Fig. 5

Mechanical properties of PI–xNa. a Stress–strain curves for PI–xNa stretched at 100 mm min⁻¹. The degree of neutralization for each sample is given. b Stress–strain curves of PI–90Na stretched at different stretching speeds. c Recovery of PI–90Na for different waiting periods performed by cyclic tensile tests at 100 min min⁻¹. d Plots of fracture strain (hollow magenta circles), Young’s modulus (hollow blue circles), fracture stress (filled magenta circles), and toughness (filled blue circles) of PI–xNa films against the degree of neutralization