Elastomers from polybutadienes

Polybutadiene. The many forms that can result from the polymerisation of butadiene, depending on the catalysts used, include high cis, medium cis, low cis, and high vinyl polybutadiene (PBD) (see Elastomers, synthetic-polybutadiene). 

Because of the absence of the methyl pendant group, cto 1,4-polybutadiene is more resistant to abrasion and develops less heat buildup as a result of cyclic flexing than Hevea rubber (polyisoprene NR). Tires made from polybutadiene do not grip the road as well as those made from Hevea elastomers. 

Elastomers from Myrcene-Based Polyols. A series of polyurethanes was formed using a PM polyol in admixture with various amounts of 1,4-butanediol (BD) and reacted with MDI (>/). For comparison, a corresponding series of elastomers based on a commercially available polybutadiene (PB) polyol (Arco R45-HT, Cornelius Chemical Company) was also prepared. Characterization data of the PB and PM polyols are given in Table III. 

FIGURE 9.17 Dependence of productivity and separation factor /3p C6H5CH3/H2O of membranes based on various rubbery polymers on the glass transition temperature of the polymer (pervaporation separation of saturated toluene/water mixture, T = 308 K) (1) polydimethyl siloxane (2) polybutadiene (3) polyoctylmethyl siloxane (4) nitrile butadiene rubber with 18% mol of nitrile groups (5) the same, 28% mol of nitrile groups (6) the same, 38% mol of nitrile groups (7) ethylene/propylene copolymer (8) polyepichlorohydrin (9) polychloroprene (10) pol3furethane (11) polyacrylate rubber (12) fluorocarbon elastomer. (From analysis of data presented in Semenova, S.I., J. Membr. Sci., 231, 189, 2004. With permission.)... 

Early attempts at preparations of synthetic rubbers resulted in developments of elastomers from 2,3-dimethylbutadiene. The material, called methyl rubber, was claimed to yield better elastomeric properties than polybutadiene. Methyl rubber was produced in Germany during World War I where the monomer was prepared from acetone. The polymerizations were carried out by free-radical mechanism and anionically, using sodium metal dispersions for initiatimi. Later, it was demonstrated that 2,3-dimethyl polybutadiene can be polymerized to very high cis-, A polymer with Ziegler-Natta catalysts [125, 126]. 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

FIGURE 1.12 Master curve of tear energy Gc versus rate R of tear propagation at Tg for three cross-linked elastomers polybutadiene (BR, Tg — —96°C) ethylene-propylene copolymer (EPR, Tg — —60°C) a high-styrene-styrene-butadiene rubber copolymer (HS-SBR, Tg — —30°C). (From Gent, A.N. and Lai, S.-M., J. Polymer Sci., Part B Polymer Phys., 32, 1543, 1994. With permission.)... 

FIGURE 9.18 (continued) (b) Resilience measurements of elastomers. Samples of chlorobutyl rubber (CIIR), polybutadiene rubber (BR), and cross-linked recombinant resilin. (From Elvin, C.M., Carr, A.G., Huson, M.G., Maxwell, J.M., Pearson, R.D., Vuocolol, T., Liyou, N.E., Wong, D.C.C., Merritt, D.J., and Dixon, N.E., Nature, 437, 999, 2005.)... 

An elastomer is a polymer that is flexible, allowing it to be distorted from one shape to another. Polyisoprene (natural mbber), polybutadiene, and butadiene-styrene copolymer are the most important commercial elastomers. All contain some CDC bonds, and their bulk properties are affected by the varying geometries about the carbon atoms that make up the polymer backbone. 

The use of lightly crosslinked polymers did result in hydrophilic surfaces (contact angle 50°, c-PI, 0.2 M PhTD). However, the surfaces displayed severe cracking after 5 days. Although qualitatively they appeared to remain hydrophilic, reliable contact angle measurements on these surfaces were impossible. Also, the use of a styrene-butadiene-styrene triblock copolymer thermoplastic elastomer did not show improved permanence of the hydrophilicity over other polydienes treated with PhTD. The block copolymer film was cast from toluene, and transmission electron microscopy showed that the continuous phase was the polybutadiene portion of the copolymer. Both polystyrene and polybutadiene domains are present at the surface. This would probably limit the maximum hydrophilicity obtainable since the RTD reagents are not expected to modify the polystyrene domains. 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

The information on physical properties of radiation cross-linking of polybutadiene rubber and butadiene copolymers was obtained in a fashion similar to that for NR, namely, by stress-strain measurements. From Table 5.6, it is evident that the dose required for a full cure of these elastomers is lower than that for natural rubber. The addition of prorads allows further reduction of the cure dose with the actual value depending on the microstructure and macrostructure of the polymer and also on the type and concentration of the compounding ingredients, such as oils, processing aids, and antioxidants in the compound. For example, solution-polymerized polybutadiene rubber usually requires lower doses than emulsion-polymerized rubber because it contains smaller amount of impurities than the latter. Since the yield of scission G(S) is relatively small, particularly when oxygen is excluded, tensile... 

The basis for cross-linking in the elastomers in these studies is the addition of thiols to olefmic double bonds. This reaction has been studied by a number of investigators.61-65 It has been shown that the addition reaction proceeds by a radical mechanism. The rate of cross-linking of polybutadiene can be strongly promoted by relatively small amounts of polythiols.66 For example, G(X) values increased from 5 (pure polybutadiene) to 29 when 1 wt. percent of dimercaptodecane was added and to more than 49 on the further addition of o-dichlorobenzene (see Table 5.2). 

It appears from the evolution of the adhesion index that a distinction has to be made between the interactions carbon blacks are able to have with unsaturated or with saturated (or near-to-saturated) elastomers. Thus, the adhesion index of butyl rubber is enhanced upon oxidation of the black, while the reverse is observed with polybutadiene 38). The improvement of the reinforcing ability of carbon black upon oxidation, in the former case, has been interpreted by Gessler 401 as due to chemical interactions of butyl rubber with active functional groups on the solid surface. Gessler, relating the reinforcing characteristics of the oxidized carbon black for butyl rubber to the presence of carboxyl groups on the surface of the filler, postulated a cationic... 

The carboxyl terminated polybutadiene (C-3000) is about equally effective to CTBN in heat distortion temperature and impact but considerably less effective in strength. From the haze data (the percent haze of ERL-4221 modified with 10 phr of CTBN and C-3000 were 17 and 85% respectively) it is quite clear that this elastomer (C-3000) is highly incompatible with the epoxy-hardener system in the cured state. A 2000 molecular weight polybutadiene elastomer, containing no carboxyl groups, was completely incompatible with the epoxy system and segregated in the cured state. 

One further example might be the thermoplastic elastomer, triblock copolymer of polystyrene, polybutadiene, and polystyrene, in that order. The polybutadiene in this case has been grafted with poly (methyl methacrylate). The proposed nomenclature is shown in Equation 17. For emphasis, the notation is read from top to bottom, and from left to right. 

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