Preparation And Properties Of Microgel Composite Hydrogels With High Mechanical Strength

Hydrogels are three-dimensional hydrophilic polymer networks used in many areas, such as soil amelioration, medicine, hygiene, biomedical applications and so on. For practical utilities, most hydrogels must be able to operate against an external load, and thus hydrogels with high mechanical strength are required. However, the industrial and biomedical applications of ordinary hydrogels made from synthetic sources are strongly limited by their poor mechanical properties. In contrast, many biological gels composites, such as cartilage, are quite strong. The development of synthetic methods encompassing traditional chemistry to molecular biology has been used in the design of hydrogels. Great deals of efforts have been made to change the inner microstructure of the hydrogels to improve their mechanical strength in recent years. In this paper, the reactive microgels were used as a multifunctional crosslinker instead of conventional crosslinkers to prepare microgel composite hydrogels with high mechanical strength. At the same time, the multilayer hydrogels and the anisotropic hydrogels were prepared by changing the polymerization methods and the other conditions. Higher mechanical strength hydrogels were also prepared using microgel composite combine interpenetrating polymer network methods. The main contents of this paper are as follows.(1) A novel post-crosslinking method by heating the microgel composite polymers with physical dispersed reactive microgels to prepare microgel composite (MC) hydrogels. The MC hydrogels were crosslinked by reactive microgels instead of traditional crosslinkers. The reactive microgels containing hydroxymethyl groups were prepared by inverse emulsion polymerization. The formed MC hydrogels had high mechanical properties. For MC hydrogels prepared by direct heating the microgel composite polymers with 75% water content, their properties were influenced by the heating conditions. When the heating conditions were 90℃at 4 h, the MC hydrogel of 90% water content had a tensile strength of 32 kPa and a high elongation of 960%. In addition, for MC hydrogels prepared by heating the partly evaporated microgel composite polymers, their properties can be adjusted by the composite polymers water content. When the microgel composite polymer with 50% water content was heated at 90℃for 3 h, the MC hydrogel had high tensile strength of 130 kPa and high tensile elongation of 503%.(2) Hydrophilic reactive microgels composite (HRM) hydrogels based on acrylamide (AM) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) were prepared using HRM as a new multifunctional crosslinker. HRM containing C=C double bonds were obtained by chemically modifying hydrophilic microgels (HM) of AM and AMPS in inverse emulsion for the first time. The HM and HRM was characterized by dynamic light scattering measurements, SEM, TEM and FTIR, respectively. The resulting HRM hydrogels had high compression strength, elasticity, and elongation under high water content. When the HRM hydrogel content 2.4 wt.% of HRM,20 wt.% of AM and 0.5 wt.% of AMPS, and water content was 93.40%, it can reach 4.60 MPa at the strain 90% and recovered, its elongations at break was 323% at high strength of 255 kPa.(3) Series of HRM hydrogels with high mechanical strength were synthesized. Chemical modifying conditions affecting the HRM double-bound content were also investigated. The maximum double-bond content was 1.82% at the optimum conditions of 70 g HM inverse emulsion, NMA 8 g,6 mol/L hydrochloric acid 0.8 ml, reaction temperature 60℃and reaction time 4 h. The mechanical properties of the HRM hydrogels could be significantly enhanced using HRM as a crosslinker. The optimum conditions for high compressive strength is HRM double bond content of 1.82%, HRM concentration of 2.5 wt.%, AM concentrations of 10 wt.%, AMPS concentrations of 2.5 wt.% and redox initiator of ammonium persulfate 1.5 mg and N,N,N’,N’-tetramethyldiamine 20μl. The HRM hydrogels with 90% water content can reach 8.08 MPa at the strain 85% and could recover after test. At the similar experimental conditions, the chemical crosslinked ordinary hydrogels is only 0.38 MP a. The soft or hard characteristic of HRM hydrogels can easily be changed mainly by altering HRM double-bond content, HRM concentration or AM concentration. Water-swelling kinetics of the HRM hydrogels was slower than the orindry hydrogels. However, water-deswelling kinetics of the HRM hydrogels was faster than the orindry hydrogels. This behavior is due to the unique microstructure of HRM hydrogels. This chapter is an important complement and gives valuable hints for the functionalization of the HRM hydrogels.(4) Multilayer HRM hydrogels with high mechanical strength were prepared by frontal photopolymerization of AM and AMPS using HRM as a multifunctional crosslinker. It was found that the resulting multilayer HRM hydrogels showed high fracture strength and high tensile elongation along parallel direction. While their fracture strength and tensile elongation along perpendicular direction was very weak. The elongations for the multilayer HRM hydrogels along perpendicular direction at break are dependent on HRM content and change from 1137% to 1420% at high strength of 24.63-21.68 kPa. The swollen multilayer HRM hydrogels were about 1.0-2.0 mm in thickness, the equilibrium swelling ratio was only 24.11-29.45. The multilayer HRM hydrogels were characterized by DSC, TEM and XRD, respectively. This might be a new approach to yield soft and wet multilayer HRM hydrogels with high mechanical strength and may provide some practical applications such as artificial skin.(5) Anisotropic HRM hydrogels based on AM were prepared by frontal photopolymerization using HRM as a crosslinker. Only one direction of the hydrogels showed excellent fracture strength and tensile elongation, but the other two directions showed little fracture strength and tensile elongation. The equilibrium swelling ratio in distilled water was only between 12.23 and 17.23. Elongations at break for the anisotropic hydrogels were from 611% to 872% at high strength of 74.45-47.86 kPa. The reason of the high mechanical performance in the given direction is that the hydrogels are crosslinked by HRM, these nanoparticles can self-assemble into anisotropic structures and the structure can be stabilized by free radical polymerization. The anisotropic HRM hydrogels could use potential materials such as artificial muscles tissue.(6) Microgels composite combine interpenetrating polymer network (MCI) hydrogels with higher mechanical strength had been synthesized by two-step solution polymerization. Firstly, HRM hydrogel (the first network) was made of P(AM/AMPS) with HRM as a crosslinker, while the second network was made of PAM by interpenetrating polymer network with no crosslinker. When 128 wt.% of AM as a mass percentage with respect to the AM and AMPS weight in the HRM hydrogel was used in the second network, the sample with 90% water content could reach 8.20 MPa at the strain 53.38% and recovereded after test. Water-swelling showed that the MCI hydrogels was slower than the HRM hydrogels, but water-deswelling kinetics showed that the MCI hydrogels was similar to the HRM hydrogels. The equilibrium swelling ratio of the MCI hydrogels in distilled water was only between 8.87 and 15.20. The MCI hydrogels are potential materials such as artificial cartiage tissue.