Morphology block sequence

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

Although short-segment sequences are expected to be the most compatible, the 1/2/1 random copolymer with an average hard-block-sequence length of only two units does exhibit a phase-separated morphology—as reflected for the as-reacted sample in hard-segment crystal-... 

It should be clear that the conclusions of this work are limited to block polymers isolated from the polymerization solvent (cyclohexane) by evaporation and subsequently processed by conventional thermal mixing and shaping techniques. Obviously, other morphologies could be realized in many instances by casting films from solvents of varying quality for the two block sequences. 

In this section the question of how the morphology is influenced by changing the block sequence for a given overall composition in linear ternary block copolsuners is discussed. At the end of this section linear block copolymers are compared with their heteroarm star terpolymer analogues. 

Fig. 21. Influence of the block sequence on the morphology in SBV and BSV triblock copolymers (OSO4/CH3I, see Table 1).

Figure 2 Schematic of morphologies for linear ABC triblock copolymer. A combination of block sequence (ABC, ACB, BAC), composition, and block molecular weights provides an enormous parameter space for the creation of new morphologies. Microdomains are colored as shown by the copol3mer strand at the top, with monomer types A, B, and C confined to regions colored blue, red, and green, respectively, (a) Lamellar phase, (b) coaxial cylinder phase, (c) lamella-cylinder phase, (d) lamella-sphere phase, (e) cylinder-ring phase, (f) cylindrical domains in a square lattice structure, (g) spherical domains in the CsCl type stmcture, (h) lamella-cylinder-II, (i) lamella-sphere-II, (j) cylinder-sphere, (k) cocentric spherical domain in the bcc structure. (Reproduced with permission from Ref. 33. American Chemical Society, 1995.)...

Figure 13.16 Morphologies of ABC and ABCB multiblock copolymers. Samples (a) ISP-4, (b) ISP-3, (c) ISP-18, (d) ISP-12. Exact block sequences are described in the text.

Mogi et al. [138] and Gido et al. [143] studied triblock terpolymers based on polystyrene (S), polyisoprene (I), and poly(2-vinylpyridine) (VP) with different block sequences. The difference in block sequence resulted in a different morphology for a similar overall composition of the systems. While polyisoprene-block-polystyrene-block-polyvinylpyridine I-S-VP with similar amounts of all three components forms lamellar stacks (Figure 4(a)) [138], polystyrene-block-polyisoprene-block-polyvinylpyridine S-I-VP forms hexagonally packed core-shell cylinders (Figure 4(b)) [143]. 

Stadler et al. studied triblock terpolymers based on polystyrene S, polybutadiene B, and poly(methyl methacrylate) M and a number of new morphologies were discovered [6,129,150-155]. For symmetric systems with the block sequence S-B-M, that is, where the end blocks have similar size, lamellar morphologies were found. Varying the volume fraction of the middle block from 0.03 up to approximately 0.3 forms spheres, cylinders, or a lamella between the lamellae of the outer blocks [150] (Figure 5). 

In comparison to binary block copolymers relatively little work on ternary block copolymers has so far been published. There are more independent variables in ternary block copolymers as compared to binary block copolymers. While in the latter only one independent composition variable and one interaction parameter exist, in ternary systems there are two independent composition variables and three interaction parameters. This leads to a richer phase diagram. In addition, the block sequence also can be changed, which introduces another tool to influence the morphology [165]. As mentioned before in the case of diblock copolymers, systematic studies of triblock copolymers became possible with the development of sequential polymerization techniques with living anionic polymerization being still the most important one. 

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. 

Our focus in this chapter is on model block copolymers those with well-defined molecular architectures, i.e., where the block sequences (AB vs. ABA vs. ABC. . . ) are practically identical across the ensemble of chains, and where the individual blocks possess narrow chain length distributions. Throughout this chapter, block copolymer chemistries will be denoted as A/B , and particular diblock copolymers as A/B n/m , where A is the abbreviation for the monomer comprising the crystallizable block (e.g., CL for s-caprolactone), B is the monomer comprising the amorphous block (e.g., S for styrene), and n and m are the crystallizable and amorphous block molecular weights, in kg/mol (rounded to the nearest kg/mol). This notation immediately connotes the approximate volume fraction of A block, and hence suggests the likely melt morphology. 

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]. 

Figures 7 and 8 show the original morphologies of the block copolymers observed by TEM selectively stained P4VP, P2VP, and polyisoprene (PIP) sequences...

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