Polyelectrolyte polycarboxylic

AB cements are not only formulated from relatively small ions with well defined hydration numbers. They may also be prepared from macromolecules which dissolve in water to give multiply charged species known as polyelectrolytes. Cements which fall into this category are the zinc polycarboxylates and the glass-ionomers, the polyelectrolytes being poly(acrylic acid) or acrylic add copolymers. The interaction of such polymers is a complicated topic, and one which is of wide importance to a number of scientific disciplines. Molyneux (1975) has highlighted the fact that these substances form the focal point of three complex and contentious territories of sdence , namely aqueous systems, ionic systems and polymeric systems. 

Polyelectrolytes are polymers having a multiplicity of ionizable groups. In solution, they dissociate into polyions (or macroions) and small ions of the opposite charge, known as counterions. The polyelectrolytes of interest in this book are those where the polyion is an anion and the counterions are cations. Some typical anionic polyelectrolytes are depicted in Figure 4.1. Of principal interest are the homopolymers of acrylic acid and its copolymers with e.g. itaconic and maleic adds. These are used in the zinc polycarboxylate cement of Smith (1968) and the glass-ionomer cement of Wilson Kent (1971). More recently, Wilson Ellis (1989) and Ellis Wilson (1990) have described cements based on polyphosphonic adds. 

Polyelectrolyte-based dental cements or restorative materials include zinc polycarboxylates, glass ionomers, a variety of organic polyelectrolyte adhesives as well as alginate-based impression materials. Dental cements are primarily used as luting (cementing) agents for restorations or orthodontic bands, as thermal insulators under metallic restorations, and as sealents for root canals, pits and fissures. They are also sometimes used as temporary or permanent (anterior) restorations. For further introduction to dental materials the reader is referred to standard texts [122,123]. 

Zinc polycarboxylate, the first polyelectrolyte dental material, was developed and used as early as 1968 [124]. These materials are formed by the reaction of a zinc oxide powder with an aqueous solution of poly(acrylic acid). The zinc ions cross-link the polyacid chains and form a cement. A few years after the development of zinc polycarboxylate cements, Wilson and Kent introduced the first glass-ionomer cement (GIC) [125]. Glass-ionomer cements are formed... 

At the same time, the industry embarked on an intensive search for phosphate substitutes. Of a very large number of experimental organic builders, a few substances reached commercialization or near-commercialization, including trisodium nitrilotriaceate (NTA), trisodium carboxymethoxysuccinate (CMOS) (181) and trisodium carboxymethyltartronate (182). As discussed above, sodium citrate ether carboxylates have achieved widespread use as phosphate substitutes. Polymeric builders (polyelectrolytes) proved to be effective calcium sequestrants, but failed to satisfy the criterion of acceptable biodegradability. Interestingly, some monomeric polycarboxylates proved to be even more powerful calcium sequestrants than sodium tripolyphosphate but were not sufficiently biodegradable (183). 

J.W. Nicholson, Studies in the setting of polyelectrolyte materials. Part 3 effect sodium salts on the setting and properties of glass polyalkenoate and zinc polycarboxylate dental cements, J. Mater. Sci. Mater. Med. 6 (1995) 404 08. 

A different effect occurs with the use of polycarboxy-lates in combination with zeolites. Small amounts of polycarboxylates or phosphonates can retard the precipitation of sparingly soluble calcium salts such as CaCOs (the threshold effect ). As they behave as anionic polyelectrolytes, they bind cations (counterion condensation), and multivalent cations are strongly preferred. Whereas the pure calcium salt of the polymer is almost insoluble in water, mixed Ca/Na salts are soluble, i.e. only overstoichiometric amounts of calcium ions can cause precipitation. Polycarboxylates are also able to disperse many solids in aqueous solutions. Both dispersion and the threshold effect result from the adsorption of the polymer on to the surfaces of soil and CaCOs particles, respectively. 

A polyelectrolyte can be utilized as a model to investigate the mechanism of ion selectivity of ion exchangers. The ion selectivity exhibited by polyelectrolytes is therefore an important property for practical applications. Our work deals specifically with polycarboxylic polyelectrolytes in salt free aqueous solutions. In the first part, the activities of monovalent and divalent counterions are determined and discussed in connection with the nature of the counterion and polyelectrolyte charge density. Then, experimental results are presented showing clearly the existence of selectivity and affinity sequences. 

The addition of polyelectrolytes with strong inhibition ability [241,242] will stabilize amorphous nanobuilding blocks in the early stage and then stimulate a mesoscale transformation [40] or act as a material depot in a dissolution-recrystallization process. This could be shown in a time resolved study on the CaCOa scale inhibition efficiency of polycarboxylates, where amorphous precursor particles were detected in the initial stages [243]. 

Addition of sodium, potassium, calcium, ammonium, etc., bases to aqueous solutions of Gantrez AN resins leads to two viscosity peaks, corresponding to 1 and 2 mole equivalents of base. The interpolymers are precipitated by Ca ions and other heavy, polyvalent metal cations, beyond 0.7 mole equivalents. With ammonium hydroxide, peaks tend to be observed at about 1 and 3 mole equivalents of base. Ionic interactions in aqueous solutions of Gantrez AN polyelectrolytes and polycarboxylates, electrophoretic mobility and viscosity of copolymer salts, counterion binding properties, etc., have received substantial <659,672.700-702,1100) polyelectrolyte salt sol-... 

Studies on the interaction between surfactants and styrene-ethylene oxide block co-polymers, however, indicate that the polymers exhibit, in the presence of surfactant, typical polyelectrolyte character. This, it has been suggested [264], is due to interaction repulsions between like charges of the NaDS ions adsorbed onto the polyoxyethylene blocks. Investigating the interaction of the same detergent with methylcellulose and poly(vinyl alcohol), Lewis and Robinson [265] also observed the polyelectrolyte character of the polymer-surfactant complexes. A complex between non-ionic surfactants and a polycarboxylic acid in water can solubilize oil-soluble dyes below the surfactant CMC [268]. The complex containing the solubilizate can be precipitated the solubilizate remains in the precipitated complex and is leached out only slowly on placing the precipitate in fresh solvent. This has potential pharmaceutical implications. Halothane uptake by coacervate systems of gelatin-benzalkonium [269] has... 

Similar structural effects also dominate the interaction of polyelectrolytes with divalent ligands, for example, of polycarboxylic acids with divalent cations. Investigations by Zwick [13] and Michaeli [14] had shown that only some 80% of the fixed charges can be neutralized and that this could be accounted for by the statistics of divalent ligand placement on a linear lattice. 

It is, perhaps, fair to say that extensive use of polyelectrolytes as biomaterials has been partly hindered by lack of fundamental and systematic information on these macromolecules pertaining to specific applications as biomaterials. In the present work, the phenomena studied are surface adsorption and ionic crosslinking of poly(alkenoic acid) aqueous solution. These two phenomena are directly relevant to the present use of zinc polycarboxylate cements (ZP) and glass ionomers (GI) in restorative dentistry. Both of these types of dental biomaterials are formulated for clinical use as two-component systems. A liquid, which is an aqueous solution of a poly(alkenoic acid), is used in both products. In the case of... 

The results on ionization kinetics, shown in Figure 6, reveal considerable differences among a zinc polycarboxylate and two glass ionomers in dental use at the present time. These results should not be necessarily interpreted as crosslinking kinetics of the polyelectrolytes by cations such as, Ca", or Al" to form the... 

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