Polybutadiene network structure

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

A structural model, based on a complex process of stretch induced ordering in the polyacetylene domains, was proposed to account for these observations. Support for this model was obtained using electron microscopic techniques. Low polyacetylene content blends (<20% PA) were found to consist of discrete polyacetylene domains dispersed in a continuous polybutadiene matrix. In the high polyacetylene content blends (>70% PA), both phases were simultaneously continuous, forming an interpenetrating network structure. Blends with intermediate compositions consist of both continuous and isolated domains of polyacetylene distributed throughout the polybutadiene matrix. 

INTRODUCTTON Polymer molecules may have a variety of architectural structures such as linear, ring, star, branched, and ladder chains as well as three-dimensional network structures. The first synthetic cychc jxjlymers to be prepared and characterized were the poly(dimethylsiloxanes) (PDMS), which were reported in 1977. Since that time a number of other cychc pwlymers have been synthesized including cyclic polystyrene, cyclic poly(phenyhnethylsiloxane), cyclic poly(2-vinylpyridine), cyclic polybutadiene, and cychc poly(vinyhnethylsiloxane). ... 

FIGURE 9.19 Plots of equilibrium modulus data for cross-linked ethylene-propylene (EP60), 1,4-polybutadiene (PBDB), and PDMS from various sources. The density of elastic chains N is represented by the symbol v. (Reprinted with permission from Graessley, W. W., Polymeric Liquids Networks Structure and Properties, Garland Science, chap. 9, New York, 2004.)... 

Hybrid versions of silicone-thermoplastic semi-IPNs have been developed (19). A hybrid interpenetrating network is one in which the cross-linked network is formed by the reaction of two polymers with structurally distinct backbones. Hydride-functionalized siloxanes can be reacted with organic polymers with pendant unsaturated groups such as polybutadienes (5) in the presence of platinum catalysts. Compared with the polysiloxane semi-IPNs discussed earlier, the hydride IPNs tend to maintain mechanical and morphologically derived properties, whereas properties associated with siloxanes are diminished. The probable importance of this technology is in cost-effective ways to induce thermoset characteristics in thermoplastic elastomers. 

Blending of polyacetylene with polybutadiene provides an avenue for property enhancement as well as new approaches to structural studies. As the composition of the polyacetylene component is increased, an interpenetrating network of the polymer in the polybutadiene matrix evolves from a particulate distribution. The mechanical and electrical properties of these blends are very sensitive to the composition and the nature of the microstructure. The microstructure and the resulting electrical properties can be further influenced by stress induced ordering subsequent to doping. This effect is most dramatic for blends of intermediate composition. The properties of the blend both prior and subsequent to stretching are explained in terms of a proposed structural model. Direct evidence for this model has been provided in this paper based upon scanning and transmission electron microscopy. 

Destruction of polybutadiene that occurs under the action of nitric acid is caused by oxidation of a polymer macromolecule. In other words, cross-section links of a spatial composite network formed by the vulcanization process are broken. The well-known oxidization ability of sulfuric acid is responsible for a decrease in double links in the rubber molecular structure, resulting in a reduction in the RubCon strength indexes. Corrosive attack by hydrochloric acid is linked to oxidation and isomerization processes and, therefore, the durability of the composite depends on the speed of these processes. 

Despite the small r values for copolymerization of styrene and the 1,2-vinyl units of polybutadiene, it is expected at styrene conversions over 95% that crosslinking copolymerization of styrene and rubber will occur. In addition, the allylic radicals formed in initiation Reaction 3 start graft copolymers of styrene monomer and polybutadiene double bonds. The existence of allylic radicals in our system is corroborated by the work of Fischer (22). In graft copolymerization, the 1,2- configuration is preferentially incorporated into the copolymer (16, 17, 18, 19) as in Structure 6. Networks are apparently formed... 

Infrared spectroscopy of peroxide cured rubbers has revealed only minimal spectroscopic information on the new cross-linked structure. What has been observed is the decrease in the intensities of the C-H out-of-plane bending modes of the olefin double bond which absorb at 837 cm l for the natural rubber and at 740 cm for cis-1,4-polybutadiene. While these bands reflect losses in the amount of unsaturation in the final material, when compared to the starting material, no evidence of the network carbon-carbon single bond absorption bands has been reported. 

Saraf, V., Glasser, W. G., Wilkes, G. L., and McGrath, G. E. Engineering plastics from lignin IV. Structure property relationships of PEG-containing polyurethane networks, Journal Applied of Polymer Science, 30, 2207 (1985). Sarkar, S. and Adhikari, B. Thermal stability of lignin-hydroxy-terminated polybutadiene-co-polyurethanes. Polymer Degradation and Stability, 73, 169-175 (2001). 

Epoxy structural adhesives which employ carboxylic polybutadiene/acrylonitrile solid and liquid (CTBN) elastomers as modifiers have increased in number and proliferated in use since their introduction in the mid- 60 s. Such adhesive systems are now used in aircraft, electronics, automotive and industrial bonding operations. In the mid- 70 s, amine-reactive versions of the liquid polymers (ATBN) were issued, thereby offering another way to introduce rubber modification into a cured epoxy network. References are cited which provide detailed discussions of nitrile rubber, carboxylic nitrile rubber and both carboxyl- and amine-terminated nitrile liquid polymers (1-4). ... 

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. 

F. De Candia, A. Taglialatela, and V. Vittoria, Structure-Property Relationships in Crosslinked Networks from cis-1, 4-Polybutadiene and Methacrylic Acid. Swelling Behavior, J. Appl. Polym. Sci. 20, 831 (1976). Simultaneous crosslinking of polybutadiene and polymerization of methacrylic acid. Swelling studied via Mooney-Rivlin equation. 

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