Regulation Study Of Structure And Properties Of Photopolymeric Hydrogels And Its Applications

Hydrogels are widely used in biomedicine,energy storage and conversion,optical devices,pollution control and other fields because of their rich water content,porosity and adjustable mechanical properties.Among many hydrogel gelling methods,photocuring is often used to construct the second or third heavy polymer network in hydrogels,form chemical crosslinking points between linear polymers,or"fix"the special structure of anisotropy,because of its advantages of fast gelling speed,low energy consumption,time and space control.It is a more widely used polymer three-dimensional network forming method.Among them,when hydrogels are prepared by free radical photopolymerization method,the selection of raw materials is more abundant and the range of performance adjustment is wider.There have been a lot of studies and applications of this method to prepare hydrogels with special functions such as strong adhesion,multi-stimulus response,high mechanical properties,high conductivity and drivability.In the past studies,researchers paid more attention to the design of special functional groups on polymer molecular chains,inorganic material enhancement,and directional arrangement structure induced by external field,but ignored the effect of free radical photopolymerization on the structure and properties of hydrogels.According to the kinetic theory of free radical polymerization in polymer chemistry,polymerization parameters have an important effect on the average molecular chain length of polymers.In polymer physics,it is proposed that the length of polymer chain segments and the interaction force between molecular chains have a decisive effect on the macroscopic mechanical properties of polymers.Therefore,as a kind of"three-dimensional polymer network aqueous solution",the microstructure and macroscopic properties of hydrogel will also be affected by the free radical photopolymerization process.Inspired by the basic theory of free radical photopolymerization kinetics,this paper uses free radical photopolymerization reaction parameters to regulate the structure and properties of hydrogels.The regulation principle is explored from the perspectives of polymer molecular segment length,interchain force of polymer molecules,force between polymer and water molecules,etc.,and the hydrogels are customized according to the principle and performance requirements in practical applications.The specific work is as follows:（1）Light intensity（I₀）,polymerization temperature（T）,concentration of photoinitiator（PI）and concentration of monomer（M）were selected as the regulatory factors.The supramolecular PNAG（poly-N-acryloyl 2-glycine）hydrogel without chemical crosslinking was selected as the research object.One step free radical photopolymerization was carried out by the mold method,and an ultra-wide control range of hydrogel properties was obtained with tensile strength ranging from 0.14 MPa to 2.58 MPa,Young’s modulus ranging from 15 to 519 k Pa and water absorption capacity ranging from 33 to 2.8*10³ g/g.（2）Explore the similarities and differences of the regulatory mechanisms of the above four regulatory factors on the structure and properties of hydrogels:Experimental results confirm that the four regulatory factors have the ability to regulate the fracture tensile strength of hydrogels,but only the polymerization temperature（T）and monomer concentration（M）have the ability to regulate the Young’s modulus of hydrogels.According to the basic theories of polymer chemistry and polymer physics,the fracture tensile strength of hydrogel is mainly affected by the number and length of polymer molecular chains,while Young’s modulus is mainly affected by the forces between polymer molecular chains and between polymer and water molecules.It is concluded that the four regulatory factors can affect the tensile strength of hydrogels by adjusting the length of polymer molecular segments.The polymerization temperature（T）and monomer concentration（M）can regulate the intermolecular forces to affect the Young’s modulus of hydrogels,while the light intensity（I₀）and the concentration of photoinitiator（PI）have no effect on the intermolecular forces.（3）Explore the mechanism of polymerization temperature（T）showing special regulatory ability:It is confirmed that the photodegradation rate and yield of photoinitiator 1173 under UV-LED（365nm）have a strong dependence on temperature,so that the lower the temperature,the lower the concentration of free radicals generated in the polymerization system in unit time,so that a longer polymer molecular chain can be obtained at low temperature;It was confirmed that the hydrogen bond interaction between molecular chains of PNAG in hydrogel had the same variation trend as that between small molecules of NAG in precursor solution.Conclusions were drawn:In the polymerization process,the interaction between small and medium molecules in the precursor solution can be retained in the three-dimensional polymer network of the hydrogel to a certain extent,and become the force between the polymer molecular chains and the polymer and water molecules,so that the polymerization temperature（T）,which affects the interaction between small and medium molecules in the precursor solution,has the ability to regulate the Young’s modulus of the hydrogel obtained by polymerization.（4）Study the universality of the special regulation mechanism of polymerization temperature（T）:The influence of polymerization temperature（T）on the structure and properties of polymeric products was investigated in aqueous solution polymerization system,organic solution polymerization system and bulk polymerization system,using common photopolymeric monomers such as acrylic acid（AA）,acrylamide（AM）,hydroxyethyl acrylate（HEA）and hydroxyethyl propyl acrylate（HPA）as research objects.The suitability of its regulatory mechanism in the three polymerization systems was verified.The experimental results confirm that the two regulatory mechanisms of polymerization temperature（T）on the structure and properties of hydrogels mentioned in work（3）are applicable to the above three polymerization systems.（5）Based on the work of the first four parts and taking the performance requirements of wound dressing in the field of biomedicine as an example,the photopolymeric hydrogel was personalized and made into a PNAG-Gly hydrogel sheet with a Young’s modulus of 20 k Pa,skin tissue adhesion strength of 100 k Pa,good biocompatibility,antibacterial activity,antioxidant activity and excellent self-healing performance.In addition,the excellent ionic conductivity provided by amphiphilic small molecules in hydrogels endows them with strain sensing properties,enabling them to monitor human movement signals on the basis of application in wound dressings.The toughest PNAG hydrogel with tensile strength of 2.58 MPa can be stretched to 14 times the original length and be bent,twisted and buckled arbitrarily,which also shows very low cyclic hysterescence under low deformation.