Changes In Viscoelastic Properties Research Articles

Monitoring changes in the viscoelastic properties of thin polymer films by the quartz crystal resonator

This paper shows that a quartz crystal resonator is applicable to the investigation of rheological property changes of polymer blend and polymer films by the simultaneous measurement of resonant frequency and resonant resistance of the quartz crystal resonator. Phase transition phenomena of PMMA/PVAc blend films were studied in the thermal cycles. Viscoelastic property changes of PMMA homopolymers including a certain amount of plasticizer and the dependence of molecular size in blend film on phase transition temperature were also investigated. Both the resonant frequency and resonant resistance of film coated quartz crystals changed critically at the glass transition temperature. The responses of the resonant resistance showed more clear change than those of the resonant frequency because the resonant resistance directly reflects the information of energy dissipation in the coated films. The responses in the thermal cycle were represented as diagrams of the resonant frequency versus the resonant resistance and d R/d F versus temperature for detailed explanations. These diagrams showed clear glass transition temperatures of the films in the thermal cycles.

Setting and stress relaxation behavior of resilient denture liners

Statement of problem. Resilient denture liners are widely used for the patients who are not comfortable with correctly made conventional hard-based dentures because of thin and relatively nonresilient mucosa or severe alveolar resorption. There are several materials used for denture liners and the efficacy in their use is influenced by their viscoelastic properties. Purpose. This study evaluated the setting behavior and viscoelastic properties of various types of resilient denture liners and the changes in viscoelasticity with the passage of time. Material and methods. Four types of resilient denture liners were used. Setting behavior of 5 autopolymerizing materials was evaluated with an oscillating rheometer. Stress relaxation tests were conducted to evaluate the viscoelastic properties of 9 materials and changes that occurred over time by means of Maxwell model analogies. Results. Significant differences were found in the setting behavior of the autopolymerizing materials. The acrylic resin and fluoroethylene materials demonstrated viscoelastic properties and the silicone and polyolephin materials were found to be elastic. The acrylic resin materials exhibited the greatest changes in viscoelastic properties over time when compared with silicone, polyolephin, and fluoroethylene materials. Conclusions. The results suggest that it is important to select denture liner materials according to clinical situations because of the wide ranges of setting behavior, viscoelastic properties, and durability over time.(J Prosthet Dent 1998;80:714-22.)

Thermal denaturation and renaturation of a fermentation broth of xanthan: rheological consequences

The rheological properties of an unpasteurised and concentrated xanthan fermentation broth (c=30 g/l 0.02 M in salt) were studied before heat treatment and after a thermal heating/cooling cycle performed at various polymer concentration conditions (10–30 g/l). At concentrations below 10 g/l heat denaturation occurs with dissociation of the native double-stranded structure into two single strands. At higher concentration, no complete dissociation happens. Changes in both viscoelastic properties and molecular weight are observed after heating above the melting order–disorder temperature (Tm). They are related to the order–disorder conformational transition of the xanthan molecules. Xanthan renatured in concentrated conditions (above 10 g/l) has a higher viscosity than that of the native sample and displays more gel-like properties. The inhibition of the dissociation in two single strands in the high concentration range is attributed to the presence of nematic phases observed by viscoelastic measurements and apolar microdomains evidenced by the addition of a neutral detergent.

Viscoelastic properties of tissue conditioners--influence of molecular weight of polymer powders and powder/liquid ratio and the clinical implications.

The effect of the molecular weight of polymer powders and the polymer powder/liquid ratio on the viscoelastic properties after gelation of tissue conditioners was studied by means of a stress relaxation test. The results are summarized as follows. The lower-molecular-weight polymer powders produced the larger flow after gelation especially at long times. The use of a lower powder/liquid ratio produced a greater flow after gelation at both short times and long times. The difference in the molecular weight of polymer powders and the powder/liquid ratio was found to have no influence on changes in viscoelastic properties with the passage of time. It would be possible to control the viscoelastic properties of tissue conditioners suitable for each clinical purpose by making variations in composition and structure.

Determination of complex shear modulus with thickness shear mode resonators

The electrical response of polymer-coated acoustic wave sensors depends on changes in the surface mass loading and changes in viscoelastic properties of the coating material. In this paper we consider the acoustic behaviour and the electrical response of a thickness-shear mode resonator on changes in shear parameters of the coating material at its fundamental frequency as well as its third and fifth harmonics. The changes in material properties were induced by temperature changes. Both a glassy and a rubbery polymer were investigated. The complex shear parameter and dynamic glass transition temperature were calculated from impedance measurements.

A new approach for tracking respiratory mechanical parameters in real-time.

A new recursive least-squares procedure for on-line tracking of changes in viscoelastic properties of respiratory mechanics is proposed and applied to artificially ventilated patients. Classical least-squares methods based on simple first-order linear models with time-constant parameters generally provide systematic residuals that hardly satisfy standard statistical tests for model validation in terms of residuals. On the other hand, high order and/or nonlinear models introduce parameters whose estimates are of difficult interpretation in a clinical context. The present procedure overcomes these limitations by using the well-known first-order model of respiratory mechanics, wherein variability of resistance and elastance during the breathing cycle is allowed to take into account nonlinear and high-order behavior. Mean and standard deviation of resistance and elastance estimates, relative to a respiratory cycle, are then determined recursively. Feasibility of the method is evaluated by applying it both to experimental and simulated pressure-airflow signals measured in an intensive care unit during mechanical ventilation of patients recovering from heart surgery. Results demonstrate that the proposed procedure provides data description satisfying statistical tests, such as residual whiteness, and reliable estimates of viscoelastic lung parameters even during substantial and fast variations in the respiratory status. In addition, unlike classical methods, the new technique provides the means for on-line evaluation of parameter variability during each respiratory cycle, by the estimate of their standard deviations. This is important in clinical practice, because only the knowledge of reliable parameter values and standard deviations enables significant changes in the respiratory viscoelastic characteristics, and thus in patient status, to be assessed.

On-line detection of organic pollutants in water by thickness shear mode resonators

A sensor system based on quartz thickness shear mode resonators is developed for the detection of organic pollutants in pure water, drinking water, and sea water. Unpolar polymer coatings with a glass transition below operating temperature and calix[8]arenes have been found as most sensitive, reversible and reliable for working in an aqueous environment. Mass changes and changes in viscoelastic properties of the coating caused by analyte sorption contribute to the sensor response.

Passive tension in rat hindlimb during suspension unloading and recovery: muscle/joint contributions.

Muscle/joint stiffness associated with disuse conditions or weightlessness may seriously impair movement and work capacity. The purposes of this study were 1) to develop a noninvasive model to measure rat hindlimb passive tension, 2) to describe changes in passive tension (i.e., flexibility) during whole body suspension and weight-bearing recovery, and 3) to determine relative contributions of the posterior hindlimb to passive tension. Male Sprague-Dawley rats were suspended (14 days) and reloaded (14 days). On days 0, 7, 14, 17, 21, and 28, animals were anesthetized and hindlimb passive tension was measured during ankle dorsiflexion. Seven days of suspension significantly increased passive tension. Recovery of passive tension occurred by 14 days of weight bearing. In suspended animals, increased passive tension was due to musculotendinous units (75%) rather than to the joint (25%). Increased passive tension did not appear to be due to a shorter muscle, but changes in muscle architecture, cytoskeletal proteins, or viscoelastic properties of the muscle and its connective tissue elements cannot be excluded.

Therapeutic effects of nitric oxide-donor isosorbide dinitrate on atherosclerosis-induced alterations in hemodynamics and arterial viscoelasticity are independent of the wall elastic component.

Whether the arterial elastic structures are involved in the beneficial effects of long-term treatment with organic nitrates on atherosclerosis-induced changes in hemodynamics and arterial wall viscoelastic properties, are case for angiotensin-converting enzyme (ACE) inhibitors, is not known. In the present study, atherogenic (A) diet, and isosorbide dinitrate (ISDN) (I) (60 mg Risordan LP, daily dose) were given concomitantly for 4 months to adult Pitman-Moore minipigs (A + I animals, n = 8), which were compared with A (n = 8) or control (C, n = 8) animals. Blood flow was investigated by hemodynamics in the hindlimb arterial bed; and wall rheology, histomorphometry and elastin; and desmosine (DES) and isodesmosine (IDE) contents in the abdominal aorta. Atherosclerosis prominently impaired the function of capacitance and resistance arteries, altered blood pressure contours, increased aortic stiffness and wall tension, and reduced parietal viscoelasticity through viscous component blunting. The treatment with ISDN significantly improved aortic pulsatility, arteriolar opposition to blood flow, and blood pressure (BP) contours by restoring, at least in part, the wall viscoelastic properties. However, there was no significant change in the area of the pressure-diameter curve hysteresis between the three animal groups. In contrast, ISDN reduced neither the cross-sectional area of lesions nor the losses in wall elastin content and had no influence on lipid accumulations in vessels and in the blood. The present results demonstrate that the beneficial hemodynamic and wall viscoelastic effects elicited by ISDN in atherosclerotic minipigs are not accounted for by therapeutic properties of the nitric oxide (NO) donor against alterations of elastic structures, but by the viscoelastic properties in the arterial wall.

Viscoelastic properties of cultured porcine aortic endothelial cells exposed to shear stress

The viscoelastic properties of cultured endothelial cells exposed to shear stress were measured by the micropipette technique and analyzed using a standard linear viscoelastic model. Cells from porcine aorta were cultured on glass coverslips. A shear stress of 2 Pa was applied using a parallel-plate flow chamber. After flow exposure, the cells were detached from the coverslips and suspended in culture medium. The micropipette experiment was performed on single cells under an inverted microscope. The desired negative pressure was applied stepwise to the tip of the micropipette by opening a solenoid valve. The deformation process of cells in the micropipette was observed through a TV camera and recorded on a videotape. To obtain the viscoelastic parameters, a half-space model of an endothelial cell was used. The cell was assumed to be a homogeneous and incompressible material, and a standard linear viscoelastic model was employed to account for the viscoelastic response. Cells exposed to shear stress for 6 h became spherical in shape after detachment from the substrate. In the case of a 24 h exposure, about half of the detached cells retained an elongated shape upon detachment, with the others taking on a spherical shape. The elastic constants, as determined based on the model, were approximately two times higher for the elongated cells than for control cells from static culture, no-flow conditions, indicating that the elongated cells became stiffer. Enhanced viscous properties also were observed for the elongated cells. These viscoelastic properties are considered to be closely related to cytoskeletal structure. Spherical cells upon detachment, even those that had been exposed to shear stress for 24 h, did not show such significant changes in viscoelastic mechanical properties.

Changes in Color and Dynamic Viscoelastic Properties of Epoxy Resin Composites by Surface Modification of Carbon Fibers

In the paper, we prepared the carbon fibers/epoxy resin composites using modified carbon fibers by a new surface modification method named photo-oxygenation. Effects of the surface modification of carbon fibers on the properties of the composites and matrix resin have been studied through the estimation of color and dynamic viscoelastic properties of the composites. The obtained results are as follows.(1) Color and the dynamic viscoelastic properties of the composites change with carbon fibers' surface modified or not. So it is seen that the surface modification of reinforcing fibers changes not only the interphase but also the structure of matrix resin and its curing behavior.(2) The color properties and crosslinking density of the matrix resin are linearly related, so the color properties will be able to be applied for the quality control as curing estimation.

Changes in color and dynamic viscoelastic properties of epoxy resin composites by surface modification of carbon fibers

In the paper, we prepared the carbon fibers/epoxy resin composites using modified carbon fibers by a new surface modification method named photo-oxygenation. Effects of the surface modification of carbon fibers on the properties of the composites and matrix resin have been studied through the estimation of color and dynamic viscoelastic properties of the composites. The obtained results are as follows.(1) Color and the dynamic viscoelastic properties of the composites change with carbon fibers' surface modified or not. So it is seen that the surface modification of reinforcing fibers changes not only the interphase but also the structure of matrix resin and its curing behavior.(2) The color properties and crosslinking density of the matrix resin are linearly related, so the color properties will be able to be applied for the quality control as curing estimation.

A rheological study on the application of carbohydrate-protein incompatibility to the development of low fat commercial spreads

The small and large deformation properties of commercial low fat spreads and traditional full fat products have been investigated in order to develop a background understanding of the changes in viscoelastic properties and the structural organisation occurring as a result of addition of biopolymers to the aqueous phase of low fat dispersions. Parameters have been derived from compression analysis, dynamic oscillation (frequency, strain and temperature sweeps) and creep compliance testing. It seems that the direct replacement of fat with a biopolymer-structured aqueous phase does not imitate the plastic rheology of butter and margarine. Thus, the ratio of plastic to maximum stress ( σ p σ m ) of butter and margarine is substantially higher (0.96–1.0) than the ratio of inflectional to maximum stress ( σ i σ m ) of commercial low fat spreads with a strong, gel-like character (up to 0.83). Additionally, some commercial embodiments with reduced amounts of structural components have stress-strain profiles resembling those of viscous solutions instead of a plastic product. Dynamic oscillatory measurements have characterised the mechanical properties of water-continuous low fat spreads. Dispersions reproduced the mechanical profile of three-dimensional biopolymer gels with a high elastic component (tan δ ≈ 0.04) and a substantial linear response to increasing amplitude of oscillation (up to 10% deformation). Products comprising hydrolysed starch as one of the functional ingredients show long melting profiles upon heating, which contrast strongly with the ‘melt in the mouth’ properties of butter. In accordance with the above, butter requires lower initial strain to exhibit negligible recovery of shape after the removal of stress, than do commercial low fat spreads with a pronounced elastic element, during a creep compliance experiment.

The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures

Presbyopia is a condition of age rather than ageing and, as such, is devolved from the lamentable situation where the normal age-related reduction in amplitude of accommodation reaches a point when the clarity of vision at near cannot be sustained for long enough to satisfy an individual's requirements. Most of our facility to accommodate has been lost by 55 years-of-age and subsequent deterioration in visual performance at near is attributable to characteristics of senescent vision familiar to the optometrist. Our understanding of the cause of presbyopia has then to be derived principally from our understanding of the mechanism of accommodation in young eyes. Hermann von Helmholtz did much to clarify these mechanisms, but despite much research in the 100 years since his death, there is still no consensus on their precise nature. This paper presents a summary of issues, past and present, which have figured in the literature on the physiology of accommodation and presbyopia, and confirms that the pathophysiology of presbyopia is likely to result from deterioration in structure and function of a number of inter-related tissues. Changes in crystalline lens dimensions with age, the associated change in geometry of zonular attachments, and changes in viscoelastic properties of the lens capsule and lens matrix would, however, appear to be the principal correlates for the onset of presbyopia. Recent models of the biomechanics of accommodation have drawn attention to the feasibility of extralenticular contributions to presbyopia and have examined properties of the elasticity and leverage provided by posterior, anterior and tensile fibre systems. The stimulus for innovation in the correction and remediation of presbyopia is strong as at least 98% of individuals from the industrialised nations will survive until 45 years-of-age; it is salutary to note that the survival rate 100 years ago was only 60%.

The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures

SummaryPresbyopia is a condition of age rather than ageing and, as such, is devolved from the lamentable situation where the normal age‐related reduction in amplitude of accommodation reaches a point when the clarity of vision at near cannot be sustained for long enough to satisfy an individual's requirements. Most of our facility to accommodate has been lost by 55 years‐of‐age and subsequent deterioration in visual performance at near is attributable to characteristics of senescent vision familiar to the optometrist. Our understanding of the cause of presbyopia has then to be derived principally from our understanding of the mechanism of accommodation in young eyes. Hermann von Helmholtz did much to clarify these mechanisms, but despite much research in the 100 years since his death, there is still no consensus on their precise nature. This paper presents a summary of issues, past and present, which have figured in the literature on the physiology of accommodation and presbyopia, and confirms that the pathophysiology of presbyopia is likely to result from deterioration in structure and function of a number of inter‐related tissues. Changes in crystalline lens dimensions with age, the associated change in geometry of zonular attachments, and changes in viscoelastic properties of the lens capsule and lens matrix would, however, appear to be the principal correlates for the onset of presbyopia. Recent models of the biomechanics of accommodation have drawn attention to the feasibility of extralenticular contributions to presbyopia and have examined properties of the elasticity and leverage provided by posterior, anterior and tensile fibre systems. The stimulus for innovation in the correction and remediation of presbyopia is strong as at least 98% of individuals from the industrialised nations will survive until 45 years‐of‐age; it is salutary to note that the survival rate 100 years ago was only 60%.

Viscoelastic Property Changes in Cheddar Cheese During Ripening

ABSTRACTThe rheological properties of pasteurized and raw milk Cheddar cheese were studied using oscillatory dynamic measurements, and a specially designed rheometer fixture that prevented specimen slippage. Dynamic measurements within the linear viscoelastic range were made throughout ripening. Within‐cheese changes, as related to ripening time, as well as between‐cheese‐type differences in G’ and G” were observed. Differences in rheological characteristics were attributed to proteolytic activities in Cheddar cheese during ripening. Specific peptide profiles associated with proteolysis during ripening may affect cheese rheological properties.

Characterization of commercial soft liners by dynamic mechanical analysis

Although there is a significant clinical interest in suitable polymer-based soft liners, none have proven fully satisfactory in actual use. As a result there has been continued interest in the development of new materials. One major weakness in the development of improved materials has been the lack of clear understanding of structure/property relationships. This paper deals with the determination of visco-elastic properties such as E′ (Storage modulus) and tan δ (damping factor) of four commercial materials. They represent the broad range of chemical types available for denture base soft liners. A DMA study of four materials: (1) Molloplast B (silicone); (2) Novus (phosphazine); (3) Kurepeet (fluropolymer); and (4) Super Soft (acrylic) was made using a Perkin-Elmer DMA-7. Samples were made following the manufacturers' instructions, in the form of sheets 1.5 mm thick and 15.0 mm square. The samples were tested compressively using a 3 mm flat tip probe cycled at a frequency of 1 Hz. Wet and dry specimens were evaluated for E′ and tan δ over a 5–95°C temperature range. Water sorption was determined gravimetrically at 37°C. Changes in E′ between the wet and dry conditions for Molloplast (B), Kurepeet and Super Soft were insignificant. A significant increase in tan δ for wet Novus was observed, suggesting that the material is capable of dissipating more energy. The ‘wet’ modulus (E′) is about 42% lower than the “dry” modulus (E′). This difference may be attributed to the very high water sorption (34%) of Novus. i.e. the significant decrease in E′ indicates plasticization due to sorbed water. Changes in visco-elastic properties seem to occur for materials which take up large amounts of water at 37°C. DMA is found to be a useful supplement for the evaluation of soft lining materials in conjunction with the standard mechanical test methods.

Alternate instrumental methods of measuring scorch and cure characteristics

After the development of the oscillating disk curemeter over 30 years ago, standard conditions for measuring scorch times (time till the onset of vulcanization) were developed. ISO 3417 and ASTM D2084 describe t s 1 and t s 2 test parameters which are accepted by the rubber industry as a method for measuring scorch times. t s 1 is the time until one torque unit rise above minimum torque and t s 2 is the time until two torque units rise above minimum torque. These parameters are arbitrary and do not measure the ‘true’ scorch time. Also t s 1 and t s 2 may not be an accurate comparison for compounds which differ greatly in their state of cure. Experience has shown that some rotorless curemeter designs are far more sensitive to physicochemical changes occurring during the cure process than the conventional oscillating curemeters. The MDR 2000 ® rotorless curemeter (MDR) and the RPA 2000 ™ Rubber Process Analyzer (RPA) measure subtle changes in viscoelastic properties during the curing process. This paper discusses the feasibility of using the time until the viscous torque (S′') peaks (t rmmaxS′') to more accurately measure the ‘true’ scorch time. Other techniques for measuring cure characteristics are also reviewed for comparison.

Monitoring of the Changes of Dynamic Viscoelastic Properties of Liquid Crystals-During the Orientation with Voltage Pulses Using a Quartz Crystal Resonator

Abstract The change of dynamic viscoelastic properties of liquid crystals was monitored using the quartz crystal resonator by measuring the resonant frequency and the resonant resistance of the quartz crystal during impression of voltage pulses. The viscoelastic change showed different responses as the effects of the applied voltage amplitude, an alignment film and the anisotropy of dielectricity. The higher applied voltage and a cell with alignment films gave larger responses than lower applied voltage and a cell without alignment film, respectively, and the positive and negative anisotropy of dielectricity gave opposite responses. The diagramed resonant frequency and resonant resistance for the voltage pulses showed hysterisis, and these changes are understood as the hysterisis of the dynamic orientation processes of liquid crystal molecules.

Viscoelastic properties of polystyrene–poly(vinyl methyl ether) blends in oscillatory and steady shear flows near the phase separation temperature

AbstractViscoelastic properties of a polystyrene‐poly(vinyl methyl ether) blend were measured in oscillatory and steady shear flows near the phase separation temperature. In the one‐phase region, the viscoelastic properties are independent of the type of flow and flow geometry, and their shear and frequency dependences are the same as those of homopolymers. When phase separation occurred, a change in viscoelastic properties was observed in all types of flow and flow geometries, and they are different in the different types of flow and/or different flow geometries in the two‐phase region.In the shear rate dependence of viscosity of the polymer blend in the two‐phase region at quiescence, a plateau region was observed, in which the shear viscosity can be regarded as the zero shear viscosity of the homogenized polymer blend at that temperature.