Blending triblock/diblock

The substantial work on polystyrene/polybutadiene and polystyrene/ polyisoprene blends and diblock and triblock copolymer systems has lead to a general understanding of the nature of phase separation in regular block copolymer systems (5,6). The additional complexities of multiblocks with variable block length as well as possible hard- and/or soft-phase crystallinity makes the morphological characterization of polyurethane systems a challenge. 

The copolymer base of the adhesives is a blend of diblock and triblock copolymers. In Fig. 16.2 we compare the rheological behavior for two blends which have different diblock contents (54% and 85%), different molecular weights for the diblock part (86000 and 104300 g mol ), and the same styrene content (16%). The molecular weight of the triblock is 128000 gmoh. Let us describe now the relaxation domains exhibited by the two blends ... 

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

The previously discussed theories were developed for monodisperse diblock copolymers, which are not TPEs. However, Leibler s mean-field theory has been extended to include polydispersity (Leibler and Benoit, 1981) and to include triblock, star, and graft copolymers (Olvera de la Cruz and Sanchez, 1986 Mayes and Olvera de la Cruz, 1989). In the former case, polydispersity corrections tend to lower x N corresponding to the ODT. As would be expected from the analogy between blends and diblocks, triblocks will phase separate at higher xN values than the corresponding diblocks. This theory predicts a monotonic increase in the critical value of x A as the symmetry of the triblock increases, to a maximum of about 18 for the symmetric triblock. Surprisingly, the minimum xN value that separates the order and disordered regions in triblocks does not necessarily correspond to the critical point. 

Fig. 18 Composite micelles consisting of antisense oligonucleotides and (a) viral capsids or (b) synthetic polymers, (a) Micelles of DNA amphiphiles loaded with either small hydrophobic compounds top left) or with hydrophilic compounds by hybridization top right) were used to template virus capsid formation at neutral pH. TEM images show micelles incorporated into virus capsids with T = 1 or 2 geometry and an empty capsid formed at pH 5.0 as control inset). Scale bars 40 nm. (b) Representation of a blend micelle. Diblock DNA copolymer PPO-h-DNA was mixed with a triblock copolymer Pluronic (PEO-h-PPO-h-PEO) composed of the same hydrophobic block, PPO [21] (figure reproduced with permission of Royal Society of Chemistry)...

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). 

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). 

Another important class of copolymers synthesized by chain polymerisation are block (or sequenced) copolymers diblock and triblock copolymers being the most important ones. They are very useful as compatibilisers (emulsifiers) in immiscible polymer blends. Another major use is as thermoplastic elastomers. Both uses are best explained through the example of butadiene-styrene block copolymers. 

As an example of blends with attractive interactions, Fig. 65 shows a superstructure in which interactions between methacrylic acid groups and pyridine side groups of a polystyrene-fc-polybutadiene-fo-poly(f-butyl methacry-late-staf-methacrylic acid) (PS-b-PB-b-P(MAA-sfaf-fBMA)) triblock quater-polymer and a PS- -P2VP diblock copolymer lead to a wavy lamellar structure with cylinders from mixed P2VP and P(MAA-sfaf-fBMA) blocks [194],... 

ABC triblock copolymers have recently proven to be useful in constructing the so-called three-layer, onion, or core-shell-corona micelles, as described in Sect. 7.2. These micelles are characterized by a centrosymmetric structure and a micellar core with two different concentric compartments. Noncentrosymmetric structures from ABC triblock copolymers blended with AC diblocks have, however, been reported in bulk by Goldacker et al. [290]. 

Given the morphological complexity of AB diblock and ABA triblock copolymers, it might be expected that the phase behaviour of ABC triblocks would be even more rich, and indeed this has been confirmed by recent experiments from a number of groups. From a practical viewpoint, ABC triblocks can also act as compatibilizers in blends of A and C homopolymers (Auschra and Stadler 1993). In addition to the composition of the copolymer, an important driving force for structure formation in these polymers is the relative strength of incompatibilities between the components, and this has been explored by synthesis of chemically distinct materials. 

The spatial distribution of homopolymer within lamellar domains formed by the same PS-PB-PS triblock blended with homopolymer has been determined (Kimishima et al. 1995). Blends with PS or PAMS with molecular weights lower than that of the PS in the diblock were investigated using SAXS and TEM. It was found that both PS and PAMS were solubilized in the PS microdomains however, the PAMS was not uniformly distributed, tending to be localized at the domain centres. This localization was diminished on increasing the temperature. In contrast, the PS homopolymer was uniformly distributed. The localization of the PAMS was ascribed to repulsive interactions between PS chains and the PAMS homopolymer, this effect decreasing with increasing temperature. 

Until recently, very little quantitative information was available on blends of block copolymers. The literature is summarized in Table 6.3. Hoffman et al. (1970) reported microscopic demixing of blends of PS-PB diblocks, with two maxima in the domain size distribution, but with no evidence tor macrophase separation. These findings must be treated with caution in the light of more recent results. Hadziioannou and Skoulios (1982) used SAXS and SANS to investigate the morphology of binary blends of PS-PI diblocks, and binary PS-PI/PS-PI-PS or PS-PI/PI-PS-PI blends or blends of the two types of triblock. They found that the blends were microphase separated, and that the sharpness of the interface was not reduced in blends compared to neat copolymers. The transition between a lamellar and a cylindrical structure was shown to depend primarily on blend composition. In contrast, the transition from a lamellar to a disordered phase at... 

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