Nonlinear rheology of poly(ethylene-co-methacrylic acid) ionomers

Abstract

Using a parallel-plate rheometer equipped with a partitioned plate, and the Sentmanat extensional rheometer fixture, a full rheological characterization of several commercial ionomers and their corresponding parent copolymers has been carried out. Particular emphasis has been placed on the distribution of the relaxation times to identify the characteristic times, such as reptation, Rouse, and sticky-Rouse, that are associated with the characteristic lifetime of the ionic and hydrogen bonding associations. As such, scaling laws have been used to calculate the order of magnitude of the lifetimes of associations and the association energy that are important parameters to gain a better understanding of their rheological behavior. To study the effects of ionic associations, the commercial ionomers were completely unneutralized and their rheological behavior was compared directly with their associative counterparts. The rheological comparison included the linear viscoelastic moduli, the damping function, steady shear, and extensional rheology demonstrating the significant effects of ionic interactions.

Stretching of Bombyx mori Silk Protein in Flow

“…The lifetime of these ionic bridges depends on their ionic environment and appears to shorten with increasing monovalent cation (mainly potassium) concentration. Usually, the sticker lifetime is described using an empirical Arrhenius form , where the activation energy is determined from the linear rheology using Van ’t Hoff plots [ 13 , 23 , 26 , 40 , 51 , 52 , 53 ]. In this section, we argue that this activation energy might not be the one that is of importance to the non-linear rheology, because a different mechanism of sticker opening takes over.…”

Theoretical rheo-physics of silk: Intermolecular associations reduce the critical specific work for flow-induced crystallisation

“…The low-energy mechanism for natural silk-spinning therefore remains to be identified. Clues may be present in the subtle electrostatically-modified rheo-physics of associating polymers [12][13][14][15][16][17][18][19][20][21].…”

“…We previously found in collaboration with Laity and Holland that the silk protein exhibits intermolecular reversible cross-links [7,22]. While these associations shift the alignment-tostretch transition to smaller strain rates by replacing the usual Rouse relaxation dynamics for 'sticky Rouse' relaxation [12][13][14][15][16][17][18][19][20][21], this is not the full story, for the Bombyx mori silkworm manages also to generate the opposite effect during pupation: when the silkworm starts spinning the material it actually chemically reduces this relaxation time through the addition of potassium cations [7,22]. Intriguingly, the group of Holland discovered that the structural features of the silk fibre is significantly enhanced through a gradient in the pH along the spinning duct, suggesting an exquisitely controlled local rheology [23].…”

Power-Law Stretching of Associating Polymers in Steady-State Extensional Flow

“…Recent work has shown that microscopic chain stretch and the consequent macroscopic strain hardening is triggered by a small number of calcium bridges [13,15] that act as 'sticky' reversible intermolecular crosslinks akin to those in synthetic 'sticky polymers' [16][17][18][19][20][21][22][23][24]. For this class of molecules, a molecular understanding of the non-linear rheology and crystallization of 'sticky polymers' has so far relied on computationally expensive (coarse-grained) molecular dynamics simulations [5,[25][26][27][28][29]. Simpler molecular models coarse-grained at the level of entanglements, but able to capture the vital slow processes, remain absent.…”

“…open [13,17,[24][25][26][27][28], and the closing rate is given by k close = k open p/(1 − p), with p the time-or ensemble-averaged fraction of closed stickers. Here, we ignore the underlying dissociation-association or bondswap mechanisms that determine the concentrationdependence of the opening and closing rates [40,41], and view p and τ s as free model parameters.…”