Comb-branched polymers

Exponential dependence of ij0 on Ma, as discussed earlier for star polymers, was also seen in the H-polymers. The crossover where rj0 of polystyrene H-polymers surpasses that of its linear homologues occurs at Mw 600000g/mol. This value is significantly lower than that observed for four-arm star polystyrenes [35]. Furthermore, the average entanglement of each of the five subchains of the H-polymers at this crossover is Ma 7Me, a value far lower than the level of arm entanglement required to reach the crossover in polystyrene star polymers. 

Roovers [34] demonstrated that the added branch point found in an H-polymer significantly alters its relaxation behavior vis-a-vis a star polymer. He showed that the value of v [Equation (5)] was approximately twice that of four-arm star polymers. The time-scales for relaxation of the free-end subchains of H-polymers are very different to the relaxation time-scale associated with the cross-bar subchain that exists between the two branch points of the H-polymer. The free-end subchains of H-polymers relax in a manner nearly identical with that of the arms of a star polymer. However, the cross-bar  

The rheological properties of polymers with more complicated branch architecture have been investigated, but unfortunately such polymers are seldom of the monodisperse variety as can be found in star and linear polymers. Work on comb polymers [37-39] has shown in general that 

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 

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Scheme 6 Representative examples of hyperbranched base polymers used for sialic acid attachment and subsequent inhibition of flu virus infection. 14 Poly(ethyleneimine) (PEI) backbone 15 comb-branch polymer 16 dendrigraft polymer 17 PAMAM scaffolded on PEI...

Comb-branched polymers, e.g., poly[2-(2-methoxyethoxy)ethylglyci-dyl ether] [383], metacrylate-based polymers [384],... 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

The comb-branched polymers based on the phosphazene backbone, —(—R2P=N-),—, where R is an oligomeric poly(ethylene oxide) side chain capped by —OCH3, are amorphous polymers with very low TgS [12, 13] (Table 3.1). As mentioned earlier, a prominent example of the phosphazene polymers is MEEP. 

The ability of comb-branched polymers with low TgS to provide electrolytes with high ambient temperature conductivity is further illustrated by the LiC104 complex of poly[2-(2-methoxyethoxy]-ethylglycidyl ether (PMEEGE) [29]. Conductivities of about 10" Scm at 20°C and 10 Scm at — 20°C were obtained. 

Conventional electrolytes derived from comb-branched polymers with low glass transition temperatures. 

Fig. 7.15 Conformational relaxation time as a function of position of the dihedral in comb-branch polymer electrolytes from MD simulation...

MD simulations of comb-branch polyethers of the structure shown in Fig. 7.14 (PEPE5) have been performed [60]. SPEs formed from this comb-branch polymer have also been studied experimentally [70-72]. A comparison of the ionic conductivity of SPEs with LiTFSI and this comb-branch polymer from simulation and experiment, along with the conductivity of a linear PEO SPE with same salt concentration is shown in Fig. 7.14 [60]. Good agreement between experiment and simulation is apparent. Furthermore, the conductivity of the comb-branch SPE is very similar to that of the linear PEO SPE. The slightly higher conductivity in the linear PEO SPE is facilitated by the lower molecular weight (2,380 Da) of the linear polymer compared to the comb-branch (around 6,000 Da). 

Analysis of the MD simulations of the PEPE5 reveal that Li" occurs primarily by hopping of the cation from one side chain to another [60]. The six oxygen atoms of the side chains facilitated coordination of Li", as anticipated. However, a fraction of Li" cations are partially coordinated by the polyether backbone. These cations have very slow dynamics and do not contribute appreciably to Li" motion. The slow dynamics of cations partially coordinated by the chain backbone can be understood by considering conformational dynamics in the comb-branch polymer. Figure 7.15 shows conformational relaxation time, based on... 

Fig. 7.18 A comparison of the lithium diffusion coefficient in PEO-based single ion conductors (Fig. 7.14) and PEO-based binary comb-branch polymer electrolytes (Fig. 7.10) from simulations...

Branched macromolecules fall into three main classes star-branched polymers, characterized by multiple chains linked at one central point (Roovers, 1985), comb-branched polymers, having one linear backbone and side chains randomly distributed along it (Rempp et al., 1988), and dendritic polymers, with a multilevel branched architecture (Tomalia and Frechet, 2001). The cascade-branched structure of dendritic polymers is typically derived from polyfunctional monomers under more or less strictly controlled polymerization conditions. This class of macromolecules has a unique combination of features and, as a result, a broad spectrum of applications is being developed for these materials in areas including microencapsulation, drag delivery, nanotechnology, polymer processing additives, and catalysis. 

Figure 9.20 Hierarchical algorithm for computing the linear relaxation of the arbitrary comb-branched polymer illustrated in (a). The molecule consists of arms and backbone segments. Initially, after a step strain, only the arms can move, by primitive path fluctuationsy from the arm tips Inward. When an arm fully relaxes, it is pruned away, and replaced by a bead at the branch point to represent the frictional drag contributed by that arm see (b). Continued arm relaxation converts the molecule into a star (c), and finally a linear chain (d).The linear chain can complete its relaxation by reptation. From Park and Larson [49].

Other comb-branch polymers have been made with methacrylate " and itaconate backbones. In the latter cases, the values of the precursor and short-chain substituted polymers were rather high, again increasing upon salt addition, and showed lower conductivities than MEEP. However, conductivity values comparable with MEEP were obtained in the methacrylates with 22 units of EO per side chain (although the samples slowly crystallized). 

Comb-branch polymers with siloxane backbones have very low values and high conductivities, but the samples prepared were liquids at room temperature because of the low molecular weight. Solid products were obtained either by crosslinking or blending with PEO. 

Dual glass transition behavior is observed in immiscible binary polymer blends or block copolymers which have undergtme microphase separation to create discrete domains of each type of polymer in the rrratrix. The itaconate structures considered here are comb-branch polymers with relatively short side chains which plasticize the polymer efficiently when the chain lengdis are Cj to Cfy but then undergo a subtle change in bdiavior at long chain lengths. While there is no apparent... 

GTP, on the other hand, offers good potential for the synthesis of star-and comb-branched polymers. The GTP chain ends are living even at temperatures as high as 70°C and, thus, are capable of participating efficiently in branching reactions. Both the arm-first and core-first approaches have been reported for the synthesis of multiarm star-branched acrylic polymers. However, very few details of experimental conditions and polymer characterization have been reported for star-branched pol)uners synthesized using the arm-first approach. 

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