FT-NIR spectroscopy for non-invasive identification of natural polymers and resins in easel paintings

In the present study, the analytical strengths and limitations of near-infrared (NIR) spectroscopy to non-invasively characterize organic components in painting materials have been investigated. In spite of the increased amount of information available today from advanced modern analytical instrumentations dedicated to cultural heritage, the non-invasive identification of materials belonging to the wide class of organic compounds historically used in paintings is still a challenging task. Near-infrared spectroscopy offers several attractive features that make this technique particularly suitable to this purpose. In fact, it is non-invasive, allows for non-contact measurements in reflectance mode, gives molecular information on complex macromolecules, and can be performed on-site by means of portable devices. First-derivative transformation of reflectance spectroscopic data has been applied to provide a simple and fast way to deduce more information from NIR spectra. This approach has allowed spectral features to be identified that can be useful to distinguish different compounds belonging to the classes of lipids, proteins, and resins. To this purpose, at first, a spectral database of pure standard has been collected. Our analytical approach was then successfully validated on pictorial models reproducing the typical stratigraphy of an easel painting. As final step, the study of a real painting has been attempted and a drying oil, animal glue, and a terpenic natural resin, as well as an earth pigment were clearly identified, as cross-validated by GC-MS analysis.     

Physicochemical characterization of chemical hydrogels based on PVA

In this paper, we report on the physicochemical characterization of hydrogels recently obtained by crosslinking poly (vinylalcohol), PVA, with telechelic PVA (telPVA, bearing terminal aldehydic groups) via acetalization in aqueous solution. These gels were studied by equilibrium swelling, compression modulus measurements, and proton relaxometry experiments. Swelling and compression modulus data allow to estimate the average molecular weight of PVA chain between crosslinks, the average mesh size of the networks, and the polymer–solvent interaction parameter χ₁. The average mesh size of PVA‐telPVA compares well with domain dimensions of diffusionally confined water as detected by NMR relaxometry. Proton relaxometry also showed different network domains in which water is compartmentalized, indicating a complex heterogeneity. The study of the temperature behavior of the nuclear spin–spin relaxation times of the confined water was also considered. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1225–1233, 1999     

Studies on radiation synthesis of polyethyleneimine/acrylamide hydrogels

Polyethyleneimine(PEI)/acrylamide(AAM) hydrogels were synthesized by γ-radiation-induced polymerization/crosslinking of aqueous mixtures containing different ratios of PEI and AAM. The gel percentage and equilibrium degree of swelling (EDS) of the synthesized hydrogels were investigated. The compositions of the hydrogels produced were found to be different from the feed composition. Ion-chromatography technique was used to determine the amount of Pb (II) and Cd (II) absorbed by the hydrogel. The maximum binding capacity of the PEI/AAM hydrogels, for Pb and Cd was found to be 19 and 12.6 mg/g, respectively, (at 100 ppm). PEI/AAM hydrogels had better metal uptake efficiency than the pure AAM hydrogel at concentrations less than 50 ppm. Pure PEI was observed to be highly degrading type polymer on exposure to gamma radiation. TGA and FT-IR techniques were used to characterize the prepared hydrogels.     

Characterization of structured acrylamide/acrylic acid hydrogels by TMDSC and microscopy

The thermal behavior of hydrogels synthesized by solution polymerization between acrylamide, acrylic acid and diglycidyl acrylate (DGA) as a crosslinking agent was investigated. The structure of the hydrogel can be tightly controlled with the reaction temperature. This method produces a new type of hydrogels, which exhibit well defined structures at various scales of length simultaneously. These multi-structured hydrogels are hydrophilic, elastic, water insoluble, and soft polymers with an anisotropic optical response. The structure was observed by scanning electron microscopy (SEM), polarized light microscopy (PLM) and macroscopic visualization (CCD camera). In addition, structural transitions in the hydrogels were monitored by temperature modulated differential scanning calorimetry (TMDSC). Severe heating tests in an adiabatic oven were performed to analyze decomposition of the material. Fourier transform infrared (FTIR) spectroscopy was used to qualitatively analyze the hydrogels samples exposed to a sudden thermal treatment.     

Swelling characterization of novel ternary semi‐IPNs: acrylamide/1‐vinylimidazole/PEG hydrogels

New ternary semi interpenetrating polymer networks (semi‐IPNs) systems containing acrylamide (AAm), 1‐vinylimidazole (VI) and poly (ethylene glycol) (PEG) have been prepared. AAm/VI hydrogels and semi‐IPN's, poly (AAm/VI/PEG) with 0.25, 0.50, 0.75 and 1.00 g of PEG (per 1.00 g AAm) were prepared by free radical solution polymerization in aqueous solution of AAm with VI as comonomer and a multifunctional crosslinker such as 1,4 butanediol dimethacrylate (BDMA). Swelling experiments were performed in water at 25°C, gravimetrically. The influence of VI and PEG content in hydrogels were examined. AAm/VI and AAm/VI/PEG hydrogels showed large extents of swelling in aqueous media, the swelling being highly dependent on the chemical composition of the hydrogels. Percentage swelling ratio of AAm/VI hydrogels and AAm/VI/PEG hydrogels was shown as 650–4167%. The values of equilibrium water content (EWC) of the hydrogels are between 0.8990 and 0.9750. Diffusion behavior was investigated. Water diffusion into hydrogels was found to be non‐Fickian in character. Copyright © 2007 John Wiley & Sons, Ltd.     

Physicochemical characterization and drug release properties of PDMAEMA/OSA Semi‐IPN hydrogels with microporous structure

A new poly(2‐(dimethylamino) ethyl methacrylate)/oxidized sodium alginate (PDMAEMA) semi‐interpenetrating network (Semi‐IPN) hydrogel with microporous structure was prepared by using PDMAEMA microgels as an additive during the polymerization/crosslinking process. The interior morphology characterized by scanning electron microscopy showed the Semi‐IPN hydrogels have different pore sizes by changing the amount of microgels. The hydrogels were also characterized by using Fourier transform infrared and DSC. The swelling behaviors of hydrogels indicated that the hydrogels have excellent pH and temperature sensitivity. Bovine serum albumin was entrapped in the hydrogels and the in vitro drug release profiles were established in different buffer solutions at various temperatures. The release behaviors of the model drug were dependent on the pore size of the hydrogels and environmental temperature/pH, which suggested that these materials have potential application as intelligent drug carriers. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and characterization of acrylamide/acrylic acid hydrogels crosslinked using a novel diacrylate of glycerol to produce multistructured materials

In this work we propose a new crosslinking agent and the method to use it for the synthesis of acrylate based hydrogels. The use of this diacrylate of glycerol, synthesized in our laboratory, allows the generation of materials with well defined micro‐structures in the dry state, unique meso‐ and macro‐structures during swelling, and enhanced mechanical properties and swelling capacity in water. These properties depend on the crosslinking agent concentration, as well as synthesis thermal history. Poly(acrylamide‐co‐acrylic acid) hydrogels are commonly crosslinked with N, N′‐methylenebisacrylamide or N‐isopropylacrylamide. Here we obtain and use a new crosslinking agent, obtained from the reaction between glycerol and acrylic acid to produce a Diacrylate of glycerol (DAG). Two synthesis methods at equivalent molar ratio of acrylamide/acrylic acid (AM/AA) were analyzed. The mechanical properties, the swelling capacity, and the morphology at microscale of these hydrogels showed a well defined transition at a critical concentration of crosslinking agent. DAG induces the generation of hydrogels with hierarchichal structure. The micro‐structure surface morphology was investigated by scanning electron microscopy, the meso‐structure by polarized light microscopy and the macro‐structure by CCD imaging. The hydrogels with hierarchical structures showed improved mechanical properties when compared with structureless hydrogels. Control of the microstructure allows the generation of materials for different applications, i.e. templates or smart materials that interact with electromagnetic radiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2667–2679, 2008     

Swelling studies of copolymeric acrylamide/crotonic acid hydrogels as carriers for agricultural uses

In this study, highly swollen acrylamide/crotonic acid hydrogels (in a rod form) containing some inorganic salts such as ammonium nitrate, potassium nitrate and ammonium sulphate used as fertilizer, an agricultural drug such as Dalapon (sodium 2,2‐dichloropropionate) and two crosslinkers such as ethylene glycol dimethacrylate and 1,4‐butandiol dimethacrylate were prepared by copolymerization of acrylamide and crotonic acid with γ‐radiation. As a result of swelling tests, the influence of γ‐ray dose and relative content of crotonic acid on the swelling properties, the diffusional behavior of water, diffusion coefficients and network properties of the hydrogel systems were examined. Acrylamide/crotonic acid hydrogels containing these salts and agricultural drug were swollen in the range 2045–400% in water, while polyacrylamide hydrogels swelled in the range 660–700%. Water intake of hydrogels followed a nonFickian‐type diffusion. Copyright © 2000 John Wiley & Sons, Ltd.     

Thermodynamics of phase equilibrium for systems containing N-isopropyl acrylamide hydrogels

Hydrogels are crosslinked polymers of hydrophilic monomers. Hydrogels can swell and shrink in aqueous solutions. The swelling behavior of hydrogels and the encountered phase behavior are of interest in many areas, e.g., in biotechnology, membrane science and controlled drug release. This contribution presents the criteria for such phase equilibria and a previously developed thermodynamic model for correlating/predicting the swelling and shrinking of hydrogels. The application of the method is demonstrated by describing the swelling equilibrium of some synthetic, non-ionic N-isopropyl acrylamide (N-IPAAm) hydrogels in aqueous solutions of sodium chloride at 298 K. Furthermore, new experimental results are presented for the degree of swelling of synthetic hydrogels that contain – besides the non-ionic monomer N-IPAAm – either a combination of a cationic comonomer (here, N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA)) and an anionic comonomer (here, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)) or a zwitterionic comonomer (here, [3-(methacryloylamino)propyl]dimethyl(3-sulfopropyl)ammonium hydroxide inner salt (MPSA)). These gels were equilibrated with aqueous solutions of sodium chloride at 298 K.     

Properties of conductive polymer hydrogels and their application in sensors

Conductive polymer hydrogels (CPHs), which combine the unique advantages of hydrogels and organic conductors, have received wide attention due to their adjustable mechanical properties, biocompatibility, self‐healing, hydrophilicity, and ease of preparation. With doping engineering and incorporation with other functional nanomaterials, CPHs have exhibited excellent physical/chemical properties. CPHs have been widely used in various electronic devices, especially in the field of sensors due to its sensitivity to external stimuli. This review summarizes recent progress in CPHs from the aspect of the CPHs' properties and their application in advanced sensor technology. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1606–1621     

Swelling of N-isopropyl acrylamide hydrogels in aqueous solutions of sodium chloride

Hydrogels are three-dimensional networks of hydrophilic polymer chains. Hydrogels can absorb/desorb water and hydrophilic solutes. This behavior is called swelling/shrinking, as it is accompanied by a volume change. The amounts of absorbed substances depend on the structure of the hydrogel and the composition of the coexisting liquid phase. This paper deals with experimental investigations of the swelling behavior of nonionic, chemically crosslinked, synthetic hydrogels of N-isopropyl acrylamide. The swelling equilibrium of some hydrogels in aqueous solutions of sodium chloride was investigated at 298 K. The experimental results are presented, discussed and correlated/predicted with a thermodynamic model which combines an expression for the Gibbs energy of a liquid phase with an expression for the Helmholtz energy of an elastic network.     

Preparation and characterization of PVA/SA/HA composite hydrogels for wound dressing

A composite hydrogel based on, by introducing, polyvinyl alcohol, sodium alginate, and hyaluronic acid was fabricated using CaCl₂ as a cross-linker. The physical properties including morphology, water vapor transmission rate, and hydrophilicity were investigated. All PVA/SA/HA composite hydrogels with different compositions had highly homogeneous and interconnected pores, and the morphologies of the PVA/SA/HA hydrogels ranged from fibrous structure to irregular structure with increasing content of SA. The introduction of sodium alginate enhanced the hydrophilicity and water vapor transmission capacity of the hydrogel; however, the hydrophilicity of the composite hydrogels decreased with the increasing cross-linker content.     

How mobile NMR can help with the conservation of paintings

The conservation of paintings is fundamental to ensure that future generations will have access to the ideas of the grand masters who created these art pieces. Many factors, such as humidity, temperature, light, and pollutants, pose a risk to the conservation of paintings. To help with painting conservation, it is essential to be able to noninvasively study how these factors affect paintings and to develop methods to investigate their effects on painting degradation. Hence, the use of mobile nuclear magnetic resonance (NMR) as a method of investigation of paintings is gaining increased attention in the world of Heritage Science. In this mini-review, we discuss how this method was used to better understand the stratigraphy of paintings and the effect different factors have on the painting integrity, to analyze the different cleaning techniques suitable for painting conservation, and to show how mobile NMR can be used to identify forgeries. It is also important to keep in mind its limitations and build upon this information to optimize it to extend its applicability to the study of paintings and other precious objects of cultural heritage.     

Microfluidic fabrication of microengineered hydrogels and their application in tissue engineering

Microfluidic technologies are emerging as an enabling tool for various applications in tissue engineering and cell biology. One emerging use of microfluidic systems is the generation of shape-controlled hydrogels (i.e., microfibers, microparticles, and hydrogel building blocks) for various biological applications. Furthermore, the microfluidic fabrication of cell-laden hydrogels is of great benefit for creating artificial scaffolds. In this paper, we review the current development of microfluidic-based fabrication techniques for the creation of fibers, particles, and cell-laden hydrogels. We also highlight their emerging applications in tissue engineering and regenerative medicine.     

Swelling equilibria and dye adsorption studies of chemically crosslinked superabsorbent acrylamide/maleic acid hydrogels

Superswelling acrylamide (AAm)/maleic acid (MA) hydrogels were prepared by free radical polymerization in aqueous solution of AAm with MA as comonomer with some multifunctional crosslinkers such as trimethylolpropane triacrylate and 1,4-butanediol dimethacrylate. AAm/MA hydrogels were used in experiments on swelling and adsorption of a water-soluble monovalent cationic dye such as Basic Blue 17 (Toluidin Blue). As a result of dynamic swelling tests, the influence of relative content of MA on the swelling properties of the hydrogel systems was examined. AAm/MA hydrogels were swollen in the range 1660-6050% in water, while AAm hydrogels swelled in the range 780-1360%. Equilibrium water content of AAm/MA hydrogels were calculated in the range 0.8873-0.9837. Water intake of hydrogels followed a non-Fickian type diffusion. The uptake of the cationic dye, BB-17 to AAm/MA hydrogels is studied by batch adsorption technique at 25 °C. In the experiments of the adsorption equilibrium, S-type adsorption in Giles's classification system was found. The binding ratio of hydrogel/dye systems was gradually increased with the increase of MA content in the AAm/MA hydrogels.     

Synthesis and characterization of poly(acrylic acid‐co‐acrylamide)/hydrotalcite nanocomposite hydrogels for carbonic anhydrase immobilization

To combine the advantages of a biopolymer with hydrotalcite in an enzyme immobilization system, the intercalation polymerization was used to prepare poly(acrylic acid‐co‐acrylamide)/hydrotalcite (PAA‐AAm/HT) nanocomposite hydrogels using sodium methyl allyl sulfonate as intercalation agent. Transmission electron microscopy, X‐ray diffraction, and Fourier transform infrared spectroscopy results revealed that sodium methyl allyl sulfonate chains entered into the interlayer of HT, the interaction between them has taken place, and HT was dramatically exfoliated into nanoscale and homogeneously dispersed in the PAA‐AAm matrix. Transmission electron microscopy and cryo scanning electron microscope results showed that dried hydrogels were regular spherical particles, and swollen hydrogels revealed homogeneous porous network structures. Then, PAA‐AAm/HT nanocomposite hydrogels were used to immobilize carbonic anhydrase (CA), and the CO₂ hydration activities of free enzyme and immobilized enzyme were evaluated. Results showed that immobilized CA retained the majority of the enzyme activity. The reason may be the formation of a microenvironment almost all of which is composed of free water inside the porous network structures. Therefore, the immobilized CA is of great potential in the removal of trace CO₂ from the closed spaces. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3232–3240, 2009     

Electrochemical characterization of hydrogels for biomimetic applications

Hydrogels are increasingly being recognized as having potential in bio‐compatible applications. In previous work, we investigated the feasibility of poly(ethylene glycol)‐dimethacrylate (PEG‐1000‐DMA) and poly(ethylene glycol)‐diacrylate (PEG‐400‐DA) polymerized using either a chemical initiator (C) or a photoinitiator (P) to encapsulate and stabilize biomimetic membranes for novel separation technologies or biosensor applications. In this paper, we have investigated the electrochemical properties of the hydrogels used for membrane encapsulation. Specifically, we studied the crosslinked hydrogels by using electrochemical impedance spectroscopy (EIS), and we demonstrated that chemically crosslinked hydrogels had lower values for the effective electrical resistance and higher values for the electrical capacitance compared with hydrogels with photoinitiated crosslinking. Transport numbers were obtained using electromotive force measurements and demonstrated that at low salt concentrations, both PEG‐400‐DA‐C and PEG‐400‐DA‐P hydrogels presented an electropositive character whereas PEG‐1000‐DMA‐P was approximately neutral and PEG‐1000‐DMA‐C showed electronegative character. Sodium transport numbers approached the bulk NaCl electrolyte value at high salt concentrations for all hydrogels, indicating screening of fixed charges in the hydrogels. The average salt diffusional permeability 〈P_s〉 and water permeability 〈P_w〉 were found to correlate with EIS results. Both PEG‐1000‐DMA‐C and PEG‐400‐DA‐C had higher 〈P_s〉 and 〈P_w〉 values than PEG‐1000‐DMA‐P and PEG‐400‐DA‐P hydrogels. In conclusion, our results show that hydrogel electrochemical properties can be controlled by the choice of polymer and type of crosslinking used and that their water and salt permeability properties are congruent with the use of hydrogels for biomimetic membrane encapsulation. Copyright © 2011 John Wiley & Sons, Ltd.     

Physicochemical and biological characterization of ceramide-containing liposomes: paving the way to ceramide therapeutic application

Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high "packing parameter" (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short- or medium-chain ceramides are unstable because of their very high (>1.3) PP. To overcome this major obstacle, we included the lipopolymer 2kPEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (>1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of 14C-labeled ceramides shows that the C6Cer, but not C16Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C6Cer. The activity of 2kPEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C6Cer-containing nanoliposomes. The 2kPEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C6Cer to reach the tumor site and to demonstrate therapeutic efficacy.     

Network structure and swelling behavior of poly(acrylamide/crotonic acid) hydrogels in aqueous salt solutions

Hydrogels with various ionic group contents were prepared from acrylamide and crotonic acid (CrA) monomers with 0–12.9 mol % CrA in aqueous solutions by radiation‐induced polymerization and gelation with γ rays from a ⁶⁰Co source. The volume swelling ratio of the poly(acrylamide/crotonic acid) hydrogels was investigated as a function of the pH and ionic strength of the swelling medium and the type of counterion in the swelling medium. The volume swelling ratio increased with an increase in pH and a decrease in the ionic strength. The volume swelling ratio of these hydrogels was evaluated with an equation, based on the Flory–Huggins thermodynamic theory, the James–Guth phantom network theory, and the Donnan theory of swelling of weakly charged ionic gels, that was modified here for the determination of the molecular weight between crosslinks (M_c) and the polymer–solvent interaction parameter (χ). The modified equation described very well the swelling behavior of the charged polymeric network. The same equation also provided the simultaneous measurement of these parameters for the systems investigated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1656–1664, 2003     

pH-thermoreversible hydrogels. I. Synthesis and characterization of poly(N-isopropylacrylamide/maleic acid) copolymeric hydrogels

N-isopropylacrylamide (NIPAAM)/maleic acid (MA) copolymeric hydrogels were prepared by irradiating the ternary mixtures of NIPAAM/MA/Water by γ-rays at ambient temperature. The influence of external stimuli such as pH and temperature of the swelling media on the equilibrium swelling properties was investigated. The hydrogels showed both temperature and pH responses. The effect of comonomer concentration and irradiation dose on the swelling equilibria and phase transition was studied. For the characterization of these hydrogels, the diffusion behaviour and molecular weight between crosslinks were investigated.