Rheological characterization of high concentrated MFC gel from kenaf unbleached pulp

Microfibrillated Cellulose (MFC) was isolated from unbleached kraft pulp derived from kenaf bast fiber. MFC gels with different concentrations were manufactured from pulp with varying initial consistencies. The MFC was diluted to a consistency of around one percent using distilled water and was further homogenized by passing through a microfluidizer. In order to gain an understanding of the relative changes in behavior of the resulting MFC gels, their rheological properties were characterized. Results show that all of the gels exhibit a shear-thinning behavior. It was also determined that the rheological characteristics improved with increasing gel concentration, which was achieved by using higher pulp suspension consistencies. Diluted MFC that was derived from highly concentrated MFC had more variable modulus under the same strain and frequency compared to poorly concentrated MFC. But such a strong effect was not observed for viscosity. Additionally, the value of G′, ranging from 76 to 5325 Pa under the studied concentrations, was found to be fourfold the value of G″. In the low frequency range, G′ was almost independent of frequency, but was dependent on gel consistency with a coefficient of 3, indicating that MFC gels are elastic. These results show that it is possible to produce MFC gels with good rheological properties from high consistency kenaf pulp suspensions.     

Comparative rheological study of ionic semi-IPN composite hydrogels based on polyacrylamide and dextran sulphate and of polyacrylamide hydrogels

Ionic semi-interpenetrating polymer networks composite hydrogels were synthesized by free-radical polymerization using dextran sulphate (DxS), acrylamide as monomer and N,N′-methylene(bis)acrylamide as cross-linking agent. The viscoelastic properties of these composite hydrogels were investigated by oscillatory shear measurements under small deformation conditions comparative with those of polyacrylamide gels. Changes of the rheological properties of composite hydrogels have been studied in terms of polymerization temperature, cross-linker ratio, initial monomer concentration and molar mass of DxS. The results showed that the stability of the composite hydrogels obtained at room temperature (22?°C) was relatively low because the storage modulus (G′) was only eight times higher than the loss modulus (G″), while for those obtained by cryopolymerization (?18?°C), the stability was improved, the G′ values being about 30 times higher than those of G″. This behaviour indicated that, by conducting the synthesis of hydrogels below the freezing point of the reaction solutions, an enhancement of the hydrogels elasticity was achieved. The network parameters, i.e. the average molecular weight between two cross-links and the cross-link density of the composite hydrogels prepared at ?18?°C, were estimated from rheological data.     

Rheometry of an androstanol steroid derivative paramagnetic organogel. Methodology for a comparison with a fatty acid organogel

This paper compares and contrasts the behavior of two different gelators using rheological and neutron scattering methods. The flow properties of a steroid-made paramagnetic organogel in cyclohexane are presented. The original gelator STNO is important in the class of organogels as being one of the most documented and as such is a good candidate for comparisons with another reference system, the 12-hydroxy stearic acid (HSA) gel. The linear viscoelastic regime of deformations of STNO gels is identified and analyzed in the context of self-assembled fibrillar networks. The linear elasticity scales with the concentration as G′ α C² similarly with HSA organogels, and both systems can be considered as cellular materials. Rheological and neutron scattering experiments show that the kinetics of gel formation exhibits long equilibration times corresponding to the elaboration of entangled fibrillar aggregates. Comparison of the linear elasticities between STNO and HSA gels demonstrates that HSA gels are much more stiffer (G′_HSA/G′_STNO∼2700). Contributions from the cross-sectional sizes, the mesh size of the networks, the solubility concentrations, and the Young's modulus of the materials are discussed. Non-linear flow properties are also compared using thixotropic loops. They indicate that the transduction of the chirality from the molecular to the supramolecular stages is more efficient with STNO gels having strong chiral junction zones. Simplified scattering and optical protocols are proposed to facilitate comparisons between different organogels.     

The Influence of the Addition of Nuts on the Thermal and Rheological Properties of Wheat Flour

The aim of the study was to assess the influence of replacing wheat flour with hazelnuts or walnuts, in various amounts, on the thermal and rheological properties of the obtained systems. The research material were systems in which wheat flour was replaced with ground hazelnuts (H) or walnuts (W) in the amount of 5%, 10%, and 15%. The parameters of the thermodynamic gelatinization characteristics were determined by the differential scanning calorimetry method. In addition, the pasting characteristics were determined with the use of a viscosity analyzer and the viscoelastic properties were assessed. Sweep frequency and creep and recovery tests were used to assess the viscoelastic properties of the tested gels. It was found that replacing wheat flour with nuts increased the values of gelatinization temperature, gelatinization, and retrogradation enthalpy, and the degree of retrogradation. The highest viscosity was characteristic of the control sample (2039 mPa·s), and the lowest for the paste with 15% addition of walnuts (1120 mPa·s). Replacing the flour with nuts resulted in a very visible reduction in the viscosity of such systems. In addition, gels based on the systems with the addition of H and W were weak gels (tan δ = G″/G′ > 0.1), and the values of G′ and G″ parameters decreased with the increased share of nuts in the systems. Creep and recovery analysis indicated that the systems in which wheat flour was replaced with hazelnuts were less susceptible to deformation compared to the systems with the addition of W.     

Effect of Soil Type and Application of Ecological Fertilizer Composed of Ash from Biomass Combustion on Selected Physicochemical,Thermal, and Rheological Properties of Potato Starch

The aim of the study was to assess the effect of soil type and the application of fertilizer composed of ashes from biomass combustion to potatoes on selected physicochemical, rheological, and thermal properties of potato starches isolated by using the laboratory method. Potatoes were grown in Haplic Luvisol (HL) and Gleyic Chernozem (GC) soil and fertilized with different doses of biomass combustion ash (D1–D6) with different mineral contents. The thermodynamic characteristics of gelatinization and retrogradation were identified by DSC. The analyses of rheological properties included the determination of the gelatinization characteristics by using the RVA method, flow curves, and assessment of the viscoelastic properties of starch gels. It was found that the starches tested contained from 24.7 to 29.7 g/100 g d.m. amylose, and the clarity of 1% starch pastes ranged from 59% to 68%. The gelatinization characteristics that were determined showed statistically significant differences between the starches analyzed in terms of the tested factors. The value of maximum viscosity and final viscosity varied, respectively, in the range of 2017–2404 mPa·s and 2811–3112 mPa·s, respectively. The samples of the potato starches studied showed a non-Newtonian flow, shear thinning, and the phenomenon of thixotropy. After cooling, the starch gels showed different viscoelastic properties, all of which were weak gels (tan δ = G″/G′ > 0.1).     

pH-responsive microgels containing hydrophilic crosslinking co-monomers: shell-exploding microgels through design

pH-responsive microgels are crosslinked polymer colloids that swell when the pH approaches the pK _a of the particles. They have potential application for injectable gels for tissue repair and drug delivery systems. This study focuses on the pH-triggered gelation behaviour of a series of poly (EA/MAA/X) microgels. EA and MAA are ethylacrylate and methacrylic acid. Here, we investigate the effect of crosslinking monomer type (X) on microgel properties. The crosslinking monomers used were poly (ethyleneglycol) dimethacrylate (PEGD), ethyleneglycol dimethacrylate (EGD) and butanediol diacrylate (BDD). The microgel containing PEGD (m-PEGD) is a new system. The microgel containing BDD (m-BDD) was used as a control system. The concentrated microgel dispersions formed physical gels when the pH was increased to 5.3?C6.7, and the polymer volume fractions (? _p ) were above about 0.05. Evidence from photon correlation spectroscopy (PCS) and dynamic rheology was presented for abrupt pH-triggered increases, and then decreases of the hydrodynamic diameters for m-PEGD and the microgel prepared using EGD (m-EGD). This appears to be tuneable through crosslinker structure. An unexpected gelation behaviour, which may involve a new gel state for microgels, was found for m-PEGD dispersions. Uniquely, those dispersions formed gels at pH values less than the microgel's pK _a . This behaviour was linked to an outer-shell electrostatic repulsive interaction. The data point to a phenomenon, whereby the m-PEGD shells appear to explode at pH values above 7.0. The control microgel prepared, using BDD (m-BDD), did not show any evidence of shell fragmentation at any pH. That microgel has potential as a model pH-responsive microgel system in that the properties measured by PCS and rheology agreed well. To probe that system in more detail, the rheological data for m-BDD was analysed using scaling theory. The variation of the storage modulus (G') with ? _p gave a scaling exponent of 2.0.     

The photoelastic behaviour and small-angle x-ray scattering of ionized gels of copolymers of 2-hydroxyethyl methacrylate with methacrylic acid

The photoelastic behaviour and small-angle X-ray scattering of ionized, loosely crosslinked gels of copolymers of 2-hydroxyethyl methacrylate (HEMA) with methacrylic acid (MA) swollen in 0.5 M NaCl are characterized by the following specific features. Increasing neutralization of gels causes a steep decrease in the volume fraction of the polymer in the gel, v₂, and in the modulus G. The time dependences of photoelastic functions, which are the most pronounced for PMA, become weaker with swelling. The slopes of the dependence of the extrapolated equilibrium modulus log G_c on log v₂ is large (φ 0.6) 1.5 for PMA. The stress-optical coefficiént C passes at about v₂φ 0.25 through a pronounced maximum, probably due to a decisive effect of the shape contribution, caused by possible correlation structures in the copolymers. Increasing ionization leads to a rise in swelling, first increasing the contrast of the correlation structures but then obviously causing their dissociation. The small-angle X-ray data indicate a considerable heterogeneity of the order 10 nm in size of gels in the ionized state: the structural characteristics calculated using a two-phase model are in agreement with the assumed shape contribution to birefringence.     

Radiation crosslinking of polyvinylidenefluoride

The crosslinking efficiency of radiation (electron beam from linear electron accelerator) in vinylidenefluoride-hexafluoropropylene and vinylidenefluoride homopolymer has been studied and quantified. The yield of crosslinking G_c and scission G_s, respectively, was determined using sol-gel analysis and swelling-elasticity measurements. The agreement between the G_c values for copolymer and homopolymer obtained by the two methods are very good. The δ parameter value for the system dimethylsulfoxide-vinylidenefluoride copolymer was determined. Different values of G_c were obtained for compounds of copolymer containing sensitizer using the two independent methods.     

Extensional rheology of cellulose/NaOH/urea/H<Subscript>2</Subscript>O solutions

The extensional flow behaviors of cellulose/NaOH/urea/H₂O solution were investigated by using capillary breakup extensional rheometry (CaBER). The effects of temperature, storage time and cellulose concentrations on both the storage modulus G′ and the loss modulus G″ were also analyzed. For 2 wt% cellulose solution, the G′, G″ and filament lifetime remained unchanged after long storage time. While, for 4 wt% cellulose solution, physical gels could form at either higher temperature or for longer storage time, and the filament lifetime, the relaxation time (λ _e ) and the initial extensional viscosity (η _e0) first increased and then decreased with increase of the storage time. The transition points of the filament lifetime shifted to lower storage time with the increase of the temperature. The η _e0 is proportional to λ _e . The results presented suggest that the extensional properties of the cellulose/NaOH/urea/H₂O solution first increase and then decrease during the gelation process, and the spinning time, which decreases linearly with the increase in the storage temperature, must be controlled below the time that η _e0 starts to decrease.     

Photon transmission technique for studying swelling and drying kinetics of heterogeneous gels formed with various cross-linker contents

Bifunctional polytetrahydrofuran (PTHF) macromonomer was synthesized by the termination of living polymerization of tetrahydrofuran (THF) initiated by triflic anhydride and subsequent termination by sodium methacrylate. PTHF macromonomer thus prepared was copolymerized with methyl methacrylate (MMA) by free radical polymerization to yield networks in various cross-linker densities. These PTHF-PMMA gels were used for swelling experiments in chloroform vapor. Drying processes were monitored after removing the gels from the solvent vapor. Photon transmission from PTHF-PMMA gels was monitored during swelling and drying processes using UV-visible spectrophotometer. Transmitted light intensities, I_tr from these gels decreased when they are subjected to chloroform vapor. Decrease in I_tr was attributed to the heterogeneous lattice structure of PTHF-PMMA gels which appeared during swelling. Decrease in I_tr was modeled using Li-Tanaka equation from which time constants, τ₁ and τ^′₁ and cooperative diffusion coefficients, D₀ and D^′₀ were determined. Increase in I_tr after removing of vapor from the cell was observed and attributed to the decrease in heterogenity of lattice structures during drying of the corresponding gels. Time constants, τ₂ for drying processes were also determined.     

Self‐Assembly of Amphiphilic Janus Dendrimers into Mechanically Robust Supramolecular Hydrogels for Sustained Drug Release

Compounds that can gelate aqueous solutions offer an intriguing toolbox to create functional hydrogel materials for biomedical applications. Amphiphilic Janus dendrimers with low molecular weights can readily form self‐assembled fibers at very low mass proportion (0.2 wt %) to create supramolecular hydrogels (G′?G′′) with outstanding mechanical properties and storage modulus of G′>1000 Pa. The G′ value and gel melting temperature can be tuned by modulating the position or number of hydrophobic alkyl chains in the dendrimer structure; thus enabling exquisite control over the mesoscale material properties in these molecular assemblies. The gels are formed within seconds by simple injection of ethanol‐solvated dendrimers into an aqueous solution. Cryogenic TEM, small‐angle X‐ray scattering, and SEM were used to confirm the fibrous structure morphology of the gels. Furthermore, the gels can be efficiently loaded with different bioactive cargo, such as active enzymes, peptides, or small‐molecule drugs, to be used for sustained release in drug delivery.     

Radiation effect on stabilized polypropylene

Radiation-induced oxidative degradation of stabilized polypropylene films using Co-60 γ irradiation at room temperature under different irradiation conditions (vacuum and oxygen) was investigated. Gas evolution, oxygen consumption, and mechanical properties were studied and analyzed quantitatively. It was found that the G values for the evolved gases during irradiation of stabilized polypropylene films under vacuum were G(H₂) = 2.5, G(CH₄) = 0.08, G(CO₂) = 0.02, and G(C₂H₆) = 0.005. In the presence of oxygen, G(H₂) was the same, and the G values for other hydrocarbons were twice those under vacuum. The G values of oxidative products and oxygen consumption were G(CO) = 0.15, G(CO₂) = 0.5, and G(O₂) = 9.0 at oxygen pressure of 500 Torr and were dependent on oxygen pressure. The stabilized polypropylene, irradiated at low doses under different conditions (air, oxygen, and vacuum), shows mechanical properties almost similar to those of the unirradiated one. However, at higher doses the mechanical properties were sharply decreased and deterioration was enhanced in the presence of oxygen due to the oxidative degradation. On the other hand, post- irradiation effect on the stabilized polypropylene irradiated up to 0.02 MGy under vacuum, was studied at various annealing temperatures and different storage time at room temperature (25°C). Molecular weight distribution curves for polypropylene samples irradiated in air atmosphere show that degradation of the high molecular weight chains is the predominant process in the range of irradiation doses under investigation.     

Radiation chemical and magnetic resonance studies of aqueous agarose gels containing ferrous ions

Aqueous agarose gels containing ferrous ions sustain a radiolytic chain reaction, producing Fe³⁺. G(Fe³⁺)-values up to 156 have been observed, independent of dose rate between 0.434 and 3.74 Gy min^-1. Dissolved oxygen is needed to maintain the chain reaction, and initial ferric yields are increased if the gel is oxygen saturated, or if the Fe²⁺ concentration is decreased. Longitudinal proton magnetic relaxation rates are increased in proportion to ferric production, permitting visualizing of dose levels in these gels by magnetic resonance imaging techniques. There is a potential for application to radiation therapy treatment planning.     

Peleg and Weibull models for water absorption of copolymer gels crosslinked on polyethylene glycol dimethacrylates

In this study, the super-absorbent copolymer gels were obtained from the aqueous solution of monomers dispersed in a continuous organic phase. The inverse suspension polymerization was accomplished with the use of aqueous solutions of partly neutralized acrylic acid with a predetermined crosslinker agent, polyethylene glycol dimethacrylates (PEGDMA)/monomer ratio in cyclohexan. The copolymer gels synthesized using PEGDMA crosslinker with three different molecular weights (PEGDMA₃₃₀, PEGDMA₅₅₀, and PEGDMA₇₅₀) were characterized by ?nfrared spectroscopy and scanning electron microscopy. Water absorption capacities of the copolymer gels were determined. The water absorption capacity of 1 g copolymer gels in water was respectively obtained as 232, 255, and 316 g water/g polymer gel for PEGDMA₃₃₀, PEGDMA₅₅₀, and PEGDMA₇₅₀, respectively. The Peleg and Weibull models were used to describe the water absorption behavior of the copolymer gels. The highest values of R ² and the lowest values of χ ² and RMSE for copolymer gels were observed with the Weibull model.     

Influences of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile solutions around the sol-gel threshold

Viscoelastic experiments were performed to study the influence of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile (PAN) solutions around the sol-gel threshold. The dynamic critical parameters around the sol-gel threshold were determined using dynamic rheometer. The sol-gel transition takes place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ holds, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. Although the gel points of PAN solutions increase with increasing H₂O content, the results show that the scaling exponent n at the gel point is found to be universal for all ternary PAN solutions, which is independent of temperature and H₂O content, indicating the similarity of the fractal structure in the critical PAN gels. The gelation of ternary PAN solutions induced by adding a nonsolvent and by decreasing the temperature is demonstrated to be a thermoreversible process, which implies that the PAN gels are physical gels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2637–2643, 2008     

Covalently Immobilized Battacin Lipopeptide Gels with Activity against Bacterial Biofilms

Novel antibiotic treatments are in increasing demand to tackle life-threatening infections from bacterial pathogens. In this study, we report the use of a potent battacin lipopeptide as an antimicrobial gel to inhibit planktonic and mature biofilms of S. aureus and P. aeruginosa. The antimicrobial gels were made by covalently linking the N-terminal cysteine containing lipopeptide (GZ3.163) onto the polyethylene glycol polymer matrix and initiating gelation using thiol-ene click chemistry. The gels were prepared both in methanol and in water and were characterised using rheology, Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). Antibacterial and antibiofilm analyses revealed that the gels prepared in methanol have better antibacterial and antibiofilm activity. Additionally, a minimum peptide content of 0.5 wt% (relative to polymer content) is required to successfully inhibit the planktonic bacterial growth and disperse mature biofilms of P. aeruginosa and S. aureus. The antibacterial activity of these lipopeptide gels is mediated by a contact kill mechanism of action. The gels are non-haemolytic against mouse red blood cells and are non-cytotoxic against human dermal fibroblasts. Findings from this study show that battacin lipopeptide gels have the potential to be developed as novel topical antibacterial agents to combat skin infections, particularly caused by S. aureus.     

Poisson's ratio of polyacrylamide (PAAm) gels

Poisson's ratio (μ₀) of polyacrylamide (PAAm) gels was estimated. The value of μ₀ for PAAm gels was found to be 0.457, which is close to that for poly (vinyl alcohol) (PVA) gels swollen in the mixture of dimethylsulfoxide (DMSO) and water, but is higher than the value for PVA hydrogels.     

Establishment and Quality Assessment of Tissue Cultures in Glycyrrhiza uralensis Fisch.

In this study, seedling, callus, cell, and adventitious root of Glycyrrhiza uralensis Fisch. have been established. In order to find the best one for producing G. uralensis active constituents, triterpenoid saponins and flavonoids in native root and tissue cultures were determined, and the contents in different G. uralensis materials were analyzed using cluster analysis. The contents of triterpenoid saponins and glycyrrhizic acid in tissue cultures were much lower than that in native G. uralensis. The total flavonoids content we determined in adventitious root was 6.31 mg?g^?1, which was close to that of native root (9.82 mg?g^?1). Based on the cluster analysis, we found that G. uralensis cultures were not suitable for production of glycyrrhizic acid, while adventitious root had a greater capability of flavonoids production comparing to seedling, callus, and cell.     

Finite element modelling of mode I delamination specimens by means of implicit and explicit solvers

The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively.TSEM is an efficient method to determine the energy release rate components G_Ic, G_IIc and G_IIIc using the experimental critical load (P_c) as input, while CZM is the most widely used method to predict crack propagation (P_c) using the critical energy release rate as input.The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates.The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models.Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB).Results showed a good agreement between both FEM models and experimental results.