Noncovalent binding interactions of polyacrylamide and clay in nanocomposite hydrogels

The interactions between organic and inorganic components in pregel solution for polyacrylamide (PAAm)/clay nanocomposite hydrogels (NC gels) and in prepared NC gels are investigated. Besides, a kind of self‐crosslinked PAAm gels with excellent mechanical properties is fabricated in the absence of any cross‐linking agents, the hydrogen bonding interactions among PAAm chains are acted as the cross‐linking force. It is revealed that the binding interactions of PAAm and clay in NC gels are owing to the noncovalent interactions between amide groups on PAAm chains and clay platelets, which afford the cross‐linking force for NC gels network formation. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Effect of polymer entanglement on the toughening of double network hydrogels

The mechanical strength of double network (DN) gels consisting of highly cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first component and linear polyacrylamide (PAAm) as the second component has been investigated by varying the molecular weight of the second polymer PAAm, M(w). The experimental results reveal that, for toughening of the DN gels, (1) M(w) is one of the dominant parameters; (2) there is a critical value of M(w) = 10(6) for a remarkable enhancement; (3) the fracture energy of DN gels with a M(w) larger than 10(6) reaches a value as high as 10(3) J/m(2). By plotting the strength of DN gels (fracture stress sigma and fracture energy G) against a characteristic parameter of c[eta], where c is the average concentration of PAAm in the DN gels and [eta] is the intrinsic viscosity of PAAm, it is found that the dramatic increase in the mechanical strength of the DN gels occurs above the region where linear PAAm chains are entangled with each other. Thus, we conclude that the entanglement between the second component PAAm plays an important role of the toughening mechanism of DN gels. This result supports the heterogeneous model, which predicts the presence of "voids" of the first network PAMPS with a size much larger than the radius of the second polymer PAAm.     

Synthesis and properties of a novel double network nanocomposite hydrogel

A novel polyacrylamide/polyacrylic acid (PAAm/PAA) double network (DN) nanocomposite (NC) hydrogel had been synthesized by two‐step solution polymerization. The PAAm network was crosslinked by inorganic clay while the PAA network was crosslinked by a chemical crosslinker. The chemical structure of the network was confirmed by Fourier transform infrared (FTIR), X‐ray diffraction (XRD), and transmission electron microscopy (TEM). The swelling and mechanical strength properties of PAAm/PAA hydrogels were examined. The results showed that a DN hydrogel achieved both a high swelling capacity of 1219 g/g in deionized water and 124 g/g in 0.9 wt% NaCl solution and high compressive stress of 21.5 kPa in a high water content of 99.58%. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

Nanocomposite Gels: New Advanced Functional Soft Materials

Summary: Nanocomposite gels (NC gels) consisting of poly(N-alkylacrylamides) and exfoliated inorganic clay were prepared by in-situ, free-radical polymerization at high yield under mild conditions (near ambient temperature, without stirring). Various shapes and different surface forms of NC gels were readily obtained using corresponding vessels and templates, since NC gels were always mechanically tough. The view that polymer/clay networks are formed in NC gels was supported by dynamic viscoelastic and swelling measurements. The entirely different mechanical properties of NC gels, compared with conventional, chemically-crosslinked hydrogels (OR gels), are discussed on the basis of their network structure. In addition to functions previously reported, NC gels exhibit further interesting characteristics, such as inherent incombustibility, good thermal conductivity, large heat capacity, they can be readily colored and fabricated as foams. NC gels can be utilized as environmentally-friendly, soft materials from the viewpoints of resources and waste, as their primary component is water.     

Novel cross-linked homogeneous polyacrylamide gels with improved separation properties: investigation of the cross-linker functionality

Polyacrylamide (PAAm) gels were synthesized using cross-linkers with their potential functionality (twice the number of double bonds of a cross-linker) varying from six to sixteen. Improved electrophoretic separation and highly desirable porosity and sieving properties were observed for most of the PAAm gels containing novel cross-linkers. An increase in the potential functionality of cross-linkers used in PAAm gels was an important factor, influencing the pore size and pore size distribution of the network.     

Semi‐interpenetrating polymer networks composed of poly(N‐isopropyl acrylamide) and polyacrylamide hydrogels

Three series of semi‐interpenetrating polymer networks, based on crosslinked poly(N‐isopropyl acrylamide) (PNIPA) and 1 wt % nonionic or ionic (cationic and anionic) linear polyacrylamide (PAAm), were synthesized to improve the mechanical properties of PNIPA gels. The effect of the incorporation of linear polymers into responsive networks on the temperature‐induced transition, swelling behavior, and mechanical properties was studied. Polymer networks with four different crosslinking densities were prepared with various molar ratios (25:1 to 100:1) of the monomer (N‐isopropyl acrylamide) to the crosslinker (methylenebisacrylamide). The hydrogels were characterized by the determination of the equilibrium degree of swelling at 25 °C, the compression modulus, and the effective crosslinking density, as well as the ultimate hydrogel properties, such as the tensile strength and elongation at break. The introduction of cationic and anionic linear hydrophilic PAAm into PNIPA networks increased the rate of swelling, whereas the presence of nonionic PAAm diminished it. Transition temperatures were significantly affected by both the crosslinking density and the presence of linear PAAm in the hydrogel networks. Although anionic PAAm had the greatest influence on increasing the transition temperature, the presence of nonionic PAAm caused the highest dimensional change. Semi‐interpenetrating polymer networks reinforced with cationic and nonionic PAAm exhibited higher tensile strengths and elongations at break than PNIPA hydrogels, whereas the presence of anionic PAAm caused a reduction in the mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3987–3999, 2004     

Synthesis and characterization of poly(N‐vinyl formamide) hydrogels—A potential alternative to polyacrylamide hydrogels

The synthesis and characterization of solution‐cast, molded gels of N‐vinyl formamide (NVF) has not been previously reported even though NVF is an isomer of acrylamide (AAm) and polyacrylamide (PAAm) hydrogels have many commercial applications. Aqueous NVF solutions were cross‐linked into gels using a novel cross‐linker, 2‐(N‐vinylformamido)ethylether, and the thermally‐activated initiator VA‐044. For a given formulation, PNVF gels swell up to twice that of PAAm gels cross‐linked with N,N′‐methylenebisacrylamide. From swelling and compression measurements, PNVF gels were found to be more hydrophilic than PAAm gels. Flory‐Huggins solubility parameters were χ = 0.38?₂ + 0.48 for PNVF and χ = 0.31?₂ + 0.49 for PAAm, where ?₂ is the polymer volume fraction. The shear moduli for PNVF and PAAm scale with ? and ? respectively, consistent with good solvent behavior, also suggesting PNVF is more hydrophilic than PAAm. Similarity of mechanical properties for both gels as a function of ?₂ suggests that network structures of PNVF and PAAm gels are similar. Fracture strains of both gels declined with ?₂ by the same linear function while fracture stresses were about 500 kPa regardless of formulation. Since NVF is a liquid monomer, less toxic than AAm and can be hydrolyzed to a cationic form, PNVF gels could become technologically significant. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Elucidation of the chemical and morphological structure of double-network (DN) hydrogels by high-resolution magic angle spinning (HRMAS) NMR spectroscopy

(1)H HRMAS NMR spectroscopy is applied to gain insight into the chemical and morphological structure of double-network (DN) hydrogels, prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(acrylamide) (PAAm). The method enables one to obtain detailed information at the molecular level about the formation of covalent bonds between the two polymer networks through non-reacted double bonds of the cross-linker N,N'-methylene bis(acrylamide) (MBAA). Evidence to the existence of strong hydrogen-bond interactions based on the N-H group of the PAMPS as a hydrogen-bond donor and the C=O group of the PAAm as a hydrogen-bond acceptor is also provided. These findings clarify the origin of the toughening mechanism and the exceptionally strong mechanical properties of DN gels, further supported by microhardness data.     

Growth of long CdS nanowires using a carboxylic acid-functionalized self-assembled monolayer as substrate

We report a novel result that long CdS nanowires with typical width of 70–80 nm and length ranging from several micrometers to tens of micrometers can be obtained by the in situ growth on substrate of a carboxylic acid-functionalized self-assembled monolayer (SAM), which formed from a rigid aromatic molecule on mica surfaces. The as-grown CdS nanowires were characterized by X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and Energy-dispersive X-ray spectroscopy (EDS). A possible mechanism that accounts for the formation of the one-dimensional (1D) nanowires was also discussed.     

PREPARATION AND PHYSICAL PROPERTIES OF BLEND FILMS FROM SODIUM ALGINATE AND POLYACRYLAMIDE SOLUTIONS

Blend films of sodium alginate and polyacrylamide (PAAm) were prepared by mixing the aqueous solution of both samples at a different ratio. All blend films obtained are optically clear to the naked eye. The structure and physical properties of the films were studied by FT-infrared (FT-IR), wide angle X-ray diffraction (WAXD), differential thermal analysis (DTA), thermogravimetic analysis (TGA), scanning electron microscopy (SEM), and tensile strength test. The results showed that the occurrence of interactions between -COO^?, -OH groups of sodium alginate and -CONH₂ groups of PAAm in the blends through hydrogen formation. The blend films exhibited the higher thermal stability and improved mechanical properties in dry states. These properties had the maximum value around 20 wt% PAAm content in the blend film. The morphological transition of the blend films from sodium alginate-like to PAAm-like was observed by scanning electron microscopy.     

New hydrogels based on the interpenetration of physical gels of agarose and chemical gels of polyacrylamide

In this paper we report a novel method for preparing interpenetrating polymer hydrogels of agarose and polyacrylamide (PAAm) in three steps. The procedure consists in (i) formation of physical hydrogels of agarose, (ii) diffusion of acrylamide, N,N′-methylene-bis-acrylamide and potassium persulfate (the initiator) from aqueous solutions inside the gel of agarose, and (iii) cross-linking copolymerization reaction of the aforementioned reactants to produce PAAm chemical gels interpenetrated with the agarose physical gels. Viscoelasticity measurements and thermal analysis have been performed in order to follow the kinetics of copolymerization. The viscoelastic, swelling and thermal properties of the resulting hydrogels confirm the formation of an interpenetrated system. Further evidence of interpenetration is obtained from inspection with atomic force microscopy. The improvement of the agarose and PAAm gel properties in the resulting interpenetrated hydrogel is analyzed in view of the results.     

Synthesis and characterization nanocomposite of polyacrylamide‐rGO‐Ag‐PEDOT/PSS hydrogels by photo polymerization method

We present a novel approach to the fabrication of advanced polymeric nanocomposite hydrogels from polyacrylamide (PAAm) by incorporation of graphene‐silver‐polyethylenedioxythiophene‐polystyrene sulfonate (rGO‐Ag‐PEDOT/PSS) by photopolymerization method. Infrared spectroscopy was employed to characterize the structure of the hydrogels. The internal network structure of nanocomposite hydrogels was investigated by scanning electron microscope. Swelling, deswelling, and mechanical properties of the hydrogels were investigated. The compressive strength of nanocomposite hydrogels reaches maximum of 1.71 MPa when the ratio of rGO‐Ag‐PEDOT/PSS to PAAm was 0.3 wt%, which is 1.57 times higher than that of PAAm hydrogels (1.09 MPa). The electrical conductivity of the PAAm‐rGO‐Ag‐PEDOT/PSS hydrogel was found to be 3.91 × 10^?5 S cm^?1. Copyright © 2015 John Wiley & Sons, Ltd.     

高性能Bi3+掺杂改性PbO2电极的制备及表征

采用电沉积法制备了金属Bi3+改性PbO2(Bi-PbO2)电极,并通过扫描电镜(SEM)、X射线能谱(EDS)、X射线光电子能谱(XPS)、X射线衍射(XRD)、紫外-可见漫反射(UV-VisDRS)、荧光光谱(FP)、莫特-肖特基(Mott-Schottky)曲线、电化学阻抗谱(EIS)和线性极化扫描(LSV)等方法表征了其微结构和电化学性能.SEM、EDS和XPS结果表明,Bi3+以Bi2O3的形式掺杂进入PbO2镀层,同时其掺杂改性可明显改善PbO2镀层的微结构,使电极表面颗粒细化;XRD和UV-VisDRS分析结果显示Bi3+掺杂改性后,PbO2晶体的晶胞参数发生变化,同时禁带宽度(Eg)变小;荧光光谱分析表明Bi-PbO2电极可促进羟基自由基的产生,增强电极降解有机物的催化活性;电化学性能测试显示,Bi3+改性PbO2电极电催化活性的提升与电极的平带电势(Efb)负移、活性表面积增大、电化学反应电阻减小和析氧电位提高有关.     

CuInS2：两步电沉积制备及性能

采用恒电位沉积法制备铜铟合金预制膜,并存管式炉中通过固态源蒸发硫化预制膜得到CuInS:薄膜.通过扫描电镜(SEM)、能量色散谱仪(EDS)和X射线衍射仪(XRD)对CuInS2薄膜的表而形貌、截面厚度、成分组成和薄膜的组织结构进行了研究,并利用紫外可见光吸收谱仪(UV-Vis)研究了不同硫化温度对CuInS:薄膜的形貌及其光学吸收性质的影响.结果表明:不同的退火温度能够影响CuInS:薄膜的表面形貌以及带隙的大小,从而影响其光学吸收特性.     

Preparation of photoluminescent nanocomposite ink toward dual-mode secure anti-counterfeiting stamps

Fluorescent photochromism has been applied as an attractive approach for the production of effective authentication substrates to show dual-mode secure encoding. In the current study, novel photochromic and fluorescent nanocomposite ink was developed to introduce a stamped film with strong dual-mode emission for anti-counterfeiting purposes. Inorganic/organic nanocomposite ink was developed from lanthanide-doped aluminate (LDA) dispersed in poly(acrylic acid)-based binder. To produce a transparent film, LDA must be dispersed well in the poly(acrylic acid)-based ink solution. The fluorescent and photochromic nanocomposite ink was stamped effectively onto cellulose documents followed with thermal fixation. Homogeneous fluorescent and photochromic layer was stamped onto paper surface providing a transparent look with the ability to switch to green beneath ultraviolet as illustrated by CIE Lab. The stamped documents were studied by photoluminescence spectra to show an absorption peak at 364 nm, and fluorescence band at 438 nm. The induced security encoding was transparent beneath visible light turning into visible greenish-yellow beneath ultraviolet light indicating a bathochromic shift. LDA was synthesized in the nanoparticle form and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The morphological properties of the stamped documents were examined by infrared spectroscopy (FTIR), scan electron microscope (SEM), EDS, and X-ray fluorescence (XRF). The stamped paper sheets displayed an efficiently reversible photochromism without fatigue under visible and ultraviolet lights. The rheologies of the prepared photoluminescent nanocomposite inks as well as the mechanical performance of the stamped sheets were investigated.     

Effect of void structure on the toughness of double network hydrogels

Introduction of soft filler in a hard body, which is one of the common toughening methods of hard polymeric materials, was applied for further toughening of robust double network (DN) hydrogels composed of poly(2‐acrylamido‐2‐methylpropanesulfonic acid) gels (PAMPS gels) as the first component and polyacrylamide (PAAm) as the second component. The fracture energy of the DN gels with the void structure (called void‐DN gels) became twice when the volume fraction of void was 1–3 vol % and the void diameter was much larger than the Flory radius of the PAAm chains. Such toughening was induced by wider range of internal fracture of the PAMPS network derived from partial stress concentration near void structure. Considering the mechanical tests and the dynamic light scattering results, it is implied that the absence of the load‐bearing PAAm structure inside the void is important for the toughening. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1246–1254, 2011     

Green synthesis of Au nanoparticles immobilized on halloysite nanotubes for surface-enhanced Raman scattering substrates

A facile and green route was introduced to synthesize Au nanoparticles immobilized on halloysite nanotubes (AuNPs/HNTs) used for surface-enhanced Raman scattering substrates. The naturally occurring HNTs were firstly functionalized with a large amount of -NH(2) groups by N-(β-aminoethyl)-γ-aminopropyl trimethoxysilane (AEAPTES), which possesses one lone electron pair and will "anchor" Au ions to form a chelate complex. Then, with the addition of tea polyphenols (TP), the Au ions were reduced on the surface of the previously formed Au-NH(2) chelate complex to form AuNPs. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) observations indicate that a large amount of AuNPs were synthesized on HNTs. The AuNPs are irregularly spherical and densely dispersed on HNTs and the diameter of the nanoparticles varies from 20 to 40 nm. The interactions between AuNPs and -NH(2) groups were verified by X-ray photoelectron spectroscopy (XPS) and the results showed that the functional groups can "anchor" AuNPs through the chelating effect. The as-prepared AuNPs/HNTs nanomaterials with several nanometers gaps among nanoparticles were used as a unique surface-enhanced Raman scattering substrate, which possessed strong and distinctive Raman signals for R6G, indicating the remarkable enhancement effect of the AuNPs/HNTs.     

Fracture Behavior of Two Highly Stretchable Double Network Hydrogels

The static and dynamic mechanical behavior of two double network (DN) hydrogels, alginate/polyacrylamide (PAAm) hybrid hydrogel and sodium poly(2-acrylamido-2-methylpropanesulfonic acid) PNaAMPS/PAAm, is presented to understand the role played by different cross-linked networks on fracture and recovery properties. Although with a smaller modulus, alginate/PAAm hybrid hydrogel had a much higher stretchability, whether with or without notches, in the tensile tests. Continuous step strain measurement by using a strain-controlled parallel-plate rheometer showed that alginate/PAAm can immediately recover its mechanical properties after breakdown, while PNaAMPS/PAAm didn't show mechanical recovery at all.     

Controlled synthesis and upconverted avalanche luminescence of cerium(III) and neodymium(III) orthovanadate nanocrystals with high uniformity of size and shape

We report on controlled synthesis of uniform LnVO4 (Ln = Ce and Nd) nanocrystals (NCs) with square-plate and H-shaped morphologies in nanosized reverse microemulsion reactors, via a facile solvo/hydrothermal strategy. The NCs were thoroughly characterized by X-ray diffraction, transmission electron microscopy (TEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), infrared absorption spectroscopy, and photoluminescence. Possible mechanisms of the rare-earth orthovanadate NC growth and size and shape evolution are proposed. A unique upconverted avalanche luminescence property pertaining to the NCs has been discovered, systematically studied, and mechanistically discussed. Our work combines synthetic and optical studies of the NCs and lays a foundation for reinventing their applications in optoelectronics among others.