W. Brostow, M. Drewniak, and N.N. Medvedev
Laboratory of Polymers and Composites
Department of Materials Science
University of North Texas
Denton, TX 76203-5308
The knowledge of the behavior of polymer chains in static and flowing solutions can enhance our understanding of polymer physics as well as provide new insights into interesting and useful phenomena such as drag reduction (DR). Although some experiments (including those pertinent to DR) indicate possible existence of polymer chain interactions and entanglements in dilute solutions, the question remains due to limited resolution of experimental techniques. We use Brownian dynamics simulation technique to investigate this problem.
The spring-bead chain model as defined already in 1953 by Rouse along with the Fraenkel spring potential was used to represent polymer chains. The equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements.
We have analyzed chain conformations and the existence of chain overlaps and entanglements in dilute polymer solutions at concentrations c < c* (where c* is the critical concentration). The results demonstrate that both chain entanglements and overlaps take place even in dilute solutions. They also confirm predictions from earlier analytical model (W. Brostow and B.A. Wolf, Polymer Commun. 1991, 32, 551.), which did not include the polymer + polymer and polymer + solvent interactions that exist in real systems. We are currently testing different parameters that may affect the number of overlaps and entanglements in the system such as: the number density, molecular weight of polymer and the viscosity of medium. The respective results will be reported in future papers.
*Macromol. Theory & Simul. 1995, 4, 745.