Grafted block copolymer networks formed

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

The second growing trend is the impact modification of polyolefin blends using styrenic block copolymers, which are known to be clear, strong, have low glass transition, compatible with PP, form interpenetrating polymer networks, and very efficient in contrast to maleic anhydride-grafted polyolefins. 

The ABA triblocks which have been most exploited commercially are of the styrene-diene-styrene type, prepared by sequential polymerization initiated by alkyllithium compounds as shown in Eqs. (99-101) [263, 286]. The behavior of these block copolymers illustrates the special characteristics of block (and graft) copolymers, which are based on the general incompatibility of the different blocks [287]. Thus for a typical thermoplastic elastomer (263), the polystyrene end blocks (-15,000-20,000 MW) aggregate into a separate phase, which forms a microdispersion within the matrix composed of the polydiene chains (50,000-70,000 MW) [288-290]. A schematic representation of this morphology is shown in Fig. 3. This phase separation, which occurs in the melt (or swollen) state, results, at ambient temperatures, in a network of... 

An interpenetrating polymer network, IPN, is defined as a combination of two polymers in network form, at least one of which is synthesized and/or cross-linked in the immediate presence of the other (1). An IPN is distinguished from other multipolymer combinations such as Polymer Blends (qv), blocks, and grafts in two ways (/) An IPN swells, but does not dissolve in solvents and (2) creep and flow are suppressed (see Block Copolymers Graft Copolymers). 

Block copolymers or graft copolymers made up of soft and rigid polymer sequences. Styrene block copolymers like polystyrene (PS) blocks [styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), and styrene-ethylene-butylene-styrene (SEBS)], and polyester TPEs belong to this family. Structurally, the thermoplastic blocks form physical network knots within the polydiene. 

If two or more monomeric units are used to make one polymer, a copolymer is formed. The statistical, random, alternating, and periodic copolymers show the relationship of two mers on an individual basis. The block, graft, AB-cross-linked, and interpenetrating polymer network copolymers comprise large portions of chain or chains containing only one mer. It must be pointed out that each of these may be subject to being composed of the various tac-ticities and so forth that describe the configurational properties. 

Mixtures of two or more polymers in polymer blends are another combination of different macromolecules. The different polymer components are normally incompatible and thus show phase separation, with the minor component usually in the form of the dispersed phase and the major component as the matrix. Close to the composition of 50 50, interpenetrating structures or networks are formed. This phase separation corresponds to the microphase separation in block copolymers but-owing to the absence of covalent bonds between the components-with coarser structures. Several processes are used to enhance the compatibility between the polymer phases, including grafting, mixing with compatibilizers, or reactive blending. Compatibilizers (block copolymers, graft polymers) cause a reduction in particle size of the minor component in the matrix. Between the components, separate interfaces (only thin boundaries) and interphases (thicker layers with often an own structure) exist. 

Figure 4.2 Macromolecular topologies of amphiphilic polymers capable of forming self-assembled networks (a) linear block copolymers, (b) star-shaped copolymers, (c) graft copolymers.

In the presence of a selective solvent, ordered block copolymers form micelles that, at sufficiently high copolymer concentrations, serve to stabilize a diree-dimensional network and promote physical gelation. This study examines the steady and dynamic rheological properties of micellar solutions composed of AB diblock, ABA triblock and bidisperse mfactures of AB and ABA copolymer molecules. Of particular interest is the unexpected improvement in network development upon addition of an AB copolymer to an ABA copolymer at constant solution composition. This behavior is observed for ABA/solvent systems above and below the critical gelation concentration, and is interpreted in terms of the volume exclusion that occurs in bidisperse mixture of grafted chains. 

Grafting terminal hydrophotes onto water soluble chains or polar groups onto nonpolar backbones produces triblock copolymers. In solvents selective for the midblock these associate to form a network in solution, adsorb onto the surfaces of colloidal particles, or absorb into the core of micelles. The reversibility of these complexes varies with size of the end blocks and the selectivity of the solvent and is sensitive to the presence of surfactants. The water soluble versions comprise the associative thickeners cited as a revolutionary advance in the coatings industry (5). 

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