Linear Viscoelastic and Uniaxial Extensional Rheology of Alkali Metal Neutralized Sulfonated Oligostyrene Ionomer Melts

Abstract

The linear viscoelastic and nonlinear extensional behavior of melts of alkali metal salts of oligomeric sulfonated polystyrene (SPS) ionomers were characterized by dynamic shear and nonlinear, uniaxial extensional flow experiments. The oligomeric SPS had a weight-average molecular weight of 4000 g/mol, a polydispersity index of 1.06, and a degree of sulfonation of 6.5 mol %. The molecular weight was below the entanglement molecular weight of PS, so all rheological effects were due to association of the ionic dipoles and nanophase separation of the ionic species that provides a transient elastic network. The SPS salts exhibited linear viscoelastic properties similar to well-entangled polystyrene (PS) melts, with a distinct rubbery region that had a shear modulus comparable to that of high molecular weight PS. Time−temperature superposition failed as a consequence of overlapping relaxations for the terminal response of the chain and ion hopping of the ionic dipoles. Unlike entangled PS melts, the modulus of the ionomer increased with increasing extensional strain rate, and a maximum in the stress occurred at a relatively low Hencky strain that was nearly independent of strain rate. The maximum in the stress during stretching was attributed to a catastrophic failure of the physical ionic network. At sufficiently high stress, the chains can pull the ionic groups out of nanophase-separated ionic domains, which significantly disrupts the network microstructure.

Linear shear and nonlinear extensional rheology of unentangled supramolecular side-chain polymers

“…Strain hardening was observed for strain rates significantly smaller than the inverse of the reptation time, indicating that the strain hardening for their studied polymer system is attributed to stretching of chain segments which are restricted by hydrogen bonding groups. Similar observations were also made for ionomers [89,90]. In ionomers, the supramolecular interactions originate from association of ionic groups covalently attached to either the polymer backbone or the side groups [92].…”

“…Supramolecular polymers are made of covalent chains connected through reversible interactions, such as hydrogen bonding , metal-ligand coordination [26][27][28][29][30][31][32][33][34][35][36][37][38], and ionic aggregation [39][40][41][42][43][44][45]. The ability to vary and control the interactions in supramolecular systems provides an efficient tool to tune the structure, dynamics and rheology [26,29,46].…”

“…However, for supramolecular polymers with side-chain functional groups, relatively few studies have been reported [89][90][91]. As an example, Shabbir and coworkers [91] have reported the extensional rheology of poly(butyl acrylate-co-acrylic acid) with varying acrylic acid content.…”

Supramolecular Multiblock Polystyrene–Polyisobutylene Copolymers via Ionic Interactions

“…An example of this type of failure is the lower tensile fracture strain of an oligomeric ionomer melt undergoing unaxial extensional flow compared with that for a high molecular weight, entangled polymer of the same backbone. 67 Nonlinear Rheological Behavior. Three consecutive shear strain−time cycles for the SMBCP are shown in Figure 11.…”

Viscoelasticity of Reversible Gelation for Ionomers

“…The gel point estimated by eq 1 is supported by the linear viscoelastic complex modulus reported by Weiss and coworkers. 7,8 For random sulfonated oligomeric styrene with low molecular weight M = 4000 g/mol (N = 38) (< entanglement molecular weight M e = 17 000 g/mol), the gel point is expected to be p c = 1/(N − 1) = 2.7 mol %. In accordance with this expectation, the ionomer having p = 2.5 mol % (ε = −0.064) shows power law viscoelasticity usually seen quite close to the gel point.…”