Highly stretchable and tough thermo-responsive double network (DN) hydrogels: Composed of PVA-borax and poly (AM-co-NIPAM) polymer networks

The development of polymeric double network (DN) hydrogels consisting of polyvinyl alcohol-borax (PVA-Borax), poly(N-isopropylacrylamide) and poly(acrylamide) are encouraging to utilize them in many applications like as soft materials for biomedical and other applications due to their unique characteristics such as stretchability, toughness, thermo-responsiveness, resilient and self-healing property. The DN hydrogels are hydrophilic in nature and also soft smart materials that have higher capability to absorb water and also, they respond external stimuli such as temperature, pressure and chemical trigging. Thus, DN hydrogels of PVA-Borax as the first network and poly (NIPAM-co-AM) as the second network are prepared and characterized. Synthesized DN hydrogels are sensitive to temperature by introducing the temperature sensitive polymer, poly (NIPAM) into the network. Poly (AM) is used for high swelling and the stiffness improvement. The degree of swelling increases with the increasing amount of AM in the network. The highest degree of swelling is achieved as high as 1272% in GNAM-1 which contains 100% AM in second network (No NIPAM). The lower critical solution temperature (LCST) was observed to be 31.5 °C for GNAM-5 (100 wt% NIPAM and no AM) DN hydrogel by swelling study. In DSC analysis volume phase transition has been observed to be 33.72 °C for this hydrogel. However, marginal increment of phase transition is observed due to incorporation of more hydrophilic AM monomer in the second network. The synthesized DN hydrogels exhibit excellent mechanical properties. We tested all the samples with different water content (WC). Such a DN hydrogel (GNAM-3) sustains high compressive stress 5.75 MPa at 87.71 % strain with 30% WC, whereas it was decreased to 1.33 MPa at 75.02% strain with 60% WC and to 0.0.86 MPa at 75.86% with 90% WC. The maximum compressive fracture energy is observed for this sample with 30% WC which is 36.72 MJ/m3. This DN hydrogel also maintained its shape after many mechanical cycles, indicating the achievement of desirable resilience properties. Furthermore, it could be stretched up to 944 % strain with the tensile strength of 0.473 MPa with 60% WC. Fracture strain of the hydrogels was increased with poly(NIPAM) amount in the DN hydrogels till the 1:1 of Poly(NIPAM): poly(AM) after that it was started to decrease. The highest fracture strength of 1.36 MPa is observed for GNAM-5 hydrogels with 30% WC.