Rheological Behavior of Block Copolymers

More recently, it has been demonstrated that many of the unusual rheological behavior of block copolymers will disappear when the measurements were carried out at temperatures higher than the separation temperature proposed by Leary and Williams (43). Figure 11 shows that for bulk SBS block copolymers with a composition of 7-43-7 (xlO3), the transition occurs around 145°C (especially clear at low frequencies) (76, 77). These data are consistent with those of Pico and Williams on plasticized block copolymers (78). 

As with cylinder- and lamellae-forming block copolymers, the rheological behavior of block copolymers that form spherical domains depends on whether or not the domains possess macrocrystalline order. If the domains are disordered, then the low-frequency moduli show terminal behavior typical of ordinary viscoelastic liquids that is, G and G" fall off steeply as the frequency becomes small (Watanabe and Kotaka 1983, 1984 Kotaka and Watanabe 1987). When the spherical domains are ordered, however, elastic behavior is observed at low frequency that is, G approaches a constant at low frequency, and a yield stress is observed in steady shearing. 

Finally, the rheological behavior of block copolymers serves as a model for well compatibilized blends, with perfect adhesion between the phases. The copolymers provide important insight into the effects of the chemical nature of the two components, and the origin of the yield phenomena. 

Owing to the presence of microdomain structures, the rheological behavior of block copolymers in an ordered state (at < odt) is much more complicated than that in the disordered state (at T > For instance, block copolymers with lamel-... 

The interest in the phase behavior of block copolymer melts stems from microphase separation of polymers that leads to nanoscale-ordered morphologies. This subject has been reviewed extensively (1,22-24). The identification of the structure of bicontinuous phases has only recently been confirmed, and this, together with major advances in the theoretical understanding of block copoljuners, means that the most up-to-date reviews should be consulted (1,24). The dynamics of block copolymer melts, in particular rheological behavior, and studies of chain diffusion via light scattering and nmr techniques have also been the focus of several reviews (1,25,26). 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

Block copolymers in selective solvents exhibit a remarkable capacity to self-assemble into a great variety of micellar structures. The final morphology depends on the molecular architecture, tlie block composition, and the affinity of the solvent for the different blocks. The solvophobic blocks constitute the core of the micelles, while the soluble blocks form a soft and deformable corona (Fig. Id). Because of this architecture, micelles are partially Impenetrable, just like colloids, but at the same time inherently soft and deformable like polymers. Most of their properties result from this subtle interplay between colloid-like and polymer-like features. In applications, micelles are used to solubilize in solvents otherwise insoluble compounds, to compatibilize polymer blends, to stabilize colloidal particles, and to control tire rheology of complex fluids in various formulations. A rich literature describes the phase behavior, the structure, the dynamics, and the applications of block-copolymer micelles both in aqueous and organic solvents [65-67],... 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

The melt rheology of amorphous block copolymers, e.g., styrene-butadiene block copolymers (Arnold and Meier, 1968 Holden et al, 1969a Meier, 1969), has been described and interpreted already (Section 4.11). It is interesting to compare the amorphous block copolymers with block copolymers that have the additional feature of crystallizable sequences. A basic study of block copolymer rheology was carried out by Erhardt et al (1970), who determined the complex modulus and tan 6, and studied melt behavior at temperatures between about 60 and 200°C. A report on dielectric behavior by Pochan (1971) is also significant. 

D.M. Baek, C.D. Han, Rheological behavior of binary-mixtures of a block copolymer and a homopolymer, Macromolecules 25 (1992) 3706-3714. 

SHORT REVIEWS ON THE RHEOLOGICAL BEHAVIORS OF UNFILLED BLOCK COPOLYMER SYSTEMS, 713... 

Even though thermoplastic elastomer has been widely studied [1-23], there has been relatively little serious research of the rheological behavior of the block copolymer systems [20-31]. Data on the effects of carbon black on the extrusion behavior are even more limited. In this work, we investigated the effects of carbon black on the triblock copolymer thermoplastic elastomers SBS (styrene-butadiene-styrene), SIS (styrene-isoprene-styrene), and SEES (styrene-ethylene/butylene-styrene). 

As previously mentioned, the growth of the block copolymer thermoplastic elastomer industry has now reached a high level of commercial importance. However, it is still difficult to study the rheological behaviors of thermoplastic elastomers because they exhibit complex behavior that combines the elastomeric final product properties with the processing characteristics of thermoplastics. Recently, the rheological behavior of the styrene block copolymers (SBS, SIS, and SEES) was studied by Kim and Han [31]. A series of systematic investigations for the viscoelastic behavior of triblock copolymer thermoplastic elastomers also were conducted by Mathew et al. [22,23]. Kim and Hyun [20] reported the viscoelastic properties of SEES. 

Unfortunately, for block copolymer thermoplastic elastomers, any trials to accommodate the viscosity changes by carbon blacks into theoretical considerations, or to fit the equations cited onto the rheological behavior of black-filled systems have not been conducted up to now. Of course, such trials will be relatively complex and time-consuming work because thermoplastic elastomer shows intricate properties due to its unique microstructure. 

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-... 

Nanophase separation of block copolymer melts introduces interesting rheological behavior. For example, the transient elongational viscosity behavior of a triblock copolymer of styrene and olefins was found to be strongly dependent on the initial orientation of the cylindrical domains [103]. 

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