Forces, 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. 

Fig. 2. The hardness of a poiymer crystal is related to the critical stress required to overcome the cohesive forces between chain molecules. Different modes arise depending on the direction of the applied force...

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. 

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. 

Gross mobility of entire chains must be low. The cohesive energy forces between chains of elastomers permit rapid, easy expansion. In its extended state, an elastomeric chain exhibits a high tensile strength, whereas at a low extension it has a low modulus. Polymers with low cross-link density usually meet the desired property requirements. The material after deformation returns to its original shape because of the cross-linking. This property is often referred to as elastic memory. 

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-... 

Highly linear, unbranched chains allow maximum interaction between chains. More interactions lead to stronger forces between chains and thus a stronger material. 

Ball and stick models contain only bond length and angle constraints and substantiate this result. It is easy to see that such a model of a twofold coordinated material will collapse because there is nothing holding the chains apart. (In reality, of course, additional constraints are provided by the weaker van der Waals forces between chains.) On the other hand the common experience of those who have constructed amorphous silicon models is that a large bond strain tends to accumulate in the process of satisfying each silicon atom s four bonds. 

Cellulose is a polymer composed of glucose units linked by J-l,4-glycosidic bonds (Fig. 1). Its linear structure is strengthened by hydrogen bonding and van der Waal s forces between chains, resulting in a crystalline structure [27],... 

An interesting calculation is that of the volume occupied by the segments themselves compared with the total volume that the chain occupies. The diameter of a sphere within which the chain spends 95% of the time is about 5Rq. Since the chain segments occupy only about 0.02% of this volume, the remaining space must be occupied by other chains of different molecules both when the polymer is under 0-conditions and in the presence of solvent molecules when it is expanded. Thus, except in very dilute solutions, polymer molecules interpenetrate one another s domains so that intermolecular forces between chains are significant. 

Most polymers are a mixture of random tangles interspersed with crystalline domains called crystallites (Figure 27 6) The degree of crystallinity of a polymer, that is, the percentage of crystallites, depends on the strength of intermolecular forces between chains. 

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. 

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