Polymer Molecules in the Melt

While this helps us to describe the conformation of the unperturbed molecule in a melt, the rheological behavior of a melt is dramatically different from that of a dilute solution if the molecular weight exceeds a certain critical value. This is the result of an extremely important phenomenon that occurs in a melt and has no counterpart in the dilute solution. Over a certain rather narrow range of molecular weights the dynamic interaction between polymer molecules starts to have a very marked effect on the dynamic behavior of the melt. This strong interaction is traditionally said to be due to entanglements, although it is now understood that 

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It is tempting to apply the ideas developed in section 2.1. to describe the dynamics of long linear polymer molecules in the melt state. The average radius of the chain is Rq =... 

Among the various approaches in use for the depiction of the interactions of the polymer molecules in the melt, these being known to be at the origin of the observed rheological behaviour, the network theories enable the building of reasonable models that fulfill the previous requirements for the sake of simplicity. 

It is common to distinguish between internal and external lubricants. The worst definition is based on who adds the lubricant if the supplier added it, it comes to the processor as an internal lubricant if the processor must add it himself, it is an external lubricant. A semimeaningful distinction an internal lubricant is compounded into the polymer an external lubricant is applied to the surface of the mold. The proper distinction is based on miscibility/immiscibility of the additive in the polymer an internal lubricant is miscible with the polymer and acts like a small amount of plasticizer to increase molecular flexibility and mobility, and to help the disentanglement and flow of tbe polymer molecules in the melt. An external lubricant is immiscible in the polymer and tends to exude to the surface of the plastic and form an abhesive (nonstick) interface between the polymer and the steel process equipment, or it tends to come to the surface of the finished plastic product and give it continual lubricity in the use of the product. 

Polymer molecules in the melt are very coiled up in fact, they are often approximately Gaussian coils. In other words, the radius of gyration for a linear chain is given by Eq. (2.2), which also applies to an isolated coil in solution. In the melt, the density of surrounding molecules prevents an individual molecule from stretching, and it adopts a compact conformation. 

ABSTRACT. Over the past 15 years, a number of transient optical grating techniques have been developed for measurements of the transport properties of materials. Such methods have been used to measure the tracer diffusion coefficients of polymer molecules which have been labeled with photochromic or fluorescent dyes. The present paper describes the common features of these techniques, and gives an example of how Fluorescence Redistribution After Pattern Photobleaching has been used to study the diffusion of polymer molecules in the melt. 

Holographic techniques have been used to measure diffusion of polymer molecules in solution by L6ger, et al of the College de France, by Coutandin and Sillescu of the Institut Fur Physikalische Chemie der Universitat Mainz, and by Nemoto, et al of the University of Wisconsin. Smith, et al at IBM Almaden Research Center have used FRAPP to measure the diffusion of polymer molecules in the melt. ... 

A study of the diffusion of polymer molecules in the melt will be used to illustrate the application of FRAPP to polymer diffusion. In this example, the diffusion coefficient of poly(propylene oxide) has been measured as a function of concentration. The results are discussed in terms of the reptation model. A more complete description of the materials and... 

De Gennes (1971) postulated that polymer molecules were constrained to move along a tube formed by neighbouring molecules. In a deformed melt, the ends of the molecules could escape from the tube by a reciprocating motion (reptation), whereas the centre of the molecule was trapped in the tube. When the chain end advanced, it chose from a number of different paths in the melt. This theory predicts that the zero-shear rate viscosity depends on the cube of the molecular weight. However, in the absence of techniques to image the motion of single polymer molecules in a melt, it is hard to confirm the theory. 

The ribbon polymer is related to the less-flexible ladder and sheet polymers discussed before. One might expect very much different viscous behavior of such molecules in the melt. The interpenetrating networks [8] can have interesting elastic properties since each network may respond differently and interact with the other. The two-dimensional flexible polymers have recently been explored. They also belong to the sheet-like polymers. 

The helix-coil transition is unique in that the coordinated action of many molecules is not required. It is by necessity restricted to the very dilute portion of the phase diagram. As the concentration of polymer molecules in the helical conformation increases, intermolecular interactions begin to manifest themselves. The cooperative character of the transihon will be further enhanced. The dimensional interdependence will increase from one in the case of a dilute soluhon to three in the more concentrated system. The transition will then become formally idenhcal to the melting of the dense crystalline phase that has been discussed previously. 

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