Mechanically Strong Functionalized-BNNS/PVA Composite Hydrogels with Excellent Thermal Conductivities

Polyvinyl alcohol (PVA) hydrogels exhibit superior viscoelasticity, flexibility, and biocompatibility and have been widely utilized in wearable devices, flexible electronics, and electronic packaging. However, their low thermal conduction ability and poor mechanical performance limit their further applications and development for the high integration and multifunctionality of flexible electronics and wearables. Herein, BNNS-OH/PVA composite hydrogels containing oriented structure were fabricated via directional freezing, salting-out, and stretching, applying PVA hydrogel as matrix and hydroxyl boron nitride nanosheets (BNNS-OH) as thermal-conductive fillers. The oriented BNNS-OH/PVA composite hydrogel containing 9 wt.% of BNNS-OH exhibited appreciably improved in-plane thermal conductivity (λ) of 3.13 W/(m·K) when stretched by 3.5 times, enhanced by 210% compared to 1.01 W/(m·K) for the unstretched hydrogel and 502% compared to 0.52 W/(m·K) for pure PVA hydrogel made applying freeze-thaw (FT) method. Additionally, the oriented BNNS-OH/PVA composite hydrogels exhibited superior elastic modulus, tensile strength, and toughness of 20 MPa, 18.8 MPa, and 9.9 MJ/m³, respectively, to 0.07 MPa, 0.4 MPa, and 0.32 MJ/m³ of pure FT PVA hydrogel.