Viscosity of polybutadienes

Figure 12. Variation of trans-i,4 content and intrinsic viscosity of polybutadiene with molar ratio of Bd to butylethylmagnesium (BEM). Conditions solvent, cyclohexane Mg-Al-Ba, 1.0/0.2/0.2 60°C.

This work examines the effect of long-chain branching on the low-shear concentrated solution viscosity of polybutadienes over a broad range of molecular weights and polydispersity. It will show that the reduction in molecular coil dimension arising from long-chain branching is more sensitively measured in concentrated than in dilute solutions for the polymers examined. 

We can test our predictions for the scaling correction S(N) to the zero-shear viscosity by comparing them against experimental data. In Fig. U we have plotted experimental values of the zero-shear viscosity of polybutadiene versus molecular weight as reported by Colby, Fetters and Graessley. The continuous curve in this figure was computed from Eqs.(43) and (33) using the polymer data in Table 1. The viscosity values were divided by to remove the pure reptation component. Therefore any non-zero slope in Fig. 4 represents a departure from reptation theory which corresponds to the function S(N) in Eq. (44). 

Urakawa, et al, report the viscosity of polybutadiene solutions and the principal relaxation time for polyisoprene and polyisoprene polybutadiene solutions(9). As seen in Figure 7.6a, p increases nearly exponentially in c, the slope a from a stretched-exponential fit increasing markedly with increasing matrix molecular weight. The corresponding relaxation times, all with near-exponential or stretched-exponential concentration dependences, appear in Figure 7.6b. In constructing this... 

Figure 12.12 Low-shear viscosity of polybutadiene methylnaphthalene against concentration for polymer molecular weights of 38 (O). 76( ), 100 ( ), 150 ( ), and 240 (A) kDa, and fits to the measurements, using results of Malkin, era/. (38).

The particle size of the dispersed phase depends upon the viscosity of the elastomer-monomer solution. Preferably the molecular weight of the polybutadiene elastomer should be around 2 x 10 and should have reasonable branching to reduce cold flow. Furthermore, the microstructure of the elastomer provides an important contribution toward the low-temperature impact behavior of the final product. It should also be emphasized that the use of EPDM rubber [136] or acrylate rubber [137] may provide improved weatherability. It has been observed that with an increase in agitator speed the mean diameter of the dispersed phase (D) decreases, which subsequently levels out at high shear [138-141]. However, reagglomeration may occur in the case of bulk... 

The molecular structure of polybutadiene prepared with BuLi and barium salts is greatly dependent on the presence of specific amounts of water and t-butanol used in the barium salt formation. The data in Figure 3 demonstrate that the greatest effect is obtained as the hydroxide content of the solution phase of the barium salt increased from 0 to 7.4 mole 7, Ba(0H)a. This particular salt results from a Hs0-t-butanol mixture containing 2.5 mole 7. water. The amount of trans-1,4 increased from 637, to 767, and accompanying this change the intrinsic viscosity increased from 1.60 to 5.22. The polymers were gel-free. 

The solution properties of dendrigraft polybutadienes are, as in the previous cases discussed, consistent with a hard sphere morphology. The intrinsic viscosity of arborescent-poly(butadienes) levels off for the G1 and G2 polymers. Additionally, the ratio of the radius of gyration in solution (Rg) to the hydrodynamic radius (Rb) of the molecules decreases from RJRb = 1.4 to 0.8 from G1 to G2. For linear polymer chains with a coiled conformation in solution, a ratio RJRb = 1.48-1.50 is expected. For rigid spheres, in comparison, a limiting value RJRb = 0.775 is predicted. 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

In several cases the melt viscosity of a series of lightly-branched polymers has been determined as a function of MW, and compared with that of linear polymers, and it has been found or may be deduced from the published data that there is a cross-over molecular weight, below which the branched polymer is less viscous, but above which it more viscous, than the linear polymer of equal MW. This behaviour is observed with some comb-shaped polystyrenes (35) and poly(vinyl acetate)s (59, 89), star polybutadienes (57, 58, 123), and randomly-branched polyethylenes (56,61). Jackson has found (141) that if the ratio ZJZC of the number of chain atoms at the cross-over point, Zx, to the number at the kink in the log 0 — logM curve, Zc, [as given in Ref. (52)], is plotted against nb, the number of branches, a reasonable straight line is obtained, as in Fig. 5.1. 

When there is no aggregation, the concentration dependence of the viscosity does not vary much with styrene content and block length. It corresponds somewhat to that of polybutadiene P 5. 

Baumgaertel M, De Rosa ME, Machado J, Masse M, Winter HH (1992) The relaxation time spectrum of nearly monodisperse polybutadiene melts. Rheol Acta 31(l) 75-82 Baumgarter A, Muthukumar M (1996) Polymers in disordered media. Adv Chem Phys 94 625-708. Eds I. Prigogine and S.A. Rice, Wiley, New York Berry GC, Fox TG (1968) The viscosity of polymers and their concentrated solutions. Adv Polym Sci 5 261—357... 

Research grade poly(styrene-b-butadiene-b-styrene), designated as TR-41-1647, TR-41-1648, and TR-41-1649, were received from Shell Development Co. These block copolymers contain 26.8, 29.3, and 48.2 wt% polystyrene (PS), respectively. The average molecular weights, determined by intrinsic viscosity measurements in toluene at 30°C, were found to be 7-36-6, 16-78-16, and 14-30-14 in units of thousands. The microstructure of polybutadiene (PB) blocks was found to contain about 40 mol% in cis-1,4, 50% in trans-1,4, and 10% in 1,2 units. 

We will finish this Section with some interesting experimental results on dilute polymer solutions. In Fig. 16.23 extensional viscosity data are shown, obtained from a spin-line rheometer on a solution of polybutadiene in decalin (Hudson and Ferguson, 1976). Here rie not only shows normal behaviour, starting with an increase, followed by a substantial decrease, but also at very high extensional rates a substantial increase. It is not clear yet whether this ultimate tension thickening also occurs for stiff polymeric systems. Concentration dependent extensional viscosities are presented in Fig. 16.24 for 0.0125-0.75%... 

FIG. 16.23 Extensional viscosity curve for a 6.44% solution of polybutadiene in decalin determined with a commercial spin-line rheometer. Analogous to Hudson and Ferguson (1976). 

Table 12.9 Effect of polybutadiene rubber viscosity on ESCR properties of an extrusion-grade HIPS...

The liquid hydroxytelechelic polybutadiene admixtures increase the viscosity of synthetic or mineral oils 89). However, the oxidation of double bonds is followed by a fast ageing of the product. 

Kraus has studied the steady flow and dynamic viscosity of the following branched butadiene t) ene block copolymers (88)3, (88)3, (88)4 in comparison with 888 and 888 copolymers. He has found higher viscosities (at constant molecular weight and total styrene content for polymers terminated by styrene blocks) for the former inespective of branchii, but for copolymers of equal molecular weight the viscosity is smaller for branched than for linear copolymer. Kraus has also studied the effect of free polybutadiene molecules on the viscoelastic behaviour of branched (88)4 block copolymers which consist of styrene domains in a butadiene matrix and verified De Gennes s theory of reptation ... 

The simplest way to correct viscosity data to constant friction coefficient is to first fit the temperature dependence of viscosity of each individual sample to the WLF equation [Eq. (8.134)], which determines 5//q. At a given reference temperature, sufficiently long chains have the same 5//o and progressively lower values of 5//o are obtained for shorter chains, since they have more free volume at a given temperature. The viscosity data at the reference temperature can then be corrected to the friction coefficient of the long chains at the reference temperature using Eq. (8.133). Viscosity data subjected to such a correction are shown in Fig. 8.17 for polybutadiene, polyisobutylene and polystyrene, roughly... 

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