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 Misfolded Proteins

Insulin protein was exposed to mildly denaturing conditions (heat and low pH) to encourage the formation of beta-sheet rich amyloid fibrils. This resulted in an increase in viscosity of our protein samples and the morphology and thermodynamics of the resulting hydrogel were monitored using environmental scanning electron microscopy and micro differential scanning calorimetry respectively. It was found that the beta-sheet fibrils aggregated further to form macrofibrils, 2 μm in diameter and several microns in length. These long, flexible macrofibrils became entangled to form hydrogels with controllable mesh size: the higher the incubation temperature the higher the number of entanglements, and consequently the smaller the mesh size.     

Gelation Studies, 3

Summary: The sol‐gel transition of one thermoreversible gelling mixture made of xanthan gum and locust bean gum has been studied by using in situ time‐resolved dynamic light scattering (DLS) and measuring the spin‐lattice relaxation time T₁ of several protons. A critical dynamical behavior was observed near the sol‐gel transition, which is characterized by the presence of power‐law spectra over four decades of the delay time in the time‐intensity correlation function g₂(t)−1 ∼ t^−μ at 48 °C. The increase in T₁ with increasing temperature becomes steeper at 50 °C indicating a significant change in the local mobility of one anomeric proton of the xanthan side chain and the anomeric protons of the locust bean gum mannose backbone.

Temperature dependence of the spin‐lattice relaxation time T₁ for the equatorial anomeric proton of the mannopyranosic unit located next to the main chain of the xanthan.     

Gelation in semiflexible polymers

Molecular dynamics simulations have been employed to study the formation of a physical gel by semiflexible polymer chains. The formation of a geometrically connected network of these chains is investigated as a function of temperature and rate of cooling. The stiffness of the molecules is controlled via a potential between beads separated by two bonds. As the temperature is lowered, a percolated homogeneous solution phase separates to form a high-density, non-percolated nematic fluid and a low-density gas phase. On further decreasing the temperature, the chains are dynamically arrested preventing the completion of the vapor-liquid (VL) phase separation. As a result, the chains are stuck in a three-dimensional network of nematic bundles forming a percolated gel. Apart from temperature, the rate of cooling also plays an important role in the formation of the gel. Cooling the system at a faster rate yields gel while slower rates result in complete VL phase separation.     

超声诱导凝胶研究进展

近年来,超声诱导凝胶作为一种在室温下形成分子凝胶的新方法受到广泛关注.以分子凝胶的形成和超声作用的原理为出发点,从超声诱导溶液转变为凝胶、超声诱导沉淀转变为凝胶、超声诱导等多种手段形成的凝胶体系、超声增强的凝胶体系、超声诱导的杂化凝胶体系等五个方面,较系统地综述了近年来超声诱导凝胶的研究进展情况,简单展望了超声诱导凝胶的研究和应用前景.     

Gelation in Multiple Link System

The critical point theory is generalized to include gelation in multilink system with f functional units and J junction points. The equations derived include, as special cases, the cyclotrimerization model of J = 3, and the R–A_f model of J = 2. The theory is applied to the recent observation of the cyclotrimerization of bisphenol‐A dicyanate. The theoretical prediction agrees exactly with the Stutz‐Simak observation, D_c = 0.504, and accords with the Georjon‐Galy‐Pascault observation, giving a confirmation of the physical soundness of the theory. Under the smoothness assumption, we derive post‐gelation relationships with loop formation, the result suggesting the formation of permanent sol molecules that resist being absorbed into gel phase throughout an entire reaction process.

Representation of binary branching.     

Gelation process of amylose-DMSO-water system

Amylose gel is obtained by cooling amylose aqueous solution at a certain cooling rate. In order to clarify the gelation process of amylose in the mixed solvent of water and DMSO, the characteristics of turbidity and dynamic viscoelasticity of the solution as a function of temperature have been studied by optical and rheological methods, respectively. Accordingly, cloud temperature (Tc) at which the aggregation of amylose occurs, and gelation temperature (Tgel) at which the elasticity began to appear were obtained. Tc and Tgel were strongly dependent on cooling rate, and these values shifted to higher temperatures with decreasing cooling rate. However, there was difference between Tc and Tgel, and Tc was higher than Tgel, indicating the gelation process occurs in two stages.     

Gelation in strongly charged polyelectrolytes

Crosslinking of strongly charged chains via multivalent ions of valence z was treated within the Flory approach, which accounts only for the treelike architecture of the clusters formed. Density–density correlations due to the electrostatic nature of the system were considered with a modified random‐phase approximation. At a certain concentration of multivalent ions that play the role of effective stickers among z monomers, an infinitely large polymer network was formed. We analyzed the gelation driven by the formation of locally neutral clusters induced by divalent and trivalent ions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 766–776, 2004     

Gelation mechanism of ionic polysaccharides

Experimental results obtained by membrane equilibria, osmotic pressure, viscosity and circular dichroism measurements on alginate and pectate solutions in the presence of Ca²⁺ ions are presented. From equilibrium dialysis data both electrostatic and cooperative interactions seem to describe the binding process of Ca²⁺ ions onto polymer chains. An increase of the number-average molecular weight M̄_n for both poly-saccharides with calcium ion concentration is observed. An increase of polymer dimensions can well account for the observed increase of the intrinsic viscosity [η] with bound Ca²⁺ ion concentration at several ionic strengths.     

Gelation of a Model Globular Protein

Hen egg white lysozyme (HEWL) was exposed to various physical and chemical denaturing environments to encourage protein denaturation and consequent gelation. Its phase behavior was examined as a function of pH, temperature and also in the presence of the reductant dithiothreitol (DTT). Transparent viscoelastic gels form at low pH values while opaque gels form under alkaline conditions. No increase in viscosity was observed for systems in pure water unless 20 mM of DTT was added, which is known to break the disulfide bridges present in HEWL. The microstructure of the gel was studied using transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Gels formed at low pH contain fibrils ∼10 nm in diameter with various lengths while at high pH the gels are dominated by particulate aggregates. Thinner fibrils that are 4–6 nm in diameter are observed in the gels formed in the presence of DTT. In this case the distinct feature of the gels is they are thermoreversible and can be melted and reformed easily by varying the temperature.     

Metal–Phenolic Supramolecular Gelation

Materials assembled by coordination interactions between naturally abundant polyphenols and metals are of interest for a wide range of applications, including crystallization, catalysis, and drug delivery. Such an interest has led to the development of thin films with tunable, dynamic properties, however, creating bulk materials remains a challenge. Reported here is a class of metallogels formed by direct gelation between inexpensive, naturally abundant tannic acid and group(IV) metal ions. The metallogels exhibit diverse properties, including self‐healing and transparency, and can be doped with various materials by in situ co‐gelation. The robustness and flexibility, combined with the ease, low cost, and scalability of the coordination‐driven assembly process make these metallogels potential candidates for chemical, biomedical, and environmental applications.     

聚苯胺溶液的凝胶化行为

通过流变学手段对聚苯胺溶液的凝胶化行为研究表明，在室温下，聚苯胺（ＰＡｎ）／Ｎ－甲基－２－吡咯烷酮（ＮＭＰ）溶液有一转折浓度Ｃｇ（０．０５ｇ／ｍＬ），浓度高于此值时，浴液凝胶化速度急剧增大，凝胶速度还与溶剂，ＰＡｎ分子量及溶液的温度有很大关系。溶液中不溶的ＰＡｎ的存在会加速凝胶化过程。ＰＡｎ／ＮＭＰ凝胶的假塑性和触变性明显地反映出ＰＡＶＮＭＰ溶液的凝胶化是溶剂参与的ＰＡｎ分子链间的物理交联过程。     

Gelation of siloxane cross-linked organic molecules

We have studied by ²⁹Si NMR and small angle neutron scattering the cross-linking by formation of siloxane bridges of two amino-epoxy-silane molecules at different concentration in acidic water. We relate our observations to the fact that one of the studied systems never gels, while the second one displays a concentration threshold for gelation that is accounted for by percolation theory.     

Heat-Induced Gelation of Globular Protein Mixtures

Heat-induced conformational changes and heat-induced gels of whey and egg white albumen, and their major components were studied under physicochemical conditions that favour protein-protein interactions. We used differential scanning calorimetry (DSC) to compare their conformational heat stability, through the characteristic temperature (Θ_max) corresponding to the maximal heat flow and the overall calorimetric heat of reaction (Δ_r H _cal). Times needed to observe sol-gel state transitions at various temperatures were determined by a tilting test and the corresponding time-temperature experimental points were best fitted to two successive Arrhenius plots intersecting at Θ～Θ_max corresponding to the major protein component for whey proteins and to a minor protein component for egg white albumen. Observations of gel-networks by scanning electron microscopy indicated a wide range of stuctural patterns, depending on the composition of protein solutions. The results are discussed in terms of the temperature of maximal rate of heat-induced conformational changes and of sol-gel state transitions of protein molecules.     

Gelation of Covalently Cross-Linked PEG-Heparin Hydrogels

We study PEG-heparin hydrogels to identify compositions that lead to gel formation and measure the corresponding gelation kinetics. The material consists of a maleimide-functionalized high molecular weight heparin (HMWH) backbone covalently cross-linked with bis-thiol poly(ethylene glycol) (PEG). Using multiple particle tracking microrheology, we investigate a broad composition space, defined by the number of maleimide functional sites per HMWH (f = 3.9-11.8), the molecular weight of the PEG cross-linker (M(n) = 2000, 5000, and 10 000), and the concentrations of the heparin and PEG polymers. Gelation kinetics are characterized by time-cure superposition, yielding the gel time, t(c), and the critical relaxation exponent, n. Gelation times range from 5 < t(c) ≤ 45 min, with the fastest kinetics occurring for the highest HMWH maleimide functionalities. t(c) depends nonmonotonically on the PEG cross-linker molecular weight, suggesting that gelation is affected by the length of the cross-linker relative to intermolecular interactions between heparin molecules. The critical relaxation exponent decreases from n = 0.52 for PEG 2000 to n = 0.39 for PEG 10 000. Finally, 219 equilibrated samples taken over the entire composition space are identified as liquid or solid, defining the "gelation envelope". The boundaries of this empirical gelation envelope are in good agreement with Flory-Stockmayer theory. In all, microrheological measurements enable characterization over a large parameter space and provide crucial insight into the gelation of complex, multifunctional hydrogelators used in therapeutic applications.     

Thermoreversible Gelation of Polybenzimidazole in Phosphoric Acid

Summary: We report a study of thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA). The PBI gel in PA exhibits fibrillar network morphology and reversible first order phase transition. The gelation rate is measured by the tube tilting method and found to depend both upon gelation concentration and gelation temperature. The UV‐vis study demonstrates that the gelation process is a two‐step process: conformational transformation and aggregation which produces crystallites for gel formation. The WAXS study supports the presence of crystallites in the gel. The PA doping level of the membrane increases significantly because of gelation.

Thermoreversible gelation of polybenzimidazole in phosphoric acid and the membrane produced from the gel.     

Thermoreversible Gelation of Syndiotactic Polystyrene in Naphthalene

Investigation into syndiotactic polystyrene/naphthalene systems of concentrations ranging from 0 to 78% (w/w) have been carried out by electron microscopy, DSC, and neutron diffraction. It is found that a fibrillar morphology is produced in this solvent, similar to that observed for thermoreversible gels. The temperature-concentration phase diagram suggests the existence of two compounds of differing stoichiometries. Neutron diffraction experiments confirm the existence of compounds in this system.     

Gelation on Heating of Supercooled Gelatin Solutions

Diluted (1.0–1.5 wt%) aqueous gelatin solutions have been cooled to –10 °C at a cooling rate 20 °C min⁻¹ without freezing and detectable gelation. When heated at a constant heating rate (0.5 –2 °C min⁻¹), the obtained supercooled solutions demonstrate an atypical process of gelation that has been characterized by regular and stochastically modulated differential scanning calorimetry (DSC) as well as by isoconversional kinetic analysis. The process is detectable as an exothermic peak in the total heat flow of regular DSC and in the nonreversing heat flow of stochastically modulated DSC. Isoconversional kinetic analysis applied to DSC data reveals that the effective activation energy of the process increases from approximately 75 to 200 kJ mol⁻¹ as a supercooled solution transforms to gel on continuous h eating.     

聚丙烯酰胺胶凝动力学的研究

聚丙烯酰胺(PA)水溶液的胶凝作用已成功地应用于油井堵水及二、三次采油的压裂工艺~[1].1979年Hessert等~[2]用Cr(Ⅵ)还原生成的Cr(Ⅲ)使PA交联成三维凝胶并用于采油,Terry等~[3]用稳态剪切粘度法对影响该过程动力学的因素作过研究.Prud'hornme等~[4]用流变模数随时间的变化来表征胶凝的速率.Sydansk~[5,6]对Cr(Ⅲ)使PA水溶液发生胶凝的速率及