Comb-shaped branched molecules

Recently, Yamamoto and Hyodo have employed the DPD method for studying Nafion membranes [20]. The systems considered in this study were built using two distinct molecular species, denoted comb-shaped polymer ip) and water (w). The polymer was presented as a branched sequence of beads. It consisted of a main chain (backbone) of iV = 20 effective monomer units (-CF2CF2CF2CF2 ) linked with rig = 5 short side chains of = 2 units [-0CF2C(CF3)F0 and F2CF2S03H] the total number of interaction sites in the macromolecule was Np= N/, + n xn = 30. A water-like particle was modeled as the same size as the units of the Nafion fragment (<7 = 6.1 A) and represented four water molecules. The x parameters were found using an atomistic calculation. The DPD simulation was performed for water volume... 

An alternative to a linear polymer is a branched one. The branches can be long or short. Low-density polyethylene, for instance, can have both short and long branches. Linear and branched molecules are illustrated in Fig. 1.1a and b. Branched polymers can also be star or comb shaped (Fig. 1.1c and d). In addition to the above, polymer molecules can also be double stranded. Such polymers are called ladder polymers (Fig. 1. le). It is also possible for polymers to have semiladder structures (Fig. l.lf). 

FIGURE 1.1. Shapes of polymeric molecules, (a) linear polymer, (b) branched polymer, (c) star>shaped polymer, (d) comb-shaped polymer, (e) ladder polymei (f) semiladder polymer, and (g) network structure. 

The values of the hydrodynamic diamet calculated according to [67] are reported in Tables 3.2 and 3.3 an increase in the diameter of the molecule d (15-40 A) reflects the characteristic increase in the section of comb-shaped macromolecules with long side branches, and they are in reasonable agreement with the geometric length of their side groups. 

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]. 

By use of chlorosilane chemistry, various branched structures can be prepared. For example, star-branched PBd can be prepared [27] and hydrogenated to produce analogs of star-branched polyethylene [46]. Hadjichristidis etal. [47] have reported the latest developments in the preparation of polyethylene analogs based on butadiene. Using the methods they describe, a remarkable array of structures can be produced, including stars, H-shaped molecules, super-H molecules (three-armed stars at both ends of a backbone segment), pom-poms (multiarmed stars at the ends of a backbone) and combs of various types. Rheological data have been published for the polymers they described [48]. 

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