TRANSMISSION ELECTRON MICROSCOPY OF AQUEOUS POLYMER GEL NETWORK MORPHOLOGY

Freeze-fracture TEM images of polymer network structure in aqueous hydroxypropyl guar (HPG) gels permit direct correlations between observed gel morphology and macroscopic chemical and mechanical properties. HPG in aqueous solution associates via hydrogen bonding, even without titanate crosslinker, to form an extended network, the structure of which depends on the electrolyte concentration. The titanate crosslinker acts only to weld together polymer molecules already associated by hydrogen bonding. Mechanical shear degrades the gel by breaking up the network inhomogeneously to form “fractured gel particles,” small domains of well-crosslinked gel separated by fluid domains.     

Phenylboronate-diol crosslinked polymer gels with reversible sol-gel transition

Covalent dynamic gels based on reversible phenylboronic acid-diol ester bonds were prepared by crosslinking of N,N-dimethylacrylamide-4-((4-vinylbenzyloxy)carbonyl)phenylboronic acid copolymer, poly(VPB-co-DMA), and poly(2,2-bis(hydroxymethyl)butyl acrylate) (PHBA) in several organic solvents at ambient temperature. These gels formed rapidly and revealed typical properties of chemical gels. Analysis of the composition-property relationships of these polymer gels, specifically considering the effects of pH, molar ratio of HBA to VPB units, and gelator concentrations on dynamic rheological properties, were performed. Additionally, the polymer gels can be switched into their starting polymer solutions by adjusting pH of the system. The reversible sol-gel phase transition can be performed for several cycles in a similar way of supramolecular gel. Moreover, the gel revealed interesting self-healing property which occurred autonomously without any outside intervention. Employing this dynamic character, it is possible to regenerate the used gel, and thus has the potential to perform in a range of dynamic or bioresponsive applications.     

氨基-酰胺类智能超分子水凝胶农药载体制备

为了提高疏水性农药的水分散性,利用氨基-酰胺类两亲分子和多元羧酸通过氢键作用自组装形成一种智能超两亲分子水凝胶,在较低浓度[5.8%(质量)]下即可形成稳定的缠绕交联三维网状结构。通过DSC、FTIR、¹H NMR、SEM、DLS、黏度测试等表征手段对比了氨基-酰胺与两种多元羧酸形成的超分子的性能和结构,发现两种超分子凝胶具有不同的刺激响应相行为及表观形貌,如氨基-酰胺/柠檬酸超分子具有热响应溶胶-凝胶可逆转换相行为,凝胶态可在70～80℃获得,临界凝胶化温度为76℃,冻干凝胶呈球状缠绕结构;氨基-酰胺/马来酸超分子具有pH诱导溶胶-凝胶可逆转换行为,pH在7～8之间可形成水凝胶态,冻干凝胶呈柱状囊泡结构。氨基-酰胺/柠檬酸超分子对农药的包封率略高于氨基-酰胺/马来酸,三元羧酸与氨基-酰胺形成的超分子与二元羧酸相比,从结构和电荷的角度,更有利于实现农药的智能缓释效果。     

Hydrogel of Radiation‐Induced Cross‐Linked Hydroxypropylcellulose

Hydroxypropylcellulose (HPC) hydrogel being a material of natural origin, combines the properties of a polymer, which make up the network, with biodegradability. In this report the effects of high energy radiation on the ether of cellulose‐HPC are presented. The polymer irradiated in its solid state or in dilute aqueous solution underwent mainly degradation, induced by the cleavage of glycosidic bonds in its main chain. Irradiation of HPC in aqueous solutions at moderate concentrations resulted in the formation of hydrogels. Chemical cross‐links bond the chains of polymer, turning it to an insoluble macroscopic gel. We have found that in addition to concentration, dosage and dose rate can affect the results of irradiation. Electron beam irradiation gave higher gel fraction, up to 90%, than gamma irradiation, which has a maximum gel fraction of 65%. Swelling of the cross‐linked hydrogels was related to the density of cross‐links and was the highest at low irradiation doses. HPC hydrogels displayed thermally reversible character in their swelling. The volume of gel underwent continuous deswelling with an increase of the solution temperature, with the deswelling rate increasing rapidly over 40°C. At elevated temperatures the hydrogel collapsed, lost its transparency and changed color to translucent white. This transition was fully reversible when the gel was placed in the medium of low temperature. The hydrogel demonstrated superior mechanical properties. Despite of the stable three‐dimensional cross‐linked network, the gels underwent biodegradation under controlled conditions when enzyme was used.     

Preparation and characterization of highly oriented poly(γ‐benzyl L‐glutamate) networks and gels with long channels with micrometer‐scale diameters

Poly(γ‐benzyl L ‐glutamate) (PBLG) gels with highly oriented α‐helix chains were prepared by the crosslinking of PBLG chains through changes in the concentration of ethylenediamine, used as a crosslinker, in 1,4‐dioxane in the presence of the strong magnetic field of an NMR magnet with a strength of 10.5 T. The experimental results showed that in one of these gels, long channels with an average diameter of about 100 μm were formed by phase separation between crosslinked PBLG and the solvent. Furthermore, three‐dimensional ¹H‐NMR imaging patterns showed that the long channels were aligned in the direction parallel to the α‐helix axis. The PBLG gel was swollen in the direction perpendicular to the α‐helix axis, but it was not swollen in the direction parallel to the α‐helix axis. The X‐ray diffraction patterns of the gel showed that the interchain distance between the two nearest neighboring PBLG chains changed from 13.4 to 18.1 Å with a change in the swelling degree. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1053–1060, 2004     

Rheological properties of poly(ethylene glycol)/poly(N‐isopropylacrylamide‐co‐2‐acrylamido‐2‐methylpropanesulphonic acid) semi‐interpenetrating networks

For the increasing demands of multifunctional materials in applications such as drug delivery system, a pH‐ and temperature‐responsive polyelectrolyte copolymer gel system was studied using rheometry. Rheological properties, determined by plate–plate rheometry in oscillatory shear, of hydrogels formed by free radical initiated copolymerization of N‐isopropylacrylamide (NIPA) and 2‐acrylamido‐2‐methylpropanesulphonic acid (AMPS) in the presence of methylene bisacrylamide (MBAA) as crosslinker are compared with the properties of semi‐interpenetrating network (SIPN) polyelectrolyte gels made by incorporation of poly(ethylene glycol) with molar mass 6000 g mol^?1 (PEG6000). Based on our systematic studies for this PEG/SIPN system, the effects of initiator and crosslinker concentration, relative proportions of comonomer units in the main chains, PEG6000 content and temperature on viscoelastic properties, unusual high storage moduli at small strain for the SIPN were discussed. The SIPN gel with characteristics of PEG molecules as well as pH and temperature responsiveness from AMPS and NIPA units has potential application in drug delivery system design. Ice‐like rheological behavior of the PEG/AMPS‐NIPA SIPN gels at low temperature was first time reported and water remains homogeneous without phase separation in PEG/AMPS‐NIPA SIPN hydrogels at low temperature may be considered as an ideal candidate for water storage material. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Autonomic self-healing in covalently crosslinked hydrogels containing hydrophobic domains

Self-healing hydrogels suffer from low mechanical strength due to their reversible breakable bonds which may limit their use in any stress-bearing applications. This deficiency may be improved by creating a hybrid network composed of a combination of a physical network formed via reversible crosslinks and a covalent network. Here, we prepared a series of hybrid hydrogels by the micellar copolymerization of acrylamide with 2 mol % stearyl methacrylate (C18) as a physical crosslinker and various amounts of N,N′-methylenebis(acrylamide) (BAAm) as a chemical crosslinker. Rheological measurements show that the dynamic reversible crosslinks consisting of hydrophobic associations surrounded by surfactant micelles are also effective within the covalent network of the hybrid hydrogels. A significant enhancement in the compressive mechanical properties of the hybrid gels was observed with increasing BAAm content. The existence of an autonomous self-healing process was also demonstrated in hybrid gels formed at low chemical crosslinker ratios. The largest self-healing efficiency in hybrids was observed in terms of the recovered elastic modulus, which was about 80% of the original value.     

Photopolymerization synthesis of poly(N-isopropylacrylamide) hydrogels

Poly(N-isopropylacrylamide) (NIPAAm) gels were formed by photopolymerization of NIPAAm in the absence of a crosslinker using a water solvent at 25°C. Factors affecting formation were the wavelength region of irradiated light, the type of photoinitiators, and the concentrations of the photoinitiator and monomer. A high-pressure mercury lamp (400 W) was used as a light source. An NIPAAm concentration of 10 wt % and irradiation time of 15 h was used for the photopolymerization. The gel (68% yield) was formed when the quartz glass system was used, but no gelation was observed for the Pyrex glass system that transmits light with π > 290 nm. The gel (100% yield) was easily formed, even in the latter system, when 30 mmol/L of hydrogen peroxide and potassium persulfate were used as the photoinitiator. Water soluble photoinitiators such as ferric chloride and sodium anthraquinone-2,7-disulfonate were not effective for the gel formation. Yield of the gel increased with increasing the potassium persulfate concentration (1–30 mmol/L), but it decreased when a high concentration of hydrogen peroxide (60 mmol/L) was used. The gel yield increased with the NIPAAm concentration (5–20 wt %). The degree of swelling of the resultant poly(NIPAAm) gels, which was measured by immersing the gels in water at various temperatures (0–50°C) for 24 h, steeply decreased at about 30°C with increasing temperature, exhibiting a temperature-responsive character. The gels swelled and shrank in water below and above the temperature, respectively. The extent of the character depended on the concentrations of hydrogen peroxide and monomer. The formation mechanism of the gel in the photopolymerization of NIPAAm using hydrogen peroxide photoinitiator was discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1313–1318, 1997     

Synthesis and characterization of graft copolymers of hydroxypropyl cellulose with acrylamide and some comonomers

To obtain new polymeric materials for environmental management, we used pine needles from the huge forest of the Western Himalayas as a source of cellulose. Cellulose was derivatized to hydroxypropyl cellulose (HPC), a useful water‐soluble cellulose ether. HPC was graft‐copolymerized with acrylamide (AAm) with benzoyl peroxide as the initiator. At optimum grafting conditions, five different concentrations of the comonomers glycidyl methacrylate, acrylic acid, 2‐hydroxyethyl methacrylate, and acrylonitrile were grafted with AAm. Networks of HPC and AAm were also synthesized by crosslinking reactions with glutaraldehyde as a crosslinker over a range of four different concentrations of crosslinker under acidic conditions. Crosslinked networks of HPC with AAm and a comonomer at one comonomer concentration were also synthesized. Graft copolymers were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and swelling behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 545–555, 2004     

Thermal gelation of the 12S canola globulin

The nature of intermolecular forces responsible for thermal gelation of the 12S canola globulin was determined by preparing gels under a variety of environmental influences. The effects of pH, sodium salts and denaturing agents were evaluated by differential scanning calorimetry, small amplitude oscillatory rheology and transmission light microscopy. Gels prepared with 6% protein at alkaline pH values were superior to gels prepared under acidic conditions. Sodium salts, which promoted protein stability, had an adverse effect on gelation. The addition of guanidine hydrochloride and dithiothreitol to protein dispersions prior to heating produced inferior gels. Hydrophobic forces and electrostatic interactions were responsible for the establishment of canola gel networks. Gel stabilization and strengthening were attributed to disulfide bonding, electrostatic interactions and hydrogen bonding.     

Brillouin scattering from polyurethane gels

The hypersonic sound speed and attenuation in a series of PPO polyurethane gels show a strong crosslink dependence in highly crosslinked samples. The temperature of maximum loss for the -relaxation is raised from 370 K to 400 K in a highly crosslinked gel whilst the β-loss maximum is unaffected. This result indicates cooperative motion at the transition with a rearranging unit which appears to involve some 3 or 4 monomer units. The hypersonic properties were also found to depend upon the relative proportions of polymer and solvent in the gel. In the dry network hydrogen bonding between the urethane links and the interposed PPO dominates the dynamics. As the proportion of solvent increases, the effect of this hydrogen bonding is reduced, lowering the relaxation temperatures.     

环氧化合物改性侧链氨基聚硅氧烷的研究

以5种单官能缩水甘油醚——腰果酚缩水甘油醚（NC-513）、烯丙基缩水甘油醚（AGE）、苄基缩水甘油醚（692）、正丁基缩水甘油醚（660）及甲基丙烯酸缩水甘油酯（GMA）和侧链氨基聚硅氧烷为原料,通过环氧基与氨基的反应对侧链氨基聚硅氧烷进行改性。研究了不同反应条件下,改性聚硅氧烷的凝胶行为,表征了改性聚硅氧烷的结构。结果表明,NC-513与侧链氨基聚硅氧烷的反应活性最大;且室温以上时,改性聚硅氧烷凝胶的形成不受反应溶剂及原料反应官能团量之比的影响,所形成的凝胶为氢键物理交联;当温度升高时,会伴有羟基缩合的化学交联,烯键不与氨基反应。由于GMA中烯键和环氧基均与氨基反应,故形成化学交联凝胶。AGE、692、660改性聚硅氧烷在有溶剂的体系形成凝胶,在无溶剂情况下不形成凝胶。     

智能高分子凝胶的研究与应用

介绍了智能高分子凝胶的定义、分类、研究历史和体积相变原理。按外界环境刺激因素如温度、pH值、光、电场和磁场等分类，介绍了智能高分子凝胶的特性，并展望了智能高分子凝胶的应用前景。     

Preparation and optical properties of an all‐polymer light modulator using colored N‐isopropylacrylamide gel particles in a gel‐in‐gel system

A novel all‐polymer light modulator with a gel‐in‐gel system was developed. The gel‐in‐gel system was constructed with colored gel particles responsive to stimuli held independently in another stimuli‐nonresponsive gel matrix. Well‐known thermoresponsive N‐isopropylacrylamide (NIPAM) gel particles containing a pigment were dispersed and fixed in an outer stimuli‐nonresponsive gel matrix. When poly(vinyl alcohol)–styrylpyridinium (PVA–SbQ) was used for the outer gel matrix, the light modulator showed excellent color‐changing properties because the PVA–SbQ matrix was selectively formed around the NIPAM gel particles and the particles exhibited a large volume change in the matrix. The temperature when the outer gel matrix was formed affected the haze of the light modulator. When the outer gel matrix was formed in the swollen state of the NIPAM gels, the haze of a light modulator increased with heating. On the contrary, the haze of a light modulator prepared in the shrunken state of the NIPAM gels decreased with heating. The response time of the color change was less than 1 s. The gel‐in‐gel system made a very fast macroscopic color change, taking advantage of the fast response of the micrometer‐sized gel particles. We believe that a light modulator with a gel‐in‐gel system may find various applications in optical devices. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2295–2303, 2007     

Effect of swelling on spatial inhomogeneity in poly(acrylamide) gels formed at various monomer concentrations

The effect of swelling on the spatial inhomogeneity in poly(acrylamide) (PAAm) gels has been investigated with the static light scattering measurements. Four sets of gels were prepared using N,N′-methylenebis(acrylamide) (BAAm) as a crosslinker at various initial monomer concentrations. The crosslinker ratio X (the mole ratio of BAAm to the monomer acrylamide) was fixed at 1/50, 1/61.5, 1/66, and 1/100 in each set of gels. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration (ν_2,cr⁰). ν_2,cr⁰ shifts toward smaller concentrations as X is decreased or, as the gel swells beyond its swelling degree after preparation. Swelling enhances the extent of spatial inhomogeneity in PAAm gels and, this enhancement mainly occurs at low crosslinker ratios. The theoretical prediction of the Panyukov-Rabin theory was found to be in qualitative agreement with the experimental findings. It was also shown that three different effects, namely crosslinker, concentration, and swelling effects determine the extent of inhomogeneities in gels formed at various monomer concentrations.     

Tough organogels based on polyisobutylene with aligned porous structures

Macroporous gels with aligned porous structures were prepared by solution crosslinking of butyl rubber (PIB) in cyclohexane at subzero temperatures. Sulfur monochloride was used as a crosslinker in the organogel preparation. The reactions were carried out at various temperatures between 20 and −22 °C as well as at various freezing rates. The structure of the gel networks formed at −2 °C consists of pores of about 100 μm in length and 50 μm in width, separated by polymer domains of 10-20 μm in thickness. The aligned porous structure of PIB gels indicates directional freezing of the solvent crystals in the direction of the temperature gradient. The size of the pores in the organogels could be regulated by changing the freezing rate of the reaction solution. The results suggest that frozen cyclohexane templates are responsible for the porosity formation in cyclohexane. In contrast to the regular morphology of the gels formed in cyclohexane, benzene as a crosslinking solvent produces irregular pores with a broad size distribution from micrometer to millimeter sizes due to the phase separation of PIB chains at low temperatures. Macroporous organogels prepared at subzero temperatures are very tough and can be compressed up to about 100% strain without any crack development. The gels also exhibit superfast swelling and deswelling properties as well as reversible swelling-deswelling cycles in toluene and methanol, respectively.     

Dynamic covalent cross-linked polymer gels through the reaction between side-chain β-keto ester and primary amine groups

A dynamic covalent gel network was constructed via the coupling reaction between the pendant primary amine groups of poly(l-leucine methacryloyloxyethyl ester) (P(H₂N-Leu-EMA)) and the side-chain β-keto ester functionalities of poly(2-(acetoacetoxy)ethyl methacrylate) (PAEMA) by varying the molar ratios of P(H₂N-Leu-EMA)/PAEMA and the concentration of polymers in different solvents at room temperature. Fourier transform infrared (FT-IR) spectroscopy and solid-state ¹³C nuclear magnetic resonance (NMR) study confirmed the newly formed enamine bonds in the polymeric gel. Rheological studies showed that the mechanical strength of gels increased with increasing weight percentage (wt.%) of polymers in solution. Furthermore, polymer gels can be transferred to the mixture of starting polymer solutions by regulating the pH of the system, which can be further transformed into the gel state by adding an external base.     

Sol–gel derived organic/inorganic hybrid gels based on poly(2-hydroxyethyl aspartamide) and silica

The sol–gel derived polymer/silicate hybrid materials have attracted considerable attention in recent years. The incorporation of silicate phase into polymeric materials may constitute an important tool to either enhance mechanical properties or provide more biocompatibility to the resulting hybrids. PHEA, α,β-poly(N-2-hydroxyethyl-dl-aspartamide), is a class of poly(amino acid)s that has been widely studied as a biodegradable functional polymer with potential biomedical and pharmaceutical applications. Hydrogels from PHEA are formed easily by a chemical or physical crosslinking reaction but the resulting gels are mechanically weak and less thermally stable. In this study, hybrid materials were prepared based on PHEA and silicate. A sol–gel process was employed using TEOS and modified PHEA to introduce inorganic silicate phase within the polymer gel matrix. FT-IR and NMR were used to analyze the chemical structure of the PHEA derivatives. In addition, the morphology, thermal and swelling properties of the hybrid gels were examined.     

Hydrogels of polyvinylpyrrolidone (PVP) and poly(acrylic acid) (PAA) synthesized by photoinduced crosslinking of homopolymers

A novel method of covalent crosslinking between polyvinylpyrrolidone (PVP) and poly(acrylic acid) (PAA) resulting in hydrogels has been developed. The hydrogels were formed by photocrosslinking in oxygen-free aqueous solutions containing hydrogen peroxide as a source of hydroxyl radicals. The crosslinking was achieved via irradiation within the broad wavelength range from 200 to 800 nm, as well as by the light cut-off at λ > 300 nm. The obtained PAA-PVP gels were sensitive to pH. Protonation of the PAA carboxylic groups with decreasing pH promoted hydrogen bonding between the PAA and PVP segments within the crosslinked structure and caused the gel to collapse. This property enabled the use of the hydrogels as a simple chemical sensor. When loaded with glucose oxidase, the PAA-PVP gel's opacity and sedimentation due to the clearly observable phase separation were triggered by the presence of glucose due to a drop in pH caused by the enzymatic reaction.