M

Substituting for vh in equation (8.10), [V] = limc ^ J ^^ = v(V2 + d,vf)

chapter in discussion of individual macro-molecules, for example, gum Arabic (acacia).

The viscosity of solutions of globular proteins (which are more or less rigid) is only slightly affected by change in ionic strength. The intrinsic viscosity of serum albumin varies only between 3.6 and 4.1 cm3 g 1 when the pH is varied between 4.3 and 10.5 and the ionic strength between zero and 0.50.

Viscosity in pharmacopoeial specifications

In cases where control of molecular weight is important, for example in the use of dextran fractions as plasma expanders, a viscosity method is specified, for example, in the BP monograph. Staudinger proposed that the reduced viscosity of solutions of linear high polymers is proportional to the molecular weight of the polymer or its degree of polymerisation, p:

Shape and solvent

As the shape of molecules is to a large extent the determinant of flow properties, change in shape due to changes in polymer-solvent interactions and the binding of small molecules with the polymer may lead to significant changes in solution viscosity. The nature of the solvent is thus of prime importance in this regard. In so-called 'good' solvents linear macromolecules will be expanded as the polar groups will be solvated. In a 'poor' solvent the intramolecular attraction between the segments is greater than the segment-solvent affinity and the molecule will tend to coil up (see Fig. 8.6). The viscosity of ionised macro-molecules is complicated by charge interactions which vary with polymer concentration and additive concentration. Flexible charged macromolecules will vary in shape with the degree of ionisation. At maximum ionisation they are stretched out owing to mutual charge repulsion and the viscosity increases. On addition of small counterions the effective charge is reduced and the molecules contract; the viscosity falls as a result. Some of the effects are illustrated later in this

This empirical law has been modified to limC20 ^ = [n] = KMa

where a is a constant in the range 0-2, which for most high polymers has a value between 0.6 and 0.8, [n] is the intrinsic viscosity as defined previously, and M is the molecular weight of the polymer. For a given polymersolvent system, K and a are constant. Values of these constants may be determined from measurements on a series of fractions of known molecular weight and hence the molecular weight of an unknown fraction can be determined by measurement of the intrinsic viscosity. The viscosity average molecular weight is essentially a weight average since the larger macromolecules influence viscosity more than the smaller ones. The intrinsic viscosity of Dextran 40 BP is stated to be not less than 16 cm3 g_1 and not more than 20 cm3 g 1 at 37°C, while that of Dextran 110 is not less than 27 cm3 g1 and not more than 32 cm3 g1.

8.3.2 Gelling water-soluble polymers

Concentrated polymer solutions frequently exhibit a very high viscosity because of the interaction of polymer chains in a three-dimensional fashion in the bulk solvent. These viscous crosslinked systems are termed gels. A gel is a polymer-solvent system containing a three-dimensional network of quite stable bonds which are almost unaffected by thermal motion. If such a polymer network is surrounded by the solvent (the system can be arrived at by swelling of solid polymer or by reduction in the solubility of the polymer in the solution) the system is a gel regardless of whether the network is formed by chemical or physical bonds. When gels are formed from solutions, each system is characterised by a critical concentration of gelation below which a gel is not formed. This concentration is determined by the hydrophile-lipophile balance of the polymer and the degree of regularity of the structure, by polymer-solvent interaction, by molecular weight and the by the flexibility of the chain: the more flexible the molecule the higher is the critical gelling concentration. The characteristic features of a gel include the considerable increase in viscosity above the gel point, the appearance of a rubber-like elasticity, and, at higher polymer concentrations, a yield point stress. Under small stress the gel should retain its shape, but at higher stress considerable deformation can occur.

Type I and type II gels

Gels can be divided into two groups, depending on the nature of the bonds between the chains of the network. Gels of type I are irreversible systems with a three-dimensional network formed by covalent bonds between the macromolecules. They include swollen networks which have been formed by polymerisation of a monomer in the presence of a crosslinking agent.

Type II gels are heat-reversible, being held together by intermolecular bonds such as hydrogen bonds. Sometimes bridging by additive molecules can take place in these type II systems. Poly(vinyl alcohol) solutions gel on cooling below a temperature known as the gel point. The gel point can therefore be influenced by the presence of additives which can induce gel formation by acting as bridge molecules, as, for example, with borax and poly(vinyl alcohol). The gel point of polymers can also be increased or decreased by the addition of solvents which alter the polymer's affinity for the solvent (Table 8.3).

Solutions of vinyl alcohol polymers in water are viscous mucilages which resemble those formed by methylcellulose; the viscosity of the mucilage is greatly increased by incorporating sodium perborate or silicate. Because of their gelling properties poly(vinyl alcohol) solutions are used as jellies for application of drugs to the skin. On application, the gel dries rapidly, leaving a plastic film with the drug in intimate contact with the skin. Plastic film (Canadian Pharmacopoeia) is prepared from poly(vinyl alcohol) and other additives and is intended as a vehicle for acriflavine, benzocaine, ichthammol and other topical drugs.

Gelation can occur either with a fall (as with poly(vinyl alcohol)) or a rise in the temperature depending on the type of temperature variation of solubility. While gels of type II are commonest in pharmacy, with the interest in polymers as drug delivery adjuvants some type I materials are being used.

Crosslinked polymeric systems

If water-soluble polymer chains are covalently crosslinked, gels will be formed when the dry material interacts with water. The polymer swells in water but cannot dissolve as the

Table 8.3 Gel points of

10% poly(vinyl alcohol)a

Solvent

Gel point (°C)

Water

0 0

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