Elastomers, branching

The authors gratefully acknowledge the interest and sponsorship of this work by T. A. Werkenthin, head of the Elastomers Branch, Bureau of Ships, Navy Department, Washington, D. C. 

The authors would like to extend their appreciation to Dr. A. L. Logothetls for the Invitation to prepare and present this paper at the Rubber Division Session of the Macromolecular Secretariat Symposium 1983, to Dow Corning Corporation for permission to write this paper, to the Fluids, Lubricants, and Elastomers Branch, Nonmetalllc Materials Division, Air Force Materials Laboratory for supporting the f1uorohydrocarbon alkyl spacer work In part under contract No. F33615-69-C-1301 and for permission to Include this work In this paper, and to their many colleagues who carried out the described work (some of which has not been previously published outside the patent literature). 

Desirable properties of elastomers include elasticity, abrasion resistance, tensile strength, elongation, modulus, and processibiUty. These properties are related to and dependent on the average molecular weight and mol wt distribution, polymer macro- and microstmcture, branching, gel (cross-linking), and... 

Polyester polyols are based on saturated aHphatic or aromatic carboxyHc acids and diols or mixtures of diols. The carboxyHc acid of choice is adipic acid (qv) because of its favorable cost/performance ratio. For elastomers, linear polyester polyols of ca 2000 mol wt are preferred. Branched polyester polyols, formulated from higher functional glycols, are used for foam and coatings appHcations. Phthalates and terephthalates are also used. 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

The properties of the solvent-bome CR adhesives depend on the molecular weight, degree of branching and rate of crystallization of the polymer. The ability of polychloroprene adhesives to crystallize is unique as compared to other elastomers. The higher the crystallization rate, the faster the adhesive strength development. 

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... 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

Spot tests, 245 Spray elastomers, 204 Star-branched polymers, 187 Star copolymers, 7... 

Markel E.J., Weng W., Peacock A.J., and Dekmezian A.H. Metallocene-based branched-block thermoplastic elastomers. Macromolecules, 33, 8541, 2000. 

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Multiarm-star polyisobutylene-polystyrene thermoplastic elastomers from a novel multifunctional initiator, Polym. Mater. Set Eng., 82,42 3, 1999. Brister, L.B., Puskas, J.E., and Tzaras, E. Star-branched PIB/poly(p-t-bu-Styrene) block copolymers from a novel epoxide initiator, Polym. Prepr., 40, 141-142, 1999. 

Isobutylene-based elastomers include HR, the copolymer of isobutylene and isoprene, halogenated HR, star-branched versions of these polymers, and the terpolymer isobutylene-p-methylene styrene-bromo-p-methyl styrene (BIMS). A number of recent reviews on isobutylene-based elastomers are available [33-35]. 

In SBR, the benzene rings protrude like branches from the mbber backbone and absorb molecular vibrations caused by the elastomer chain touching the road surface thereby improving the traction properties. In ENR, the epoxide group performs a similar function like the benzene rings pendant from the backbone. 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

Poly(olefins), long chain branching 1980s 1990s Elastomers, plastomers ... 

These are the linear or slightly branched long ehaln moleeules capable of repeatedly softening on heating and hardening on eoolmg. These polymers possess intermolecular forces of attraetlon intermediate between elastomers and fibres. Some eommon thermoplastics are polythene, polystyrene, polyvinyls, etc. 

Macromers have been used to produce thermoplastic elastomers. Generally, the backbone serves as the elastomeric phase while the branches serve as the hard phases. These structures are often referred to as comb -shaped because of the similarity between the rigid part of the comb and its teeth and the structure of these graft polymers. 

The macroradicals of natural rubber react with those of the styrene elastomer, due to the presence of the very reactive 1,2 pendent vinyl groups in the latter. This mechanism leads to a structure where the styrene rubber forms a gel network with grafted branches of natural rubber. 

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