Deformation Behavior of Block Copolymers

Block copolymers are made up of more than two different polymers connected by covalent bonds. Due to thermodynamic repulsion between the component polymers, microphase separation occurs and various types of stmctures (spheres, cylinders, lamellae, and gyroids) are formed [37-39] (see Fig. 18.15), which have been studied extensively and are described in several chapters in this book. Here, deformation of block copolymers has been described in terms of microstructure/morphology of block copolymers. As there are many reports concerning simple deformation, here the emphasis is placed on the mechanical deformation (mechanical properties) and their relationships with microstructures [41]. 

The main factors influencing the deformation behavior may be molecular weight, composition, morphology, chain architecture, orientation of the microdomains, and thermal history of the block copolymer. Two examples are chosen for demonstrating the effect of morphology on the deformation behavior of block copolymers. 

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Figure 18.15 Molecular and morphological factors affecting the deformation behavior of block copolymers. Adhikari and Michler [40]. Reproduced with permission of Elsevier.

Block copolymers provide various types of morphologies that influence deformation behavior. The concept of effective entanglement density due to the existence of a hard phase (and physical cross-links ) is used to explain the deformation behavior of block copolymers. 

The macroscopic properties such as mechanical behavior of block copolymers or polymer blends depend directly on the relative concentrations of different constituents and their meso-structures. How to predict the exact macroscopic properties of polymer blends or block copolymers with meso-phase separation structures from pure component properties remains a big challenge. Some theoretical efforts have been explored. For example, Buxton et al. found that the deformations and fractures of polymer blends can be described by the... 

Most studies of the behavior of block copolymers as compatibilizing agents consider two opposing effects during deformation a reduction in critical droplet size due to a reduction in the interfhcial tension (droplet breakup) proposed by Taylor, and an increase in droplet size due to increased collision frequency between droplets (droplet coalescence) studied by Smoluchowski. The problem of droplet breakup in a... 

Block copolymers in selective solvents exhibit a remarkable capacity to self-assemble into a great variety of micellar structures. The final morphology depends on the molecular architecture, tlie block composition, and the affinity of the solvent for the different blocks. The solvophobic blocks constitute the core of the micelles, while the soluble blocks form a soft and deformable corona (Fig. Id). Because of this architecture, micelles are partially Impenetrable, just like colloids, but at the same time inherently soft and deformable like polymers. Most of their properties result from this subtle interplay between colloid-like and polymer-like features. In applications, micelles are used to solubilize in solvents otherwise insoluble compounds, to compatibilize polymer blends, to stabilize colloidal particles, and to control tire rheology of complex fluids in various formulations. A rich literature describes the phase behavior, the structure, the dynamics, and the applications of block-copolymer micelles both in aqueous and organic solvents [65-67],... 

Finally, as an example showing significant influence of morphology on deformation behavior, block copolymers are chosen. Because of the microphase separation occurring between different blocks, block copolymers form various stmctures (morphologies), which in turn influence their deformation behavior. This is described in Section 18.4, in which only a small number of examples have been chosen to demonstrate the morphology-deformation relationships, because a vast amount of hterature and work cannot be covered. Morphology and characterization of block copolymers are also described in several chapters in this book. 

Again, the importance of entanglement density is noted for understanding the deformation of block copolymers this is applicable to all polymeric materials from amorphous polymers, to semicrystalline polymers, and to block copolymers. It is expected that many of the deformation behavior will be explained systematically by using the concept of entanglement density. This needs more systematic investigation both experimentally and theoretically. 

Adhikari R, Michler GH, Lebek W, Goerhtz S, Weidisch R, Knoll K. Morphology and micromechanical deformation behavior of SB-block copolymers. II. Influence of molecular architecture of asymmetric star block copolymers. J Appl Polym Sci 2002 85(4) 701-713. 

Adhikari R, Michler GH, Henning S, Godehardt R, Hny TA, GoerlitzS, et al. Morphology and micromechanical deformation behavior of styrene-bntadiene block copolymers. IV. Structnre-property correlation in binary block copolymer blends. J Appl Polym Sci 2004 92 1219-30. 

Weidisch R, Schreyeck G, Ensslen M, Michler GM, Stamm M, Schubert DW, Budde H, Horing S, Jerome R (2000) Deformation behavior of weakly segregated block copolymers. 2. Correlation between phase behavior and deformation mechanisms of diblock copolymers. Macromolecules 33 5495-5504... 

SAXS and WAXS are particularly efficient in the study of amorphous polymers including microstructured materials, hence their use in block copolymers (see also Chapters 6 and 7). The advent of synchotron sources for X-ray scattering provided new information, particularly on the evolution of block copolymer microstructures with time resolution below one second. In particular, the morphology of TPEs is most often studied with these techniques Guo et al. [108] applied SAXS to the analysis of the phase behavior, morphology, and interfacial structure in thermoset/thermoplastic elastomer blends. WAXS is often associated with SAXS and some other methods, such as electron microscopy, and various thermal and mechanical analyses. It is mainly used in studies of the microphase separation [109,110], deformation behavior [111], and blends [112]. 

The hysteresis behavior of the diblock copolymer HBI-50 is not shown but is very similar to that of HIBI-49. In summary then, the difference in hysteresis behavior of the HBIB series to that of HIBI and HBI is related to the ability of the members of the first series to form permanent entanglements, by entrapment of the end blocks in the semicrystalline domains, whereas no such arrangment is possible for neither HIBI nor HBI series. The permanent entanglement serves as a physical crosslink which promotes recovery of the polymer after the deforming stress has been removed. At the same time, much less energy is lost as heat. 

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