Flexibility, chain

The flexibility of the chain is undoubtedly the most important factor influencing T. It is a measure of the ability of a chain to rotate about the constituent chain bonds hence, a flexible chain has a low T, whereas a rigid chain has a high T.  

A further class of polymer does not contain carbon atoms in the main chain such polymers are called inorganic polymers. The most important of these polymers are the silicone rubbers containing silicium rather than carbon. These polymers are often built up through a sequence of — Si—O— units. Another group of inorganic pol mer. are the polyphosphazene.s which contain phosphorus in the main chain ((—P=N—]i. Whereas the —Si—O— chain i. crv flexible the —P=N— chain is quite rigid. 

Chain flexibility is also determined by the character of the side groups, which determine to some extent whether rotation around the main chain can take place readily or whether steric. hindrance occurs. In addition, the character of the side group has a strong effect on interchain interaction. The smallest possible side group is the hydrogen atom (—H). This has no influence on the rotational freedom of the bonds in the main chain and its affect on interchain distance and interaction is also minimal. On the other hand a side group such as the phenyl group (—C H,) reduces rotational freedom in the main chain while the distance between the various chains is also increased. 

The significant differences found in the backbone of BR and that of the recently solved G PC Rs also underline the importance of main-chain relaxation. This can also be achieved by M D simulations with explicit membrane and water molecules. In this method, however, it is difficult to decide which frame of the simulation to use for further investigations and, furthermore, it is difficult to judge whether the system reached the global minimum. 

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Leach A R 1994. Ligand Docking to Proteins With Discrete Side-chain Flexibility. Journal Of Molecula Biology 235 345-356. 

It is generally recognized that the flexibility of a bulk polymer is related to the flexibility of the chains. Chain flexibility is primarily due to torsional motion (changing conformers). Two aspects of chain flexibility are typically examined. One is the barrier involved in determining the lowest-energy conformer from other conformers. The second is the range of conformational motion around the lowest-energy conformation that can be accessed with little or no barrier. There is not yet a clear consensus as to which of these aspects of conformational flexibility is most closely related to bulk flexibility. Researchers are advised to first examine some representative compounds for which the bulk flexibility is known. 

The formalism that we have set up to describe chain flexibility readily lends itself to the problem of hindered rotation. Figure 1.8a shows a sawhorse representation of an ethane molecule in which the angle of rotation around the bond is designated by electron repulsion between the atoms bonded to... 

Poly(ethylene oxide). Although AH j is more than double that of polyethylene, the effect is offset by an even greater increase for AS j. The latter may be due to increased chain flexibility in the liquid caused by the regular insertion of ether oxygens along the chain backbone. 

From the preceding considerations it is appreciated that the intrinsic chain flexibility is determined by the nature of the chain backbone and by the nature of groups directly attached to the backbone. 

Long side chain Interaction in long side chain Flexible backbone... 

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg s of commercial polymers (approx. 104°C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups. 

Introduction of aromatic or cycloaliphatic groups at R and/or Rj gives further restriction to chain flexibility and the resulting polymers have transition temperatures markedly higher than that of the bis-phenol A polycarbonate. 

The polymer structure bears a clear resemblance to that of the polycarbonate of bis-phenol A and of the polysulphones so that there are a number of similarities between the materials. The greatest difference arises from the substantial aliphatic segment, which enhances chain flexibility and hence leads to comparatively low softening points. This has placed severe restrictions on the value of these materials and they have found difficulty in competing with the more successful polycarbonates. 

The ester link appears to enhance chain flexibility of an otherwise polymethylenic chain. At the same time it generally increases interchain attraction and in terms of the effects on melting points and rigidity the effects appear largely self-cancelling. 

It is reasonable to consider that in an ester group the in-chain ether link —C—O—C— increases the chain flexibility compared with a polymethylene chain to decrease the heat of fusion. At the same time there will be some increase in interchain attraction via the carbonyl group which will decrease the entropy of fusion. Since these two effects almost cancel each other out there is almost no change in melting point with change in ester group concentration. 

The presence of ether linkages in the polymer molecule imparts chain flexibility, lowers glass transition temperature, and enhances solubility while maintaining the desired high temperature characteristics [192]. Recently, polyether imines were prepared by the reaction of different diamines with 4,4 -[l,4-phenylene bis(oxy)] bisbenzaldehyde [184]. The polymers synthesized by the solution method were yellow to white in color and had inherent viscosities up to 0.59 dl/g in concentrated H2SO4. Some of these polyimines can be considered as... 

As was shown by Floryl), starting from a certain concentration, it is impossible in principle (for purely geometric reasons) to fill a larger fraction of the volume with these chains when they are randomly arranged. This critical concentration depends on chain flexibility which Flory characterized by the fraction f of folded (gauche) isomers in a polymer molecule... 

This condition means that for f < 0.63 the disordered arrangement of molecules is thermodynamically unstable and the system is spontaneously reorganized into an ordered liquid crystalline phase of a nematic type (Flory called this state crystalline ). This result has been obtained only as a consequence of limited chain flexibility without taking into account intermolecular interactions. 

Hence, Flory s theory offers an objective criterion for chain flexibility and makes possible to divide all the variety of macromolecules into flexible-chain (f > 0.63) and rigid-chain (f < 0.63) ones. In the absence of kinetic hindrance, all rigid-chain polymers must form a thermodynamically stable organized nematic phase at some polymer concentration in solution which increases with f. At f > 0.63, the macromolecules cannot spontaneously adopt a state of parallel order under any conditions. 

When the chain undergoes extension, the stretching fields increases the number of trans isomers in the chain and, hence, decreases the effective chain flexibility which is related to by the equation (see Eq. (11))... 

Applying the TABS model to the stress distribution function f(x), the probability of bond scission was calculated as a function of position along the chain, giving a Gaussian-like distribution function with a standard deviation a 6% for a perfectly extended chain. From the parabolic distribution of stress (Eq. 83), it was inferred that fH < fB near the chain extremities, and therefore, the polymer should remain coiled at its ends. When this fact is included into the calculations of f( [/) (Eq. 70), it was found that a is an increasing function of temperature whereas e( increases with chain flexibility [100],... 

Much fewer experiments are available in solution where the few reported data are generally more concerned about the effect of molecular structure than about bond dissociation energy. In simple shear, it is generally agreed that chain flexibility dominantly influences the rate of bond scission, with the most rigid polymers being the easiest to fracture [157]. The results are interpreted in terms of the presence of good and poor sequences in the chain conformation. 

Reactants with flexible links (O, CO, S02, CH2, etc.) which disrupt the conjugation and increase the chain flexibility.10,14... 

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

Noting the influence of temperature on the intrinsic viscosity is given by the parameter of chain flexibility (dln[ j]/d7), which gives information about the conformation of the macromolecule chain in solution (Kasaii 2007, Chen and Tsaih 1998). The chain flexibility parameter in the temperature range of 20-29°C is dln[t]]/dT = 4,404.11K-i, 0.9993 and in... 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

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