Block copolymers basic principles

Generalities about block copolymer micelles have been reviewed by Ham-ley [2] and Riess [14], based on previous works from the 1980s and 1990s. This topic will not be covered in detail, but the basic principle, as well as some important practical issues, will be reviewed. The essential experimental techniques used for block copolymer micelle characterization will also be outlined briefly. 

Block copolymer micelles have been the subject of an enormous body of work during the last 30 years. Although the basic principles of block copolymer mi-cellization have been already discovered and experimentally investigated in the 80s, intense research on this topic has been performed since the mid-90s by many research groups. Because it was not possible to include every contribution to that field in the frame of the present review, only selected examples were discussed. 

In an earlier section, we have shown that the viscoelastic behavior of homogeneous block copolymers can be treated by the modified Rouse-Bueche-Zimm model. In addition, the Time-Temperature Superposition Principle has also been found to be valid for these systems. However, if the block copolymer shows microphase separation, these conclusions no longer apply. The basic tenet of the Time-Temperature Superposition Principle is valid only if all of the relaxation mechanisms are affected by temperature in the same manner. Materials obeying this Principle are said to be thermorheologically simple. In other words, relaxation times at one temperature are related to the corresponding relaxation times at a reference temperature by a constant ratio (the shift factor). For... 

Figure 12.5 shows some of the possible types of compatibiliser, which are all forms of block or graft copolymers. The basic principle is that the... 

Block copolymers are efficient reinforcers of weak interfaces, but they are costly and difficult to get to the interface. Other reinforcing strategies, which may in some cases be more practical, do exist. Two examples are the use of reactive grafting at interfaces and the use of random copolymers. In both of these methods one uses the same basic principle of reinforcement as that which operates in block copolymers a single, covalently bonded molecule straddles the interface in such a way that it is well entangled with the homopolymers on both sides of the interface. 

The assembly of amphiphilic block copolymers to generate discrete nanoscale structures is primarily driven by the hydrophobic effect, with micelle size and shape governed by a set of basic principles rooted in surfactant phase-separation behavior [22-28]. Important parameters that control the size of micelles are the degree of polymerization of the polymer blocks and the Flory-Huggins interaction parameter [28]. 

Since then, the research has dramatically grown and many goals have been achieved. A review reports an organized and detailed overview on theoretical aspects and basic principles of self-assembly and micellization of block copolymers in solution, together with a wide number of examples concerning the methods for the stabilization of macromolecular aggregates and their applications, mainly focused on biomedical field, in the perspective of smart nano-objects production [37]. The self-organization of block copolymers in different shapes is depicted in Fig. 1.4. 

The transition line which separates the homogeneous phase from the various microphase-separated structures has an appearance similar to the binodal of a polymer mixture. There is, however, a basic difference In the block copolymer case, we are dealing with a one-component-system, rather than a binary mixture. The line therefore relates to a phase transition rather than to a miscibility gap. It should also be noticed that, in contrast to the binodal of a mixture, the transition line tells us nothing about the internal composition of the microphases. In principle, these could be mixed states, practically, however, compositions are mostly close to pure A- or B-states. 

In principle, what has just been stated for surfactant micelles also holds for the larger and more complex self-assembhes that surfactants and amphiphific block copolymers can form microemulsion droplets, vesicles, and mesophases. The lifetimes of these assembhes are much longer than for micelles, mainly when they involve block copolymers. Nevertheless, exchanges and other processes can also take place. Vesicles and lyotropic mesophases can be considered as permanent objects. However, vesicles can be transformed into micelles, and vice versa. Likewise, a lyotropic mesophase may be transformed into another mesophase or in a micellar solution by an appropriate change brought to the system. The kinetics of these transformations is of basic as well as of practical interest. 

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