Copolymer radial

Transparent toughened polystyrene polymers are produced by blending polystyrene with SBS block copolymers (see Section 11.8). During the 1970s and 1980s most development was with block copolymers with a radial (or star) shape. Two types were developed block copolymers with a central butadiene block, and block copolymers with a central polystyrene block. 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Fig. 5. Representative block copolymer structures, (a) Routes to SIS i, coupling reaction ii, sequential, (b) Three and four-arm SI stars, (c) (Sllih radial, (d) (SEB)2B2 radial.

Figure 10.1. USAXS observation during straining of an SBS block copolymer. Right monitor Intensity maxima on an ellipse. Raw-data coordinate system (x,y) and radial cuts for data analysis are indicated. Middle Videotaping of sample. Left Stress-strain curve. Control booth of beamline BW4, HASYLAB, Hamburg...

Fig. 18 Schematical representation of different types of micelles formed by ABC triblock copolymers. Core-shell-corona micelles with insoluble core and shell (a), core-shell-corona micelles with radially compartmentalized corona (b), and Janus micelles with laterally compartmentalized corona (c)...

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

Random copolymers of VF2/F3E when crystallized from the molten state above the Curie temperature show a microstructure in the form of very thin needle-like morphological units which are probably semicrystalline. Figure 5a illustrates the needle-like microstructure of the copolymer 80/20 melt crystallized in the paraelectric phase observed at 140 °C. After codling at room temperature the microstructure of the ferroelectric crystals is such that what appear in the optical microscope as radial fibers are, in fact, stacks of thin platelet-like morphological units (see Fig. 5b). 

Fig. 13 Radial distribution of hydrophobic and hydrophilic segments (with respect to the center of the globule) for a copolymer chain of 512 segments with a 1 1 HP composition. o is the size of the chain segment. Adapted from [76]...

Figure 21.9 2D SAXS (logarithmic scale) from a shear-aligned specimen of the parent ABC block copolymer (a, c). The shear direction is horizontal in (a) and along the X-ray beam direction in (c), while the surface normal of the sample is vertical in both images. Radial averages (b, d) in both cases show a main peak at s = (2.63 + 0.05) x 10 2 nm4 (repeat spacing of 38.0 0.7 nm) and the dotted vertical lines indicate the allowed reflections for a hexagonal lattice.37 (Reprinted with permission from G. E. S. Toombes et al., Chem. Mater. 2008, 20, 3278-3287. Copyright 2008 American Chemical Society.)...

Starblock (or radial star) copolymers form another kind of amphiphilic nanoparticles which can be regarded as unimolecular micelles. Alternatively, cylindrical core-shell brushes can be regarded as unimolecular cylinder micelles. Due to the covalent attachment of the block copolymers at one end, frustrated micellar structures can be made which would never form spontaneously. The cylindrical systems will be reviewed in Sect. 4.2. 

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