Cross-links glassy domains

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

The situation is quite different with block copolymers. As an example we again take a copolymer of styrene and butadiene, but now as a three-block copolymer, SBS. The incompatibility of polystyrene and polybutadiene now results in a phase separation, which is enabled by the circumstance that the blocks can live their own life . The polystyrene chain ends clog together into PS domains, which lie embedded in a polybutadiene matrix. These glassy domains act as physical cross-links, so that the polymer has the nature of a thermoplastic rubber. The glass-rubber transitions of PS and BR both remain present in between these two temperatures the polymer is in a, somewhat stiffened, rubbery condition (see Figure 3.8). This behaviour is dealt... 

The properties of block copolymers differ from those of a blend of the correponding homopolymers or a random copolymer (Chapter 7) with the same overall composition. An important practical example is the ABA-type styrene/butadiene/styrene triblock copolymer. These behave as thermoplastic elastomers. Ordinary elastomers are cross-linked by covalent bonds, e.g., vulcanization (see Chapter 2) to impart elastic recovery property, as without this there will be permanent deformation. Such cross-linked rubbers are therraosets and so cannot be softened and reshaped by molding. However, solid thermoplastic styrene/butadiene/styrene triblock elastomers can be resoftened and remolded. This can be explained as follows. At room temperature, the triblock elastomers consist of glassy, rigid, polystyrene domains... 

Styrene-butadiene block copolymer belongs to the A-B-A type thermoplastic elastomer. The principal structure of this type of polymer involves the thermoplastic rubber molecules terminated by the hard, glassy end blocks. The A and B copolymer block segments are incompatible and, consequently, separate spontaneously into two phases. Thus in the solid state, the styrene-butadiene (S-B-S) thermoplastic elastomer has two phases a continuous polybutadiene rubber phase and the dispersed glassy domains of polystyrene. The styrene plastic end blocks, called domains, act as cross-links locking the rubber phase in place. 

The outstanding behavior of these rubbers arises from the natural tendency of two polymer species to separate. However, this separation is restrained in these polymers since the blocks are covalently linked to each other. In a typical commercial SBS triblock copolymer with about 30% styrene content, the styrene blocks congregate into rigid, glassy domains which act effectively to link the butadiene segments into a network (Figure 4.7) analogous to that of cross-linked rubber. 

The materials are tough and elastic below the glass-transition temperature of the outer blocks, the glassy domains acting as both cross-links and fillers for the rubbery matrix, which would otherwise behave like an unvulcanised rubber. SBS compositions usually have tensile strengths in the... 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be molded at high temperatures because of the presence of the glassy domains that act as physical cross-links. In solution, attachment of a water-soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution, that differ from either parent polymer. 

Nstwork Structure. Chains must be joined by permanent bonds called cross-links, as illustrated in Figure 5. The network structure thus obtained is essential so as to avoid chains permanently slipping by one another, which would result in flow and thus irreversibility, ie, loss of recovery. These cross-links may be chemical bonds or physical aggregates, eg, glassy domains in multiphase block copolymers (55,56). 

Since the ends of the polyisoprene chains are covalently bonded into the polystyrene glass, they cannot move away, and the glassy polystyrene phase effectively cross-links the rubber polyisoprene chains. However, at temperatures above the glass transition of polystyrene, the chains can be pulled out of the polystyrene domains, and so the article can be reshaped, becoming cross-linked once again on cooling. 

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