Gelling Behavior of PVA or its Blends with Other Polymers in Presence of Borax and Thermochromism in Such Gel Network Materials

Gelling behavior of PVA or its blends with other polymers such as polyvinyl pyrrolidone (PVP), polyethylene oxide (PEO), hydroxyl propyl cellulose (HPC), agar and gelatin A has been studied as a function of borax concentration from viscosity measurements. The pH of the borax cross linked hydrogels of PVA or its blends showed an interesting behavior with temperature - it initially increases with temperature up to a maximum and then decreases with further increase in the temperature. Such temperature sensitive transparent hydrogels when embedded with a suitable acid-base indicator like phenolphthalein showed reversible thermochromism. However, thermochromism was not observed in the hydrogels of polymer blends if the amount of PVA in the blend was less than 60%. The results suggest that thermochromism in dye embedded borax cross-linked hydrogels is due to the formation of didiol cross-links between hydroxyl groups in PVA and the borax, which is governed by change in temperature. The effect of the addition of an anionic (SDS), or a non ionic (TX-100) surfactant on the thermochromic behavior of the hydrogels has also been studied.