Copolymers, triblock type

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). 

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

Taking into account that pK°a=5.15 and pK° i3=8.58 for MAA and N,N-dimethylaminoethylmethacrylate (DMAEM), respectively, an S-shaped curve of pHiEp vs. the acid content has been obtained theoretically. The theoretical curve fits well with the experimental one for A-type copolymers. Small deviations from the theoretical curve are observed for copolymers of type B and C and can be explained by variations of pK° g and pK° i, due to the nearest-neighbour interaction which depends on the distribution of the acidic and basic monomers. The values of pHipp of the DMAEM-MMA-MAA triblock copolymer [26] calculated from Eq. 4.14 form the theoretical curves of Fig. 2 ... 

One straightforward way to obtain such microemulsion-mediated polymer networks is to add a triblock copolymer of type ABA to the microemulsion, where the A blocks are favorable to the dispersed phase and the B block to the continuous phase of the microemulsion. By doing so one may expect that the different A blocks will be located in different aggregates of the microemulsion and will thereby lead to a transient network treated theoretically in some detail in Refs. 129-133. 

Although the synthetic strategy using non(homo)polymerizable monomers has been shown to be highly effective for the synthesis of a variety of di- or triblock copolymers, ABA type linear triblock copolymers can also be prepared by coupling... 

RAFT-CTAs can be prepared in a similar way and thus combined with ROMP for the synthesis of well-defined block copolymers. ABA-type triblock polymers of 1,5-cyclooctadiene (by ROMP) and St or tBA (by RAFT) were synthesized by Mahanthappa et In the first step of the process, the synthesis of telechelic PBs possessing trithiocarbonate-functionalized termini was performed using the specially designed difanctional CTA containing double bond in the middle. The RAFT polymerizations of conventional monomers mediated by the telechelic polymers gave rise to PSt-b-PB-l7-PSt and PtBA-l -PB-l7-PtBA (Scheme 68). 

Further examples of polymers used to solubilise hydrophobic compounds are polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock-type copolymers. Such polymers form micelles in solution with the more hydrophobic PPO chains forming the iimer core and the more hydrophilic PEO chains the outer shell. Hydrophobic materials are able to dissolve within the core of the micelles and such systems are finding increasing... 

As discussed previously, thermoplastic elastomers are materials which have the functional properties of conventional vulcanized rubbers but which may be processed as normal thermoplastics (see section 2.9). Effects of this kind are shown by styrene-butadiene block copolymers. Two types of styrene-butadiene block copolymers are produced commercially, namely triblock and radial block copolymers. The triblock copolymers (denoted by SBS) consist of a centre block of butadiene units with two terminal blocks of styrene units. The radial block copolymers (denoted by (SB) X) consist of three or more styrene-butadiene diblock copolymers radiating from a central... 

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). 

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. 

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-... 

Similar types of lamellar morphologies were observed for triblock copolymers of diphenylsiloxane and dimethylsiloxane having 40 wt% polydiphenylsiloxane, using electron microscopy, 47-148>. The lamellae thickness was approximately equal to the chain length of the rigid polydiphenylsiloxane blocks. These copolymers showed elastomeric properties comparable to those of conventional silica-reinforced, chemically crosslinked silicone rubbers. Tensile tests yielded an initial modulus of 0.5-1 MPa, tensile strength of 6-7 MPa and ultimate elongation between 400 and 800 %. 

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] ... 

Linear triblock copolymers of the type styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) are produced commercially by anionic polymerization through sequential addition of monomers in the reaction chamber [10] as shown below ... 

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... 

Recently, Tong et al. [195] have shown that in methyl methacrylate-b-alkyl acrylate-b-methyl methacrylate (MAM) and SIS-type triblock copolymers, the ultimate tensile strength is inversely proportional to the molecular weight between the chain entanglements in the middle soft block at comparable proportion of the outer block. 

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

---