Force attractive between chains

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

For condensation polymers, the attractive forces between chains and chain units are greater so that physical chain entanglement is not necessarily the limiting factor, but rather other factors including localized crystallization become important. 

Also polymers with high attractive forces between chains will require more heat energy to go from a glassy to a rubbery or a plastic state. On the other hand, polymeric chains with loose hanging side chains that tend to loosen the polymer structure and increase the free volume for segmental movement will have lower Tg. For instance, the glass transition temperature of poly(methyl methacrylate) is higher than that of poly( -butyl methacrylate) as can be seen from Table 2.2. 

If R in the CH2=CHR monomer is a methyl group, —CH3, the monomer is propylene, CH2=CHCH3, and the polymer is polypropylene. We expect the slightly larger R group to increase attractive forces between chains. Polypropylene is used to make plastic bottles, automobile battery cases, fabrics such as Herculon, and indoor-outdoor carpeting. 

For a given rate of denaturation, the smaller the attractive forces between chains of denatured protdn, the slower the second step of the... 

X-ray diffraction studies have revealed that alkane chains with an even number of carbon atoms pack more closely in the crystalline state => attractive forces between individual chains are greater and melting points are higher. 

Estimates of the ultimate shear strength r0 can be obtained from molecular mechanics calculations that are applied to perfect polymer crystals, employing accurate force fields for the secondary bonds between the chains. When the crystal structure of the polymer is known, the increase in the energy can be calculated as a function of the shear displacement of a chain. The derivative of this function is the attracting force between the chains. Its maximum value represents the breaking force, and the corresponding displacement allows the calculation of the maximum allowable shear strain. In Sect. 4 we will present a model for the dependence of the strength on time and temperature. In this model a constant shear modulus g is used, thus r0=gyb. 

Nonpolar molecules such as heptane and PE are attracted to each other by weak London or dispersion forces that result from induced dipole-dipole interactions. The temporary or transient dipoles are due to instantaneous fluctuations in the electron cloud density. The energy range of these forces is fairly constant and about 8 kJ/mol. This force is independent of temperature and is the major force between chains in largely nonpolar polymers, for example, those in classical elastomers and soft plastics such as PE. 

In general, the absorption of smaller molecules such as water acts to decrease the intermolecular forces between chains. Absorption is an attraction property in which likes attract. Thus nonpolar polymers such as PS, poly-... 

Intermolecular Forces. It is the principal function of the plasticizer to interpose itself between the polymer chains. The main obstacles to this endeavor are the attractive forces between the polymer molecules, which depend on the chemical and physical structure of the polymer. 

Although bisphenol A polycarbonate is plasticized by a 30% concentration of dibutyl phthalate or dibenzyl sebacate (Table III), as has been reported (6, 7), a higher concentration is required for effectively plasticizing the polycarbonates of bisphenols I and II (Tables I and II). Perhaps the reason for this is the appreciably lower mobility of polymer chains containing bulky, three-dimensional, polycyclic structures than chains of the bisphenol A polycarbonate. So even though a plasticizer may weaken the secondary attraction forces between polymer chains,... 

Polymer complexation frequently leads to dehydration and precipitation of polymer [15-32] as shown in Fig, 2. To cause polymer complexation, attractive forces between polymers are needed, and the force must overwhelm the strength of interaction between polymer chains and hydrating water molecules. In this process, water molecules must be replaced by competing polymer contacts. 

All adsorption processes result from the attraction between like and unlike molecules. For the ethanol-water example given above, the attraction between water molecules is greater than between molecules of water and ethanol As a consequence, there is a tendency for the ethanol molecules to be expelled from the bulk of the solution and to concentrate at die surface. This tendency increases with the hydrocarhon chain-length of the alcohol. Gas molecules adsorb on a solid surface because of die attraction between unlike molecules. The attraction between like and unlike molecules arises from a variety of intermolecular forces. London dispersion forces exist in all types of matter and always act as an attractive force between adjacent atoms and molecules, no matter how dissimilar they are. Many oilier attractive forces depend upon die specific chemical nature of the neighboring molecules. These include dipole interactions, the hydrogen bond and the metallic bond. 

The structural characteristics that are most important to determining the properties of polymers are (1) the degree of rigidity of the polymer molecules, (2) the electrostatic and van der Waals attractive forces between the chains, (3) the degree to which the chains tend to form crystalline domains, and (4) the degree of cross-linking between the chains. Of these, cross-linking is perhaps the simplest and will be discussed next. 

An amorphous polymer is one with no crystallites. If the attractive forces between the chains are weak and if the motions of the chain are not in... 

A hydrogen bond between the two chains is a weak attractive force between a covalently bounded hydrogen atom (H—N) and a negatively charged keto oxygen atoms (C=0). 

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