Polymer junction zone

Consistent with their chemical differences, the molecular structures of i- and K-carrageenans are not identical. A shorter pitch and an offset positioning of the two chains in the kappa helix is compatible with the lack of sulfate group on every 3,6-anhydrogalactose residue. The variations in molecular structures mirror the types of junction zones formed by these polymers and relate to the observed gelation properties. 

Rees and coworkers158 showed that, at 15°, i-carrageenan forms a gel whose 13C-n.m.r. signals are so broad that they cannot be detected, in contrast to those given by the solution at 80° (see Fig. 28). At the lower temperature, segmental motion is restricted by frequent, interunit junction-zones in a double-helix structure, in contrast to the gel of a /8-D-(l— 3)-linked D-glucopyranan, where the intermolecular association is not so complete, and portions of the polymer are sufficiently mobile to provide broad signals.159... 

This result is directly proportional to the minimum number of carboxylic acid sites required to form a stable junction zone on polymers. The number of calcium cations bound must also be directly related to the stability of the junction and its thermoreversibility. Finally, it must be also pointed out that it is not the pH, but the degree of neutralization, a, which controls calcium binding. 

Inserted L-rhamnopyranosyl units may provide the necessary irregularities (kinks) in the structure required to limit the size of the junction zones and produce a gel. The presence of side chains composed of D-xylosyl units may also be a factor that limits the extent of chain association. Junction zones are formed between regular, unbranched pectin chains when the negative charges on the carboxylate groups are removed (addition of acid), hydration of the molecules is reduced (addition of a cosolute to a solution of HM pectin), and/or pectinic acid polymer chains are bridged by multivalent, eg, calcium, cations. 

Thin polymer films have many possible technical applications. Transistors and light-emitting diodes are the obvious ones. In ultra-thin films, one may even approach an electronics of molecular dimension. Molecular electronics will be a future challenge for basic and applied science. Nature applies it on a large scale in the reaction centers of the photosynthetic process, where photoinduced mobile charges are separated in some analogy to the separation of the photo-(p-n)-pair in the junction zone of a semiconductor (see Section 13.3.1). 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

Oakenftill (1984) developed an extension of Equation 6.1 for estimating the size of junction zones in noncovalently cross-linked gels subject to the assumptions (Oakenfull, 1987) (1) The shear modulus can be obtained for very weak gels whose polymer concentration is very low and close to the gel threshold, that is, the polymer chains are at or near to maximum Gaussian behavior. (2) The formation of junction zones is an equilibrium process that is subject to the law of mass action. Oakenfull s expression for the modulus is (Oakenfull, 1984) ... 

Micelle-like junction zones are formed by methylcel-lulose and polyethyloxylene polypropyloxylene block copolymers (poloxamers). Although the polymers differ in chemical structure, both have hydrophobic regions in their chains the di- and trimethyl-o-glucose residues of methylcellulose and the polypropyloxylene block of poloxamer. Another feature common to the... 

Fig. 2 Microstructures associated with physically bonded polymer gels. (A) Multihelical junction zones of agar gels and (B) egg-box model junction zones of calcium alginate gels.

Molecular associations between polymer segments occur through the cooperation of several intermolecular forces such as hydrogen bonding, van der Waals forces, and electrostatic attractive and repulsive forces. The disruption of junction zones is associated with a high activation energy, further indicating that many intermolecular forces cooperate to retain the structure of each junction.f ... 

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