Synthesis of strong and stretchable double network (DN) hydrogels of PVA-borax and P(AM-co-HEMA) and study of their swelling kinetics and mechanical properties

Strong and stretchable double network (DN) hydrogels are reported here which are consisting of first network of PVA-borax double strand and crosslinked P(AM-co-HEMA) as the second network. The first network is formed due to PVA-borax complexation in which the second network of AM and HEMA was embedded by in-situ polymerization. These DN hydrogels are having high degree of equilibrium swelling and mechanical properties. The swelling behavior of the hydrogels increased with AM content and decreased with HEMA content because of more hydrophilic nature of AM unit than HEMA unit in DN hydrogels. The tensile strengths of such hydrogels were observed to be 363.0 kPa, 183.4 kPa and 36.3 kPa respectively for the 20 wt%, 40 wt% and 60 wt% water content in the DN hydrogels (having 100% AM content in the second network). On the other hand, the tensile strength of the control single network crosslinked polyacrylamide (CPAM) hydrogels (no double network) was observed to be 280.9. kPa for 20 wt% and 112.6 kPa for 40 wt% water content in the hydrogel. Therefore, about 30% and 62% improvement of mechanical properties is observed in these cases due to DN hydrogels. High tensile strength of the DN hydrogels may be attributed to the reinforcing influence of PVA-borax complex onto the P(AM-co-HEMA) network. Such a DN hydrogel is observed to be 13 times of the stretchable cycle of its double length. The mechanical properties of the DN hydrogels varies not only with the composition of second network of P(AM-co-HEMA) but also by the influence of H-bonding. Optimum mechanical properties are observed at the composition of AM/HEMA 30/70 to 75/25 in the DN hydrogels. The influence of H-bonding is more prominent in case of lower water content in the hydrogels than that of the hydrogels having more water content.