Self-Reinforcement of PNIPAm-Laponite Nanocomposite Gels Investigated by Atom Force Microscopy Nanoindentation

Strain hardening and self-reinforcement were observed from poly(N-isopropylacrylamide)(PNIPAm)-Laponite nanocomposite hydrogels (NC gel) after large deformation of either stretching or tearing. These phenomena were investigated with atomic force microscopy (AFM) nanoindentation in nanoscale for the first time. Strong attractive force was detected from the indentation force curve of the as-prepared and swollen NC gels due to the capillary effect of water between the AFM tip and gel surface. The Young's modulus E of the NC gels was evaluated by the AFM nanoindentation using the modified Hertz model, which increased with increasing laponite and decreased after swelling. After the NC gels suffered stretching to 900% strain or tearing to break, the Young's modulus was substantially increased, implying the self-reinforcement of the gel samples. This effect was enhanced by increasing clay content. On relaxation of deformed samples containing a small amount of clay, the modulus almost recovered its original value (before application of large deformation) within 10 h at rest. However, for the NC gels with high clay content, this recovery was slowed down and the residual strain remained even after 190 h. The strain hardening of the NC gels during deformation was attributed to the orientation of the clay platelets by pulling connected polymer network chains during elongation. The interparticle distance L related to the diameter d of the platelets was adopted to interpret the recovery of the NC gels: L > d at low clay concentrations (<= 69% w/v), and the clay platelet disorientation resulted in the recovery; while L < d at high clay concentrations (>6% w/v), the clay platelet movement was strictly limited to induce self-reinforcement for the NC gels.