Basic Nature Of Polymers
The term polymer denotes a molecule that is made up of many(poly) parts(mers). The mer ending represents the simplest repeating chemical structural unit from which the polymer is composed. Thus poly(methy1 methacrylate) is a polymer having chemical structural units derived from methyl methacrylate, as indicated by the simplified reaction and structural formula I.
The molecules from which the polymer is constructed are called monomers (one part). Polymer molecules may be prepared from a mixture of different types of monomers. They are called copolymers if they contain two or more different chemical units and telpolymers if they contain three different units, as indicated by the structural formulas 11 and 111.
As a convenience in expressing the structural formulas of polymers, the mer units are enclosed in brackets, and subscripts such as n, m, and p represent the average number of the various mer units that make up the polymer molecules. Notice that in normal polymers the mer units are spaced in a random orientation along the polymer chain. It is possible, however, to produce copolymers with mer units arranged so that a large number of one mer type are connected to a large number of another mer type. This special type of polymer is called a blockpolymer. It also is possible to produce polymers having mer units with a special spatial arrangement with respect to the adjacent units; these are called stereospeczfic polymers.
The molecular weight of the polymer molecule, which equals the molecular weight of the various mers multiplied by the number of the mers, may range from thousands to millions of molecular weight units, depending on the preparation conditions. The higher the molecular weight of the polymer made from a single monomer, the higher the degree of polymerization. The term polymerization is often used in a qualitative sense, but the degree of polymerization is defined as the total number of mers in a polymer molecule. In general, the molecular weight of a polymer is reported as the average molecular weight because the number of repeating units may vary greatly from one molecule to another. As would be expected, the fraction of low-, medium-, and high- molecular-weight molecules in a material or, in other words, the molecular weight distribution, has as pronounced an effect on the physical properties as the average molecular weight does.
Therefore two poly(methy1 methacrylate) samples can have the same chemical composition but greatly different physical properties because one of the samples has a high percentage of low- molecular-weight molecules, whereas the other has a high percentage of high-molecular weight molecules. Variation in the molecular weight distribution may be obtained by altering the polymerization procedure. These materials therefore do not possess any precise physical constants, such as melting point, as ordinary small molecules do. For example, the higher the molecular weight, the higher the softening and melting points and the stiffer the plastic.
In addition to chemical composition and molecular weight, the physical or spatial structure of the polymer molecules is also important in determining the properties of the polymer. There are three basic types of structures: linear, branched, and cross-linked. They are illustrated in Figure as segments of linear, branched, and cross-linked polymers. The linear homopolymer has mer units of the same type, and the random copolymer of the linear type has the two mer units randomly distributed along the chain. The linear block copolymer has segments, or blocks, along the chain where the mer units are the same. The branched homopolymer again consists of the same mer units, whereas the graft-branched copolymer consists of one type of mer unit on the main chain and another mer for the branches. The cross-linked polymer shown is made up of a homopolymer cross-linked with a single crosslinking agent.
The linear and branched molecules are separate and discrete, whereas the cross-linked molecules are a network structure that may result in the polymer's becoming one giant molecule. The spatial structure of polymers affects their flow properties, but generalizations are difficult to make because either the interaction between linear polymer molecules or the length of the branches on the branched molecules may be more important in a particular example. In general, however, the cross-linked polymers flow at higher temperatures than linear or branched polymers. Another distinguishing feature of some cross-linked polymers is that they do not absorb liquids as readily as either the linear or branched materials.
An additional method of classifying polymers other than by their spatial structure is according to whether they are thermoplastic or thermosetting. The term thermoplastic refers to polymers that may be softened by heating and solidify on cooling, the process being repeatable.
Typical examples of polymers of this type are poly(methy1 methacrylate), polyethylene- polyvinylacetate, and polystyrene. The term thermosetting refers to plastics that solidify during fabrication but cannot be softened by reheating. These polymers generally become nonfusible because of a crosslinking reaction and the formation of a spacial structure. Typical dental examples are cross-linked poly(methy1 methacrylate), silicones, cis-polyisoprene, and bisphenol A-diacrylates. Polymers as a class have unique properties, and by varying the chemical composition, molecular weight, molecular-weight distribution, or spatial arrangement of the mer units, the physical and mechanical properties of polymers may be altered.