Dynamics in Supramolecular Polymer Networks Formed by Associating Telechelic Chains

Abstract

We present hybrid molecular dynamics/Monte Carlo simulations of supramolecular networks formed by unentangled telechelic chains with sticky end monomers (or stickers) of finite functionality. The reversible bonding between sticky monomers leads to the formation of sticker clusters with well-defined size distribution, which in turn work as cross-links for transient polymer networks. We study the kinetics of sticky monomer association, the topological structure and the resulting dynamic and rheological behavior of the supramolecular systems as a function of the sticker bonding energy ε and the parent polymer chain length. Percolated transient networks are formed above a threshold bonding energy around 4.3k B T. At high bonding energies ε ≥ 10k B T , the majority of the stickers are fully reacted and the fraction of open stickers is less than 1%. The conventional picture of a single sticker hopping from one cluster to another is energetically unfavorable. We find the dynamic and rheological behavior of such strongly associated supramolecular networks are dominated by a partner exchange mechanism in which the stickers exchange their associated partners, and so release the imposed topological constraints, through the association and disassociation of sticker clusters. The characteristic time of the partner exchange events grows exponentially with the bonding energy and is up to 2 orders of magnitude longer than the average lifetime of the reversible bonds. As 1 a result, three relaxation regimes can be clearly identified in the stress and chain end-to-end vector relaxation functions as well as the mean-squared displacements of the stickers, which are the initial Rouse regime, the intermediate rubbery regime and the terminal relaxation regime. A phantom chain hopping model based on the microscopic understanding is proposed to describe the chain relaxation dynamics in the supramolecular networks, which provides numerical predictions in reasonably good agreement with our simulation results.

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 .…”

“…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 . Indei and Takimoto derived the relaxation time distribution by a transient network theory of APs with multiple associating groups .…”

“…Hoy and Fredrickson found the crossover from diffusion‐limited to kinetically limited physical bond recombination . Amin and others showed the Rouse relaxation time and the exchange time of associating groups . However, relationships between the association behavior of APs and rheological properties at dense micellar phase still remain unclear.…”

“…A lot of theoretical and computational studies of rheological properties of physical gels consisting of APs have been reported. 16,17,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] 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. [24][25][26] For the relatively high shear rates, where the finite extensible nonlinear elasticity effect of the polymer is significant, the shear thickening induced by the nonlinear stretching of elastically effective chains was calculated by a transient network theory [27][28][29][30] and a nonequilibrium molecular dynamics simulation.…”

“…The multiple relaxation times of the physical gels have also been theoretically studied. 31,35,38,40 Indei and Takimoto derived the relaxation time distribution by a transient network theory of APs with multiple associating groups. 31 Bedrov and others calculated the stress relaxation and elucidated two-step relaxation due to the translational motion of the associative groups within the clusters and the hopping of associative groups between neighboring clusters.…”