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Structure II Poly(HEMA) crosslinked with N,N'-methylene-bisacrylamide (BIS)

increasing the viscosity of the medium and retarding the drainage of the tear fluid from the eye.

Heterogels

As it is possible to produce macromolecular chains with segments that have different solubilities in a given solvent (copolymers), one would expect that concentrated solutions of such copolymers would behave in a manner different from that of a simple polymer. In block copolymers of the type AAABBBAAA, in which A is water-soluble and B is water-insoluble, the insoluble parts will tend to aggregate. If, for instance, a polystyrene -poly(oxyethylene) copolymer, comprising 41% polystyrene and 59% poly(oxyethylene), is dissolved at 80°C in butyl phthalate (a good solvent for polystyrene), a gel with a microscopic layer structure is formed at room temperature; in nitromethane the form is somewhat different (Fig. 8.8a) as the nitromethane preferentially dissolves the poly(oxyethylene) chains.

Poly(oxyethylene) -poly(oxypropylene) -poly(oxyethylene) block copolymers, known commercially as Pluronic or poloxamer surfactants, are used as emulsifiers. Some form micellar aggregates and in aqueous solutions above a critical micelle concentration, in which the hydrophobic central block associates with other like blocks, leaving the hydrophilic poly(oxyethylene) chains to the outside and protecting the inner core. Packing of these micelles in solution of high concentration leads to the reversible formation of gels as shown in Fig. 8.9.

8.3.3 Syneresis

Syneresis is the term used for the separation of liquid from a swollen gel. Syneresis is thus a form of instability in aqueous and non-aqueous gels. Separation of a solvent phase is thought to occur because of the elastic

(a)

Figure 8.8 Structure of a copolymer of type A-B made from polystyrene and polyoxyethylene (a) in nitromethane (cylindrical structure) and (b) in butyl phthalate (layer structure). Nitromethane dissolves the poly(oxyethylene) part preferentially, but butyl phthalate dissolves the polystyrene part. (--) Polystyrene; (---), poly(oxyethylene); (O)

solvent.

Reproduced from F. Sadron, Angew. Chem., 2, 248 (1963).

contraction of the polymeric molecules; in the swelling process during gel formation, the macromolecules involved become stretched and the elastic forces increase as swelling proceeds. At equilibrium, the restoring force of the macromolecules is balanced by the swelling forces, determined by the osmotic pressure. If the osmotic pressure decreases, for example on cooling, water may be squeezed out of the gel. The syneresis of an acidic gel from Plantago albicans seed gum2 was decreased by the addition of electrolyte, glucose and sucrose and by increasing the gum concentration; pH had a marked effect on the separation of water. At low pH marked syneresis occurs, possibly due to suppression of ionisation of the carboxylic acid groups, loss of hydrating water and the formation of intramolecular hydrogen bonds. This would reduce the attraction of the solvent for the macromolecules.

8.3.4 Polymer complexes

The varied structure and chemistry of polymers provide ample opportunity for complexes to form in solution. One example occurs when an aqueous solution of high molecular weight polyacids is mixed with polyglycols. The viscosity and pH of the solution of the equimolar mixture of polyacid and glycol remain the same with the increase in oligomer chain length up to a critical point. The nature of the interaction is shown in (III); this occurs only

Figure 8.9 ABA-type copolymers: (a) micelles in dilute solution, (b) formation of a cubic-phase gel, by packing of micelles.

Figure 8.9 ABA-type copolymers: (a) micelles in dilute solution, (b) formation of a cubic-phase gel, by packing of micelles.

when the polyethylene glycol molecules have reached a certain size.

Complexes between polyvinylpyrrolidone and poly(acrylic acid)s are also possible (IV). Such macromolecular reactions are highly selective and strongly dependent on molecular size and conformation. On mixing, some of the macromolecules might be involved in the complex while the rest will be free. The reason for compositional heterogeneity of the products could be the conformational transitions of macromolecules in the course of complex formation.

Interactions between macromolecules can occur in formulations, for example when preparations are mixed. They can be put to good advantage in the synthesis of novel compounds. Polyethyleneimine and poly(acrylic acid) form a polyelectrolyte complex with saltlike bonds as shown in (V). If the complex is heated as a film, interchain amide bonds are formed between the groups which formed electrostatic links. The nonionised -COOH and -NH groups in the chain are the points of structural defects in the film.

Biological macromolecules undergo complex reactions which are often vital to their activity. Recent studies have established a specific interaction between hyaluronic acid (VI) and the proteoglycans in the intracellular matrix in cartilage. An understanding of these macromolecular interactions is sometimes of value in elucidating the effects of drugs or formulations in vivo. The essential feature of the proposed proteoglycan-hyaluronic acid (PG-HA) complex is that many proteoglycans are able to bind along the entire length of the hyaluronic acid chain (at saturation there is one to each 20 disaccharide units). Each proteoglycan can bind to only one hyaluronic acid chain, so the system does not readily form a network or gel by an interaction of the type HA-PG-HA, but instead the PG-HA aggregates interact electrostatically (via polysaccharide side-chains) with collagen to form the molecular organisation in cartilage.

Structural investigations of the anticoagulant macromolecule heparin (VII) currently favour a linear polydisaccharide.

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