Gelation of cassia gum by freezing and thawing

Aqueous solution of cassia gum (CG), which is categorized as a galactomannan polysaccharide having mannose/galactose ratio = 5/1, forms hydrogels by freezing and thawing. When frozen CG aqueous solution was thawed, transparent sol was separated from a turbid gel, i.e. syneresis occurred. Gel concentration ({(Mass of dry gel) / (Mass of gel)} × 100) increased with increasing CG concentration. Viscoelastic properties of CG hydrogels formed by freezing and thawing were investigated by thermomechanical analysis (TMA) in water using an oscillation mode at 0.05 Hz. Dynamic modulus (E′) increased from 3 kPa to ca. 5 kPa with increasing freezing rate. In contrast, E′ maintained a constant value regardless of repeating number of freezing and thawing. From TMA results, it is concluded that the density of cross-linking network structure depends on the size of ice formed by freezing. At the same time, the low E′ value of CG gels is ascribed to the fact that association of galactosyl side group is disturbed by the stiff chain attributed to the unsubstituted region of CG.     

Effect of thermal history on kappa-carrageenan hydrogelation by differential scanning calorimetry

The effect of thermal history on gel-sol transition was investigated by highly sensitive differential scanning calorimetry (DSC) in order to clarify the non-equilibrium state of κ-carrageenan hydrogels. κ-Carrageenan with a concentration from 0.5% to 5.0% was used. When concentration of solution was lower than 2.0%, homogeneous κ-carrageenan gel was formed when aqueous solution was fully equilibrated. When concentration exceeded 3.0%, a sub-peak could be observed at the low temperature side of the main peak. It was indicated that helices having various sizes and different kinds of defects are present in the junction zone. Thermal histories, such as cooling rate from the sol state or annealing at around gel-sol transition temperature, markedly affect the junction zone formation. A large junction zone is formed at a slow gelation rate, and also that many small junction zones form at a fast gelation speed.     

超声作用对超分子水凝胶结构及性能的影响

采用一种简便方法,以戊二酸酐和3-羟基2-氨基吡啶合成了一种新的凝胶因子3-羟基-2-[N-(羧丙基羰基)氨基]吡啶(简称SWG),并采用IR、1H-NMR和元素分析确认其结构.红外光谱中2576cm-1和1906cm-1峰的出现证明羧基与吡啶基间形成了氢键.于20°C的超声仪中冷却1.5wt%的SWG水溶液,在1min内形成了超分子水凝胶.基于扫描电子显微镜(SEM)、示差扫描量热仪(DSC)及凝胶-溶液破坏温度(Tgel)分析表明,在凝胶化过程中,SWG自组装形成纤维状网络结构.随着超声功率由200W增加到500W,纤维宽度由8μm逐渐减小到2μm,凝胶网络密度逐渐增大,可冻结水含量逐渐增加,Tgel从60℃增加到70℃.因此,通过改变超声功率可有效控制凝胶的结构及性能.进一步增加SWG的浓度,可使纤维宽度及网络密度均增加,并导致凝胶的Tgel升高.采用多晶X射线衍射仪(XRD)分析表明,在凝胶形成过程中,SWG分子自组装出现明显的择优取向.     

Preparation and characterization of novel physically cross-linked hydrogels composed of poly(vinyl alcohol) and amine-terminated polyamidoamine dendrimer

Poly(vinyl alcohol) (PVA) and polyamidoamine (PAMAM) dendrimers are water-soluble, biocompatible and biodegradable polymers, which have been widely applied in biomedical fields. In this paper, novel physically cross-linked hydrogels composed of PVA and amine-terminated PAMAM dendrimer G6-NH(2) were prepared by cyclic freezing/thawing treatment of aqueous solutions containing PVA and G6-NH(2). The FT-IR analysis and elemental analysis indicated that PAMAM dendrimer G6-NH(2) was successfully introduced into the formed hydrogels, possibly via hydrogen bonds among hydroxyl groups, amide groups and amino groups in PVA and PAMAM dendrimer in the process of freezing-thawing cycle. Compared with physically cross-linked PVA hydrogel, PVA/G6-NH(2) hydrogels show higher swelling ratios and faster re-swelling rate due to the higher hydrophilicity of PAMAM dendrimer G6-NH(2). Higher contents of G6-NH(2) in PVA/G6-NH(2) hydrogels resulted in higher swelling ratios and faster re-swelling rates. With increasing freezing/thawing cyclic times, the swelling ratios and re-swelling rates of PVA/G6-NH(2) hydrogels decreased, which is similar to that of physically cross-linked PVA hydrogel. Combining the special host property of polyamidoamine dendrimer, these novel physically cross-linked hydrogels are expected to have potential use in drug delivery, including improving drug-loading amounts in hydrogels and prolonging drug release time. Swelling ratios of physically cross-linked PVA/G6-NH(2)-50 hydrogels prepared by three, six, nine freezing/thawing cycles. The swelling equilibrium experiments were carried out in distilled water at 25 degrees C.     

Characterization of water in polysaccharide hydrogels by DSC

Water molecules in hydrogels were classified into three categories according to phase transition behavior; non-freezing, freezing bound and free water. Melting, crystallization, and glass transition of water in hydrogels reflected the state of the water interacting with polysaccharides. Freezing bound water formed metastable ice by slow cooling and formed amorphous ice by quenching. From the isothermal crystallization measurement, nucleation rate and crystal growth rate were obtained. The crystal growth rate of freezing bound water was about ten times slower than that of free water. The DSC characterization of water in hydrogels was summarized.

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Zusammenfassung In übereinstimmung mit dem Phasenum wandlungsverhalten werden Wassermoleküle in Hydrogelen in drei Kategorien eingestuft; nicht gefrierendes, gefrierendes gebundenes und freies Wasser. Das Schmelzen, die Kristallisation und die Glasumwandlung von Wasser in Hydrogen wiederspiegeln den Zustand von Wasser, welches in Wechselwirkung mit Polysacchariden tritt. Gefrierendes gebundenes Wasser bildet metastabiles Eis durch langsames Abkühlen und amorphes Eis durch Abschrecken. Anhand isothermer Kristallisationsmessungen wurden Keimbildungsgeschwindigkeit und Kristallwachstumsgeschwindigkeit erhalten. Die Kristallwachstumsgeschwindigkeit von gefrierendem gebundenem Wasser liegt zehnmal niedriger als die von freiem Wasser.

     

物理交联聚乙烯醇/羟基末端聚酰胺-胺树型高分子水凝胶的制备与性质研究

聚乙烯醇(PVA)是一种水溶性的生物相容性好并可降解的合成高分子,PVA通过化学或物理方法交联可以形成水凝胶,PVA与聚丙烯酸(PAA)、聚N-异丙基丙烯酰胺(PNIPA)、聚乙烯基吡咯烷酮(PVP)及壳聚糖形成互穿网络型水凝胶,改善凝胶的性质,另一方面,树型高分子是一     

Amphoteric poly(vinyl alcohol) gel as a force generator: Observation of microporous gel formation with cryo-SEM

Amphoteric PVA hydrogels were made from mixed aqueous solutions of poly(acrylic acid), poly(allylamine) hydrochloride and poly(vinyl alcohol) by repetitive freezing and thawing. The hydrogen bond induced gelation produced an elastic and tough microporous hydrogel which generated tension as high as 0.3MPa corresponding to the solvent exchange from water to acetone. In this paper we show the microscopic structure of hydrogels determined by means of cryo-SEM technique and X-ray analysis.     

Analysis of swelling process of protein by positron annihilation lifetime spectroscopy and differential scanning calorimetry

The change in the nanoscopic structure and bound state of water in the protein gel were investigated using positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). Gelatin was used as a protein. To examine the bound state of water in gelatin gel, the amount of freezing and nonfreezing water in gelatin gels were evaluated by fusion enthalpy of DSC curves. Below water content of 40% (w/w), the whole amount of water was nonfreezing water, whereas above water content of 40% (w/w), the amount of freezing water increased according to increase in water content. To investigate the nanoscopic spatial structure under coexistence of polymer and water, positron annihilation lifetime measurement was performed. The lifetime of o‐Ps or the pore size increased according to increase in water content, particularly below the water content of 40% (w/w). When the water penetrates into the gelatin network, the water molecules form hydrogen bonds with hydrophilic groups inside the helical structure in gelatin gel. The water molecules inside the helical structure expand the structure outward, leading to increase in pore size. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2031–2037, 2007.     

热分析技术研究疏水缔合微区对含氟丙烯酸酯共聚物凝胶中水状态的影响

丙烯酸 (AA)与不同含量的 2 (N 乙基全氟辛烷基磺酰胺 )乙基甲基丙烯酸酯 (FMA)在叔丁醇中进行自由基共聚 ,合成了一系列疏水改性共聚物凝胶 .用示差扫描量热法 (DSC) ,热失重分析 (TGA)研究了含氟疏水改性凝胶中水的状态 ,证明FMA改性凝胶中存在 3种状态的水 ,即不可冻结水 ,可冻结结合水和自由水 .对于未改性凝胶和FMA含量为 1mol%的疏水改性凝胶 ,在凝胶中水含量分别低于 0 60 9mol%和 0 698mol%时 ,DSC升温曲线上观察不到熔融峰 ,说明凝胶中存在不可冻结水 .提出计算凝胶中 3种状态的水含量和熔融焓的理论假设 .计算出FMA含量为 1mol%的疏水改性凝胶中自由水和可冻结结合水的摩尔熔融焓分别为75 89 0 3和 6864 0 6J mol.计算结果表明疏水改性后 ,单位聚合物单元上结合的可冻结水数目 (n1 )和非冻结水的数目 (n0 )都增加     

Morphology of optically anisotropic agarose hydrogel prepared by directional freezing

Agarose hydrogels which showed optical anisotropy were obtained by the directional freezing of starting isotropic gels under a temperature gradient. The directional freezing caused a crystallization of many isolated ice crystal phases, leaving a honeycomb-like gel phase with a higher polymer content. The crystallographic c-axis of the ice crystals was directed to the temperature gradient. X-ray and optical analyses showed that agarose chains had a strong planar orientation along the walls'side surfaces, which were parallel to the equatorial planes of the ice crystals.Scanning electron microscopy showed that the wall consisted of a large number of sheets stacked along the wall thickness; in each sheet, agarose fibrillar structures were found to be densely aligned. With the application of repeated freezing and thawing, the anisotropy of the segregated gel phases increased.     

States of water in poly(vinyl alcohol) hydrogels

Poly(vinyl alcohol) (PVA) hydrogels with various water contents were prepared from 10 wt% aqueous solutions of mixtures of PVA and anionic poly(styrene sulfonic acid) sodium salt (NaPSS) by casting, dehydrating, and then extracting NaPSS. The existence of three types of ice were suggested by differential scanning calorimetry (DSC) measurements for every frozen hydrogel. In the frozen hydrogels the states of water, except unfrozen water, were ice of free water and disordered ice crystals. The mobility and activation energy for motion of water molecules in unfrozen hydrogels were investigated by using pulsed nuclear magnetic resonance (PNMR) measurements. It was concluded that there are two states of water in the unfrozen hydrogel, i.e. unfrozen water and disordered water which is mainly formed in narrow apertures in the hydrogel. The discrepancy between the DSC and PNMR measurements was explained by a structural transformation during cooling.     

超分子均苯四甲酸/对羟基吡啶水凝胶中纤维状聚集体结构与性能的关系

采用1H-NMR和FT-IR表征了在摩尔比1∶4条件下,由均苯四甲酸和对羟基吡啶合成的一种凝胶因子(G2).通过在室温下冷却G2的水溶液,形成了超分子水凝胶.采用扫描电子显微镜(SEM)、示差扫描量热仪(DSC)和流变仪等多种技术研究了冷却速率对凝胶的组装纤维结构及宏观性能的影响.随着冷却速度的降低,纤维尺寸变大而凝胶的稳定性降低.因此,可以通过环境因素来控制凝胶的性能.采用流变仪分析表明凝胶具有高的机械强度.DSC分析结果表明随着凝胶因子浓度的增加,凝胶中可冻结水的含量降低.相对于在摩尔比1∶2条件下,由均苯四甲酸和对羟基吡啶合成的凝胶因子G1,在相同浓度下,G2在更高的最低凝胶因子浓度(MGC)使水凝胶,并且得到的凝胶具有更低的凝胶-溶胶破坏温度(Tgel).利用环境扫描电镜(ESEM)直接观测了实际含水状态下凝胶的形貌,结果表明采用常规SEM观测到的纤维状网络与ESEM的结果一致,这说明在干燥过程中形貌并未发生太大变化.组装体结构和性能关系有助于认识凝胶形成机理并使凝胶满足不同的应用.     

Effect of non-associated electrolyte solutions on the behaviour of poly(vinyl alcohol)-based hydrogels

The aim of this paper is to present the behaviour of the poly(vinyl alcohol) hydrogels [PVA-HG] in sodium and potassium chlorides aqueous solutions, due to their interactions. The tested [PVA-HG]-s have been obtained by repeated freezing and thawing cycles. White, heterogeneous hydrogels have been obtained. These hydrogels exhibit a mechanical active behaviour at their contact with electrolytes aqueous solutions, manifested by important changing in mass, volume and density of the hydrogel samples. These modifications could be explained by water elimination from the hydrogels that initially reached the equilibrium of swelling. The kinetic of the water desorption and the reversibility of this process, have been studied and some of the factors that influence this behaviour have been evidenced. The sensitivity of PVA hydrogels to electrolyte nature and concentration could be used in sensors design and also could explain some aspects of electrolytes diffusion through PVA membranes and targeted drugs delivery.     

The effect of hydration on the thermal behaviour of hydrophilic non-aqueous gels stabilised by Carbopol 974P

A thermal analysis of the effect of hydration of non-aqueous polymer-stabilised gels was investigated using differential scanning calorimetry (DSC). The interaction of water with the polymer and its distribution within the gel are critical to the physicochemical behaviour of the gel, and consequently affects the utility of the gel matrix as a drug delivery vehicle. Addition of water at levels up to and including 50% (w/w) did not result in an observable freezing event in the thermogram. However, at 60 and 80% (w/w) water, phase transitions were observed, the magnitude of which were found to be independent of the annealing time within the range used. The observed melting enthalpies increased as the water concentration increased for all formulations, but were always smaller than that of pure water. There was no evidence of multiple transitions that might be attributed to different populations of water molecules. However, the results demonstrate that DSC can be employed to differentiate between freezable and non-freezable water, in these particular formulations.     

水在纤维素水凝胶中的存在状态及对纤维素结晶的影响

制备了高度水合状态的纤维素凝胶, 研究了水在凝胶中的存在状态及其对纤维素结晶的影响. 结果表明, 水在纤维素水凝胶中存在非冻结水、 可冻结水和自由水3种状态. 非冻结水饱和含量为一般纤维素吸附水中不可冻结水的5倍以上, 高达1.6 g/g. 纤维素在水合状态下结晶受到抑制, 随着水含量的减小, 结晶会趋于完善. 在环境温度下, 当纤维素中只存在非冻结水时, 其与纤维素分子链间氢键作用力不稳定, 对纤维素结晶抑制作用较弱, 纤维素结晶比较完善, 导致纤维素断裂时表现为脆性断裂. 水介质的引入有望为纤维素的利用开发提供一种新的思路.     

Rational Fabrication of Anti‐Freezing,Non‐Drying Tough Organohydrogels by One‐Pot Solvent Displacement

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti‐freezing, non‐drying CPA‐based organohydrogels with long‐term stability by partially displacing water molecules within the pre‐fabricated hydrogels. CPA‐based Ca‐alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA‐based organohydrogels remain unfrozen and mechanically flexible even up to −70 °C and are stable under ambient conditions or even vacuum.     

Rational Fabrication of Anti‐Freezing,Non‐Drying Tough Organohydrogels by One‐Pot Solvent Displacement

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti‐freezing, non‐drying CPA‐based organohydrogels with long‐term stability by partially displacing water molecules within the pre‐fabricated hydrogels. CPA‐based Ca‐alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA‐based organohydrogels remain unfrozen and mechanically flexible even up to ?70 °C and are stable under ambient conditions or even vacuum.     

Synthesis and Properties of Hydrogels Based on Chitosan and Poly(Vinyl Alcohol) Crosslinked by Genipin

Semi‐interpenetrating polymeric networks of chitosan and poly(vinyl alcohol) [PVA] were prepared by varying the ratio of the constituents. The hydrogels were crosslinked using genipin, a naturally occurring nontoxic cross‐linking agent. The swelling behavior of these hydrogels was studied by immersing the films in deionized water at various temperatures and in buffer solutions of different pH. The states of water in the hydrogels, swollen at 25°C and pH 7, were determined using Differential Scanning Calorimetry (DSC). The swelling behavior of the gels was found to be dependent on temperature and pH of the medium. The amount of freezing water in the swollen hydrogels increased, whereas the amount of nonfreezing bound water remained more or less the same with increasing PVA concentration.     

温度敏感聚(N-异丙基丙烯酰胺)/聚(乙烯醇)水凝胶的制备及性能研究

聚 (N -异丙基丙烯酰胺 ) (PNIPA)水凝胶具有温度敏感性 ,其在 33℃左右有一个相转变温度或较低临界溶解温度 (L CST) [1,2 ] .当外界温度低于 LCST时 ,PNIPA水凝胶吸水溶胀 ;而当外界温度高于L CST时 ,PNIPA水凝胶剧烈收缩失水 ,发生相分离 .这种相分离特性应用于药物的控制释放 [3] .固定化酶[4 ] 和循环吸收剂 [5] 等领域 .然而 ,通常的 PNIPA水凝胶是通过化学键交联而成的三维网络聚合物 ,很难发生解体或进行生物降解 ,其在某些特定场合 (如药物的体内释放等 )受到一定限制 .聚乙烯醇 (PVA)的亲水性和生物相容性较好 ,是…     

聚乙烯醇水凝胶溶胀特性研究

在前文对聚乙烯醇水溶液冰冻凝胶化浓度依赖性研究基础上，对接触浓度（Ｃ）以上聚乙烯醇水溶液通过冰冻－融化处理，制得了一种含水率高达９５～９８％的水凝胶．系统研究了该水凝胶在蒸馏水中的溶胀及溶解特性．得到了一个与实验结果相吻合的溶胀动力学方程：Ｑ１＝Ｑｅ－（Ｑｅ－ＱＯ）／ｅｋｔ，及平衡溶胀比Ｑｅ与浓度之间的定量关系：Ｑｅ＝６０．３－４．４５×１０２Ｃ．发现当冰冻－融化次数Ｎ≤５时，平衡溶胀比Ｑｅ及溶解量Ｗ与冰冻－融化次数（Ｎ）间满足幂函数关系：Ｑｅ。Ｗ通过对聚乙烯醇水凝胶平衡溶胀比与经冰冻处理的聚乙烯醇水溶液特性粘数进行比较，发现反映链间氢键凝聚缠结效应与反映链内氢键凝聚缠结效应的定量指标具有等效性．