Uncoiling

As noted above, this force is counterbalanced by an entropy-based, elastic force which prevents the molecule from uncoiling. Entropy elasticity was discussed in Sec. 3.4, where the elastic (subscript el) force between crosslinks is given by Eq. (3.19) to be... 

The polymers exist in saline solution as tightly coiled chains and are readily adsorbed owing to relatively low solubiUty in hard water. Subsequent injection of soft, low salinity water uncoils the adsorbed polymer chains increasing water viscosity and reducing rock permeabiUty. This technology could also be used to reduce the permeabiUty of thief 2ones adjacent to injection wells. However, mechanical isolation of these 2ones may be necessary for cost-effective treatments. 

We have divided the total process line into three sections Uncoiler section... 

With the help of bridle no. I driven by motors M and M4, the uncoiler section speed is controlled by monitoring the tension of the travelling sheet and hence maintaining constant speed of the sheet in the uncoiler section. The tensile difference of T tind Ti determines the speed of the uncoiler. Speed and tension of the sheet must remain constant for absolute synchronization between the uncoiler process and the recoiler sections. 

This section is almost the same as the uncoiler section. 

Chain uncoiling, and the converse process of coiling, is conveniently considered as a unimolecular chemical reaction. It is assumed that the rate of uncoiling at any time after application of a stress is proportional to the molecules still coiled. The deformation Dhe(0 at tinie t after application of stress can be shown to be related to the equilibrium deformation Dhe( ) by the equation... 

Figure 3.2. Application of stress to a highly elastic body. Rate of chain uncoiling with time...

The rate constant is a measure of the ease at which the molecule can uncoil through rotation about the C—C or other backbone bonds. This is found to vary with temperature by the exponential rate constant law so that... 

When polymer melts are deformed, polymer molecules not only slide past each other, but they also tend to uncoil—or at least they are deformed from their random coiled-up configuration. On release of the deforming stresses these molecules tend to revert to random coiled-up forms. Since molecular entanglements cause the molecules to act in a co-operative manner some recovery of shape corresponding to the re-coiling occurs. In phenomenological terms we say that the melt shows elasticity. 

If we consider the total deformation (T>totai) occurring during flow to be almost entirely composed of a viscous flow (Dyisc) and a high elastic deformation due to chain uncoiling then we may write... 

Thus it is found that extrusion die swell, when compared at equivalent shear rates, tends to go down with an increase in temperature and go up with molecular weight. As shear rates are increased, polymer uncoiling is increased and die swell increases Figure 8.9). A point is reached, however, where the die swell no longer increases and in fact it can be argued that it goes down. At this point it is... 

A deformation due to chain uncoiling which is not instantaneous and whose rate depends on temperature (high elastic deformation, Dhe)-... 

The diffusion process in natural and polychloroprene rubber adhesives can be explained by Campion s approach [1] which considers the concept of molecular free volume. This free volume is mainly affected by the solvent mixture of the adhesive (which will determine the degree of uncoiling of rubber chains) and by the ingredients in the formulation (mainly the amount and type of tackifier). 

Abrohr, n. outlet pipe, discharge pipe. abroUen, v.t. roll off, unroll, unfold, uncoil. abrSschen, v.t. air (paper), abrosten, v.i. rust off or away, corrode, abrdsten, v.t. roast thoroly. 

Coil coating Coil coating is the technique of depositing a film of liquid on to a continuously moving thin steel or aluminium sheet. The sheet is uncoiled from a roll at the start of the process and recoiled at the end. The coils are then cut to length and formed into the required shape. 

Replication (Section 28.3) The process by which double-stranded DNA uncoils and is replicated to produce two new copies. 

Transcription (Section 28.4) The process by which the genetic information encoded in DNA is read and used to synthesize RNA in the nucleus of the cell. A smal I portion of double-stranded DNA uncoils, and complementary ribonucleotides line up in the correct sequence for RNA synthesis. 

Quite large elastic strains are possible with minimal stress in TPEs these are the synthetic rubbers. TPEs have two specific characteristics their glass transition temperature (7 ) is below that at which they are commonly used, and their molecules are highly kinked as in natural TS rubber (isoprene). When a stress is applied, the molecular chain uncoils and the end-to-end length can be extended several hundred percent, with minimum stresses. Some TPEs have an initial modulus of elasticity of less than 10 MPa (1,500 psi) once the molecules are extended, the modulus increases. 

Japanese researchers20 have attained a high degree of uncoiling of molecules and a high orientation of the latters in fibres by extension below the glass transition temperature... 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

The application of an extending field to the melt simultaneously uncoils and orients the molecules. The extension of the molecules leads to a decrease in their effective flexibility (see Eq. (4)). The essential question is how high should the values of melt orientation and the changes in flexibility be to ensure formation of ECC in subsequent... 

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