Branched polystyrenes rheology

Unlike radical polymerization, branched polystyrenes having a variety of controlled structures have been synthesized (Figure 24.10). This is because termination can be precisely controlled. The branched polystyrenes synthesized using anionic chemistry have been used to study the effect of branch structure on rheology [15]. As will be discussed in the next section, branch architecture (like those presented in Figure 24.10) can influence the rheological properties of polystyrene resins. 

T. Neidhoefer, S. Sioula, N. Hadjichristidis, and M. Wilhelm. Distinguishing linear from star-branched polystyrene solutions with Fourier-transform rheology. Macromol. Rapid Communications, 25 (2004), 1921-1926. 

Styrene polymerizes spontaneously on heating, but samples for use in research are made by anionic polymerization. Of particular interest are structures having well-defined branching structures [28-31]. Several rheological studies of branched polystyrenes are discussed in later chapters. 

Ferri, D., LomoUini, P. Melt rheology of randomly branched polystyrenes. /. Rheol. (1999) 43, pp. 1355-1372... 

Namba, S., Tsukahara, Y., Kaeriyama, K., Okamoto, K., Takahashi, M. Bulk properties of multi-branched polystyrenes from polystyrene macromomers Rheological behavior I. Polymer (2000)... 

In a later study, Roovers and Graessley [39] investigated the role that molecular weight between branches plays in the rheology of polystyrene combs. Two backbone polymers with molecular weights of 275000 and 860 000 g/mol... 

Very few studies have been performed investigating the effect of branching on the extensional rheological properties of polystyrenes. Such investigations can be valuable because many of the fabrication operations associated with commercial applications of polystyrene include operations in which the polystyrene melt undergoes an extensional deformation. Some examples are extruded foam sheet, blown film, oriented (tentered) sheet, and thermoforming. The types of deformations associated with these processing operations are best described as... 

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-... 

Polymers with a star-like topology have attracted interest for many years. The rheological behavior in the melt and in solution of starpolymers differs from the behavior of linear polymers [172]. Polystyrene starpolymers with selectively deuterated core or corona chains were investigated by SANS and it was found that the chains are more stretched within the core (or close to the branching point), while the outer parts of the chains follow the single chain behavior of linear polymers [173]. This result confirmed theoretical predictions by Daoud and Cotton [174] and Birshtein et al. [175]. A similar behavior was found for the chain conformations in star-like block copolymer ionomer micelles, which were studied by SANS, too [176]. 

T. Masuda, Y. Ohto, M. Kitamura, etal. Rheological properties of anionic polystyrenes. 7. Viscoelastic properties of six-branched star polystyrenes and their concentrated solutions. Macromolecules, 14 (1981), 354-360. 

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]. 

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