Thermoplastic elastomers from polybutadienes

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. 

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. 

This investigation started as a continuation of research into aspects of grafting from1). Our original intention was to prepare thermoplastic elastomers by grafting polystyrene branches from lightly chlorinated polybutadiene backbones in conjunction with alkylaluminum coinitiators ... 

FIGURE 13.8 Schematic illustration of phase separation in a thermoplastic elastomer based on a styrenic triblock copolymer such as SBS. The isolated spherical domains containing the polystyrene end blocks form the hard phase, which acts as both intermolecular tie point ("physical crosslinks ) and filler. The continuous phase from the polybutadiene midblock imparts the elastomeric characteristics to this polymer. 

Important commercial uses of block copolymers depend on phase separation in the solid state. For example, triblock copolymers PS-b-PB-b-PS (PB = polybutadiene) that contain a long PB block form glassy domains of PS (Tg=100°C) within a matrix of low Tg PB (Tg -100°C). The glassy PS domains function as physical crosslinks, which prevent the PB chains from slipping past one another under deformation. This generates elastomeric properties but, unlike normal elastomers which are permanently chemically crosslinked, heating above the Tg of the PS block allows the material to be reprocessed. This reversibility has led to the term thermoplastic elastomer for these materials, which are known as Kratons and are sold commercially [39]. 

The flow behavior of block copolymers differs from that of the parent homopolymers. Let us first examine the temperature dependence of the viscosity rj for the thermoplastic elastomers. Below the glass transition temperature of polystyrene (about 110 C) the triblock material has a viscosity intermediate between that of the parent homopolymers, as shown in Figure 4.22. This is normal and expected. However, at a temperature where flow is well developed in the polystyrene, 140 C, an inversion occurs, the block copolymer assuming the higher viscosity (Holden et a/., 1969b). The reason for this inversion lies in the difficulty of pulling styrene blocks out of their normal phase and into and through the polybutadiene phase, and vice versa. Motions of this type are required for viscous flow, and... 

The most extensively studied block copolymers prepared by anionic polymerization are the styrene-butadiene or styrene-isoprene rubbers. Shell Chemical Company s Kraton thermoplastic elastomers are ABA block copolymers of this type. Their elastomeric properties are excellent, yet they differ from other rubbers in that vulcanization is not required. These elastomers consist of a rubbery polybutadiene matrix with the styrene segments serving as anchors in thermoplastic microdomains. 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be molded at high temperatures because of the presence of the glassy domains that act as physical cross-links. In solution, attachment of a water-soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution, that differ from either parent polymer. 

It was pointed out in Section 2.16.9 that anionic living polymerisation can be used to prepare ABA tri>block copolymers suitable for use as thermoplastic elastomers. In such copolymers the A blocks are normally of a homopolymer which is glassy and the B block is of a rubbery homopolymer (e.g. a polydiene such as polybutadiene or polyisoprene). The characteristic properties of these materials stems from the fact that two polymers which contain repeat units of a different chemical type tend to be incompatible on the molecular level. Thus the block copolymers phase separate into domains which are rich in one or the other type of repeat unit. In the case of the polystyrene-polydiene-polystyrene types of tri-block copolymers used for thermoplastic elastomers (with about 25% by weight polystyrene blocks), the structure is phase-separated at ambient temperature into approximately spherical polystyrene-rich domains which are dispersed in a matrix of the polydiene chains. This type of structure is shown schematically in Fig. 4.36 where it can be seen that the polystyrene blocks are anchored in the spherical domains. At ambient temperature the polystyrene is below its Tg whereas the polydiene is above its Tg. Hence the material consists of a rubbery matrix containing a rigid dispersed phase. 

The characteristic properties of the styrene-butadiene thermoplastic elastomers stem from the inherent incompatibility of the polystyrene and polybutadiene blocks. In the bulk material the two different blocks aggregate into separate domains leading to a dispersion of polystyrene domains (as the lesser component) in a continuous matrix of polybutadiene. At ordinary temperatures the polystyrene domains are rigid and immobilize the ends of the polybutadiene segments, in effect serving both as cross-links and filler particles. Thus the system has the strength of a filler-reinforced vulcanized amorphous elastomer (see Table 20.1). At temperatures above the glass-transition temperature of polystyrene (100°C), the polystyrene domains are readily disrupted and the material may be melt-processed. The maximum service temperature of components fabricated from the copolymers is about 60°C. 

The anionic arm-first methods can also be applied to the synthesis of star block copolymers [59]. The procedure is identical except that living diblock copolymers (arising from sequential copolymerization of two appropriate monomers, added in the order of increasing nucleophilicity) are used as living precursor chains. The active sites subsequently initiate the polymerization of a small amount of a bis-unsaturated monomer (DVB in most cases) to generate the cores. If polystyrene and polyisoprene (or polybutadiene) are selected, the resulting star block copol)miers behave as thermoplastic elastomers because of their different glass transition temperatures. 

We have previously alluded to two major commercial applications of block copolymers the addition of AB diblock copolymers in blends of homopolymers, A and B, to improve the processability and mechanical properties of the mixture, and the use of ABA-type triblock copolymers as thermoplastic elastomers when block A (e.g., PS) is below its glass transition, whereas block B (e.g., polybutadiene) is above its glass transition at ambient temperature. Other products made from block copolymers include pressure-sensitive adhesives, oil additives, and automobile parts. 

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). 

The choice of date range is arbitrary. The number of journal articles for each year was obtained from a search of electronic version of English-based polymer and polymer-related journals using the keywords polyolefin and blends. Within polyolefin keyword, the subkeywords used in the search were polyethylene (PE, LLDPE, LDPE, HDPE, UHMWPE, PE, etc.), polypropylene (PP, iPP, sPP, aPP, etc.), polybutene-1, poly-4-methylpentene-l, ethylene-diene monomer, ethylene-propylene-diene terpolymer, ethylene propylene rubber, thermoplastic olefins, natural rubber (NR), polybutadiene, polyisobutylene (PIB), polyisoprene, and polyolefin elastomer. For the polyolefin blends patent search, polymer indexing codes and manual codes were used to search for the patents in Derwent World Patent Index based on the above keywords listed in the search strategy. 

Methods have also been developed to evaluate the average MWs by analyzing the fragments produced with SIMS from the surfaces of polymer materials. Studies conducted by Galuska on a variety of hydrocarbon elastomers (polyisoprene [PIP], polybutadiene [PBD], and PIB) and thermoplastics (polyethylene [PE], PS, polypropylene [PP], and poly(1-butene) [Pl-B]) [133] demonstrated that the relative intensity of the protonated monomer (F) can be correlated with the average MWs (M ), according to the general relationship ... 

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