Utilizing a novel, hybrid molecular dynamics, Monte Carlo simulation, we report on microstructural changes in a polymer network that arise in response to oscillatory shear deformation. We model telechelic selfassociating polymers as a course-grained, bead−spring system. The stress response of the system is obtained from rheological experiments and is reported as a function of frequency and amplitude in both the linear and nonlinear regimes. The frequency-dependent material properties are then correlated with observed changes in the topological network structure. While only minimal structural variations are observed in the elastic regime, a substantial rearrangement occurs in the low frequency, large amplitude viscous regime. Aggregates tend to break apart, resulting in an increased density of free chains. Additionally, the network tends to break and form larger structural elements with an increase multiplicity of chains bridging between the same two aggregates.
■ INTRODUCTIONAssociating polymers have the unique ability to span a large spectrum of rheological properties, from fluid-like viscosity to near solid-like elastic dynamics. Telechelic polymers are one class of associating polymers. These triblock polymers consist of two differing chemical groups. The backbone has a high molecular weight and is constructed from multiple, repeating units. The two functionalized ends of the molecule, referred to as end groups, are members of a different chemical group and comprise a small fraction of the total molecular weight. In solution, end groups tend to aggregate by gathering into localized domains. At low enough temperatures and at concentrations above the micelle transition, a space spanning network is formed. The nodes of this network consist of aggregates of end groups, while links between aggregates are formed by one or more bridging polymer chains. End groups associate and dissociate from aggregates frequently; therefore, the topological structure within the network exhibits transient behavior.The general fluid thickening characteristics of telechelic polymers have long since had utility as a rheological modifier in industrial applications within coatings 1 such as paints, adhesives, plastics, and sealants. Over the past few decades, they have come into considerable interest in a multitude of fields. Most recently, composite materials, partially composed of telechelic polymers, have found their respective place in medical and biological applications. Examples include a temporary matrix for bone tissue regeneration 2 and an injectable drug delivery method 3,4 along with regenerative tissue engineering for wound dressing. 5 Within these applications a water-soluble polymer is desirable, constructed as a hydrophilic backbone terminated by hydrophobic groups. 6−9 Because of the fact that these materials are primarily water by weight, they can exhibit biocompatible and biodegradable properties. They can also be subject to external parameters such as temperature and pH. 10,11 The behavior of these materials is highly s...