Clearing block copolymers

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Where transparency is required, a range of polymers is available. Polystyrene is the least expensive but polymethylmethacrylate has an outstanding high light transmission combined with excellent weathering properties. Also to be considered are the polycarbonates, glass-clear polyamides, SAN, butadiene-styrene block copolymers, MBS polymers, plasticised PVC, ionomers and cellulose esters such as cellulose acetate. 

Hedrick et al. reported imide aryl ether ketone segmented block copolymers.228 The block copolymers were prepared via a two-step process. Both a bisphenol-A-based amorphous block and a semicrystalline block were prepared from a soluble and amorphous ketimine precursor. The blocks of poly(arylene ether ether ketone) oligomers with Mn range of 6000-12,000 g/mol were coreacted with 4,4,-oxydianiline (ODA) and pyromellitic dianhydride (PMDA) diethyl ester diacyl chloride in NMP in the presence of A - me thy 1 morphi 1 i nc. Clear films with high moduli by solution casting and followed by curing were obtained. Multiphase morphologies were observed in both cases. 

This molecular weight response clearly indicates that chain-shuttled ethylene-octene block copolymers, rather than blends, are formed upon introduction of DEZ. The Mn can also be used in conjunction with the DEZ feed and polymerization rate to calculate the number of chains produced per Zn molecule. The low DEZ level of sample 4 results in the production of ca. 12 chains/Zn. However, the reaction is practically stoichiometric at higher DEZ (no H2), with production of sample 6 resulting in 1.9 chains/Zn (or ca. one chain per Zn-alkyl moiety). This example indicates that nearly every polymer chain exited the reactor bound to the CSA, with very little chain termination, demonstrating the efficiency of the chain shuttling reaction. 

Figure 1 shows the DSC cooling scan of iPP in the bulk after self-nucleation at a self-seeding temperature Ts of 162 °C (in domain II). The self-nucleation process provides a dramatic increase in the number of nuclei, such that bulk iPP now crystallizes at 136.2 °C after the self-nucleation process this means with an increase of 28 °C in its peak crystallization temperature. In order to produce an equivalent self-nucleation of the iPP component in the 80/20 PS/iPP blend a Ts of 161 °C had to be employed. After the treatment at Ts, the cooling from Ts shows clearly in Fig. 1 that almost every iPP droplet can now crystallize at much higher temperatures, i.e., at 134.5 °C. Even though the fractionated crystallization has disappeared after self-nucleation, it should also be noted that the crystallization temperature in the blend case is nearly 2 °C lower than when the iPP is in the bulk this indicates that when the polymer is in droplets the process of self-nucleation is slightly more difficult than when it is in the bulk. In the case of block copolymers when the crystallization is confined in nanoscopic spheres or cylinders it will be shown that self-nucleation is so difficult that domain II disappears. 

Several block copolymer systems have shown only domains I and III upon self-nucleation. This behavior is observed in confined crystallizable blocks as PEO in purified E24EP57EO1969 [29]. Crystallization takes place for the PEO block at - 27 °C after some weak nucleating effect of the interphase. Domain II is absent and self-nucleation clearly starts at Ts = 56 °C when annealed crystals are already present, i.e., in domain III (Fig. 17b). The absence of domain II is a direct consequence of the extremely high... 

It is important to define clearly the characteristic features of block copolymer micelles. We mentioned above that the insoluble blocks formed a micellar core surrounded by a corona. Depending on the composition of the starting block copolymer, two limiting structures can be drawn (1) starlike micelles with a small core compared to the corona and (2) crew-cut micelles with a large core and highly stretched coronal chains. Both situations are schematically depicted in Fig. 2. 

The micellization behavior of copolymers containing two hydrophobic blocks, or double-hydrophobic block copolymers, has been shown to be mainly controlled by the solvent and its interaction with the copolymer blocks. It is thus possible to tune the micellization of these copolymers by changing the organic solvent. In this respect, large differences in Z, i h, Rc, etc. are expected whenever the interaction parameter between the polymer and the solvent is varied. This is illustrated by, e.g., the work of Pit-sikalis et al. [87] for PS-PSMA diblock copolymers dissolved in either ethyl-or methylacetate. The effect of temperature has been studied by Quintana et al. [88,89], who have clearly shown that CMC decreases with increasing temperature for PS-PEB copolymers in alkanes. 

Although many investigations have been carried out over the last 30 years on block copolymer micelles, some basic questions have not yet received a clear answer. In this respect, there is a need for libraries of block copolymers in which the relative lengths of the different constituent blocks are systematically varied. Most of the investigations on block copolymer micelles have indeed been realized on a limited number of samples. Synthetic strategies based on the use of macroinitiators or the recent lego approach... 

It is important to point out that the improvement in adhesion did not result from an increase in the solubihty of the imidized polymer containing the flexible coblock after the Tj cure cycle. In fact, the block copolymers demonstrated less than 2 % swelling (72 h) in the casting solvent, whereas PMDA/ODA polyimide homopolymer swells approximately 20-30 % (72 h). Clearly these data suggest that the improved auto-adhesion results from melt flow at 400 °C [44]. 

Clear impact-resistant polystyrene is a commercial plastic with the desirable combination of toughness and exceptional clarity. It is a styrene-1,3-butadiene multiblock copolymer containing more than 60% styrene. Most of these products are mixtures of block copolymers formed by incremental additions of initiator and monomers followed by coupling (Sec. 5-4c). The products generally have a tapered and multiblock composition with branching (due to the coupling agent). 

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