Thermal gelation of cellulose in a NaOH/thiourea aqueous solution

Utilizing a novel solvent of cellulose, 6 wt % NaOH/5 wt % thiourea aqueous solution, for the first time, we prepared the thermally induced cellulose gel. We investigated the thermal gelation of cellulose solutions with rheometry and the structure of the gel with 13C NMR, wide-angle X-ray diffraction, environmental scanning electron microscopy, and atomic force microscopy. The cellulose solutions revealed an increase in both the storage modulus (G') and the loss modulus (G") with an increase in the temperature during gelation. The temperature at the turning point, where G' overrides G" because of the onset of gelation, decreased from 38.6 to 20.1 degrees C with an increase of cellulose concentration from 4 to 6 wt %. Given enough time, G' of all solutions can exceed G" at a certain temperature slightly lower than the gelation temperature, indicating that the occurrence of the gelation is also a function of time. Each of the assigned peaks of NMR of the cellulose gel is similar to that of the cellulose solution, suggesting that the gelation resulted from a physical cross-linking. The gels were composed of relatively stable network units with an average diameter of about 47 nm. At either a higher temperature (at 60 degrees C for 30 s) or a longer gelation time (at 30 degrees C for 157 s), the gel in the 5 wt % cellulose solution could form. A schematic gelation process was proposed to illustrate the sol-gel transition: the random self-association of the cellulose chains having the exposed hydroxyl in the aqueous solution promotes the physical cross-linking networks.     

Rheological study of the sol-gel transition in silica alkoxides

The sol-gel transition of the system tetramethylorthosilicate-water-methanol under basic conditions has been studied by rheological measurements. The gelation time t(g) has been determined from the measurements of the elastic G' and viscous G' moduli as a function of time at different frequencies according to Winter's criterion. At gelation time the frequency dependence of both moduli follows a power law with an exponent related to the fractal dimension of the network structure. Different initial monomer concentrations, hydrolysis molar ratios, and temperatures are studied. The decrease of the gelation time with an increase of initial monomer concentration or hydrolysis molar ratio is well-described by a power law. An apparent activation energy is deduced from the temperature dependence of gelation time.     

Controllable gelation of methylcellulose by a salt mixture

The effects of a salt mixture consisting of a salt-out salt (NaCl) and a salt-in salt (NaI) on the sol-gel transition of methylcellulose (MC) in aqueous solution have been studied by means of micro differential scanning calorimetry and rheometry. The salt mixture was found to have a combined effect from the salt-out and salt-in salts in the mixture, and the salt effect was dependent on the water hydration abilities of the component ions and ion concentration. At a fixed total salt concentration, the sol-gel transition temperature nicely followed a rule of mixing: Tp = m1Tp1 + m2Tp2 where Tp, Tp1, and Tp2 are the gelation peak temperatures for the MC solutions with a salt mixture, NaCl, and NaI, respectively, and mi is the molar fraction of the salt component i in the salt mixture. The linear rule of mixing proved that the effects of NaCl and NaI on the sol-gel transition of MC are completely independent. In addition, the presence of a single salt or a salt mixture in a MC solution does not change the essential mechanism of MC gelation. Therefore, the sol-gel transition of MC can be simply controlled by a salt mixture consisting of a salt-out salt and a salt-in salt. The rheological results supported the micro thermal results excellently. But the gel strength of MC containing salts was influenced by both salt type and salt concentration.     

Efficient formation of multicompartment hydrogels by stepwise self-assembly of thermoresponsive ABC triblock terpolymers

The gelation behavior of a poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PON) triblock terpolymer and a poly(N-isopropylacrylamide)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (NON) triblock copolymer was studied by rheology over the concentration range 1-5 wt %. In comparison to the NON copolymer, gelation of the PON terpolymer was achieved at a much lower concentration, with a much sharper sol-gel transition. This is due to a stepwise gelation of PON terpolymers involving micellization at room temperature and gelation at elevated temperatures. The separation of micellization and gelation leads to the formation of a two-compartment network as observed by cryoTEM. The results highlight the intricate and tunable nanostructures and new properties accessible from ABC terpolymer hydrogels.     

Viscoelastic properties of polyacrylonitrile gels: dependence of sol-gel transition on concentration and aging time

Dynamic viscoelasticity was observed for polyacrylonitrile solution in dimethylacetamide in the process of sol-gel transition. The gel was prepared by the freezing and thawing method, i.e., the solution was frozen at −50°C and then kept at 25°C so as to undergo gelation. The longer the freezing time, the shorter was the gelation time. Also the higher the concentration, the shorter the gelation time. The strength of the network, S and critical exponent n at the transition point were evaluated. The S value increased with increasing concentration and was constant in value in spite of the variation of freezing time when the concentration of the solution was constant.     

κ-卡拉胶热可逆凝胶化行为研究

用固体小角激光散射方法研究κ 卡拉胶 (KC)的热可逆凝胶化行为 .以散色斑点的突停点温度为体系的凝胶化点Tgel,考察了溶液中加入Na+ ,K+ ,NH+4,Ca2 + ,Cu2 + ,Zn2 + 等抗衡离子对Tgel的影响 .结果是随抗衡离子浓度增大Tgel上升 ;Tgel与Na+ 的浓度呈线性关系 ,与K+ ,NH+4,Ca2 + ,Cu2 + ,Zn2 + 等离子浓度的平方根成线性关系 ;另外 ,还得到 30℃时KC在KCl盐溶液中的溶胶 凝胶相图 ,并对比了KC在NaCl溶液中透析前后Tgel的变化 .     

水溶性高分子弱凝胶体系凝胶化过程的Monte Carlo模拟

为研究弱凝胶的形成过程,并把高分子弱凝胶用于三次采油,采用三维Monte Carlo模拟了高分子溶液凝胶化过程. 模拟预测了凝胶化开始的时间,得到了凝胶化过程中分子量分布的演化规律和胶团生长的三维图像. 发现生成溶胶与凝胶团的歧化过程,初始聚合物的浓度对能否形成凝胶至关重要,低于临界浓度不能形成凝胶. 模拟了凝胶化速度和聚合物浓度以及交联剂浓度的关系,并与粘度随凝胶化时间变化的实验结果进行比较, 结果表明, 聚合物浓度较高时,浓度对交联反应的影响减弱,这一趋势与实验结果相一致.     

Thermoreversible gelation of poly(vinylidene fluoride) in phthalates: the influence of aliphatic chain length of solvents

Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) has been studied in a new series of solvents (phthalates), for example, dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), and dihexyl phthalate (DHP) as a function of temperature and polymer concentration, both by test tube tilting and dynamic light scattering (DLS) method. The effect of aliphatic chain length (n) of diesters on the gelation kinetics, structure/microstructure and morphology of PVDF gels has been examined. Gelation rate was found to increase with increasing aliphatic chain length of diester. DLS results indicate that the sol-gel transformation proceeds via two-steps: first, microgel domains were formed, and then the infinite three-dimensional (3D) network is established by connecting microgels through polymer chains. The crystallites are responsible for 3D network for gelation in phthalates, and alpha-polymorph is formed during gelation producing higher amount of crystallinity with increasing aliphatic chain length of diester. Morphology of the networks of dried gels in different phthalates showed that fibril thickness and lateral dimensions decrease with higher homologues of phthalates. The scattering intensity is fitted with Debye-Bueche model in small-angle neutron scattering and suggested that both the correlation length and interlamellar spacing increases with n. A model has been proposed, based on electronic structure calculations, to explain the conformation of PVDF chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length.     

Tuning thermal gelling behavior of <Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide based copolymer through introducing cucurbit[8]uril ternary complex on side-chain

Thermo-gelation polymers have attracted increasing attention over decades. However, rare facile tuning method of sol-gel transition temperature restricted the wider application. Preceding study indicated that supramolecular interactions demonstrated a powerful means to control the structure and property of polymeric materials. Here we designed an N-isopropylacrylamide (NIPAM) based thermo-sensitive copolymer with naphthyl (Np) on its side chain. Positive-charged side-chain ternary complex was formed with cucurbit[8]uril (CB[8]) and methylviologen (MV²⁺) via CB[8]-enhanced intermolecular charge-transfer (CT) interaction. Introducing the ternary complex CB[8]/MV²⁺/Np on side-chain altered microstructure of macromolecular chains and led to a strong tendency for thermo gelation. Altering content of CB[8] and MV²⁺ changed content of the positive-charged side-chain ternary complex and varied gelation temperature. Therefore, introducing supramolecular interaction endowed the hydrogel with tunable gelation property.     

聚N-异丙基丙烯酰胺/类水滑石复合水凝胶的制备及温敏性

以类水滑石(LDHs)和N-异丙基丙烯酰胺(NIPA)为原材料,采用自由基引发聚合制得了有机无机PNIPA/LDHs温度敏感复合水凝胶。 通过热重分析仪(TGA)、示差扫描量热仪(DSC)和扫描电子显微镜(SEM)等技术手段表征了材料的结构和性能。 结果表明,PNIPA/LDHs复合水凝胶在33 ℃左右可实现溶胶-凝胶的可逆性变化,LDHs质量分数基本不影响复合水凝胶的胶凝化温度和胶凝时间。 LDHs添加可使PNIPA/LDHs复合水凝胶的热稳定性较NIPA有大幅度提升。 随LDHs质量分数及n(Mg):n(Al)的增加,复合凝胶的吸热峰值稍有增加。 所合成PNIPA/LDHs复合水凝胶表面粗糙不平,具有一定的孔洞结构。     

Aqueous sol-gel synthesis route for the preparation of YAG: Evaluation of sol-gel process by mathematical regression model

Yttrium aluminium garnet (Y₃Al₅O₁₂, YAG) polycrystalline samples have been prepared by a simple aqueous sol-gel methodology. The influence of nineteen sol-gel processing variables on the formation of YAG has been investigated. Effects of different fabrication parameters on the phase purity and morphological properties of the compounds were studied by energy-dispersive spectrometry (EDS), X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). The parameters of the sol-gel processing such as pH of starting solution, concentration and nature of complexing ligand, temperature and duration of gelation, powder rehomogenization during annealing, duration and temperature of the final heat treatment were found to be the most significant. For the evaluation and verification of the experimental results the Brandon's model of a multiple regression was successfully used.     

Thermoreversible gelation of isotactic-rich poly(N-isopropylacrylamide) in water

We report the experimentally determined phase diagram for an aqueous solution of isotactic-rich poly(N-isopropylacrylamide) (PNiPAM) composed of the sol-gel transition curve and the cloud-point curve. The meso diad content of isotactic-rich PNiPAM is 64%, and it is soluble in water at low temperatures, but undergoes a sol-to-gel transition with increasing temperature in the investigated concentration range of 1.8 wt. %-6.0 wt. %. With a further increase in temperature, the system becomes turbid. The gel formation and clouding behavior are thermally reversible. This is the first observation of thermoreversible gelation under the cloud-point temperature for an aqueous solution of PNiPAM. On the basis of the determined phase diagram, we carried out light scattering experiments to characterize the sol-gel transition behavior as a function of temperature.     

Mechanisms of silk fibroin sol-gel transitions

Silk fibroin sol-gel transitions were studied by monitoring the process under various physicochemical conditions with optical spectroscopy at 550 nm. The secondary structural change of the fibroin from a disordered state in solution to a beta-sheet-rich conformation in the gel state was assessed by FTIR and CD over a range of fibroin concentrations, temperatures, and pH values. The structural changes were correlated to the degree of gelation based on changes in optical density at 550 nm. No detectable changes in the protein secondary structure (FTIR, CD) were found up to about 15% gelation (at 550 nm), indicating that these early stages of gelation are not accompanied by the formation of beta-sheets. Above 15%, the fraction of beta-sheet linearly increased with the degree of gelation. A pH dependency of gelation time was found with correlation to the predominant acidic side chains in the silk. Electrostatic interactions were related to the rate of gelation above neutral pH. The overall independencies of processing parameters including concentration, temperature, and pH on gel formation and protein structure can be related to primary sequence-specific features in the molecular organization of the fibroin protein. These findings clarify aspects of the self-assembly of this unique family of proteins as a route to gain control of material properties, as well as for new insight into the design of synthetic silk-biomimetic polymers with predictable solution and assembly properties.     

溶剂效应与海藻酸钠溶液溶胶-凝胶相转移

以溶质、溶剂间的协同作用作为高分子电解质体系的凝胶化条件，以松软粒子结构（Blob）间的三维修滤过程作为体系的凝胶化模型，从而将高分子电解质体系的凝胶化分为松软粒子的形成和渗滤过程．结合不同浓度海藻酸钠溶液的Ca2＋离子深加体系的凝胶化，分析了高分子电解质溶胶-凝胶相转移的实质，得到了体系相对粘度的临界指数k＝0．90～0．97和相转移的临界点fc与聚合物浓度间的幂次关系fc～c-0.65，明确了临界点附近相对粘度的幂次公式ηrel∝（fc-f）-k的适用范围．     

Rheological study of physical cross-linked quaternized cellulose hydrogels induced by β-glycerophosphate

As a weak base, β-glycerophosphate (β-GP) was used to spontaneously initiate gelation of quaternized cellulose (QC) solutions at body temperature. The QC/β-GP solutions are flowable below or at room temperature but gel rapidly under physiological conditions. In order to clarify the sol-gel transition process of the QC/β-GP systems, the complex was investigated by dynamic viscoelastic measurements. The shear storage modulus (G') and loss modulus (G″) as a function of (1) concentration of β-GP (c(β-GP)), (2) concentration of QC (c(QC)), (3) degree of substitution (DS; i.e., the average number of substituted hydroxyl groups in the anhydroglucose unit) of QC, (4) viscosity-average molecular weight (M(η)) of QC, and (5) solvent medium were studied by the oscillatory rheology. The sol-gel transition temperature of QC/β-GP solutions decreased with an increase of c(QC) and c(β-GP), the M(η) of QC, and a decrease of the DS of QC and pH of the solvent. The sol-gel transition temperature and time could be easily controlled by adjusting the concentrations of QC and β-GP, M(η) and DS of QC, and the solvent medium. Gels formed after heating were irreversible; i.e., after cooling to lower temperature they could not be dissolved to become liquid again. The aggregation and entanglement of QC chains, electrostatic interaction, and hydrogen bonding between QC and β-GP were the main factors responsible for the irreversible sol-gel transition behavior of QC/β-GP systems.     

A Systematic Study of the Use of DL-Tartaric Acid in the Synthesis of Silica Materials Obtained by the Sol-Gel Method

DL-tartaric acid was used as a template for the formation of silica nanotubes and spheres by the sol-gel method from tetraethylorthosilicate (TEOS) as silica source. The reactions were carried out in ethanol/water mixtures in the presence of aqueous ammonia, between 0°C and 75°C, using both stirred and non-stirred conditions. TEM and SEM images show that the yield and microstructure of the silica is influenced by the synthetic conditions (temperature, ammonia (aq) concentration, gelation time, solvent mixture). It was observed that the chiral form of the tartaric acid used and the diffusion of TEOS to the template determines the eventual silica structure.     

The Effect of HF in a Two-Step Sol-Gel Process of TEOS

The addition of HF to a TEOS-ethanol-water mixture results in faster gelation compared to a system without HF. When HF is added in the basic step the effect is most pronounced. The addition of HF in the acid step only results in an enhancement of the condensation reactions. The effect of HF in the acid step is largely determined by the H+ concentration due to the low acidity of HF. At low HF concentrations a maximum in gelation time is found as a function of the H+ concentration. At low H+ concentrations the F– enhances gelation; at high H+ concentrations the influence of F– is negligible but gelation is enhanced by the protons.     

用荧光光谱跟踪钙-海藻酸水溶液的Sol-gel转变

为了克服传统方法在测定凝胶化点的同时, 作用力对物理交联点的破坏和对大分子链运动的干扰, 探索和建立不施加应力的凝胶化点测定方法, 采用荧光光谱跟踪了异硫氰酸荧光黄(FITC)标记海藻酸与钙离子在水溶液中螯合的物理凝胶化. 随着凝胶化的进行, 荧光相对强度和各向异性比在凝胶化时间曲线的80 min时出现了明显的转折点, 与Winter方法得到的凝胶化点(80 min)完全一致. 因此可以利用FITC标记的荧光发射相对强度及各向异性比来决定钙-海藻酸体系凝胶化点.     

Self-assembly of water-soluble dendrimers into thermoreversible hydrogels and macroscopic fibers

Hydrogels and macroscopic fibers are formed through the salt-induced self-assembly of water-soluble polycationic phosphorus dendrimers. Interestingly, the hydrogels are thermoreversible and the sol-gel transition temperature can be easily tuned in a wide range of temperatures (approximately 2-80 degrees C). The effects of different parameters, such as salt nature, dendrimer generation, concentration, and temperature, on dendrimer aggregation are examined. The macroscopic fibers are prepared by flocculation under flow and observed using scanning electron microscopy (SEM) which reveals a microscopic fibrillar substructure. We interpret the gelation and flocculation of the polycationic dendrimers in terms of colloidal flocculation.     

Preparation of YSZ Microporous Membranes on Porous a-Alumina with Metal Chlorides as Precursor

A sol-gel process for YSZ membrane prepared with less expensive chemicals, ZrOCI₂·8H₂0 and YC1₃, has been developed. The sol viscosity as a function of concentration, acidity and temperature was brane formation processes were also studied. investigated. And gelation and Based on an optimized procedure memhole and crack-free YSZ membranes with the pore size less than 100 nm have obtained on coarse porous a-alumina.