Polybutadiene and Its Copolymers

Homopolymers of polybutadiene can consist of three basic isomeric forms (czs-1,4, trans-1,4, and 1,2 vinyl), and these can be present in different sequential order. Copolymers may obtain a variety of co-monomers, such as styrene, acrylonitrile, etc. Depending on their distribution in the chain, random copolymers or block copolymers of different types and perfection can be produced. There are many synthetic elastomers based on butadiene available commercially. 

Upon irradiation, 1,4 polybutadienes and poly(butadiene-styrene) form free radicals relatively readily, and their concentration has been found to increase linearly proportional to dose up to approximately 100 Mrad (1,000 kGy).  

During radiolysis of polybutadiene and butadiene-styrene copolymers hydrogen and methane evolve. The incorporation of styrene as comonomer strongly reduces the total gas yield. Small amounts (typically 2 phr) of N-phenyl- 3-naphtylamine greatly reduce the gas yield and at the same time reduce G(X) considerably. 

Similar to NR, polybutadiene and its copolymers exhibit decay of unsaturation during radiolysis. Its extent depends greatly on microstructure.i Another effect observed is cis-trans isomerization.i i  

The yield of cross-links depends on the microstructure and purity of the polymer as well as whether it was irradiated in air or in vacuo2 The rate of degradation was found to be essentially zero when polybutadiene or poly(butadiene-styrene) was irradiated in vacuo, but increased somewhat when irradiated in air. 

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Tensile Strength Data from Electron Beam Cross-Linked Polybutadiene and Its Copolymers... 

Natural Rubber and Synthetic Polyisoprene Polybutadiene and Its Copolymers Polyisobutylene and Its Copolymers Ethylene-Propylene Copolymers and Terpolymers Polychloroprene Silicone Elastomers Fluorocarbon Elastomers Fluorosilicone Elastomers Electron Beam Processing of Liquid Systems Grafting and Other Polymer Modifications... 

The chains must be crosslinked to form a network (cf. Fig 7.16). In most elastomers containing double bonds, covalent bonds are introduced between chains. This can be done either with sulfur or polysulfide bonds (the well known sulfur vulcanisation of natural rubber is an example), or else by direct reactions between double bonds, initiated via decomposition of a peroxide additive into radicals. Double bonds already exist in the chemical structure of polyisoprene, polybutadiene and its copolymers. When this is not the case, as for silicones, ethylene-propylene copolymers and polyisobutylene, units are introduced by copolymerisation which have the property of conserving a double bond after incorporation into the chain. These double bonds can then be used for crosslinking. This is how Butyl rubber is made from polyisobutylene, by adding 2% isoprene. Butyl is a rubber with the remarkable property of being impermeable to air. It is used to line the interior of tyres with no inner tube. 

Polymers are seriously subject to oxygen attack. This limits their upper operating performance temperature. For elastomers these temperatures are quite low, being around 100 °C for natural rubber, 120 to 130 °C for polybutadiene and its copolymers as well as polychloroprene. EPDM at 200 °C is somewhat more stable. 

Curatives are introduced into compounds to crosslink polymer chains. The most important curative is sulfur which produces (polymer)-S -(polymer) crosslinks. This primarily involves unsaturated elastomers based on isoprene and butadiene such as natural rubber, polybutadiene, and its copolymers with styrene and acrylonitrile (see Section 1.3). After crosslinking, the polymer networks show increased retractive force and reduced creep. The cured rubber becomes insoluble and it cannot be processed in the molten state. The concentration of curatives and their reactivity affect the degree of crosshnking. 

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