Describe the connection issue. SearchWorks Catalog Stanford Libraries. Introduction to polymer science and chemistry : a problem solving approach. Responsibility Manas Chandra. Physical description xviii, p. Online Available online. Full view. Science Library Li and Ma. C Unknown. More options. Those polymers that can be heat-softened in order to process into a desired form are called thermoplastics. Waste thermoplastics can be recovered and refabricated by application of heat and pressure.
Polystyrene is an important example of a commercial thermoplastic. Other major examples are the polyolefins e. In comparison, thermosets are polymers whose individual chains have been chemically linked by covalent bonds during polymerization or by subsequent chemical or thermal treatment during fabrication. Once formed, these crosslinked networks resist heat softening, mechanical deformation, and solvent attack, but cannot be thermally processed.
Such properties make thermosets suitable materials for composites, coatings, and adhesive applications. Principal examples of thermosets include epoxy, phenol—formaldehyde resins, and unsaturated polyesters that are used in the manufacture of glass-reinforced composites such as Fiberglas see Section 7. In addition to classifying polymers on the basis of their processing characteristics, polymers may also be classified according to their mechanism of polymerization. An early scheme classifies polymers as either addition or condensation —a scheme attributed to Wallace Carothers 2 , a pioneer of the polymer industry working at DuPont from until his untimely death in Polystyrene, which is polymerized by a sequential addition of styrene monomers see Figure , is an example of an addition polymer.
Most important addition polymers are polymerized from olefins and vinyl-based monomers. A few other polymers that are traditionally recognized as belonging to the addition class are polymerized not by addition to an ethylene double bond but through a ring-opening polymerization of a sterically strained cyclic monomer. An example is the ring-opening polymerization of trioxane to form polyoxymethylene an engineering thermoplastic , which is illustrated in Figure Table lists the chemical structure of the repeating units and the commonly used nomenclature of some of the most important addition-type polymers derived from substituted ethylene.
Figure Polymerization of styrene.
Condensation polymers are obtained by the random reaction of two molecules. A molecule participating in a polycondensation reaction may be a monomer, oligomer, or higher-molecular-weight intermediate each having complementary functional end units, such as carboxylic acid or hydroxyl groups. Typically, condensation polymerizations occur by the liberation of a small molecule in the form of a gas, water, or salt. Any high-yield condensation reaction such as esterification or amidation can be used to obtain a high-molecular-weight polymer.
An example of a condensation polymerization is the synthesis of nylon-6,6 by the polycondensation of adipic acid and hexamethylenediamine as illustrated in Figure A. This polymerization is accompanied by the liberation of two molecules of water for each repeating unit.
Another important example of a polycondensation, illustrated in Figure B , is the preparation of polycarbonate from bisphenol-A and phosgene. In this case, two molecules of hydrogen chloride are formed for each repeating unit. Alternatively, if the sodium salt of bisphenol-A was used in the polymerization, the by-product of the condensation would be sodium chloride rather than hydrogen chloride.
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The salt will precipitate out of the organic solvent used for the polymerization and, therefore, can be easily and safely removed. Some other examples of condensation polymers are given in Table Figure Two examples of a condensation polymerization. Polyamidation of nylon-6,6. Some are tough undergoing larie permanent deiormations without breaking, sume are stiff and strong, some aresot'r and flcxihlr, and otherscan withstand considerable imoact without hreakine. All of these "mechanical properties" are peculiar to the polymer and are not characteristic of the monomer from which it was prepared.
Ethylene gas, for example, does not form very good films! These intrr;xtions consist of various types of intermolecular bonds and physical entanglements.
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The magnitude of these interactions is dependent i p o n the nature of the intermolecular bonding foices, the molecular weight, the manner in which the chains are packed together, and the flexibility of the polymer chain. Thus, the amount of interaction is different in different polymers and quite often different in different samples of the same polymer I. Chemlcai Properties of Polymers The chemical. A functional group attached to a.
For example, an acid group Ean be esterified, an aromatic ring can be sulfonated, and an allylie hydrogen can be abstracted by free radicals. The rates a t which these pendant groups undergo reactions, however, can be quite different. For example, due to steric effects and the hydrophobicity of its surroundings, the ester group in poly methyl methacrylate Table 1 is considerably more resistant to hydrolysis than the ester group in methyl propionates.
Nature of intermolecular Bonding Forces The secondary bonding forces present in polymers, e. In oolvmers, however, all the tvpes of electrostatic forces can in. The strenath of these bonds Increases with increasing polarity and decreases sharply with increasing distance. Although the individual enerries are low, ranging hetween 0. These honds arise from extremely short-lived dipoles, which result from the motion of electrons in the molecules. Linear, nonpolar polymers, such as polyethylene, that have only van der Waals attractions between the chains, must have relatively high molecular weights and he packed very close together to have useful mechanical properties.
I t should not he too surprising that many commercial polymers contain polar functional groups that provide stronger dipole-dipole interactions between the chains Fig. Ester xruups.
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I'dm rther and e s k r hnkages are also incornorated in manv oolvmer chains. Since dinoiethe dipole interactions are depenieni on the alignment dipoles, the interactions between polar polymer molecules can he enhanced considerably by properly orienting the chains.
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The strongest tvoe.. In lwt. For example, aliphatic polyamides nylons have properties that permit them to he used in many applications. Whereas, the properties of aliphatic polyesters do not warrant their commercial production. Other polymers that display hydrogen honding include poly viny1 alcohol and cellulose, with their pendant hydroxyl groups, and polyurethanes, with their carbarnate linkages.
A relatively new class of polymers called ionomers actually have ionic interactions between the chains Fig. These polyolefins contain pendant carhoxylate groups associated with free Group I and Group I1 metallic cations. This results in outstanding strength and impact resistance.
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Hence, one must talk in terms of average chain lengths and average molecular weights. The number-average degree of polymerization X, is defined as the average number ofrepeat units in the polymer chains. Closely associated with X , i s a e number-average molecular weight M, which is equal to X , multiplied by the molecular weight octhe repeat unit. If the intensity of electrostatic forces per unit length for a collection of molecules is the same, such as in the homologous series, then the total amount of attractive force increases as the molecular weight increases.
The increase in interaction results first in changes in physical state. At a molecular weight of approximately 1,, the molecules begin to decompose before boiling. This means that the total honding force between the nonpolar molecules has become stronger than their covalent intramolecular honds.
None of the relatively low molecular weight hydrocarbons, however, display the mechanical properties of polyethylene. The question then becomes: How high must the molecular weight he before the molecule exhibits "polymer properties"? Oligomers have relatively no strength until a critical X, is reached Fig. At this point, which depends on the type of secondary honding forces present, the molecule begins to develop mechanical properties, such as tensile strength, elongation to break and impact strength.
For polymers containing hydrogen honding, e. Above this X, the mechanical properties increase rapidly with increasing molecular weight until a second critical X , is reached. After this point further increases in molecular weight result in very little change in a particular property. The value of this second critical X, is also very dependent upon the type of intermolecular bonding present. For polyamides this point occurs near degrees of polymerization of Polyhydrocarhons require X , values of ereater than Althourh rhe second critical point is also sliahtls different fur different properties, once ihe X, is reached; the property becomes characteristic of the polymer.
Polymer chemists Table 3. Plot of selected properties versus? Journal of Chemical Education Figure 3. Schematic twodimensional representation of lamellae model Figure 4. Extended, planar, zig-rag conformation of polyethylene. Amtic Figure 5. Stereochemical Configurationsof monosubstituted vinyl polymers. It is tempting to conclude that chemists simply prepare polymers with as high a molecular weight as possible in order to maximize their properties.
This is usually not the case, however, because polymers also become much harder to process as the molecular weight increases. In most industrial processing operations the polymer must undergo considerable flow, which is dependent on the melt viscosity, another property that increases with increasing molecular weight.
Fortunately, the rate a t which the melt viscosity increases is low until a critical molecular weight is reached Fig. After this point the melt viscosity increases rapidly. This behavior is due to the fact that low-molecular-weight polymers are free to flow as single molecules. As the chain length increases, the chains begin to entangle and "network flow" occurs. As the molecules become still larger the flow network and, hence, the resistance to flow, rapidly increases. In fact, the resistance to flow eventually will becomeso high that the polymer cannot be worked mechanically M, lo7.
In practice, the upper limit on the polymer's molecular weight is usually set by the flow requirements of the processing operation employed.
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The final product often reflects a compromise between optimum properties and ease of processing.