Polystyrene—polybutadiene star-block

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]. 

Figure 12. FT-IR spectra of unsaturated and hydrogenated polystyrene/polybutadiene star-block copolymer (SBD-2).

Figure 9. Synthesis scheme for polystyrene/polybutadiene (SBD) star-block copolymers.

Figure 11. Gel permeation chromatograms of a polystyrene/ polybutadiene block copolymer (SBD-1) arm and three-arm star. The star sample was fractionated once.

Tuzar and coworkers [294] investigated the micellization behavior of styrene-butadiene star-block copolymers with four arms and polybutadiene inner blocks in the mixed solvent tetrahydrofuran/ethanol, selective for polystyrene blocks. 

Self-assembled block copolymers are basically amphilic molecules which contain distinctively different polymers. This block copolymer contains two or more polymers quantitatively in the form of blocks. Some of the block copolymers are polyacrylic acid, polymethylacrylate, polystyrene polyethylene oxide, polybutadiene, polybutylene oxide, poly-2-methyloxazoline, polydimethyl sUoxane, poly-e-caprolactone, polypropylene sulfide, poly-A -isopropylacrylamide, poly-2-vinylpyridine, poly-2-diethylamino ethyl methacrylate, poly-2-(diisopropylamino) ethyl methacrylate, poly-2-(methacryloyloxy) ethyl phosphorylcholine, and polylactic acid. These copolymers contain more than polymers to form certain configurations like linear, branched, patterned. For example, if we take three polymers named A, B, and C, they can be combined to form arrangements AB, BA, AA, BAB, ABCAB, ABCABC, ABABAB, etc. in the form of branched configuration it forms (ABQa, (ABA)a, (AB)4, etc. Depending on the above-mentioned number of blocks, they are named as AB diblock copolymers, ABC triblock copolymers, ABC star block copolymers, etc. The covalent linkage between these different blocks of polymers makes macroscopic phase separation impossible, that is, in its place the phase separation... 

Styrene-butadiene block copolymers (SBC) with a high (70-85 %) styrene content are commercially produced and marketed as transparent, stiff, and tough thermoplastic resins under the trade names of Styrolux (Styrolution), K-Resin (Chevron-Phillips), Finaclear (Total petrochemical), and Clearene (Denka-Kaguku). Unlike other more elastomeric types of styrene-butadiene block copolymers, the rigid SBC resins contain only <25 % polybutadiene rubber content. Structurally, these SBC polymers are composed of polystyrene (S) and polybutadiene (B) blocks, linked together in an unsymmetrical star-block [(S-B)x] structure. 

Among branched polymers, star polymers represent the most elementary way of arranging the subchains since each star contains only one branching point, and as such, they serve as useful models for experimental evaluation of theories about solution properties and rheological behavior of branched polymers (Angot et al., 1998). Star polymers nd applications as additives in various areas such as rheology modi ers, pressure sensitive additives, etc. Besides serving as additives, star polymers can also be used as such to achieve sped c properties. For instance, star block copolymers with polystyrene-fc-polybutadiene (PSt-fo-PB) arms have better processability and me-... 

Figure 17.8a is the TEM image of a styrene/butadiene star block copolymer containing 74% (by volume) styrene. The nanostmctured morphology arises from the fact that the constituents - that is, polystyrene (PS) and polybutadiene (PB) chains -are connected chemically by covalent bonds which do not permit a macroscopic segregation of the polymer chains. Due to the presence of a unique molecular architecture of the copolymer, the star block copolymer was found to show a cocontinuous arrangement of the nanostmctures [69-72]. The star block copolymer specimen was treated -with osmium tetroxide vapor prior to TEM imaging such treatment... 

By means of anionic polymerization, it is possible to produce in the laboratory linear polymers that are nearly monodisperse and have many types of branching such as multi-armed stars and combs and H-shaped molecules. For example, there have been reports of studies of anionically polymerized polystyrene, polybutadiene, and polyisoprene. An example of the anionic polymerization of a branched polymer is the technique of Roovers and Toporowski [22] for making comb polystyrenes. The varieties of model branched polymer that can be produced today by means of block polymerization and coupling chemistries include stars, H-shaped molecules, super-H molecules (multi-armed stars at both ends of a backbone segment), and combs of various types [23]. So-called pom-pom polymers are of special interest, because their rheological behavior has been modeled by McLeish and Larson [24]. These molecules have several arms at each end of a central crossbar, and polybutadienes having this structure have been synthesized [25,26]. 

The anionic arm-first methods can also be applied to the synthesis of star block copolymers [59]. The procedure is identical except that living diblock copolymers (arising from sequential copolymerization of two appropriate monomers, added in the order of increasing nucleophilicity) are used as living precursor chains. The active sites subsequently initiate the polymerization of a small amount of a bis-unsaturated monomer (DVB in most cases) to generate the cores. If polystyrene and polyisoprene (or polybutadiene) are selected, the resulting star block copol)miers behave as thermoplastic elastomers because of their different glass transition temperatures. 

Similar polymers, slightly less perfect, are the umbrella star copolymers [101]. These polymers are based on a central polystyrene star with 25 arms. An average of five polybutadiene or poly(2-vinylpyridine) branches are grafted onto the end of each arm. Since these polymers are models for block copolymer micelles their properties have been studied in selective solvents. In particular, the PBd-PS umbrella-star copolymers are monomolecularly dissolved in non-solvents for the core-forming polystyrene. 

In the case of mass ABS, the variety of rubber particle morphology is less diverse. Typical examples of morphology are shown in Figure 14.6. If polybutadiene rubber is used (linear or star), cellular particles are obtained with SAN occlusions. In the case of styrene-butadiene block rubber (typically 30% styrene) also cellular particles are obtained but besides the SAN occlusions, polystyrene domains are clearly visible in the particles. To be able to make the other morphologies that are possible in HIPS, the interfacial tension has to be manipulated. Controlling the grafting reaction is a way to achieve this but the possibilities are limited with the tools (mainly initiator) that are currently available. 

Now, if 75% styrene and 25% butadiene are polymerized together in a different way then, a totally different material results it is a plastic rather than a rubber. This is done by sequential, anionic polymerization in solution. As a result a material is produced which contains long lengths (or blocks) of polystyrene and long lengths (or blocks) of polybutadiene may contain up to, for example, six blocks. The molecules of the final plastic are star shaped as the initial multiblocks are coupled together by a polyfunctional material, for example, epoxidised linseed oil. (Because of the polymerization technique employed, plastics with a narrow molecular weight distribution may be produced if required). 

After the formation of the star-shaped blocks there is a phase separation of the polystyrene and the polybutadiene into domains or regions the links between the two phases are... 

The chemical modification of homopolymers such as polyvinylchloride, polyethylene, poly(chloroalkylene sulfides), polysulfones,poly-chloromethylstyrene, polyisobutylene, polysodium acrylate, polyvinyl alcohol, polyvinyl chloroformate, sulfonated polystyrene block and graft copolymers such as poly(styrene-block-ethylene-co-butylene-block-styrene), poly(1,4-polybutadiene-block ethylene oxide), star chlorine-telechelic polyisobutylene, poly(lsobutylene-co-2,3-dimethy1-1,3-butadiene), poly(styrene-co-N-butylmethacrylate) cellulose, dex-tran and inulin, is described. 

It is interesting to note that some of their electron micrographs exhibit features similar to those of a linear polystyrene-block-poly((4-vinylbenzyl)di-methylamine)-block-polyisoprene triblock copolymer with almost equal amounts of the three components, when cast from benzene [168] (Fig. 16c,d). Also a quaternary star copolymer consisting of polystyrene, polyisoprene, polybutadiene, and poly(4-methylstyrene) was reported, but no morphological characterization was given [79]. 

Fig. 3.26 Universal calibration curve for crosslinked polystyrene gels with tetrahydrofuran as solvent %y linear polystyrene 0 branched polystyrene (comb type) +, branched polystyrene (star type) A, branched block copolymer of styrene methyl methacrylate x, poly (methyl methacrylate) poly (vinyl chloride) V, graft copolymer of styrene methyl methacrylate , polybutadiene (reprinted with permission from Comprehensive Polymer Science, copyright 1989, Pergammon Press pic).

---