Application of sensitive hydrogels in chemical and pH sensors

In the present work, pH-sensitive poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blends as well as hydrogels based on poly(N-isopropylacrylamide) (PNIPAAm), which are sensitive to organic solvent concentration in aqueous solutions, were used in silicon micromachined sensors. A sensitivity of approximately 15 mV/pH was obtained for a pH sensor with a 50 μm thick PVA/PAA hydrogel layer in a pH range above the acid exponent of acrylic acid (pK_a=4.7). The output voltage versus pH-value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed.     

Fabrication and characterization of poly(vinyl alcohol)/chitosan hydrogel thin films via UV irradiation

A series of poly(vinyl alcohol)/chitosan (PVA/CTS) hydrogel thin films were prepared via ultraviolet (UV) irradiation, with acrylic acid (AA) monomer added as a crosslinker without the addition of any other photo-initiator. The swelling behaviors, intermolecular chemical bonds, molecular structures, thermal behaviors, degrees of crystallinity, morphologies of the surfaces and internal structure, and their relationship to the AA content were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Poly(acrylic acid) (PAA) and its chemical crosslinks formed in hydrogel films via free-radical reactions were confirmed using FTIR and DSC analyses. The XRD patterns indicated that the degree of crystallinity of the hydrogel films decreased as the PAA content was increased. SEM micrographs showed that a uniform interconnected pore structure was formed through the entire hydrogel structure, and a gradient in the crosslinking density through the film thickness was observed to result from extended irradiation times. The swelling behaviors revealed that the formation of PAA and its crosslinking in the hydrogel thin films improved the pH stability and controlled the degree of swelling while retaining a high swelling rate. The successful formation of chemical crosslinking without any specific photo-initiator improves the natural characteristics of CTS and PVA and imparts the resulting PVA/CTS hydrogel thin films with properties that make them very promising in biomedical applications.     

PVA-PAMPS-PAA三元互穿网络型水凝胶的合成及其性能研究

以2-丙烯酰胺基-2-甲基丙磺酸(AMPS)、丙烯酸(AA)以及聚乙烯醇(PVA)为原料,制备了PVA-PAMPS-PAA三元互穿网络型(T-IPN)水凝胶.红外分析表明,PVA与PAA以及PAMPS之间形成了较强的氢键,使得PVA分子上的C—O伸缩震动吸收峰移向了低波数处.X射线衍射以及电镜分析表明,当PVA用量较低时,PVA能均匀的穿插于凝胶网络中,形成完善的互穿网络结构,当PVA用量过高时,部分的PVA结晶而使得凝胶出现相分离.研究了该三元互穿网络型水凝胶的溶胀性能,结果表明,该水凝胶的平衡溶胀比在200至340之间,并且随着AA以及AMPS用量的增加,凝胶的溶胀速率以及平衡溶胀比均升高.该三元互穿网络型水凝胶在酸性溶液中和在碱性溶液中表现出截然不同的消溶胀性能;并且随着溶液pH的升高,凝胶在pH=9.0附近出现体积突变,表现出pH敏感性.通过研究T-IPN水凝胶的抗压缩性能发现,利用线型高分子、柔性高分子网络以及刚性高分子网络制备的三元互穿网络型水凝胶能在高溶胀比下保持较高的强度.溶胀比为180的T-IPN水凝胶,其最大抗压缩强度可达12.1 MPa.进一步研究发现,凝胶的组成以及溶胀比均对凝胶的抗压缩强度和压缩应变均存在较大的影响.     

PH effects in the complex formation and blending of poly(acrylic acid) with poly(ethylene oxide)

The effect of pH on the complex formation between poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) has been studied in aqueous solutions by turbidimetric and fluorescent methods. It was shown that the formation of insoluble interpolymer complexes is observed below a certain critical pH of complexation (pH(crit1)). The formation of hydrophilic interpolymer associates is possible above pH(crit1) and below a certain pH(crit2). The effects of polymer concentrations in solution and PEO molecular weight as well as inorganic salt addition on these critical pH values were studied. The polymeric films based on blends of PAA and PEO were prepared by casting from aqueous solutions with different pHs. These films were characterized by light transmittance measurements and differential scanning calorimetry. The existence of the pH value above which the polymers form an immiscible blend was demonstrated. The transitions between the interpolymer complex, miscible blend, and immiscible blend caused by pH changes are discussed. The recommendations for preparation of homogeneous miscible films based on compositions of poly(carboxylic acids) and various nonionic water-soluble polymers are presented.     

Synthesis,characterization, and swelling behaviors of chitosan‐g‐poly(acrylic acid)/poly(vinyl alcohol) semi‐IPN superabsorbent hydrogels

A series of granulated semi‐interpenetrating polymer network (semi‐IPN) superabsorbent hydrogels composed of chitosan‐g‐poly(acrylic acid) (CTS‐g‐PAA) and poly(vinyl alcohol) (PVA) were prepared by solution polymerization using ammonium persulfate (APS) as an initiator and N,N′‐methylenebisacrylamide (MBA) as a crosslinker. The effects of reaction conditions such as the concentration of MBA, the weight ratio of AA to CTS, and the content of PVA on water absorbency were investigated. Infrared (IR) spectra and differential scanning calorimetry (DSC) analyses confirmed that AA had been grafted onto CTS backbone, and PVA semi‐interpenetrating into CTS‐g‐PAA networks. SEM analyses indicated that CTS‐g‐PAA/PVA has improved porous surface and PVA was uniformly dispersed in CTS‐g‐PAA network. The semi‐IPN hydrogel containing 10 wt% PVA shows the highest water absorbency of 353 and 53 g g^?1 in distilled water and 0.9 wt% NaCl solution, respectively. Swelling behaviors revealed that the introduction of PVA could improve the swelling rate and enhance the pH stability of the superabsorbent hydrogel. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of thermal annealing on a bilayer polyvinyl alcohol/polyacrylic acid electrospun hydrogel nanofibres loaded with doxorubicin and clarithromycin for a synergism effect against osteosarcoma cells

Polyvinyl alcohol/polyacrylic acid (PVA/PAA) bilayer hydrogel nanofibres were successfully fabricated by electrospinning and physically crosslinked via heat treatment. The effects of the thermal annealing process on the structure, morphology, swelling, thermal properties and hydrophilicity of electrospun nanofibres were investigated. In addition, these membranes were also used to incorporate doxorubicin and clarithromycin for osteosarcoma treatment, one in each layer. These drugs were used because it is hypothesized in this work that a synergism occurs between both drugs. So, these membranes were analyzed towards their dual-drug release and potential cytotoxicity towards the U2OS human osteosarcoma cell line. Moreover, the water contact angle, disintegration, swelling and weight loss studies confirmed the rapid swelling and improved water stability of the annealed PVA/PAA bilayer nanofibres. The annealed bilayer nanofibres exhibited an increase in the average diameter and degree of crystallinity. In addition, the results revealed that a variation occurred in the degree of hydrophilicity of annealed PVA/PAA bilayer nanofibres. The PAA nanofibres surface exhibited higher hydrophilicity than the PVA nanofibres surface. Drug delivery presented to be as fast rate release for clarithromycin and slow-rate release for doxorubicin, which may be advantageous because both drugs exhibited to be synergetic for certain dosages presenting the combination of the drugs higher than 50% of cell inhibition, while these membranes had higher inhibition values (up to 90%), which was attributed to the PAA but also the drugs. These unique properties are of potential interest in drug delivery applications for dual drug delivery where the tunability of surfaces is desirable.     

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 role of hydrogen-bonding interaction in poly(vinyl alcohol)/poly(acrylic acid) blending solutions and their films

<正>The aim of this work is to investigate the hydrogen-bonding interaction in poly(vinyl alcohol)(PVA)/poly(acrylic acid)(PAA) blending system and its influence on rheological properties in solution and the physical properties in solid state. Introducing PAA into PVA solutions resulted in a thickening behavior of blend solutions.The viscosity of the solutions increased with PAA content increasing,and a maximum viscosity could be obtained when the ratio of PVA/PAA was 70/30. The intermolecular hydrogen-bonding and miscibility between PVA and PAA in solid state were investigated by differential scanning calorimetry(DSC),Fourier transform infrared spectroscopy(FTIR) and mechanical measurements.The results displayed the great influence of introducing PAA on the properties of blending films.The tensile strength increased from 89.31 MPa to 119.8 MPa and Young's modulus improved by over 300%with increasing PAA concentration compared with those of pure PVA films.By systematically studying the rheological behaviors of solutions and the physical properties of films,the influence of hydrogen-bonding in solutions and solid states were discussed.     

活性炭复合改善聚丙烯酸凝胶耐盐性及其溶胀机理

在活性炭(Ac)存在的情况下通过自由基溶液聚合,以过硫酸铵为引发剂,N,N′-亚甲基双丙烯酰胺为交联剂,制备了活性炭复合聚丙烯酸凝胶(PAA/AC)。 考察了凝胶在蒸馏水、生理盐水和不同pH值缓冲溶液中的平衡溶胀比以及溶胀动力学,结果表明,活性炭能有效提高凝胶的平衡溶胀比,在实验设计的pH值范围内复合凝胶具有比PAA凝胶更高的平衡溶胀比,蒸馏水和生理盐水中PAA/AC凝胶的平衡溶胀比分别可达到303和60 g/g,约为PAA凝胶的2.4倍。 讨论了凝胶的溶胀机理,结果表明,活性炭成分的介入破坏了聚合物链段之间的聚集态结构,减弱了聚合物链段之间的相互作用,提高了凝胶的溶胀能力。 示差扫描量热仪测定复合前后凝胶的玻璃化转变温度,扫描电子显微镜观察了复合前后凝胶的断面网络结构,结果进一步表明活性炭复合后聚合物链段之间的作用力减弱。     

Needleless electrospinning of poly(acrylic acid) superabsorbent: Fabrication,characterization and swelling behavior

In this work, nanofibrous hydrogel has been fabricated from needleless electrospinning of Poly(acrylic acid) (PAA) in an aqueous solution with different concentrations. First, all solution samples were characterized for pH, surface tension, conductivity and viscosity. Next, electrospinnability of the PAA-water dope solution was investigated using the needleless electrospinning technique under constant conditions. Results indicated that the PAA-water solution was not electrospinnable. Therefore, the neutralization of carboxylic groups in the PAA chemical structure using the NaOH solution was investigated to enhance the PAA electrospinnability. Morphology observation revealed that the fiber diameters ranged from 40 to 250 nm and increased with increasing the solution concentrations. Increasing the neutralization degree (10%, 15% and 20% with 50 wt% NaOH solution) led to increase the dope viscosity and conductivity. The resultant nanofibers could be rendered water-insoluble by incorporating 1,4-butanediol diglycidyl-ether in the PAA-water dope solution, then heat-induced crosslinking was performed using a microwave at different curing times (1–5 min) and temperatures (45–105 °C). The nanofibrous hydrogel mat was then characterized by FTIR. The resulting nanofibrous hydrogel showed remarkable water absorption capacity up to 17,000% and 51,000% (within 15 min) in the standard saline solution and distilled water, respectively.     

pH effects on the complexation, miscibility and radiation-induced crosslinking in poly(acrylic acid)-poly(vinyl alcohol) blends

The effect of pH on the complexation of poly(acrylic acid) with poly(vinyl alcohol) in aqueous solution, the miscibility of these polymers in the solid state and the possibility for crosslinking the blends using gamma radiation has been studied. It is demonstrated that the complexation ability of poly(vinyl alcohol) with respect to poly(acrylic acid) is relatively low in comparison with some other synthetic non-ionic polymers. The precipitation of interpolymer complexes was observed below the critical pH of complexation (pH(crit1)), which characterizes the transition between a compact hydrophobic polycomplex and an extended hydrophilic interpolymer associate. Films prepared by casting from aqueous solutions at different pH values exhibited a transition from miscibility to immiscibility at a certain critical pH, pH(crit2), above which hydrogen bonding is prevented. It is shown here that gamma radiation crosslinking of solid blends is efficient and only results in the formation of hydrogel films for blends prepared between pH(crit1) and pH(crit2). The yield of the gel fraction and the swelling properties of the films depended on the absorbed radiation dose and the polymer ratio. [Diagram: see text] SEM image of an equimolar PAA-PVA blend cast from a pH 4.6 solution.     

温度及pH敏感性水凝胶的合成及性能研究

乙烯基吡咯烷酮与丙烯酸共聚物／聚丙烯酸β-羟基丙酯互穿网络水凝胶具有温度及PH双重敏感特性。经验证,在蒸馏水中,聚丙烯酸β－羟基丙酯与丙烯酸间存在络合作用,在缓溶液中,没有这样络合作用。     

PVA/PAA thermo-crosslinking hydrogel fiber: preparation and pH-sensitive properties in electrolyte solution

The preparation of thermo-crosslinking hydrogel fibers composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was presented. The hydrogel fiber was prepared by extruding the spinning dope from in situ polymerization of acrylic acid in the presence of PVA into coagulating bath of saturated ammonium sulfate aqueous solution. The network was formed by thermally heating the dried fibers under vacuum. The final hydrogel fibers exhibit pH-sensitive behavior and show hysteresis loop in the pH range from 2.5 to 12.5. The pH value at which the swelling ratio of the fiber had a jump shifted to lower value with increasing the PAA content within the network. Increasing the heating temperature and time for the fibers, the swelling ratio decreased and the jump point pH shifted to higher pH value. The oscillatory swelling/contracting behavior of the hydrogel fiber exhibited a well reversible pH-responsive property.     

pH敏感的聚(L-谷氨酸)-交联聚丙烯酸水凝胶的合成与性能研究

通过在聚L-谷氨酸侧链部分接枝甲基丙烯酸2-羟乙酯得到含有双键的聚(L-谷氨酸),将其与丙烯酸共聚得到由聚(L-谷氨酸)侧链接枝并交联聚丙烯酸的pH敏感水凝胶.研究水凝胶在不同pH的缓冲溶液中的溶胀性、溶胀动力学,并通过SEM观察水凝胶的微观结构.结果表明,水凝胶在低pH环境下的溶胀率明显低于高pH环境中的溶胀率,不同...     

Hydrogen‐Bonded Complexes and Blends of Poly(acrylic acid) and Methylcellulose: Nanoparticles and Mucoadhesive Films for Ocular Delivery of Riboflavin

Poly(acrylic acid) (PAA) and methylcellulose (MC) are able to form hydrogen‐bonded interpolymer complexes (IPCs) in aqueous solutions. In this study, the complexation between PAA and MC is explored in dilute aqueous solutions under acidic conditions. The formation of stable nanoparticles is established, whose size and colloidal stability are greatly dependent on solution pH and polymers ratio in the mixture. Poly(acrylic acid) and methylcellulose are also used to prepare polymeric films by casting from aqueous solutions. It is established that uniform films can be prepared by casting from polymer mixture solutions at pH 3.4–4.5. At lower pHs (pH < 3.0) the films have inhomogeneous morphology resulting from strong interpolymer complexation and precipitation of polycomplexes, whereas at higher pHs (pH 8.3) the polymers form fully immiscible blends because of the lack of interpolymer hydrogen‐bonding. The PAA/MC films cast at pH 4 are shown to be non‐irritant to mucosal surfaces. These films provide a platform for ocular formulation of riboflavin, a drug used for corneal cross‐linking in the treatment of keratoconus. An in vitro release of riboflavin as well as an in vivo retention of the films on corneal surfaces can be controlled by adjusting PAA/MC ratio in the formulations.     

Colloid surface engineering via deposition of multilayered thin films from polyelectrolyte blend solutions

Multilayer thin films were constructed on polystyrene colloidal particles by depositing alternating layers of poly(allylamine hydrochloride) (PAH) at pH 7.5 and varying composition blends of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS) at pH 3.5. Following the deposition of each layer, microelectrophoresis experiments showed alternating zeta-potentials, suggesting the formation of multilayered films on the particles. Scanning and transmission electron microscopy were used to examine the surface morphology of the colloidal particles, with homogeneous surface coatings apparent for films deposited from PAA/PSS blend solutions containing up to 90 wt % PAA. The colloidal stability of these particles is greater than those coated with individual PAH and PAA layers. In the case of the blend PAA/PSS = 25:75 wt %, up to 20 layers were assembled without compromising the colloidal stability of the dispersion. The results demonstrate that the deposition of layers from PE blend solutions containing a strong and weak PE can be used as a facile method for controlling the surface properties and hence the colloidal stability of core-shell particles, as well as the thickness and morphology of the coatings. Control of these parameters is important for subsequent processing and application of these particles in controlled delivery, photonics, catalytic, and separation applications.     

pH-induced changes in the fabrication of multilayers of poly(acrylic acid) and chitosan: fabrication, properties, and tests as a drug storage and delivery system

Multilayers of poly(acrylic acid), PAA, and chitosan, CHI, have been built by the layer-by-layer (LbL) method from aqueous solutions at different pH values and analyzed by the dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results showed that under all of the assembly conditions considered the growth of the films is nonlinear. The thickness of the PAA layers increases as the pH of the assembling solutions decreases, whereas the adsorption of CHI is almost unaffected by the pH conditions. The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films, showing that the multilayers are highly hydrated, with an average water content higher than 20%. The analysis of D-QCM data has provided high-frequency values of the complex shear modulus that are in the megapascal range and shows a transition from mainly viscous to mainly elastic behavior for the added PAA layers, depending on the pH. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the monomer density depends on the assembly conditions. It was found that the adsorption kinetics is a bimodal process, with characteristic times that depend on the number and nature of the layers. Finally, the possibility of using of these multilayers as a drug storage and delivery system has been evaluated.     

Synthesis and characterization of a novel controlled release nitrogen-phosphorus fertilizer hybrid nanocomposite based on banana peel cellulose and layered double hydroxides nanosheets

In the present study, a novel environment-friendly hybrid nanocomposite of Banana Peel Cellulose-g-poly(acrylic acid)/PVA (BPC-g-PAA/PVA) hydrogel and Layered double hydroxides (LDH) nanosheets was developed using in situ graft polymerization for slow release of NP (nitrogen, phosphorous) fertilizers and water retention. The hybrid nanocomposite hydrogel containing NP fertilizers was characterized using FTIR, and SEM. The effects of pH changes and different saline solutions on the swelling behavior, fertilizer release and water retention properties of the nanocomposite were investigated.The nanocomposite hydrogel showed a pH dependent swelling, as in the pH range of 7–10, the hydrogel had higher water absorbency. However, pH had opposite effects on the release of fertilizers. Phosphorus release had an increasing trend from pH 2 to 7 and it reached its maximum value at normal pH while nitrogen had a higher release rate at acidic pH and by increasing pH from 2 to 7, the release of nitrogen decreased gradually. Water absorption and fertilizer release of hydrogel was influenced by different cations in the order of Ca²⁺ < K⁺ < Na⁺. Water retention study in loamy sand soil showed that the nanocomposite hydrogel significantly improved the water retention of the soil for a longer period of time, compared to neat BPC-gPAA. This result indicated that incorporation of LDH nanosheets in hydrogel matrix improved its water retention property. The obtained results revealed that the nanocomposite of BPC-g-PAA/PVA hydrogel and LDH nanosheets can be a promising controlled release fertilizer formulation with enhanced water retention properties for agricultural applications.     

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.     

Characteristics and degradation of hybrid composite films prepared from PVA,starch and lignocellulosics

A research cooperation between USDA and the University of Pisa led to the development of several composite blends of poly(vinyl alcohol) (PVA) and lignocellulosic fibers. The cast films were prepared by blending orange fibers (OR-fibers) and PVA with and without cornstarch to yield flexible and cohesive films. To improve properties, films were also prepared by crosslinking PVA, starch and OR-fibers with hexamethoxymethylmelamine (HMMM). Films were evaluated for their thermal stability, water permeability and biodegradation. Thermal gravimetric analyses indicated the potential usefulness of such blends in several thermoplastic applications. Films were permeable to water, and retained the moisture content in the soil while retaining their integrity. Films generally biodegraded within 30 days in compost, achieving between 50-80% mineralization. Both neat PVA and blends that had been crosslinked showed comparatively slow degradation. A possible stimulating effect of lingocellulosic fillers on the biodegradation of PVA in blends has been observed.