Star-block copolymer materials

Consider a polystyrene-( )-polybutadiene star block copolymer with four arms coupled by a central Si-atom. Or consider a metal catalyst (e.g., Au) supported in activated carbon. Then the scattering of only the selected element (Si, Au, respectively) can be extracted [242], Even the distribution of the elements in the material can be mapped based on ASAXS data. A concise review of the ASAXS method in combination with AXRD and AWAXS has been published by Goerigk et al. [243]. 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

Thomas and co-workers [8] discovered an entirely new morphology on examination of multiple-arm star block copolymers. For the right combinations of arm lengths and compositions, these materials organize themselves into a bicontinuous structure, with each phase fully interpenetrating the other in a diamond lattice arrangement. In these star molecules, the arms were diblock polymers a considerably wider range of structural variation is conceivable and even possible. 

The synthesis and bulk and solution properties of block copolymers having nonlinear architectures are reviewed. These materials include star-block copolymers, graft copolymers, mik-toarm star copolymers, and complex architectures such as umbrella polymers and certain dendritic macromolecules. Emphasis is placed on the synthesis of well-defined, well-characterized materials. Such polymers serve as model materials for understanding the effects of architecture on block copolymer self-assembly, in bulk and in solution. 

In this case, an excess of living diblock arm is used to ensure complete reaction and the star material is isolated by fractionation. Star-block copolymers with very narrow molecular weight distributions have been prepared in this way (I < 1.1). These materials also exhibit a well defined functionality, close to the func-... 

The most studied materials are the star-block copolymers since they were the first synthesized. The morphology of DVB linked poly(styrene-h-isoprene) star-blocks has been studied by Bi and Fetters [ 12]. Some more recent studies on these materials have been presented by other groups [332]. 

The effect of surface constraints on the morphology of the star-block copolymers was studied by Thomas and coworkers [337]. Thin film droplets of samples with various functionalities were studied, and the ones that exhibited the OBDD structure in the bulk were found to be cylinders in this case. In an independent study, the lamellar domain spacings of 4-arm and 12-arm star-block copolymers of styrene and isoprene were found, by TEM and SAXS, to be the same as those of the arm material [338]. 

Herein, in addition to the existing baroplastics, we have shown that baroplastic tri- and star-block copolymers can be recycled multiple times. For example, recycling of P6 (Figure 8) was achieved by chopping and remolding at 400 kg cm for 30 min which was repeated five times. The fact that the resulting material is still remoldable is an indication of the recyclability of the material. 

It was also shown that baroplastic materials can be used as a processing aid to process high Tg homopolymer such as polystyrene (PS). To optimize the conditions, polystyrene and baroplastic materials in powder form were blended at different compositions and pressed using compression or extrading at room temperature. The results indicate that a room temperature moldable polystyrene/baroplastic blend can be obtained at 50 wt% baroplastic content with the di-block, tri-block and star-block copolymers. Figure 10 shows that the 50 wt% blend of PS (45K) and 4-arm star-block copolymer (98K, P8) can be... 

Baroplastic di-, tri- and star-block copolymers were shown to act as processing aids in the processing of homo-polystyrene. Detailed studies with different baroplastic materials as processing aids and the mechanical properties of the obtained materials are under investigation. 

Block copolymers adopt diverse morphologies in concentrated solutions, thin films, and bulk materials (33). Metal-containing molecular structure translates into morphological positioning on the nanoscale. PMCs are ideally suited for selective positioning of chromophore placement in films. Depending on the type of star block copolymer architecture and the nature of the arms, metal cores can be localized to one domain (star blocks), or to the inter ce between domains (heteroarm stars). 

SBS Star-block copolymer with 74% PS content in gyroid morphology (starting material for the blends of Figs. 3.39 and 3.40) [15] ... 

Star block copolymers composed of two crystallizable blocks and an amoiphous block offer new possibilities to control the block crystallization at the synthesis level and in the lab by changing the crystallization conditions. We have shown that in asymmetric stars only the longer block will crystallize. In more symmeti ic stars both blocks can crystallize but not within the same molecule. Furthermore, we have shown here with the use of time-resolved synchrotron SAXSAVAXD, that in stars with comparable block lengths of the crystallizable blocks, it is possible to delay the crystallization of a particular block for long times with a. suitable choice of temperature. These features could be important in the design of new materials where a selectivity in block crystallization is required. 

Star-block copolymers can be envisioned as star polymers where each arm is actually a diblock or a triblock copolymer. The presence of a central connecting point of the polymer chains is expected to bring about differences in the properties of the material compared to the linear diblock and triblock copolymers. Several synthetic approaches including sequential monomer addition and mechanism transformation have been used to synthesize star-block copolymers. 

Star polymers are excellent models for block copolymers dissolved in selective solvents and for end-associating polymers that form polymeric micelles [1,2]. These polymers, with their dense core and swollen corona, are of great technical importance but are difficult to study because their degree of association is highly variable. Knowledge of the properties of stars is also important for the design of the properties of star block copolymers, which are an important class of industrial materials. The conformation of the arms of a star is also the limiting conformation of polymers attached to a surface with infinite curvature (l/R ), Therefore, the properties of star polymers are related to the properties of polymer-coated colloidal particles. 

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

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