Radiation Synthesis And Characterization Of Temperature-and PH-sensitive Copolymer/clay Hydrogels

Intelligent hydrogels are three-dimensional hydrophilic polymer network or IPN that exhibit volume or phase transition in response to external environmental changes, such as temperature, pH, ionic strength, pressure, electronic and magnetic field. They have attracted considerable attention in recent research work for their potential applications in biomedical and biotechnological field, including controlled drug release, sensor, tissue engineering, and separation of cells, etc.. Most of the hydrogels studied previously are responsive to only one kind of stimulus. However, temperature and pH are two important issues of physiology, biologic and chemistry system, the intelligent hydrogel which can response to temperature and pH at the same time were most intensively studied.However, some of their potential applications are hindered by their low mechanical strength, low swelling ratio, bad biocompatibility and low purity because of catalysts or additives in chemosynthesis. Liang synthesized PNIPA/clay composite hydrogel to improve its mechanical strength. However biocompatibi-lity and purity weren't improved. In recent years, considerable researches have been made on the mechanical strength of interpenetrating polymer networks hydrogels, however , the equilibrium swelling ratio and purity of most of IPN hydrogels are bad. So it is a important to study temperature- and pH-sensitive hydrogels with outstanding properties.In this paper, We choose N-isopropylacrylamide(NIPA) as temperature-sensitive monomer,acrylic acid (AA) as pH-sensitive monomer,N-vinyl-2-pyrrolidone(NVP) as biocompatibility comonomer, and organic clay modifided by Hexadecyl trimethyl ammonium bromide(HTMAB) as modifying agent and prepared a series of temperature- and pH-sensitive hydrogels or interpenetrating polymer networks(IPN) hydrogels based on ⁶⁰Co-γ-ray irradiation technique. In this paper, more attention was paid to three respects------equilibrium swelling ratio,stimuli-response velocity and mechanical strength to improve intelligent properties of hydrogels for their potential applications. Firstly, hydrogels which were responsive to only one kind of stimulus (temperature or pH value) were synthesized. Secendly, the cohydrogel and IPN hydrogels which can response to temperature and pH at the same time were prepared byγ-ray radiation. The influence of irradiation dose,irradiation dose rate,monomer ratio contents of clay and NVP on the swelling ratio (SR) and compressive properties of the hydrogels were investigated in detail.The primary results of swelling properties,deswelling properties and compressive properties texts are as followed.In this work, the four system (including 28 series) of temperature-sensitive,pH-sensitive,temperature- and pH-sensitive copolymer,temperature- and pH-sensitive IPN hydrogels were synthesized by radiation.1. Temperature-sensitive hydrogelsThe optimized radiation synthesis condition of hydrogels were that: radiation dose was 6KGy and with 97.51 Gy/min radiation dose rate at the room temperature.The layer distance between Na-clays is changed from 1.1 to 2.6 nm because Na-clay pieces can be intercalated or exfoliated by HTMAB, and that between P(NIPA-co-NVP)/Clay pieces is 3.6nm.The LCST of P(NIPA-co-NVP) hydrogel is higher than PNIPA when added NVP in sysrem. And with the increase of NVP content, LCST became higher, as the ratio of NIPA/NVP is 90/10, hydrogel shows the best swelling property and LCST is 37°C.The compressive properties the composite hydrogels with clay is better than the conventional P(NIPA-co-NVP) hydrogels'. In addition, with the increase of clay content, the maximum compression force, compression strength of hydrogel composites increase at the same condition. When the content of clay is 15%, the compression strength of P(NIPA-co-NVP)/clay hydrogel is 3 times of P(NIPA-co-NVP) hydrogel's, and the maximum compression force is 17 times.2. pH-sensitive hydrogelsThe optimized radiation synthesis condition of hydrogels were that: radiation dose was 14KGy and with 97.51 Gy/min radiation dose rate at the room temperature.The compressive properties the composite hydrogels with clay is better than the conventional P(AA-co-NVP) hydrogels'. When the content of clay is 15%, the maximum compression force of the P(AA-co-NVP)/clay hydrogel is 62.87N which is 14 times of P(AA-co-NVP) hydrogel; and compression strength is 23 times. Moreover, in the whole compressive process, when the content of clay changes from 5% to 15%, the hydrogels were not damaged, and it was found that the hydrogel can recover to about 70% of its original length at room temperature after the compressive process. The result indicates that the hydrogels are hardly damaged by compressive deformation, similar to rubber.3. Temperature- and pH-sensitive copolymer hydrogelsIn acidic solution, the swelling rate of P(NIPA-co-AA) hydrogel was slow. The time was 5.5h when the ratio of imbibling water was 80% of the total amount of imbibling water, while it was just 3h in alkaline and weak alkaline solutionThe swelling properties and compressive properties of P(NIPA-co-AA)/clay hydrogel were better than P(NIPA-co-AA) hydrogels'. When the content of clay is 15%, the compression strength of P(NIPA-co-AA)/clay hydrogel is 2.4 times of P(NIPA-co-AA) hydrogel's, and the maximum compression force is 2.1 times.4. Temperature- and pH-sensitive IPN hydrogelsThe surface of P(NIPA-co-AA) copolymer hydrogel was smooth ,compact and no phase separation. The surface and cross-section of P(NIPA- AA)IPN hydrogel were loose and macroporous structure. The surface of P(NIPA- AA) IPN hydrogel treated with pH=9.8 buffer solution was discrete and with visible phase separation because of the destruction of hydrogen bond which certified the formation of IPN structure.In alkaline and weak alkaline solution, the rate of swelling of PNIPA-AA IPN hydrogels was controlled by relaxation movement of polymer chain, while in acidic solution, the rate was controlled by diffusion of small molecule.The compressive properties of N-10%/A-x% IPN hydrogels were rising with AA concentration increasing. When AA concentration is 3%, the compressive properties of N-10%/A-3% IPN hydrogel is best, its compression strength is 3.48MPa, which is 34.8 times of N-10%/A-0.5% hydrogel's, and compressive yield stress is 83 times.