A-B block copolymers

In a somewhat wider sense, one can define amphiphiles as molecules in which chemically very different units are linked together. For example, the structures formed by A B block copolymers in demixed A and/or B homopolymer melts and their phase behavior are very similar to those of classical amphiphiles in water and/or oil [13,14]. Copolymers are used not only to disperse immiscible homopolymer phases in one another, but also to create new, mesoscopically structured materials with unusual and interesting properties [15]. 

Meier D.J., Theory of block copolymers Domain formation in A-B block copolymers, J. Polym. Sci., Part C, 26, 81, 1969. 

More detailed investigations with planar brushes, obtained from A-B block copolymers, where one component is able to crystallize on the surface of crystalline PE cores of platelet-like aggregates, have been reported recently [175]. 

SYNTHESIS OF A-B BLOCK COPOLYMERS OF PIPAAm WITH VARIOUS HYDROPHOBIC SEGMENTS... 

Hoffman (37) has offered a variety of circumstantial evidence supporting the random coil model. In A-B block copolymers of styrene and butadiene, for instance, the characteristic dimension of the dispersed phase particles depends on the molecular weight of blocks in the dispersed phase according to ... 

Near the ODT, the composition profile of ordered microstructures is approximately sinusoidal (Fig. 2.1).The phase behaviour in this regime, where the blocks are weakly segregated, can then be modelled using Landau-Ginzburg theory, where the mean field free energy is expanded with reference to the average composition profile. The order parameter for A/B block copolymers may be defined as (Leibler 1980)... 

Block copolymers are also used as compatibilizers An A-B block copolymer, when added to a blend of A and B homopolymers, reduces the tendency of the two homopolymers to phase separate, and thus it stabilizes the blend. It does so by migrating to the interface between A and B regions, where the A block can happily mix in the A domain while the B block can mix in the B domain. 

Fig. 4.1G The direct synthesis of A-B block copolymers using the naked nickel catalyst with polymeric chain transfer agents.

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesised [6] and used for the emulsion polymerisation of styrene. These molecules are ideal as the polystyrene (PS) block is compatible with the PS formed, and thus it forms the best anchor chain. The PEO chain (the stabilising chain) is strongly hydrated with water molecules and extends into the aqueous phase where it forms the steric layer necessary for stabilisation. 

Some A-B block copolymers have been reported (55) by using the approach of the living oxonium ion system and procedure (Reaction 9) ... 

PO/PHB/A-B block copolymers. for disposable napkins, ostomy bags. ... 

Nanospheres of PEC-Polycaprolactone A-B Block Copolymer as a Novel Drug Carrier... 

Fig. 5 Phase diagrams of A-B block copolymers with the A block adsorbed on a surface for (5 = (/3 - y2)lyi = 0 while y =0.1 (a) or yi = 0.75 (b) (Reproduced with permission from [36]. Copyright 1999 American Chemical Society)...

Pedersen and coworkers [74, 80, 81, 86] have modified Eq. 78 based on Monte Carlo simulation results from chains exhibiting excluded volume effects. Written in terms of a micelle constituted of a A-B block copolymer, this can be written independently of morphology (spherical, ellipsoidal, or cylindrical) ... 

Figure 4.19. A-B block copolymers at the AB phase boundary. The effect is that of oil-in-oil surfactant emulsification.

E. Ruckenstein and D.B. Chung. Surface modification by a two-liquid process deposition of A-B block copolymers. /. Colloid Interface ScL 123 170-185 (1988). 

For an A-B block copolymer, there can be only two segments, poly-A and poly-B, and the influence of A-B units can be taken to be minimal (6) and may... 

Sankar et al [48] have studied A/B block copolymers of polystyrene (PS) and poly(methylmethacrylate) (PMMA) by observation of C-detected where neither, one or the other, and both blocks were perdeuterated. They found that the carbons in each block of the copolymers were experiencing primarily their own protons, indicating some degree of phase separation, but the for each block differed somewhat from the measured for the PS and PMMA homopolymers. Annealing quenched samples apparently increased the degree of phase separation, because their values more closely approached the homopolymer values. 

FIGURE 14.3 Various conformations of polymeric surfactants adsorbed on a plane surface (a) Random conformation of loops-trains-tails (homopolymer) (b) preferential adsorption of short blocks (c) chain lying flat on the surface (d) A-B block copolymer with loop-train configuration of B and long tail of A (e) A-B-A block as in d and (f) BA graft with backbone B forming small loops and several tails of A (teeth). 

A-B-block-copolymers (A = functional monomer, B = cross-linker from BET measurements, calculated from GPC data, average particle size = 40 10 pm. 

Structurally, transparent PUs are —(A—B) — block copolymers containing alternating soft segments (A), with flexible chains of low glass transition temperatures, and hard, polar blocks (B). Linear polyols from the soft segments, diisocyanates the hard blocks and the two are linked or chain-extended, preferably with a linear diol. Diamine chain extenders usually lead to coloured urethanes which darken further on exposure to light. 

Radicalized particulates from dispersion FRRPP have been found to be viable intermediates for block copolymer formation. An example recipe and procedure for the formation of an RB1-200 A-B block copolymer is shown in Table 4.2.1, which will be a starting point for the development of other A-B block copolymer procedures. We studied the feasibility of second-stage block copolymer formation from radicalized particulates in the following environments (a) solution, (b) dispersion, and (c) in situ polymerization. The formation of RB 1-200 is believed to involve a second-stage solution polymerization process, due to its single peak in the fractional precipitation plot (similar to Fig. 3.2.7). 

Table 4.2.1 Recipe and procedure for the formation of RB1-200 A-B block copolymer...

The overall stmcture of the diagram is similar to that of the A/B blends with added A B block copolymers, but it differs in detail near the concenttation where the number of A groups coincides with that of B groups. We call this concentration the stoichiometric concentration 4>st (vertical broken-dotted line). It is explicitly given by... 

---