The MFT approach proposed by Leibler was also used in the calculation of the triblock copolymer phase diagram. [53] A first-order transition to bcc spheres was found for all compositions except for f =fc. Here, τ defined as f1/f, and the cases of τ = 0 or 1 reduce to a pure diblock case. The triblock copolymer phase diagrams below are highly
asymmetric. The reason for this asymmetry is the high deformation of the central B blocks in order to accommodate the outer A blocks into A domains. Increasing t gives rise to important differences in the window of stability for each morphology at a given composition. For example, at f < 0:5, a diblock copolymer goes from the disordered phase directly to the lamellar, whereas, for a triblock copolymer, with τ =0.25 at the same composition, there are large zones of bcc and hex phases. These regions become
narrower for the more symmetric triblock copolymers. These differences suggest that transitions between different morphologies with decreasing temperature are more likely to be seen experimentally in asymmetric triblock copolymers with τ =0.25 and f<0:5.
Figure 1-6 shows phase diagrams for ABA triblock copolymer melts with τ = 0.25 (left) and τ = 0.5 (right). Solid lines give the disorder-to-order transition as (χNt) (f). Dotted lines give the transitions between bcc and hex, and dashed lines the transition from hex to lam. For an ABC triblock, however, there are three interaction parameters (χAB, χBC, χAC)
and two composition variables (fA, fB) that are needed for positioning the particular microdomain morphology. Furthermore, the block sequence plays an important role and can strongly affect the phase behavior. As a result the four equilibrium morphologies known from AB diblocks are replaced by the more complex structures shown in Figure 1-7.
1-2-4-1-3 Others
When the number of blocks of block copolymer and the complexity of the architecture are increasing, the phase diagram will be more complicated. The morphology is not easy to studies. But from many studies, triblock, star, and, recently, graft and miktoarm
copolymers have been synthesized and theoretical and experimental efforts revealed that, in general:
(χNt)c,blend < (χNt)ODT, diblock < (χNt)ODT, graft < (χNt)ODT, triblock < (χNt)ODT, start
From the general rule, we can preliminary predict the phase behavior and the ODT temperature.
1-3-4-2 Solutions
There are two basic processes that characterize the phase behavior of block copolymers in solution: micellization and gelation. Micellization occurs when block copolymer chains associate into, often spherical, micelles in dilute solution in a selective solvent.
The core of the micelle is formed by the insoluble or poorly solvated block, whilst the corona contains the selectively solvated block. At a fixed temperature, micellization occurs on increasing concentration at the critical micelle concentration (CMC). The cmc is usually determined from the sharp decrease in the surface tension as a function of concentration, although other properties such as viscosity also exhibit pronounced changes.
In concentrated solutions, micelles can order into gels. Soft and hard gels are
distinguished from each other and from micellar solutions by their flow properties, gels being characterized by a finite yield stress. The hard gels seem to be associated with the formation of cubic phases of spherical micelles, whereas soft gels are usually lamellar or hexagonal-packed rod micellar phase. The phase behavior of these materials has only recently begun to be elucidated using small angle scattering. It promises to be even richer than that of block copolymer melts, at least if results for analogous conventional
surfactants are any guide. The flow behavior of these gels is the basis for many of their applications, and study of the rheology and behavior under shear of these materials will enhance the fundamental understanding underpinning future developments.
1-3-4-3 Solids
The structure of block copolymer melts is usually trapped upon vitrification. The mechanisms underlying the glass transition are similar to those of the constituent homopolymers. Thus there are little distinct physicals associated with the formation of solid phase by glassy block copolymers.
In contrast, crystallization of one or both components of a block copolymer is accompanied by profound structure and dynamic changes. The fundamental process in crystallization of chains in a crystallizable block copolymer is the change in block conformation, i.e. the adoption of an extended or a folded structure rather than a coiled configuration found in the melt or solution. Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt.
Crystallization from solution can lead to non-lamellar crystalline structures, although these may be often be trapped non-equilibrium morphology. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains.
1-3-5 Applications
The most important and popular application of block copolymers is their use as thermoplastic elastomers (TPEs). These materials are so versatile that they can be used for wine bottle stoppers, jelly candles, outer coverings for optical fiber cables, adhesives, bitumen modifiers, or in artificial organ technology. In recent years, the major
applications of block copolymers are based on their ability to self-assemble. Although in bulk or in selective solvents, block copolymer can assemble into ordered nanostructures, with dimensions comparable to chain dimensions.
The use of block copolymer self-assembly to gain structured organic-inorganic composites on the nanometer scale is appealing since no special machining other than combining the right components under the right conditions is required. For example, block copolymers were used as templates for fabricating waveguide,[54-55]
aluminosilicate mesostructures,[56] ordered silica structure[57] or as a nanolithography tool. [58] Symmetrical coil-coil diblock copolymer can also self-assemble into periodic lamellar structures for photonic crystal applications.[59-60] Moreover, tunable photonic crystal can be attained from blends of block copolymer and homopolymers.[61] This unique control over the nanometer scale stimulates our thinking of using block copolymer as templates for manipulating the otherwise randomly arranged quantum dots by colloidal chemistry for both fundamental interests and optical applications.
1-4 Nanostructured materials
1-4-1 Introduction
In the early 1980s Dr. Louis Brus at Bell Laboratories, and Drs. A. Efros and A.I.
Ekimov of the Yoffe Institute in St. Petersburg in the former Soviet Union.[62-64] Brus and collaborators experimented with nanocrystal semiconductor materials and observed solutions of strikingly different colors made from the same substance, which contributed
to the understanding of the quantum confinement effect that explains the correlation between size and color for these nanocrystals. This transition happens when the structures themselves become smaller than a fundamental scale intrinsic to the substance. In
nanocrystals' size regime, the Bohr radius of the electron-hole pair determines the scale length. For two decades, Drs. Moungi Bawendi (MIT) and Paul Alivisatos (UC Berkeley) have been investigating optical properties of Q-dots.[65-66] They not only have found ways to make the Q-dots water-soluble, but also discovered that adding a passivating inorganic "shell" around the nanocrystals, and then shining blue light, caused the quantum dots to light up brightly.