<i>Viscoelastic Properties of Polymers</i>

Ferry, John D.; Rice, Stuart A.

doi:10.1063/1.3057989

Cited by 2,282 publications

(4,011 citation statements)

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“…Dividing the spring constant by the shape factor results in the complex shear modulus G=G′+i·G″. The real part of the complex shear modulus (storage modulus, G′) represents the elastic component and the imaginary part of the complex shear modulus (loss modulus, G″) represents the energy dissipated or lost as heat [14]. The shape factor is defined by the ratio between the shear modulus of the material and the force applied by the oscillating probe and is derived from the geometry of the sample.…”

Section: Discussionmentioning

confidence: 99%

“…The correction factor was calculated using the average of 'cylindrical shear' ( [14], pp 97-98) and 'cylinder compression', which were modified in order to incorporate a nonuniform shear modulus (see Appendix). The correction factor (Table 2) is a function of the measured (uncorrected) ratio between the shear modulus at the lens equator (G o ) and in the lens centre (G i ).…”

Section: Discussionmentioning

confidence: 99%

“…The stiffness of a viscoelastic material depends on the deformation rate [14]. Therefore, we used dynamic measurements, using multiple deformation rates.…”

Section: Introductionmentioning

confidence: 99%

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Stiffness gradient in the crystalline lens

Weeber

Eckert

Pechhold

et al. 2007

Graefes Arch Clin Exp Ophthalmol

141

147

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Background While the overall stiffness of the lens has been measured in a number of studies, the knowledge about the stiffness distribution within the lens is still limited. The purpose of this study was to determine the stiffness gradient in the human crystalline lens. A secondary purpose was to determine whether the stiffness gradient depends on age. Methods The local dynamic stiffness was measured in 10 human crystalline lenses (age range: 19 to 78 years). The lenses were stored at −70°C before being measured. The influence of freezing on the mechanical properties has been determined in a previous study. A small oscillating probe was used to measure the local dynamic shear modulus as a measure of lens stiffness. The measurements were taken in the cross-sectional plane through the lens equator. Results The local dynamic shear modulus varied with location for all tested lenses. The central stiffness of the oldest lens (78 years) was 10 4 times higher than the youngest (19 years) lens. The equatorial stiffness of the oldest lens was 10 2 times higher than the youngest lens. For the older lenses, the centre was 5.8-210 times stiffer than the periphery, as opposed to earlier results described by Fisher (1971), who found that the periphery was up to 3 times softer than the centre for lenses younger than 70-years-old. For the three youngest lenses (19 to 49 years), the periphery was 2.2-16.6 times stiffer than the centre. Conclusions The dynamic stiffness of the crystalline lens varies with location within the lens. The stiffness gradient depends on the age of the lens. The results of the 10 lenses indicate that the stiffness of both centre and periphery increase with age, but at a different rate.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Stiffness gradient in the crystalline lens

Weeber

Eckert

Pechhold

et al. 2007

Graefes Arch Clin Exp Ophthalmol

141

147

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“…The plateau elastic moduli (G N ) of the aqueous gels were determined from the value of the storage modulus where the loss moduli exhibit a minimum (18) (Table II). For sample AQ8, the crossover frequency (the frequency at which GЈ and GЉ cross each other) is identified as 0.00158 rad/s from which a longest relaxation time of 3960 s (relaxation time, 0 ∼2 / c ) can be evaluated (18).…”

Section: Low Amplitude Oscillatory Shear Measurementsmentioning

confidence: 99%

“…For sample AQ8, the crossover frequency (the frequency at which GЈ and GЉ cross each other) is identified as 0.00158 rad/s from which a longest relaxation time of 3960 s (relaxation time, 0 ∼2 / c ) can be evaluated (18). For other samples, the crossover frequencies could not be accessed within the frequency range for which measurements were conducted.…”

Section: Low Amplitude Oscillatory Shear Measurementsmentioning

confidence: 99%

Rheological Characterization of Topical Carbomer Gels Neutralized to Different pH

et al. 2004

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Purpose. The primary objective of this study is to perform detailed and extensive rheological characterization of rheology of carbomer (Carbopol) microgels formulated using a solvent system typically used in topical gel formulations. Solvents like glycerin and propylene glycol can alter rheology and drug delivery characteristics of topical gels owing to their different viscosities and due to the change in solvent-polymer and solvent-solvent interactions. Methods. Aqueous gels with different pH were prepared by dissolving cross-linked Carbopol polymers in a co-solvent system comprising water, propylene glycol, and glycerol and subsequently neutralizing the carboxylic groups of the polymers with triethanolamine (TEA). Oscillatory, steady, and transient shear measurements were performed to measure viscoelastic properties, temperature dependency, yield strength, and thixotropy of carbomer pharmaceutical gels. Results. The topical pharmaceutical gels exhibit remarkable temperature stability. Flow curves obtained at different temperatures indicate Carbopol microgels show much more pseudoplastic behavior (lower power law index) compared to Carbopol gels dissolved only in water. Substantial yield strength is required to break the microgel network of the topical gels. The gel samples exhibit modest thixotropy at higher deformation rates. Conclusions. The rheological behavior of the Carbopol microgels do not change appreciably in the pH range 5.0-8.0, and the gels can be used as effective dermatological base for topical applications.

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