This series of condensation reactions can be related to the simple esterification reaction shown above if X is a carboxylic acid group and Y is an alcohol group. The reaction product, XY, produced as a result of every condensation reaction then becomes water for that specific example.

It can be seen that at each stage of the reaction the chain grows by one monomer unit and there is one molecule of byproduct XY evolved. In addition, the growing polymer chain retains two reactive groups at each stage. The resulting polymer is a regular copolymer of the monomers M1 and M2 which are arranged in sequence along the chain. Chain branching and cross-linking can be produced by introducing some trifunctional monomer, e.g.

X

|

X − M1 − X

into the reaction.

By using a monomer which carries two reactive groups it is possible to produce a homopolymer as follows:

X − M1 − Y + X − M1 − Y →

X − M1 − M1 − Y + XY

X − M1 − M1 − Y + X − M1 − Y →

X − M1 − M1 − M1 − Y + XY

X − M1 − M1 − M1 − Y + X − M1 − Y → X− M1 − M1− M1− M1 − Y + XY etc.

Examples of the use of condensation polymerisation include the production of nylon 6,6 as illustrated in Fig. 12.8, and the synthesis of polydimethysiloxane (silicone rubber) as illustrated in Fig. 12.9. The latter example illustrates the simplest type of condensation polymerisation, in which each molecule contains two identical reactive groups (hydroxyl groups in this case) which are capable of reacting to eliminate water.

12.3 Physical changes occurring during polymerisation

Phase changes: Most monomers are gases or liquids at normal temperatures and pressures. Some liquid dental monomers are mixed with inert fillers, such as glass powders, to form pastes which can be easier to handle than the monomer alone. In addition, the filler often has a very beneficial effect on the properties of the set material. During polymerisation, as the average molecule weight increases and chain entanglement occurs, the viscosity begins to increase. This is most noticeable when polymerisation is being carried out in the absence of solvents, which is the case for the vast majority of dental polymers. When

Fig. 12.9 The formation of hydroxyl-terminated polydimethylsiloxane by condensation of hydrolysed dimethylsiloxane.

the polymerisation has reached an advanced stage the chain entanglements may become so great that the material develops a degree of rigidity and effectively behaves more like an amorphous solid than a liquid. At this stage the rate of conversion of monomer declines rapidly for two reasons. First, the quantity of unreacted monomer is now very small. Secondly, the rate of diffusion of the monomer through the highly viscous material to the reactive polymerisation sites becomes very slow. The result is that polymer produced as a result of the reaction inevitably contains a small concentration of unreacted or residual monomer. The majority of dental polymers have amorphous structures. Other polymers are capable of forming crystalline structures in which the chains are arranged in regular patterns.

Temperature changes: The majority of polymerisation reactions are exothermic in nature and cause a marked increase in temperature of the polymerizing material. This temperature rise may have important consequences for both industrial polymers and dental polymers.

Industrial polymers are generally produced in large quantities using huge reaction vessels. An uncontrolled increase in temperature in such a vessel could be catastrophic. The heat liberated will cause an acceleration in the rate of polymerisation which causes heat to be liberated more rapidly and so on. If such a process continues an explosion may result. At the very least, the polymer may solidify in the reaction vessel causing the requirement for a massive cleaning operation or the scrapping of the reaction vessel. Naturally, attempts are made to avoid such events which may be both dangerous and expensive. One of the most effective ways of dissipating heat during polymerisation is to either dissolve the monomer in a suitable solvent or to form a suspension of the monomer in water (suspension polymerisation). The heat of polymerisation is dissipated through the solvent or water. The resulting polymer, normally in the form of a powder, may be collected either by precipitation from the solvent or by sieving beads from the water. Suspension polymers formed in this way are normally moulded into the required shape by softening the polymeric moulding powder with heat and shaping under pressure (e.g. injection moulding). Condensation polymers are often cured in bulk in the absence of water or solvent. The reaction is allowed to proceed under controlled conditions and is halted (e.g. by cooling) when only a small proportion of the reactive groups of the monomer have been converted. At this stage the partially polymerised monomer is still relatively fluid and can be moulded. Polymerisation is completed during the moulding stage.

Dental polymers are used in much smaller quantities than industrial polymers and temperature rises are therefore much smaller and less likely to cause the reaction to go out of control. Polymerisation in bulk is therefore possible and this enables the conversion of monomer to polymer to be carried out in situ (e.g. in the mouth) in many cases. Due consideration must still be given to the temperature rise during polymerisation however, as a relatively small increase in temperature within a filling material may damage the dental pulp. Another occurrence in some materials is that elevated temperatures during polymerisation cause vaporization of some unconverted monomer producing spherical voids, known as gaseous porosity, in the set material.

Dimensional changes: Each addition or condensation reaction which takes place during polymerisation results in a small contraction since the resulting species occupies less space than the two initial reacting species. Since many hundreds of reactions of this type take place during polymerisation, the overall shrinkage which takes place can be very marked. For example, when methylmethacrylate is converted to polymethylmethacrylate a volumetric shrinkage of about 21% occurs. Dimensional changes of this magnitude are unacceptable for most dental applications and methods are normally sought to reduce the shrinkage.

Common methods involve blending the monomer with an inert material such as glass powder or polymer (e.g. polymethylmethacrylate beads in the case of methylmethacrylate), overfilling moulds to allow for some contraction, polymerizing under pressure and using monomers which undergo less polymerisation contraction. The latter method is based on the principle that if larger monomer molecules are used the concentration of reactive groups within them is decreased and the subsequent contraction is less. Inspection of Fig. 22.3, for example, shows that the concentration of reacting carbon–carbon double bonds in Bis-GMA is much lower than in the equivalent amount of methylmethacrylate. The polymerisation shrinkage for Bis-GMA is therefore considerably lower than that for methylmethacrylate. In some urethane acrylates and methacrylates the space between the two terminal reactive double bonds is greater still, effectively diluting them further and reducing the shrinkage to a greater extent.

12.4 Structure and properties

Factors which control the structure and therefore the properties of polymers include:

(1)The molecular structure of the repeating units including the use of copolymers.

(2)The molecular weight or chain length.

(3)The degree of chain branching.

(4)The presence of cross-linking and the crosslink density.

(5)The presence of plasticizers or fillers.

Two basic properties which characterise polymers are glass transition temperature (Tg) and melting temperature (Tm). Crystalline polymers exhibit both a glass transition temperature and a

melting temperature. Amorphous polymers, on the other hand, exhibit only a glass transition temperature. The latter materials are more widely used in dentistry. The glass transition temperature is the temperature at which molecular motions become such that whole chains are able to move. This temperature is close to the softening temperature, which can be observed in practice for amorphous polymers. The glass transition temperature can be estimated mechanically by noting the temperature at which a sudden change in elastic modulus occurs.

Amorphous polymers below their glass transition temperature generally behave as rigid solids whilst above the glass transition temperature they may behave as viscous liquids, flexible solids or rubbers, depending on the molecular structure and the degree of branching or cross-linking.

From a practical point of view the value of Tg has great significance. For example, if a denture were constructed from a polymer which had a Tg value of 60ºC, the denture would be rigid at normal mouth temperature but might soften and become flexible on taking a hot drink at 70ºC.

Molecular structure is the factor which, naturally, has the greatest influence over polymer properties. For example, polymer backbones which contain phenyl groups are more rigid than those which contain only carbon–carbon bonds whilst backbones with silicon–oxygen bonds tend to be even more flexible. Hence the introduction of a phenyl group into a polymer backbone has the effect of increasing Tg whilst the introduction of carbon–oxygen links has the reverse effect. It is not only the atomic groups forming the backbone of the polymer chain which have an effect on Tg. Pendant groups may have an equally marked effect. This is illustrated by considering the Tg values of a series of poly N-alkyl methacrylates where the alkyl group can be methyl, ethyl, propyl or butyl (Table 12.1). It can be seen that altering

Table 12.1 Glass transition temperatures of N-alkyl methacrylate polymers.

the pendant group in this series of polymers has a significant effect. Polymethylmethacrylate is rigid at mouth temperature whereas polybutylmethacrylate is relatively soft and flexible.

Molecular weight is another factor which affects Tg. The two properties are related by an equation of the type

Tg = Tg0 − K

M

where K is a constant, M is the average molecular weight and Tg0 is the glass transition temperature for a polymer of infinite molecular weight. It can be seen that the value of Tg increases with increasing molecular weight. As M becomes large Tg approaches Tg0. The reduction of the average molecular weight of a polymer by low molecular weight chains or residual, unreacted monomer may have a considerable effect on the Tg value and hence on material performance. In addition to influencing the glass transition temperature, molecular weight has an effect on other fundamental properties, such as modulus of elasticity. Many polymers exhibit a linear relationship between molecular weight and modulus up to fairly high values of molecular weight (105 or above) then reach a plateau region in which further increases in molecular weight have little or no effect.

Optimizing mechanical properties often involves producing polymers with a high value of average molecular weight and a low concentration of residual monomer. Chain branching may have an effect on polymer properties. Increasing the concentration of branches generally lowers the glass transition temperature. The mechanism of this lowering is probably related to the fact that chain branches prevent adjacent chains from drawing close enough together to undergo interchain attraction and the formation of interchain bonds. This can be viewed as an extension of the trend seen in Table 12.1, where the pendant alkyl groups can be viewed as small chain branches. Unsuccessful attempts to form cross-links can result in a branched structure if only one end of a difunctional monomer becomes polymerised into a chain. The other end of the monomer is left unreacted and may have the opposite effect to that which would be expected.

The effect of cross-linking is of considerable practical importance. Increasing the number of cross-links increases the glass transition tempera-

ture. In addition, alteration of the cross-link density can have a considerable effect on mechanical properties. A material with a low value of Tg and a small cross-link density may behave as a rubber at room temperature. On the other hand, a very high cross-link density normally produces a rigid, brittle polymer in which it is not possible to detect a glass transition since the material often decomposes thermally before softening. An example of this behaviour is the effect of crosslinking on the properties of natural rubber. When natural rubber (polyisoprene) is lightly crosslinked (vulcanized) it has rubbery, elastic properties. When the same material is highly cross-linked however, it becomes hard, rigid and brittle. This highly cross-linked material, vulcanite, has been used as a denture base material.

Certain additives such as plasticizers and fillers can have a profound effect on the properties of polymers. Plasticizers such as di-n-butylphthalate (Fig. 12.10) have an effect on both the glass transition temperature and the modulus of elasticity of some polymers. They are said to ‘lubricate’ the movements of polymer chains and are sometimes added to help moulding characteristics. They also lower the Tg and elastic modulus. For this reason the inclusion of plasticizers is a common method used by manufacturers to produce ‘soft’ polymers.

The inclusion of particulate or fibrous inorganic fillers has an equally significant effect on polymer properties. The modulus of elasticity and strength are generally increased although, in the case of fibre-filled polymers, a degree of anisotropy may exist; that is, the strength depends on the orientation of the fibres in the polymer.

Resin matrix composite materials are now a most important group of products in dentistry. Much research and scientific debate surrounds issues such as the ideal particle size, particle size distribution and filler loading of the discrete glass or ceramic filler to achieve an optimum value of key properties. Filler loading is often maximized

Fig. 12.10 Structural formula of di-n-butylphthalate, a commonly used plasticizer.