... 

Micelles of type (1) were the first investigated examples of ABC triblock copolymer micelles. These micelles are generally characterized by the so-called onion, three-layer, or core-shell-corona structures, i.e., the first insoluble A block forms the micellar core, the second insoluble B block is wrapped around the core, and the third soluble C block extends in the solution to form the micellar corona (Fig. 18). To the best of our knowledge, there are no known examples of ABC block copolymer micelles with A and C insoluble blocks and a B soluble block. 

Core-shell-corona micelles were formed by PEHA-PMMA-PAA triblock copolymers in water, as demonstrated by Kriz et al. [266]. Ishizone et al. [267] synthesized ABC triblock copolymers containing 2-(perfluorobutyl)ethyl methacrylate, tBMA, and 2-(trimethylsilyloxy) ethyl methacrylate with various block sequences. These copolymers were converted into amphiphilic sys-... 

Fig. 18 Schematical representation of different types of micelles formed by ABC triblock copolymers. Core-shell-corona micelles with insoluble core and shell (a), core-shell-corona micelles with radially compartmentalized corona (b), and Janus micelles with laterally compartmentalized corona (c)...

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

---