Rheology of sodium and zinc ionomers: Effects of neutralization and valency

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 (sodium and zinc) 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 lifetime of associations that are associated with entanglements, ionic and hydrogen bonding associations. As such, scaling laws have been used to calculate the order of magnitude of these characteristic times that are important parameters to gain a better understanding of their rheological behavior. To study the effects of ionic reversible associations, the commercial ionomers were completely un-neutralized and their rheological behavior was compared directly with their associative counterparts. The rheological comparison included the linear viscoelastic moduli, the damping function, and extensional rheology, demonstrating the significant effects of ionic interactions. Moreover, the rheological properties of sodium and zinc ionomers are also compared addressing the effect of valency of ions (Na+ vs Zn++).

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

“…To solve Eqs. (20)(21) in the long-time limit, we make the useful approximation that at an intermediate time t = t * the distribution is equilibrated for small stretches λ ≤ λ * , while the large-stretch tail of the distribution is unoccupied. Hence, at t = t * the distribution is given by P 0 (0, y) = c c P eq 0 Θ(−y + y * )…”

“…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].…”

“…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].…”

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.…”