Block copolymers solid state phase behavior

Schlaad et al. reported in greater detail on the solid-state phase behavior of linear and bottle-brush shaped polystyrene-block-poly(Z-L-lysine) in a wide range of molecular weight, composition, and architectures [76]. According to earlier reports on rod-coil block copolymers with polypeptides, mainly (undulated) lamellar morphologies were observed (see Figure 21). 

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 contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Polyferrocenylsilane block copolymers also phase separate in solid state to generate periodic, nanoscopic iron-rich domains that can be observed by TEM without resorting to staining techniques.24 This bulk self-assembly behavior has been studied with various block copolymers, such as PS-fe-PFS,24,52 PI-h-PFS,49 and PFS-b-PMMA.22 Additional self-assembly complications... 

In contrast to the situation found for dilute solutions, the behavior of nonlinear block copolymers in the solid state seems to have attracted great attention. Many theoretical publications appeared in recent years, dealing mainly with the phase behavior of star-block, simple graft and comb copolymers. Issues like the nature of the phase diagram and the order-disorder transition have been studied in considerable detail. The compatibilizing effects of complex copolymers, in comparison to simple diblock copolymers, were also investigated. 

Much experimental work has been devoted so far to the study of the solid state and melt behavior of nonlinear block copolymers. Most of the studies are concerned with the microphase separation morphology of copolymers with various architectures as well as the phase transition temperature. Other solid state and melt properties have been examined in a few cases. 

Polyferrocenylsilane block copolymers in which the blocks are immiscible (which is generally the case) would be expected to self-assemble to form phase-separated organometallic domains in the solid state. Based on the classical behavior of organic block copolymers, thin films of polyferrocene diblock copolymers would be expected to form domains such as spheres, cylinders, double diamonds (or gyro-ids) (or their antistructures), or lamellae (Chapter 1, Section 1.2.5). The preferred domain structure would be expected to be controlled by the ratio of the blocks, their degree of immiscibility (as defined by the Flory-Hu ins interaction parameter x), and the overall molecular weight of the block copolymer [159]. 

Block copolymers, particularly of the A-B-A type, can exhibit properties that are quite different from those of random copolymers and even from mixtures of homopolymers. The physical behavior of block copolymers is related to their solid state morphology. Phase separation occurs often in such copolymers. This can result in dispersed phases consisting of one block dispersed in a continuous matrix from a second block. Such dispersed phases can be hard domains, either crystalline or glassy, while the matrices are soft and rubber-like. 

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