Polyelectrolytes cement

In the 1870s more effective liquid cement-formers were found ortho-phosphoric acid and eugenol (Wilson, 1978). It was also found that an aluminosilicate glass could replace zinc oxide, a discovery which led to the first translucent cement. Thereafter the subject stagnated until the late 1960s when the polyelectrolyte cements were discovered by Smith (1968) and Wilson Kent (1971). 

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. 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

These early views are, perhaps, too simplistic to explain in full the rheological changes that occur in polyelectrolyte cement pastes before and at gelation. There are several physicochemical processes that underlie... 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

The polyelectrolyte cements may be classified by the type of basic powder used to form the cement. 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

The molecular structure of the polyelectrolyte cements has been examined by a number of workers using infrared spectroscopy (Crisp et al., 1974 Crisp, Prosser Wilson, 1976 Wilson, 1982 Nicholson et al., 1988a,b). The asymmetrical COO stretching modes in particular can be used to... 

Gelation involves an extended structure and some type of linking between chains. The concept of salt-like crosslinks has already been described (Section 5.5). Other possibilities may be considered. Hill, Wilson Warrens (1989) examined the possibility that chain entanglements might account for the strength of polyelectrolyte cements. They used in particular... 

Cook, W. D. (1982). Dental polyelectrolyte cements. I. Chemistry of the early stages of the setting reaction. Biomaterials, 3, 232-6. 

Cook, W. D. (1983c). Degradative analysis of glass-ionomer polyelectrolyte cements. Journal of Biomedical Materials Research, 17, 1015-27. 

Watts, D. C., Combe, E. C. Greener, E. H. (1979). Effect of storage conditions on the mechanical properties of polyelectrolyte cements. Journal of Dental Research, 58, Special Issue C, Abstract No. 18. 

Wilson, A. D., Crisp, S., Prosser, H. J., Lewis, B. G. Merson, S. A. (1980). Aluminosilicate glasses for polyelectrolyte cements. Industrial Engineering Chemistry Product Research Development, 19, 263-70. 

This chapter is devoted to a miscellaneous group of aqueous acid-base cements that do not fit into other categories. There are numerous cements in this group. Although many are of little practical interest, some are of theoretical interest, while others have considerable potential as sustained-release devices and biomedical materials. Deserving of special mention as biomedical materials of the future are the recently invented polyelectrolyte cements based on poly(vinylphosphonic adds), which are related both to the orthophosphoric acid and poly(alkenoic add) cements. 

Ellis, 1989 Ellis, 1989 Ellis Wilson, 1990, 1991, 1992 Ellis, Anstice Wilson, 1991). They are polyelectrolyte cements related to the poly-alkenoate cements and represent an attempt to improve on them. PVPA (Figure 8.2) has a structure similar to that of PAA (Figure 5.2). 

The chemistry of polyelectrolyte cement liquids has been studied using NMR. Watts (1979) used this technique to distinguish between the homopolymer of acrylic acid and its copolymer with itaconic acid in various commercial polyelectrolyte dental cements. This was readily achieved because of the ability of NMR to differentiate between carbon atoms in chemical environments that are only slightly different. 

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