Effects of phenolic compounds on gelation behavior of gelatin gels

The effects of phenolic additives on the gelation behavior of gelatin gels were investigated using thermomechanical analysis (TMA) for study of gel‐melting temperature, dynamic mechanical analysis (DMA) for study of gel‐storage modulus and gel‐aging stability, viscometry for study of gelation time, and texture analyzer for study of gel strength and gel melting. Thermodynamically, the addition of 1,3‐benzenediol, 1,4‐benzenediol or 1,3,5‐benzenetriol favored the gelation process of gelatin solutions (increases in T_m and aging stability) due to the introduction of extra physical crosslinks among gelatin chains through hydrogen bonding, while the addition of 1,2‐benzenediol had a negative effect (decreases in T_m and aging stability) possibly due to intra‐hydrogen bonding of the additive molecule itself. All the phenolic compounds had little effect on gel moduli. Kinetically, the introduction of 1,2‐benzenediol or 1,4‐benzenediol slowed the gelation process, while introduction of catechin, a polyphenol, accelerated the first stage of the gelation process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 224–231, 2001     

Tightening of gelatin chemically crosslinked networks assisted by physical gelation

Developing the use of polymers from renewable sources to build hydrogels with tailored mechanical properties has become an increasing focus of research. The impact of the thermo‐reversible physical networks of gelatin (arising from the formation of triple‐helices) on the structure formation of a chemical network, obtained by crosslinking with glutaraldehyde (a non‐catalytic crosslinker), was studied using optical rotation, oscillatory rheology, and large strain mechanical deformation. We observed a direct correlation between the storage shear modulus of the chemical network grown in the gel state (i.e., simultaneously with the physical network) and the amount of gelatin residues in the triple‐helix conformation (χ). Since χ is directly affected by temperature, the value of the storage modulus is also sensitive to changes in the temperature of gel formation. χ values as low as 12% lead to an increase of the shear storage modulus of the crosslinked gel by a factor of 2.7, when compared to a chemical network obtained in the sol state (i.e., in the absence of a physical network). Our results show that the physical network acts as a template, which leads to a greater density of the chemical crosslinks and a corresponding higher elastic modulus, beyond what is otherwise achieved in the absence of a physical network. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1850–1858     

Differential size‐exclusion chromatography for the analysis of gelatin–polyelectrolyte complexes

Differential size‐exclusion chromatography (SEC) is used to characterize complexes formed between gelatin and two synthetic polyelectrolytes, sodium poly(styrenesulfonate) and sodium poly(2‐acrylamido‐2‐methylpropanesulfonate). The analysis is performed under aqueous, low‐salt conditions where maximum complexation between gelatin and the polyelectrolytes occurs. The adsorption effects that are commonly encountered in conventional SEC for gelatin and other charged polymers chromatographed under these solution conditions are minimized, because the columns are constantly equilibrated with the analytes in the mobile phase. Analyte solutions of identical composition, but of higher or lower concentration than that contained in the mobile phase, are injected, resulting in positive or negative detector responses, respectively. This method can separate the complexes from individual components, and can be used to determine relative sizes and stoichiometries of the complexes as a function of both the input ratio of gelatin to polyelectrolyte and the molecular weight of the polyelectrolyte. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 275–280, 1999     

Molecular Weight Influence on Gelatin Gels: Structure,Enthalpy and Rheology

Summary: This paper deals with the gelation properties of gelatins with various molecular weights. The samples are 4 different extracts from limed bones. We investigated firstly the structural properties: the helix amounts by optical rotation measurements and the enthalpy of helix formation and melting, with the same thermal protocols and for various concentrations. Comparison between the two methods allowed deriving the enthalpy of helix-coil transition and melting. Rheology was performed under the same conditions with a stress controlled rheometer working with a fixed, very small, strain during gel formation, maturation and melting. Correlation between all the measurements confirms the existence of the master curve for storage modulus versus gelatin concentration for all samples investigated.     

Polyelectrolyte complexes. II. Specific aspects of the formation of polycation/dye/polyanion complexes

We report on the formation of the polycation/dye/polyanion (PC/D/PA) complexes by the interaction between nonstoichiometric polycation/dye (PC/D) complexes with polyanions. Polycations differed in their content of the (N,N‐dimethyl‐2‐hydroxypropylene ammonium chloride) units in the main chain. Poly(sodium acrylate) (NaPA), poly(sodium 2‐acrylamido‐2‐methylpropane sulfonate) (NaPAMPS) and poly(sodium styrenesulfonate) (NaPSS) were used as polyanions. Crystal Ponceau 6R (CP6R) and Ponceau 4R (P4R) with two or three sulfonic groups were used as anionic dyes. The interaction between nonstoichiometric PC/D complexes and polyanions was followed by UV‐VIS spectroscopy, viscometry, and conductometry measurements. Formation of PC/D/PA complexes takes place mainly by the electrostatic interaction between the polyanion and the free positive charges of the nonstoichiometric PC/D complex. The stoichiometry and the stability of the tricomponent complexes depended on the polycation structure, the structure and molecular weight of polyanion, the dye structure, and the P/D molar ratio. A high amount of the dye was excluded from the complex before the end point when a branched polycation was used. The higher the solubility of the dye the lower the stability of the PC/D/PA complexes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 409–418, 1999     

Mechanism and kinetics of the crosslinking reaction between biopolymers containing primary amine groups and genipin

The reaction mechanism of chitosan, bovine serum albumin (BSA), and gelatin with genipin (a natural crosslinking reagent) was examined with infrared, ultraviolet–visible, and ¹³C NMR spectroscopies; protein‐transfer reaction mass spectrometry; photon correlation spectroscopy; and dynamic oscillatory rheometry. Two reactions that proceeded at different rates led to the formation of crosslinks between primary amine groups. The fastest reaction to occur was a nucleophilic attack on genipin by a primary amine group that led to the formation of a heterocyclic compound of genipin linked to the glucosamine residue in chitosan and the basic residues in BSA and gelatin. The second, slower, reaction was the nucleophilic substitution of the ester group possessed by genipin to form a secondary amide link with chitosan, BSA, or gelatin. A decreased crosslinking rate in the presence of deuterium oxide rather than water suggested that acid catalysis was necessary for one or both of the reactions to proceed. The behavior of the gel time with polymer concentration was consistent with second‐order gelation kinetics resulting from an irreversible crosslinking process, but was complicated by the oxygen radical‐induced polymerization of genipin that caused the gels to assume a blue color in the presence of air. The lower elastic modulus attained after a given time during crosslinking of the globular protein BSA as compared to the coiled protein gelatin, despite possessing more crosslinkable basic residues, demonstrated the importance of protein secondary and tertiary structures in determining the availability of sites for crosslinking with genipin in protein systems. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3941–3953, 2003     

Thermoreversible gelation on cooling and on heating of an aqueous gelatin–poly(N-isopropylacrylamide) conjugate

A polymer conjugate composed of 43 wt% gelatin and 57 wt% poly(N-isopropylacrylamide) (PNIPAAm) was prepared. The dynamic viscoelastic properties of an aqueous solution of the conjugate at the concentration of 5 wt% were examined. The solution was viscous fluid at 30°C and turned into an elastic homogeneous hydrogel upon heating above 34°C or upon cooling below 10°C. The resultant hydrogels turned back into a solution at the opposite temperature cycles of the gelation. It is considered that the driving force of the gelation is the intermolecular hydrophobic interaction of PNIPAAm blocks or the intermolecular helix association of gelatin blocks, respectively, on heating or on cooling. © 1998 John Wiley & Sons, Ltd.     

A comparative study of the rheological and structural properties of gelatin gels of mammalian and fish origins

From a long time, the traditional source of collagen has been mammals wastes, mainly bovine and porcine skins and bones. This study undertakes a comparative analysis of the viscoelastic behaviour and the structural properties of gelatin gels from both mammalian and fish origins. All the rheological measurements were carried out within the linear viscoelastic region for gelatin gels and tentatively related to structural characteristics. At a given concentration the pig gelatine exhibits a high storage modulus, G′, which indicates that the gel is stiffer than the fish one. The helical structures content measured by the specific rotation, [α], is also higher for the pig gelatin. Moreover, the gel is denatured in the same range of temperature, namely, around 30°C but the transition is sharper for fish gelatine. These differences between the gelatins lie in the amino-acid composition and molecular weight distribution which may influence the nature of the interchain junctions yielding to various gel mesostruture.     

Nanostructures of polyelectrolyte gel–surfactant complexes

Small‐angle X‐ray scattering was used to investigate the nanostructures of complexes formed by slightly crosslinked anionic copolymer gels of poly(sodium methacrylate‐co‐N‐isopropylacrylamide) [P(MAA/NIPAM)] with cetyltrimethylammonium bromide (CTAB), and didodecyldimethylammonium bromide (DDAB), respectively, at room temperature (∼ 23°C). Several highly ordered supramolecular structures were observed in the polyelectrolyte gel–surfactant complexes. In P(MAA/NIPAM)–CTA systems, in sequence with decreasing charge density of the P(MAA/NIPAM) copolymer chains, structures of the Pm3n space group cubic, face‐centered cubic close packing of spheres, and hexagonal close packing of spheres were determined at a charge content of ≥ 75, 67, and 50%, respectively. The spheres and rods in these structures were the spherical and cylindrical micelles formed by the self‐assembly of CTA cations with their paraffin chains inside. Both the aggregation number and the size of the micelles decreased with a decreasing charge density of the copolymer chains. In the P(MAA/NIPAM)–DDA systems, the bilayer lamellar structures formed at charge contents ≥ 75% transferred to bicontinuous cubic structures of the Ia3d space group at charge contents of 50–67%. The rods in the Ia3d cubic structures were formed by the self‐assembly of double‐tailed DDA cations with polar moieties inside. The formation of these highly ordered structures were driven by both electrostatic and hydrophobic interactions of the charged copolymer chains/surfactants and the surfactants/surfactants inside the charged gels. The structures became less ordered by further decreasing the charge content of the P(MAA/NIPAM) chains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2165–2172, 1999     

Thermoreversible gelation of poly(ethylene oxide) biodegradable polyester block copolymers

The gel to sol transition of aqueous solutions of di‐ and triblock copolymers consisting of poly(ethylene oxide) and biodegradable polyesters was studied as a function of temperature. The molecular weight and the chemical composition of the biodegradable blocks, (poly(l ‐lactic acid), poly(dl ‐lactic acid), poly(dl ‐lactic acid‐co‐caprolactone), and poly(dl ‐lactic acid‐co‐glycolic acid)) were varied to investigate the effects of chain packing and relative hydrophobicity on the gel to sol transition. The block copolymers studied formed micelles at lower concentrations in water, while the concentrated solutions experienced a gel to sol transition as the temperature increased. Further increase in temperature resulted in the precipitation of polymers. With increasing molecular weight and chain packing tendency of hydrophobic biodegradable block, the gel to sol transition occurred at lower concentrations and the transition temperature ranged from 0°C to over 90°C in a relatively narrow concentration range. The results obtained in this study confirm the relationship between gelation properties and polymer structure, as well as provide more information for these polymers in drug delivery applications. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 751–760, 1999     

Determination of the Molecular Weight and Particle Radius of Gelatin Micelles by Sedimentation and Optical Mixing Spectroscopy

Micelles formed from gelatin solutions were studied by sedimentation and optical mixing spectroscopy (OMS). We sampled gelatin solutions at various centrifugal speeds. Single exponential autocorrelation functions of the gelatin solutions were measured at 27°C and at 90° scattering angle by OMS. Calculation of the diffusion coefficient D from the measured autocorrelation function of scattered light has been carried out in this studied. A calculation of the micellar radius from the Stokes-Einstein equation was carried out. We also calculated the sedimentation coefficient and molecular weight of gelatin micelles and found that the polydisperse gelatin micelles were formed at 50mg/mL.     

Self‐assembled graphene oxide–gelatin nanocomposite hydrogels: Characterization,formation mechanisms,and pH‐sensitive drug release behavior

Novel physically crosslinked graphene oxide (GO)‐gelatin nanocomposite hydrogels were obtained by self‐assembly. The hydrogels with various ratios of GO to gelatin were prepared, and characterized by X‐ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy. The static and dynamic rheological properties of the hydrogels were investigated, along with the underlying hydrogel formation mechanisms. The storage modulus of the hydrogels (containing 98–98.5 wt % water) reached 114.5 kPa, owing to the relatively strong physical bonding (i.e., hydrogen bonding and electrostatic forces) between GO and gelatin. Drug release tests showed that the drug release from the hydrogel was pH‐dependent, with 96% of the model drug released in a neutral environment, compared to 28% released in an acidic medium. These hydrogels could have potential in pH‐sensitive drug delivery. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 356–367     

Viscometric study of gelatin in dilute aqueous solutions

The viscosities of aqueous solutions of gelatin at different temperatures were carefully measured in a common glass‐capillary Ubbelohde viscometer at dilute to extremely dilute concentrations. The adsorption effect that occurred in the viscosity measurements was theoretically analyzed and discussed. A theory based on Langmuir isotherms could adequately describe the existing data. Some structural information was obtained by the use of an iterative fitting procedure to treat the reduced viscosity data, which disclosed that individual gelatin chains underwent a coil‐to‐helix transition as the solution cooled from 40 to 15 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1804–1812, 2006     

Effect of gelatin molecular charge heterogeneity on formation of intermolecular complexes and coacervation transition

We investigate the molecular charge heterogeneity of gelatin molecules at three different pHs: isoelectric pH = 5 (pI), and at pHs = pI ± 1 by measuring the zeta potential distributions. Its effect on the formation of soluble intermolecular complexes induced by the presence of a nonsolvent, ethanol, was explored. The charge distributions were found to be symmetric at pH = 5, where the onset of binding, and formation of soluble complexes were observed to be facilitated by the presence of a small net charge (close to zero) with the molecules exhibiting polyampholyte (PA) behavior. These distributions turned asymmetric at pH = 6, yet complex formation and coacervation occurred. On the other hand, for pH = 4 samples, these distributions were found to be strongly asymmetric with the molecules possessing very high net positive charge, such a system did not yield coacervation. The PA to polyelectrolyte transition and its repercussion on coacervation has been discussed in the light of the experimental results obtained from electrophoresis, turbidimetry, atomic force microscopy, and nanoindentation studies. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1511–1520, 2007     

Thermal and hydrolytic degradation of electrospun fish gelatin membranes

The thermal and hydrolytic degradation of electrospun gelatin membranes cross-linked with glutaraldehyde in vapor phase has been studied. In vitro degradation of gelatin membranes was evaluated in phosphate buffer saline solution at 37 °C. After 15 days under these conditions, a weight loss of 68% was observed, attributed to solvation and depolymerization of the main polymeric chains. Thermal degradation kinetics of the gelatin raw material and as-spun electrospun membranes showed that the electrospinning processing conditions do not influence polymer degradation. However, for cross-linked samples a decrease in the activation energy was observed, associated with the effect of glutaraldehyde cross-linking reaction in the inter- and intra-molecular hydrogen bonds of the protein. It is also shown that the electrospinning process does not affect the formation of the helical structure of gelatin chains.     

Synthesis of polyisocyanide derived from phenylalanine and its temperature‐dependent helical conformation

Screw‐sense‐selective polymerization of the chiral isocyanide monomers derived from phenylalanine with NiCl₂ as a catalyst in methanol to yield helical‐conjugated polyisocyanide was investigated with respect to the thermal stability of its helical conformation. Poly(1‐tert‐butoxycarbonyl‐2‐phenylethyl isocyanide) (poly 1c ) took a stable helical conformer independent of the polymerization temperature. In poly(1‐ethoxycarbonyl‐2‐phenylethyl isocyanide) (poly 2c ), which had slightly smaller side groups, the helical conformation was thermally destabilized. The specific rotation and circular dichroism of poly 2c prepared at temperatures greater than 40 °C were considerably depressed in comparison with the values for poly 2c prepared at or below room temperature. Additionally, poly 2c prepared at low temperatures exhibited reversible temperature‐dependent specific rotation and circular dichroism, whereas poly 1c showed few changes. It is suggested that polyisocyanide derived from phenylalanine takes various helical conformers (i.e., from tightly to loosely coiled helices), the thermal stability of which depends on the size of the side group. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 399–408, 2002     

Helix formation and formaldehyde crosslinking in gelatin solutions

Optical rotation measurements on 5% gelatin solutions containing 0, 2, and 5% formaldehyde (w/w gelatin) are carried out at a series of temperatures between 20°C and 50°C. The solutions without formaldehyde show a diminished growth in rotation with rising temperatures, but even at 50°C there is still a steady rise in rotation with time. The addition of formaldehyde diminishes the optical rotation growth, and at 40°C and above solutions with 5% formaldehyde show no change in rotation at all. The specific rotation and time figures are fitted to a series of third-degree polynomial equations. It is shown that the data do not conform to any pattern of simple reaction kinetics. The mechanism of the formaldehyde–gelatin interaction is discussed.     

Thermal transitions in and structures of dried polyelectrolytes and polyelectrolyte complexes

Dynamic mechanical analysis (DMA) was used to explore the thermomechanical properties of dried polyelectrolytes and polyelectrolyte complexes (PECs) with different thermal and humidity histories. Although differences in the amount of water remaining in polyelectrolytes and PECs were small for ambient versus dessicator storage, the properties of polyelectrolyte‐based materials were drastically different for different humidity histories. Glass transition temperatures (T_gs) of poly(diallyldimethylammonium chloride) (PDADMAC) were shown to vary by 100 °C, depending on humidity and thermal histories. These parameters also change glassy storage modulus values by 100%. Furthermore, we observe that dried PDADMAC is highly lossy. DMA of dried poly(styrene sulfonate) (PSS) was more complex and did not exhibit a glass transition in the tested range. DMA of a PEC of PDADMAC and PSS revealed a humidity history‐dependent water melt in the first heating cycle, as well as storage modulus values of dried and annealed PECs that only varied by 17–26% over a 275 °C temperature range. Based on these results, we report for the first time humidity history as controlling structure and properties of polyelectrolyte‐based materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 684–691     

Crosslinking of gelatin membranes with ferulic acid or glutaraldehyde: Relationship between gas permeability and renaturation level of gelatin triple helices

This study investigated the gas separation membranes made with gelatin, crosslinked with ferulic acid (FA) and blended with polyethylene glycol (PEG) 200, by using a solvent‐free procedure. Gas permeation properties (He, N₂, O₂, and CO₂) of these “green membranes” were studied and discussed in relation with their structure. Differential scanning calorimetric measurements were carried out to determine the gelatin triple helical renaturation level. The lowest permeability values [He and CO₂ permeability (4.5 × 10^?2 Barrer) with CO₂/O₂ selectivity of 14.5] were reached with gelatin/PEG 200 uncrosslinked membranes showing the highest renaturation level (40%). Crosslinking with FA lead to less rigid and brittle materials than GTA and to 10 times more permeable membranes compared with uncrosslinked membranes. Membranes crosslinked with glutaraldehyde broke during gas permeation measurements. Results demonstrated that higher gas permeability values were closely related to lower renaturation level of gelatin. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 280–287     

An adsorption effect on the gel strength of dilute gelatin-stabilized oil-in-water emulsions

The incorporation of emulsion oil droplets into a gelatin gel leads to an initial increase in shear modulus at 25 °C for a gelatin concentration of 8 wt % but an initialdecrease for a concentration of 5 wt %. The latter result is consistent with a net lowering of the gelatin concentration available for gelation in the aqueous phase due to adsorption at the oil-water interface.