Triblock polymers

Polymer-templated titania gels were also prepared. Monolithic titania wet gels were immersed in a solution of hydrophilic polymers, triblock-copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly (ethylene oxide) [Pluronic, M.W. 12,600, Sigma Chemical Co.] or a PEG (M.W. = 4,000 and 20,000, Sigma Chemical Co.), under an atmospheric pressure before drying [29]. The dried gel films were annealed at 500°C for 2 h. 

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

Because graft copolymers are much "easier" to obtain synthetically than heterogeneous diblock or triblock copolymers, they have also been used as compatibiUzers ia polymer blends. Theoretically, they are not as efficient as the diblocks (60), but they are successhilly and economically used ia a number of commercial systems (61). 

The outstanding morphological feature of these rubbers arises from the natural tendency of two polymer species to separate one from another, even when they have similar solubility parameters. In this case, however, this is restrained because the blocks are covalently linked to each other. In a typical commercial triblock the styrene content is about 30% of the total, giving relative block sizes of 14 72 14. At this level the styrene end blocks tend to congregate into spherical or rod-like glassy domains embedded in an amorphous rubbery matrix. These domains have diameters of about 30 nm. 

Hydrogenated SBS triblock polymers have become increasingly important (Kraton G by Shell). With the original polybutadiene block comprised of 65% 1,4-and 35% 1,2-structures the elastomeric central block is equivalent to that of a high-ethylene ethylene-butene rubber. 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

With these polymers hard blocks with T s well above normal ambient temperature are separated by soft bloeks which in the mass are rubbery in nature. This is very reminiscent of the SBS triblock elastomers discussed in Chapter 11 and even more closely related to the polyether-ester thermoplastic elastomers of the Hytrel type deseribed in Chapter 25. 

We present here a simple experiment, conceived to test both the reptation model and the minor chain model, by Welp et al. [50] and Agrawal et al. [51-53]. Consider the HDH/DHD interface formed with two layers of polystyrene with chain architectures shown in Fig. 5. In one of the layers, the central 50% of the chain is deuterated. This constitutes a triblock copolymer of labeled and normal polystyrene, which is, denoted HDH. In the second layer, the labeling has been reversed so that the two end fractions of the chain are deuterated, denoted by DHD. At temperatures above the glass transition temperature of the polystyrene ( 100°C), the polymer chains begin to interdiffuse across the... 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

A new process to develop interface vulcanization is grafting of selective accelerators onto a polymer chain, which in the subsequent process of vulcanization acts as an effective cure accelerator for the second polymer component in the blend. Beniska et al. [6] prepared SERFS blends where the polystyrene phase was grafted with the accelerator for curing SBR. Improved hardness, tensile strength, and abrasion resistance were obtained. Blends containing modified polystyrene and rw-1,4-polybutadiene showed similar characteristics as SBS triblock copolymers. 

In the preceding sections, our discussion has been limited to softer grade elastomer-plastic vulcanizates. Commercial interest, however, also centers on another major family of polymer blends, semi-rigid impact resistant polyolefins. Thus, we report some of our findings on PRP triblock copolymer and EVA rubber blends without... 

The preparation of ABA triblock polymers requires use of a telechelie bisthiol prepared by termination of anionic polymerization initiated by a difunctional initiator. The relative yields of homopolymer, di- and triblock obtained in these experiments depend critically on conversion.273... 

Poly(dimethyl siloxanc) with vinyl or hydrosilanc (Si-H) chain ends have been converted to ATRP initiator ends e.g. Scheme 9.62) by hydrosilylalion, Bis-functional dimethyl siloxane polymers prepared in this way were used in polymerizations of S, MA, tsobornyl acrylate and BA to form ABA triblock copolymers. 

AABB polyimides, synthesis of, 300-302 AA-BB-type polymers, 135 AA-BB-type sulfonylation, 330 AA monomers, 11-12 A-B-A triblock copolymers, 7 A-B copolymers, 7 AxBy monomers, 8 AB polyamides, 173-180 AB polyimides, 304-307 syntheses of, 306 Abrasion resistance test, 243-244 ABS. See Acrylonitrile-... 

The gel permeation chromatogram shown in Fig. 6 illustrates the purity of a block copolymer obtained by ion coupling. It is seen that about 5% of uncoupled block copolymer contaminates a triblock copolymer of narrow molecular weight distribution. The synthesis of star block polymers owes its recent development to the use of new coupling agents412. ... 

Polystyrene-polytetrahydrofuran block copolymers121122 are an interesting case of coupling between functional polymers The mutual deactivation of living anionic polystyrene and living cationic polyoxolane occurs quantitatively to yield polystyrene-polyoxolane block copolymers. Since either of the initial polymer species can be mono- or difunctional, diblock, triblock or multiblock copolymers can be obtained. 

Vinyl copolymers contain mers from two or more vinyl monomers. Most common are random copolymers that are formed when the monomers polymerize simultaneously. They can be made by most polymerization mechanisms. Block copolymers are formed by reacting one monomer to completion and then replacing it with a different monomer that continues to add to the same polymer chain. The polymerization of a diblock copolymer stops at this point. Triblock and multiblock polymers continue the polymerization with additional monomer depletion and replenishment steps. The polymer chain must retain its ability to grow throughout the process. This is possible for a few polymerization mechanisms that give living polymers. 

Triblock copolymers of ABA type, where B is the central elastomeric block and A is the rigid end-block, are well-known commercially available polymers [7,8]. The chemical structures of some common TPEs based on styrenic block copolymers are given in Eigure 5.1. Synthesis of such ABA-type polymers can be achieved by three routes [9] ... 

Weiss et al. [75] have synthesized Na and Zn salt of sulfonated styrene(ethylene-co-butylene)-styrene triblock ionomer. The starting material is a hydrogenated triblock copolymer of styrene and butadiene with a rubber mid-block and PS end-blocks. After hydrogenation, the mid-block is converted to a random copolymer of ethylene and butylene. Ethyl sulfonate is used to sulfonate the block copolymer in 1,2-dichloroethane solution at 50°C using the procedure developed by Makowski et al. [76]. The sulfonic acid form of the functionalized polymer is recovered by steam stripping. The neutralization reaction is carried out in toluene-methanol solution using the appropriate metal hydroxide or acetate. 

This polypeptide is structurally identical to ABA-type triblock copolymer with a central hydrophdic elastomeric end-block capped with two hydrophobic plastic end-blocks and exhibits amphiphilic characteristics. The end-blocks of the polymer were chosen in such a way that their LCST would reside at or near room temperature. Thus the polymer exhibits phase separation, which is analogue to conventional TPEs, and offers TPE gels under physiological relevant conditions [104]. Glutamic acid residue is placed periodically in the elastomeric mid-block to increase its affinity towards the aqueous... 

FIGURE 5.7 Phase separation in styrene-butadiene-styrene (SBS) triblock copolymer. The isolated spherical styrene domains form the hard phase, which act both as intermolecular tie points and filler. The continuous butadiene imparts the elastomeric characteristics to this polymer. MW = molecular weight. (From Grady, B.P. and Cooper, S.L., Science and Technology of Rubber, Mark, J.E., Erman, B., and Eirich, F.R. (eds.). Academic Press, San Diego, CA, 1994. With permission.)... 

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