Viscous and Elastic Behavior Attributed to Polymer Entanglements

Author:

Porter Roger S.1,MacKnight William J.1,Johnson Julian F.2

Affiliation:

1. 1University of Massachusetts, Amherst, Massachusetts

2. 2Chevron Research Company, Richmond, California

Abstract

Abstract Polymer chains are separated and behave as individual hydrodynamic units in sufficiently dilute solutions. A minimum polymer molecular weight, dependent on concentration, is necessary to produce the characteristic rheological effects generally attributed to entanglements. The minimum polymer molecular weights and concentrations for which entanglement effects are observed are called the characteristic entanglement compositions. Undiluted polymers exhibit such effects only above some minimum molecular weight. The common observation of entanglement effects indicates that they are not due solely to chemical or structural inhomogeneities. Polymer composition, e.g., polarity and perhaps tacticity, can lead, however, to changes in frequency and strength of entanglements. Entanglements appear to govern many important polymer characteristics, thus providing a strong motivation for their study. Characteristic chain spacings between entanglements have been reported from various viscoelastic experiments, low shear viscometry, nonNewtonian flow, and from relaxation times measured by nuclear magnetic resonance. The different techniques generally give concordant values, although with a wide variation in precision. For a few polymers, e.g., polydimethylsiloxane, the characteristic entanglement spacing has been calculated by each of the four techniques. For others, e.g., polyisobutylene and polystyrene, entanglement spacings have been reported by all except NMR. Entanglement effects have been treated theoretically by analogy with theories of rubber elasticity. Other theories have been developed based on breakage and reformation of entanglements and on polymer chain slippage. Certain of these theories have been shown to have the same formalism and yield similar conclusions. In general, the entanglement hypothesis provides a consistent interpretation for a variety of rheological data on concentrated systems of amorphous polymers, this despite the fact that an entanglement has not as yet been directly “seen”. A discussion of entanglements and the first method of calculating entanglement spacings was given by Mark and Tobolsky. A review in the field of polymer viscosities for concentrated systems has been recently prepared. Experimental details and theoretical derivations are given in texts. The notations used are defined in the Appendix.

Publisher

Rubber Division, ACS

Subject

Materials Chemistry,Polymers and Plastics

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