聚(N-异丙基丙烯酰胺)水凝胶微球体积相变的研究

窄分散的聚（Ｎ 异丙基丙烯酰胺）水凝胶微球用乳液聚合方法制备，并用动态和静态光散射对其体积相变进行了研究．与水中聚（Ｎ 异丙基丙烯酰胺）线性单链比较，水中凝胶微球的体积相变温度较高，对温度的响应比较平缓．相变是连续的，有别于大块凝胶非连续的体积变化．在体积相变过程中，凝胶微球始终是密度均一的热力学稳定球体．从相变过程网络密度的变化可以确定，绝大部分的水在收缩过程被排了出来，但在紧缩的凝胶微球中仍含有约７０％的水．     

Preparation and characterization of pH‐responsive and thermoresponsive hybrid microgel particles with gold nanorods

We report on pH‐responsive and thermoresponsive hybrid materials based on the assembly of gold nanorods, Au NRs, into multiresponsive, crosslinked copolymer microgel particles. These microgel particles were prepared by the surfactant‐free emulsion polymerization of N‐isopropylacrylamide and acrylic acid using N, N′‐methylene bis‐acrylamide as a crosslinker, which produces particles measuring approximately 160 nm that are interconnected to one other. Cetyltrimethyl ammonium bromide‐stabilized Au NRs were also prepared independently using a seed‐mediated growth method and then loaded into swollen, deprotonated, acrylic acid‐containing microgel particles using the electrostatic interactions between the oppositely charged particles. Transmission electron micrographs of the as‐prepared hybrid Au NR–microgel particles confirmed that the Au NRs were attached to the surface of the microgel particles. The size‐dependent temperature‐responsive characteristics of the hybrid microgel particles were studied by dynamic light scattering, and it was found that as the temperature increased across the phase transition temperature, the particle size decreased to 56% of the original volume. The thermoresponsive and pH‐responsive optical properties of the hybrid microgel particles were also systematically investigated. The thermo‐ and pH‐induced shrinkage of the microgel led to an increase in the UV–vis absorption intensity and caused a significant blue shift in the longitudinal surface plasmon bands of the Au NRs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Temperature dependent phase behavior of PNIPAM microgels in mixed water/methanol solvents

Temperature dependent phase behavior of poly(N-isopropylacylamide) (PNIPAM) microgels in water/methanol mixtures of different composition was studied with dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Using DLS, it is possible to measure the diffusion coefficient, and thus the size of particles exactly and directly; the variation of the phase transition temperature in the different solvents is also easy to detect by this method. With SANS measurements in D₂O/MeOD mixtures, some of the DLS results were confirmed. Moreover, SANS measurements give valuable information on the particle structure in different solvents. The experiments were compared with the theory of competitive hydration introduced by Tanaka et al. We found a good agreement of theory and experiment, and obtained the theoretical predictions: around the transition temperature, the composition of the bound methanol along the chains is higher than that of the outer solution, while the whole methanol composition inside the gel is lower. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym. Phys. 2013, 51, 1100–1111     

Effects of surfactants on the phase transition of poly(N-isopropylacrylamide) in water

The effects of both anionic (sodium dodecyl sulfate, SDS) and cationic (dodecylpyridine bromide, DPB) surfactants on the phase transition of narrowly distributed poly(N-isopropylacrylamide) (PNIPAM) microgel particles were investigated by laser light scattering. The addition of SDS swells the particles and increases the phase transition temperature, while DPB has a much smaller effect. This difference cannot be due to an association between the surfactant hydrophobic tail and PNIPAM because DPB and SDS have an identical hydrophobic tail. The amide groups in PNIPAM are slightly protonized in deionized water (pH ∼ 5.5). Our results contradict a previous prediction that oppositely charged surfactants will collapse a polyelectrolyte gel. After adding SDS, a two-step phase transition of the PNIPAM gel is observed. This suggests that SDS forms micelles inside the microgel with the help of the immobilized counter ions on the gel network. The SDS micelles are broken into individual SDS molecules in the first step of phase transition, while in the second step individual SDS molecules are gradually expelled. Surfactant effects on the microgel particles are compared with those of individual PNIPAM chains. © 1996 John Wiley & Sons, Inc.     

Thermoresponsive core-shell microgels with silica nanoparticle cores: size, structure, and volume phase transition of the polymer shell

Core-shell microgels made of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) and silica nanoparticles as inorganic cores were investigated by dynamic light scattering (DLS) and small angle neutron scattering (SANS). In order to study the response of the particles upon changes of temperature, experiments were done in a temperature interval close to the volume phase transition temperature of the PNIPAM shell. While DLS probes the hydrodynamic dimensions of the particles, determining their centre of mass diffusion, SANS provides the correlation length xi of the PNIPAM network. Additionally, the composite particles were characterised by electron microscopy as well as atomic force microscopy to reveal the core-shell structure and at the same time the approximate dimensions and the shape of the microgels.     

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     

NMR investigations into heterogeneous structures of thermosensitive microgel particles

The internal properties of submicron poly(N‐isopropylmethacrylamide) latex particles were investigated as a function of the methylene bisacrylamide (MBA) concentration used as a crosslinker. Two experimental approaches were performed. First, quasi‐electric light scattering measurements provided the size variation of the particles as a function of temperature, from which the swelling capacity of the particles as a function of MBA were estimated. In addition, the broadening and lowering effects of the volume phase transition temperature were detected from the turbidity of the solutions versus the MBA concentration. Second, observations of the transverse relaxation of protons gave evidence for heterogeneous structures inside the particles; several structural parts were discriminated from one another from different proton mobilities detected through magnetic relaxation rates. Corresponding to the concentration gradients of the crosslinker, the internal particle structures were looser and looser from the core to the shell. The state of the gelation of the polymer particles was governed by the initial amount of the crosslinker introduced into the latex recipe. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 889–898, 2000     

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.     

Spectral time moment analysis of microgel structure and dynamics

The structure and dynamics of crosslinked nanoparticles (microgels) made out of hydroxypropylcellulose (HPC) polymer chains were studied using dynamic light scattering spectroscopy. The microgel light scattering spectra were found to be highly nonexponential requiring a spectral time moment analysis in which the spectra were fit to a sum of stretched exponentials. Each term offers three parameters for analysis and represents a single spectral mode. At room temperature microgel spectra reveal three modes. Two faster modes are almost diffusive and correspond to apparent sizes of 25 and 450–650 nm. The slowest mode is independent of scattering angle and is reminiscent of the slow polymer mode observed in identical non‐crosslinked polymer solutions. When solution temperature is varied from 23 to 45°C and back, the microgel undergoes a reversible volume phase transition between 40 and 45°C. According to the time‐moment analysis, above the transition temperature two faster modes collapse into one with apparent hydrodynamic radius of 100–150 nm, while the slow mode remains largely unchanged. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 771–781, 2008     

Thermo-responsive behavior and microenvironments of poly(N-isopropylacrylamide) microgel particles as studied by fluorescent label method

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with a fluorescent monomer 4-N-(2-acryloyloxyethyl)-N-methylamino-7-N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole (DBD-AE) were prepared by emulsion polymerization under various crosslinker concentrations. The thermo-responsive behavior and the microenvironment of the microgel particles were studied in water by turbidimetric and fluorescence analyses. For the microgel particles prepared under the crosslinker concentration of 1 mM, the turbidity began to increase at ca. 32.5 degrees C, but the relative fluorescence intensity dramatically increased and the wavelength at the maximum fluorescence intensity (lambda(max)) was dramatically blue-shifted both at ca. 31.5 degrees C with increasing the temperature, suggesting the hydrophobicity around the DBD-AE unit was dramatically increased and the subsequent shrinking of the microgel particles occurred. As the crosslinker concentration increased from 0.5 to 20 mM, the transition temperature determined by turbidimetric analysis was constant upto 2 mM, rose between 2 and 10 mM, leveled off above 10 mM, and was ca. 34 degrees C at 20 mM. The temperature-induced microenvironmental change inside the microgel particles was also reduced at high crosslinker concentrations. The results obtained from the fluorescence of the DBD-AE unit and another fluorescent monomer unit 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) suggested that the heterogeneity inside the microgel particles prepared under the crosslinker concentration of 20 mM became high.     

Structure and polymer dynamics within PNIPAM-based microgel particles

The synthesis of temperature-responsive microgels of poly(N-isopropylacrylamide) (PNIPAM) was first reported in 1986 and, since then, there have been hundreds of publications describing the preparation, characterization and applications of these systems. This paper reviews the developments concerning the study of the structure of PNIPAM-based microgels performed over the last years using small angle neutron scattering (SANS) and also the investigations of the polymer-chain dynamics within the microgels carried out with incoherent elastic and quasielastic neutron scattering, and pulse field gradient nuclear magnetic resonance (PFG-NMR) techniques. Furthermore, the self-diffusion coefficient of the water molecules within the microgel, determined by means of solvent relaxation NMR, is also discussed as a function of the polymer volume fraction of the microgels.     

Coil-to-globule-type transition of poly(N-isopropylacrylamide) adsorbed on colloidal silica particles

 The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) adsorbed on two different colloidal silica particles was studied with dynamic light scattering. The hydrodynamic diameter was measured when the temperature was varied stepwise from 10 to 60 °C. PNIPAM molecules free in solution undergo a conformational transition at the θ temperature. We have found that PNIPAM adsorbed onto silica particles also undergoes a transition below the θ temperature. When a small amount of polymer was adsorbed the coil-to-globule transition at the θ temperature did not occur. Potentiometric titrations showed that the surface charge of the silica particles was not affected by the polymer adsorption. Sodium dodecyl sulfate (SDS) (100–1200 mg/l) was added to improve the stability. The particles with a higher zeta potential required a smaller addition of SDS to prevent coagulation compared to the particles with a smaller surface potential. For low additions of SDS the transition curves of adsorbed PNIPAM were unaffected. For larger additions of SDS the collapse of PNIPAM was shifted to higher temperatures. When as much as 1200 mg/l SDS was added, two regions with weak transitions were observed before the collapse. It was also observed that the presence of SDS results in a smaller adsorption of PNIPAM onto the particles. The addition of SDS strongly increased the magnitude of the electrophoretic mobility of the polymer–particle unit. From the electrophoretic measurements an electrokinetic layer thickness was calculated and it was found to be smaller than the corresponding hydrodynamic layer thickness, as obtained by dynamic light scattering. Received: 14 December 1999/In revised form: 22 February 2000/Accepted: 6 March 2000     

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     

Dispersion of polystyrene inside polystyrene‐b‐poly(N‐isopropylacrylamide) micelles in water

The addition of mixture of polystyrene‐b‐poly(N‐isopropylacrylamide) (PS‐b‐PNIPAM) and polystyrene homopolymer (h‐PS) in tetrahydrofuran dropwise into water leads to nanoparticles with a PS core and a thermally sensitive PNIPAM shell. The effects of the ratio of the homopolymer to copolymer and temperature on the formation and stabilization of the dispersion were investigated by using a combination of static and dynamic laser light scattering. PNIPAM shell continuously collapses as temperature increases in the range 20–40 °C. Such formed particles are stable even at temperatures much higher than lower critical solution temperature (LCST ～ 32 °C) of PNIPAM. Our results reveal that the area occupied per hydrophilic PNIPAM chain on the hydrophobic PS core remains nearly a constant regardless of the amount of h‐PS in the polymer mixture. This clearly indicates that the surface area occupied per hydrophilic group is a critical parameter for stabilizing particles dispersed in water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749–755, 2010     

Direct imaging of temperature-sensitive core-shell latexes by cryogenic transmission electron microscopy

We present a comprehensive investigation of the volume transition in thermosensitive core-shell particles. The particles consist of a solid core of poly (styrene) (radius: 52 nm) onto which a network of crosslinked poly(N-isopropylacrylamide) (PNIPAM) is affixed. The degree of crosslinking of the PNIPAM shell effected by the crosslinker N,N ^′-methylenebisacrylamide was varied between 1.25 and 5 mol%. Immersed in water, the shell of these particles is swollen at low temperatures. Raising the temperature above 32°C leads to a volume transition within the shell. Cryogenic transmission electron microscopy (Cryo-TEM) and dynamic light scattering (DLS) have been used to investigate the structure and swelling of the particles. The Cryo-TEM micrographs directly show inhomogeneities of the network. Moreover, a buckling of the shell from the core particle is evident. This buckling increases with decreasing degree of crosslinking. A comparison of the overall size of the particles determined by DLS and Cryo-TEM demonstrates that the hydrodynamic radius provides a valid measure for the size of the particles. The phase transition within the network measured by DLS can be described by the Flory–Rehner theory. It is shown that this model captures the main features of the volume transition within the core-shell particles including the dependence of the phase transition on the degree of crosslinking. All dispersions crystallize at volume fractions above 0.5. The resulting phase diagram is identical to the phase behavior of hard spheres within the limits of error. This demonstrates that the core-shell microgels can be treated as hard spheres up to volume fractions of at least 0.55.     

Spectral time moment analysis of microgel deswelling. Effect of the heating rate

Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through the self-association of amphiphilic HPC molecules and the subsequent cross linking at room temperature. Dynamic Light Scattering was used to study the transport properties of HPC microgels below and above the volume phase transition. Highly nonexponential, multimodal microgel spectra were observed and successfully analyzed by spectral time moment analysis. This article expands earlier results and focuses on the effect of the heating rate on microgel deswelling. During the fast heating two identified microgel modes with apparent hydrodynamic radii (R_H) of 25–30 nm and 400–650 nm collapse into one mode with R_H = 100–150 nm. This indicates the shrinkage of microgel size distribution and an apparent decrease in the radius of larger microgels. During the slow heating, however, both microgel-identified modes remain present above T_c. Although equally represented below the transition, the dominance of larger microgels' mode increases almost two fold with rising temperature above 40°C. Moreover, R_H for this mode increases from 250–300 nm to about 800–850 nm with a multi-step temperature change from 40 to 42.5°C, indicating the growth (and not shrinkage) of microgels. The second mode is represented by the temperature independent R_H, but its contribution goes down from about 50% to less than 10%. © 2008Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2792–2802, 2008     

激光光散射研究聚(N-异丙基丙烯酰胺)单链及其智能凝胶微球在水中的相变(下)

（接上期）２聚（Ｎ－异丙基丙烯酰胺）微凝胶在水中的体积相变２．１理论部分凝胶体积相变热力学：聚合物凝胶的溶胀和蜷缩可以用膨胀因子α＝（Ｖ／Ｖ０）１／３＝（ΦＴ／ΦΘ）１／３来表征，其中ΦΘ的ΦＴ分别是温度Θ和Ｔ下凝胶网络的体积分数。在平均场理论中，中...     

Morphology evolution of polystyrene-core/poly(N-isopropylacrylamide)-shell microgel synthesized by one-pot polymerization

One-pot polymerization with macroinitiator is supposed to be a robust, facile way to synthesize well-defined coreshell nanoparticles with fixed shell thickness. To testify this, we investigated the temperature-depending morphology evolution of polystyrene(PS) core/poly(N-isopropylacrylamide)(PNIPAM) shell microgel synthesized by one-pot polymerization with PNIPAM-RAFT as macroinitiator in dimethylformamide(DMF) by transmission electron microscopy(TEM), dynamic/static light scattering(DLS/SLS) and small angle neutron scattering(SANS). It is revealed that the microgel has a core-shell structure, i.e., the core is made of pure PS, but the shell is composed of both PNIPAM-RAFT macroinitiator and crosslinked PS. In fact, there are 92.0 wt% D2 O, 6.7 wt% PNIPAM and 1.3 wt% PS in the shell in its aqueous dispersion at 21 °C; therefore, its shell thickness is much larger than the extended chain length of the macroinitiator as revealed by both SANS and DLS observations. Competitive growth of styrene, divinylbenzene and PNIPAM macroinitiator as well as possible chain transfer from amine proton of PNIPAM side chain may lead to the larger shell thickness, compared with the extended chain length of the macroinitiator. Our work can shed light on the real morphology control in one-pot polymerization.     

Hydrophilic stimuli‐responsive particles for biomedical applications

Hydrophilic and stimuli‐responsive submicronic latex particles based on polyalkyl(meth)acrylamide can be prepared owing to simple radical‐initiated polymerizations in heterogeneous media using a water‐soluble initiator and a crosslinker (methylenebisacrylamide). The paper aims at reviewing the synthesis and properties of functionalized polystyrene‐polyN‐isoprpylacrylamide core‐shell particles or polyN‐isopropylmethacrylamide microgel particles. Particle size of analysis showed that a short nucleation period afforded the synthesis of highly monodispersed latexes. The dramatic change of the colloidal properties (particle size, electrophoretic mobility) was found to reflect the thermal sensitivity of such particles. The hydrophilic nature of the particles below the volume phase transition temperature was found to drastically reduce the physical adsorption of proteins. Some examples of biomedical applications of these stimuli‐responsive particles are briefly reported.