Atomic level picture of stress relaxation in polymer melts

✍ Scribed by G. Loriot; J. H. Weiner

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 273 KB
• Volume: 36
• Category: Article
• ISSN: 0887-6266
• DOI: 10.1002/(sici)1099-0488(19980115)36:1<143::aid-polb16>3.0.co;2-c

✦ Synopsis

We have been developing a physical picture on the atomic level of stress relaxation in polymer melts by means of computer simulation of the process in model systems. In this article we treat a melt of freely jointed chains, each with N Å 200 bonds and with excluded-volume interactions between all nonbonded atoms, that has been subjected to an initial constant-volume uniaxial extension. We consider both the stress relaxation history s(t) based on atomic interactions, and the stress history s e (t; N R ) based on subdividing the chain into segments with N R bonds each, with each segment regarded as an entropic spring. It is found that at early times s(t) ú s e (t; N R ) for all N R , and that, for the remainder of the simulation, there is no value of N R for which s(t) Å s e (t; N R ) for an extended period; by the end of the simulation s(t) has fallen just below the value s e (t; 50). The decay of segment orientation, »P 2 (t; N R )…, and of bond orientation »P 2 (t; 1)…, is computed during the simulation. It is found that the decay of the atom-based stress s(t) is closely related to that of »P 2 (t; 1)…. This result may be understood through the concept of steric shielding. The change in local structure of the polymer melt during relaxation is also studied.