Polymers have molecules that are very long and chainlike, usually extending over several thousand angstroms. Because of their great length these molecules, which are usually organic, are referred as macromolecules. A true macromolecule is a set of atoms bound together by covalent bonds. In addition, two neighboring macromolecules are held together by lateral weak van der Waals forces. These large molecules may be linear, slightly branched or highly interconnected. In the later case the structure develops into a large three-dimensional network.
The structural arrangement of a single macromolecule is shown schematically in . The small molecule used, as a basic building block (M) for these large molecules, known as monomers. A common polymer having a simple structure is polyethylene, which is shown in. The monomer used here is vinyl chloride group, CH2-CHCl-, whose structure is show in , where as shows the structure of free polyethylene molecule. The binding force holding the monomers together are usually covalent or ionic, in nature, and consequently very strong. The common commercially used polymers are Polyethylene, Polyvinyl chloride (PVC), Nylons, Epoxy resin, Phenol-Formaldehyde polymers etc.
The size of polymer molecule may be defined either by its mass or by the number of repeat units in the molecule; in the later indicator the number of monomers in a single macromolecule is called the degree of polymerization (DP), which is typically 104, or even more.
As the characteristic of covalent compounds, in addition to primary valence forces, polymer molecules are also subject to various secondary intermolecular forces. These include dipole forces between oppositely charged ends of polar bonds and dispersion forces which arise due to perturbations of the electron clouds about individuals atoms within the polymer molecule and a week van der Waals forces due to which two neighboring macromolecules are held together.
CLASSIFICATION OF POLYMERS
Polymers can be classified into two main branches, Thermoplastic and Thermosets, depending upon their response to thermal treatment and according to the amount of cross-linkage present. Thermoplastics are polymers, which melt when heated and resolidify when cooled, while thermosets are those, which do not melt when heated, but at sufficiently high temperature decompose irreversibly. Thermoplastics are essentially linear or lightly branched polymer molecules, while thermosets are substantially cross-linked materials, consisting of an extensive three-dimensional network of covalent chemical bonding.
Another classification system is based on the nature of the chemical reactions employed in the polymerization. The two major groups are the Condensation and Addition polymers. Condensation polymers are those prepared from monomers where reaction is accompanied by the loss of a small molecule, usually of water. For example polyesters, which are formed by the condensation shown in reaction (1-1)
STRUCTURE OF EPOXY RESIN
Epoxy resins contain the epoxied group, also called the epoxy, oxirane, or ethoxyline group, which is a three membered oxide ring. The simplest compound in which it is found is ethylene oxide, and gives the formula for a number of substituted ethylene oxides which are related to epoxide resin technology.
The resins can be regarded as compounds which contain, on average, more than one epoxide group per molecule; and they are polymerized through these epoxide groups, using a cross linking agent, to form a tough three dimensional network
The resin-curing agent combination often has other materials added to it, such as inert fillers, diluents, flexibilisers, and flame-retardents, which are intend to achieve certain physical or chemical properties in the cured resin, or to cheapen it.
The changes caused in epoxide resin systems by the incorporation of fillers are low cost of production, reduced shrinkage on curing, loss of transparency, increased thermal conductivity, increased dielectric constant and power factor, reduced water absorption, increased electrical strength. Most epoxide resin systems have filler incorporated, the choice being determined by the property it is wished to improve, the composition of the resin, the curing agent, other components and practical considerations. Clearly the type and amount of filler in an electrical application will differ from that in a trowel ling mixture or in an adhesive formation.
In electrical casting systems non-conducting fillers are used whereas metallic fillers are used to degrade the electrical properties. On the other hand, powdered copper or powder silver has been used to prepare electrical conducting castings.
Covalent chemical bonds that occur between macromolecules are known as crosslinks. Their presence and density have a profound influence on the chemical, mechanical and electrical properties of the polymer.
Epoxy resins are those materials prepared from polymers containing at least two 1,2-epoxy groups per molecule. The epoxy ring is unstable because of a high degree of strain within it and so readily undergoes reaction with a large range of substances. Most commonly employed of all the reactions undergoes by the epoxy group is addition of proton donor species (HX) as illustrated in reaction (1-3)
This reaction is quite general and, since the organic group R can be aliphatic, cycloaliphatic, or aromatic, there is wide scope for variation in the composition of epoxy resins. In practice, however, the most frequently used materials are those based on bisphenol A and epichlorohydrin, which represent over 80% of commercial resins.
As is usually characteristics of crosslinked polymers of commercial importance, epoxy resins are prepared in two stages, with the initial reaction leading to a prepolymer and the subsequent reaction introducing the crosslinks between the molecules.
Crosslinking of epoxy resin may be brought about in one of two ways, either by using catalytic quantities of curing agent or by using stoichiometric crosslinkers.
For commercial application, diepoxides such as those derived from bisphenol A are employed, and they are cured via ring-opening cross-linking reactions, into which the epoxy group enter readily. Bisphenol A is so-called because it is formed from two moles of phenol and acetone. (reaction (1-4))
The cross-linking reactions, illustrated in reaction (1-5), and they demonstrate that, in principle, only a trace of curing agent is necessary to bring about cure of epoxy resins. Selection of curing agent depends on various considerations, such as cost, ease of handling, pot life, cure rates, and the mechanical, electrical, or thermal properties required in the final resin.
Epoxy resins are relatively expensive, but despite this, they are firmly established in a number of important applications. These include adhesives, protective coatings, laminates, electrical insulation and a variety of uses in building and construction.
The popularity of epoxy resin is related to the broad variety of possible materials resulting from the use of different polyphenol and different sizes of prepolymers as well as different curing agents. Another advantage is in the ease of the curing reaction, which starts spontaneously after mixing the prepolymer and curing agent and often does not require a higher temperature post cure.
Most pure polymers exhibit very small electrical conductivity; in fact, some of them are used for insulation purpose. The addition of impurity may significantly increase the electrical conductivity. Many hydrophilic polymers show good conduction when wet, and poor conduction when dry, whereas hydrophobic polymers are highly resistive. In hydrophilic polymers conductivity is seems to be associated with the ionic conductivity of the protons. Epoxies are bonded by either linkages, so they are unaffected by water. Epoxies use a prepolymer that can be modified to control the curing time. In the absence of the hardening agent, they have a long shelf life. The most common epoxy resins use a prepolymer made from bisphenol A and epichlorohydrin.
When the atoms or molecules of a dielectric are placed in an external electric field, the nuclei are pushed with the field resulting in an increased positive charge on one side while the electron clouds are pulled against it resulting in an increased negative charge on the other side. This process is known as polarization and a dielectric material in such a state is said to be polarized. There are two principal methods by which a dielectric can be polarized: stretching and rotation.
- Stretching an atom or molecule results in an induced dipole moment added to every atom or molecule.
- Rotation occurs only in polar molecules those with a permanent dipole moment
In all molecules there is a distribution of positive and negative electric charges. When an electric field E acts on the molecule, these charges are mutually and slightly displaced; the molecules are polarized and acquire an induced dipole .
This displacement of charges can follow the changes in the electric field at all frequencies. The magnitude of the induced dipole is proportional to the polarizing field.
Where the polarizability a describes the case with which the electrons are displaced with respect to the molecular skeleton and depends on the chemical structure of the molecules.
An electric field, which changes more slowly, can induce additional polarization by another mechanism: the orientation of molecular dipoles. These dipoles try to lower their potential energy in the electric field by orienting themselves along the field. This orientation requires molecular movement and is opposed by the friction forces. At very high frequencies, the molecular dipoles cannot follow the changes in the field; electronic shifts are solely responsible for the polarization and the dielectric is equal to the square of the refractive index. At some intermediate frequencies, orientation of the molecules lags behind the changes of the field; the polarization is out of phase with the exciting field.
If the polymers carry polar groups, the electric field tries to orient them. The magnitude of the opposing forces will depend on the physical state of the polymer and on the location of the polar group within the polymer molecule. Movement of the side groups is obviously hindered much less than the movement of the backbone. Hence dipoles within the side groups display maximum out-of-phase polarization at higher frequencies than dipoles within the polymer backbone.
Reorientation of the dipole upon a change of electric field is known as dielectric relaxation. Thus a complex dielectric coefficient e*(ω) is defined as
e*(ω) = eo(ω) exp(ε) = e'(ω) +i e"(ω)
where the real part e'(ω) describes the component when the field and the polarization are in phase; the imaginary part e"(ω) reflects the out-of-phase component. The loss angle d defined as
tan d = e"(ω)/ e'(ω)
d is a measure of power dissipation in the dielectric.
The complex modulus and complex dielectric constant reflect the same phenomenon: mobility of the molecular segments and groups. Both loss tangents usually display a maximum in the region of the main glass-rubber transition (Tg). Polymers with side groups capable of independent motion usually have a secondary maximum at low temperatures when this motion is frozen. However there are differences between mechanical and dielectric dependences. Any kind of movement contributes to mechanical relaxation, while dielectric relaxation requires movement of groups carrying the dipoles. Thus polymers with non-polar side chains display only a very weak if any secondary loss peak. The epoxy resins belong to the later one in which dielectric relaxation requires movement of groups carrying the dipoles.
EFFECT OF TEMPERATURE
One of the important characteristics of polymers is their sensitivity to temperature. When a thermosetting polymer is heated, many of the links are broken and the strength is reduced; is phenomenon is referred to as degradation. The other type of polymer is thermoplastic. It has a fairly limited cross-linking. The material is weak and can readily be molded into any desired shape.
At high enough temperature, a polymer exists in the liquid state in which it usually has a thick rubbery texture. Each molecule is folded around it self and around others, many times over, resulting in a very complex molecular arrangement.
At very high temperature the molecules are constantly twisting and wriggling, due to thermal excitation, so that each molecule constantly changes its shape and position, but at any one instant the result is an amorphous distribution of molecular matter. When temperature is lowered, changes take place in the system.The volume decreases gradually until the melting point Tm is reached, whereupon, if the cooling is accomplished slowly, the polymer under goes a discontinuous decrease in volume. The system is now in crystalline state, and further reduction of temperature causes a further decrease in volume. The system is composed not of only single crystal, but of a large number of crystallites separates from each other by regions of supercooled liquid. This type of polymeric crystalline state is referred as to the fringed micell model. In this crystallite, macromolecules are aligned parallel to each other, somewhat as in regular crystal. Under most circumstances a liquid polymer does not actually crystallize at the temperature Tm, but enter in a supercooled liquid state. Here the system behaves as a
highly viscous liquid. The molecules are arranged randomly so that the structure is an amorphous one, but they continue to move and wriggle, though in a lesser extent than in a true liquid state. At some yet lower temperature Tg (glass transition temperature), the system undergoes another change to a new glassy, or vitreous state. Here the system behaves as an amorphous solid, which is strong and brittle, much as an ordinary glass is. The value of the temperature Tg and Tm with respect to room temperature are for vital importance. If T <Tg, the substance is in the glassy state, and is strong and brittle. If Tg<T <Tm, the substance act as a highly viscous liquid, in a plastic ductile.
The value of Tm and Tg shown in depend on the nature of the molecular bonds of the side-group molecules, and on the length and flexibility of the molecules. The stronger the bonds the higher are these temperatures. However since the bonding is due to the weak forces, these temperatures are relatively low (100-200o C). The temperatures Tm and Tg may be raised if side molecules with polar bonds are introduced. It is usually hard to achieve crystallization in polymers (is primarily that the length of molecules and the complexities of the side group make it hard for the molecules to enter an ordered state. The crosslinkage that may be present also inhibit the tendency of the molecule to go into the ordered state demanded by crystallization.
Polymers exhibit a diverse range of the physical properties in which the most important is the mechanical properties. Mechanical properties depend on the state of the polymers. If the tensile strength is applied on the supercooled polymer, the substance flow plastically
as shown in , which depicts the strain as a function of time: the substance act as a viscous liquid. The response of the system also depends more precisely on the time scale of the applied stress, and that, if a rapidly alternating stress is applied, supercooled polymer shows elasticity. This property combining both viscosity and elasticity is referred to as viscoelasticity. A polymer in the glassy state also exhibits viscoelasticity, except that the viscoelasticity strength is much larger than the strength in the supercooled.
Another property is the great extensibility and flexibility. Under the tensile strength, the sample may be increase to several times its original length. A material capable of this behavior is known as an elastomer. The molecules in a polymer are coiled in a very complicated manner around themselves and around each other, and furthermore, above Tg, they are wriggling about due to thermal excitation. When a tensile strength is applied, it acts by pulling at the end of each molecule, causing it to uncoil. This is how the molecule, and consequently the sample, elongates. If the stress is maintained for a long time, the molecule, after the uncoiling process is completed, begins to slide past neighboring molecules. This sliding is irreversible process. Once it has occurred, the polymer never returns to its original shape, however before sliding takes place, the uncoiling process is reversible. When the stress is removed, each molecule coils back to its original shape.
Another model based on the fact that the uncoiling process is accomplished by rotations of the various segments in the backbone of the polymer molecule around the C - C bond shown in . The right side of the molecule can rotate around the axis as shown, and may take up several positions, or conformations. These conformations are not necessarily of the same energy, but if the energy differences involved are less than, or comparable to, kT, then all conformations are accessible, and the molecules flip back and forth between them as a result of the thermal excitations. The speed of rotation increases rapidly with temperature. In a long molecule various segments of the molecule are incessantly rotating between available conformations, in a random manner. When a stress is applied, the molecule accommodates this by rotating those conformations, which make the molecules longest without sliding take place. Conversely, when the stress is removed, the molecule returns through segmental rotations to the shape with the greatest disorder, which is, more or less, the original shape.