l8O%&JIAR zyxwvutsrqp LIQUIDS Journal of Molecular Liquids 91 (2001) 205-217 www.elseviecnl/locate/molliq Rheologlcal properties of self-associating polymer systems: Nonequilibrium molecular dynamics simulation Pave1 G. Khalatur and Dmitrii A. Mologin Department of Physical Chemistry, Tver State University, Tver 170002, Russia We present the results of nonequilibrium molecular dynamics (NEMD) studies of self- associating polymer systems composed of flexible linear telechelic chains with strongly at- tracting end-groups. Formation of micellar aggregates, their structure and structural charac- teristics of associative polymer network (micellar gel) are studied under the influence of ex- ternal shearing forces. Interconnection between structural and rheological properties of the steady-state system is discussed. We find that a critical factor in the shear behavior of self- associating polymers is the space distribution (topology) of chain abridges,) connecting mi- cellar aggregates. Dependii on the topology of the bridges, the system may exhibit shear thickening or shear thinning. Furthermore, the applied external force may cause transitions from continuous (space-fillii) associative network, which spans the whole space, to fibrous textures - threadlike objects which align along the shear field. These structural reorganiza- tions lead to dramatic changes in the shear dependent steady-state viscosity. 0 2001 Elsevier Science B.V. All rights reserved. 1. Introduction There is a growing interest in understanding the rheology of solutions of associative poly- mers in terms of the properties of the constituent molecules. Associative polymers represent one of the examples of polymeric systems with strongly interacting groups which associate in good solvents for the polymer backbone [l-3]. The most common are hydrophobic polar or ionic groups which are capable of association and form aggregates similar to those formed by surfactant molecules. ORen the associating groups, referred to as c(stickerD+ are randomly distributed along the polymer chain One of the simplest examples of associative polymers is a telechelic polymer, i.e., a water-soluble polymer which contains hydrophobic sticker sites only at the chain ends. Such polymers can act as model systems for associative polymers in gen- eral. In solution, telechelic polymers usually aggregate in micelles or muhiplets. The important feature of telechelic micelles is the possibility of bridging that gives rise to an effective attrac- tion between micelles [4]. As a result, telechelic polymers can form a temporary (reversible) network with hydrophobic aggregates as junction points. This implies that the junctions of the network can ctopeml and ctclose)) in a dynamic equilibrium. The main dynamical effect related to the network (gel) formation is a strong increase of the solution viscosity at low concentra- tions where the viscosity of corresponding unmrmtionahzed polymer solutions is still close to 0167-7322/01/$ - see front matter 0 2001 Elsevier Science B.V. All rights reserved. PII SO167-7322(01) 00164-7