含β-CD单元VCL共聚物微凝胶的合成与性能研究

采用无皂沉淀聚合法,在水溶液中通过N-乙烯基己内酰胺(VCL)与一种单取代乙烯基β-环糊精单体(GMA-EDA-β-CD)的共聚反应,合成出了含有β-CD结构单元的温敏性VCL/GMA-EDA-β-CD共聚物微凝胶.用红外光谱仪(FTIR)、1H核磁共振仪(1H-NMR)、透射电镜(TEM)及激光粒度仪(DLS)对其结构、形态和性能进行了表征.研究结果表明,β-CD结构单元的加入使共聚物微凝胶的粒径减小,粒径分布变窄,但其温敏性降低.     

Unresolved issues in the preparation and characterization of thermoresponsive microgels

The preparation and properties of colloidal thermal responsive microgels based on poly(N-isopropylacrylamide) are described with an emphasis on the unresolved issues which will impair commercial implementation. The main difficulties are: 1) removal of sol, surfactants and other impurities; 2) obtaining colloidally stable particles less than 50 nm in diameter; 3) synthesis of concentrated, colloidally stable microgel dispersions; 4) accurate measurement of water content; and, 5) control of microgel particle morphology.     

Copolymer microgels by precipitation polymerisation of N-vinylcaprolactam and N-isopropylacrylamides in aqueous medium

In this work, we designed copolymer microgels by the copolymerisation of N-vinylcaprolactam (VCL) and two acrylamides (N-isopropylacrylamide (NIPAAm) and N-isopropylmethacrylamide (NIPMAAm)) under precipitation conditions in aqueous phase. In synthesis protocols, the ratio between monomers was varied from 1:5 to 5:1 mol/mol. By NMR and Raman spectroscopy, we determined the chemical composition of PVCL/NIPAAm and PVCL/NIPMAAm copolymer microgels reflecting the initial monomer ratio in the reaction mixture. The hydrodynamic radii of PVCL/NIPAAm microgels are around 375 nm (at 25 °C) and do not vary with the copolymer composition. On the contrary, for PVCL/NIPMAAm microgels, the size decreases from 450 to 250 nm with an increase of the VCL amount in copolymer structure. The heterogeneity of the microgel structure in terms of the distribution of the monomer units was probed by ¹H transverse magnetization relaxation NMR, showing that the VCL, NIPAAm and NIPMAAm units are unorderly distributed in the colloidal networks. The investigation of volume phase transition temperature (VPTT) for copolymer microgels was performed using dynamic light scattering, NMR and differential scanning calorimetry. It has been found that PVCL/NIPAAm microgels show VPTT around 35 °C independently from the copolymer composition; however, PVCL/NIPMAAm particles exhibit a nonlinear increase of VPTT from 34 to 45 °C as the NIPMAAm fraction in copolymer structure increases.     

N‐vinylcaprolactam‐based microgels: Synthesis and characterization

Poly(N‐vinylcaprolactam) (PVCL) is well known for its thermoresponsive behavior in aqueous solutions. PVCL combines useful and important properties; it is biocompatible polymer with the phase transition in the region of physiological temperature (32–38 °C). This combination of properties allows consideration of PVCL as a material for design biomedical devices and use in drug delivery systems. In this work, PVCL based temperature‐sensitive crosslinked particles (microgels) were synthesized in a batch reactor to analyze the effect of the crosslinker, initiator, surfactant, temperature, and VCL concentration on polymerization process and final microgels characteristics. The mean particle diameters at different temperatures and the volume phase‐transition temperature of the final product were analyzed. To obtain information about the inner structure of microgel particles, semicontinuous polymerizations were carried out and the evolution of the hydrodynamic average particle diameters at different temperatures of the microgel synthesized was investigated. In the case of microgel particles obtained in a batch reactor the size and the swelling‐de‐swelling behavior as a function of the temperature of the medium can be tuned by modulating the reaction variables. When the microgel particles were synthesized in a semibatch reactor different swelling‐de‐swelling behaviors were observed depending on particles inner structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2510–2524, 2008     

Dynamic control of volume phase transitions of poly(N‐isopropylacrylamide) based microgels in water using hydrazide‐aldehyde chemistry

We demonstrate that the volume phase transition temperature (VPTT) of copolymer microgel particles made from N‐isopropylacrylamide (NIPAm) and methacryloyl hydrazide (MH) can be tailored in a reversible manner upon the reaction of the hydrazide functional groups with aldehydes. The microgels were synthesized by precipitation polymerization in water. Due to the water‐soluble nature of the MH monomer, the VPTT at which the microgel particles contract shifts to higher values by increasing the incorporated amounts of methacryloyl hydrazide from 0 to 5.0 mol %. The VPTT of the copolymer microgel dispersions in water can be fine‐tuned upon addition of hydrophobic/hydrophilic aldehydes, which react with the hydrazide moiety to produce the hydrazone analogue. This hydrazone formation is reversible, which allows for flexible, dynamic control of the thermo‐responsive behavior of the microgels. The ability to “switch” the VPTT was demonstrated by exposing hydrophilic streptomycin sulfate salt incubated microgel particles to an excess of a hydrophobic aldehyde, that is benzaldehyde. The temperature at which these microgels contracted in size upon heating was markedly lowered in these aldehyde exchange experiments. Transformation into benzaldehyde hydrazone derivatives led to assembly of the microgel particles into small colloidal clusters at elevated temperatures. This control of supracolloidal cluster formation was also demonstrated with polystyrene particles which had a hydrazide functionalised microgel shell. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1745–1754     

Functionalised Microgels for Acrylic Coatings

Summary: Aqueous acrylic dispersions of hydroxy-functionalised copolymer microgel particles crosslinked with allyl methacrylate were synthesized by emulsion polymerization. The microgels were investigated as reactive polymer fillers in mixtures with a water-borne film-forming dispersion. Properties of coatings cast from mixtures of aqueous dispersion of hard microgel particles and film-forming water-borne dispersion were investigated. The swelling behaviour of microgels in selected solvents (aliphatic ketones) as a function of microgel composition is discussed as well. It was found that the swelling ability of microgels decreased with growing degree of crosslinking. Microgels comprising copolymerised butyl methacrylate swelled less in aliphatic ketones than microgels without this comonomer. This work was focused mainly on the influence of microgels incorporated in the commercial solvent-borne acrylic binders on the properties of coatings. It was shown that the application of microgels that were redispersed in acetone did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors accelerated film curing at ambient temperature and improved the final hardness of coatings.     

Dual stimuli‐responsive microgels based on photolabile crosslinker: Temperature sensitivity and light‐induced degradation

The synthesis and characterization of a new photocleavable crosslinker is presented here. Dual stimuli‐responsive P(VCL‐co‐NHMA) microgels were prepared by precipitation polymerization of vinylcaprolactam (VCL) with N‐hydroxymethyl acrylamide (NHMA) and the new crosslinker. The microgels had distinct temperature sensitivity as observed in the case of PVCL‐based particles and their volume phase transition temperature (VPTT) shifted to higher temperature with increasing NHMA content. Photolytic degradation experiments were investigated by irradiation with UV light, which led to microgel disintegration caused by cleavage of the photolabile crosslinking points. The degradation behavior of the microgels was conducted with respect to degradation rates by means of the relative turbidity changes. Hence, the microgels could totally degrade into short linear polymers by UV light, thus representing a great potential as new light and temperature dual responsive nanoscale materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1676–1685     

Equilibrium and kinetic aspects of the uptake of poly(ethylene oxide) by copolymer microgel particles of N-isopropylacrylamide and acrylic acid

The use of microgels for controlled uptake and release has been an area of active research for many years. In this work copolymer microgels of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc), containing different concentrations of AAc and also cross-linking monomer, have been prepared and characterized. These microgels are responsive to pH and temperature. As well as monitoring the equilibrium response to changes in these variables, the rates of swelling/de-swelling of the microgel particles, on changing either the pH or the temperature, have also been investigated. It is shown that the rate of de-swelling of the microgel particles containing AAc is much faster than the rate of swelling, on changing the pH appropriately. This is explained in terms of the relative mobilities of the H(+) and Na(+) ions, in and out of the particles. It was observed that the microgels containing AAc, at pH 8, de-swelled relatively slowly on heating to 50 degrees C from 20 degrees C. This is attributed to the resistance to collapse associated with the large increase in counterion concentration inside the microgel particles. The swelling and de-swelling properties of these copolymer microgels have also been investigated in aqueous poly(ethylene oxide) (PEO) solutions, of different MW (2000-300 000). The corresponding absorbed amounts of PEO from solution onto the microgels have also been determined using a depletion method. The results, as a function of AAc content, cross-linker concentration, PEO MW, pH, and temperature, have been rationalized in terms of the ease and depth of penetration of the PEO chains into the various microgel particles and also the H-bonding associations between PEO and either the -COOH of the AAc moeities and/or the H of the amide groups (much weaker). Finally, the adsorption and desorption of the PEO molecules in to and out of the microgel particles have been shown to be extremely slow compared to normal diffusion time scales for polymer adsorption onto rigid surfaces.     

Hybrid microgels photoresponsive in the near-infrared spectral range

We report for the first time a photothermally responsive composite material based on polymer microgel particles doped with gold nanorods. We used the dependence of the longitudinal surface plasmon of the gold nanorods on their aspect ratio to synthesize nanoparticles with strong absorption in the near-IR spectral range (in the "water window"). The nanoparticles were incorporated in the interior of temperature-responsive poly(N-isopropylacrylamide-acrylic acid) microgels. Upon irradiation at lambda = 810 nm, hybrid microgel particles doped with Au nanorods underwent a strong deswelling phase transition. These photothermally responsive microgels can be used to carry and release small molecules (e.g., small protein molecules and drugs).     

Cationic polyvinylamine binding to anionic microgels yields kinetically controlled structures

Polyvinylamine (PVAm) binding (absorption and adsorption) to carboxylated microgels gave colloidally stable, cationic microgels that can be centrifuged, washed, freeze dried, and redispersed in water with no loss in colloidal stability. Because both PVAm and the carboxylated microgels are pH sensitive, changes in microgel swelling and electrophoretic mobility in response to pH change can be positive or negative depending upon pH and the PVAm content of the microgels. For a given PVAm molecular weight, the steady-state saturated mass fraction of bound PVAm in the microgels varied by a factor of four in our experiments. We proposed that the PVAm content at saturation was controlled by the relative rates of the initial attachment of PVAm chains versus the rate of attached chain spreading on and into the microgel structure. This explanation was further supported by a series of quartz crystal microbalance measurements. Finally, PVAm binding to two types of PNIPAM microgels shows general features recently reported for other polyelectrolyte types. Specifically: (1) for surface localized anionic charges on the microgels, the mass fraction of bound PVAm increased with PVAm molecular weight and vice versa; (2) in virtually all conditions, the quantity of adsorbed cationic ammonium groups was much greater than the carboxylate content of the microgel; and (3) sodium chloride additions lowered the mass fraction of bound PVAm.     

Temperature-sensitive hybrid microgels with magnetic properties

In the present paper, we report the preparation of hybrid temperature-sensitive microgels which include magnetite nanoparticles in their structure. Polymeric microgels have been prepared by surfactant-free emulsion copolymerization of acetoacetoxyethyl methacrylate (AAEM) and N-vinylcaprolactam (VCL) in water with a water-soluble azo-initiator. The obtained microgels possess a low critical solution temperature (LCST) in water solutions, with a rapid decrease of the particle size being observed at elevated temperatures. Magnetite was deposited directly into microgels, leading to the formation of composite particles which combine both temperature-sensitive and magnetic properties. The influence of magnetite load on microgel size, morphology, swelling-deswelling behavior, and stability is discussed.     

Symmetric and asymmetric adsorption of pH-responsive gold nanoparticles onto microgel particles and dispersion characterisation

The interaction between carboxylic acid-stabilised gold nanoparticles (AuNP) and pH-responsive microgels is shown. The microgel particles are a copolymer of N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and N-isopropylacrylamide (NIPAM). The microgel properties are presented by their hydrodynamic diameter and electrophoretic mobility in response to pH. These microgel particles are pH-responsive under neutral conditions decreasing in diameter beyond pH 7. The dispersion characteristics of AuNP adsorbed onto the microgel network are shown with respect to adsorbed amount and the pH-responsive properties of the AuNP. This data is presented between pH 3 and 6 where the microgel properties remain constant. Asymmetric adsorption of AuNP onto poly(DMAPMA-co-NIPAM) microgels is achieved by adsorption of nanoparticles, from the aqueous phase, onto microgel-stabilised oil-in-water emulsions. These asymmetrically modified microgels display very different dispersion behaviour, in response to pH, due to their dipolar nature.     

Metal nanocrystals incorporated within pH-responsive microgel particles

Cross-linked sterically stabilized latexes of approximately 250 nm diameter were synthesized by emulsion polymerization of 2-(diethylamino)ethyl methacrylate using a bifunctional oligo(propylene oxide)-based diacrylate cross-linker and a poly(ethylene oxide)-based macromonomer as the stabilizer at pH 9. These particles exhibit reversible swelling properties in water by adjusting the solution pH. At low pH, they exist as swollen microgels as a result of protonation of the tertiary amine units. Deswelling occurs above pH 7 [the effective pK(a) of poly(2-(diethylamino)ethyl methacrylate)], leading to the formation of the original compact latex particles. The swollen microgels can be used as nanoreactors: efficient impregnation with Pt nanoparticles can be achieved by incorporating precursor platinum compounds, followed by metal reduction. Dynamic light scattering was used to compare two methods of Pt nanoparticle impregnation with respect to the size and stability of the final Pt-loaded microgel particles. In the first method, the H2PtCl6 precursor was added to hydrophobic latex particles at high pH, followed by metal reduction. In the second method, H2PtCl6 was added to hydrophilic swollen microgel particles at low pH, and then this metal salt was reduced in situ using NaBH4 and the pH was raised by the addition of base. Both the Pt salt-loaded (metalated) microgels and the final Pt nanoparticle-loaded microgels had well-defined structures that were independent of the synthesis route. Polymer-metal interactions were investigated by UV-visible absorption spectroscopy, which confirmed that the Pt salt was completely reduced to zero-valent Pt. Transmission electron microscopy and X-ray diffraction studies verified the formation of nanometer-sized Pt nanoparticles within these microgels, which can be used as recoverable colloidal catalyst supports for various organic reactions.     

Preparation and Characterization of pH-Sensitive Microgels of Poly((2-dimethylamino) ethyl methacrylate)

Summary: pH-sensitive microgels of poly((2-dimethylamino) ethyl methacrylate) (PDMAEMA) were prepared by dispersion polymerization of 2-dimethylamino ethyl methacrylate in a mixed solvent of water/ethanol. ¹HNMR, FTIR and SEM were used to confirm the chemical structure and morphological properties of the resulting microgels. Dynamic Light Scattering (DLS) was used to measure the hydrodynamic diameter of the particles. SEM micrographs showed that the microgel particles have a diameter of about 100–200 nm in dry state. Mean hydrodynamic diameter of the particles at their collapsed state at pH = 9.5 was found to be about 150 nm. DLS measurements at various pH values showed that the prepared microgels have a volume phase transition around pH = 8 at which the hydrodynamic diameter decreased from about 470 nm to around 150 nm corresponding to a 32 fold change in the mean volume of a microgel particle.     

Synthesis of pH and temperature sensitive, core-shell nano/microgels, by one pot, soap-free emulsion polymerization

The synthesis and properties of thermal/pH-sensitive core-shell copolymer nano/microgels were investigated. The crosslinked core consisted of N-isopropylacrylamide (NIPAAm) while the shell was stabilized by poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 2-methacryloyloxybenzoic acid (2MBA) using a "one pot" soapless emulsion polymerization method. Monodisperse particles were produced with average hydrodynamic diameters ranging from 40 to 880 nm, as determined by dynamic light scattering (DLS) in water at 25°C, depending on the synthetic recipe used. The influence of PEGMA and 2MBA content on size and temperature transition at different pH values was studied. Zeta potential measurements and acid-base titration studies demonstrated almost complete incorporation of acid comonomer (2MBA) into the nano/microgels. Two different crosslinkers, a stable and an acid labile, were compared. The crosslinker used has a major influence on the size and charge density of the nano/microgels produced. Microscopic studies confirmed the core-shell morphology of the nano/microgels.     

Photo-, thermally, and pH-responsive microgels

Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.     

Thermal response of a PVCL–HA conjugate

The synthesis and self‐assembling of a thermoresponsive conjugate of hyaluronic acid (HA) and poly(N‐vinylcaprolactam) (PVCL) is reported. Both polymers were end functionalized: HA via reductive amination, thereby introducing an azide endgroup to the chain end, and PVCL via thioetherification to introduce a propargyl group. The two were coupled with a copper assisted “click” reaction into a bioconjugate composed of HA blocks with the molar mass 3,600 g mol^?1 (1618 saccharide units) and PVCL blocks of 3,500 g mol^?1 (～25 repeating units). The cloud point temperature measured by transmittance was 50–51 °C in water. The calorimetrically observed phase transition temperature of PVCL in the conjugate increased by 2 °C to 47.7 °C, whereas the enthalpy of the phase transition was unaffected by the conjugation. HA‐PVCL conjugate self‐assembles in water upon heating into monodisperse, colloidally stable, hollow spherical particles whose size may be tuned with the heating rate of the solution. Slow and fast heating resulted in vesicles with the hydrodynamic radii of 443 or 275 nm, respectively. The heating rate did not, however, affect the cloud point. Salt did not noticeably affect the size of the polymer particles, presumably because of interactions between the HA and PVCL blocks. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 425–436     

Interfacial layers of stimuli-responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) microgels characterized by interfacial rheology and compression isotherms

Stimuli-sensitive emulsions stabilized by microgel particles consisting of poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) and being responsive to both pH and temperature have been investigated with respect to the visco-elastic properties of the interfacial layer. Properties of the interfacial layer were probed by means of shear and dilatational rheology as well as by compression isotherms and are related to the microgel packing at the interface as visualized by cryogenic scanning electron microscopy. The corresponding pH dependent emulsion stability is strongly correlated with the visco-elastic properties of the microgel covered oil-water interface. At high pH when the microgels are charged, a structure of partially interconnected microgels is found that provides elastic, soft gel-like interfaces. At low pH, however, the uncharged microgels are densely packed and the interface is rather brittle. Obviously, these pH dependent visco-elastic properties of the microgel layer at the oil-water interface play a determining role in the stability of emulsion droplets and allow us to prepare very stable emulsions when the microgels are charged and to break the emulsion by changing the pH.     

Connection of poly(methyl methacrylate) microgel particles dispersed in poly(vinyl alcohol) matrix by seed polymerization

Poly(methyl methacrylate) microgels covered with poly(hydroxyethyl methacrylate) thin layer was dispersed in poly(vinyl alcohol) matrix. Homogeneous and regular arrangement of the microgel particles was suggested by Bragg diffraction for the films prepared by varying the PVA/microgel ratio (from 6/4 to 3/7 (w/w)). It was proved that the regular arrangement and connection of the microgels by seeded polymerization in poly(vinyl alcohol) were possible. © 1996 John Wiley & Sons, Inc.     

Unperturbed volume transition of thermosensitive poly-(N-isopropylacrylamide) microgel particles embedded in a hydrogel matrix

The thermoresponsive behavior of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in a covalently cross-linked polyacrylamide hydrogel matrix was investigated using ultraviolet-visible (UV-vis) spectroscopy, small-angle neutron scattering (SANS), and confocal laser scanning microscopy. The hydrogel synthesis was performed at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or fully collapsed PNiPAM microgel particles during the incorporation step. UV-vis spectroscopy experiments verify that the incorporation of thermosensitive microgels leads to temperature-sensitive optical properties of the composite materials. SANS measurements at different temperatures show that the thermosensitive swelling behavior of the PNiPAM microgels is fully retained in the composite material. Volume and structure criteria of the embedded microgel particles are compared to those of the free microgels in acrylamide solution. To visualize the temperature responsive behavior of larger PNiPAM particles, confocal fluorescence microscopy images of PNiPAM beads, of 40-microm size, were taken at two different temperatures. The micrographs also demonstrate the retained temperature sensitivity of the embedded microgels.