| Classic colloids can be divided into three categories:flexible macromolecules,micelles(self-assembled particles),and rigid colloids.Microgels are a special kind of colloids that do not belong to the above three.They are cross-linked networks of polymers with size between 100 nm and 100 μm that are fully swollen in solvents(mainly water).Strictly speaking,gel particles less than 100 nm should be called nanogels.However,in domestic and foreign works and literature,microgel also refers to gel particles of nano size.Microgels are unique microopen systems because of their deformability,permeability,and architectural versatility.They can be used as good carriers for controlled release of drugs.And they can become microreaction "factories",protecting the activity of enzymes and metal catalysts,and controlling the"on and off" of catalytic reactions.They can also become the environmental responder of sensing devices.Recently,bioinks based on microgels have been applied to 3D printing of soft tissue materials,demonstrating its potential as "building units" to construct complex advanced materials.Among a wide variety of microgels,thermoresponsive microgels composed of poly(Nisopropylacrylamide)(PNIPAm)has been studied the most widely.This is not only because temperature is an environmental stimulus which is relatively easy to control,but also because researchers can easily obtain well-defined microgel particles with narrow size distribution in laboratory conditions through precipitation polymerization or other synthetic methods.However,the simplicity also brings troubles.First,the selection of main components of thermoresponsive microgels is limited,including the selection of functional units.Second,the selection of crosslinking agents and crosslinking methods are limited,so microgels usually have a high crosslinking density core and a low crosslinking density shell.Third,thermal responsive microgels are generally dominated by spherical solid structures,which have isotropy in mechanical response,making it difficult to use in mechanical response environments requiring anisotropy.To solve these problems,researchers at home and abroad have developed a series of microgels with special structures in recent years.Different from the classical spherical solid microgels,the special structure microgels are more abundant in the changes of functional unit,crosslinking methods,and swelling properties.For example,core-shell microgels can have functional shells and responsive cores,which perform multistage discrete swelling properties.Hollow microgels can hybridize functional core and load drug molecules.These two types of microgels show unique advantages in the fields of catalysis and drug delivery.Janus microgel has anisotropy in mechanical changes and can be used as a special mechanical probe in sensing devices.In addition,a class of ultra-low crosslinked microgels with excellent deformability has been proved to be used as platelet mimics for rapid hemostasis.Although microgels with special structures have shown broad application prospects,except core-shell microgels with relatively mature preparation methods,there are few reports on preparation strategies of microgels with special structures.Taking hollow microgel as an example,the preparation process still needs to rely on template and complex preparation path,so its preparation method cannot be popularized.Bulilt the above background,the study of this paper takes the cononsolvency effect of thermoresponsive polymers as the entry point,and reports two strategies for preparing functional ultra-low cross-linked microgels and hollow microgels.Inspired by the above exploration,the cononsolvency effect of thermoresponsive microgels is further utilized to improve their adhesion ability on material surface.This trial lies a foundation for the application of microgels in the field of biomedical surface interface modification.The research work carried out is as follows:(1)We found that PNIPAm synthesized by free radical polymerization in the organic phase can form stable microgels in water by solvent exchange(dialysis)without chemical crosslinking agents.These microgels maintain good stability in water and have large swelling ratio and sharp size shrinkage above the critical temperature,which are obvious characteristics of"ultra-low crosslinking" microgel(ULC-microgel).The results of 1H NMR and FTIR showed that the self-crosslinking structure of these microgels was derived from the hydrogen abstracting behavior of free radicals on the isopropyl groups and vinyl groups.Different organic solvents determined the hydrogen abstraction ratio by free radicals.Our discovery revealed that the self-cross-linking of PNIPAm chains is a common phenomenon within their free-radical polymerization process in organic phase.The by-product is an unwanted impurity for the preparation of linear PNIPAm.From another perspective,however,this is a simple method to prepare ULC-microgels.We then used this strategy to prepare a variety of functional ULCmicrogels by random or block polymerization.Among them,ULC-poly(Nisopropylacrylamide-b-methacrylic acid)(P(NIPAm-b-MAA))microgel showed intuitive catalytic activity after internal reduction of silver nanoparticles.It benefited from the high packing density of metallic nanoparticles,the good permeability of ULC microgels,as well as the rapid recyclability.We believe this versatile and easily prepared nanosystem has a wide range of applications in catalysis,detection,and other environmental engineering fields.(2)In the process of preparation of ULC-microgel,we found that the cononsolvency effect of PNIPAm is commonly happened in the component solvent.The so-called cononsolvency effect means the organic solvent and water alone are good solvents for PNIPAm,but a certain proportion of their mixture is bad for it.In previous studies on cononsolvency effect,methanol was added into PNIPAm solution as an additive.We employed an experimental operation contrary to the mixed steps by dropping PNIPAm methanol solution into water instead of adding methanol into PNIPAm aqueous solution.It was found that PNIPAm could form a metastable liquid layer composed of microspheres at the methanol/water mixing interface.These microspheres could be preserved by chemical crosslinking and a hollow structure was demonstrated by electron microscopy and fluorescence microscopy.We proposed a new mechanism called "interfacial cononsolvency" to explain the formation of hollow microvesicles.This mechanism could guide a novel self-assembly strategy for a range of nonamphiphilic polymers.And it was demonstrated that the addition of a few mole ratio of functional monomers has no effect on the self-assembly of copolymers to form hollow microspheres.As an application case,a copolymer consisted of glucose monomer and NIPAm was synthesized in our work to fabricate "sweet" hollow microspheres,which could well mimick the cells with proteoglycan membrane.These artificial cells show a good affinity for concanavalin A and can selectively regulate the aggregation behavior of Escherichia coli and Staphylococcus aureus,which are potential functional models for studying intercellular interface interactions.(3)Due to the spatial repulsion and hydrophilicity of the microgels,it is difficult to form a solid microgel film on the material surface.Inspired by the hydrophilic/hydrophobic changes of PNIPAm in cononsolvents,we tried to improve the binding ability of PNIPAm microgel to the hydrophobic surface by using the hydrophobicity of microgels in cononsolvent pairs.The experimental results showed that PNIPAm microgel films can be successfully prepared on the surface of silicon wafers and hydrophobic medical polymer substrates.Besides,the microgel films can remain stable in the saline environment for over a week and does not separate from the substrate.The hydrophobic interaction between the microgel films and the substrate was preliminarily confirmed by the destruction test of guanidine hydrochloride solution.For comparison,we designed three other kinds of microgels with different components,including N,N-diethyl-2-acrylamide(DEAAm),N-vinyl caprolactam(NVCL),and 2-methyl-2-acrylate2-(2-methoxyethoxy)ethyl ester(DEGMA).Due to the differences in composition,the four microgel films are not only different in film forming performance,but also have significant effects on biological functions.In summary,in this paper,two novel preparation methods of microgels are developed according to the cononsolvency effect.They show potential to be used as microreactors or artificial cell models.The film formation properties of microgels regulated by cononsolvency effect were also evaluated,which further promoted the transformation of microgels from a single reactor to a functional module.These advanced micro/nano materials can further become the cornerstone of biological surface construction,and have great value in optical sensing,surface detection,drug delivery,biomedical materials modification,and other fields. |
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