Block copolymers interpenetrating polymer

Multicomponent polymeric materials consist of polymer blends, composites, or combinations of both. A polymer blend has two definitions The broad definition includes any finely divided combination of two or more polymers. The narrow definition specifies that there be no chemical bonding between the various polymers making up the blend. Table 2.5 and Section 2.7 summarize the basic types of polymer blends based on the broad definition primarily these are the block, graft, star, starblock, and AB-cross-linked copolymers (conterminously grafted copolymers), interpenetrating polymer networks, as well as the narrow definition of polymer blends. More complex arrangements of polymer chains in space can be shown to be combinations of these several topologies. 

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. 

Steric stabilisers are usually block copolymer molecules (e.g. poly (ethylene oxide) surfactants), with a lyophobic part (the anchor group) which attaches strongly to the particle surface, and a lyophilic chain which trails freely in the dispersion medium. The conditions for stabilisation are similar to those for polymer solubility outlined in the previous section. If the dispersion medium is a good solvent for the lyophilic moieties of the adsorbed polymer, interpenetration is not favoured and interparticle repulsion results but if, on the other hand, the dispersion medium is a poor solvent, interpenetration of the polymer chains is favoured and attraction results. In the latter case, the polymer chains will interpenetrate to the point where further interpenetration is prevented by elastic repulsion. 

Thomas and co-workers [8] discovered an entirely new morphology on examination of multiple-arm star block copolymers. For the right combinations of arm lengths and compositions, these materials organize themselves into a bicontinuous structure, with each phase fully interpenetrating the other in a diamond lattice arrangement. In these star molecules, the arms were diblock polymers a considerably wider range of structural variation is conceivable and even possible. 

Multicomponent polymeric materials with microheterogeneous mophologies include a number of polymer blends and block copolymers, however, an especially easy way to bring about the desired morphology is through interpenetrating polymer networks. Several papers in the symposium are concerned with IPN s and related materials. 

A random co-polymer or a blend of compatible polymers will have a single glass transition temperature intermediate between those of the two homopolymers. An example is shown in Figure 14 for nitrile-butadiene-rubber (22). The specific weight percents shown are those of commercial interest for NBR. In contrast, most polymer blends, graft and block copolymers, and interpenetrating polymer networks (IPN s) are phase separated (5) and exhibit two separate glass transitions from the two separate phases. Phase separated systems will not be considered here. 

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. 

This part of the monograph will examine systems containing mixtures of two distinguishable kinds of polymer molecules. Such mixtures, known as polymer blends, polyblends, or simply blends, include mechanical blends, graft copolymers, block copolymers, and interpenetrating polymer networks. 

Fig. 1. Hybrid organic-inorganic polymer systems can be devised for all structural paradigms of polymer chemistry including (a) homopolymer, (b) block copolymer, (c) copolymer, (d) graft copolymer, and (e) interpenetrating polymer networks, including, shown as geometrical abstractions, (f) true and (g) semi-interpenetrating versions, where crosslinks are depicted as junctions of horizontal and vertical lines.

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