ABC block terpolymers

The experimental studies on phase behavior and pattern formation reviewed here have been done on substrate-supported films of cylinder-forming polystyrene- foc -polybutadiene diblock (SB) [36, 43, 51, 111-114] and triblock (SBS) [49, 62, 115-117] copolymers (Table 1), lamella-forming polystyrene- /ocfc-poly(2-vinyl pyridine) diblock copolymers (SV) [118, 119] and ABC block terpolymers of various compositions [53, 63, 120-131], In simulation studies, a spring and bid model of ABA Gaussian chains has been used (see Sect. 2) [36,42, 58, 59],... 

A large variety of ABC block terpolymers have been synthesized by anionic polymerization using in most of the cases sequential monomer addition as described recently in book chapters and review articles.Herein only some selective examples concerning the various pathways and recent results dealing with important solution properties will be given. 

Stefik. M.. Mahajan, S., Sai, H., Epps, T, Bates, E.S., Gruner, S.M. et al. (2009) Ordered three- and five-ply nanocomposites from ABC block terpolymer raicrophase separation with niobia and aluminosilicate sols. Chemistry of Materials, 21, 5466-5473. 

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... 

Polymers containing three types of monomers are called terpolymers (Fig. 1.8). Examples of random terpolymers are polyampholytes containing positive, negative and neutral monomers. An example of block terpolymers are ABC triblocks shown in Fig. 1.8. 

An ABC triblock terpolymer was successfully synthesized by a sequential three-step RAFT polymerization process of N-n-propylacrylamide, N-isopropylacrylamide and N,N-ethylmethylacrylamide monomers [41], The different cloud points of the respective blocks, present in the terpolymer, are responsible for the rich temperature depended solution self-assembly of the sample in aqueous media. 

In the concept of water soluble ABC block copolymers one has also to mention a huge amount of work that has been presented on the synthesis and solution behavior of copolymers with two hydrophilic blocks and one hydrophobic. However, the presence of a permanently hydrophobic block in this type of block polymers makes difficult their categorization as DHBCs. The synthesis of ABC triblock terpolymers with at least two hydrophilic blocks has been realized via a number of polymerization methodologies, like GTP and cationic polymerization, and has been studied in detail [42,43,44,45,46,47,48,49]. In most of the cases, the terpolymers were based on suitably functionalized methacrylate monomers and have been produced by the sequential monomer addition method. 

In contrast, the order of monomer addition is critical among monomers with different reactivities. As described in Section 5.1, a more-reactive chain-end anion is produced by a less-reactive monomer, and vice versa. Accordingly, less-reactive monomers should first be polymerized, followed by the polymerization of more-reactive monomers. In the block copolymer of styrene and MMA, for instance, it is necessary first to polymerize styrene, after which MM A is polymerized to prepare the second block, as the chain-end enolate anion produced by MMA cannot initiate the polymerization of styrene. Similarly, and for the same reason, the synthesis of P(2)-b-PMMA is possible only by the addition of 2-vinylpyridine first, and then MMA. For the successful design and synthesis of block copolymers, the pJ values of the conjugated acids of chain-end anions, as well as the e- and a-values of monomers (as mentioned above) are valuable guides. The details of almost all block copolymers synthesized to date, using living anionic polymerization, have been summarized by Quirk and Hsieh [190]. With the monomer addition order in mind, ABC triblock terpolymers composed of PS (A), PB (B), and PMMA (C), as well as PS (A), poly(2-vinylpyridine) (P(2VP)) (B), and P BMA (C), could be successfully... 

PEO-Br macroinitiators are often used to prepare ABC terpolymers through the macroinitiator approach. An interesting example wherein this approach has been utilized is the preparation of PEO-PDEAEMA-PHEMA (HEMA 2-hydroxyethyl methacrylate). The synthesis was performed in a one-pot reaction with sequential addition of monomers DEAEMA and HEMA. Accordingly the PHEMA block was converted by esterification, using excess succinic anhydride in pyridine, yielding the final product PEO-PDEAEMA-PSEMA. This triple-hydrophilic block terpolymer exhibits pH-responsive self-association. It forms three types of micelles, the corona of which changes its nature from cationic protonated PDEAEMA (low pH) to natural PEO (intermediate pH) and to anionic neutralized PSEMA (high pH), as a function of solution pH. ... 

A remarkable example reported by Huang indicates versatility of the transformation approaches in block copolymer synthesis. In his recent work, an ABC triblock terpolymer (PSt-Z -PEO-l7-PtBA) was prepared by a combination of three... 

This methodology was also employed for the synthesis of ABC triblock terpolymer of L-lactide (LL), N, N-dimethylacrylamide (AAm), and St. Degradation of the PEL segment of the block copolymer resulted in the formation of nanoporous material.In the synthetic process, first anionic polymerization of lactide initiated by benzyl alcohol in the presence of triethyl aluminum yielded PEL with hydroxyl terminus, which was converted to a CTA with the aid of thionyl chloride. The macro-CTA was then utilized in the successive polymerization of AAm and St in the presence of a free radical source generating a triblock copolymer, PEE-l -PAAm-l7-PSt (Scheme 46). 

Another type of remarkable double crystalline materials that have been synthesized and characterized by Balsamo et al. are ABC triblock terpolymers composed of polystyrene, polyethylene and poly(ir-caprolactone) (PS-5-PE-6-PCL or SEC) [51-54,57,63]. The morphology, nucleation and crystallization of such copolymers have been recently reviewed [3]. It is interesting to mention that in such terpolymers the PE block induces an antinucleation effect [3,63] on its covalently bonded neighboring PCL block, a remarkable effect that has only been observed in this tjq >e of triblock terpolymer. Diblock copolymers or triblock terpolymers with two crystallizing blocks can display aU possible effects from the nucleation point of view of one crystallizing block on the other. One block can cause nucleation of the other, or cause no effect, or in the other extreme of behavior induce antinucleation. In addition to the... 

Rgure 5 Schemes for different lamellar morphologies of ABC triblock terpolymers. Upon decreasing the volume fraction of the middle block it changes from lamellae via cylinders to spheres. 

This morphological transition is induced by a change of the interfacial tensions between the middle block and the end blocks. While the interfacial tension between S and B is close to the one between B and M, the situation changes strongly for S and EB, and EB and M. This leads to a displacement of the spheres or cylinders at the lamellar interface in the S-B-M block copolymers and induces curvature into the interface between the outer blocks. This scenario is schematically shown for an ABC triblock terpolymer in Eigure 10. 

Figure 10 Scheme for the change of curvature of the intermaterial dividing surfaces by changing the relative interactions between the middle and the outer blocks in an ABC triblock terpolymer via chemical modification of B to B (xab = Xbc. Xab < Xb c)-... 

An interesting question relating AC diblock and ABC triblock terpolymers is the influence of the B block on the microphase separation between A and C. Annighofer and Gronski [163,164], as well as Hashimoto et al. [165], reported on the morphological properties of ABC triblock terpolymers where B consisted of a random or tapered block of A and C. Kane and Spontak [130] found in their theoretical work that a random A/C middle block can enhance the mixing of the outer blocks due to an increase of the conformational entropy of the middle block. A similar result was obtained for symmetric ABC triblock terpolymers, where B forms either spheres, cylinders, or a lamella between the lamellae of the A and C blocks [166]. Erukhimovich et al. [167] studied the influence of a very short strongly incompatible C block on the ODT of an ABC and ACB block copolymer within the WSL. It was found that in both cases, for certain compositions and certain relative incompatibilities between C and the other two blocks, a stabilization of the disordered phase can occur as compared to the pure AB diblock copolymer. 

Rgure21 Schemes for blends of lamellar ABC and BC block copolymers (A gray, B dark, C white), (a) Macrophase separation between BC diblock copolymer and ABC triblock terpolymer, (b) centrosymmetric double layers of BC and ABC, (c) centrosymmetric mixed layers of BC and ABC, (d) nonlamellar superstructure of BC and ABC. 

Besides a macrophase separation between the two block copolymers, also blends with the sequence ABC CB BC CBA can occur (a centrosymmetric structure of double layers of both diblock and triblock terpolymers). Another possibility is a kind of random sequence between BC and ABC block copolymers, which will occur when the C blocks of both diblock and triblock terpolymer do not show any preferential mixing with either C block. In this case, an aperiodic superstructure will be obtained. Another possible superstructure is the incorporation of the BC diblock with the same molecular orientation into the ABC structure, which will lead to a real effective increase of the volume fractions of both C and B with respect to A. Thus, a lamellar superstructure may only be expected for small volume fractions of diblock copolymer. For larger amounts of diblock chains a lamellar superstructure will be disfavored with respect to a superstructure with curved intermaterial dividing surfaces, such as cocontinuous, cylindrical, or spherical morphologies. Experiments on blends of lamellar S-B-M with lamellar B-M [242], and also of lamellar S-B-T with lamellar B-T, when all blocks were of about the same length proved the existence of the last case in Figure 22. 

Either the A blocks prefer the formation of common microdomains with A blocks of the other species, while the corresponding C blocks tend to phase separate from each other, or vice versa. The different possibilities are shown in Figure 23. The formation of mixed domains of different A blocks or C blocks is due to an entropic gain caused by the reduction of chain stretching for one species. This leads to a depression of the overall free energy as compared to the macrophase separated state (where only similar blocks from the same species form common microdomains). In fact, in ABC triblock terpolymers there are AB and BC interfaces, while in AC diblock copolymers there is an AC interface. 

Adding C block, a third component, to the end of an AB diblock copolymer produces an ABC linear triblock terpolymer. The syntheses of ABC triblock terpolymers in good quality are not so easy because each step of the sequential living polymerization of A, B and C monomers contains a chance to produce byproducts such as homopolymers and AB diblock copolymers that contaminate the system. This may be one of the reasons why the number of systems studied on ABC triblock terpolymers is much smaller than that on AB diblock copolymers. Another reason is that the number of parameters that defines the mesoscopic organizations of block copolymers increases dramatically just by adding another component to diblock copolymers. Whereas an AB diblock copolymer is defined by three parameters, (/>a, Xab and A, a triblock terpolymer consisting of A, B and C blocks requires six parameters, (/>a, b = a b) ... 

Systematic smdies on the microdomain morphology of linear ABC Mblock terpolymers (hereafter called linear ABC) are limited to several systems. Linear ABC samples were predominantly synthesized by sequential living anionic polymerizations, which limit the kinds of monomers. Thus, polystyrene (PS) and polyisoprene (PI) or polybutadiene (PB) as well as their hydrogenated polymers such as poly(ethylene-a/t-propylene) (PEP) or poly(ethylene-aft-butylene) (PEB) were used as the A and B blocks, while poly(2-vinylpyridine) (P2VP), poly(4-vinylpyridine) (P4VP), poly(methyl methacrylate) (PMMA), poly(tert-buthyl methacrylate) (PtBMA), poly(ethylene oxide) (PEO) or polydimethylsiloxane (PDMS) was used as... 

Figure 18.9 An illustration to explain how network formation in ABC triblock terpolymers is induced the flat interfaces are preferred for symmetric compositions (left and right). Intermediate amounts of the C block destabilize the flat B/C interface, leading to a saddle surface element. (Reprinted with permission from T.H. Epps, E.W. Cochran, T.S. Bailey et al., Ordered network phases in linear poly(isoprene-b-styrene-b-ethylene oxide) triblock copolymers, Macromolecules, 37, 22, 8325-8341, 2004. 2004 American Chemical Society.)...

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