a, Lap-joint geometry. Displacement was measured by a video extensometer that followed two markers (white dots), which were placed at a distance of 5 mm from the edge of the lap joint. The total length of the assembled ribbons was 40 mm.w denotes the width andh the thickness of gel ribbons.l is the overlap length.b, Comparison of force–displacement curves for PDMA S0.1 ribbon (black line) and for the lap joint glued by spreading 15 μl of TM-50 silica solution (blue circles). Displacement was measured by optical extensometer from two markers, initially spaced by 20 mm and centred on the joint. The PDMA S0.1/S0.1 assemblies broke outside the joint (inset).c, Lap-shear adhesion test reproducibility. Force–displacement curves of PDMA S0.1/S0.1 lap joint illustrating lap-shear test reproducibility. All gel ribbons were cut from the same plate. Bulk failure outside the joint was systematically observed.

a, Lap-shear test geometry in which interfacial failure by peeling was observed for S0.1 gel ribbons glued by spreading 6 μl of TM-50 silica solution (l = 5 mm,w = 2 mm andh = 5 mm).b, Force–displacement curves for PDMA S0.1/S0.1 lap joints. Adhesive failure by interfacial peeling was observed. All gel ribbons were cut from the same gel plate and the tensile modulus was measured to beE ≈ 8.1 ± 1.0 kPa (error is s.d.). From the measured failure force, the adhesion energy can be estimated to be 6.2 ± 1.4 J m⁻² (error is s.d.).

In order of increasing deformation at the breaking point are shown the adhesive failure by interfacial peeling in CNT-Thy (grey), CNC1 (green), SM-30 (yellow) and HS-40 (purple). Fracture outside the junction occurred for TM-50 (blue) and AL-30 (red). Lap-joint dimensions werel = 5 mm,w = 5 mm andh = 2 mm. 6 μl of solution was spread to make the junction.

a, Force–displacement curve for an assembly made of soft PDMA S0.1 and rigid PDMA S1.5 (red triangles) gels glued by TM-50 solution. For comparison the results obtained under identical conditions for the symmetric PDMA S0.1/S0 assembly are plotted (blue circles). Lap-joint dimensions werel = 10 mm,w = 5 mm andh = 2 mm. 15 μl of TM-50 solution was spread to make the junction.b, Glued at their preparation state, both PDMA S0.1 and PDMA S1.5 gels initially had the same size (diameter of about 10 mm). The picture shows gels after 5 h of swelling in deionized water. The highly crosslinked PDMA S1.5 gel (top piece) is less swollen than the PDMA S0.1 gel (bottom piece). Interfacial stresses induced by heterogeneous overswelling considerably exceed the shear stresses applied in the mechanical lap-shear test ofa and lead to interfacial failure during immersion and overswelling in water. De-bonding was slow.c, Lap-shear force–displacement trace for the gelatine and S0.1 PDMA gel assembly (green stars) glued by spreading TM-50 silica solution. The failure occurred outside the lap joint and the fatal crack propagated in tension mode.

a, Adhesion energy of joints made of PDMA S0.1 hydrogels swollen before being glued with AL-30 silica solutions to various degrees of swellingQ (in blue) and adhesion energy of joints made of S0.1 hydrogels glued with TM-50 silica solutions at as-synthesized swelling degree (Q₀ ≈ 8.5) and after being immersed in water and swollen to reach the maximum, equilibrium swelling degree,Q_e ≈ 41 (in red) (error bars are s.d.). When S0.1 gels were glued with AL-30 particles at the as-synthesized state, bulk failure systematically occurred outside the junction even when the joints were very short, narrow and thick. To induce peeling (interfacial failure) making cuts at the interface was necessary.b, Lap-shear test for PDMA S0.1/S0.1 assembly glued by TM-50 silica at the preparation state,Q₀ (blue circles) and after swelling in water for three days and attaining maximum equilibrium swellingQ_e (red circles).c, Mechanical behaviour of a nanocomposite sample that can be considered as a model of the interfacial adhesive layer. Engineering stress is plotted as a function of strain. The loading–unloading cycle at the gel preparation conditions (green line) shows a large hysteresis. At equilibrium swelling (red dashed line) the hysteresis and dissipation were very weak. Low dissipation at swelling equilibrium is responsible for a relatively weaker adhesion after immersion in water.

Failure by fracture occurs outside the junctions (MOV 4958 kb)

The PDMA S0.1/S0.1 lap joint glued with TM-50 silica solution at as-synthesized conditions, immersed in water for two days to reach swelling equilibrium fails by interfacial peeling. (MOV 986 kb)