Stimulus response and molecular structural modification of polyacrylamide gel in nitric acid: A study by Raman,FTIR, and photoluminescence techniques

Stimulus response of photopolymerized 1% and 0.5% N,N′‐methylene bisacrylamide (MBA) crosslinked 10% polyacrylamide (PAAm) hydrogels was studied in nitric acid. The hydrogels swelled exponentially to saturation in 13 h due to the osmotic pressure arising from diffusion of ions in to the gel. MBA (0.5%) gels swell more with larger time constant than 1% MBA gels due to lower bulk modulus. Diffusion coefficient of nitric acid in the hydrogel and polymer‐solvent interaction parameter were estimated from the swelling behavior and discussed. At longer times, the hydrogels deswelled linearly in nitric acid due to molecular modification of amide group by acidic hydrolysis. Degree of swelling and deswelling increase with nitric acid concentration. Raman and FTIR investigations revealed the formation of carboxylic acid due to acidic hydrolysis. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 710–720, 2008     

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.     

Determination of the ratio of the shear modulus over the longitudinal modulus for swollen networks

The ratio of the shear modulus over the longitudinal osmotic modulus has been determined by different experimental methods. Osmotic deswelling, swelling kinetics as well as deswelling induced by unidirectional compression were performed. For the experiments chemically cross-linked poly(vinyl acetate) gels swollen in toluene and acetone (good solvents) and iso-propyl alcohol (poor solvent) have been used. The results are in agreement with the theoretical predictions.     

电场敏感性聚(丙烯酸-co-丙烯酸甲酯)凝胶的合成和性能

以丙烯酸、丙烯酸甲酯为单体,亚甲基双丙烯酰胺为交联剂,过硫酸铵为引发剂,N,N-四甲基乙二胺为促进剂,合成了一系列电场敏感性凝胶.研究了氯化钠溶液,pH缓冲溶液对凝胶平衡溶胀度的影响,并测定了所得电场敏感性凝胶在电场作用下的脱水行为.研究结果表明,所得凝胶具有很好的电场敏感性.此外,初步讨论了该类电场敏感性凝胶的电致收缩机理.     

Effect of additives on swelling of covalent DNA gels

The volumetric response of polymer gels on cosolute addition depends on the interaction of the polymer with the cosolute and can be used as a simple and sensitive way of elucidating these interactions. Here we report on DNA networks, prepared by crosslinking double-stranded DNA with ethylene glycol diglycidyl ether (EGDE); these have been investigated with respect to their swelling in aqueous solution containing different additives, such as metal ions, polyamines, charged proteins, and surfactants. The deswelling on addition of metal ions occurs at lower concentrations with increasing valency of the counterion. The collapse of the gels in the presence of trivalent ions seems to follow the same kind of mechanism as the interaction in solution, but addition of these ions leads to DNA denaturation and formation of single-stranded DNA. Striking features were found in the deswelling of DNA gels by chitosan, spermine, spermidine, lysozyme, poly-l-lysine and poly-l-arginine. Chitosan is the most efficient cosolute of those investigated with respect to DNA gel collapse. The effect of the cationic surfactant tail length on the volume phase transition of DNA gels was studied as a function of surfactant concentration. Cationic surfactants effectively collapsed the gel from the critical aggregation concentration (cac), decreasing with increasing length of the hydrophobic tail. In several cases, the deswelling as a function of cosolute concentration shows a pronounced two-step behavior, which is interpreted in terms of a combination of DNA chain condensation and general osmotic deswelling. The studies included investigations on the state of the DNA chain after deswelling, on the reversibility of the deswelling as well as on the kinetics. With the exception for the trivalent lanthanide ions, it appears that the DNA chain always retains a double-helix conformation; with these metal ions, single-stranded DNA is found. The deswelling appears to be reversible as exemplified by addition of anionic surfactant subsequent to gel collapsed by cationic surfactant and addition of sodium bromide to gels collapsed by a polycation. An investigation of the kinetics shows that an increase in the surfactant tail length gives a pronouncedly slower deswelling kinetics.     

Structural effects of crosslinking a biopolymer hydrogel derived from marine mussel adhesive protein

In an effort to explore new biocompatible substrates for biomedical technologies, we present a structural study on a crosslinked gelatinous protein extracted from marine mussels. Prior studies have shown the importance of iron in protein crosslinking and mussel adhesive formation. Here, the structure and properties of an extracted material were examined both before and after crosslinking with iron. The structures of these protein hydrogels were studied by SEM, SANS, and SAXS. Viscoelasticity was tested by rheological means. The starting gel was found to have a heterogeneous porous structure on a micrometer scale and, surprisingly, a regular structure on the micron to nanometer scale. However disorder, or "no periodic structure", was deduced from scattering on nanometer length scales at very high q. Crosslinking with iron condensed the structure on a micrometer level. On nanometer length scales at high q, small angle neutron scattering showed no significant differences between the samples, possibly due to strong heterogeneity. X-ray scattering also confirmed the absence of any defined periodic structure. Partial crosslinking transformed the viscoelastic starting gel into one with more rigid and elastic properties.     

Swelling behavior of strong polyelectrolyte poly(N-t-butylacrylamide-co-acrylamide) hydrogels

Temperature-sensitive ionic hydrogels based on N-t-butylacrylamide (TBA), acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) and N,N^′-methylenebis(acrylamide) (BAAm) monomers were prepared. The molar ratio of TBA to the monomers AAm and AMPS was fixed at 60/40, while the AMPS content of the hydrogels was varied. The elastic modulus of the hydrogels was in the range of 347-447 Pa, much lower than the modulus of PAAm or poly(N-isopropylacrylamide) hydrogels due to the reduced crosslinking efficiency of BAAm in TBA/AAm copolymerization. The hydrogels exhibited swelling-deswelling transition in water depending on the temperature. Increasing ionic group (AMPS) content resulted in shifting of the transition temperature interval in which the deswelling takes place. The higher the ionic group content, the broader the temperature interval at the phase transition. Ionic hydrogels exhibited first-order reentrant conformational transitions in ethanol-water and in dimethylsulfoxide (DMSO)-water mixtures. The higher the ionic group content of the hydrogels the narrower the ethanol (or DMSO) range in which the reentrant phenomena occur. By taking into account the difference of the solvent mixture composition inside and outside the gel, the equilibrium swelling theory provided a satisfactory agreement to the experimental swelling data of the hydrogels immersed in the solvent mixtures.     

Synthesis and properties of multifunctional thiol crosslinked gels containing disulfide bond in the network structure

Joint crosslinked gels containing disulfide linkage have been synthesized by oxidation reaction of multi‐functional thiol monomers, trimethylolpropane tris(3‐mercaptopropionate), tris‐[3‐mercaptopropionyloxy‐ethyl]‐isocyanurate, pentaerythritol tetrakis (3‐mercaptopropionate), and dipenta‐erythritol hexakis (3‐mercaptopropionate) in dimethyl sulfoxide (DMSO). Both the oxidation reactions with DMSO at 85 °C and Albright‐Goldman oxidation in the presence of acetic anhydride at 50 °C yielded the corresponding gels. The oxidation reaction with DMSO showed higher reaction conversion than that with Albright‐Goldman oxidation. Network structure of the gels was quantitatively characterized by means of a scanning microscopic light scattering. The reactions formed homogeneous network structure with about 0.5 nm of mesh in the gels. Mechanical properties of the obtained gels were investigated by compression test. Increasing of the crosslinking density with increasing of the monomer concentration, number of thiol group of the monomer or reaction conversion, raised Young's modulus, and breaking stress of the gels. Cogelation of the tri‐, tetra‐, of hexa‐thiol monomer and dithiol monomers yielded soft and flexible gels. Reduction of the disulfide bonds in the gels by dithiothreitol turned the gel into solution. Heating of the resulting solution induced the regelation by reforming of the disulfide bonds. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3749–3756     

低温水凝胶力学性能的研究

本文采用单向压缩和单向拉伸法测定了聚乙烯醇(PVA)低温水凝胶的力学性能。结果表明,其机械强度随凝胶浓度和冷冻时间的增加而增强。得到弹性模量与凝胶浓度的关系符合标度律。说明采用低温处理方法可使PVA水溶液凝胶化,制得含水量高、具有较高强度、类似于橡胶的弹性材料,认为其力学强度来自于物理交联网络的贡献。     

Swelling behaviors of polyelectrolyte hydrogels containing sulfonate groups

Series of maleate monoester and diester monomers based on poly(ethylene glycol) monomethyl ether (MPEG) were copolymerized using the ionizable 2‐acrylamido‐2–methyl propane sulfonic acid (AMPS) via different dose rate of electron‐beam irradiation (40–150 kGy). The crosslinking of the copolymers were carried out in aqueous acidic solutions at pH 1 or in the presence of 1% N,N‐methylene bisacrylamide (MBA) as crosslinking agent. The final equilibrium water content and swelling capacities for the prepared hydrogels were determined in aqueous solutions at pH 1, 6.8, and 12 and in aqueous salt solutions at 298 K. Swelling equilibria for prepared hydrogels were determined in different molar salt solutions of NaCl, KCl, CaCl₂, Na₂SO₄, K₂SO₄, and CaSO₄. The swelling ratios of gels in pure water and in the salt solutions were found to depend on the counterion species in the increasing sequence of Ca²⁺, Na⁺ and K⁺. Copyright © 2002 John Wiley & Sons, Ltd.     

Quasi-elastic neutron scattering in gels and solutions

Measurements of swelling pressure, neutron spin-echo scattering, and dynamic light scattering were made in an end-linked poly(dimethylsiloxane) (PDMS) gel swollen to equilibrium in a good solvent (toluene) and also in the equivalent solution. The macroscopic osmotic modulus is depressed in the gel. Neutron spin echo observations at intermediate and high values of the scattering vector Q reveal that the mobility of the monomers is unaffected by cross-linking. Elastic neutron scattering at small Q detects non-uniformities in the polymer concentration distribution, which are absent from the solution. These non-uniformities play a major role in the dynamic response of the system at lower Q, and and appear to be the cause of the observed reduction in osmotic pressure.     

Globular protein gelation

In this paper, significant advances in the structure and rheological properties of globular protein gels have been described. The achievements of scattering methods, which have provided information over a range of distances from the monomer to the micron scale, are reviewed. The kinetics of gelation measured by rheological methods have also been discussed, including currently applied models for the gelation time, the gel elastic modulus and the critical gel concentration.     

Synthesis and characterization of thermoresponsive poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide-co-<Emphasis Type="Italic">N</Emphasis>-hydroxymethylacrylamide-co-2-hydroxyethyl methacrylate) hydrogels

Thermoresponsive hydrogels based on N-isopropylacrylamide, N-hydroxymethylacrylamide, and 2-hydroxyethyl methacrylate, poly(NIPAM–co-NHMAAm–co-HEMA), have been synthesized and their swelling—deswelling behavior studied as a function of NIPAM concentration, NIPAM/NHMAAm and NIPAM/HEMA mole ratio, and total monomer concentration. Copolymers varying in composition have been obtained by redox copolymerization of these three monomers. Temperature has been changed in the ranges from 4 to 70 °C at fixed pH and total ionic strength. Equilibrium swelling ratio, dynamic swelling ratio, and dynamic deswelling ratio were evaluated for all hydrogel systems. The equilibrium swelling ratios of the copolymeric gels decrease with increasing NHMAAm and HEMA content. The formation of the intermolecular hydrogen bonding between hydroxyl and amido groups decreases the hydrophilic group numbers of the gel and the affinity of the gel towards water decreases. The copolymer gels also showed rapid volume transitions with time. The time required for equilibrium shrinking increased with increasing NHMAAm and HEMA content in the gel.     

纳米复合水凝胶的力学行为与刺激响应性

纳米复合水凝胶（nanocomposite hydrogels,NC凝胶）由于其简便的制备方法、独特的组成、优异的力学性能、高光学透明度以及良好的溶胀/去溶胀性等,引起了广泛的关注.近年来,本课题组在NC凝胶的力学行为及刺激响应性NC凝胶的制备方面取得了可喜的研究成果.观察到新合成的和溶胀平衡的聚N-异丙基丙烯酰胺（PNIPAm）-锂藻土LaponiteNC凝胶的超拉伸性,发现了在大应变下的应变硬化现象;发现利用Mooney-Rivlin方程可以描述NC凝胶的压缩应力-应变,但不能描述较大的应变硬化;Creton模型能很好地描述NC凝胶在大形变下的应力-应变曲线,特别是凝胶的应变硬化;从NC凝胶小应变下的平衡剪切模量得到了有效交联密度,观察到NC凝胶形变-回复过程的迟滞现象,测定了NC凝胶的松弛指数.我们认为NC凝胶具有超拉伸性的原因是其较低的交联密度和适度的松弛速率;在响应性NC凝胶的制备方面,发现了溶胶型Laponite的水分散液的稳定窗口,避免了加入离子性单体引起的聚集沉淀;通过共聚制备了具有超拉伸性、pH响应或温度和pH双响应的透明NC凝胶.本文主要综述了我们课题组在NC凝胶力学行为及响应性NC凝胶领域的一些研究进展,并分析了NC凝胶研究领域仍然未解决的科学问题,以及今后可能的发展方向.     

Chemically Crosslinked pH- and Temperature-Sensitive (N-isopropylacrylamide-co-1-vinyl-2-pyrrolidone) Based on New Crosslinker: I. Swelling Behavior

Novel pH- and temperature-sensitive polymer matrices based on N-isopropylacrylamide have been developed. The hydrogels were prepared by bulk radical polymerization of N-isopropylacrylamide and 1-vinyl-2-pyrrolidinone in appropriate amounts of distilled water using different mol% of traditional N,N-methylene bisacrylamide (MBA) and the new synthesized N,N,N-tris acryloyl melamine (MAAm) crosslinkers. Lower critical solution transition temperatures (LCST) were measured by differential scanning calorimetry. The synthesized hydrogels have LCST lower than 40°C. The influence of environmental conditions such as temperature and pH on the swelling behavior of these polymeric gels was investigated. The swelling behaviors of the resulting gels show pH sensitivity. The crosslinked NIPAAm/VP with MAAm hydrogels exhibited more rapid deswelling rate than NIPAAm/VP hydrogels crosslinked with MBA in pure water in response to abrupt temperature changes from 20°C to 50°C.     

Rheology and Permeability of Crosslinked Polyacrylamide Gel

Gels produced by crosslinking polyacrylamide solutions with chromium (III) have been characterized by dynamic rheology studies. To vary the gel strength, different polymer concentrations were used, while keeping the temperature, salinity, and crosslinker concentration constant. Both the loss and storage moduli increased with the polymer concentration for this gel system. The storage modulus at the end of the gelation was used to characterize the gel strength. Steady-state water flow experiments through gel-filled capillary tubes were performed, with the aim of linking the gel strength and flow behavior. The permeability was found to be a function of the water flow rate (velocity) and polymer concentration. Two parameters were used to characterize the flow behavior, intrinsic gel permeability and elasticity index, which are each functions of the polymer concentration. However, only one parameter is needed to fully identify the flow and rheological gel properties, as the elasticity index and storage modulus are linked by a power-law relationship. The loss modulus and intrinsic permeability are correlated with the storage modulus and elasticity index, respectively. A theoretical model for this behavior linking both gel properties based on the dual domain structure was used to demonstrate that the flow and rheological behavior of the gel are indeed related and that the gel strength controls the water permeability. Implications for prediction of flow of water through gels emplaced in a porous medium are discussed.     

Osmotic deswelling of weakly charged poly(acrylic acid) solutions and gels

The osmotic pressure of weakly charged aqueous poly(acrylic acid) (PAA) solutions and the swelling pressure PAA gels were studied by osmotic deswelling at different degrees of ionization (α). In solution, the osmotic pressure was found to scale linearly with concentration, whereas the scaling power of the swelling pressure of gels was higher (1.66). The effect of the ionization degree on the osmotic coefficient in PAA solutions was in agreement with the theory of Borue and Erukhimovich [Macromolecules, 21 , 3240 (1988)]. Ionization increases the swelling capacity of the PAA gels until a plateau is reached at about 35% neutralization. The concentration at equilibrium swelling scales as C_e ～ α^?0.6. The contribution of the network to the gel swelling pressure is evaluated by subtracting the osmotic pressure of the polymer solution at the same concentration and degree of ionization. In swollen gels the extended network opposes swelling. As the gel is osmotically deswelled, a state of zero network pressure exists at a certain concentration, below which the network elasticity favors swelling. The crossover concentration shifts to lower values as the degrees of ionization increases. © 1995 John Wiley & Sons, Inc.     

Swelling,elasticity and structure of hydrogels prepared via bis-macromonomers of poly(ethylene oxide)

The effect of swelling on the shear modulus was studied for hydrogels prepared by radical polymerization of methacrylate-terminated poly(ethylene oxide) (PEO) bis-macromonomers of different molecular weight. Gels made of long chains (M = 12000 or 6000) display classical softening upon swelling, whereas gels made of shorter chains (M = 4000 or 2000) remain rigid or even stiffen. The abnormal behaviour is explained by a specific character of network junctions presented by polymethacrylate chains in which each unit is linked with a PEO network chain. It is assumed that the interactions among densely grafted PEO chains result in their stretching on polymerization and non-affine deformation on swelling, which stiffen the gel. This is verified by the data on copolymer (macromonomers - 2-hydroxyethyl methacrylate) gels that have lesser densities of PEO chains attached to the junctions and show weaker stiffening on swelling. The osmotic pressure of gels was estimated from the swelling pressure and shear modulus. Similar to the mixing pressure of equivalent PEO solutions, it varies as the 9/4 power of polymer concentration. At the same time, it is lower than the mixing pressure. This indicates that the junctions make only quantitative changes in the osmotic properties of macromonomer chains.     

The effect of methyl group on the mechanical properties of hydrophobic association hydrogel

Hydrophobic association hydrogels were fabricated via micellar copolymerization of acrylamide and hydrophobic monomers lauryl (meth)acrylate (LA or LMA) in an aqueous solution of sodium dodecyl sulfate. The effect of methyl groups of hydrophobic monomers on the crosslinking network structure and mechanical behavior of the gels was investigated on the basis of rubber elastic theory. It was found that the LMA-gel exhibited higher effective crosslink density and elastic modulus. The methyl groups of hydrophobic monomers limited the flexibility of the methacrylate backbone in the association domain, which resulted in the increment of chains constraints. With the increase of stretch rate, the dissipated energy of LMA-gel increased more highly than that of LA-gel. In addition, the methyl group hindered the movement of polymer chains, leading to the lower recovery efficiency of dissipated energy for LMA-gel. In contract, the LA-gel exhibited a rapid response to external force, and possessed better elasticity and self-recovery property. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1505–1512     

Monte Carlo simulations of cross-linked polyelectrolyte gels with oppositely charged macroions

The volume and structural changes upon replacement of oppositely charged network counterions for oppositely charged macroions in cross-linked polyelectrolyte gels have been investigated by Monte Carlo simulations using a coarse-grained model. Initially, the gel deswells, but after an approximately equivalent amount of macroions, the gel starts to swell again. The deswelling effect is greatest for small and highly charged macroions. The role of different network properties on the deswelling has also been examined. The initial deswelling is understood in terms of a replacement of confined counterions with macroions, thereby reducing the osmotic pressure originating from the counterions. At these conditions, macroions are located near network nodes with various degrees of network chains wrapping them. At charge equivalence, a profound change in the network structure has appeared. At these conditions, the cohesive electrostatic interaction and the excluded volume effect of the macroions strongly influence the equilibrium volume of the gel. Our model system reproduces many characteristic experimental observations of polyelectrolyte gels containing oppositely charged surfactants.