Composite aqueous microgels: an overview of recent advances in synthesis,characterization and application

In the last decade there has been a steady increase in the number of publications describing the synthesis, characterization and applications of hybrid particles. Colloidal aqueous microgels form an important subdivision of polymer colloids used for the deposition of different functional materials. Due to their numerous attractive properties, microgel particles have been used as templates for the synthesis, storage and transportation of nanostructured materials. This paper reviews the important developments in the area of composite microgels over the last decade, and is limited to microgel particles operating in aqueous media. The patent literature is not included in the review. Different synthetic approaches as well as some properties and applications of composite microgels are discussed. Copyright © 2006 Society of Chemical Industry     

Synthesis and solution properties of single‐chain microgels from poly(N‐isopropylacrylamide) copolymer

“Single‐chain” microgels were synthesized successfully from the cross‐linkable poly(N‐isopropylacrylamide) (PNIPAM) copolymer. This type of microgel has the exact chemical structure, molecular weight and molecular weight distribution of its precursor. It provides a direct way to compare the properties of linear polymers with those of their networks. The viscosity properties show that the microgels have lower critical solution temperatures (LCST) that are even higher than those of the corresponding linear copolymers. This can be attributed to the crosslinking points, which retard the change of the conformation of the network chains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2179–2183, 2003     

Fast, multi-responsive microgels based on photo-crosslinkable poly(2-(dimethylamino)ethyl methacrylate)

Multi-responsive microgels based on poly(2-(N,N-dimethylamino)ethyl methacrylate) were developed and their properties were investigated. The primary goal of this research was to speed up the stimulus-response time of the hydrogels to a level usable for actuator applications, by reducing the diffusion distance of water. The gels were prepared by a UV induced photodimerization of a copolymer of 2-(dimethylamino)ethyl methacrylate and 4-cinnamoyl-phenyl methacrylate. Patterning studies showed that these materials can be used as photo-resist materials with high resolution at short exposure times. They showed lower critical solution temperature behavior in water, as well as pH dependent solubility and swelling ratios. While 1 mm thick gels showed response times to temperature and pH-changes of several hours, Si-supported microgels of 300 nm thickness had response times in the range of only a few seconds. The copolymer was prepared by free radical copolymerization, and the reactivity ratios were determined with the extended Kelen Tudos method. Spin-coating of this copolymer on Si supports and subsequent UV-irradiation yielded microgels of variable thickness (200 nm-15 μm), which was determined by confocal scanning laser microscopy. Surface plasmon resonance spectroscopy measurements demonstrated the fast, stimuli-responsive swelling behavior, while differential scanning calorimetry gave insight into the morphology of the networks.     

Flower-Like SiO2-Coated Polymer/Fe3O4 Composite Microspheres of Super-Paramagnetic Properties: Preparation via A Polymeric Microgel Template Method

N -isopropylacrylamide (NIPAM) and acrylic acid (AA) copolymer microgels P(NIPAM - co -AA) with different amount of AA were prepared by using a reverse suspension polymerization technique. The polymeric microgels were used firstly as micro-containers to include Fe₃O₄ nanoparticles and then as micro-reactors to control the hydrolysis of tetraethyl orthosilicate (TEOS). In this way, various super-paramagnetic composite microspheres (P(NIPAM-AA)/Fe₃O₄/SiO₂) with different morphologies were prepared. It was demonstrated that the morphologies of the composite microspheres could be tailored to a certain extent by either varying the ratio of the two monomer units in the template microgels or the amount of SiO₂ deposited. The sensitivity of the composite microspheres to an external magnetic field was determined by the amount of Fe₃O₄ included. The magnetic responsibility of the composite microspheres, and the ease of modifying the surfaces may make the microspheres of important use in the mild separation of bioactive materials, loading of active materials, and radiation and shock absorption, etc.     

聚N-异丙基丙烯酰胺微凝胶的合成和应用进展

介绍了聚N-异丙基丙烯酰胺(PNIPAM)微凝胶的合成方法和环境响应性能,重点论述了PNIPAM微凝胶作为先进功能材料的最新进展。作为"智能"材料,PNIPAM微凝胶在不同领域具有广泛的应用,论述了PNIPAM微凝胶在作为微反应器模板合成纳米粒子、光学材料以及药物的定向释放和控制释放载体中的应用。     

分散聚合法合成聚(N-异丙基丙烯酰胺)温敏性微凝胶

以PVP为稳定剂,在水或醇/水介质中通过分散聚合法合成出聚（N-异丙基丙烯酰胺）（PNIPAM）温敏性微凝胶。研究结果表明,可通过改变稳定剂用量和分散介质极性来控制PNIPAM微凝胶的大小,这两种因素对PNIPAM微凝胶的溶胀比有明显的影响,而对其相转变行为影响不大;交联剂用量对PNIPAM微凝胶的粒径、溶胀比和相转变行为都有比较明显的影响。PNIPAM微凝胶可能主要通过接枝共聚物聚结机理成核,通过初级粒子之间的聚并完成粒子增长过程。     

Synthesis and characterization of reactive copolymeric microgels

Hydrogels whose diameters range in the nanosize scale were synthesized by precipitation copolymerization of 4‐nitrophenol acrylate (NPA) with methacrylamide (MeAM) and N‐isopropylacrylamide (NIPAM). The polymerization reaction process was followed by attenuated total reflectance Fourier‐transform infrared spectrometry (ATR‐FTIR) and the conversion was studied by UV‐vis spectrometry. Poly(NPA‐co‐MeAM) and poly(NPA‐co‐NIPAM) microgels were characterized by proton nuclear magnetic resonance (¹H‐NMR) spectrometry. The thermal properties were studied by both thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. Transmission electron microscopy (TEM) of these microgels showed that the copolymer is located in spherical particles with a moderate polydispersity. The average particle diameter and the particle size distribution were measured in acetone by quasi‐elastic light scattering (QELS), which indicated a mean diameter close to 50 nm. Copyright © 2005 Society of Chemical Industry     

Hollow poly(N‐isopropylacrylamide)‐co‐poly(acrylic acid) microgels with high loading capacity for drugs

A convenient approach has been developed for the preparation of microsize hydrogels composed of crosslinked poly(acrylic acid) (PAA) and poly(N‐isopropylacrylamide) (PNIPAm). First, semi‐interpenetration polymer networks of hydropropylcellulose (HPC) and PNIPAm‐co‐PAA copolymer are formed through the copolymerization and crosslinking of monomer acrylic acid and N‐isopropylacrylamide in HPC aqueous solution. After the selective removal of HPC from networks due to ionization of PAA units and disruption of hydrogen bonding with increasing pH, PNIPAm‐co‐PAA microgels are obtained, whose volume is confirmed to be responsive to both temperature and pH. Doxorubicin hydrochloride (Dox) can be encapsulated in PNIPAm‐co‐PAA microgels with high drug loading driven by the electrostatic interaction, and a sustained‐release characteristic of Dox from the microgels is observed under physiological pH value and temperature. In vitro cell experiments, the drug‐loaded microgels can be taken up by LoVo cells and release their payload in cell cytoplasm without loss of drug efficacy. This indicates that PNIPAm‐co‐PAA microgels might be a potential drug delivery carriers especially for water‐soluble or polypeptide drugs. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The electrostatic self‐assembly of microgels on polymer brushes and its effects on interfacial friction

In this article, a series of monodisperse poly(N‐isopropylacrylamide‐co‐acrylic acid) [P(NIPAm‐AA)] microgels were prepared with different content of acrylic acid (AA) by surfactant‐free emulsion polymerization, and their electrostatic self‐assemble and tribological behavior on polymer brushes were investigated. The ζ‐potential of microgels became more negative with the increase content of AA, which means a stronger hydration capability. For cationic poly[2‐(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes, negative P(NIPAm‐AA) microgels adsorbed on the surfaces of brushes as a result of the electrostatic interaction, and more AA content means stronger absorption ability. However, compared to the polymer brushes, P(NIPAm‐AA)_2:1 and P(NIPAm‐AA)_5:1 microgels possessed the weaker hydration capability, which led to a concomitant increase in friction of interface. In terms of P(NIPAm‐AA)_10:1 microgels, due to the weak adsorption, they could be sheared off easily, leading to the PMETAC brushes swell again, and thus, a lower friction of interface was obtained. Moreover, the tribological behavior of microgels was significantly affected by the pH, especially the P(NIPAm‐AA)_2:1 microgels exhibited good lubrication property in high pH solution due to high hydration of deprotonated carboxylic acid groups. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44215.     

反应性微凝胶分散液的流变性能

为了在实际应用中更好地利用反应性微凝胶分散液的流变特性,采用自稳定分散聚合的方法合成了带有环氧基或羧基的反应性微凝胶,并研究了其在不同溶剂中的流变性能。结果表明,与相应的线形聚合物相比,当相对分子质量一定时,反应性微凝胶分散液的特性黏度较低,对聚合物相对分子质量依赖性较小;随着反应性微凝胶交联度的增加,特性黏度变小。在二甲苯中,随着反应性微凝胶质量分数的增加,分散液逐渐由牛顿流体转变为非牛顿流体,且随着反应性微凝胶质量分数的增加,假塑性增加。在甲苯、苯乙烯(St)或甲基丙烯酸异冰片酯(IBOMA)稀溶液中,反应性微凝胶的官能团种类对特性黏度的影响较弱,官能团种类对高浓度溶液的流变性能有较大的影响。     

Electrophoresis of functionalized microgels: morphological insights

The electrophoretic mobility of thermosensitive poly(N-isopropylacrylamide) (PNIPAM) microgels, functionalized to contain the same bulk carboxylic acid content using acrylic acid (AA), methacrylic acid (MAA), vinylacetic acid (VAA) and acrylamide hydrolyzed in situ (H-AM), is fit to the soft particle mobility model developed by Ohshima. Good model fits were achieved for experimental mobility data as a function of both ionic strength and temperature. The model deconvolutes the underlying contributions to soft particle mobility, identifying large morphology differences between microgels with similar absolute mobility values (i.e. AA-NIPAM and VAA-NIPAM). The differences in the predicted morphologies can be correlated to the physical properties of the carboxylic acid-containing comonomers and independent light scattering data. Theoretical softness models significantly overestimate the best-fit softness of the microgels but give reasonable estimates of how the softness changes as a function of temperature. Based on simultaneous comparisons of the best-fit softness, volumetric charge density, mobility, and particle size profiles, functionalized microgels appear to deswell primarily through a consecutive core-shell mechanism.     

Review on vinyl acetic acid-based polymer microgels for biomedical and other applications

Vinyl acetic acid comonomers-based microgels exhibit exceptional behavior and morphological properties as compared to other comonomer polymerized microgels. High hydrophobic character of vinyl acetic acid induces core-shell type structure in poly(N-isopropylacrylamide-vinyl acetic acid) microgels. This review article describes the preparation methodologies and various properties of poly(N-isopropylacrylamide-vinyl acetic acid)-based microgels. Applications of poly(N-isopropylacrylamide-vinyl acetic acid) microgels in the field of nanotechnology, drug loading and delivery vehicle, morphology-tuned applications, optical etalon formation and catalysts have been discussed in detail. Future aspects of these specific types of polymer microgels have also been elaborated in detail.     

Preparation and thermal response behavior of poly(N‐isopropylacrylamide‐co‐a crylic acid) microgels via soap‐free emulsion polymerization based on AIBN initiator

Poly(N‐isopropylacrylamide‐co‐acrylic acid) (poly(NIPAM‐co‐AA)) microgels with different copolymer compositions were prepared through soap‐free emulsion polymerization at 80°C, and 2, 2′‐azobisisobutyronitrile (AIBN) was used as initiator. Scanning electron microscope (SEM) characterization shows that the prepared microgels are regular and smooth and not easy to distort. Result of ¹H‐NMR characterization shows that with increasing of the initial concentration of AA (AA in feed), the AA content in polymer chains increases. The thermal response of microgels latex was investigated by UV‐3010 spectrophometer through detecting the transmittance of the latex at different temperature in the range of 190–900 nm. The thermal response of the poly(NIPAM‐co‐AA) microgels was tested by dynamic light scattering (DLS). The results show that with the increase of AA content in polymer chains, the low critical solution temperature (LCST) of microgels latex first decreases and then increases. Still, with increasing of AA in poly(NIPAM‐co‐AA) microgels, the LCST of microgels first increases and then decreases. The basic reasons causing the changes of LCST of microgels latex and microgels are interpreted clearly in this article from the perspective of hydrogen bonding interaction. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Flow and film properties of coatings containing microgels

Microgels have been prepared via an emulsion polymerization process involving the use of the amphiphilic oligosoap, ZEO. The particle sizes of these microgels were < 100 nm and particle properties such as crosslink density, copolymer composition and functionality could be varied over a wide range. Their versatility and usefulness as coating modifiers has been demonstrated.

Dispersions of the microgels are stable in both aqueous and organic media. Furthermore, solid powdered microgels can be produced by drying emulsions. These powdered microgels are generally added to photoreactive coatings or can be redispersed in the above media.

Such microgels may be applied in various other fields besides coatings because of the diversity of their form and properties.     

Preparation and characterization of thermosensitive organic-inorganic hybrid microgels with functional Fe3O4 nanoparticles as crosslinker

A methodology is described for the preparation of thermosensitive organic-inorganic hybrid microgels with functional Fe₃O₄ nanoparticles as the crosslinker and N-isopropylacrylamide (NIPAm) as the monomer. Magnetic Fe₃O₄ nanoparticles were first prepared via a redox reaction in aqueous solution and then modified with 3-(trimethoxysilyl)propylmethacrylate (TMSPMA) via the silanization. The bonding of multiple TMSPMA monomers on the surface of Fe₃O₄ nanoparticles renders them as crosslinker. Surfactant-free emulsion polymerization (SFEP) of NIPAm was then carried out with the presence of TMSPMA-modified Fe₃O₄ nanoparticles at 70 °C in aqueous solution, leading to the formation of thermosensitive PNIPAm-Fe₃O₄ hybrid microgels crosslinked with Fe₃O₄ nanoparticles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), dynamic light scattering (DLS) and physical properties measurement system (PPMS) were then used to characterize the resultant hybrid microgels. The experimental results show that the PNIPAm-Fe₃O₄ hybrid microgels were spherical in shape with a large size distribution and the Fe₃O₄ nanoparticles were randomly distributed inside the microgels. The PNIPAm-Fe₃O₄ hybrid microgels were thermosensitive, exhibiting a reversible swelling and deswelling behavior as a function of temperature. The PNIPAm-Fe₃O₄ hybrid microgels also show superparamagnetic behavior at room temperature (300 K).     

Synthesis and characterization of novel,temperature‐sensitive microgels based on N‐isopropylacrylamide and tert‐butyl acrylate

A series of temperature‐sensitive microgels based on N‐isopropylacrylamide as the main monomer, tert‐butyl acrylate (tBA) as the comonomer, and N,N′‐methylene‐bis(acrylamide) as the crosslinker were synthesized with a modified surfactant‐free emulsion polymerization method. The chemical structure and global shape with an excellent monodispersity of the microgels were confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The temperature‐sensitive behavior of the microgels was investigated by dynamic light scattering and ultraviolet–visible spectrophotometric analysis. The results show that the volume phase‐transition temperature of the poly(N‐isopropylacrylamide‐co‐tert‐butyl acrylate) [poly(NIPAM‐co‐tBA)] microgels were tuned over a broad range by the incorporated amount of tBA comonomer and their temperature sensitivity decreased with increasing content of tBA units incorporated into the microgel network. Furthermore, the swelling ratios of the poly (NIPAM‐co‐tBA) microgels were lowered gradually with increasing tBA unit content within the microgel network. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2962–2967, 2007     

Effects of end groups on the thermal response of poly(N‐isopropylacrylamide) microgels

Poly(N‐isopropylacrylamide) (PNIPAM) microgels were prepared through soap‐free emulsion polymerization using 2, 2′‐ azobisobutyronitrile and potassium persulfate as initiator respectively. The thermal response of microgels was researched by measuring the transmittance and the hydrodynamic diameter of the microgels at different temperatures. The result shows that the different structure of the end groups of polymer that come from residues of initiator result in the different thermal response of PNIPAM microgels. The LCST (lower critical solution temperature) of AIBN‐initiator microgels is 5°C lower than that of the KPS‐initiator microgels, whereas the AIBN‐initiated PNIPAM microgels have better thermal response sensitivity. The scanning electron microscope characterization shows that the morphology of AIBN‐initiated PNIPAM microgels is more regular than that of KPS‐initiated. Furthermore, the T_g of the microgels was measured by differential scanning calorimeter and the result indicates that the end groups influences the T_g of microgels severely. This work demonstrated that the hydrophobic end group coming from initiators can decreases the LCST of PNIPAM microgels and increases the thermal response sensitivity, which providing a newly simple but effective method to regulate the thermal response of PNIPAM microgels. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1164‐1171, 2013     

pH值对N-异丙基丙烯酰胺与甲基丙烯酸共聚形成微凝胶的影响

在pH值为4．0和7．0的水介质中分别采用乳液聚合法使N-异丙基丙烯酰胺（NIPAM）和甲基丙烯酸（MAA）共聚,合成了具有温度／pH双重刺激响应性的P（NIPAM—CO—MAA）共聚微凝胶。傅立叶红外变换光谱、元素分析和电位滴定分析结果表明,在pH为4．0条件下合成的微凝胶中MAA单元含量偏高,微凝胶的粒径随着MAA用量的增加而增大,而pH为7．0时合成的微凝胶的粒径随着单体MAA用量的增加而减小。通过动态激光光散射测试P（NIPAM—CO—MAA）微凝胶的动力学直径随介质温度或pH值的变化关系发现,pH为4．0条件下合成的微凝胶具有更明显的pH刺激响应性,其温度刺激响应性受介质pH值的影响较大,表明MAA单元有可能更均匀地分布于微凝胶聚合物网络中。     

Properties of pH‐dependent tertiary amine‐based gels as potential drug delivery matrices

The rheological and morphological properties and in vitro theophylline release of tertiary amine‐based microgels were evaluated. The testing of such a formulation through in vitro diffusion experiments revealed that the release of theophylline from the microgels was pH‐dependent and differs significantly with respect to a nonresponsive gel like scleroglucan (Scl). The microgels were obtained from 2‐(diethyl amino) ethylmethacrylate (DEA) in the presence of a bifunctional crosslinker at pH 8–9. As the resulting microgels are pH‐responsive and an increase in viscosity from high to low pH range is exhibited, the in vitro release of theophylline as model drug was studied at different pHs of both the matrix and the receptor medium. The release behaviors of PDEA‐based microgels were compared to nonresponsive natural gel Scl, studied previously. For microgels, diverse release patterns were found at different acidity conditions. This observation seems to be related to complex diffusion phenomena and the different gel structure obtained for samples prepared at dissimilar pH. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4035–4040, 2007     

Novel Cross-Linking Mechanism with Diol Type Cross-Linkers,to Prepare PAA Microgels via Precipitation Polymerization Method

This article describes a new cross-linking mechanism for synthesizing PAA microgels by precipitation polymerization method. The post curing stage was defined, for the first time, in this study. Effect of cross-linker concentration, and the time and temperature of post curing stage on the samples properties was investigated. A new relationship between gelation rate in the post curing stage and hydroxyl content was defined. Also a new relationship between M_c, hydroxyl concentration and temperature of post curing stage was suggested too. According to rheological characterization, microgels synthesized by novel mechanism have better rheological properties than microgels synthesized by conventional mechanism.