Results for block copolymers

Clearly, all these simulations of surface effects on polymer blends are very stimulating, but at this point they may be considered to be first steps only. The effect of long range polymer wall interactions should also be considered. 

While the simulation of phase transitions in polymer blends dates back to 1987 and the necessary tools (finite size scaling, etc.) are well developed, studies of mesophase formation in block copolymers have only just begun, - and only rather short chain lengths have been accessible N 60). We consider this work as rather preliminary, and hence this section can be relatively short. 

---

Table 2 Summary of thickness and contact angle measurement results for block copolymers [72]...

Dynamic light scattering has traditionally been applied to polymer solutions, and DLS results for block copolymer solutions are discussed in Chapters 3 and 4. A number of recent papers have described the application of the technique to disordered block copolymer melts (Anastasiadis et al. 1993a,6 Boudenne et al. 1996 Floudas et al. 1995 Fytas et al. 1993 Jian et al. 1994a Stepanek and Lodge 1996 Vogt et al. 1994). Due to the limited range of dynamic time-scales that can... 

Tetraethylthiuram disulfide (13) induces St polymerization by the photodissociation of its S-S bond to give the polymer with C-S bonds at both chain ends (15). The C-S bond further acts as a polymeric photoiniferter, resulting in living radical polymerization. Eventually, some di- or monosulfides, as well as 13, were also examined as photoiniferters and were found to induce polymerization via a living radical polymerization mechanism close to the model in Eq. (18), e.g., the polymerization of St with 35 and 36 [76,157]. These disulfides were used for block copolymer synthesis [75,157-161] ... 

The more recently developed cryo-TEM technique has started to be used with increasing frequency for block copolymer micelle characterization in aqueous solution, as illustrated by the reports of Esselink and coworkers [49], Lam et al. [50], and Talmon et al. [51]. It has the advantage that it allows for direct observation of micelles in a glassy water phase and accordingly determines the characteristic dimensions of both the core and swollen corona provided that a sufficient electronic contrast is observed between these two domains. Very recent studies on core-shell structure in block copolymer micelles as visualized by the cryo-TEM technique have been reported by Talmon et al. [52] and Forster and coworkers [53]. In a very recent investigation, cryo-TEM was used to characterize aqueous micelles from metallosupramolecular copolymers (see Sect. 7.5 for further details) containing PS and PEO blocks. The results were compared to the covalent PS-PEO counterpart [54]. Figure 5 shows a typical cryo-TEM picture of both types of micelles. 

In general, block copolymers are heterogeneous (multiphase) polymer systems, because the different blocks from which they are built are incompatible with each other, as for example, in diene/styrene-block copolymers. This incompatibility, however, does not lead to a complete phase separation because the polystyrene segments can aggregate with each other to form hard domains that hold the polydiene segments together. As a result, block copolymers often combine the properties of the relevant homopolymers. This holds in particular for block copolymers of two monomers A and B. 

Figures 2 and 3 as additional proof to Equation 5. The two solid lines in the upper left corner of Figure 2 are the M - [77] M curves for styrene and butadiene homopolymers. The data points for block copolymers shown as cross and open circle fall in between these two curves. When plotted as M — [77] M curve, all points fall on or near the curve for polystyrene shown as a solid line in the lower right part of Figure 2. The behavior of these copolymers in toluene and dioxane is shown in Figure 3. Since these block copolymers cover a wide range of composition (% S = 3.6-45.9) as well as molecular weight (M = 34,000-620,000), these results prove unequivocally the adequacy of Equation 5. Tliis equation will make it possible to interpretate the chromatogram of block copolymer without preparing monodispersed copolymers which is something difficult, if not impossible.

Results from TEM experiments on solutions of a series of poly(styrene)-poly(cinnamoylethyl methacrylate) (PS-PCEMA) diblocks with short PS blocks and long PCEMA blocks have been compared (Tao et al. 1997) to the theories for block copolymer micelles described above. Micelles of type IV in the Zhulina-Birshtein classification (Fig. 3.18) formed in cyclopentane, which is a selective solvent for PCEMA (coronal A block), when the ratio of was... 

Addition of sec-BuLi to 21a resulted in a hexane-insoluble dilithium initiator that could be solubilized with 1,3-butadiene, and subsequently used for block-copolymer synthesis. Once again, the starting material for the initiator based upon 21a is available only via special syntheses. 

S. Krause46, 47 has studied the general conditions of phase segregation and found (in agreement with Meier s results) that the phase separation is more difficult for block copolymers than for homopolymers and that this difficulty increases with the number of blocks. 

Five fundamental domain structures are possible for block copolymers consisting of two types of blocks. Generally lamellar structures will form at compositions with approximately equal proportions of the two components. As the proportion of one component increases at the expense of the other, cylindrical morphologies will result. The matrix phase will... 

Fig. 15. Phase diagram of block copolymers and random copolymers. Circles are results for homopolymers squares are results for diblock copolymers diamonds are results for triblock copolymers up triangles are results for a mixture of A-B-A and B-A-B triblock copolymers left triangles are results for tetrablock copolymers X s are results for random copolymers with average sequence length 1 = 55 crosses are results for random copolymers with / = 20 asterisks are results for a completely random copolymer.

---