Ag Nanocomposite Particles: Preparation,Characterization and Application

Summary Herein, we report that different core-shell particles could be successfully used as the carrier systems for the deposition of silver nanoparticles. Firstly, thermosensitive core-shell microgel particles have been used as the carrier system for the deposition of Ag nanoparticles, in which the core consists of poly (styrene) (PS) whereas the shell consists of poly (N-isopropylacrylamide) (PNIPA) network cross-linked by N, N′-methylenebisacrylamide (BIS). Immersed in water the shell of these particles is swollen. Heating the suspension above 32 °C leads to a volume transition within the shell, which is followed by a marked shrinking of the network of the shell. Secondly, “nano-tree” type polymer brush can be used as “nanoreactor” for the generation of silver nanoparticles also. This kind of carrier particles consists of a solid core of PS onto which bottlebrush chains synthesized by the macromonomer poly (ethylene glycol) methacrylate (PEGMA) are affixed by “grafting from” technique. Thirdly, silver nanoparticles can be in-situ immobilized onto polystyrene (PS) core-polyacrylic acid (PAA) polyelectrolyte brush particles by UV irradiation. Monodisperse Ag nanoparticles with diameter of 8.5 nm, 7.5 nm and 3 nm can be deposited into thermosensitive microgels, “nano-tree” type polymer brushes and polyelectrolyte brush particles, respectively. Moreover, obtained silver nano-composites show different catalytic activity for the catalytic reduction of p-nitrophenol depending on the carrier system used for preparation.     

The volume transition in thermosensitive core-shell latex particles investigated by small-angle x-ray scattering and dynamic light scattering

We present a survey over recent studies of the volume transition in colloidal core-shell particles composed of a solid poly(styrene) core and a shell of a thermosensitive crosslinked polymer chains. The thermosensitive shell is built up from poly(N-isopropylacrylamide) chains (PNIPA) crosslinked by N,N′-methylenbisacrylamide (BIS). In addition, particles containing acrylic acid (AA) as comonomer have been synthesized and investigated. The volume transition of these particles have been studied by dynamic light scattering (DLS) and by small-angle X-ray scattering (SAXS). In all cases analyzed so far the volume transition was found to be continuous. This finding shows that the core-shell microgels behave in a distinctively different manner than ordinary thermosensitive gels: The crosslinked chains in the shell are bound to a solid boundary independent of temperature. The spatial constraint by this boundary decreases the maximum degree of swelling but also prevents a full collapse of the network above the volume transition.     

Thermosensitive Au-PNIPA yolk-shell particles as “nanoreactors” with tunable optical properties

We demonstrate that Au-silica-poly(N-isopropylacrylamide) (PNIPA) trilayer composite particles with controllable thickness of PNIPA out-layer can be developed via free radical polymerization using Au-silica core-shell particles as seed. The presence of Au-silica particles does not influence of thermosensitivity of PNIPA shell, which exhibits a similar swelling behavior as pure PNIPA microgels. The etching of silica midlayer by NaOH treatment leads to Au-PNIPA particles with yolk-shell structure. The obtained yolk-shell particles can work as “nanoreactors” for the further growth of Au core within the PNIPA shell via seeded growth approach, which is followed with interesting optical properties. In addition, the optical properties of the Au cores can be modulated by the volume transition of the PNIPA shell.     

Comparative Study on the Collapse Transition of Poly(N-isopropylacrylamide) Gels and Magnetic Nanoparticles Loaded Poly(N-isopropylacrylamide) Gels

A novel polymer gel exhibiting simultaneous temperature and magnetic field sensitivity has been prepared and studied. Poly(N-isopropylacrylamide) (PNIPA) and magnetic nanoparticles (magnetite, Fe₃O₄) loaded PNIPA gel beads with mm size and monolith gels with cm size were prepared. The dependence of swelling degree on the temperature has been studied. The effects of cross-linking density and the presence of magnetic nanoparticles on the equilibrium swelling degree as well as on the collapse transition have been investigated. Swelling kinetic measurements were also made. By comparing the equilibrium swelling properties of PNIPA and magnetite loaded PNIPA gels it was found that the built in magnetic nanoparticles do not modify the temperature sensitivity of these gels. Within the experimental accuracy the temperature of the collapse transition was not sensitive to the presence of magnetic particles. We have compared the swelling behaviour of mm size gel beads to the cm size monolith gels in order to study the influence of surface skin layer on the swelling equilibrium. It was established that the extent of surface skin formation was decreased by the presence of magnetic particles.     

Graphical analysis for gel morphology. III. Gel size and temperature effects on the volume phase transition of gels

Just after poly(N-isopropylacrylamide) (PNIPA) gels are immersed into hot water over the critical temperature, they undergo a two- or three-step shrinking accompanied with the whitening. The size of gels and the destination temperature were changed and their effects on the shrinking process of PNIPA gels were studied. The following important knowledge in the volume phase transition process of PNIPA gels was obtained. The first is the fact that the fraction of first shrinking is determined by the shrinking rate in the first shrinking stage. The second is the fact that PNIPA gels are divided into small parts by the pinning of density fluctuation in the final stage of the first shrinking and that the shrinking process of each small part undergoes in the secondary shrinking stage. The volume phase transition process of PNIPA gels is briefly discussed from the viewpoint of the scaling theory.     

Preparation and characterization of sodium carboxymethylcellulose/poly(N-isopropylacrylamide)/clay semi-IPN nanocomposite hydrogels

A new kind of pH-/temperature-responsive semi-interpenetrating polymer network hydrogels based on linear sodium carboxymethylcellulose (CMC) and poly(N-isopropylacrylamide) (PNIPA) cross-linked by inorganic clay (CMC/PNIPA/Clay hydrogel) was prepared. The temperature- and pH-responsive behaviors, the mechanical properties of these hydrogels were investigated. The CMC/PNIPA/Clay hydrogels exhibited a volume phase transition temperature around 32 °C with no significant deviation from the conventional PNIPA hydrogels. The swelling ratio of the CMC/PNIPA/Clay hydrogels gradually decreased with increasing the contents of clay. The influence of pH value on swelling behaviors showed that there is a maximum swelling ratio at pH 5.9. Moreover, the CMC/PNIPA/Clay hydrogels exhibited excellent mechanical properties with high tensile stress and elongation at break in excess of 1200%.     

The volume transition in thermosensitive core–shell latex particles containing charged groups

An investigation of the volume transition in thermosensitive core–shell particles by dynamic light scattering (DLS) is presented. The core of the particles consists of polystyrene (diameter 118 nm), whereas the thermosensitive shell is composed of a network of poly (N-isopropylacrylamide) containing 2 mol% acrylic acid counits. The hydrodynamic radius of these particles as determined by DLS decreases in a continuous manner when raising the temperature. It is shown that the volume transition in the core–shell microgels remains continuous for a wide range of ionic strengths and pH values. This behavior is opposite to that of macrogels of the same chemical composition, which undergo a discontinuous volume transition. The present investigation therefore demonstrates that affixing the network to solid colloidal particles profoundly alters the volume transition of thermosensitive networks. The reason is that shrinking can take place only along the radial direction of the particles. The solid core thus exerts a strong spatial constraint onto the network, which leads to the observed behavior. Received: 29 March 1999 Accepted in revised form: 16 July 1999     

Stimuli-responsive nanocomposite gels

A new class of polymer hydrogels, nanocomposite hydrogels (NC gels), consisting of a unique organic (polymer)/inorganic (clay) network structure, was synthesized by in situ free-radical polymerization in the presence of exfoliated clay nanoparticles in an aqueous system. The resulting NC gels overcame most of the disadvantages associated with chemically cross-linked hydrogels, such as mechanical fragility, structural heterogeneity, and slow de-swelling rate. By using thermo-sensitive poly(N-isopropylacrylamide) (PNIPA) as a constituent polymer, NC gels with remarkable mechanical, optical, and swelling properties as well as thermo-sensitivity were obtained. The various properties of NC gels, such as transparency, gel volume, cell culturing, and surface friction changed significantly in response to the temperature and surrounding conditions. All the excellent properties and new stimuli-responsive characteristics of NC gels are attributed to the unique PNIPA/clay network structure. The thermo-sensitivities and the transition temperature can largely be controlled by varying the clay content and by the addition of solutes.     

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.     

Influence of a crown ether comonomer on the temperature-induced phase transition of poly(N-isopropylacrylamide) hydrogels

Copolymerization of thermosensitive hydrogels based on poly(N-isopropylacrylamide) (PNIPA) is a possible route to enhanced storage capacity of guest molecules. This article describes the synthesis of the amphiphilic crown ether N9-propenoyl-3,6,12,15-tetraoxa-9,21-diazabicyclo[15.3.1]heneicosa-1(21),17,19-triene (CE) and its incorporation into a PNIPA hydrogel (PNIPA/CE). Mechanical measurements on the gel show that the CE units contribute to the elasticity of the network, but the swelling ratio in water is reduced compared to the unmodified system. The comonomer reduces the temperature of the volume phase transition (VPT), TVPT, and broadens the transition. Both the enthalpy and the entropy associated with the VPT decrease. Scattering measurements indicate that the local structural features on the scale of 10 A are unchanged, but the CE units form large clusters, the size of which increases with rising temperature. In the phase-separated state above TVPT these clusters are distributed on the polymer-water interface.     

Ethylene (Co)Polymerization with Metallocene Catalysts Encapsulated in Gel‐Type Poly(styrene‐co‐divinylbenzene) Beads

Gel‐type poly(styrene‐co‐divinylbenzene) beads (PS bead) were used as a carrier to encapsulate metallocene catalysts through a simple swelling‐shrinking procedure. The catalytic species were homogeneously distributed in the PS bead particle. The catalyst exhibited high and stable ethylene polymerization and ethylene/1‐hexene copolymerization activity affording uniform spherical polymer particles (1 mm). Polymerization rate profiles exhibited slow initiation and stable increase in polymerization activity with time.     

Imaging the volume transition in thermosensitive core-shell particles by cryo-transmission electron microscopy

We report a study of colloidal thermosensitive core-shell particles by cryo-transmission electron microscopy (cryo-TEM). The particles consist of a solid core of poly(styrene), onto which a network of cross-linked poly(N-isopropylacrylamide) (PNIPAM) is affixed. In water, the shell of these particles swells when the temperature is low. Raising the temperature above 32 degrees C leads to a marked shrinking of the shell. In this letter, we present the first study of these core-shell particles by cryo-TEM in situ, that is, in aqueous solution. We demonstrate that the core-shell particles are well-defined and exhibit a narrow size distribution. In particular, the PNIPAM shell is compact and has a defined outer surface of a slightly irregular shape. The micrographs show that there are density fluctuations within the network. Cryo-TEM of the system above and below the transition temperature furnishes information about the thermosensitive particles that had not been available through other methods employed in previous investigations.     

Thermosensitive core-shell particles as model systems for studying the flow behavior of concentrated colloidal dispersions

We report on a comprehensive investigation of the flow behavior of colloidal thermosensitive core-shell particles at high densities. The particles consist of a solid core of poly(styrene) onto which a network of cross-linked poly(N-isopropylacrylamide) is affixed. Immersed in water the shell of these particles will swell if the temperature is low. Raising the temperature above 32 degrees C leads to a volume transition within this shell which leads to a marked shrinking of the shell. The particles have well-defined core-shell structure and a narrow size distribution. The remaining electrostatic interactions due to a small number of charges affixed to the core particles can be screened by adding 0.05M KCl to the suspensions. Below the lower critical solution temperature at 32 degrees C the particles are purely repulsive. Above this transition, a thermoreversible coagulation takes place. Lowering the temperature again leads to full dissociation of the aggregates formed by this process. The particles crystallize for effective volume fractions between 0.48 and 0.55. The crystallites can be molten by shear in order to reach a fluid sample again. The reduced shear stress measured in this metastable disordered state was found to be a unique function of the shear rate and the effective volume fraction. These reduced flow curves thus obtained can be described quantitatively by the theory of Fuchs and Cates [Phys. Rev. Lett. 89, 248304 (2002)] which is based on the mode-coupling theory of the glass transition.     

Thermosensitive pluronic micelles stabilized by shell cross-linking with gold nanoparticles

Gold nanoparticles were employed to prepare shell cross-linked Pluronic micelles that exhibit a reversibly thermosensitive swelling/shrinking behavior. Two terminal hydroxyl groups of Pluronic F127 were thiol-functionalized to form self-assembling Pluronic micelles in aqueous solution with exposed -SH groups in an outer shell layer. The thiol groups present in the outer shell were cross-linked by gold nanoparticles synthesized through NaBH4 reduction of gold precursor anions. The resultant shell cross-linked gold-Pluronic micelles exhibited a temperature-dependent volume transition: their hydrodynamic diameter was changed from 157.1 +/- 15.6 nm at 15 degrees C to 53.4 +/- 5.5 nm at 37 degrees C as determined by dynamic light scattering. The critical micelle temperature measured by a pyrene solubilization technique suggested that the reversible swelling/shrinking behavior of the micelles was caused by hydrophobic interactions of cross-linked or grafted Pluronic copolymer chains in the micelle structure with increasing temperature. Transmission electron microscopy directly revealed that the shell cross-linked micelles were indeed produced by gold nanoparticles covalently clustered on the surface. These novel self-assembled organic/inorganic hybrid micelles would hold great potential for diagnostic and therapeutic applications.     

Temperature-sensitive polyamidoamine dendrimer/poly(N-isopropylacrylamide) hydrogels with improved responsive properties

Novel temperature-sensitive poly(N-isopropylacrylamide)/amine-terminated polyamidoamine dendrimer G6-NH2 hydrogels with fast responsive properties were synthesized by forming semi-interpenetrating polymeric networks. In contrast to the conventional PNIPA hydrogel, these new gels showed rapid shrinking rate at the temperature above lower critical solution temperature (LCST), and exhibited higher equilibrium swelling ratio at room temperature. All these properties might be attributed to the incorporation of polyamidoamine dendrimer G6-NH2, which forms water-releasing channels and increases the hydrophilicity of PNIPA network. The novel hydrogels have potential applications in drug and gene delivery.     

Preparation and properties of poly(N‐isopropylacrylamide)/poly(N‐isopropylacrylamide) interpenetrating polymer networks for drug delivery

A novel poly(N‐isopropylacrylamide) (PNIPA)/PNIPA interpenetrating polymer network (IPN) was synthesized and characterized. In comparison with conventional PNIPA hydrogels, the shrinking rate of the IPN hydrogel increased when gels, swollen at 20 °C, were immersed in 50 °C water. The phase‐transition temperature of the IPN gel remained unchangeable because of the same chemical constituent in the PNIPA gel. The reswelling kinetics were slower than those of the PNIPA hydrogel because of the higher crosslinking density of the IPN hydrogel. The IPN hydrogel had better mechanical strength because of its higher crosslinking density and polymer volume fraction. The release behavior of 5‐fluorouracil (5‐Fu) from the IPN hydrogel showed that, at a lower temperature, the release of 5‐Fu was controlled by the diffusion of water molecules in the gel network. At a higher temperature, 5‐Fu inside the gel could not diffuse into the medium after a burst release caused by the release of the drug on the surface of the gel. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1249–1254, 2004     

Preparation and characterization of pH- and temperature-responsive semi-IPN hydrogels of carboxymethyl chitosan with poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide) crosslinked by clay

A new kind of pH- and temperature-responsive semi-interpenetrating polymer network hydrogel based on linear carboxymethylchitosan (CMCS) and poly (N-isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The pH-and temperature-responsive behaviors, the deswelling kinetics, and the mechanical properties of the hydrogel were investigated. The hydrogels exhibited a volume phase transition temperature around 33 °C with no significant deviation from the conventional PNIPA hydrogels. The results of the influence of pH value on the swelling behaviors showed that the minimum swelling ratios of the hydrogels appeared near the isoelectric point (IEP) of CMCS, and when pH deviated from the IEP, the hydrogels behaved as polycations or polyanions. The novel hydrogels had much higher response rate than the conventional CMCS/PNIPA hydrogels. Moreover, the semi-IPN hydrogels crosslinked by clay could be elongated to more than 800% and the elongation could be recovered almost completely and instantaneously.     

Double hydrophilic block copolymer monolayer protected hybrid gold nanoparticles and their shell cross-linking

This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.     

快速响应的温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成及表征

快速响应的温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成及表征;N-异丙基丙烯酰胺;水凝胶;温敏性;快速响应     

负载纳米银复合微球制备及其催化性能

以具有温度和pH双重敏感性能的N-异丙基丙烯酰胺(NIPAM)共聚丙烯酸(AA) P(NIPAM-co-AA)高分子微凝胶为模板, 以乙醇为还原剂, 原位还原得到负载纳米银的微米尺度Ag/P(NIPAM-co-AA)复合微凝胶材料. 通过扫描电子显微镜(SEM)、X射线衍射(XRD)仪和紫外-可见(UV-Vis)分光光度计等对复合材料的形貌、组成和催化性能进行表征. 研究结果表明, Ag/P(NIPAM-co-AA)复合微球具有均一的表面结构, 微凝胶的限域作用显著提高了纳米银的分散性和稳定性. 另外, Ag/P(NIPAM-co-AA)复合微球对对硝基苯酚(4-NP)的还原具有较好的催化活性, 且其催化活性与微凝胶网络结构的溶胀、收缩行为有一定关系, 即模板微凝胶的温敏特性可以实现对对硝基苯酚催化反应活性的调控作用.