Enough evidence has been gathered to conclude that the changes in an environmental variable like temperature exhibit a remarkable discontinuous change in the volume phase transition of polymer gels. Among the polymer gels exhibiting volume phase transition with changes in temperature, poly-(N-isopropylacrylamide) (PNIPA)-based hydrogels have particularly attracted research attention. It exhibits the crosslink density changes during the volume phase transition. Regarding the role of crosslink density, its relationship with the volume phase transition has not been fully explored. This is, however, an important consideration due to the rapid growth of the poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) (PEO-b-PPO-b-PEO) block copolymers in the pharmaceutical industries.
Recently, Kyoto University scientists: Nobuhiro Takaoka, Dr. Jun-ichi Horinaka and Professor Toshikazu Takigawa from Department of Material Chemistry investigated the temperature dependence of volume and the modulus reflecting the crosslink density of the systems. In particular, the authors were determined to clarify the origin of the crossover on the temperature dependence curve of gel volume taking into account the gel modulus. Their work is currently published in the journal, Colloid and Polymer Science.
In brief, the authors commenced their work by first preparing hydrogels composed of a PEO-b-PPO-b-PEO copolymer to be used in the experimental investigation. Secondly, mechanical tests for the gels at various temperatures were carried out to examine the emergence of the physical crosslinks into the gel systems. To gain more insights about the gels, phase diagrams deduced from the mechanical tests were presented and discussed.
Three different PEO-b-PPO-b-PEO triblock copolymer-based hydrogels were considered. The authors observed that the polymer volume fraction of the gels increased monotonically with the increase in the temperature. However, hysteresis occurred on the gel-temperature curves only during the heating and cooling processes. On the same gel-temperature curves especially around the critical micelle temperature of the constituent copolymers, a leveling-off tendency was observed. As such, it was possible to clarify the relationship between the volume phase transition and the changes in crosslink density. For instance, conducting the mechanical tests at different temperatures enabled the analysis of the Youngs modulus as a function of the temperature even though it also depended on the volume fraction of the gels. Furthermore, the plots of Youngs Modulus versus temperature indicated that the crossover between two gels states differed in crosslink density.
The gel network structures were further classified based on three different temperature regions. At temperatures lower than the critical micelle temperature of the constituent copolymers the network structure was formed only by the chemical crosslinks while at higher temperatures, the poly (propylene oxide) aggregates worked as physical crosslinks. On the other hand, the degree of crosslink formation was dependent on the temperature at a medium temperature around the critical micelle temperature. The crossover was qualitatively explained by the free energy functions of two states with different crosslink numbers.
In summary, the study successfully investigated the temperature dependence of the volume of hydrogels. This further presented more insights about the relationship between the volume phase transition and the change in crosslink density. As explained by Professor Toshikazu Takigawa the lead author in a statement to Advances in Engineering, the study will advance hydrogel-based applications.
Takaoka, N., Horinaka, J., & Takigawa, T. (2019). Crossover behavior on temperature dependence of volume of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide)–based hydrogels. Colloid and Polymer Science, 297(9), 1177-1182.