Computational Study of the Structure and Rheological Properties of Self-Associating Polymer Networks

Abstract

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...

Temperature-dependent self-assembly and rheological behavior of a thermoreversible pmma-PnBA-PMMA triblock copolymer gel

“…The evolution of structure and rheological properties of the telechelic polymers subjected to shear flow have been investigated using hybrid molecular dynamics, Monte Carlo simulations . It has been reported that the size and the number of aggregates change with the applied frequency and strain …”

“…This will cause the shifting of peak position at q ≈ 0.02 Å −1 . In some instances, some of the aggregates can break in forming smaller aggregates . In addition, during the data collection step, the sample can relax (through chain exchange) and as a result, no significant differences in time‐averaged scattering profiles would be observed.…”

“…16 The evolution of structure and rheological properties of the telechelic polymers subjected to shear flow have been investigated using hybrid molecular dynamics, Monte Carlo simulations. 46,47 It has been reported that the size and the number of aggregates change with the applied frequency and strain. 46,47 Thermoreversible gels consisting of PMMA-PnBA-PMMA in mid-block selective solvents display interesting mechanical properties, such as strain-stiffening behavior.…”

Molecular simulation of structure formation and rheological properties of mixtures of telechelic and monofunctional associating polymer

“…A lot of theoretical and computational studies of rheological properties of physical gels consisting of APs have been reported . Tanaka and Edwards proposed a transient network theory to physical gels consisting of APs and explained the shear thinning under the steady shear flow and the Maxwellian behavior controlled by the dissociation of associating groups from micelles in the linear viscoelasticity regime .…”

“…In addition, Ianniruberto and Marucci recently suggested the shear thickening by the interpenetration of flower micelles at low concentration . Computational studies have revealed the structural changes, including the shear banding and the shear‐induced phase separation, of physical gels by deformation. The multiple relaxation times of the physical gels have also been theoretically studied .…”