Polyisoprene star-branched

Fig.5. The elastic modulus G (co) and dissipative modulus G (co) for linear top) and three-arm-star branched (bottom) polyisoprene from [5]. Note the broad range of relaxation times indicated by the width of the peak in the star-polymer...

A Chromatix low-angle light scattering G.P.C. detector was also employed for the determination of the weight average molecular weight (Mw), as well as a sensitive detector, in order to assay absolutely the Mw versus elution volume profile for a series of star-branched polyisoprenes and polybutadienes. The results indicate that under optimum conditions a relatively well defined number of arms can be achieved with DVB linking. 

In 1965, Milkovich (. ) reported that divinylbenzene could be utilized for the formation of star-branched macromolecules. Later, Rempp and coworkers (2, 3, 4) successfully applied this method for the synthesis of star-branched polystyrenes. Moreover, Fetters and coworkers (54 ) used this procedure for the synthesis of multi-arm star-branched polyisoprene homopolymers and poly-... 

In comparison to the polybutadiene stars under similar reaction conditions, the polyisoprene stars showed slightly lower degrees of branching. The added steric hindrance from the methyl group on the polyisoprene anion perhaps makes entry into the DVB "microgel" nodule difficult. 

Masuda et a/.[30] reported data collected for a series of polystyrene star polymers that seemingly conflict with the discovery made by Quack and Fetters [27]. They showed that the viscosity of polystyrene star polymers was dependent on the number of arms. Specifically, they showed that viscosity increased with the number of branches for a series of polystyrene stars with Mw, arm = 55 000 g/mol and the number of arms ranging from 7 to 39. However, the level of arm entanglement for the polystyrene stars was far lower than that of the polyisoprene stars studied by Quack and Fetters [27]. 

The efficiency of the linking reactions of polychlorosi-lanes with poly(dienyl)lithium compounds has been documented by synthesis of well-defined, narrow molecular weight distribution, 18-armed star-branched polyisoprenes, polybutadienes, and butadiene end-capped polystyrenes by linking reactions with a decaoctachlorosilane [(SiCl)ig] [256, 257]. The linking reactions of poly(butadienyl)lithium (Mjj = 5.3-89.6 x 10 g/mol) with carbosilane dendrimers with up to 128 Si-Cl bonds have been reported to proceed... 

Fig. 2. Angular scattering functions for tegular star-branched polymers with f = 2, 4,6, 30. Experimental points, six-braiK ted star polyisoprene (Ref.

Similar results are obtained for linear, four and six-branched polyisoprenes at a concentration of 0.145 g/ml. In this case, however, at hi r concentrations more serious deviations occur from theory. The higher molecular weight samples can have zero shear viscosities higher than linear polymers of the same molecular wei t Such behavior was first noted in a study of melt viscosity of regular star-branched polybutadienesViscosities of the order of one hundred times that of a linear equivalent could be observed, but the effect decreased rapidly on dilution with solvents i.e. the viscosities of branched polymers were more sensitive to concentration than those of linear polymers. Star-branched polyisoprenes show viscosity enhance-... 

In solutions of star-branched polyisoprenes, the characteristic time r, is greater than for linear polymers of the same molecular weight when c and M are sufficiently high that both ijp and are enhanced by the branching. However, the ratio cf. equation 21 of Chapter 13) is not only nearly independent of c and M but also nearly the same for linear, four-arm, and six-arm branched polymers. ... 

E. von Meerwall, D. H. Tomich, N. Hadjichristis, and L. J. Fetters. Phenomenology of self-diffusion in star-branched polyisoprenes in solution. Macromolecules, 15 (1982), 1157-1163. 

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

Pearson, D. S., Mueller, S. J., Fetters, L. J., Hadjichristidis, N. Comparison of the rheological properties of linear and star-branched polyisoprenes in shear and elongational flows. /. Polym. Set, Polym. Phys.Ed. (1983) 21, pp. 2287-2298... 

Watanabe, H., Matsumiya, Y, Osaki, K. Tube dilation process in star-branched cis-polyisoprenes. /. Polym. Sci. (2000) 38, pp. 1024-1036... 

Changes in molecular architecture can have an effect on domain structure and dimensions but is not as important as might be expected. Price et al showed that the structure and dimensions of linear and star-branched (polystyrene-h/oc/c-polyisoprene) X copolymers are indpendent of the value of n for n= 1, 2, 3 and 4. This result has recently been confirmed by Alward et al However, changes in domain structure are observed when n > 8, in which case the formation of an ordered bicontinuous structure occurs. 

For star polymers the temperature at which 2 = 0 is lower than the Floiy 0-temperature of the linear polymer and depends on the molecular weight and the functionality [103]. Several studies have confirmed this phenomenon for polystyrene stars [37,38] and polyisoprene stars [41-43]. All studies have assumed that the lower temperature of the star polymer is exclusively due to their branched structure and is not affected by the increased weight fraction of foreign chemical groups present in the multiple chain ends or in the central coupling unit of the star. There is experimental evidence that such groups affect the observable [104]. Furthermore, in the discussion it should be kept in mind that determination of 0 usually has a 1 to 2 K accuracy. 

Polystyiene-polybutadiene Polybutadiene-poly(a-methyl styrene) Polybutadiene-poly(vinyl naphthalene) Polystyrene-polybutadlene-polystyrene Polybutadiene-polystyrene-polybutadiene Polystyrene-polyisoprene Pblystyrene-polyisoprene-polystyrene Polyia>prene-poly(vinyl-2-p dine) PofyiK>prene-poly(vinyl-4-pyridine) Polyisoprene-poly(methyl methacrylate) Polystyrene-poly(butadiene or ia>prene)-polystyrene Star polystyrene-polybutadiene with 4 branches Star polybutadiene-polystyrene with n branches Star polystyrene-polybutadiene with n branches Star polystyrene-polyisoprene with n brandies Polystyrene-polyisoprene-poly(vinyl-2-pyridine) Polystyrene-poly vinyl-2-pyridine) Polystyrene-poly(vinyl-4-pyridine) Poly(vinyl-2-pyridine)-poly(vinyl-4-pyridine)... 

For the diblock copolymer, which exhibits a flow region at longer times than pure polyisoprene, the relaxation of the isoprene sequence is treated like the relaxation of the arm of a star polymer. We have followed the description proposed by McLeish [17, 18] for star homopolymers. The distribution of relaxation times is given by Eq. (9), where Mb is the molecular weight of one branch (here the molecular weight of the polyisoprene sequence), s ranges be-... 

FIGURE 1.7 Plots of viscomelric branching parameter, g, versus branch functionahty, p, for star chains on a simple cubic lattice (unfilled circles), together with experimental data for star polymers in theta solvents , polystyrene in cyclohexane , polyisoprene in dioxane. Solid and dashed lines represent calculated values via Eqs. (1.70) and (1.71), respectively. (Adapted... 

---