Toward a Design for Flowable and Extensible Ionomers: An Example of Diamine-Neutralized Entangled Poly(styrene-co-4-vinylbenzoic acid) Ionomer Melts

Processing ionomers is complicated by their ability to exhibit brittle fracture even in the melt state. This work introduces a new strategy for providing ionomers with good flowability, extensibility, and superior strain hardening. Diamine-neutralized entangled poly­(styrene-co-4-vinylbenzoic acid) ionomers were studied using small-amplitude oscillatory shear and nonlinear uniaxial extension measurements. The parent molecule, poly­(styrene-co-4-vinylbenzoic acid), has a molar mass of 85,400 g/mol, well above the entanglement molar mass of polystyrene (13,300 g/mol). Neutralization was performed using “Jeffamine” type diamines with different molar masses. The resulting neutralized ionomers presented relaxation processes similar to entangled polymers but with faster terminal relaxation, suggesting negligible ionic cluster formation and indicating a diluting effect of the introduced diamines. This feature provides the ionomers with good flowability and facilitates their processing. In extensional measurements, these ionomers displayed superior strain hardening compared to the parent molecule, which also proved to be adjustable via changing diamine length. The stress growth curves showed a maximum stress, followed by stress overshoot and steady state at larger strain. The stress maximum and overshoot were correlated with ionic sticker disassociation, as evidenced by phase separation-induced color change during filament stretching. At high stretch rates, the stickers disassociate abruptly to accommodate the strain, so that the sticker disassociation time decreased with increasing stretch rates. Good extensibility (up to Hencky strain 7) was achieved via ionic sticker reassociation and entanglements.