Rheological characterization of chitosan matrices: Influence of biopolymer concentration

Viscoelastic properties of chitosan (CH), chitosan‐poly(ethylene glycol) 400 (CH‐PEG), and chitosan‐poly(ethylene glycol) 400 with glyoxal as crosslinking agent (CH‐PEG‐Gly) systems were studied to analyze the effect of chitosan concentration (from 0.83 to 1.67%). Dynamic moduli increase as chitosan concentration increases for all systems. For CH and CH‐PEG systems the loss modulus (G″) is greater than the storage modulus (G′) with predominance of the viscous over the elastic behavior. This corresponds to the characteristic behavior of solutions (nonstructured systems). The presence of PEG 400 induces a complementary reinforcement of the mechanical properties of the system. Except for the lowest chitosan concentration, when glyoxal was added to the CH‐PEG systems, a gelled matrix was obtained. In this case, G′ is greater than G″, and practically independent of frequency. This behavior is typical of three‐dimensional networks and indicates true gel formation, showing clear elastic behavior (tan δ < 1). In creep and recovery analysis, CH‐PEG‐Gly systems exhibited distinct regions that were mathematically modeled using Burger's model. This analysis shows that the CH‐PEG‐Gly matrices (from 1.25 to 1.67%) recover almost totally (100%). Therefore, these matrices could be useful as systems for the development of films for topical hydrophilic drug delivery, and the levels of the residual viscosity (η₀) or the complex viscosity (η*) could be used to control drug release. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Dynamic mechanical properties of oil-extended reinforced SBR rubbers

The dynamic mechanical behavior of six samples of styrene–butadiene random copolymers (SBR with 24% styrene) containing different amounts of carbon black and oil extender has been investigated in the temperature range ?50° to 80°C. The measurements were carried out using a Fitzgerald apparatus working in the frequency range of 0.1 to 1.0 kHz. Master curves for all materials were obtained by the method of reduced variables. The master curves show that increasing amounts of carbon black in SBR in crease both the elastic (G′) and the viscous (G″) components of the complex shear modulus. The loss factor (tan δ) decreases markedly with increasing amounts of carbon black. Increasing amounts of oil extender increase the loss factor within the reduced frequency range here reported (2 < log ωa_T < 7).     

Rheological properties of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) dilute solution

Viscoelastic behavior of the dilute solution of ultrahigh molecular weight syndiotactic poly(vinyl alcohol) (UHMW s-PVA)/dimethyl sulfoxide (DMSO)/water was investigated through rheological response. Below a critical temperature, the dynamic storage modulus (G′) of the solution became greater than the dynamic loss modulus (G″) and the viscoelastic exponent for G′ became smaller than that for G″ before macroscopic gelation, which indicates the evolution of viscoelastic solid properties at the sol state. Also, the loss tangent (tan δ) of the solution below the critical temperature increased with increasing frequency. Consequently, the dilute solution of UHMW s-PVA/DMSO/water showed the rheological behaviors as can be observed in general chemical or physical gel systems below the critical temperature. These results suggest that solid-like heterogeneity prevailed in the solution before macroscopic gelation with decreasing temperature. Such heterogeneity was considered as phase-separated domains attributed to spinodal decomposition. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 569–576, 2001     

Rheological properties of PLGA-PEG-PLGA copolymers for ophthalmic injection

Polymer rheological property is one of intrinsic properties for the design and preparation of intravitreal injection systems. Rheological behaviors of thermosensitive poly(lactic acid-co-glycolic acid)-poly(ethylene glycol)-poly(lactic acid-co-glycolic acid) (PLGA-PEG-PLGA) triblock copolymers were investigated in this article. The rigidity phase angle (δ), elastic modulus (G′), viscous modulus (G″), and complex viscosity (η*) were determined. The injectability of polymers was evaluated as well. The results indicated both temperature and concentration were key parameters influencing elasticity of polymers. Owing to low complex viscosity (below 1 Pa s), PLGA-PEG-PLGA polymers can be successfully injected at room temperature. When the temperature was raised to 37°C, the complex viscosity increased (over 4 Pa s). Thus, suitable rheological properties (G′ > G″; tan δ < 1) were obtained for injection administration. Elastic modulus (G′), viscous modulus (G″), and complex viscosity (η*) were diminished when polymer solutions were ejected through syringe needles (25 gauge, G). Ejection force (from 4.21 to 19.42 N) was required in the process of injecting administration through syringe needles (24, 25, and 26 G) for polymer solutions at 20, 25, and 30% (w/v) concentration. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dynamic rheological properties of high‐density polyethylene/polystyrene blends extruded in the presence of ultrasonic oscillations

The linear rheological properties of high‐density polyethylene (HDPE), polystyrene (PS), and HDPE/PS (80/20) blends were used to characterize their structural development during extrusion in the presence of ultrasonic oscillations. The master curves of the storage shear modulus (G′) and loss shear modulus (G″) at 200°C for HDPE, PS, and HDPE/PS (80/20) blends were constructed with time–temperature superposition, and their zero shear viscosity was determined from Cole–Cole plots of the out‐of‐phase viscous component of the dynamic complex viscosity (η″) versus the dynamic shear viscosity. The experimental results showed that ultrasonic oscillations during extrusion reduced G′ and G″ as well as the zero shear viscosity of HDPE and PS because of their mechanochemical degradation in the presence of ultrasonic oscillations; this was confirmed by molecular weight measurements. Ultrasonic oscillations increased the slopes of log G′ versus log G″ for HDPE and PS in the low‐frequency terminal zone because of the increase in their molecular weight distributions. The slopes of log G′ versus log G″ for HDPE/PS (80/20) blends and an emulsion model were used to characterize the ultrasonic enhancement of the compatibility of the blends. The results showed that ultrasonic oscillations could reduce the interfacial tension and enhance the compatibility of the blends, and this was consistent with our previous work. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3153–3158, 2004     

Influence of drying conditions on the gelling properties of the 7S and 11S soy protein fractions

Soy protein fractions rich in β-conglycinin (7S) or glycinin (11S) were freeze dried or spray dried at temperatures of 120, 150 or 180 °C. The fractions were characterized for their particle size distribution, sorption isotherms and by scanning differential calorimetry. The gelling capacity of the protein fractions was studied at pH values of 3 and 7 using oscillatory measurements, mechanical properties and water holding capacity. The rheological measurements showed that viscous modulus (G″) predominated at low temperatures and the elastic modulus (G′) at high temperatures. At pH 3, the G′–G″ crossover occurred at lower temperatures when compared to pH 7. This behaviour was more accentuated for the 11S fractions due to its capacity to form stronger gels. An increase of drying temperature led to a displacement of the gel point to higher temperatures and decreased the elasticity modulus or gelling capacity of protein fractions. These results were confirmed by the mechanical properties, since at higher temperatures the gels were more fragile and brittle, especially when formed at pH 7.     

Solute and solvent effects on the thermorheological properties of poly(oxyethylene)–poly(oxypropylene) block copolymers: Implications for pharmaceutical dosage form design

Despite their widespread use as platforms for topical drug delivery systems, there is a relative lack of information concerning the thermorheological and viscoelastic properties of poloxamer systems and the effects of formulation components on these properties. To address this deficit, we examined the effects of the poloxamer concentration (25 and 35% w/w), molecular weight blend (poloxamer 407 and poloxamer 188), cosolvents (ethanol, propylene glycol, and glycerol), and presence of inorganic and organic electrolytes (sodium chloride and tetracaine hydrochloride, respectively) on these properties. The rheological properties were examined with a rheometer (4‐cm‐diameter, stainless steel, parallel‐plate geometry) in either thermal sweep (0.5 Hz) or frequency sweep (0.01–1.0 Hz and 37°C) modes. Increasing the poloxamer concentration increased the elasticity [i.e., increased the storage modulus (G′) and reduced the loss tangent (tan δ)] and reduced the sol–gel transition temperature (T_m) of all the formulations. Decreasing the ratio (407:188) increased T_m and reduced the elasticity of all the formulations. Increasing the concentration of ethanol, propylene glycol, or glycerol in the solvent reduced T_m. The presence of ethanol reduced G′ and increased tan δ in a concentration‐dependent fashion, whereas the viscoelastic properties of the poloxamers were more tolerant of glycerol (in particular) and propylene glycol. The elasticity of the formulations containing up to 10% glycerol and 5% propylene glycol was increased with respect to their aqueous counterparts. The presence of sodium chloride reduced T_m and, at lower concentrations (1 and 3%), increased G′ and reduced tan δ for aqueous poloxamer systems. Conversely, the addition of a model therapeutic agent, tetracaine hydrochloride (5 and 7% w/w), significantly increased T_m and altered the viscoelastic character of the poloxamer system, notably reducing G′ and increasing the loss modulus and tan δ. Alterations in the viscoelastic and thermorheological properties of aqueous poloxamer systems will have implications for their clinical performance. This study, therefore, has highlighted the need for the rational selection of components in the formulation of poloxamer systems as platforms for topical drug delivery. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1016–1026, 2003     

Preparation of highly concentrated inverse emulsions of acrylamide‐based anionic copolymers as efficient water rheological modifiers

Highly concentrated inverse anionic polymeric emulsions (with a solid content of up to 63 wt %) were prepared using a two‐step methodology: (i) First, acrylamide, acrylic acid, and its ammonium salts crosslinked copolymers were obtained by inverse emulsion polymerization, (ii) The water/volatile oil mixture was then separated from the heterogeneous system by vacuum distillation. To maintain sufficient stability during the reaction and distillation processes, a ternary surfactant mixture was used. A surface response methodology was employed to obtain the optimal values of the factors involved in both process and product specifications, and to maximize the high performance of these inverse anionic polymer emulsions. This yielded a product containing up to 63.2 wt % solids capable of achieving Brookfield viscosities as high as 40.3 Pa·s, using an aliquote of these concentrated inverse polymer emulsions (1.8 wt % in deionized water). Rheological characterization (oscillatory and rotational measurements) was carried out to evaluate the behavior of the diluted inverse anionic polymer emulsion in water thickening. The methodology developed can be used to formulate a wide range of viscoelastic (G″/G′) water‐based products from anionic water soluble polymers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43502.     

Dielectric relaxation behavior of conducting carbon black reinforced ethylene acrylic elastomer vulcanizates

The dielectric relaxation characteristics of conductive carbon black (CCB) reinforced ethylene acrylic elastomer (AEM) vulcanizates have been studied as a function of frequency (10¹–10⁶ Hz) at different filler loading over a wide range of temperatures (30–120°C). The effect of filler loadings on the dielectric permittivity (ε′), loss tangent (tan δ), complex impedance (Z*), and electrical conductivity (σ_ac) were studied. The variation of ε′ with filler loading has been explained based on the interfacial polarization of the fillers within a heterogeneous system. The effect of filler loading on the imaginary (Z″) and real (Z′) part of Z* were distinctly visible, which may be due to the relaxation dynamics of polymer chains at the polymer–filler interface. The frequency dependency of σ_ac has been investigated using percolation theory. The phenomenon of percolation in the composites has been discussed in terms of σ_ac. The percolation threshold (?_crit) occurred in the range of 20–30 phr (parts per hundred) of filler loading. The effect of temperature on tan δ, ε′, σ_ac, and Nyquist plots of CCB‐based AEM vulcanizates has been investigated. The CCB was uniformly dispersed within the AEM matrix as studied from the transmission electron microscope (TEM) photomicrographs. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Mechanistic studies in tackified acrylic emulsion pressure sensitive adhesives

Twenty‐three wt % aqueous tackifier dispersion based on glycerol ester abietic acid (T_g = 64°C, M_w = 940) was added to emulsion polymer 50/32/15/3 poly(2‐ethyl hexyl acrylate‐co‐vinyl acetate‐co‐dioctyl maleate‐co‐acrylic acid) pressure sensitive adhesive (PSA). From these latices, 25 μm thick films were cast. The films were dried at 25°C for 24 h or at 121°C for 5 min. Dynamic mechanical analysis (DMA) of the films included measuring elastic modulus (G′) and damping factor (tan δ). Under the above drying conditions, the films did not produce significant differences in their DMA and PSA properties as measured by loop tack, peel, and shear holding power. DMA of the tackified acrylic film showed thermodynamic miscibility between the tackifier and polymer regardless of the drying conditions. Microgels formed during emulsion polymerization of the acrylic PSA brought inherent weakness to the tackified film properties. In the neat acrylic PSA film, these discrete networks entangled with the uncrosslinked chains while in the tackified film, these networks could not form entanglements due to the increased molecular weight between entanglements for the uncrosslinked chains. This lack of network entanglements caused shear holding power of the tackified acrylic PSA film to be 4× lower than that of the neat acrylic PSA film. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1965–1976, 2000     

Dynamic rheological properties of Yam starch/hectorite composite gels

The rheological properties of composite gels of starch extracted from Yam roots (Discorea sp) and hectorite (a mineral clay belonging to the smectite group) were studied in aqueous solutions as a function of hectorite concentration. The elastic (G′) and viscous (G″) moduli of the composite gels were dependent on the clay content, and for all samples G′ was greater than G″. Composite gels with clay contents of <30% presented higher G′, G″ and viscosities |η*| when compared with pure starch gel, while those with a content of 50 % showed lower values. The addition of hectorite significantly inhibited the creeping properties in relation to pure starch at 25 °C but, after heating/cooling cycles between 25 and 85 °C, this effect was not observed and the composite gel showed similar behavior to that of pure starch. Copyright © 2005 Society of Chemical Industry     

Sonocrystallization of a Tristearin‐Free Fat

The objective of this study was to fractionate a purified interesterified fat to eliminate tristearin (SSS) and to evaluate the crystallization behavior of the tristearin‐free fat. The fractionated sample was crystallized with and without the application of high‐intensity ultrasound (HIU) by supercooling the sample at 2 °C. In the absence of SSS, the crystallization process was driven by low‐melting‐point triacylglycerols (TAG) such as OSS and OOS (O, oleic; S, stearic acid). There were no differences observed in the crystallinity in the sample based on the solid fat content (P > 0.05) along with any microstructural differences. In addition, an increase in the enthalpy of melting was observed upon sonication, indicating higher crystallinity (P < 0.05). Stronger intramolecular forces were formed in the sonicated samples as evidenced by increased viscoelastic parameters such as the elastic modulus (G′) and storage modulus (G″) (P < 0.05). G′ values increased from 138.25 ± 41.30 to 939.73 ± 277.45 Pa while the G″ values increased from 39.15 ± 8.98 to 149.77 ± 16.00 Pa (P < 0.05). Change in viscosity was not observed as a consequence of sonication (P > 0.05). This study showed that HIU was effective in changing the crystallization behavior of SSS‐free fats with low‐melting TAG.     

The rheological behavior and dynamic mechanical properties of syndiotactic 1,2‐polybutadiene

The rheological behavior and the dynamic mechanical properties of syndiotactic 1,2‐polybutadiene (sPB) were investigated by a rotational rheometer (MCR‐300) and a dynamic mechanical analyzer (DMA‐242C). Rheological behavior of sPB‐830, a sPB with crystalline degree of 20.1% and syndiotactic content of 65.1%, showed that storage modulus (G′) and loss modulus (G″) decreased, and the zero shear viscosity (η₀) decreased slightly with increasing temperature when measuring temperatures were lower than 160°C. However, G′ and G″ increased at the end region of relaxation curves with increasing temperature and η₀ increased with increasing temperature as the measuring temperatures were higher than 160°C. Furthermore, critical crosslinked reaction temperature was detected at about 160°C for sPB‐830. The crosslinked reaction was not detected when test temperature was lower than 150°C for measuring the dynamic mechanical properties of sample. The relationship between processing temperature and crosslinked reaction was proposed for the sPB‐830 sample. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Dynamic mechanical analysis study of the curing of phenol‐formaldehyde novolac resins

The curing reaction of typical commercial phenol‐formaldehyde novolac resins with hexamethylentetraamine (HMTA) was followed by dynamic mechanical analysis. The evolution of the rheological parameters, such as storage modulus G′, loss modulus G″, and tanδ (G″/G′), as a function of time, for samples of the phenolic resins on cloth, was recorded. The curing reaction, leading to the formation of a crosslinked structure, is described by a third‐order phenomenological equation. This equation takes into account a self‐acceleration effect, as a consequence not only of the chemical reaction of crosslinking after the gel point but of phase segregation as well. This rheokinetic model of the curing of phenolic novolac resins permits the determination of the numerical values of the kinetic equation constants. The influence of the composition, structure, and physical treatment on the curing kinetics of the novolac resins is evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1902–1913, 2001     

Rheological behaviors of PVA/H2O solutions of high‐polymer concentration

The dynamic viscoelastic properties of poly(vinyl alcohol) (PVA)/H₂O solutions with concentrations of 10 to 25 wt % have been characterized by controlled‐stress rheometry at 30°C. Parameters relating to the linear and nonlinear viscoelasticities include complex viscosity (η*), storage modulus (G′), loss tangent (tan δ), relaxation time (λ), thixotropy, and creep. Change curves of η*, G′, tanδ, and λ with frequency (ω) have been obtained for the PVA/H₂O solutions. Creep and recovery testing yielded compliance (J′) curves with loading and unloading. Shear stress versus rate profiles of the PVA solutions have been obtained through thixotropic measurements. The PVA concentration has been found to have a profound influence on the rheological properties of the aqueous solutions. Four aqueous solutions of 10, 15, 20, and 25 wt % PVA at 30°C exhibited shear‐thinning and showed different transition behaviors of η* and G′ with frequency, and different degrees of creep under constant stress to recovery with time. The 10 wt % PVA solution was viscous and displayed the lowest η* and G′; the 25 wt % PVA solution was viscoelastic and displayed the highest η* and G′; the 15 and 20 wt % PVA solutions showed η* and G′ values and creep to recovery behaviors intermediate between those of the 10 wt % and 25 wt % PVA solutions. The different rheological properties of these PVA/H₂O solutions are considered to correlate with interchain hydrogen bonds and shear‐induced orientation in the solutions. Shearing is able to break the intrachain and interchain hydrogen bonds, and, at the same time, the orientation creates new interchain hydrogen bonding. The reorganization of hydrogen‐bonding mode contributes to the transitions of the macroscopic viscoelasticity with frequency. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of nano‐organomontmorillonite on the viscoelasticity of the poly(trimethylene terephthalate)/glass fiber/organomontmorillonite hybrid nanocomposites materials

The influences of organically modified montmorillonite (OMMT) on the viscoelasticity of poly(trimethylene terephthalate)/glass fiber/OMMT (PTT/GF/OMMT) hybrid nanocomposite materials at liquid, elastic and glassy states, respectively, were investigated by using the rotational rheometer and dynamic mechanical analyzer (DMA). The viscoelasticity results suggest that OMMT has many important influences on the structure, modulus and toughness of the hybrid nanocomposite materials. At melton state, the shear‐thinning phenomena of the hybrid composite melts become remarkable with increasing OMMT content. At low frequency, the shear storage modulus (G′) and shear loss modulus (G″) of the melts increase with increasing OMMT content. The melt's elastic response increases by OMMT, and OMMT improves the creep resistance of the melts; in addition, the stress relaxation of the hybrid composite melts become slow with increasing OMMT content, and the stress leavings becomes much higher with increasing OMMT content. At glassy state, the storage modulus of the hybrid nanocomposites increases with increasing OMMT content, while the materials' loss modulus increases first and then decreases with increasing OMMT content; therefore, OMMT nanosheets have reinforcement effect on the composites, and it also has definite toughening effect on the hybrid composite when the OMMT content is no >2 wt%. At rubbery state, the hybrid composites show lower decreasing storage modulus but have lower cold‐crystallization ability than that of pure PTT and PTT/GF composite. POLYM. COMPOS., 35:795–805, 2014. © 2013 Society of Plastics Engineers     

Rheological studies on the phase dissolution in a block copolymer

The microphase transition in a styrene-butadiene-styrene triblock copolymer was studied by rheometric mechanical spectroscopy. A high-temperature-melt rheological transition from the highly elastic, nonlinear viscous behavior typical of a multiphase structure to linear viscous behavior with insignificant elasticity typical of a single-phase structure was observed. The transition temperature is determined according to the discontinuity of the rheological properties across the transition region, which agrees well with the results obtained from the small angle X-ray scattering data and the expectation of the random phase approximation theory. Maybe for the first time, microphase dissolution was investigated rheologically. The storage modulus (G′) and the loss modulus (G′) increase with time during the process. An entanglement fluctuation model based on the segmental density fluctuations is presented to explain the rheological behavior in this dissolution process. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1155–1164, 1997     

Short jute fiber‐reinforced polypropylene composites: Dynamic mechanical study

Short jute fiber‐reinforced polypropylene (PP) composites were prepared using a high‐speed thermokinetic mixer. A compatibilizer was used to improve the molecular interaction between jute and PP. Both the percent weight fraction of the jute fiber and compatibilizer were varied to study the dynamic mechanical thermal (DMT) properties. Dynamic parameters such as storage flexural modulus (E′), loss flexural modulus (E″), storage shear modulus (G′), loss shear modulus (G″), and loss factor or damping efficiency (tan δ) were determined in a resonant frequency mode. The transition peak nature, amplitude, and temperature of E′, E″, G′, G″, and tan δ of different compositions were shown to indicate possible improvements of molecular interaction in the presence of a compatibilizer. The modulus retention term, a plot of the reduced modulus with the weight fraction of the jute fiber, also indicate its improvement. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 531–539, 1999     

Preparation and rheological investigation of tough PAAm hydrogel by adding branched polyethyleneimine

In this article, tough hydrogels are prepared by introducing the polyethyleneimine (PEI) with branched structure and a large number of  NH₂ and  NH groups into permanently crosslinked polyacrylamide (PAAm) hydrogels matrix. To investigate the effects of B-PEI and chemical crosslinking agent (Bis) on the strength and toughness of hydrogels, a series of B-PEI/PAAm hydrogels with different mass percentage of Bis and B-PEI were manufactured and the rheological and swelling properties were compared. For all hydrogels, the storage modulus (G′) was much higher than the loss modulus (G″) in the linear viscoelastic region through the whole frequency range. The solid-like behavior and elastic nature (G′ > G″) are attributed to the permanent covalence crosslinking. Therefore, G′ increased when more Bis was added. For the nonlinear oscillatory shear measurement, hydrogels with B-PEI broke at larger γ than the pure PAAm hydrogels, indicating that the toughness of B-PEI/PAAm hydrogels has been improved by introducing B-PEI. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48541.