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Volume Publication Date Sponsoring Divisions: Rubber Division; Division of Polymer Chemistry. Articles Selected Elastomer Stereospecific Polymerization, Copyright, Advances in Chemistry Series, FOREWORD. Robert F . Gould.
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- Polymers: Chemistry and Physics of Modern Materials, Third Edition
A numerical example serves to highlight the differences in the various averages. For many polymerizations, the most probable value is about 2. An alternative method of describing the chain length of a polymer is to measure the average degree of polymerization x. Hence, the x average depends on which average is used for M.
To avoid confusion between the mole fraction x and the average degree of polymerization x, the latter will always be subscripted as xn or xw to indicate the particular M used in Equation 1. We can begin to answer these questions by first considering a simple molecule such as butane and examining the behavior when the molecule is rotated about the bond joining carbon 2 to carbon 3. This is the most stable conformation with the greatest separation between the two methyl groups. Although the gauche states are slightly less stable, all three minima can be regarded as discrete rotational states.
The maxima correspond to the eclipsed positions and —CH3 are angles of maximum instability. These diagrams will vary with the type of molecule and need not be symmetrical, but the butane diagram is very similar to that for the simple polymer polyethylene — CH 2 —CH 2 — n , if the groups are replaced by the two sections of the chain adjoining the bond of rotation. The backbone of this polymer is composed of a chain of tetrahedral carbon atoms covalently bonded to each other so that the molecule can be represented as an extended all trans zigzag chain.
This means that polyethylene is a long threadlike molecule, but how realistic is the extended all trans conformation? As every group of four atoms in the chain has a choice of three possible stable rotational states, a total of , shapes are available to this particular chain, only one of which is the all trans state. So, in spite of the fact that the all trans extended conformation has the lowest energy, the most probable conformation will be some kind of randomly coiled state, assuming that no external ordering forces are present and that the rotation about the carbon bonds is in no way impeded.
This is shown more clearly see Figure 1. The distribution of trans t and gauche g states along a chain will be a function of the temperature and the relative stability of these states. Consequently, there is an unequal distribution of each. Because of the possibility of rotation about the carbon bonds, the chain is in a state of perpetual motion, constantly changing shape from one coiled conformation to another form, equally probable at the given temperature.
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The speed of this wriggling varies with temperature and from one polymer to another and dictates many of the physical characteristics of the polymer, as we shall see later. Realistically, then, a polymer chain is better represented by a loosely coiled ball Figure 1. For the magnified-polyethylene chain considered earlier, a ball of about 4 cm diameter is a likely size. The term conformation has been used here when referring to a three-dimensional geometric arrangement of the polymer, which changes easily when the bonds are rotated.
There is a tendency to use the term configuration in a synonymous sense, but as far as possible, this will be reserved for the description of chains in which the geometric variations can only be interchanged by breaking a bond. Configurational isomers of importance for polymers are discussed in the following section 1. From Treloar, L. With permission. As shown in Figure 1. Two important polymers that show this type of isomerism are 1,4-polybutadiene and 1,4-polyisoprene.
The regu- larity of the trans configuration makes this type of isomer more crystalline, with a higher melting point compared to the cis configuration. If the polymer chain contains carbon atoms with two different substituents, then the C atom is asymmetrical. This situation is encountered for vinyl monomers such as polypropylene.
For a chain containing asymmetric centers, three different stereo- isomers are possible Figure 1. The isotactic configuration in which each substituent is placed on the same side of the chain. The syndiotactic structure in which substituents lie on alternate sides of the backbone. A disordered, termed atactic, configuration in which substituent groups are placed randomly on either side of the chain.
Of the three stereoisomers, atactic polypropylene was the first to be synthesized. This is because it is the atactic structure that is obtained by conventional polymer- ization of the monomers when no optically active catalyst is used.leondumoulin.nl/language/archetypes/7635-banged-by-the.php
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It was only when the Ziegler—Natta catalysts were introduced in the s that the production of stereoregular polymers became possible. It is interesting to note that polypropylene did not find any commercial use until the Ziegler—Natta catalysts became available. In fact, the atactic structure that was originally produced leads to a viscous liquid at room temperature of limited use. Isotactic polypropylene instead is one of the most important commercial polymers. As the temper- ature rises, each polymer eventually obtains sufficient thermal energy to enable its chains to move freely enough for it to behave like a viscous liquid assuming no degradation has occurred.
There are two ways in which a polymer can pass from the solid to the liquid phase, depending on the internal organization of the chains in the sample. The different types of thermal response, illustrated by following the change in specific volume, are shown schematically in Figure 1. A polymer may be completely amorphous in the solid state, which means that the chains in the specimen are arranged in a totally random fashion. The volume change in amorphous polymers follows the curve A—D. In the region C—D the polymer is a glass, but as the sample is heated, it passes through a temperature Tg, called the glass transition temperature, beyond which it softens and becomes rub- berlike.
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This is an important temperature because it represents the point where important property changes take place, i. A continuing increase in temperature along C—B—A leads to a change of the rubbery polymer to a viscous liquid. In a perfectly crystalline polymer, all the chains would be incorporated in regions of three-dimensional order, called crystallites, and no glass transition would be observed because of the absence of disordered chains in the sample. Perfectly crystalline polymers are not encountered in practice, and instead polymers may contain varying proportions of ordered and disordered regions in the sample.
These semicrystalline polymers usually exhibit both Tg and Tm, corre- sponding to the ordered and disordered portions and follow curves similar to F—E—G—A. These imperfections act to depress the melting temperature, and experimental values of Tm can depend on the previous thermal history of the sample. Nevertheless, both Tg and Tm are important parameters, which serve to charac- terize a given polymer. These can be grouped into three major classes: plastics, fibers, and elastomers; but there is no firm dividing line between the groups.
However, some classification is useful from a technological viewpoint, and one method of defining a member of these categories is to examine a typical stress—strain plot Figure 1. Rigid plastics and fibers are resistant to deformation and are characterized by a high modulus and low percentage elongations. Elastomers readily undergo deformation and exhibit large reversible elongations under small applied stresses, i.
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The flexible plastics are intermediate in behavior. An outline of the structure—property relations will be presented later, but before proceeding further with the more detailed science, we can profitably familiarize ourselves with some of the more common polymers and their uses. Some of these are presented in Table 1. A polymer normally used as a fiber may make a perfectly good plastic if no attempt is made to draw it into a filament.
Similarly, a plastic, if used at a temperature above its glass transition and suitably cross-linked, may make a perfectly acceptable elastomer. In the following text, a brief account of some of the more common plastics, fibers, and elastomers is given. The classification is based essentially on their major technological application under standard working conditions.
It is significant that these are polymers of long standing, and it has been suggested that further fiber research may involve the somewhat prosaic task of attempting to improve, modify, or reduce the cost of existing fibers, rather than to look for new and better alternatives. The commercially important fibers are listed in Table 1. The polyamides are an important group of polymers, which include the naturally occurring proteins in addition to the synthetic nylons.
The term nylon, originally a trade name, has now become a generic term for the synthetic polyamides, and the numerals which follow, e.
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Thus, nylon-6,10 is prepared from two monomers and has the structure NH CH2 6NHCO CH2 8CO n with alternative sequences of six and ten carbon atoms between the nitrogen atoms, whereas nylon-6 is prepared from one monomer and has the repeat formula — [ NH CH 2 5 CO —] n with regular sequences of six carbon atoms between the nitro- gen atoms. A nylon with two numbers is termed dyadic indicating that it contains both dibasic acid or acid chloride and diamine moieties, in which the first number represents the diamine and the second the diacid used in the synthesis.
The monadic nylons have one number, indicating that synthesis involved only one type of mono- mer. Terylene is an important polyester.
The harsh feel of the fiber, caused by the stiffness of the chain, is overcome by blending it with wool and cotton. The acrylics and modacrylics are among the most important of the amorphous fibers. Vinyl chloride and vinylidene chloride are the most important comonomers, and the copolymers produce high-bulk yarns, which can be subjected to a controlled shrinking process after fabrication.