Ordered Microdomains

Other explanations for the drop in conductivity were related to the distribution of dopanu, which may be no longer random. For certain compositions and choice of dopants, long-range ordered structures, or structures containing ordered microdomains can be obtained. These have been reported to affect negatively conductivity ... 

Figure 5.19 Small-angle neutron scattering intensity obtained with a styrene-butadiene diblock copolymer having spherical butadiene microdomains. The peaks at very small q are due to a body-centered cubic lattice structure of ordered microdomains. The solid curve is the calculated intensity of independent scattering from solid spheres of mean radius 124 A. (From Bates etal.34)...

LC-BCPs provide theorists with novel model systems for developing new theories of polymer behavior [41]. Consider the process of microphase separation in LC-BCPs starting from the disordered BCP structure (or isotropic solution) (Scheme 1). As the temperature decreases, the system will undergo microphase separation to form ordered microdomain structures. As temperature falls past the LC clearing transition temperature (Jj) the LC block will shift from the isotropic phase to a LC phase. If this temperature is lower than the Tg of the other... 

In order to study the detailed behavior of LC block copolymers, it would be ideal to create monodispersed LC-BCP samples with well defined architecture for each block over a wide range of molecular weights. LC-BCP systems with narrow polydisper-sity should form well-ordered microdomain structures while LC-BCPs with broad molecular weight distributions would probably not. Synthesis of such materials still remains a challenge. 

The theoretical methods to investigate the evolution kinetics of ordered microdomain structures are those in the atomic-scale including molecular dynamics simulations, Monte Carlo simulations, dynamic SCFT, dynamic density functional theory (DDFT), and those in the meso-scale including dissipative particle dynamics (DPD) simulations, etc. More details of these approaches can be found in the literatures. 

The transition between the ordered, microdomain structure and the disordered, homogeneous structure, induced by temperature change, can be oberserved by rheological measurement or by small-angle scattering. The transition, often called the order-disorder transition or microphase separation transition, resembles the solid-liquid transition, and should be of a first order according to the theory of Leib-ler, but experimentally its character has not yet been established clearly. 

Before discussing the theories describing the mixture of block copolymer and homopolymer, it is necessary to review the theories dealing with pure block copolymer very briefly. The latter can be divided into three categories, the first dealing with ordered microdomain structure, the second dealing with the disordered phase and the third dealing with the transition between them. 

Fig. 16. Phase diagram of a mixture containing polystyrene (M = 2400) and styrene/butadiene diblock copolymer (27 % styrene, M = 28000). Liquid phases L, and Lj represent mixtures of disordered block copolymer and polystyrene. Mesophase M, consists of ordered microdomains of the block copolymer swollen with polystyrene. Mesophase Mj probably contains aggregates of block copolymer micelles within the medium of polystyrene. The features on the lower right, drawn in broken lines, are more speculative. (From Roe and Zin...

The phase diagrams of solutions of diblock copolymers A-B may be quite complex and depend both on the chemical nature of two blocks and on the solvent. > In a selective solvent, good for the B-block and poor for the A-block, intermolecular aggregates are formed in the dilute regime so that the number of unfavorable contacts A-S is limited. The shape of aggregates (e.g, spherical or wormlike micelles, vesicles), their size and polydispersity depend very much on chain composition and length. In more concentrated solutions, aggregates order in space and form mesophases, i.e., ordered microdomains rich in A (in B) (e.g., lamellar. 

Bendejacq D et al. Well-ordered microdomain structures in polydisperse poly(styrene)-poly(acrylic acid) diblock copolymers from controlled radical polymerization. Macromolecules 2002 35(17) 6645-6649. 

Figure 15.5 Ordered microdomain structures of 3-miktoarm star terpoiymers in three-dimensional space. A and B > 0.5, pg > 0.5) are shown in gray and black, respectively, (a) Honeycomb structure (b) tetragonally packed cylinders (c) parallel plane structure (d) cylinder-shell structure. Schematic of section perpendicular to the cylinders, J is the rich junctions region and I the region containing the mixtures of blocks A-C and junctions. He et al. [35]. Reproduced with permission of Elsevier.

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