Crosslinking, ionic

Polymers can be modified by the introduction of ionic groups [I]. The ionic polymers, also called ionomers, offer great potential in a variety of applications. Ionic rubbers are mostly prepared by metal ion neutralization of acid functionalized rubbers, such as carboxylated styrene-butadiene rubber, carboxylated polybutadiene rubber, and carboxylated nitrile rubber 12-5]. Ionic rubbers under ambient conditions show moderate to high tensile and tear strength and high elongation. The ionic crosslinks are thermolabile and, thus, the materials can be processed just as thermoplastics are processed [6]. 

Finally, a new water-soluble polyphosphazene was recently synthesized that has the structure shown in 36 (46). This polymer has two attributes as a biomedical macromolecule. First, the pendent carboxylic acid groups are potential sites for condensation reactions with amines, alcohols, phenols, or other carboxylic acid units to generate amide, ester, or anhydride links to polypeptides or bioactive small molecules. Second, polymer forms ionic crosslinks when brought into contact with di- or trivalent cations such as Ca or Ai3+. The crosslinking process converts the water-soluble polymer to a hydrogel, a process that can be reversed when the system... 

Allcock, H. R., and Kwon, S., An ionically-crosslinkable poly-phosphazene Poly[di(carboxylatophenoxy)phosphazene] and its hydrogels and membranes. Macromolecules. 22. 75, 1989. 

The concept of ionic crosslinking is in accord with the idea that a gel must possess a coherent structure. However, although crosslinking may be essential to gel formation it does not necessarily have to be a simple ionic salt bridge. 

Michaeli (1960) opposed these views. He concluded that whatever the exact mechanism was, the binding of divalent cations caused contraction and coiling of the polyelectrolyte as was the case with adds. He disagreed with the concept of ionic crosslinking. The phenomenon of precipitation could be explained simply in terms of reduced solubility. From this he concluded that precipitation took place in an already coiled molecule and the matrix consisted of spherical macromolecules containing embedded cations. 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

Attempts to obtain permanently hydrophilic surfaces by other types of crosslinking chemistry were not successful. The other methods investigated included forming ionic crosslinks, using cured... 

Ghosh S, Rasmusson J, Inganaes O. Supramolecularself-assembly for enhanced conductivity in conjugated polymer blends. Ionic crosslinking in blends of poly(3,4-ethylenedioxythiophene)-poly(styiene sulfonate) and poly(vinylpyrrolidone). Adv Mater 1998 10 1097-1099. 

These polar copolyr ers are more transparent and. as a result of the presence of the polar acrylic acid moiety, have better adhesion to metallic surfaces than ldpb. The commercial copolymer, which is usually 50% neutralized to salts of sodium or zinc, is stiff and strong because of ionic crosslinking at room temperature, but it is readily proccssible when heated. 

Most simply, ionic crosslinking is achieved by the action of multivalent metal ions. Ionic crosslinks are weaker than covalent crosslinks. The shear-stress necessary to break the ionic crosslinks and, thus,... 

In general, the added metal ion reacts stoichiometrically with the carboxylic acid in the polymer up to 90% neutralization. Small excess quantities of the crosslinking agent are necessary to carry the neutralization to completion. However, a large excess of the cross-linking agent do not add to the properties of the ionic copolymer, since once all carboxylic acid groups have been ionically crosslinked, no further crosslinks are formed. 

The polar groups in ionomers are suppressing the tendency of crystallization. Moreover, a ionic crosslinking is effected. Thus, both secondary valency forces and ionic forces are active. The special types of bonds effect a special toughness of the materials. However, ionomers are true thermoplastic materials. 

Y.F. Xu, R.Y.M. Huang, Pervaporation separation of ethanol-water mixtures using ionically crosslinked blended poly(acrylic acid) (PAA)-nylon 6 membranes, J. Appl. Polym. Sci. 36 (1988) 1121— 1128. 

S. Matsui and D.R. Paul, Pervaporation Separation of Aromatic/Aliphatic Hydrocarbons by a Series of Ionically Crosslinked Poly(n-alkyl acrylate) Membranes, J. Membr. Sci. 213, 67 (2003). 

Berger J, Reist M, Mayer JM, Felt O, Peppas NA, Gurny R. Structure and interactions in covalently and ionically crosslinked chitosan hydrogels for biomedical applications. European Journal of Pharmaceutics and Biopharmaceutics 2004, 57, 19-34. 

ITPE is due to the formation of strong intermolecular ionic crosslinks, which act as a compatibiliser. The IR spectra [75] of Zn-mHDPE and Zn-XNBR in the range of 1750-1250 cm 1 are shown in Figure 5.14. The absence of the band corresponding to hydrogen bonded carboxylic acid pairs (1700-1720 cm 1) indicate almost complete neutralisation of the acid groups in both polymers. 

The maximum zinc content of the finished ionic crosslinked mixed polymerization material is about 3.5 %. Polymer mixtures of ethylene, propylene, butylene, vinyl esters and unsaturated aliphatic acids as well as salts and esters can be used in the manufacture of ionomer food contact materials. The materials are then differentiated according to whether they contain no crosslinking or have by peroxides ionic or physical crosslinked materials. 

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