Miktoarm star copolymer

In miktoarm-star copolymers (or heteroarm star copolymers ), the unlike blocks are connected at one junction point, as shown in Fig. 34. As in linear-block copolymers, exactly two blocks are linked. Hence, it seems tempting to approximate those copolymers as a set of diblocks with the free ends of one block component joined together in a cluster [111]. 

However, the central junction of miktoarm-star copolymers exhibits more crowding compared to linear diblocks. The penalty for blocks bending at the interface will be higher (Fig. 34a,c), resulting in a shift of the phase boundaries and different spacings compared to their linear analogues of similar composition. 

Recently, Grason and Kamien calculated the phase diagrams in the weak and strong segregation limit for AB miktoarm-star copolymers using both... 

Fig. 35 Phase diagrams AB miktoarm-star copolymers for n = 2, n = 3, n = 4 and n = 5. mean-field critical point through which system can transition from disordered state to Lam phase via continuous, second-order phase transition. All other phase transitions are first-order. From [112]. Copyright 2004 American Chemical Society...

Fig. 37 Morphology of 4-miktoarm-star copolymers of PS(P2MP)3 type, a TEM images, b SAXS data. From [113]. Copyright 2003 American Chemical Society...

Fig. 38 Milner s diagram for morphologies of A-b-B copolymers as reported in literature [111]. Observed morphologies of linear and miktoarm-star copolymers of BS-b-P2MP system are represented. Lamellar structure, cylinders of P2MP in PS matrix spheres of P2MP in PS matrix, biphasic structure of lamellar/double gyroid microdomains. From [113]. Copyright 2003 American Chemical Society...

In the same study the coexistence of two morphologies (L and G) exhibited by a four-miktoarm-star copolymer was reported. This biphasic structure... 

The influence of the architecture on the phase behaviour of symmetric miktoarm stars AnBn (Fig. 34c) was investigated by Grayer et al. [119]. Symmetric miktoarm-star copolymers PS-arm-P2VP having a mean func-... 

In conclusion, it can be suggested that the lamellar spacing of miktoarm-star copolymers is controlled by two parameters (i) the molecular weight of the corresponding AB diblock, which also controls the segregation strength, and (ii) the functionality of the central core. 

Fig. 43 Plot of normalized lamellar long periods, Dn/Rg,m of (PS) -arm-(PI)M miktoarm-star copolymers (n = 1, 2, 4 and 16) divided by corresponding diblocks of same series, Di/Rg,u against respective star functionality, n. Normalization factor PgiM represents unperturbed radius of gyration of diblock consisting of one PS block, one PI block and average number of bonds linking these two arms through core. From [121]. Copyright 2003 American Chemical Society...

Despite the fact that Milner s theory was originally developed for miktoarm-star copolymers, it can also be adopted for more complex branched structures. This empirical concept termed constituting-block copolymers approximates the architecture of branched molecules to be composed of an array of A2B and A2B2 miktoarms. This approach is capable of predicting the morphology of architectures as complex as centipedes or barbed wires, as shown in a very recent publication [125]. 

Fig. 1. a Star polymer, b Comb polymer, c Brush, d Miktoarm star copolymer, e Star copolymer, f Star chain center-adsorbed in a plane, g Dendrimer... 

The recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

A special technique was employed by Naka et al. for the preparation of A2B stars, A being PEO and B polyoxazoline (POX) [26] according to Scheme 8. Ru(III) complexes with bipyridyl terminated polymers were utilized in this method. Characterization data were not provided for these miktoarm star copolymers. 

A different approach but still in the frame of the chlorosilane method was adopted by Tsiang for the synthesis of (A-b-B)B3 miktoarm star copolymers, where A is PS and B is PB [29]. Living PB chains were reacted with SiCl4 in a molar ratio 3 1, followed by the addition of the living diblock PS-b-PBLi. The key step of the method is the succesfull synthesis of the (PB)3SiCl intermediate product. The reduced steric hindrance of the PBLi chain end poses questions about the purity of this polymer, since several byproducts, such as (PB)2SiCl2, (PB)4Si, PBSiCl3 can be formed in the first step of the synthesis. SEC analysis was performed to monitor the reaction sequence. 

Miktoarm star copolymers of the type A5B were prepared [30] in a similar manner to the A2B and A3B type stars. The reaction sequence is outlined in Scheme 11. 

Tsitsilianis and coworkers [41] presented results on the micellization behavior of anionically synthesized amphiphilic miktoarm star copolymers with PS... 

Few theoretical studies deal with the behavior of miktoarm star copolymers in the solid state. Issues like the phase diagram and the order-disorder transitions however have been studied in considerable detail. 

A miktoarm star copolymer carrying one PS arm and two polypropylene oxide) arms126 was prepared by the method given in Scheme 56. Living PS chains were end-capped with one unit of DPE, to reduce the reactivity of the chain end, followed by the addition of epichlorohydrin to produce the epoxide functionalized polymer. It was shown by SEC analysis and chemical titration that the epoxide content of the final product was 95 wt %. The desired functionalized... 

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