Block copolymers, separation mechanism

In block copolymers [8, 30], long segments of different homopolymers are covalently bonded to each otlier. A large part of syntliesized compounds are di-block copolymers, which consist only of two blocks, one of monomers A and one of monomers B. Tri- and multi-block assemblies of two types of homopolymer segments can be prepared. Systems witli tliree types of blocks are also of interest, since in ternary systems the mechanical properties and tire material functionality may be tuned separately. 

Block copolymers are closer to blends of homopolymers in properties, but without the latter s tendency to undergo phase separation. As a matter of fact, diblock copolymers can be used as surfactants to bind immiscible homopolymer blends together and thus improve their mechanical properties. Block copolymers are generally prepared by sequential addition of monomers to living polymers, rather than by depending on the improbable rjr2 > 1 criterion in monomers. 

The dynamic mechanical behavior indicates that the glass transition of the rubbery block is basically independent of the butadiene content. Moreover, the melting temperature of the semicrystalline HB block does not show any dependence on composition or architecture of the block copolymer. The above findings combined with the observation of the linear additivity of density and heat of fusion of the block copolymers as a function of composition support the fact that there is a good phase separation of the HI and HB amorphous phases in the solid state of these block copolymers. Future investigations will focus attention on characterizing the melt state of these systems to note if homogeneity exists above Tm. 

In the polymerization of MM A with 26, the molecular weight of the resulting copolymer increased with the polymerization time (conversion). The St polymerization provided a multiblock copolymer by recombination. It was revealed that the length of the poly(St) segment as well as the mechanical properties of the block copolymer depended on the chain length of the poly(DMS) segments because of phase separation [127]. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

Apparently the above results indicate that the chromatographic behavior of copolymers is different for different chain architecture. When the Rf values are compared at equimolar composition, the sample migration occurs first for the statistical copolymer and belatedly for the alternating, whereas the block copolymer remains immobile on the starting point. Such observations allow us to conclude that the mechanism of chromatographic separation of copolymers is related not only to the chemical composition but also to the chain architecture. 

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