The properties of macromolecules result from the subtle interplay of processes operating at several different structural levels. These include the nature of the atoms in the molecules and the way in which molecules come to together in chains or other units to form the bulk material; whether in regular, ordered crystalline arrangements or irregular, amorphous arrangements and on the presence of complex microstructures, vacancies and defects within the bulk material.
Of particular importance is the stereo regularity or tacticity of the polymer. Polymers in which all units have a spatially identical arrangement of atoms are called isotactic, those in which a random arrangement prevails are called atactic, and those which show a regular alteration in configuration along the chain are called syndiotactic. The tacticity of polymer molecules affects the way in which adjacent molecules can fit together in the bulk material and hence controls the strength of forces between molecules from which the mechanical properties arise. Stereoregular homopolymers with strong inter-chain forces, and certain block co-polymers, tend to show significant crystallinity whereas atactic homopolymers and random co-polymers are amorphous.
As with most materials, the physical state of polymers is temperature-dependent. Unlike changes of state in simple molecules, which generally occur at definite temperatures, changes of state in polymers are less well defined and often occur over a finite temperature range. Most thermoplastics are either amorphous or only slightly crystalline and exist in different states according to average molecular weight and temperature. Generally, increasing molecular weight or degree of polymerization increases toughness, viscosity in solution and softening point, and decreases rate of solution. At low molecular weights these polymers are solid below a certain temperature and liquid above it. At higher molecular weights a clearly defined melting point no
Low molecular weight
High molecular weight
Low molecular weight
High molecular weight
Figure 4.2 Glass transition temperature - the relationship of physical state to temperature for amorphous polymeric materials of different molecular weights longer occurs and a rubbery intermediate zone is observed. In the solid state, amorphous and moderately crystalline polymers have a glassy transparent appearance, sometimes called the glassy state. The transition from a glassy or brittle state to a rubbery state occurs at the glass transition temperature, Tg (Figure 4.2). The glass transition temperature is also sometimes referred to as the second order transition temperature. Increase in temperature above Tg causes progressive softening until the material becomes a viscous fluid. In the glassy state, molecular movements other than bond vibrations are very limited. Above Tg the molecules have more energy and movement of molecular segments becomes possible. However, above a certain crosslink density movement of the complete molecule as a unit does not take place. Softening under heat is the basis of moulding, extrusion and heat sealing of these materials and is important in their use as coatings and adhesives in conservation. When heated, thermosets do not soften to a truly fluid condition but some become sufficiently flexible to allow them to be bent into simple shapes.
Many properties apart from hardness are affected by the change from the glassy to the rubbery state. These include specific volume, specific heat, thermal conductivity, dynamic modulus and simple stress/strain characteristics. It is important to be aware of this since a polymer in use near its Tg may show quite marked changes in properties with small changes in temperature. For example, many grades of PVAC and some methacrylates have a Tg close to room temperature.
Transition temperatures additional to Tg may occur (e.g. in some methacrylates) because at temperatures below Tg the side chains and sometimes small segments of the main chains require less energy for mobility than the main segments associated with Tg. These are designated alpha, beta, gamma etc., with alpha being Tg and the others following in order of decreasing temperature. Secondary transitions confer useful properties on particular polymers, for example, materials that have secondary transitions below room temperature tend to be tough.
For successful use a polymer must have appropriate rigidity, toughness, resistance to long term deformation, and recovery from deformation on release of stress over the range of operating conditions. The same factors will apply to selecting a polymer for use in conservation as to the care and conservation of objects made of these materials.
The rigidity of a polymer is determined by the ease with which its molecules are deformed under load. In the absence of secondary transitions, at temperatures below Tg, the load is taken by bond bending and stretching. Secondary transitions below Tg allow more response to stress resulting in a decrease in elastic modulus (i.e. a more elastic material). Changes due to such secondary transitions are usually small but the change in behaviour that occurs at Tg is highly significant. Young's modulus, the most widely quoted measure of elasticity, indicates the resistance of a material to reversible longitudinal extension (see also section 2.5). At the Tg in an amorphous polymer the modulus may drop from typical values of 3500 MN/m2 to less than 1 MN/m2. The effect of further temperature increases depends on molecular weight. In lower molecular weight polymers the modulus drops rapidly towards zero. In higher molecular weight material a significant rubbery modulus may be maintained up to the decomposition temperature. It should be noted that fracture frequently occurs at defect sites which may be due to additives or processing and not just to the properties of the bulk polymer.
Similar effects occur in crosslinked polymers; the greater the degree of crosslinking, the higher the modulus. Molecular movement above the glass transition temperature is restricted by crystallinity; the greater the degree of crystallinity the more rigid the polymer. Additives may also affect the modulus (Roff and Scott, 1971). Generally, plasticizers reduce it and fillers increase it. The elastic modulus of unplasticized poly(vinyl butyral) is about 1500 MN/m2. With plasticizer this may fall below 10 MN/m2. The elastic modulus of phenol formaldehyde systems may rise from around 3000-6000 MN/m2 unfilled to over 20000 MN/m2 filled. Similar effects are noted on tensile strength (the maximum tensile stress to which a material may be subjected before breaking). The properties of some polymers (for example, poly(vinyl alcohol)) are markedly affected by humidity changes.
The toughness of a material is indicated by its resistance to the sudden application of a mechanical load. Tough materials will show greater elongation before breaking. Polymers that fracture with very little deformation are said to be brittle. Toughness is influenced by polymer structure and temperature and by the method and rate of stressing. A polymer may be tough when subjected to a tensile load but brittle when assessed by an Izod type test in which a notched sample is subjected to a bending load. In real life situations, toughness is also affected by stress concentrations due to design and manufacture. It is possible to obtain materials that are both rigid and tough by blending polymers with different properties, co-polymerization, the use of additives and by choosing the right processing conditions.
The deformation and recovery from deformation of linear polymers is a complex process having components that can be described as (1) normal elastic behaviour (Hooke's law obeyed). (2) highly elastic and (3) viscous (Brydson, 1991; Horie, 1987) One aspect of this behaviour is the phenomenon described as 'creep' or cold-flow in which a fixed stress can bring about permanent deformation. Plastic deformation of this kind is much enhanced at elevated temperatures and its presence in certain polymers (e.g. PVAC and PVAL) even at room temperature is undesirable and renders them unsuitable for applications involving prolonged periods of stress. The closer a thermoplastic material is to its Tg at room temperature, the greater the likelihood of creep.
As with all categories, the clear distinction between thermoplastic and thermosetting tends to break down if examined closely enough. Linear polymers can oxidize and crosslink as they age, becoming in essence thermosetting, while even a highly crosslinked polymer such as oriental lacquer (urushi) is somewhat thermoplastic. Although linear polymers are generally more soluble in various solvents than crosslinked polymers some, such as high density polyethylene, are insoluble due to very high molecular weight (long chain length) and/or crystallinity due to the arrangement of the chains. Properties of polymer materials relevant to their use as adhesives, coatings and consolidants are further discussed below. Chain branches and cyclic polymer structures also strongly influence solubility.
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