Flexibility of a polymer

FIG. 2.14 The effect of fixed bond angle in restricting the flexibility of a polymer coil. For tetrahedral bonds, 6 = 109°. 

The minimum polymer chain length or critical molecular weight, M for the formation of stable entanglements depends upon the flexibility of a polymer chain which is affected by the polarity and shape of the polymer. Relatively flexible polymer chains, such as polystyrene, have a... 

These factors were mentioned soon after the first experimental works on liquid crystalline state of polymers were published. For example, Frenkel considered the effect of these factors by studying the change in the statistical flexibility parameter/ introduced by Flory in his analysis of flexible chain polymers Frenkel proposed the following equation to describe the effect of the type of the solvent on the flexibility of a polymer chain ... 

The thermal energy of the molecular environment provides the energy required to overcome the rotational energy barrier. Consequently, the shape (flexibility) of a polymer molecule is temperature dependent. At sufficiently high temperatures, the polymer chain constantly wiggles, assuming a myriad of random coil conformations. As we shall see later, the flexibility of polymer molecules, which is a function of substituents on the backbone, has a strong influence on polymer properties. 

A compoimd of lower molecular weight that can reduce the Tg and increase the flexibility of a polymer. 

It is important to be able to regulate the degree of chain stiffness, as rigid chains are preferred for fiber formation whereas flexible chains make better elastom. The flexibility of a polymer depends on the ease with which the backbone chain bonds can rotate. Highly flexible chains will be able to rotate easily into the various available conformations, whereas the internal rotations of bonds in a stiff chain arc hindered and impeded. 

The action of the plasticizer is one of a lubricant, where the small molecules ease the movement of the polymer chains by pushing them further apart. As this lowers both 7g and the modulus, their main use is to increase the flexibility of a polymer for use in tubing and in films. 

As we know, the flexibility of a polymer chain is not very noticeable at smaller scales, but it starts showing up as the scale increases. This means... 

Two approaches of flexible bonding philosophy. In flexible bonding the proper selection of polymer properties is very important. The most important is modulus of elasticity which account for flexibility of a polymer joint and also polymer strength. There are exist two approaches in projecting process of flexible polymer joint. 

It may be reasoned that each cross-link causes an additional restriction to the flexibility of a polymer molecule. This leads to a relation of the form ... 

A common method of measuring the flexibility of a polymer coating is to take a film of standard thickness and bend it around a series of cylinders, or a conical mandrel (BS FN ISO 6860,2006). The diameter of the smallest mandrel that the film can be bent around without cracking d onstrates the flexibility of the polymer. 

The persistence length provides a measure of the flexibility of a polymer and is particularly useful in the case of semiflexible filaments. In the case of filaments that do not coil in solution, a measure of the end-to-end distance or the radius of gyration is not particularly useful in characterizing the molecule. The persistence length can be defined using the following formula ... 

Ideally a plasticiser should maintain the flexibility of a polymer over as wide a temperature range as possible. In practice, a certain proportion of low temperature plasticisers such as sebacates or adipates is often added to the phthalate or other primary plasticiser to give optimum low temperature properties, although certain phthalates derived from straight chain alcohols are appreciably better than DOP (di-octyl phthalate) or DiOP (di-iso-octyl phthalate) in this respect. 

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