Linear Viscoelastic and Nonlinear Extensional Rheology of Diamine Neutralized Entangled Poly(styrene-co-4-vinylbenzoic acid) Ionomer Melts

Ionomers are a class of polymers with a small amount (<10 mol%) of bonded ionic species attached to a relative nonpolar backbone. They have been used in a variety of applications, such as membranes, packaging and films, compatibilizers, shape memory materials, and self-healing materials.1,2 However, the processing of ionomers may be challenging due to ionic clusters formed by ion dipole interaction. In this work, a new strategy to prepare ionomers with good flowability, stretchability and extensibility is presented. Entangled poly(styrene-co-4-vinylbenzoic acid) with 5 mol% acid (Mw=85kg/mol, number of entanglements Z=6.4) was neutralized with diamine of different molar masses. The resulting neutralized ionomers presented a relaxation process similar to entangled polymers, but with faster terminal relaxation, suggesting no ionic cluster formation. This feature provides good flowability to the ionomers and facilitates their processing. In extensional measurements, better stretchablility is achieved, manifesting as higher strain hardening compared to non-ionic parent molecule. The stress-strain curves showed a maximum stress, followed by stress overshoot and steady state at larger strain. The stress maximum and overshoot are correlated to ionic sticker disassociation as evidenced by phase separation induced color change during filament stretching. The sticker lifetime decreases with increasing stretch rates. The good extensibility (up to Hencky strain 7) is obtained thanks to ionic sticker reassociation and entanglements.