吡嗪二羧酸-三聚氰胺双组分水凝胶的制备及性能

以3,6-二甲基-2,5-吡嗪二羧酸(P)和三聚氰胺(M)为组分,采用不同的摩尔比(1∶1,1∶2,1∶3)混合配制了3个样品PM11,PM12和PM13,并对其凝胶性能进行了测试.实验结果表明,PM能在水中及部分含水有机溶剂中形成稳定的凝胶,这些凝胶对酸碱具有良好的响应性能.采用扫描电子显微镜分析了3种水凝胶的微观形貌,均为纤维状的网络结构;红外光谱及紫外光谱测试结果表明氢键是形成凝胶的关键驱动力;XRD测试结果显示凝胶为层状结构.对PM12在不同pH值的水中的凝胶性能测试结果表明,在pH=3~11的范围内PM12均能形成凝胶.测试了PM12在混合溶剂中的凝胶性能,并将测试结果与混合溶剂的Hansen溶解度参数关联,以便用于分析溶剂与凝胶因子间的相互作用,所得结果亦表明氢键在凝胶形成的过程中起重要作用.     

Formation of supramolecular hydrogels with controlled microstructures and stability via molecular assembling in a two-component system

Two isomeric building units, 4-oxo-4-(2-pyridinylamino) butanoic acid (defined as G1) and 4-oxo-4-(3-pyridinylamino) butanoic acid (defined as G2) formed fiber- and tree-like crystals in aqueous solutions, respectively. The crystal formation process of G1 was suggested based on the layered cross section of an individual crystal and the single crystal structure. Through cooling the aqueous solutions of their mixtures under G1/G2 molar ratios ranged from 7/1 to 1/3, a series of supramolecular hydrogels were formed based on hydrogen bonds as the main driving force. As decreasing G1/G2 ratios, the first observed aggregates in solution changed from fiber to particle form, while the gelating time became longer and longer. At the collapsing temperature, the gels formed at G1/G2 ratio 3/1 kept the original gel shape but released water, while at G1/G2 ratio 2/1 they broke into pieces without releasing water. The "dropping ball" experiment indicated that the highest gel-to-sol dissociation temperature (T(gel)) is obtained at G1/G2 ratio of 2/1. As measured by UV-vis spectroscopy, the two building units distributed uniformly within the gel formed at G1/G2 ratio of 1/1, indicating they assembled together in forming hydrogel. The scanning electron microscope (SEM) and infrared spectrometer (FT-IR) analysis of the dried samples indicated that the backbone shape changed from fiber to sheet and the content of free carboxyl groups increased with decreasing G1/G2 ratios, therefore resulting in hydrogels with different stability. The simple gelator structures and the possibility in controlling gel structure and stability make the hydrogels suitable for various uses.     

Studies on preparation and properties of NIPAAm/hydrophobic monomer copolymeric hydrogels

A series of thermoreversible copolymeric hydrogels with various molar ratios of N-isopropylacrylamide (NIPAAm) and hydrophobic monomers such as 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (OFPMA) and n-butyl methacrylate (BMA) were prepared by emulsion polymerization. The effect of hydrophobic monomer on the swelling behavior and mechanical properties of the present copolymeric hydrogels was investigated. Results showed that the equilibrium swelling ratio and critical gel transition temperature (CGTT) decreased with an increase of the content of hydrophobic monomer, but the gel strength of the gel increased with an increase of the content of hydrophobic monomer. Due to stronger hydrophobicity of OFPMA, the NIPAAm/OFPMA copolymeric hydrogels had lower swelling ratios and higher gel strengths than NIPAAm/BMA copolymeric gels.     

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.     

Hydrogen bond-reinforced double-network hydrogels with ultrahigh elastic modulus and shape memory property

Development of tough hydrogels has greatly expanded their applications as load-bearing materials. However, the elastic modulus of tough hydrogels is usually lower than 1 MPa. It remains a challenge to design tough hydrogels with high modulus. We report here a series of tough double-network (DN) hydrogels with ultrahigh elastic modulus (up to 200 MPa) by forming robust hydrogen bonds between the first poly(acrylic acid) network and the second poly(N-isopropyl acrylamide) network. The dense cooperative hydrogen bonds greatly reduce the segmental mobility and thus improve the rigidity of gel matrix. Owing to the dynamic nature of hydrogen bonds, the modulus of hydrogels is strongly influenced by temperature and pH, affording the gels shape memory property. The strategy by forming robust noncovalent bonds between interpenetrating networks should be applicable to other systems for designing tough and versatile hydrogels with diverse promising applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1281–1286     

Two-component hydrogels comprising fatty acids and amines: structure, properties, and application as a template for the synthesis of metal nanoparticles

Stearic acid or eicosanoic acid mixed with di- or oligomeric amines in specific molar ratios form stable gels in water. The formation of such hydrogels depends on the hydrophobicity of the fatty acid, and also on the type of amine used. The gelation properties of these two-component systems were investigated using electron microscopy, FTIR spectroscopy, 1H NMR spectroscopy, differential scanning calorimetry (DSC), and both single-crystal and cast-film X-ray diffraction. Results of FTIR spectral analysis suggest salt formation during gelation. 1H NMR analysis of the gels indicates that the fatty acid chains are immobilized in the gel state and when the gel melts, these chains regain their mobility. Analysis of DSC data indicates that increase in the spacer length in the di-/oligomeric amine lowers the gel-melting temperature. Two of these gelator salts developed into crystals and structural details of such systems could be secured by single-crystal X-ray diffraction analysis. The structural information of the salts thus obtained was compared with the XRD data of the self-supporting films of those gels. Such analyses provided pertinent structural insight into the supramolecular interactions that prevail within these gelator assemblies. Analysis of the crystal structure confirmed that multilayered lamellar aggregates exist in the gel and it also showed that the three-dimensional ordering observed in the crystalline phase is retained in only one direction in the gel state. Finally, the hydrogel was used as a medium for the synthesis of silver nanoparticles. The nanoparticles were found to position themselves on the fibers and produced a long, ordered assembly of gel-nanoparticle composite.     

CdS量子点的一步法合成及量子产率

以油酸为配体,十八烯为溶剂,采用一步法合成了CdS量子点,研究了反应温度、反应时间和Cd/S的摩尔比对量子点光谱性能的影响.X射线衍射(XRD)和高分辨透射电镜(HRTEM)测试结果表明,所获得的CdS量子点为立方闪锌矿结构,且尺寸分布均一,结晶度高,其较强的带边发光、尖锐的紫外吸收峰以及狭窄的荧光发射峰进一步表明量子...     

Influence of Diffusion on the Kinetics of an Enzyme-Catalyzed Reaction in Gelatin-Based Gels

The influence of mass-transport limitations on the initial reaction rates of a lipase-catalyzed stereoselective esterification reaction has been investigated for two structurally different gelatin-based gels. The time to reach equilibrium is much longer in pelleted hydrogels (pseudo-solid aqueous gels; PAGs) than in pelleted microemulsion-based gels (MBGs). R/S-(+/-)-2-Octanol and hexanoic acid were used as substrates. The reaction takes place by imbibition of the substrate-containing organic solvent into pores of the pelleted gel. To minimize the diffusion distances, the macroscopic surface areas of the gels were increased by granulating the gel pellets. The experimentally obtained initial reaction rates in granules were in good agreement with the theoretically obtained values from extrapolation to infinitely large areas. However, the still low initial reaction rates in the hydrogels compared to those in microemulsion-based gels cannot be explained by diffusion limitations. This finding was supported by the similar activation energies in both gels in granulated form. Changes in apparent molar standard enthalpy, entropy, and Gibbs energy for the activated complex formation were also estimated. The low reaction rate in hydrogels might thus be due to partial denaturation of the enzyme during the preparation step, to higher surface energy, or to the influence of a different solvent environment on the enzyme in these gels than in the microemulsion-based gels. Copyright 2001 Academic Press.     

A new pH and thermo-responsive chiral hydrogel for stimulated release

Chirality of the amphiphile to promote gelation in the given solvent medium is narrated in a new catanionic surfactant mixture from a twin-chiral, twin-tailed surfactant derived from tartaric acid and cetyltrimethylammonium bromide (CTAB). The surfactant bis(decyloxy) succinic acid (BDSA), a chiral Gemini-type surfactant with a rigid spacer, in association with CTAB formed pH and temperature responsive vesicles and hydrogels. Molecular chirality gave rise to supertwisted fibrillar hydrogels at a BDSA:CTAB molar ratio of 1:2 and in 31% water content. The hydrogels from enantiomeric BDSA were reversibly pH and irreversibly temperature responsive at pH<6.2 and at 55 degrees C, respectively, whereas the corresponding sodium succinates formed transparent clear gels reversible to both pH and temperature. The hydrogels were able to entrap and release model dye molecules, Rhodamine B, and Congo red, responding to thermal and pH stimuli. Circular dichroism unraveled the chiro-optical behavior of the assembled fibers, allowing monitoring of aggregation and packing. The presence and the relative configuration of the stereogenic centers in the structure of this low molecular weight gelator have been observed to be critical to form gels. The high curvature Gaussian gel network was modeled based on the chiral elastic membrane approach and the pitch angle of the Gaussian twist was estimated to be 45 degrees.     

The influence of cold treatment on properties of temperature-sensitive poly(N-isopropylacrylamide) hydrogels

Poly(N-isopropylacrylamide) (PNIPAAm) hydrogel exhibits a response to external temperature variation and shrinks in volume abruptly as the temperature is increased above its lower critical solution temperature. It has great potential applications in biomedical fields. A rapid response rate is essential, especially when this material is designed as an on-off switch for targeted drug delivery. However, due to the appearance of a thick, dense skin layer on the hydrogel surface during the shrinking process, the deswelling rate of conventional PNIPAAm gels is low. In this article, a novel method is proposed to modify the surface morphology of PNIPAAm gel, in which the swollen gels are frozen at low temperature (-20 degrees C). The scanning electron micrographs revealed that a fishnet-like skin layer appeared on the surfaces of the cold-treated gels. Dramatically rapid deswelling was achieved with the cold-treated gels since the fishnet-like structure with numerous small pores prevented the formation of a dense, thick skin layer during the deswelling process, which commonly occurs in normal PNIPAAm hydrogels. Prolonging the cold treatment from 1 day to 10 days resulted in a slightly higher deswelling rate. Rearrangement of the hydrogel matrix structure during the freezing process might contribute to the formation of the fishnet-like skin layer. The water uptake of the hydrogels increased nearly in proportion to the square root of time, indicating that the reswelling rate of hydrogels was controlled predominantly by water diffusion into the network. However, there were no significant differences in the equilibrated swelling ratio and reswelling kinetics at room temperature (22 degrees C) between normal gels and cold-treated gels, which implied that cold treatment did not change bulk porosity and gel tortuosity much.     

The incorporation of carboxylate groups into temperature‐responsive poly(N‐isopropylacrylamide)‐based hydrogels promotes rapid gel shrinking

Aqueous gel deswelling rates for copolymer hydrogels comprising N‐isopropylacrylamide (IPAAm) and 2‐carboxyisopropylacrylamide (CIPAAm) in response to increasing temperatures were investigated. Compared with pure IPAAm‐based gels, IPAAm–CIPAAm gels shrink very rapidly in response to small temperature increases across their lower critical solution temperature (their volume is reduced by five‐sixths within 60 s). Shrinking rates for these hydrogels increase with increasing CIPAAm content. In contrast, structurally analogous IPAAm–acrylic acid (AAc) copolymer gels lose their temperature sensitivity with the introduction of only a few mole percent of AAc. Additionally, deswelling rates of IPAAm–AAc gels decrease with increasing AAc content. These results indicate that IPAAm–CIPAAm copolymer gels behave distinctly from IPAAm–AAc systems even if both comonomers, CIPAAm and AAc, possess carboxylic acid groups. Thus, we propose that the sensitive deswelling behavior for IPAAm–CIPAAm gels results from strong hydrophobic chain aggregation maintained between network polymer chains due to the similar chemical structures of CIPAAm and IPAAm. This structural homology facilitates aggregation of chain isopropylamide groups for both IPAAm and CIPAAm sequences with increasing temperature. The incorporation of AAc, however, shows no structural homology to IPAAm, inhibiting chain aggregation and limiting collapse. A functionalized temperature‐sensitive poly(N‐isopropylacrylamide) hydrogel containing carboxylic acid groups is possible with CIPAAm, producing rapid and large volume changes in response to smaller temperature changes. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 335–342, 2001     

Enhancement effect of lauric acid on the rectal absorption of propranolol from suppository in rats.

In a previous paper, we have demonstrated that medium chain fatty acids significantly enhance the in vitro rectal absorption of propranolol (PL) and that the enhancement may be partly due to the formation of a complex with a fatty acid at a 1:1 molar ratio. To confirm in vivo the enhancement effect of lauric acid on PL absorption, PL suppositories with lauric acid at various molar ratios were administered to rat rectum. PL absorption from Witepsol and macrogol suppositories with lauric acid at a 1:1 molar ratio was much larger than that after PL alone and the 1:2 or 1:3 molar ratio ones. The bioavailability (BA) after administration of the 1:1 molar ratio suppository (PL, 4 mg/kg) was 1.6- and 2.1-fold for the Witepsol and macrogol formulations respectively, compared with that after PL alone. A similar result was obtained with the PL solid dispersion suppository with lauric acid at a 1:1 molar ratio, showing a 1.7-fold higher BA compared with PL alone. The release of PL from the macrogol suppository was significantly faster at a 1:1 molar ratio than that of other preparations, but not so in the solid dispersion suppository. There was not good agreement between the release rates of PL from the suppositories and the plasma levels after dosing. These results supported the concept that a portion of PL, by forming a 1:1 complex with lauric acid, would penetrate across the rectal mucosa more easily than PL alone.     

Synthesis and characterization of thermoresponsive isopropylacrylamide-acrylamide hydrogels

In this study, hydrogels of poly(N-isopropylacrylamide-co-acrylamide) having a thermoresponsive character were prepared by a redox polymerization method. NIPAM-co-AAm hydrogels with different thermoresponsive properties were obtained by changing the initial NIPAM/AAm mole ratio and crosslinker concentration.Equilibrium-swelling ratio, dynamic swelling ratio and dynamic deswelling ratio were evaluated for all hydrogel systems. The fast shrinking was observed with all gels. The time required for equilibrium shrinking increased with the increase of acrylamide content in the gel.     

PAMPS/SiO_2杂化水凝胶的合成及电场敏感性研究

以2-丙烯酰胺-2-甲基丙磺酸(AMPS)为有机原料,正硅酸乙酯(TEOS)为无机原料,过硫酸钾为引发剂,N,N'-亚甲基双丙烯酰胺为交联剂,通过原位-凝胶水溶液聚合法合成了一系列不同二氧化硅含量和不同聚离子浓度的聚(2-丙烯酰胺-2-甲基丙磺酸)/二氧化硅杂化电场敏感性水凝胶.通过扫描电子显微镜(SEM)表征凝胶的结构,研究水凝胶在去离子水以及氯化钠溶液中的溶胀和消溶胀行为.结果表明,系列凝胶的平衡溶胀度介于224.9至325.6之间,复合凝胶的溶胀速率随TEOS用量的增加而降低;除理想杂化凝胶外,随着聚离子浓度的升高,凝胶在氯化钠溶液中的消溶胀速率逐渐减小.对凝胶的电场敏感性研究表明,当聚离子浓度大于氯化钠溶液浓度时,凝胶进一步溶胀,反之则消溶胀,其中杂化凝胶的再溶胀性能减弱,而消溶胀行为变得更为明显.同时制得的理想杂化凝胶,较纯有机凝胶具有更为理想的力学性能,最大抗压缩强度可达23.4 MPa.     

Swelling behavior of ionically cross-linked polyampholytic hydrogels in varied salt solutions

Ionically cross-linked polyampholytic hydrogels were synthesized by redox copolymerization of acrylamide and an ionic complex of (N,N-diethylamino)ethyl methacrylate and acrylic acid (designated as PADA hydrogel). The swelling behavior of the hydrogels in water indicated that a minimal equilibrium swelling ratio is found when the molar ratio of anionic/cationic monomers was 1.55. In NaCl solution, the hydrogels exhibited the typical swelling behavior of conventional polyampholytic gels. Their equilibrium swelling ratios increased with an increase in the NaCl concentration. In solutions of multivalent ions (CaCl₂ and trisodium citrate solutions), the equilibrium swelling ratios of the hydrogels increased first and were then followed by a decrease with an increase in salt concentration. Interestingly, an unexpected abrupt swelling phenomenon was observed when the fully swollen hydrogels in salt solution were transmitted to pure water. The unique swelling behavior of PADA hydrogels depends not only on the molar ratio of the anionic/cationic monomers but also on the valency of the ions.     

Swelling and drug releasing properties of poly(N-isopropylacrylamide) thermo-sensitive copolymer gels

A series of N-isopropylacrylamide (NIPAAm) copolymer gels with different hydrophilicities were prepared from NIPAAm, hydrophilic acrylamide (AAm) and hydrophobic butyl methacrylate (BMA). The swelling and thermo-responsive properties of PNIPAAm P (NIPAm-co-BMA) and P(NIPAm-co-AAm) copolymer hydrogels were investigated. The drug loading and releasing behaviors for two kinds of model drug with different hydrophilicities were studied. The result shows that the copolymer gels present negative thermo-sensitivities. The lower critical solution temperature (LCST), equilibrium swelling degree and the initial swelling rate increase as the hydrophilicity of gels increases when the temperature is below the LCST. With increasing gel hydrophilicity the loading ratio for sodium salicylate increases, while for salicylic acid, the reverse is observed. The initial drug releasing rate of sodium salicylate and salicylic acid also increase with increasing gel hydrophilicity. The initial drug releasing rate of sodium salicylate is significantly higher than that of salicylic acid. For salicylic acid which is less hydrophilic, the equilibrium releasing ratio at high temperature is lower than that at low temperature while for sodium salicylate which is more hydrophilic, the equilibrium releasing ratio at high temperature is almost the same as that at low temperature. Equilibrium releasing ratios of the three gels are significantly different from each other for salicylic acid when the temperature is below LCST while the equilibrium releasing ratios of the three gels are all 100% for sodium salicylate. __________ Translated from Journal of Central South University (Science and Technology), 2007, 38(5): 906–911 [译自: 中南大学学报(自然科学版)]     

A mechanistic approach on the self-organization of the two-component thermoreversible hydrogel of riboflavin and melamine

The riboflavin (R) and melamine (M) supramolecular complex in the mole ratio of 3:1 (RM31) produces a thermoreversible gel in aqueous medium. The gelation mechanism has been elucidated from morphological investigations using optical, electron, and atomic force microscopy together with time-dependent circular dichroism (CD) and photoluminescence (PL) spectroscopy. Optical microscopy indicates spherulitic morphology at lower gelation temperature (相似文献   

Gel properties as a hydrogel of crosslinked poly (l-ornithine) using organic crosslinking agents

Gel formations of water-soluble cationic ornithine polypeptides were examined using organic aliphatic crosslinking agents such as dialdehydes and diketones in water systems. When 1/20-5 equivalent molar amounts of organic crosslinking agents were added to the ornithine polypeptide systems, the corresponding gels were formed. Among the organic crosslinking agents used, glutaraldehyde was the most effective for the gel formation. As a whole, the molecular weight of the samples, the amino acid compositions, the crosslinking agents used, the molar ratios between crosslinking agents and functional residues, and system pH levels were found to play roles in the gel formation. The gels formed were characterized by swelling properties and by the selective adsorption ability of some amino acids. The polyornithine gels exhibited reversible, but hysteretic swelling in a water-acetone mixed solvent. Due to the cationic δ-amino moieties which remain unreacted, the acidic amino acid, aspartic acid, was adsorbed into the gels' matrix, exhibiting the predominant adsorption. Biodegradable characteristics of the copoly (ornithine tyrosine) [copoly (Orn Tyr)] gels by chymotrypsin were also investigated.     

Fast swelling/deswelling kinetics of comb-type grafted poly(N-isopropylacrylamide) hydrogels

The synthesis and characterization of comb-type grafted thermo-sensitive hydrogels is presented. These hydrogels were synthesized by polymerization of N-isopropylacrylamide (IPAAm) with a PIPAAm macromonomer. This process leads to a crosslinked IPAAm backbone polymer, copolymerized with highly mobile comb-type PIPAAm chains. These new thermo-responsive copolymers displayed higher equilibrium swellings at lower temperatures and rapid deswelling kinetics at elevated temperatures. The swelling/deswelling for comb-type gels is dependent on the graft chain lengths, in contrast to normal PIPAAm gel lacking the graft chains. As the temperature is increased above the critical temperature, the dehydrated graft chains aggregated due to hydrophobic attraction. Rapid and reversible kinetics of the graft-type gel were observed in response to stepwise temperature changes within short time cycles: phenomena not observed in normal crosslinked thermo-sensitive gels. The influence of freely mobile graft chains on both the equilibrium and dynamic properties of comb-type PIPAAm gel is demonstrated. Possible application of graft-type gel is discussed for actuator systems.     

Swelling-shrinking behavior of chemically cross-linked polypeptide gels from poly(α-L-lysine), poly(α-DL-lysine), poly(ɛ-L-lysine) and thermally prepared poly(lysine): effects of pH, temperature and additives in the solution

We synthesized the glutaraldehyde cross-linked hydrogels using four kinds of poly(lysine)s (PLs) and measured the equilibrium swelling ratio (Q) as a function of pH. Also measured was the temperature change of Q at a fixed pH (11.6) in the absence and presence of additives (LiBr, methanol and urea) that affect the secondary structure of PLs. The swelling data were examined using a force balance approach in which the repulsive and attractive interactions among the cross-linked PL chains were considered based on the conformational properties of PLs in aqueous solutions. It was found that the formation of the helical segments in the cross-linked chain has little effect in the gel collapse, but their association acts as the attractive interaction causing the gel to shrink. The formation of the beta-sheet structure within the network also acts as the attractive interaction. These attractive interactions are mainly due to the hydrogen bonding, but hydrophobic interactions between the lysine side chains should be considered. In addition, in the swelling behavior of all the PL gels the polyampholyte nature appears due to electrostatic interactions of the basic groups with the C-terminal carboxyl group.