Bioinspired High-Strength Montmorillonite-Alginate Hybrid Film: The Effect of Different Divalent Metal Cation Crosslinking

The natural nacre has a regular ordered layered structure of calcium carbonate tablets and ion crosslinking proteins stacked alternately, showing outstanding mechanical properties. Inspired by nacre, we fabricated different divalent metal cation-crosslinked montmorillonite-alginate hybrid films (MMT-ALG-X(2+); X(2+) = Cu(2+), Cd(2+), Ba(2+), Ca(2+), Ni(2+), Co(2+) or Mn(2+)). The effect of ionic crosslinking strength and hydrogen bond interaction on the mechanical properties of the nacre-mimetics was studied. With the cations affinities with ALG being increased (Mn(2+) < Co(2+) = Ni(2+) < Ca(2+) < Ba(2+) < Cd(2+) < Cu(2+)), the tensile strength of nacre-mimetics showed two opposite influence trends: Weak ionic crosslinking (Mn(2+), Co(2+), Ni(2+) and Ca(2+)) can synergize with hydrogen bonds to greatly increase the tensile properties of the sample; Strong ionic crosslinking (Ba(2+), Cd(2+), Cu(2+)) and hydrogen bonding form a competitive relationship, resulting in a rapid decrease in mechanical properties. Mn(2+) crosslinking generates optimal strength of 288.0 ± 15.2 MPa with an ultimate strain of 5.35 ± 0.6%, obviously superior to natural nacre (135 MPa and 2%). These excellent mechanical properties arise from the optimum synergy of ion crosslinking and interfacial hydrogen bonds between crosslinked ALG and MMT nanosheets. In addition, these metal ion-crosslinked composite films show different colors, high visible transparency, and excellent UV shielding properties.