| Sensitive hydrogels, with excellent properties such as hydrophilicity, release, etc, are widely used in industrial and agricultural production, biological and medical fields, to become the research hotspot now. And the pH and temperature sensitive hydrogels are the most research, since pH and temperature are not only easy to control, but also two very important factors in physical, biological and chemical systems. Common hydrogels are limited in many fields by their slow responsive rate and weak mechanical performance, and improving the responsive rate is an important subject about the research and development of hydrogels. The preparation of macroporous hydrogels is the most familiar, and phase separation to prepare macroporous hydrogel is a very effective method especially for thermo-sensitive hydrogel, but which is rarely used in pH sensitive hydrogel.In this study, three different systems of sensitive macroporous hydrogels are successfully prepared by phase separation to improve their responsive rate.With different concentration of sodium chloride(NaCl) aqueous solutions as the polymerization medium, the typical pH sensitive polyacrylic acid (PAAc) macroporous hydrogels are firstly prepared, and their various properties are studied. On this basis, in order to expand the application of acrylic acid in hydrogel, pH sensitive poly(acrylic acid-co-acrylamide)[P(AAc-co-AM)] as well as pH and temperature dual sensitive poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPA-co-AAc)] macroporous hydrogels are prepared. The formation mechanism of these macroporous hydrogels is discussed, the chemical structures and pore morphologies of these hydrogels are characterized by means of Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy(SEM).The influence of different concentration of NaCl solutions on the properties of the resulting hydrogels are investigated by measuring the swelling behaviors at various pH, temperature and ionic strength, deswelling kinetics and reswelling kinetics, etc. Through systematic study, the results show:(1) With NaCl aqueous solutions as the polymerization medium, different concentrations do not affect the chemical compositions and structures of a system of the resulting hydrogels; (2) In the polymerization process of P(NIPA-co-AAc) hydrogels, phase separation occurs, and there is a significant morphology change, that is, transparent becomes white opaque, even after purification, the gel's morphology changes little; while for these PAAc and P(AAc-co-AM) hydrogels, this change does not appear; (3) In the polymerization process of hydrogels, the different NaCl concentrations lead to that the extent of phase separation has obvious differences; When the NaCl concentration is not less than 0.3mol/L, a clear phase separation occurs, and the hydrogel is synthesized with a porous network structure; As the NaCl concentration continues to increase, the number and size of pores grows in the resulting hydrogel; (4) Swelling studies show that an increase in equilibrium swelling ratio in distilled water with the NaCl concentration increasing in the polymerization medium.The hydrogels exhibit stronger sensitivity to pH, temperature and ionic strength(I), and show much faster reswelling and deswelling responsive rates in the more NaCl concentration; (5) For the P-(NIPA-co-AAc) hydrogels, the introduction of AAc can significantly enhance their phase transition temperature and broaden the range of their phase transition temperature; (6) During the swelling process of the P(NIPA-co-AAc) dry gels in distilled water, the swelling rate appears turning point, that is, firstly the gel slowly swells, and then swelling appears to accelerate; while for these PAAc and P(AAc-co-AM) hydrogels, this phenomenon does not appear; (7) Due to fast responsivity of macroporous hydrogels, it is expected that three sensitive macroporous hydrogels might have potential applications in biomedical fields for stimuli-responsive drug delivery systems. |
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