Diblock, linear

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

The linear diblocks are then coupled by a polyfunctional coupling agent such as epoxidised linseed oil to give a star-shaped polymer. As already mentioned, commercial materials of this type have a tetramodal distribution. 

Ruths and Granick [95] have studied the self-adhesion of several monolayers and adsorbed polymers onto mica. For loose-packed monolayers, the adhesion, in excess of a constant value observed at low rate, increased as a power law with the square root of the separation rate. In the case of adsorbed diblocks, the excess adhesion increased linearly with logarithmic separation rate. The time effects were ascribed to interdigitation and interdiffusion between the contacting layers. 

The main results of this miero-mechanical model in the quasi-static regime have been compared with experimental results obtained by placing polystyrene (PS)-polyvinyl pyridine (PVP) diblock copolymers with a short PVP block between PS and PVP homopolymers. The fracture toughness was found to increase linearly with E from that of the bare PS/PVP interface, while the slope of the line increased with the degree of polymerization of the block being pulled out. If the data for the different copolymers were plotted as AG vs. (where... 

Abstract This review highlights recent (2000-2004) advances and developments regarding the synthesis of block copolymers with both linear [AB diblocks, ABA and ABC triblocks, ABCD tetrablocks, (AB)n multiblocks etc.] and non-linear structures (star-block, graft, miktoarm star, H-shaped, dendrimer-like and cyclic copolymers). Attention is given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

The best-known and simplest class of block copolymers are linear diblock copolymers (AB). Being composed of two immiscible blocks, A and B, they can adopt the following equilibrium microphase morphologies, basically as a function of composition spheres (S), cylinders (C or Hex), double gyroid (G or Gyr), lamellae (L or Lam), cf. Fig. 1 and the inverse structures. With the exception of the double gyroid, all morphologies are ideally characterized by a constant mean curvature of the interface between the different microdomains. 

The comparison of a linear PS-fr-PI-fr-PS triblock with its linear analogue was performed by Takano et al. [97]. As in the diblock case the cyclic copolymer exhibits a smaller domain spacing however, the effect is not so pronounced (Table 3). This finding may be explained by the tendency of the ABA triblock to exhibit a higher curvature compared to an AB diblock, which in consequence reduces the differences between a cyclic diblock and its linear ABA counterpart. 

In miktoarm-star copolymers (or heteroarm star copolymers ), the unlike blocks are connected at one junction point, as shown in Fig. 34. As in linear-block copolymers, exactly two blocks are linked. Hence, it seems tempting to approximate those copolymers as a set of diblocks with the free ends of one block component joined together in a cluster [111]. 

However, the central junction of miktoarm-star copolymers exhibits more crowding compared to linear diblocks. The penalty for blocks bending at the interface will be higher (Fig. 34a,c), resulting in a shift of the phase boundaries and different spacings compared to their linear analogues of similar composition. 

Fig. 41 Theoretical phase diagram calculated by Matsen et al. [20] for diblock copolymers in intermediate segregation regime. PS-arm-P2VP miktoarm polymers PS- -P2VP linear polymer. From [119]. Copyright 2000 American Chemical Society... 

Fig. 42 Double-logarithmic dependence of first-order reflection peak, q, on total degree of polymerization, N. o AB diblocks A2B2 miktoarm stars. Solid lines fitting results for power law scaling of q N a, with a = 0.73 0.04 and 0.70 0.04 for linear and stars respectively. From [120]. Copyright 2000 American Chemical Society...

Besides linear-block copolymers, also blends with miktoarm-star terpolymers have been reported [193]. Blending a PS-arm-PB-arm-P2VP miktoarm-star terpolymer showing hexagonal symmetry with a PS- -P2VP diblock... 

Fig. i Schematic representation of chain conformation in micelles from a linear PEO-PBO diblock copolymers, b linear PEO-PBO-PEO triblock copolymers, c linear PBO-PEO-PBO triblock copolymers and d cyclic PEO-PBO diblock copolymers... 

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