Asymmetrical copolymers

Fig. 2.53 Computer simulation results, using lime-dependent Ginzburg-Landau dynamics, of a lattice model of an asymmetric copolymer forming a hex phase subject to a step-shear along the horizontal axis (Ohta et al. 1993), The evolution of the domain pattern after the application of the step-shear is shown, (a) t = 1 (the pattern immediately after the shear is applied) (b) t = 5000 (c) t = 10000 (d) t = 15 000. The time-scale corresponds to the characteristic time for motion of an individual chain, t = R M. 

In subsequent work, Langmuir films of more asymmetric copolymers of the same type at the air-water interface were investigated (Zhu et al. 1992h.c). Cicular or starfish micelles were observed for copolymers with less than 86% PS,... 

Addition of A homopolymer to type IV micelles was found to lead to a shift in the regions of stability of cylindrical and spherical morphologies towards more symmetric compositions compared to the neat copolymer. For sufficiently long homopolymers, the structure formed by asymmetric copolymer chains was anticipated to be copolymer micelles in a matrix of unstretched homopolymer chains. In contrast to the experimental results of Winey et al. (1991c) (who observed... 

Matsen (1995ft) also calculated sections through the phase diagram in the weak segregation limit for a fixed value for blends with given values of/ . Because asymmetric copolymers were considered, ordered phases other than lamellae... 

The influence of the molecular weight distribution of diblock copolymers on their segregating properties is considered in Sect. 4.2.4. It describes first experimental study [255] on a bimodal mixture of short symmetric and long asymmetric copolymers added to a polymeric matrix. Shorter copolymers were found to adsorb preferentially at the homopolymer interfaces in accord with brush formation observed from a solvent host matrix [274-276]. The mean field model is able [255] to predict the segregation isotherm of the bimodal mixture of copolymers, basing on single component adsorption data [254]. 

Tire morphology of a micelle is primarily determined by the composition of the copolymer and the incompatibility between the blocks and the solvent.. Symmetric block copolymers produce micelles in which the core and the corona have comparable volume, leading to colloidal particles akin to the sterically stabilized particles described above. By contrast, very asymmetric copolymers form star-like particles... 

A different approach was used by Milner [326] in order to predict the phase diagram for asymmetric copolymer architectures (for example A2B, A3B etc. types of miktoarm stars). The free energy of the system can be calculated by summing the free energies of the polymer brushes existing on the two sides of the interphase. Milner described the effects of both chain architecture (i.e., number of arms) and elastic (conformational) asymmetry of the dissimilar chains, in the strong segregation limit, by the parameter... 

For strongly asymmetric copolymers, Na > Nb, the structure of a micelle is controlled by the balance of the coronal free energy, corona, and the excess free energy of the core-corona interface, Finterfaoe-... 

Strongly asymmetric copolymers with long hydrophobic blocks form crew-cut micelles with //corona -C Rcoce- The coronae of such micelle can be treated as quasi-planar PE brushes [25-33]. 

For the crew-cut quasi-neutral micelles with //corona < Rears, that are formed by strongly asymmetric copolymers with short coronal blocks, Na < one finds ... 

Density profiles for lamellae have been presented in Refs. 2,5, and 35, for cylinders and spheres in Ref. 35, and for all stable phases in Ref. 37. The results show some interesting common behavior as functions of x c-Except for highly asymmetric copolymers, the density profiles are quasi-sinusoidal only if xiVc < 15. At higher x the maximum volume fractions reach unity, and the profiles begin... 

The combination of silanes and silanols is not limited to simple methylsilicones or arylsilicones but can be extended to much more complicated systems. Kawakami and coworkers described the synthesis of complex double decker siloxanes and their alternating copolymerization with a dimethylsilicone chain (Fig. 15a) [65]. They have also shown that asymmetrical copolymers can be prepared enantiose-lectively using this strategy (Fig. 15b) [66]. 

IV.3 Very Asvnmetrical CoDolvmers From the preceding results it is evident that, in order to separate S(q) from the copolymer scattering one has to use very asymmetrical copolymers. It will therefore be assumed that in the example studied above the central part of the copolymer is small compared to and can be considered as one scattering point. In the frame of this hypothesis one can write ... 

---