Thixotropic behavior of gel-like systems

The changes of shear stress with time under constant shear rate and temperature of several pigmented acrylic polymer gels were examined. This behavior, known as thixotropy, was interpreted based on a model which separates the stress into two regions. Region I covers the growth of the stress up to the yield point, represented by a peak stress, F₀. Region II covers the decay of the stress from the peak to the point it attains an asymptotic value, F_∞. The material in region I is assumed to be elastic and in region II is represented by a dynamic equilibrium between structure A, which is non-Newtonian and structure B, which is Newtonian. The transitions between A and B represent structure degradation and structure recovery. The concentrations of A and B are solved explicitly. The total shear stress is then evaluated following a theory proposed by Ree and Eyring and is shown to give relationship between shear stress–shear rate–shearing time in good qualitative agreement with experimental observations. The ratio (F₀ ? F_∞)/F_∞, defined as the coefficient of thixotropy, is sensitive to the fundamental physical chemical parameters of the system and is easy to measure. It is shown to be useful in the characterization of thixotropic materials such as gels. Methods for evaluating the elastic modulus and yield stress of the material in the gel state are also illustrated.     

Synthesis and properties of novel polyvinyl alcohol–lactic acid gels

This article describes a process for esterifying polyvinyl alcohol (PVA) with L‐lactide (LLA) and D,L‐lactic acid (LA), using ethyl acetate and N,N′‐dimethyl formamide at temperatures varying from 120 to 150°C. The grafting process was carried out under nitrogen to avoid possible oxidation or other degradation of the process ingredients and product. Lower T_g values were obtained for the PVA/LLA graft copolymers of higher LLA content suggesting some compatibility in the amorphous phase. Higher T_g values were observed for PVA/LA graft copolymers that yielded tough polymer films. The structure of the copolymers was studied by solid‐state ¹³C‐NMR, infrared spectroscopy, and differential scanning calorimetry (DSC). The PVA/LA films exhibited melt processability and good mechanical properties such as yield strength, tensile energy at break, modulus, and elongation at break. The polymer films produced through compression molding at 100°C showed good swelling properties. The transport coefficient (n) values determined from the plot of log(M_t/M_∞) vs. log t indicate Fickian behavior, and they are consistent with the reported literature values for other PVA systems. The nature of water in gels [bound water (W_b), freezing (W_f), and freezing bound (W_fb), and water content (W_t)] was evaluated from DSC data. The results demonstrate that PVA/LA hydrogels with good combination of thermal, physicomechanical, and swelling properties can be prepared via the lactic acid esterification of PVA polymer process described. Besides being melt processable, the PVA/LA gels exhibit a melting point, which indicates possibly use of higher temperatures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Relationship between segment structures and elastic properties of segmented poly(urethane‐urea) elastic fibers

Studies on segmented poly(urethane‐urea) (SPUU) elastic fibers having various segment structures were done in terms of elastic recovery and stress‐strain relationship (S‐S). Three kinds of segment structures were used: 1) the same composition having different sequences of segment units, 2) the same length of soft segments having different molecular weights of polyol, and 3) different segment structures having almost the same stress at 350% elongation. The SPUU elastic fibers having higher sequence numbers of both soft and hard segment units, that is, greater block structures, show better elastic recovery properties, especially delayed elastic recovery. The SPUU elastic fibers showing better elastic recovery take an optimum value for the number‐average molecular weight (M_n) of soft segments jointed with urethane bonds. Here the optimum M_n depends on the molecular weight of polytetramethyleneglycol (PTMG) as a starting material. The hysteresis loss in S‐S for the pre‐elongation decreases with an increase of M_n of PTMG. The SPUU elastic fibers having greater block structures show lower stress with lower 2C₁ and 2C₁ + 2C₂ of Mooney‐Rivilin plot constants for elastic fibers having the same composition. This indicates a lower density of crosslinks for finite deformation. An increase of the urea bonds or the molar ratio of urea bond to urethane bond raises the stress. It is found that the polymerization process, as well as composition, is important for design structures of SPUU elastic fibers.     

Development of techniques to compare mechanical properties of reversible hydrogels with spherical, square columnar and ocular lens geometry

We have developed a new experimental technique for determining the elastic modulus of the ocular lens by using hydrogel phantoms. We successfully prepared disulfide-reversible polyacrylamide hydrogels in the forms of a square column, a sphere, and a lens. The M_n (number average molecular weight) of the reduced copolymer ranged from 696,800 to 870,900 Da. The physical and swelling properties of the gels were independent of shape. The M_c (molecular weight between cross-links) ranged from 1776 to 1887 compared with the theoretical M_c of 1638 Da. The gels exhibited non-linear rubber elasticity, but at low strains the elastic moduli (E) were 4680±150, 5010±280, and 4870±220 Pa for the square column, sphere, and lens, respectively. The shear modulus (G) was 1531±70 Pa with an E/G ratio of approximately 3:1, indicating an incompressible gel at low strains. At high strains (∼15%) the Mooney-Rivlin plot was linear and the magnitude of 2C₁ was 1515 Pa, which was comparable to the shear modulus of the gels. Finally, the Tatara mechanical model for large deformation of rubber spheres was successfully applied to extract the elastic modulus of the lens. The modulus of the lens obtained with this technique was consistent with the moduli for the square column and sphere. The new technique will be used to determine the mechanical properties of the ocular lens.     

Post-yield compressed semicrystalline poly(butylene terephthalate): energy storage and release

Post yield deformation of semicrystalline poly(butylene terephthalate) (PBT) is studied by differential scanning calorimetry (DSC) measurements on compressed specimens after unloading. In particular, the effects of strain level, loading-unloading rate, and deformation temperature are analyzed. DSC traces indicate that a remarkable fraction of the mechanical work of deformation (in the range from 25 up to 62%) is stored in the material after unloading. Final strain dependence of stored energy values for specimens deformed up to 40% follows the general S-shaped trend observed for many amorphous and semicrystalline polymers. The ratio of the stored energy to the mechanical work of deformation (ΔU_ST/W) is decreasing as the final deformation level increases. For a given final strain level, the amount of energy stored in specimens deformed under T_g increases as either loading or unloading rates increase: in particular, both ΔU_ST and ΔU_ST/W values are linearly increasing with the logarithm of loading rate. On the other hand, energy storage for specimens deformed at T_g results to be practically independent from the loading rate. Moreover, as the deformation temperature increases from 25 to 100 °C, ΔU_ST values markedly decrease, while the ratio ΔU_ST/W is almost constant around an average value of about 51%.     

Structure-property relations of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies

The relations between the chemical and physical network structure, the deformation and failure processes and the tensile mechanical properties of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies are reported for a series of epoxy glasses prepared from a range of polyethertriamine concentrations. Near-infra-red spectroscopy indicates that these glasses form exclusively from epoxide-amine addition reactions. Their T_g exhibits a maximum and swell ratio a minimum at the highest crosslink density. Stress-birefringence studies reveal that these highly crosslinked glasses are ductile and undergo necking and plastic deformation. The plastic deformation initially occurs homogeneously but ultimately becomes inhomogeneous and shear bands develop. Tensile failure occurs in the high strain shear band region. The ultimate tensile strain of these epoxies attains a maximum of 15% for the highest crosslinked glass. Off stoichiometric networks fail at lower strains because such networks inherently contain more defects in the form of unreacted ends. The density, yield stress, tensile strength, and modulus of these glasses all decrease with increasing polyethertriamine concentration as a result of increasing free volume because of the poor packing ability of the amine molecule. A slight minimum is superimposed on this downtrend in density and modulus with increasing amine content at the highest crosslink density because of geometric constraints imposed on segmental packing by the network crosslinks. The ability of these crosslinked glasses to undergo deformation is discussed in terms of the free volume and the crosslinked network topography. Network failure is considered in terms of stress-induced chain scission which is determined by the concentration ad extensibility of the least extensible network segments.     

Autonomic self-healing in covalently crosslinked hydrogels containing hydrophobic domains

Self-healing hydrogels suffer from low mechanical strength due to their reversible breakable bonds which may limit their use in any stress-bearing applications. This deficiency may be improved by creating a hybrid network composed of a combination of a physical network formed via reversible crosslinks and a covalent network. Here, we prepared a series of hybrid hydrogels by the micellar copolymerization of acrylamide with 2 mol % stearyl methacrylate (C18) as a physical crosslinker and various amounts of N,N′-methylenebis(acrylamide) (BAAm) as a chemical crosslinker. Rheological measurements show that the dynamic reversible crosslinks consisting of hydrophobic associations surrounded by surfactant micelles are also effective within the covalent network of the hybrid hydrogels. A significant enhancement in the compressive mechanical properties of the hybrid gels was observed with increasing BAAm content. The existence of an autonomous self-healing process was also demonstrated in hybrid gels formed at low chemical crosslinker ratios. The largest self-healing efficiency in hybrids was observed in terms of the recovered elastic modulus, which was about 80% of the original value.     

Mechanical behavior of cold-water fish gelatin gels crosslinked with 1,4-butanediol diglycidyl ether

In this work, chemical crosslinking with 1,4-butanediol diglycidyl ether (BDDGE) is used as strategy to enhance mechanical performance of fish gelatin (FG) gels in order to meet the properties' range of mammalian gelatin physical gels. Joint analysis of free amino groups, swelling ratio, and total soluble material indicates that crosslinking degree increases with increasing FG concentration and it is favored by a 0.2 BDDGE/FG ratio. Increasing crosslinking degree enhances gel indentation strength and shear modulus (μ) while decreases fracture toughness (G_IC). Measured μ and G_IC values lies within the range exhibited by mammalian gelatin physical gels, but the relationship between these parameters is opposite. This is due to the different fracture mechanisms occurring in chemically crosslinked and physical gels.     

Characterization of strong polyelectrolyte hydrogels based on poly(vinyl alcohol)

Maleic anhydride (MA) was grafted onto both partially and fully hydrolyzed poly(vinyl alcohol) (PVA) in the presence of an initiator. Strong polyelectrolyte polymers were prepared by sulfonation of PVA–MA grafts. The sulfonation was completed by reaction of hydroxyl groups of PVA–MA grafts with two different sulfonating reagents (chlorosulfonic acid and pyridine sulfonic acid). The sulfonation degree was evaluated by acid–base titration and ¹H NMR analysis. The solution behaviour of the prepared grafts was evaluated from viscosity measurements. Four kinds of water‐insoluble PVA–MA and PVA–MA‐SO₃H hydrogels were prepared by heat treatment, physical gelation and chemical crosslinking with different weight ratios of N,N‐methylene bisacrylamide (MBA) crosslinker. The swelling parameters were measured for all prepared gels in deionized water and aqueous solutions at different pH values from 2 to 12 having constant ionic strength (I = 0.1). All gels exhibit a different swelling behaviour upon environmental pH changes. Copyright © 2004 Society of Chemical Industry     

Determination of the first and second normal stress differences in the truncated-cone-and-plate apparatus

Summary The steady state radial pressure distribution in the truncated cone-and-plate (TCP) apparatus has been measured for a highly elastic polyisobutylene solution in cis-trans decalin (M_W=4300 kgmol^–1 M_W/M_n=2) at eight shear rates from 1 to 270 s^–1 and temperatures of 3.0, 20.0, 30.0 and 46.0° C. The slope of pressure, corrected for inertia by the classical formula, versus the logarithm of radius provides a combination of normal stress differences N₁+2N₂ with a typical experimental scatter of 1 % at all measured temperatures and shear rates. The plots of N₁+2N₂ versus shear rate can be shifted, without loss of accuracy, by means of the time-temperature superposition principle, the shift factors strictly adhering to a WLF equation. Extrapolated rim pressures, when corrected for elastic hole pressure with a theoretical equation, give values of the second normal stress difference with typically less than 10 % of experimental scatter. Contrary to previous analyses of the TCP apparatus, the center-hole pressure is found to give a useful estimate of the second normal stress difference, although there are insufficient data at present to perform the differentiation of a nonlinear function required in the analysis of the data. Instrument imperfections are reviewed briefly in the Appendix.     

Crosslinking structure of keratin. II. Intermolecular and intramolecular crosslinks in potassium-cyanide-treated wool fibers

The crosslinking structure of keratin fibers has been investigated. The reaction of wool with aqueous KCN was studied by means of chemical and physical methods. Quantitative conversion of disulfide (SS) groups of the cystine (Cys) residues into stable thioether (S) linkages was confirmed. In terms of mechanically effective and noneffective crosslinkages, the amounts of intermolecular and intramolecular crosslinks in the KCN-treated wools were determined from the analysis of the corresponding amino acids and mechanical experiment in which the shear modulus of swollen, rubberlike samples is determined from the relation between equilibrium stress and strain for simple extension of the fiber in a diluent. A modified elastic equation of state was used for the calculation of the number of intermolecular crosslinks. The front factor in the equation used was determined by combining the results obtained from purely chemical kinetics and the values of shear modulus of the swollen wools. The ratio of the number of intermolecular and intramolecular SS crosslinkages in the Lincoln wool, was found to be 64:36. The reactivity of the former is much higher than that of the latter, and these two types of SS crosslinks form substantially the same type of S crosslinks. The fraction of the Cys residues accessible to cyanideions depends mainly on the reaction temperature.     

Stability of multiple emulsions under shear stress

Multiple liquid emulsions of the water in oil in water (W₁/O/W₂) type are used in a variety of consumer or technical applications, for instance in the encapsulation of certain active ingredients. The encapsulation process and release mechanisms of the inner phase of the carrier drops are important in order to properly process and formulate such liquid-liquid systems. In this work the stability and breakage of multiple W₁/O/W₂ emulsions under mechanical shear stress are investigated for emulsions with different surfactants and surfactant concentrations of the internal emulsion. Stressing the emulsions in a mechanical stirring process is compared to the membrane emulsification process. The membrane emulsification process results in higher encapsulation efficiencies than the stirring process. The emulsion droplets were subjected to shear stress below and above the critical capillary number for drop breakup. The results show that stable inner emulsions with sufficient surfactant concentrations increase the overall encapsulation efficiency for multiple emulsions subjected to shear stress, although the effect is not prominent. The depletion of the carrier oil droplets could be achieved for Ca numbers below the critical limit, reducing the encapsulation efficiency below 10 %. This shows that even a low shear stress can result in content release from the internal droplet phase. The experimental emulsion release study is supported by a numerical simulation of drop deformation and break-up under shear stress.     

Influence of cure system concentration on crosslink structure in SBR sulphur vulcanizates

A series of pure gum styrene-butadiene copolymer mixes, compounded with a $\text{sulphur}\text{N-}\text{cyclohexyl}\text{-2-}\text{benzothiazyl}$ sulphenamide (CBS) curing system and vulcanized for various times at 150°C has been subjected to extraction and chemical probe treatments to cleave polysulphidic and disulphidic crosslinks. The changes in the value of C₁ of the Mooney-Rivlin equation, obtained from stress-strain data, and the degree of swelling in n-heptane are reported. The weight losses occurring during these treatments have also been determined. If the proportion of vulcanizing ingredients is varied with the ratio of sulphur to CBS held constant, the ratio of monosulphidic to disulphidic crosslinks varies considerably at cure times from 45 to 120 min; no polysulphidic crosslinks were present in the vulcanizates examined. The paper contains values of C₂ (Mooney-Rivlin equation) and of χ, the polymer-liquid interaction constant; the latter varies with the cure system and is altered by the chemical probe treatments.     

Designing crosslinked hyaluronic acid hydrogels with tunable mechanical properties for biomedical applications

This study develops a simple copolymerization/crosslinking technique to control the swelling and mechanical properties of hyaluronic acid‐based hydrogels. Because of the widespread acceptance of poly(ethylene glycol) in biomedical applications, functionalized oligomers of ethylene glycol (EG) were used as comonomers to crosslink methacrylated hyaluronic acid (MHA). The swelling degree, shear and elastic moduli, and fracture properties (stress and strain) of the gels were investigated as a function of the crosslinking oligomer length and reactive group(s). It was hypothesized that acrylated oligomers would increase the crosslink density of the gels through formation of kinetic chains by reducing the steric hindrances that otherwise may limit efficient crosslinking of hyaluronic acid into gels. Specifically, after crosslinking 13 wt % MHA (47% degree of methacrylation) with 0.06 mol % of (EG)_n‐diacrylate, the swelling ratio of the MHA gel decreased from 27 to 15 g/g and the shear modulus increased from 140 to 270 kPa as n increased from 1 to 13 units. The length and functionality (i.e., acrylate vs. methacrylate) of the oligomer controlled the crosslink density of the gels. The significant changes in the gel properties obtained with the addition of low levels of the PEG comonomer show that this method allows precise tuning of the physical properties of hyaluronic acid (HA) gels to achieve desired target values for biomedical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42009.     

Rheological Characterization of Metalized and Non‐Metalized Ethanol Gel Propellants

The gellation of metalized and non‐metalized ethanol with a methylcellulose gelling agent and its effect on the rheological properties (flow and dynamic study) of these gels is reported herein. The rheological study shows that increasing the shear rate reduces the apparent viscosity for a given yield stress (for a shear rate range of 1 to 12 s⁻¹) for both shear rate ranges (1 to 12 and 1 to 1000 s⁻¹) covered in present experiment. The gellant and metal particle concentrations significantly influence the gel apparent viscosity. Distinct changes in thixotropic behavior were observed, while decreasing the concentration of MC gellant and Al metal particles in the ethanol gels. The dynamic study showed that all of the linear viscoelastic regions (LVE) of the gel samples were independent of strain percentage (1 to 10). The G′ values depended on the frequency and exceeded the G′′ values, which indicated a gel‐like highly structured material. The tanδ values showed that all of the ethanol gels were elastic and weak physical gels with a high degree of cross‐linking.     

PH-induced structure change of poly(vinyl alcohol) hydrogel crosslinked with poly(acrylic acid)

The structure of the hydrogel of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was investigated by small angle X-ray scattering (SAXS) of synchrotron radiation. A physically crosslinked blend gel, which was prepared by repetitive freezing and thawing of an aqueous solution of PVA and PAA, could be chemically crosslinked by esterfication of PVA with PAA even in the hydrogel state. The chemical crosslinking induced the destruction of physical crosslinks into a folded structure, indicating that the chemical crosslinking proceeds at the sites around the physical crosslinks that contain PVA and PAA in much higher concentration than other portion of the gel. The pH-induced structure changes of the PVA hydrogels, chemically crosslinked with poly(acrylic acid) (PAA) were investigated by SAXS on the samples of various chemical crosslinking time. The gels were shrunk at pH4, and swollen at pH8. The results of SAXS showed, that the Porod slope changed with chemical crosslinking time from -3.5 to ?2.9 at pH4, and from ?2.9 to ?2.4 at pH8. The results suggest that a folded structure as a structural domain, which is characterized by fractally rough interface, tends to change into the structure that corresponds to percolation cluster, particularly at pH8. The gels immersed in pH8 showed a remarkable structure change accompanying swelling. The results revealed that a conformational change of PAA chains, induced by the pH change, can be explained by the presence of a structural domain in the gel network, where both PVA chains and PAA chains get entangled and partially form a interpenetrating polymer network(IPN).     

Nanoindentation study on nanomechanical characteristics of a-CN film deposited by shielded arc ion plating

Mechanical properties of the a-CN films including elastic modulus (E_r), hardness (H), elastic recovery (R), contact stiffness (S) and deformation energies were measured by a nanoindentation system. We also evaluated wear resistant behavior of the layered a-CN films in nanometer scale by the same nanoindentation system. All the a-CN films, irrespective of V_b, showed better wear-resistance characteristics than sapphire and quartz. The a-CN (− 300 V)/Si sample showed the best wear-resistance, although its hardness was lower than the a-CN (− 300 V)/a-C (− 100 V)/Si. The wear resistant characteristic of the films can be understood by considering the other mechanical properties including that of R, hardness-to-elastic modulus ratio (H / E_r), and elastic deformation energy (W_e) obtained from the nanoindentation. These various nanomechanical properties certainly govern the wear-resistance of the film.     

Sheet formation during drop deformation and breakup in polyethylene/polycarbonate systems sheared between parallel plates

A polycarbonate drop was sheared inside a polyethylene matrix in a transparent rotating parallel plate device at 220 °C and low shear rates. A flat sheet was formed during the initial shearing of the drop. The drop then developed into either a thin thread or a sheet with a thin cylindrical tip. Sheet formation was found to occur at a critical strain or time. A stress ratio (S_r) between the matrix breakup stress, made up of the matrix normal stress and viscous stress, and the drop restoring stress, made up of the drop normal stress and the interfacial stress, is used to characterize the sheet formation during the drop deformation and breakup process. It was found that the viscosity ratio (η_r), stress ratio (S_r) and Deborah number (De) of the system could be used to predict the drop deformation and breakup.     

Synthesis of boronate-containing copolymers of N,N-dimethylacrylamide, their interaction with poly(vinyl alcohol) and rheological behaviour of the gels

Cross-linking of polyvinylalcohol (PVA) by boronate-containing copolymer of N,N-dimethylacrylamide (DMAA, 1) was studied and compared to cross-linking of PVA by borate buffers in weakly alkaline solutions. The copolymer of M_w=19,000 g mol⁻¹ containing 9 mol% N-acryloyl-m-aminophenylboronic acid (NAAPBA, 2) was prepared by free radical polymerization of the monomers, exhibiting copolymerization constants r₁=0.84 and r₂=2.2. Due to multipoint interaction of the copolymer with PVA via monodiols, the intermolecular cross-linking required for seven-fold and 10-fold lower boron concentrations as compared to borate buffers of pH 8.6 and 7.5, respectively. In rheological measurements, PVA-copolymer gels exhibited storage moduli (G′_max) comparable to those of PVA-borate gels prepared at 7.5-fold higher boron concentration and the same pH 8.6, what testified to the similar concentration of cross-links in the gels. Therefore, DMAA-NAAPBA copolymer is a more effective cross-linker of PVA than borate. The PVA-copolymer gel exhibited much higher relaxation time (97 s) compared to PVA-borate gels (≤20 s) indicating a longer lifetime of junction zones. The ‘shape stability’ of the gel is suggested to originate in the structure of junctions, containing several boronate-diol complexes, between the macromolecules of PVA and the copolymer.