Polydiene elastomers

In early communications it was reported that the efficiency of dicumylperoxide (DCP) crosslinking of NR amounts to unity [85, 86], which means that one mole of chemical crosslink is obtained as the result of the thermal decomposition of one mole of DCP. This was explained by assuming that crosslinking results from the combination of polyisopropenyl radicals only and crosslinking by macroradical addition does not take place. 

In more recent studies from Gonzalez and co-workers [88-90] it was concluded from dynamic mechanical analysis of peroxide-cured NR that a non-uniform crosslinked network results if a large amount of peroxide is used. This result seems to be in line with the optical spectroscopy studies discussed. 

Fujimoto and co-workers reported in 1969 on the use of a new high-temperature ATR apparatus that they constructed for studying, among other things, the role of the type of third monomer on the peroxide curing-chemistry of EPDM [73, 74]. EPDM grades containing either ENB, DCPD, HD or MNB as a third monomer were compounded with 

DCP and uncured films were deposited on a multi-reflectance prism. Next, the ATR cell was heated to 140 °C and spectra were recorded as a function of curing time, while the angle of incidence was fixed at 45°, which minimised loss of radiation. The decrease of the absorption relating to the third monomer pendent unsaturation was monitored at 1685, 3045, 966 and 870 cm 1 for EPDM polymers containing ENB, DCPD, HD and MNB, respectively. The absorption was normalised with the methyl absorption at 1380 cm 1 which was not affected during curing. 

In the case of MNB-EPDM the absorption of the pendent unsaturation rapidly decreased within 25 minutes to about 20% of its initial value and then very slowly decreased to about 15% of its initial value. Peroxide decomposition data [95] indicate that a significant amount of peroxide was still present after 25 minutes, which means that apparently two kinetic regimes exist. Although less obvious, the data presented for the consumption of the pendent unsaturation in the case of the other EPDM grades suggest the same. 

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In view of the abundance of unsaturation in polydiene elastomers it may be expected that this type of elastomers can be cured very efficiently with peroxides. This section will show that a very high crosslinking efficiency (= moles of crosslinks formed per mole of peroxide decomposed) can indeed be obtained. However, the substitution pattern of the unsaturation plays a major role in the actual efficiency found. 

This is still the most popular method for graft copolymerization of elastomers via free radicals. Free radicals (I) are generated from the same types of initiators which are used for free radical polymerization and copolymerization (see Section 2.4). In general, these radicals are formed in the presence of a polydiene elastomer and a monomer therefore, there are several possible reactions of these initiator-derived radicals which can occur as shown in Eqs. (2.93)-(2.96). The competition between initiation of monomer polymerization (Eq. 

Styrenic block copolymers are special polydiene elastomers (polyisoprene or polybutadiene) of moderate molecular mass (7x10 to 2.5x10 ) with end blocks of polystyrene [(7-25)x 10 g/mol). They are thermoplastic elastomers At low temperature the polystyrene blocks form spherical hard domains that act as cross-links in the adhesive and result in very high cohesion. Above about 90 °C they soften to give a melt of comparatively low viscosity like a thermoplastic [211, pp. 317-373). 

In terms of stability the polydiene elastomers are the most critical due to their main-chain double bonds. Oxidative degradation is stimulated by elevated temperatures and the presence of unsaturated resins, typically used as tackiliers. 

The polydiene rubber-carbon black compoimd is one of the simplest in terms of component interaction. The major components are polymers, carbon black, and oil. The elastomer compounds are miscible or nearly so. The hydrocarbon oils dissolve roughly equally in the amorphous rubbery polydienes. The various polydiene elastomers and oils are almost as one phase. It appears they dissolve in each other at elevated temperatures and form a cross-Hnked structure together while miscible [7,8]. One problem is that the oils can distribute themselves differently among the various elastomers (Section 6.4). 

Reinforcing fillers are incorporated to improve the mechanical properties of the original polymer by strongly interacting with its molecular groups. Carbon black is often utilized for this purpose. Added to polydiene elastomers, the latter considerably improve the abrasion resistance in addition to stabilizing the polymer. 

Another group of block copolymers which has been studied is that of poly (e-caprolactam)-iilock-polydiene elastomers. Originally (see Ref [52]), hydroxy-terminated polybutadienes (PBDs) were functionalized by diisocyanates, mostly by toluene diisocyanate (TDI), 23. Polymerizations were performed first in solvents for polydienes, such as hexane or decalin [54], and later in the monomer phase [55], but neither the yields nor the mechanical properties of the products proved satisfactory. 

Legge, N.R., Holden, G. and Schroeder, H.E., Thermoplastic elastomers based on polystyrene-polydiene block copolymers. Thermoplastic Elastomers, Hanser Publishers, New York, 1987. 

Of particular interest to us was to find a method to surface modify elastomers. G. B. Butler and co-workers have demonstrated that 4-substituted-l,2,4-triazoline-3,5-diones, RTDs, readily undergo ene reactions with polydienes at ambient temperatures (13). They found that the solubility and solution properties of the modified... 

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. 

ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene B blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of elastic network, held together by the polystyrene "junctions", results in thermoplastic elastomer properties. 

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

These products find use in specialty applications requiring better thermal stability than available in the sulfur vulcanized elastomers. Other processes are also used to crosslink polydiene rubbers (Secs. 9-2c and 9-2d). 

There is a growing interest in the synthesis of star-block thermoplastic elastomers (TPEs) on account of their unique mechanical and rheological properties [71-73]. PIB-based TPEs exhibit excellent mechanical properties and have superior thermal and oxidative stabiHties relative to polydiene-based TPEs [73,74]. 

One of the most important discoveries relating to synthesis and physical behavior was made by Dr. Milkovich while at the Shell Development Co. He and his colleagues showed that triblock copolymers containing polystyrene-polydiene-polystyrene blocks in appropriate sizes could behave as a physically cross-linked but linear thermoplastic elastomer. Thus Dr. Milkovich was involved with two very crucial discoveries in this field. Interestingly, he received his M. S. degree at Syracuse with Professor Szwarc and his Ph.D. at Akron with Professor Morton. I was pleased that Dr. Milkovich accepted my invitation to be a plenary speaker at the symposium, along with Professors Szwarc and Morton. 

The living character of organolithium polymerizations makes such processes ideally suited for the preparation of pure as well as tapered-block copolymers. Diene-olefin pure-block copolymers have become important commodities because of their unique structure-property relationships. When such copolymers have an ABA or (AB) X [A = polyolefin, e.g., polystyrene or poly(a-methylstyrene) B = polydiene, e.g., polybutadiene or polyisoprene and X = coupling-agent residue] arrangement of the blocks, the copolymers have found use as thermoplastic elastomers (i.e., elastomers that can be processed as thermoplastics). 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

Elastomers are prepared by chain extension of hydroxyl-terminated low-molecular-weight polymers followed by vulcanization 180). The most important work concerns the use of hydroxy telechelic polybutadienes and polyisoprenes in the tire industry 249 252>. The hydroxylated polydienes of molecular weight 1000-20000 are mixed with a diisocyanate, a catalyst, vulcanization agent (sulfur), and accelerator, reinforcing additives (carbon black), and surface-active agents. The reaction takes place in two steps simultaneously or consecutively ... 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

An elastomer that is entirely or mostly polydiene is, of course, highly unsaturated. All that is required of an elastomer, however, is enough unsaturation to permit cross-linking. In making butyl rubber (Sec. 32.5), for example, only 5% of isoprene is copolymerized with isobutylene. 

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