凝胶化反应中标度行为及凝胶化点关系

在苯乙烯(St)-二乙烯苯(DVB)自由基聚合的凝胶化反应过程中,定时取样,得到凝胶化点前后及直至反应终点的一系列溶胶样品,利用光散射技术研究了溶胶相的重均分子量M_w、尺寸均方旋转半径R_g的变化过程,建立了M_w、R_g和反应时间t的标度关系,并在此基础上提出一神新的准确求取凝胶化时间t_gel的方法.     

Study of Gelation in Aqueous Solutions of Cysteine and Silver Nitrate

A gel based on aqueous solutions of cysteine and silver nitrate is obtained for the first time. The possibility of gelation in the presence of competing ions and ligands is studied. Structural transformations occurring at an early stage of the gelation are studied by the dynamic light scattering, IR Fourier spectroscopy, and viscometry. The process of the self-organization in the system under consideration is assumed to be accompanied by the formation of fractal structures.     

PVP改性氧化硅溶胶的微结构和流变性

以正硅酸乙酯为前驱体,聚乙烯吡咯烷酮（PVP）为聚合物改性添加剂,在碱性条件下制备氧化硅溶胶,通过小角X射线散射测量了改性溶胶的微结构参数,辅以动态光散射观测溶胶颗粒生长,研究了聚合物量、碱催化剂量和水量对溶胶颗粒度、分形特征以及流变性的影响规律.由于PVP链中强极性的分子内酰胺基团和氧化硅颗粒表面的羟基形成氢键,致使溶胶颗粒被聚合物链包裹,严重阻碍了溶胶颗粒的生长,使凝胶时间延长,流变性发生变化,同时对溶胶颗粒的微结构产生影响.     

Microstructure Evolution of Nonhydrolytic Alumina Gels

Nonhydrolitic sol-gel processes of aluminum chloride and aluminum bromide with isopropyl ether and aluminum sec-butoxide were performed at various temperatures. Based on the Arrhenius type variation of the gelation time with temperature, activation energies for the gelation were found to be in the range 19–25 Kcal/mol range. The energies were found to be sensitive to the nature of the aluminum ligands and the chemical scheme. Due to the large activation energy, it is possible to stop the reaction at any time before gelation by cooling the sol to room temperature. Small angle X-ray scattering (SAXS) of sols from the AlClAlCl₃/Pr O system shows unique development of a fractal like structure with nanometer scale order, demonstrated by discrete peaks in the SAXS data. A fractal dimension D = 1.64 was found. An aggregation scheme is proposed to explain this phenomenon. A fractal dimension of 2.4 without small scale ordering found for xerogels prepared from the AlCl₃/ASB system reflects the effect of the different precursors on the microstructure of nonhydrolytic gels.     

Dynamic scaling properties of TeO2-based gels

Homogeneous, transparent, and mechanically rigid gels have been successfully synthesized in the tellurium isopropoxide-isopropanol-citric acid and water system. The sol to gel transition and the gels microstructure have been studied by using small angle X-ray scattering (SAXS) experiments. For any value of the two key synthesis parameters, which are the citric acid ratio and the alkoxide concentration, very small Te-rich elementary particles, about 1-1.5 nm in radius, form immediately when the water is added, leading to colloidal sols. During gelation, these elementary particles stick progressively together to build up fractal aggregates by a pure hierarchical aggregation process which has been identified as a reaction-limited cluster aggregation (RLCA) mechanism. The SAXS curve analysis, based on scaling concepts, shows that the gelling network exhibits a time and length scale invariant structure factor characterized by self-similarity. This self-similarity is also displayed for a wide range of chemical compositions and the gel microstructures only differ in their fractal aggregate size according to the tellurium isopropoxide concentration as well as the citric acid ratio.     

Structure of Hydrous Ferric Oxide Aggregates

The small angle light scattering behavior of hydrous ferric oxide flocs is examined here and found to provide useful insights into the nature of the aggregates formed despite the large size of these aggregates at later times. The flocs appear to exhibit fractal properties over a significant size range though the aggregates appear to be easily disrupted through mixing effects resulting in breakup and/or restructuring to denser assemblages. Background electrolyte concentrations also have some impact on floc structure but mixing effects and apparent destabilization by ferric ions limit the effect of added electrolytes on the stability and structure of ferric oxyhydroxides. Similar estimates of fractal dimensions of these hydrous ferric oxide flocs are obtained both by static light scattering analysis and by a cluster mass scaling approach. The choice of density distribution cutoff function has some impact on derived size and structure parameters and further refinement in this area is needed. Copyright 2000 Academic Press.     

In situ small-angle neutron scattering and rheological measurements of shear-induced gelation

The microscopic structure of shear-induced gels for a mixed solution of 2-hydroxyethyl cellulose and nanometer-size spherical droplets has been investigated by in situ small-angle neutron scattering (SANS) with a Couette geometry as a function of shear rate gamma. With increasing gamma, the viscosity increased rapidly at gamma approximately 4.0 s(-1), followed by a shear thinning. After cessation of shear, the system exhibited an extraordinarily large steady viscosity. This phenomenon was observed as a shear-induced sol-gel transition. Real-time SANS measurements showed an increase in the scattering intensity exclusively at low scattering angle region. However, neither orientation of polymer chains nor droplet deformation was detected and the SANS patterns remained isotropic irrespective of gamma. It took about a few days for the gel to recover its original sol state. A possible mechanism of gelation is proposed from the viewpoint of shear-induced percolation transition.     

光散射技术在研究高分子溶液和凝胶方面的应用

从溶液中的高分子、凝胶粒子及微乳胶粒子形态结构的表征和凝胶化过程、微乳液聚合及大分子缔合等动态过程的跟踪等方面的研究综述了近十几年来光散射技术在高分子溶液和凝胶领域的应用,并简单介绍了光散射技术的基本原理、发展简史和仪器及使用方法。     

Fractal behaviour of carbon black and smectite dispersions by small angle neutron scattering

From dielectric spectroscopy on mechanically sheared dispersions of carbon black in mineral oil it was known that the distribution of relaxation times was invariant irrespective of the applied shear rate. From this observation fractal behaviour was suspected and a small angle neutron scattering experiment was set up for confirmation. Indeed, by this experiment a fractal dimension has been found of d=2.19, a value which is perfectly comparable tod=2.0 found by dielectric spectroscopy.The dielectric behaviour of the smectite dispersion is different from the carbon black and small angle neutron scattering confirms this. Ad-value of 2.8 has been found, pointing to a more compact object than could be expected from a fractal.     

Critical behavior of trifunctional randomly branched polycyanurates

The behavior near the gelation threshold of trifunctional randomly branched polycyanurates is studied by static and dynamic light scattering. By static measurements the critical exponents γ, σ and η were obtained, which describe the divergence of the weight average (M_w) and the cutoff (M*) molecular weights and the radius of gyration (R_g) respectively. All these independently measured exponents together with τ, characterizing the power law behavior of the molecular weight distribution and measured by size exclusion chromatography coupled with light scattering, confirm the predictions of the three-dimensional percolation theory. With the help of size exclusion chromatography coupled with a light scattering and a viscosity detector, a fractal dimension D = 2.24 is obtained. On the other side, from the corresponding exponent for the whole unfractionated samples a fractal dimension D = 2.21 results, using a theory of Daoud. This suggests that the fractal dimension of the polycyanurates in dilute solution lies between the theoretical predictions D = 2.5 for the unswollen and D = 2.0 for the completely swollen state. Furthermore, it is shown by dynamic light scattering that the power law behavior over some decades in time of the time autocorrelation function and the divergence of the mean relaxation time are characteristics of the gelpoint. The development with increasing reaction time of the time correlation function of the gelling system from the pregel through the gelpoint into the gel state is analyzed quantitatively by a hybrid of a stretched exponential and a power law function.     

聚丙烯腈溶液的凝胶化研究

聚丙烯腈是用途最广泛的聚合物之一,其溶于适当溶剂中形成的聚丙烯腈溶液是制备聚丙烯腈纤维、渗透膜等高分子材料的原料。聚丙烯腈溶液的物理化学性质对所制备材料的性能有很大的影响。本文对高分子溶液的凝胶化和高分子凝胶的特点做了简要介绍,并介绍了聚丙烯腈及其凝胶的特点。根据高分子浓溶液体系的特点提出用于表征聚丙烯腈溶液凝胶化的主要方法。从浓度和温度对聚丙烯腈溶液凝胶化行为的影响、熟化和非溶剂对聚丙烯腈溶液凝胶化行为的影响、聚丙烯腈溶液凝胶化的热可逆性、聚丙烯腈溶液凝胶化的分形特征以及聚丙烯腈凝胶的交联机理这几个方面对已有聚丙烯腈溶液的凝胶化研究成果和最新进展进行了综述。最后对聚丙烯腈溶液凝胶化和聚丙烯腈凝胶的研究前景做了展望。     

Gelation of tenuous titania gels in a reverse micellar medium

Tenuous gels of TiO₂ are synthesized from titanium isopropoxide in a reverse micellar medium. The volume fraction of TiO₂ in these gels is equal to 0.2%. The chemical and aggregative mechanisms are studied using ¹H NMR, small-angle X-ray scattering, dynamic and static light scattering. The alkoxide hydrolysis occurs at the first stage of the gelation. Small clusters (70–80 Å) are produced. The aggregation induces mass fractal structures with fractal dimensions depending on the solvent used in the synthesis: 1.4<D _f<1.9. The interpretation of these results is based on interactions between the growing inorganic chains and the surfactant molecules.     

大孔交联聚苯乙烯分形结构的图像分析法研究

采用图像分析法测定大孔交联聚苯乙烯树脂的 现大孔交联聚苯乙烯的外表面具有分形结构,分形维数在２和３之间,将图像分析法测定的分形维数与小角Ｘ－射线散射法测得的结果对照。发现两种方法测定结果虽然不同,但在不同树脂之间的变化趋势是一致的,因此,可用图像分析法比较不同大孔树脂分形结构的差别。     

分形理论在催化剂研究中的应用

介绍了分形和分形维数在催化剂研究中的应用.概述了催化剂研究中常用的分形维数测定方法,包括盒子维法、散射强度法和吸附法.从四个方面分析了近年来催化剂分形研究的方向和进展:分形维数与催化剂评定和筛选,催化剂分形孔结构,分形与反应动力学,模型建立.从分形维数的测量、催化剂分形生长模型以及多重分形研究三个方面对分形在催化剂方面的应用进行了评价和展望.     

Aggregation Behavior of Alkoxide-Derived Silica in Sol-Gel Process in Presence of Poly(ethylene oxide)

Growth behavior of silica in an acid catalyzed sol-gel process from silicon alkoxide in the presence of poly(ethylene oxide), PEO, was investigated by in situ small angle X-ray scattering, SAXS, and ²⁹Si NMR measurements. The results of SAXS, that aggregation and gel formation behaviors of silica were affected by the presence of PEO, suggested a strong attractive interaction between silica oligomers and PEO. A possible reaction scheme of silica in the presence of PEO is as follows; (1) PEO and small silica oligomers coexist in the solution without specific interaction just after hydrolysis of the silicon alkoxide. (2) With the progress of condensation, a ramified aggregated complex between PEO and silica oligomers is formed, which is characterized by larger apparent value of radius of gyration and smaller fractal dimension than in the PEO-free system. (3) After gelation, the fractal dimension of scatterers remains to be smaller than that in the PEO-free system, because PEO associated with the silica network inhibits aggregation within the gel networks. Furthermore, PEO inhibits the condensation in the aging and in the drying process, leading to less strongly crosslinked dry gel. A temporal maximum in the time evolution of R_g was observed for the samples separated into two phases with their characteristic domain size being larger than several micrometers. This is considered to be a phenomenon related to increase and divergence of correlation length near and at the critical point.     

On the role of block copolymer additives for calcium carbonate crystallization: small angle neutron scattering investigation by applying contrast variation

The role of the double-hydrophilic block copolymer poly(ethylen glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) on the morphogenesis of calcium carbonate (CaCO3) was studied by applying the contrast variation small angle neutron scattering technique. The morphology and size of CaCO3 crystals is strongly affected by the addition of PEG-b-PMAA. In order to determine the partial scattering functions of the polymer and CaCO3 mineral, we developed both an experimental and theoretical approach with a sophisticated method of their determination from the scattering intensity. Partial scattering functions give detailed information for each component. In particular, the partial scattering function of the polymer, Spp, shows a monotonic slope with Q(-2 to -3) where the scattering vector Q is low (Q < 0.01 Angstrom(-1)), which is a clear evidence that the polymer within the CaCO3 mineral has a mass fractal dimension. The other partial scattering functions reflected the geometry of the CaCO3 particles or the "interaction" of polymer and CaCO3 on a microscopic scale, which leads to a coherent view with Spp.     

Application of fractal geometry for studies of heterogeneity properties of nanomaterials

Calculations based on the fractal geometry in the estimation of surface heterogeneity are superior compared with conventional calculation methods (e.g. from the data of gas adsorption or X-ray radiation scattering) as they can be applied without limitation as far as the range of surface sizes of the studied structures is concerned. This paper presents structural characteristics of carbon and carbon- free nanomaterials based on the determined surface and volumetric fractal coefficients. Fractal coefficients were determined from the data obtained by means of two independent methods: sorptometry and atomic force microscopy (AFM). Correlation between porosity parameters and fractal coefficients is presented.     

On techniques for the measurement of the mass fractal dimension of aggregates

A review is presented of a number of techniques available for the characterisation of the structure of aggregates formed from suspensions of sub-micron particles. Amongst the experimental techniques that have been commonly used are scattering (light, X-ray or neutron), settling and imaging and these are the focus of this work. The theoretical basis for the application of fractal geometry to characterisation of flocs and aggregates is followed by a discussion of the strengths and limitations of the above techniques. Of the scattering techniques available, light scattering provides the greatest potential for use as a tool for structure characterisation even though interpretation of the scattered intensity pattern is complicated by the strong interaction of light and matter. Restructuring further complicates the analysis. Although settling has long been used to characterise particle behaviour, the absence of an accurate permeability model limits the technique as a means of determining the porosity of fractal aggregates. However, it can be argued that the determination of fractal dimension is relatively unaffected. The strength of image analysis lies in its ability to provide a great deal of information about particle morphology and the weaknesses lie in the difficulties with image processing and sample size as this is a particle counting technique. There are very few papers which compare the fractal dimension measured by more than one technique. Light scattering potentially provides a useful tool for checking settling results. However, further work is required to develop proper models for aggregate permeability and flow-through effects.