Triblock

Mortensen K 1996 Structural studies of PEO-PPO-PEO triblock copolymers, their micellar aggregates and mesophases a small-angle neutron scattering study J. Phys. Condens Matters A103-A104... 

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. 

Moreover, commercially available triblock copolymers designed to be thermoplastic elastomers, not compatihilizers, are often used in Heu of the more appealing diblock materials. Since the mid-1980s, the generation of block or graft copolymers in situ during blend preparation (158,168—176), called reactive compatibilization, has emerged as an alternative approach and has received considerable commercial attention. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

Ring-Opening Metathesis Polymerization. Several new titanacyclobutanes have been shown to initiate living ring-opening metathesis polymerization (ROMP) systems. These have been used to make diblock and triblock copolymers of norbomene [498-66-8] (N) and its derivatives (eg, dicyclopentadiene [77-73-6] (D)) (Fig. 2) (41). 

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). 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

The product can be converted to a triblock copolymer by further addition of A ... 

Tbe system may be used for homopolymers and for block copolymers. Some commercial SBS triblock thermoplastic rubbers and the closely related K-resins produced by Phillips are of this type. Anionic polymerisation methods are of current interest in the preparation of certain diene rubbers. 

Figure 3.8. Schematic representation of the polystyrene domain structure in styrene-butadiene-styrene triblock copolymers. (After Holden, Bishop and Legge )...

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

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. 

Whilst, chemically, SBS triblocks are similar to SBR, for example they do not show measurable breakdown on mastication, they are seriously deficient in one respect, they show a high level of creep. This would indicate that the concept of all the styrene blocks being embedded in the domains with all of the polybutadiene blocks being in the amorphous matrix is rather too simplistic. It has also resulted in these materials not being used extensively in traditional rubber applications. One exception from this is in footwear, where blends of SBS and polystyrene have been used with noted success for crepe soles. 

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. 

Somewhat less well known are the styrene-isoprene-styrene (SIS) triblocks. The commercial grade (Cariflex TR-1107) is stated to have a styrene-isoprene... 

In addition to the somewhat sophisticated triblock thermoplastic elastomers described above, mention should be made of another group of thermoplastic diene rubbers. These are physical blends of polypropylene with a diene rubber such as natural rubber. These may be considered as being an extension to the concept of thermoplastic polyolefin rubbers discussed in Section 11.9.1 and although extensive experimental work has been carried out with these materials they do not yet appear to have established themselves commercially. 

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. 

Styrene-butadiene-styrene triblocks and the related S-I-S and SEBS materials (Section 11.8). 

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... 

Fig. 5. Labeled triblocks used in the HDH + DHD experiments. The HDH chains have their centers deuterated 50% and the DHD chains have their ends deuterated 25% on each end for a total of 50%.

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. 

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