POM-Based Composites with Enhanced Mechanical Strength and UV-Induced Regional Stiffening

Abstract

Organic–inorganic composites have attracted considerable interest for their ability to combine various properties, yet achieving robust and tunable mechanical properties remains challenging. Herein, we incorporate silicotungstic acid (STA), a polyoxometalate with abundant three-dimensional hydrogen-bonding sites and intrinsic photochromic functionality, as a dynamic hydrogen-bond cross-linker into polymer networks, proposing a strategy that enhances mechanical properties of the composite. A polyacrylamide (PAAm)-STA composite was synthesized via a simple one-pot synthesis, and the composite films PAS2 exhibited a tensile strength of 130.0 ± 2.8 MPa, representing a 4.8-fold enhancement over pristine PAAm (28.5 ± 1.4 MPa), while maintaining an average optical transmittance exceeding 90%. To balance strength and toughness, ethylene glycol (EG) was incorporated into the synthesis process at varying concentrations, resulting in a series of composite films with tunable elongation (4–920%) and tensile strength (∼3.4–130.0 MPa). Upon ultraviolet-induced photochromic transition, the transmittance of the PAS2 film can be tuned to as low as 1.7%, the tensile strength can increase to 150.3 ± 2 MPa, and the toughness rises from 4.03 ± 0.11 to 5.18 ± 0.05 MJ/m3. By applying a photomask, region-specific mechanical reinforcement was readily achieved. The composite can be conveniently recycled by dissolving in water, and the reprocessed film retains its outstanding mechanical and optical properties. This strategy of synergistic reinforcement was successfully extended to poly(vinyl alcohol), poly(vinylpyrrolidone), and commercially available 3D-printable polylactic acid resin, confirming its broadly applicable. The present approach not only improves the overall mechanical performance of the material but also imparts postfabrication tunability, offering insights into the design of adaptive functional materials.