Block copolymers weakly segregated systems

The triblock copolymer styrene-butadiene-c-caprolactone, the last being the crystallizable block, serves as example of a weakly segregated system.(67) Although both the overall crystallization and spherulite growth rates are reduced relative to the crystallizing homopolymer, the temperature ranges for isothermal crystallization are approximately the same for the copolymer and homopolymer. Consequently, the Avrami exponent is only slightly reduced from that of the parent homopolymer. 

A weakly segregated rod-coil block copolymer system composed of poly(2,5-diethylhexyloxy-l,4-phenylene-vinylene) (DEH-PPV) rod blocks and PI coil blocks was used to map the equilibrium phase behavior with respect to temperature, molecular weight, and coil ffaction. The phase diagram is shown in Figure 17. These results show lamellar and hexagonal nanostructured phases and both nematic and isotropic phases where the rods and coils are miscible. 

Figure 3.12 (left) shows how the ciystallization and melting temperature of the PLLA block within PLLA-Z -PCL diblock copolymers is depressed when the PCL content increases. As opposed to the blends, the PLLA-Z -PCL diblock copolymer systems are partially miscible or weakly segregated. Therefore, a... 

Models predict that block copolymers are not totally segregated, but that there is an interphase in which both components interpenetrate, which increases the conformational entropy of the two parts. In highly dissimilar polymers, this will be very limited in extent, tending towards the strong segregation limit, whereas if they are quite similar, in a 50 50 system the composition profile may be almost sinusoidal, towards the weak segregation limit. 

Within the PRISM approach, one can formally view a block copolymer as a special case of the multisite systems discussed in Section III.B. However, the block structure results in novel physical phenomena, not displayed by simple polymer melts, which is important both scientifically and from a materials engineering viewpoint. The most prominent feature is the ability of such a fluid to spontaneously self-assemble into microdomains of variable purity, spatial symmetry, and fV-dependent size. Ultimately a first-order microphase separation transition, or weak crystallization, into an ordered superlattice-type structure can occur where the characteristic domain size or lattice constant is typically 50-500 A. Such microscopic segregation represents a dramatic structural reorganization of the fluid and can be driven by changing temperature, increasing degree of polymerization, or increasing copolymer density in solution. 

In block copolymer/homopolymer blends there is some evidence for lamellar systems (Quan and Koberstein, Macromolecules. 1987) that the homopolymer is localized in the center of the microdomain. This effect would lead to the kind of diffusion resistance that you have observed, that is, a greater resistance than that caused simply by dilution by the second component. Secondly, the solubility of the homopolymer in the microdomains falls off rapidly when its MW is about 1/2 that of the like copolymer sequence. In your case, however, you still get diffusion up to the case of equal MW s, This may be related to your conclusion that your system behaves as if it is in the weak segregation limit. 

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