Viscoelastic properties of poly(vinyl alcohol) hydrogels and ferrogels obtained through freezing-thawing cycles

We report the preparation of poly(vinyl alcohol) (PVA) hydrogels obtained through freezing-thawing cycles. The viscoelastic properties of these gels using parallel-plate shear mode were evaluated as a function of temperature, time, degree of swelling, concentration and the number of freezing-thawing cycles. The storage modulus was analyzed on the basis of a theoretical model based on the scaling approach. These results provide additional evidence for a non-crystalline nature of the structure of PVA cryogels.Furthermore, PVA ferrogels have been prepared from PVA aqueous solutions and a ferrofluid through freezing-thawing cycles. The viscoelastic properties of these materials have been evaluated. It is shown that the variation of the storage modulus with ferrofluid concentration cannot be fitted using classical theories what has been attributed to the small dimensions of the particles in the ferrogels and to the magnetic interactions between particles.     

Complexation Equilibrium Constants of Poly(vinyl alcohol)-Borax Dilute Aqueous Solutions – Consideration of Electrostatic Charge Repulsion and Free Ions Charge Shielding Effect

Equilibrium dialysis experiments of dilute poly(vinyl alcohol) (PVA) in borax aqueous solution (PVA concentration was 2 g/L and borax concentration ranged from 0.0 M to 0.05 M) were carried out to determine the concentration of free borate ions in equilibrium with the PVA-borate complex at 25°C, and hence the amount of borate bound to PVA. The binding isotherm of borate ion on PVA molecules was investigated. The concentration of borate ion bound on PVA was found to increase with increasing borax concentration and reached an asymptote at high borax concentrations. The complexation equilibrium constant between borate and PVA diol was found to decrease initially then increases and finally decreases with increasing borax concentration while PVA concentration was fixed in a constant value. The equlibrium constant of free borate ion with borate-PVA diol complex was a balance result of excluded volume of PVA molecular chains, electrostatic charge repulsion between free borate ions and borate-PVA diol complexes, and electro-shielding of free Na⁺ ions on negatively charged PVA-borate complexes. In this study, the electrostatic charge repulsion between free borate ions and borate-PVA diol complex ions and free Na⁺ ions shielding effect are considered in the estimation of equilibrium constants of PVA diol-borate complexation. A fixed value of equilibrium constant is obtained with increasing borax concentration while borax concentraion is below a critical concentration [borax]_crt. As borax cocentration is higher than [borax]_crt, a lots of PVA diols form complexes with borate ions and a high shielding of free Na⁺ ions on negatively charged borate-PVA diol complexes causes the PVA-borate complexes to behave like neutral polymers. Few free PVA diol binding sites are available for reaction with free borate ions while borax concentration is higher than [borax]_crt resulting in a decrease in equilibrium constant with increasing borax concentration.     

High performances of dual network PVA hydrogel modified by PVP using borax as the structure-forming accelerator

A dual network hydrogel made up of polyvinylalcohol (PVA) crosslinked by borax and polyvinylpyrrolidone (PVP) was prepared by means of freezing-thawing circles. Here PVP was incorporated by linking with PVA to form a network structure, while the introduction of borax played the role of crosslinking PVA chains to accelerate the formation of a dual network structure in PVA/PVP composite hydrogel, thus endowing the hydrogel with high mechanical properties. The effects of both PVP and borax on the hydrogels were evaluated by comparing the two systems of PVA/PVP/borax and PVA/borax hydrogels. In the former system, adding 4.0% PVP not only increased the water content and the storage modulus but also enhanced the mechanical strength of the final hydrogel. But an overdose of PVP just as more than 4.0% tended to undermine the structure of hydrogels, and thus deteriorated hydrogels’ properties because of the weakened secondary interaction between PVP and PVA. Likewise, increasing borax could promote the gel crosslinking degree, thus making gels show a decrease in water content and swelling ratio, meanwhile shrinking the pores inside the hydrogels and finally enhancing the mechanical strength of hydrogels prominently. The developed hydrogel with high performances holds great potential for applications in biomedical and industrial fields.     

Dynamic light scattering of dilute PVA-borax aqueous solutions

Poly(vinyl alcohol)-borate (PVA-borate) complexes in dilute aqueous solutions were investigated by dynamic light scattering (DLS). Two relaxation modes obtained, were scattering vector-q² dependent and diffusive. The amplitude of slow mode was independent of borax concentration. In the very dilute PVA concentration regime ( [PVA] < 5 g/L), the PVA di-diol-borate complexation was dominated by intra-molecular reaction, and the fast mode correlation length ς_f which corresponds to the polymer chain dimension increased rapidly with borax concentrations lower than 0.06 M, and reached an asymptote in the higher borax concentration regime. However, for a dilute PVA aqueous solution with higher PVA content (i.e., [PVA] = 9 g/L), both intra-and inter-molecular PVA di-diol-borate complexations might happen. The chain expansion and shrinkage of PVA-borate complex with increasing borax concentration was observed due to the balance between the electrostatic repulsion of the charged diol-borate complexes and the intra-molecular crosslink induced by the intra-molecular di-diol-borate complexation.     

Novel collagen‐based hydrogels with injectable,self‐healing,wound‐healing properties via a dynamic crosslinking interaction

Collagen‐based hydrogels have gained significant popularity in biomedical applications; however, traditional collagen hydrogels are easily disabled for lack of self‐healing properties due to their non‐reversible bonds. Here, a self‐healing collagen‐based hydrogel has been developed based on dynamic network chemistry, consisting of dynamic imine linkages between collagen and dialdehyde guar gum, as well as diol‐borate ester bonds between guar gum and borax. In addition, macromolecular interactions amongst macromolecules are involved. The above‐mentioned interactions were validated by Fourier transform infrared spectroscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis and DSC. The as‐prepared collagen‐based hydrogels showed good injectability and rapid self‐healing capacity (within 3 min) as reflected from injection tests, optical microscope observations, rheological measurements, as well as self‐healing studies. In addition, the collagen‐based hydrogels showed accelerated wound‐healing properties. This study offers a facile strategy to endow self‐healing ability on collagen‐based hydrogels without any external stimulus, which show great application potential as wound dressings. © 2020 Society of Chemical Industry     

Highly Stretchable,Fast Self‐Healing,Responsive Conductive Hydrogels for Supercapacitor Electrode and Motion Sensor

Conductive hydrogels as potential soft materials have attracted tremendous attention in wearable electronic devices. Nonetheless, manufacturing intelligent materials that integrate mouldability, stretchability, responsive ability, fast self‐healing ability, as well as mechanical and electrochemical properties is still a challenge. Here, multifunctional conductive hydrogels composed of poly(vinyl alcohol) (PVA) and polypyrrole (PPy) nanotube are prepared using borax as cross‐linker. The existence of multicomplexation, entangled PVA chains, and interconnected PPy nanotubes, as well as extensive hydrogen bonding results in the fabrication of hierarchical network of PVA‐PPy hydrogels. PVA‐PPy hydrogels exhibit high stretchability (more than 1000%), multiresponsiveness, low density (0.95 g cm^?3), high water content (96%), and 15 s self‐healing features. Furthermore, the self‐healing supercapacitor electrode and motion sensor based on PVA‐PPy hydrogels demonstrate ideal performances. This facile strategy in this work would be promising to construct an excellent multifunctional soft material for various flexible electrode and biosensor.     

Multi-crosslinked,ecofriendly flame-retardant starch-based composite aerogels with high compression-resistance

To achieve ecofriendly aerogels with high mechanics and fire resistance, naturally-occurring pea starch was employed as the major substrate, and starch-based composite aerogels were fabricated by freeze–drying method. The corresponding hydrogels were obtained through successive gelatinization/dissolution of pea starch and polyvinyl alcohol (PVA), formation of precursor hydrogels via cyclic freeze–thawing, followed by treatment with borax aqueous solution. Borax treatment not only results in aerogels with high mechanics, but also endows the aerogels flame-retardant property. The final aerogels show high compression-resistance, with a maximum specific compressive modulus achieved at ~44 MPa and the yield strength 665 kPa, owing to the formation of a multi-crosslinked hybrid network. The maximum limiting oxygen index (LOI) of the aerogels exceeds 34. The peak heat release rate, the heat release capacity, and the total heat release decrease 74.5%, 76.2%, and 73.8% after borax treatment. Moreover, the obtained aerogels demonstrate good thermal stability and thermal insulation. The good performance is conducive for practical application.     

Polymer Effects on the Chemorheological and Drying Behavior of Alumina–Poly(vinyl alcohol) Gelcasting Suspensions

A new gelcasting system based on aqueous alumina (Al₂O₃)–poly(vinyl alcohol) (PVA) suspensions cross-linked via titanium ion complexation has been developed as a feedstock for bulk casting and solid freeform fabrication (SFF) routes. Its chemorheological properties, as measured by stress viscometry and oscillatory techniques, exhibited a strong dependence upon polymer hydrolysis and molecular weight. The gelation time of systems of constant PVA volume fraction (φ_PVA^soln) and cross-linker concentration decreased with increasing degree of hydrolysis and molecular weight, whereas their steady-state elastic modulus ( G' ) exhibited the opposite dependence. Stress evolution during drying of gelcast layers was measured in situ using a controlled-environment, cantilever deflection apparatus. Both the maximum and residual drying stresses increased with increasing degree of hydrolysis, with only a modest molecular weight effect observed.     

pH sensitivity and swelling behavior of partially hydrolyzed formaldehyde-crosslinked poly(acrylamide) superabsorbent hydrogels

Poly(acrylamide) superabsorbent hydrogel was synthesized through crosslinking method. Formaldehyde was used as a crosslinking agent. To achieve a hydrogel with high swelling capacity, the resulted hydrogels were saponified using NaOH solution at high temperature. During saponification, ammonia gas is produced from hydrolysis reaction of amide groups. The arising of ammonia produces porous structure in hydrogels, which is confirmed using scanning electron microscopy. The conversion of amide groups to carboxylate groups was identified by FTIR spectroscopy. The reaction variables in both crosslinking and hydrolysis reactions that affect the swelling of hydrogels were optimized. The swelling of the hydrogels in various salt solutions with various valencies and radii was studied. Also, the absorbency under load was measured. The hydrogels exhibited pH-sensitivity characteristics. A sharp swelling change was observed in lieu of pH variations in a wide range (1–13). The swelling variations were explained according to the swelling theory based on the hydrogel chemical structure. The pH-reversibility and on–off switching behavior makes the intelligent hydrogels as good candidates for considering as potential drug carries. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Rheological investigation of poly(vinyl alcohol)/poly(N-vinyl pyrrolidone) mixtures in aqueous solution and hydrogel state

Rheological behavior of poly(vinyl alcohol) (PVA) and poly(N-vinyl pyrrolidone) (PVP) mixtures in aqueous solutions and hydrogel state was investigated. The complex dependence of the viscosity on PVA/PVP mixture composition could be attributed to cumulative effects of electrostatic interactions, hydrogen bonding or association phenomena. Physical hydrogels were prepared by freezing/thawing method and their viscoelastic properties were followed as a function of number of cryogenic cycles and aging time at 37 °C. From swelling experiments, it was observed that the diffusion of water molecules into the hydrogel pores is Fickian (for low number of cryogenic cycles) and it becomes pseudo-Fickian as the sample is submitted to more than 10 freezing/thawing cycles. PVA/PVP hydrogels obtained by physical interactions present a high degree of tailorability and they are suitable candidates for biomedical applications.     

生物酶破胶压裂液破胶速度控制方法研究

针对压裂液采用生物酶破胶时存在的破胶速度快的问题,以羟丙基瓜胶冻胶压裂液为研究对象,通过压裂液成胶及破胶特性测定实验,优化了生物酶的浓度,筛选了生物酶反应的金属离子抑制剂类型,并优化了抑制剂浓度。研究表明,生物酶的最佳浓度为0.01%,抑制生物酶破胶速度的最佳离子为锰离子,浓度为0.5%~1.0%,对应的破胶时间为4~8 h,可以选择不同的生物酶浓度和抑制剂浓度来满足现场施工对压裂液破胶速度的要求。     

Synthesis of dual physical self-healing starch-based hydrogels for repairing tissue defects

Hydrogels have the potential to simulate and permeate body tissues. They can be used in many biomedical applications, such as drug delivery, wound dressings, contact lenses, synthetic implants, biosensors, and tissue engineering. Despite recent significant advances in hydrogel fabrication, with the introduction of double network hydrogels, with ionic or hydrogen bonds, there is still the challenge of achieving optimal mechanical properties with appropriate self-healing ability. To solve the above problem, in this study, a new type of starch/chitosan/PVA/borax hydrogel was synthesized by adopting the one-pot method. The effect of concentration and ratio of raw materials on the final properties of hydrogels, such as the degree of hydrophilicity, morphology, degradation, mechanical strength, and drug release rate, was investigated. The properties of hydrogels were examined by scanning electron microscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray diffractometry, and contact angle, which confirmed the composite synthesis and uniform distribution of HNT and curcumin. In addition, the composite hydrogel showed excellent mechanical properties. Drug release studies confirmed that the drug is slowly released from the nanocomposite hydrogels. The results showed that starch-based nanocomposite hydrogels could provide appropriate repairing potential for defects exposed to changeable parameters.     

甲苯二异氰酸酯和硼砂交联改性聚乙烯醇胶水的研究

探讨了硼砂和甲苯二异氰酸酯 ( TDI)对聚乙稀醇 ( PVA)胶水进行交联改性。采用正交实验法研究硼砂和 TDI的加入量对 PVA粘合剂的耐水性能和剥离强度的影响。利用硼砂、TDI与 PVA反应产物的水不溶性判断该交联反应的进程。研究结果表明 ,少量硼砂和 TDI能较大程度地提高 PVA胶水的耐水性能和粘结性能     

Boron Nitride Nanosheets Strengthened PVA/Borax Hydrogels with Highly Efficient Self-Healing and Rapid pH-Driven Shape Memory Effect

Self-healing hydrogels often possess poor mechanical properties which largely limits their applications in many fields. In this work, boron nitride nanosheets are introduced into a network of the poly(vinyl alcohol)/borax (PVA/borax) hydrogels to enhance the mechanical properties of the hydrogel without compromising the self-healing abilities. The obtained hydrogels exhibit excellent mechanical properties with a tensile strength of 0.410 ± 0.007 MPa, an elongation at break of 1712%, a Young's Modulus of 0.860 ± 0.023 MPa, and a toughness of 3.860 ± 0.075 MJ m⁻³. In addition, the self-healing efficiency of the hydrogels is higher than 90% within 10 min at room temperature. Benefiting from the excellent self-healing properties, the shapeability of the hydrogel fragments is observed using different molds. In addition, the hydrogels display rapid pH-driven shape memory effects and can recover to their original shape within 260 s. Overall, this work provides a new approach to hydrogels with integrated excellent mechanical properties, self-healing abilities, and rapid pH-driven shape memory effects.     

Comparative study of PEO and PVA hydrogels for removal of methylene blue dye from wastewater

In this study, poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) hydrogels were synthesized and evaluated as adsorbent for dye removal from wastewater using methylene blue (MB) in aqueous solution as probe. PEO samples were photochemically prepared by varying irradiation time (1–16 h), while PVA samples were synthetized with different concentration of glutaraldehyde (GA) (0.03–0.48%). The obtained hydrogels were obtained through the analysis of a swelling test, scanning electron microscopy, and adsorption studies. PEO hydrogels adsorption capacity was dependent on the irradiation time, while the PVA hydrogel adsorption capacity reduces with GA concentration. Both hydrogels demonstrated a Langmuir isotherm adsorption model at the equilibrium and pseudo‐second order kinetic fits properly. pH studies showed that when pH reaches 12, the adsorbed MB amount is close to 8 and 2 times higher than pH = 2 both hydrogels. Photochemical preparation of hydrogels shows an easier way of tuning their properties in order to maximize dye removal. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45043.     

Enhanced selective adsorption of cation organic dyes on polyvinyl alcohol/agar/maltodextrin water-resistance biomembrane

In this study, we designed a novel hydrogel composite membrane based on the combination of polyvinyl alcohol (PVA), agar, and maltodextrin through a facile solution-casting router. From Fourier-transform infrared spectroscopy, contact angle, scanning electron microscopy, and swelling analyses, the formation of hydrogen bonds between surface functional groups of PVA, agar, and maltodextrin was confirmed. As a result, the PVA/agar/maltodextrin membranes exhibited a more hydrophobic nature compared with pure PVA. The thermal stability and integrity of such obtained composite membranes were also elucidated by the evaluation of thermogravimetric analysis and mechanical behavior. Besides, the composite membrane exhibited high selective adsorption for cationic dyes, namely 20.2 mg g⁻¹ for methylene blue and 19.17 mg g⁻¹ for crystal violet at initial dye concentration of 100 mg/L, an adsorbent dosage of 0.1 g, contact time of 180 min, and solution pH 7, while anionic dyes such as congo red and methyl orange are approximately zero. The adsorption kinetics and isotherm of the as-prepared composite membranes were well fitted to the pseudo-second-order and Temkin model. The effect of factors, including contact time, solution pH, PVA content, and initial dye concentration on the adsorption capacity of the as-prepared composite membrane was also investigated in detail. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48904.     

油层水配制压裂液影响因素研究

本文在对油层水进行分析的基础上,研究油层水中各种金属离子、原油及悬浮物对压裂液基液粘度以及交联的影响。实验结果表明,悬浮物对基液粘度影响很大;羟丙基胍胶与硼砂交联实验得出Ca2+、悬浮物对其交联影响很大。     

Sticky poly(vinyl alcohol) hydrogels

Low-temperature gelation of poly(vinyl alcohol) (PVA) solution was performed in the presence of CaCl₂. The resulting PVA hydrogels showed high stickiness and excellent water-holding ability. These properties became more remarkable with increasing CaCl₂ concentration in solvent. Furthermore, both the peel strength on stainless steel and the degree of swelling of the PVA hydrogel drastically increased as CaCl₂ concentration was increased from 20 to 30 wt %. It is concluded that the sticky and water-holding properties of PVA hydrogels are intimately related to the state of hydration of Ca ions in aqueous environment. © 1993 John Wiley & Sons, Inc.     

Microstructural evaluation of poly(vinyl alcohol)‐based hydrogels obtained by freezing‐thawing technique: Thermal analysis and positron annihilation

In this work, semi‐interpenetrating polymer network (s‐IPN) hydrogels of poly(vinyl alcohol) (PVA) with different contents of water‐soluble sulfonated polyester (PES) were obtained by freezing and thawing cycles. The samples were characterized by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). PALS was used to determine the average free volume radius through lifetime measures of the ortho‐positronium (o‐Ps). Degree of crystallinity of the PVA/PES hydrogels was evaluated using the melting enthalpy ratios between the samples and the 100% crystalline PVA. The results show that an increase on the PES content leads to a decrease on the degree of crystallinity of the samples, reflecting an increase on the lifetimes (τ₃). These structural changes could be interpreted as a result of different polymer‐polymer interactions between PVA and PES in the hydrogels. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Swelling and mechanical properties of polyvinylalcohol hydrogels

Summary The swelling and mechanical properties of Poly(vinylalcohol) hydrogels were examined. It was found that the degree of swelling of PVA hydrogels depends on annealing temperature, but is almost independent of the initial polymer concentration. Mechanical properties of the hydrogels were also influenced by the degree of swelling. A shoulder was observed in double-logarithmic plots of stress vs. strain for the hydrogels, and became clearer as annealing temperature increased. This shoulder was closely related to the breakdown of the microcrystalline domains acting as crosslinks. Also, the shape of stress-strain curves plotted double-logarithmically for the hydrogels changed with the extension rate.