Block copolymers solid-state morphology

Besides its effects on morphology, comonomer sequence distribution also affects copolymer crystallization kinetics. In statistical copolymers, due to the broad distribution of crystaUizable sequence lengths, bimodal melting endotherms are typically observed. In block copolymers, the dynamics of crystallization have features characteristic of both homopolymer crystallization and microphase separation in amorphous block copolymers. In addition, the presence of order in the melt, even if the segregation strength is weak, hinders the development of the equihbrium spacing in the block copolymer solid-state structure. 

Alward DB, Kinning DJ et al (1986) Effect of arm number and arm molecular weight on the solid-state morphology of poly(styrene-isoprene) star block copolymers. Macromolecules 19 215-224... 

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. 

Besides the aforementioned block copolymers 6-7 reported by Stupp and co-workers, the solid-state morphologies of a number of representative rod-coil oligomers composed of perfectly monodisperse rod segments, in particular 42, 48, and 49, have been studied in great detail using scattering and/or microscopic techniques. ... 

H. Schlaad, H. Kukula, B. Smarsly, M. Antonietti, T. Pakula, Solid-state morphologies of linear and botdebmsh-shaped polyst5oene—poly(Z-L-lysine) block copolymers. Polymer 43, 5321-5328 (2002)... 

Other peptide-polymer conjugates with interesting optical properties are rod-rod systems with 7i-conjugated aromatic polymers. The latter are appealing systems because of their optoelectronic and photoconductive properties, which strongly depend on the solid state morphology. Jenekhe and coworkers reported on triblock copolymers with a polyfluorene middle block and PBLG outer blocks... 

Copolymers are macromolecules composed of two or more chemically distinct monomer units, covalently joined to form a common polymer chain [1,2], In these materials, the sequence distribution of the monomer counits plays a critical role in determining the copolymer s crystallization behavior, and consequently influences its solid-state morphology and material properties [1,2], At one extreme, different types of monomer units may be randomly incorporated into the polymer chain, resulting in a statistical copolymer. At the other extreme, blocks of homopolymer sequences of different chemical nature and chain length may be joined together to form what is known as a block copolymer. In this chapter, we wiU review the key effects of comonomer incorporation on the solid-state morphology and crystallization kinetics in both statistical and block copolymers. 

In semicrystalline block copolymers, the crystallization behavior is often more complex than that observed in statistical copolymers because the solid-state morphology adopted by block copolymers can be driven either by block incompatibility or by crystallization of one or more blocks [5-8]. In this chapter, we will cover only block copolymers with homogeneous or weakly segregated melts, such that crystallization is always the dominant factor in determining solid-state morphology. Crystallization of block copolymers from strongly segregated melts is covered in Chapter 12. Furthermore, the... 

Figure 11.14 Schematic of solid-state morphology for poly(ethylene-Z -(ethylene-fl/Cpropylene)) diblock copolymers crystallized from a homogeneous melt. Presence of ethyl branches within the polyethylene block leads to the formation of short crystal stems, such that several crystals may be accommodated within one crystalline domain. Reprinted with permission from Reference [105]. Copyright 1993 American Chemical Society.

The solid state morphology of the OBCs was characterized previously. The WAXD patterns showed that all the block copolymers crystallized as orthorhombic polyethylene crystals. Optical and Atomic Force microscopy showed spherulitic and lamellar morphologies for all copolymers down to even H12 with only 7 % crystallinity. The solid state morphology was consistent with crystallization of hard blocks from a miscible melt. It appeared that crystallization of the hard blocks forced segregation of the noncrystallizable soft blocks into the interlamellar regions. 

It is well known that block copolymers and graft copolymers composed of incompatible sequences form the self-assemblies (the microphase separations). These morphologies of the microphase separation are governed by Molau s law [1] in the solid state. Nowadays, not only the three basic morphologies but also novel morphologies, such as ordered bicontinuous double diamond structure, are reported [2-6]. The applications of the microphase separation are also investigated [7-12]. As one of the applications of the microphase separation of AB diblock copolymers, it is possible to synthesize coreshell type polymer microspheres upon crosslinking the spherical microdomains [13-16]. 

The examples discussed above illustrate the importance of block copolymer chain segment incompatibilities for the phase separation of bulk materials, combined with the ability to perform chemistry within specific nanoscale domains to impose permanence upon those self-assembled nanostructured morphologies. Each is limited, however, to crosslinking of internal domains within the solid-state assemblies in order to create discrete nanoscale objects. To advance the level of control over regioselective crosslinking and offer methodologies that allow for the production of additional unique nanostructured materials, the pre-assembled structures can be produced in solution (Figure 6.4), as isolated islands with reactivity allowed either internally or on the external... 

It is important to mention that the structure/properties relationships which will be discussed in the following section are valid for many polymer classes and not only for one specific macromolecule. In addition, the properties of polymers are influenced by the morphology of the liquid or solid state. For example, they can be amorphous or crystalline and the crystalline shape can be varied. Multiphase compositions like block copolymers and polymer blends exhibit very often unusual meso- and nano-morphologies. But in contrast to the synthesis of a special chemical structure, the controlled modification of the morphology is mostly much more difficult and results and rules found with one polymer are often not transferable to a second polymer. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

So far in this book, we have focused on aspects of polymer synthetic chemistry and what can be considered local structure, the arrangements of units in a chain and how these can be characterized spectroscopically. In the next few chapters our focus shifts to a more global scale and involves the physics and physical chemistry of polymer materials. We will start with the shapes or conformations available to chains in solution and the solid state, how these chains interact with one another and other molecules (e.g., solvents), and the- conditions under which chains can organize and aggregate into larger scale structures, as in crystallization (or, more briefly, some of the fascinating morphologies formed by block copolymers). 

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological properties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. 

There have been numerous studies employing calorimetric(19), dynamic mechanical, ( ) dielectric, ( ) and morphological(23,24) techniques to elucidate the solid-state behavior of styrene-ethylene oxide block copolymers. These measurements have focused on transition-temperature phenomena, and they have provided reference data on the bulk properties of the copolymers. The evidence accumulated to date indicates that PS and PEO are incompatible in the bulk. While this appears true, in general, one cannot rule out the possibility that PS and PEO have some limited degree of miscibility in the copolymers. It is also unknown, at this time, what influence an interface (e.g., the air-polymer interface) has... 

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