Phase Transition Effects on Mechanical Properties of NIPA Hydrogel.

Ni Zhang,Zishun Liu,Shoujing Zheng,Zhouzhou Pan

doi:10.3390/polym10040358

Abstract

Due to its excellent temperature sensitivity, the Poly(N-isopropylacrylamide) (NIPA) hydrogel has attracted great interest for a wide variety of applications in tissue engineering and regenerative medicine. NIPA hydrogel undergoes an abrupt volume phase transition at a lower critical solution temperature (LCST) of 30–35 °C. However, the mechanical behaviors of NIPA hydrogel induced by phase transition are still not well understood. In this study, phase transition effects on mechanical properties of NIPA hydrogel are quantitatively studied from experimental studies. The mechanical properties of NIPA hydrogel with the LSCT around 35 °C are systemically studied with varying temperatures (31–39 °C) under a tensile test. We find that the mechanical properties of NIPA hydrogel are greatly influenced by phase transition during the tension process. The maximum nominal stress and maximum stretch above the LCST are larger than those of below the LCST. The Young’s modulus of NIPA hydrogel is around 13 kPa at 31 °C and approximately 28 kPa at 39 °C. A dramatic increase of Young’s modulus values is observed as the temperature increases through the phase transition. The samples at a temperature around the LCST are easy to rupture, because of phase coexistent. Additionally, NIPA hydrogel displays toughening behavior under a cyclic load. Furthermore, the toughening characteristic is different between the swollen state and the collapsed state. This might originate from the internal fracture process and redistribution of polymer chains during the tension process.

Highlights

Intelligent hydrogels have attracted considerable attention because of their abilities to change their volumes and properties in response to external stimuli [1,2,3,4]
The toughening characteristic is different between the swollen state and the collapsed state
It can be observed that the lower critical solution temperature (LCST) lies between 30 and 35 ◦ C, and the exact temperature is a function of the detailed microstructure of the macromolecule [5]

Summary

Introduction

Intelligent hydrogels have attracted considerable attention because of their abilities to change their volumes and properties in response to external stimuli (temperature, pH, light, ionic strength, electric fields, magnetic fields) [1,2,3,4]. (NIPA) is one of the widely studied hydrogels due to its excellent temperature sensitivity. It undergoes an abrupt volume phase transition at a lower critical solution temperature (LCST). May interact with water molecules through hydrogen bonding below the LCST, which leads to water absorption by the hydrogel. The hydrophobic interactions among the hydrophobic moieties (–CH(CH3 )2 ) grow to be strong, which induces the freeing of the entrapped water molecules from the network.

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Conclusion