Polymer chains flexibility

The diffusion of larger organic vapor molecules is related to absorption. The rate of diffusion is dependent on the size and shape of the diffusate molecules, their interaction with the polymer molecules, and the size, shape, and stiffness of the polymer chains. The rate of diffusion is directly related to the polymer chain flexibility and inversely related to the size of the diffusate molecules. 

The Tg is related to chain stiffness and the geometry of the polymer chain. Flexible polymers with methylene and oxygen atoms in the chain, such as polyethylene, polyoxymethylene, and polysiloxane (silicone), have relatively low Tg values. The Tg of polyoxymethylene is somewhat higher than would be anticipated because of the dipole character of the C—O—C group, which increases the intermolecular forces and restricts segmental motion. 

Jl o design the proper polymer for the right application efficiently, the polymer chemist must establish a conceptual bridge between the structure of the polymer and its physical properties. One of the most important structural features of polymer chains is the flexibility of their backbones. Polymer chemists routinely deal intuitively with polymer chain flexibility, and they modify this flexibility through structural alterations of the polymer. How-... 

Speaking of polymer chain flexibility we mean the properties of an isolated single molecule exhibited in a dilute polymer solution. Under these conditions, two concepts of flexibflity (and, hence, of rigidity) of chain molecules may be distinguished equilibrium (static) flexibility and kinetic flexibility. 

A polymer is in the glassy state at room temperature if the Tg is above room temperature, and it behaves as an elastomer if the Tg is considerably below room temperature. The T of a polymer depends on the freedom of rotation available about bonds in the polymer chain. Flexible polymers are made by the introduction of groups about which rotation is facile, like -0-, -Si-O-Si-, CHg-S-, = CH-CH2 . Discussions of different adhesive systems provide a number of examples of such groups being introduced in otherwise intractable polymers to make them flexible and tough. Plasticizers are also employed to make rigid polymers pliable. Plasticizer molecules, by their interaction with polymer chains, reduce interchain interactions and consequently, their rigidity. 

Copolymers of ethylene and vinyl acetate are commercial materials prepared by free-radical polymerization. Explain how copolymeiization with vinyl acetate affects the degree of crystallinity of polyethylene and the polymer chain flexibility at room temperature. How would you expect to vary with increasing vinyl acetate content ... 

Fig. 2.5 A worm-like polymer chain (a) and a freely jointed polymer chain (/>) — two simple and most common models of polymer chain flexibility.

Rabinowitz, S., Duckett, R. A. and Ward, I. M., Unpublished Proceedings of the British Polymer Physics Group Conference on Polymer Chain Flexibility , Colchester, (1969). 

Poly(sulfo-l,4-phenylene), or poly(p-phenylene sulfone), with the monomeric unit -(-SO2 —C6H44-> has a very high melting temperature of 520° C, and so can only be worked with difficulty. Products of good processability were only obtained after introducing ether groups which make the polymer chains flexible. 

The presence of ether-oxygen linkages gives the polymer chain flexibility to permit fabrication by conventional melt-processing techniques. The polymer can be injection molded, provided the cylinder and nozzle are capable of reaching 425 C. It also may be extruded. The polymer must be dried before processing. Injection molding barrel temperatures should be 270-360°C at the rear, 295-390°C in the middle, and 300-395°C at the front. 

The most basic approach to measuring polymer chain flexibility is to use the concept of a completely flexible chain molecule and then introduce the ways in which structures can restrict rotation and hence alter flexibility. This, however, presents some problems. The first is that poor results are obtained unless all the ways that structures can reduce flexibility are considered. 

Conversely, an increase in cross-linking ultimately decreases impact strength and elongation at break, whereas an increase in polymer chain flexibility decreases the polymer s modulus of elasticity. 

Polymer chain flexibility can also be changed during processing. This basically results from the action of smaller molecules taken up in the polymer. These molecules, which make the polymer more flexible and less rigid, are termed... 

The rotational isomers considered so far define an important mechanism of polymer chain flexibility leading to perpetual variations of the chain conformation due to thermal... 

In solution, an additional factor is involved the interaction between polymer and solvent. This feature is detailed in Volume 2, Chapter VIIB. The related effect of polymer chain flexibility in solution has also been demonstrated in ultrasonic degradation by a lower degradation of natural (c/5-isoprene) rubber when compared with the trans conformer [30],... 

Experimental Results for Polymer Chain Flexibility and Correlation with Structure... 

In this section, we will try to elucidate certain trends regarding the effect of macromolecular structure on polymer chain flexibility by examination of experimental data. There have been attempts to eorrelate chain flexibility parameters (q, Coo > < ) with the molar volume of the substituent [84-86]. These empirical correlations have met with limited success but only when the correlation is restricted to a narrow subclass of macromolecules. The most successful attempts to predict conformational characteristics from structure are certainly those based on RIS models. Unfortunately, such models have appeared for only a limited number of macromolecules, and the development of reliable RIS models remains a job for the specialist. In addition, such models become quite involved for polymers with complex substituents or with complex backbone structures. 

As discussed in the previous section, polymer chain flexibility is important for cooperative adsorption. Both a single polymer chain and 3D networks in a salt solution exhibit flexibility. In the case of the adsorption of N-pyridinium chloride (CnPyCl, = 4, 8, 10, 12, 16, and 18) onto such flexible polymers, the cooperativity increases in proportion to the hydro-phobicity (the length of the aliphatic chain) [40, 41]. In this section, the character of the adsorbed surfactant will be discussed. 

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