Effects of interaction strength of associating groups on linear and star polymer dynamics

Abstract

A small number of associating groups incorporated onto a polymer backbone have dramatic effects on the mobility and viscoelastic response of the macromolecules in melts. These associating groups assemble, driving the formation of clusters, whose lifetime affects the properties of the polymers. Here, we probe the effects of the interaction strength on the structure and dynamics of two topologies, linear and star polymer melts, and further investigate blends of associative and non-associating polymers using molecular dynamics simulations. Polymer chains of approximately one entanglement length are described by a bead–spring model, and the associating groups are incorporated in the form of interacting beads with an interaction strength between them that is varied from 1 to 20 kBT. We find that, for all melts and blends, interaction of a few kBT between the associating groups drives cluster formation, where the size of the clusters increases with increasing interaction strength. These clusters act as physical crosslinkers, which slow the chain mobility. Blends of chains with and without associating groups macroscopically phase separate for interaction strength between the associating groups of a few kBT and above. For weakly interacting associating groups, the static structure function S(q) is well fit by functional form predicted by the random phase approximation where a clear deviation occurs as phase segregation takes place, providing a quantitative assessment of phase segregation.

Entropic Mixing of Ring/Linear Polymer Blends

“…For comparison, S ( q ) for a linear/linear and a ring/ring pseudo-blend in which half the chains are randomly chosen to be type A and the other half type B, is fit nicely by the RPA formula with χ = 0. For linear/linear blends, the RPA formula with χ = 0 was also found to fit S ( q ) very well for the fully flexible model . This entropic mixing of ring/linear blends occurs for all chain lengths studied as shown in Figure c, with χ decreasing slightly from χ = −0.02 for N = 50 to −0.027 for N = 400 for chains with k θ = 1.5ε.χ is sensitive to chain stiffness as seen in Figure for N = 200, χ decreases from −0.005 for k θ = 0 to −0.053 for k θ = 3.0ε.…”

“…For linear/ linear blends, the RPA formula with χ = 0 was also found to fit S(q) very well for the fully flexible model. 43 This entropic mixing of ring/linear blends occurs for all chain lengths studied as shown in Figure 2c, with χ decreasing slightly from χ = −0.02 for N = 50 to −0.027 for N = 400 for chains with k θ = 1.5ε.χ is sensitive to chain stiffness as seen in Figure 3 for N = 200, χ decreases from −0.005 for k θ = 0 to −0.053 for k θ = 3.0ε. As k θ increases, the ring conformations become larger and the number of linear chains threading a ring increases, resulting in increased entropic mixing and smaller ring and linear domains as seen in TOC graphic.…”

Phase Behavior and Morphology of Blends Containing Associating Polymers: Insights from Liquid-State Theory and Molecular Simulations

“…Using a coarse-grained (CG) model in which hydrogen bonds between PTAD CG beads were modeled using the Lennard-Jones (LJ) potential, they showed that the intrachain loop formation impacts melt structure and chain conformations even at moderate strengths of association and low functional group composition . Similarly, Grest and co-workers studied the effects of randomly placed associating groups along polymer chains with linear and star architectures in melts and in blends containing polymers with and without associating groups, respectively . Their simulations showed that increasing the association strength leads to reduction in chain mobility due to cluster formation driven by associative interactions .…”

“…64 Similarly, Grest and co-workers studied the effects of randomly placed associating groups along polymer chains with linear and star architectures in melts and in blends containing polymers with and without associating groups, respectively. 66 Their simulations showed that increasing the association strength leads to reduction in chain mobility due to cluster formation driven by associative interactions. 66 Further, the effect of ionic associative interactions on the structure and dynamics of ionomers has also been studied using CG MD simulations where interactions between ions were modeled using simple Coulombic interactions.…”

“…66 Their simulations showed that increasing the association strength leads to reduction in chain mobility due to cluster formation driven by associative interactions. 66 Further, the effect of ionic associative interactions on the structure and dynamics of ionomers has also been studied using CG MD simulations where interactions between ions were modeled using simple Coulombic interactions. 29 Although such CG models based on simple isotropic potentials reduce computational intensity when simulating polymers with associative interactions, a limitation with these models is that the isotropic treatment of directional associative interactions like hydrogen bonding, π−π stacking, etc., leads to omission of effects arising due to the directionality of interactions.…”