Fig. 3

Stiffening of the PIC/PNIPAM hybrids. a Storage modulus G′ from different architecture hybrids. The stiffening response is identical for a covalently cross-linked network (green), IPN (blue) and a semi-IPN (orange). b Reversibility and small hysteresis observed after 10 cycles (only even cycles displayed). c Stiffening transition at different heating rates (0.1–10 °C min⁻¹). d Stiffening rates G′_T+1/G′_T (orange) and transition temperature T (red) as a function of heating rate. The concentrations in a, b and c, d are PIC/PNIPAM 1/17, 2/17 and 1/40 mg mL⁻¹. e, f Increasing the PNIPAM (e) or PIC (f) concentration increases the storage modulus G′ of the PIC/PNIPAM hybrid after thermally induced stiffening. g Differential modulus K' against the external pre-stress σ for a 1 mg mL⁻¹ PIC hydrogel at T = 33 °C. The dotted lines correspond to the modulus of the PIC/PNIPAM hybrid gels at T = 37 °C and the external pre-stress σ corresponding to this modulus. Direct comparison between G′ and K′ is allowed, because of the strong similarity between the experiments: in both cases, we determine δσ/δγ as a function of static pre-stress that is induced either by PNIPAM or by externally applied stress. h The average internal stress σ_int generated by the PNIPAM network scales linearly with the product c_PNIPAM·c_PIC for all studied samples. The dashed line is a power law fit to the experimental data (slope 0.97). i, j The average force per fibre as a function of c_PNIPAM (i, c_PIC = 1 mg mL⁻¹) and c_PIC (j, c_PNIPAM = 17 mg mL⁻¹) only depends (linearly) on the PNIPAM concentration (note the logarithmic x-axes). Lines are power law fits