Polystyrene segment

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Adhesives, Coatings, and Sealants. Eor these appHcations, styrenic block copolymers must be compounded with resins and oils (Table 10) to obtain the desired properties (56—58). Materials compatible with the elastomer segments soften the final product and give tack, whereas materials compatible with the polystyrene segments impart hardness. The latter are usually styrenic resins with relatively high softening points. Materials with low softening points are to be avoided, as are aromatic oils, since they plasticize the polystyrene domains and reduce the upper service temperature of the final products. 

In general, block copolymers are heterogeneous (multiphase) polymer systems, because the different blocks from which they are built are incompatible with each other, as for example, in diene/styrene-block copolymers. This incompatibility, however, does not lead to a complete phase separation because the polystyrene segments can aggregate with each other to form hard domains that hold the polydiene segments together. As a result, block copolymers often combine the properties of the relevant homopolymers. This holds in particular for block copolymers of two monomers A and B. 

Table 4.7. The molar masses of the polyazoester , the polystyrene containing azo groups and the individual polystyrene segments 58)...

The dynamic viscoelasticity and the thermal behaviour of films of Thermoelastic 125 cast from solutions in four solvents - toluene (T), carbon tetrachloride (C), ethyl acetate (E), and methyl ethyl ketone (M) — have been studied by Miyamato133 The mechanical loss tangent (tan 8) and the storage modulus E dependences exhibit two transitions at —70 °C and 100 °C which have been attributed to onset of motion of polybutadiene and polystyrene segments, respectively. The heights of the polybutadiene peaks on tan 6 curves decrease in the order C > T > E > M, while for polystyrene the order is reversed C < T < E < M. These phenomena have been related to the magnitude of phase separation of the polystyrene and polybutadiene blocks. 

Ionomers are copolymers in which a small portion of the repeat units have ionic pendant groups on usually a nonpolar backbone. The ionic groups tend to separate themselves into domains similar to the polystyrene segments in the SBS rubber because they are insoluble in the nonpolar polymer chains. Therefore, these ionic clusters serve as cross-links up to temperatures where they tend to disassociate. Most commercial grades of ionic elastomers are based on ethylene and propylene monomers. 

In this system using a polystyrene containing block copolymer, the polystyrene segment should readily partition into the lipophilic polystyrene particle core while the poly(FOA) or PDMS block is solubilized in the CO2 continuous phase to provide steric stabilization and prevent coagulation. In comparison of the polystyrene-b-poly(FOA) diblock copolymers to the polystyrene-b-PDMS diblock copolymers, it was found that the use of a polystyrene-b-PDMS stabilizer gives much more monodisperse particles. This most likely arises from the synthetic technique employed in the surfactant synthesis. The blocks in the polystyrene-b-PDMS block copolymers have a much narrower polydispersity than the blocks in the polystyrene-b-poly(FOA) block copolymers. It was noted that the particles obtained in... 

The copolymers of styrene and DVB discussed so far were made via free radical polymerization, which is known to produce crosslinked polymer with a very broad distribution of X in the polystyrene segments between cross-link junctions. On the other hand anionic polymerization of styrene monomer [140] to give the corresponding living dicarbanion n-mer1 followed by addition of DVB monomer gives a crosslinked network with a very narrow range of X (i.e. twice the number of monomer units) between nodules of (DVB)y. 

Swelling data for Sty-co-DVB polymers that have very-narrow-range molecular weight distributions for the poly(styrene) segments between poly(DVB) nodules have been reported by Rempp [141-143] and his coworkers. The size of these nodules and the number of polystyrene segments covalently bonded to a given... 

Fig. 16. Correlation of S in mL of sorbed liquid per gram of poly(Sty-Wock-DVB) (prepared via anion polymerization by Rempp [143] in 1970) with the corresponding calculated cube root of the number of backbone carbon atoms in the polystyrene segments between nodules of DVB...

Another example of an alternative rubber system is the asymmetric radial polymer (ARPS). ARPS has four equal arms of polybutadiene, with a polystyrene segment attached to one of the polybutadiene arms. A HIPS product made with ARPS blends polybutadiene produces two separate rubber phases with different morphologies and particle size distributions. The ARPS produces a capsular morphology and the polybutadiene produces a normal cellular morphology surrounded by a lamellar structure that provides a reactor product with both high gloss and high impact. 

A transformation method can introduce some functional groups at the junction as in B-63, which bear a fluorescent dye between the polyisoprene and polystyrene segments.375 The preparation is based on quenching the living anionic polymerization of isoprene with l-(9-phenanthryl)-l-phenylethylene followed by addition of excess a,a -dibromo-/>xylene, which affords a C—Br terminal effective for the copper-catalyzed radical polymerization of styrene. 

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