Conformation of Polymer Molecules

Once a polymer molecule has been formed, its configuration is fixed. However, it can take on an infinite number of shapes by rotation about the backbone bonds. The final shape that the molecule takes depends on the intramolecular and intermolecular forces, which, in turn, depend on the state of the system. For example, polymer molecules in dilute solution, melt phase, or solid phase would each experience different forces. The conformation of the entire molecule is first considered for semicrystalline solid polymers. Probably the simplest example is the conformation assumed by polyethylene chains in their crystalline lattice (planar zigzag), as illustrated in Fig. 1.4. A polymer molecule cannot be expected to be fully extended, and it actually assumes a chain-folded conformation, as 

The best known example is deoxyribonucleic acid (DNA), which has a weight-average molecular weight of 6-7 million. Even in aqueous solution, it is locked 

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The system described in this investigation is polystyrene-14C adsorbed on Graphon carbon black (graphitized Spheron 6) from six solvents comprising a wide spectrum from good to poor solvent power. Well-characterized materials were selected to elucidate the conformation of polymer molecules at the solid/liquid interface. So far two models have been postulated to describe the conformation of the adsorbed polymer molecules at the solid/liquid interface (9, 13, 14, 18, 19, 21, 27). In the first model the polymer assumes a loop or coil structure in which only a fraction of the polymer segments are attached directly at the interface, and in the second model the polymer forms a relatively flat and compressed interfacial layer with many segments attached to the solid substrate. 

The study of hydrodynamic properties (sedimentation, diffusion and viscosity) of dilute polymer solutions is the most widely used method permitting the characterization of geometric properties (size and conformation) of polymer molecules. 

The effect of the length and structure of side groups on the conformation of polymer molecules and the equilibrium rigidity of the main chain has been studied systematically for homologous series of poly(alkyl acrylate)s... 

The possibility of using the Kerr effect for the study of the structure and conformation of polymer molecules greatly depends on whether it is used for solutions of flexible-chain or rigid-chain molecules ... 

Virk and Wagger classified DR types based on the conformation of polymer molecules under flow. Type A occurs in random-coiling systems and Type B with fully extended macromolecules. Type A DR is seen in Fig. 6, where a series of solutions of increasing concentration yield friction factor segments fanning outward from a common onset point on the PK line. Their slopes increase with increasing concentration... 

Flocculation and subsequent stabilization of sols can also be caused by polymers that do not adsorb on the particle surface. In this case the mechanism of polymer action is different from the one described above and is related to the state of conformation of polymer molecules and change in the free energy of the system upon the transfer of polymer coil from gap between the particles into the solution bulk (a so-called depletion flocculation) [69,70]. 

Flory (1969 1971 1974) has developed the rotational isomeric state theory for predicting the conformation of polymer molecules. This incorporates the interdependence of the bond rotational potentials. This theory is fully explained in Flory s comprehensive monograph (Flory, 1969) and only the barest details will be mentioned here. 

Wei, G. 2006. Radius and chirality dependent conformation of polymer molecule at nanotube interface. Nano Letters 6 1627-1631. 

The Monte Carlo sample then reflects the number of conformations of polymer molecules. This means that observable parameters describing the solution behavior of polysaccharides are averages of the properties of individual conformations. This approach yields properties corresponding to the equilibrium state of the chain. Results refer to a model for an unperturbed chain that ignores the consequences of the long range excluded volume effect, because only nearest-neighbor interactions are accounted for in the computation of the, 4 surfaces. 

Firstly, the presence of a solid snrface will restrict the conformation of polymer molecules in its vicinity. This type of effect is widely recognised in colloid science where, as already mentioned, it has been extensively stndied with regard to steric stabilisation of suspensions. Detailed treatment can be found in works such as those of Napper [74].The basic findings are that, in the absence of mnltipoint attachment, chain molecules respond to this restriction by expanding in a direction parallel to the surface. This expansion can double the effective molecular size in that direction. 

In this chapter we consider the properties of synthetic polymers. First, the main techniques of polymer synthesis are outlined (Section 2.2). Then the conformation of polymer molecules is discussed in Section 2.3. We move on to a summary of the main methods for characterization of polymeric materials in Section 2.4. Then the distinct features of the main classes of polymer are considered, i.e. solutions (Section 2.5), melts (and glasses) (Section 2.6) and crystals (Section 2.7). Then the important properties of plastics (Section 2.8), rubber (Section 2.9) and polymer fibres (Section 2.10) are related to microscopic structure and to rheology. Polymer blends and block copolymers form varied structures due to phase separation, and this is compared and contrasted for the two types of system in Section 2.11. Section 2.12 is concerned with dendrimers and hyperbranched polymers. Section 2.13 and 2.14 deal with polyelectrolytes and (opto)electronic polymers respectively. 

Specifically folded conformations of polymer molecules occur only in special circumstances. As an example (which is also of importance in the discussion which is to follow) we mention the a-helical conformation of synthetic poly (a-amino acids). Such a folded conformation is only realized when the intramolecular polymer-polymer interactions, such as van der Waals contacts, hydrogen bonds and other dipole-dipole interactions (and, in watCT, hydrophobic contacts) in the folded conformation provide a sufficknt lowering of the energy to offset the decrease in entropy. 

A. Peterlin, Conformations of Polymer Molecules, Polymer Science and Materials. Ed. by A. V. Tobolsky and H. Mark,... 

The conformation of polymer molecules in solution is affected by intermolecular forces between polymer molecules and solvent molecules and between solvent molecules and other solvent molecules. Polar polymer molecules will tend to form tight balls in nonpolar solvents, but can attract solvent molecules and can swell in polar solvents. A nonpolar polymer molecule will attract polar solvent molecules less strongly than the polar solvent molecules attract each other and will tend to form a tight ball in a polar solvent such as water. Similarly, nonpolar polymers can swell in nonpolar solvents (try placing a rubber object in benzene or toluene). 

The fibrous state is particularly well suited to the investigation of structural transitions in the conformation of polymer molecules and in their arrangement. Such investigations are of increasing importance in the study of both naturally occurring and synthetic polymers. 

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