Linear block copolymer architectures

Previous sections discussed the micellization behavior of AB or ABA linear block copolymers. With the recent progress achieved in the field of controlled polymerization techniques, more sophisticated block copolymer architectures are now available. Investigation of the micellization behavior of such... 

The criterion of stability (R) is the constant limit value of the emulsified volume percentage at 20°C (32, 32.). It appears that 7/3 water toluene emulsions are more efficiently stabilized by star-shaped block copolymers whereas linear block copolymers give better results for 3/7 water toluene emulsions the stability of the l/l water toluene emulsions seems to be insensitive to the molecular architecture of PTBS/PO block copolymers. 

These results serve to illustrate that molecular architecture significantly influences phase behaviour compared to linear block copolymers due to changes in molecular packing, and particularly interfacial area per block. This is most graphically illustrated in the phase diagram in Fig. 2.34. 

Advancements in synthetic polymer chemistry have allowed a remarkable range of new nonlinear block copolymer architectures to be synthesized. The result is a wide variety of new materials with the capacity to form self-assembled phases in bulk and in solution. At present our synthetic capabilities exceed our understanding, both theoretical and experimental, of the properties of such macro-molecular systems. We anticipate that a better understanding of structure-property relationships for these materials will lead to impressive new polymers with applications such as structural plastics, elastomers, membranes, controlled release agents, compatibilizers, and surface active agents. From the synthetic standpoint it seems likely that recent advances in living free radical polymerization will make the syntheses of many non-linear block copolymers more commercially appealing. 

FIGURE 2.1 Various possible architectures for amphiphilic copolymer (a) linear block copolymers with different numbers of A and B blocks, (b) cyclic block copolymers, (c) star block copolymers, (d) graft block copolymers, (e) block copolymers with dendritic or hyper-branched blocks, and (f) semitelechelic polymer (upper), telechelic polymer (middle), and asymmetrical telechelic polymer with different hydrophobic chain ends [9]. 

While not related exclusively to block copolymer synthesis, the formation of many of the more complex architectures available through RAFT polymerization - including those based on a single monomer - shares the characteristics and caveats of linear block copolymer formation. One technique to obtain such structures (aldn to the triblock synthesis mentioned above) is the use of higher-level, multifunctional RAFT agents. A synthetic approach with a multifunctional core or a RAFT agent-functionalized polymer backbone allows... 

Miktoaim stars consisting of one thermoresponsive PNIPAAM arm and four pH-responsive PDMAEMA arms were synthesized and their micellization behavior in aqueous solutions was compared with the corresponding linear PNIPAAM-b-PDMAEMA block copolymers.PNIPAAM-core micelles were obtained in acidic solutions at elevated temperatures, whereas PDMAEMA-core micelles were formed in slightly alkaline solutions at room temperature. Furthermore, the kinetics of pH-induced micellization of the AB4 miktoarm stars and the linear block copolymers was studied by the stopped-flow LS technique upon a pH jump from 4 to 10. The data of both types of copolymers could be fitted with double-exponential functions yielding a fast (xj) and a slow (T2) relaxation process. For both copolymers xj decreased with increasing polymer concentration. However, xj was independent of polymer concentration for the AB4 stars, whereas it decreased with increasing polymer concentration for the linear block copolymer. This result indicates that the macromolecular architecture may greatly influence the kinetics of micellization. 

On surfaces, extension of the backbone due to steric repulsion of the adsorbed side chains results in a rod-like conformation. This gives a nanometer-sized building block with well-defined shape and multiple chemical functionalities. Depending on the strength of adsorption and the molecular architecture, bmsh molecules may undergo both association and dissociation upon their adsorption to surfaces. Figure 46 demonstrates the physical association of bmsh-linear block copolymers on a solid substrate due to crystallization of octadecyl tail... 

The most popular click reaction is Huisgen 1,3-dipolar cycloaddition of azides to alkynes applicable to a very wide range of macro molecular architecture. It has been employed for the preparation of various polymer topologies including linear, star, hyperbranched, and H-shaped polymers. The general approach is illustrated in Scheme 70 for the preparation of linear block copolymer of EO with MMA and St. Anionically prepared PEO was functionalized with azide and used in copper-catalyzed click reaction with PMMA or PSt with alkyne moiety synthesized by using alkyne-flinctional ATRP initiator. It should be noted that alkyne functionality of hetero-functional ATRP initiator was protected with a trimethylsilyl... 

One advantage of the RAFT process is its eompatibility with a wide range of monomers, including fnnctional monomers. Thus narrow polydispersity block copolymers have been prepared with monomers containing acid (e.g., acrylic acid), hydroxy (e.g., 2-hydroxyethyl methacrylate), and tertiary amino [e.g., 2-(dimethylamino) ethyl methacrylate] functionality (Chiefari et al., 1998). Linear block copolymers are the simplest polymeric architecmres achievable via RAFT process. There are two main routes for the synthesis of block copolymers by the RAFT process, viz., (i) sequential monomer addition (chain extension) and (ii) synthesis via macro-CTAs (by R- or Z-group approaches). These are schematically shown in Fig. 11.37. Linear block copolymers are the simplest polymeric architectures achievable via RAFT process. 

Figure 1 Example block copolymer architectures available, (a) Linear diblock copolymer, (b) linear multiblock copolymer, (c) miktoarm copolymer, (d) star copolymer, (e) linear-graft copolymer, and (f) cyclic diblock copolymer. In all cases, the different colors represent different block chemistries.

Figure 15 Some block copolymer architectures AB diblock, ABC trl-block, ABC star, linear-graft terpolymer, two-length-scale multiblock terpolymer, and two-length-scale binary multiblock copolymer.

Fig. 10.1 Nomenclature for linear and branched block copolymer architectures of diblock and...

The mechanical behavior of these linear block copolymers is directly related to the composition and morphology (correlated stress-strain curves are presented in Fig. 3.8 in Part II). By changing the macromolecular architecture and processing conditions in block copolymers, very different morphologies with modified mechanical properties and partly new micromechanical mechanisms can be obtained (see Chapter II.3). 

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