Changes In Viscoelastic Properties Research Articles

Rheological study of structural transitions in triblock copolymers in a liquid crystal solvent.

Rheological properties of triblock copolymers dissolved in a nematic liquid crystal (LC) solvent demonstrate that their microphase separated structure is heavily influenced by changes in LC order. Nematic gels were created by swelling a well-defined, high molecular weight ABA block copolymer with the small-molecule nematic LC solvent 4-pentyl-4'-cyanobiphenyl (5CB). The "B" midblock is a side-group liquid crystal polymer (SGLCP) designed to be soluble in 5CB and the "A" endblocks are polystyrene, which is LC-phobic and microphase separates to produce a physically cross-linked, thermoreversible, macroscopic polymer network. At sufficiently low polymer concentration a plateau modulus in the nematic phase, characteristic of a gel, abruptly transitions to terminal behavior when the gel is heated into its isotropic phase. In more concentrated gels, endblock aggregates persist into the isotopic phase. Dramatic changes in network structure are observed over small temperature windows (as little as 1 °C) due to tccche rapidly changing LC order near the isotropization point. The discontinuous change in solvent quality produces an abrupt change in viscoelastic properties for three polymers having different pendant mesogenic groups and matched block lengths.

Relationship between structural evolution and rheological measurements for polymer + organoclay nanocomposites

AbstractNanocomposites with synthetic biodegradable aliphatic polyesters (BAP) were synthesized by the solution intercalation method with addition of organically modified montmorillonite. The hybrid structural evolution, caused by nanoscopic interaction between matrix polymer and layered silicate, was analyzed using viscoelastic properties obtained from oscillatory rheological measurements. Changes of viscoelastic properties to more solid-like, especially in the terminal region, were explained by both formation of specific BAP chain mesostructures with organoclay incorporated and a rheological percolation threshold. This behaviour was analyzed via a modified Cole-Cole plot evaluating the structural changes with filler concentration as well as the frequency dependence of dynamic modulus, complex modulus, and phase angle. Furthermore, the stress relaxation behaviour successfully demonstrated structural changes and critical concentration of nanocomposites with a jump shift of the longest relaxation time towards longer time.

Ultrasonic Dynamic Mechanical Analysis of Polymers

Abstract The propagation of ultrasonic waves in polymers depends on their viscoelastic behaviour and density, resulting significantly affected by phase transitions occurring with changing temperature and pressure or during chemical reactions. Therefore, the application of low intensity ultrasound, acting as a high frequency dynamic mechanical deformation applied to a polymer, can monitor the changes of viscoelastic properties associated with the glass transition, the crystallization, the physical or chemical gelation, the crosslinking. Thanks to the non-destructive character (due to the very small deformation amplitude), low intensity ultrasound can be successfully used for polymer characterization. Moreover, this technique has a big potential as a sensor for on-line and in-situ monitoring of production processes for polymers or polymer matrix composites. Recently, in the laboratory of Polymeric Materials of Lecce University a custom made ultrasonic set-up for the characterization of polymeric material, even at high temperatures, has been developed. The ultrasonic equipment is coupled with a rotational rheometer. Ultrasonic waves and shear oscillations at low frequency can be applied simultaneously on the sample, getting information on its viscoelastic behaviour over a wide frequency range. The aim of this paper is to present the potential and reliability of the ultrasonic equipment for the ultrasonic dynamic mechanical analysis (UDMA) of both thermosetting and thermoplastic polymers. Three applications of UDMA to different polymeric systems will be reviewed, concerning the cross-linking of a thermosetting resin, the crystallisation from melt of a semicrystalline polymer and the water sorption in a dry hydrogel film. From the ultrasonic velocity and attenuation measurements, the viscoelastic properties of the tested polymers are evaluated in terms of complex longitudinal modulus and compared with the results of conventional dynamic mechanical analysis, carried out at low frequency.

Toughening of a carbon fibre reinforced epoxy anhydride composite using an epoxy terminated hyperbranched modifier

An epoxy terminated hyperbranched polymer (HBP) has been used to toughen a carbon fibre reinforced epoxy anhydride composite. This report describes the changes in viscoelastic properties used to measure the effect of HBP upon the reaction kinetics, viscosity and phase separation process occurring during cure. An appropriate cure profile was determined to ensure an optimum morphology development during fabrication of the composite containing varying amounts of HBP. The increases in the mode I and II interlaminar fracture toughness of the composite material were found to be 224% and 265%, respectively, at a loading of 10 wt% of HBP additive. Other properties such as interlaminar shear strength, flexural modulus and strength were found to decrease modestly while the glass transition, thermal stability (measured via thermogravimetric analysis) and the transverse coefficient of thermal expansion were largely unaffected. The substantial increase in toughness was attributed to the high level of interaction between the HBP and epoxy anhydride matrix measured from the varying positions of the β relaxation peaks using dynamic mechanical thermal analysis.

Effects of Fat Reduction and Curd Dipping Temperature on Viscoelasticity, Texture and Appearance of Gaziantep Cheese

The effects of varying fat content (from 13.5 to 50.4% w/w) and curd dipping temperatures (75, 85 and 95°C) on changes in viscoelastic properties, texture and appearance of Gaziantep cheese were examined. Viscoelastic properties of cheeses were studied using creep and recovery tests. Creep measurements showed that fat reduction from 50.4% (w/w) to 13.5% (w/w) decreased viscoelasticity while dipping into hot whey increased that of low-fat cheeses. Textural characteristics (hardness, gumminess, cohesiveness, springiness) were determined by texture profile analysis (TPA). Hardness, gumminess, cohesiveness and springiness values increased with decreasing fat content (P < 0.05). The application of curd dipping also increased hardness, gumminess, cohesiveness and springiness. Textural parameters were correlated with each others, except cohesiveness. Fat in dry matter correlated only with hardness and springiness, however curd dipping process correlated with all TPA parameters considered in this study. Colour of the cheese samples was affected by fat content as L value and b value decreased with decreasing fat content (P < 0.05). The decrease in curd dipping temperature increased the L value (P < 0.05) and decreased the b value.

Molecular Evolution of Polymers through Photoageing: A New UV in situ Viscoelastic Technique

AbstractSummary: Dienic elastomers are highly sensitive towards oxidation. During UV exposure, both scission and/or recombination reactions compete which involves an evolution of the molecular structure of the material. We developed an in situ technique to monitor the viscoelastic property changes upon UV irradiation of polymers in a single experiment. We illustrated this photorheology technique in the case of two kinds of elastomers. From our findings, this method is assumed to bring significant new advantages.Photograph of the photorheometer used in this study.magnified imagePhotograph of the photorheometer used in this study.

Investigating the properties of supported vesicular layers on titanium dioxide by quartz crystal microbalance with dissipation measurements

Adsorption of phospholipid vesicles on titanium dioxide was studied by a combination of quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy techniques. Vesicle size, concentration in solution, and bilayer composition were systematically varied. A strong dependence of the QCM-D response (magnitude of the frequency and dissipation factor shifts) on the vesicle concentration in solution was observed. QCM-D data were compared with a linear viscoelastic model based on the Voight element to determine layer thickness, density, elastic modulus, and viscosity. Based on the results of this comparison, it is proposed that (i) layer thickness and density, as sensed by QCM-D, saturate much earlier (in time) than the actual surface coverage of the vesicles (number of vesicles per unit area); (ii) changes in surface coverage that occur after the density and thickness, as sensed by QCM-D, have saturated, are interpreted by the model as changes in the layer's viscoelastic properties. This is caused by the replacement of the viscous media (water) between the vesicles by viscoelastic media of similar density (vesicles); (iii) viscoelastic properties of layers formed at different vesicle concentrations differ significantly, while the vesicle surface coverage in those layers does not. Based on the comparison between the atomic force microscopy images and QCM-D data acquired at various vesicle concentrations it is proposed that QCM-D response is not directly related to the surface coverage of the vesicles.

Principles of quartz crystal microbalance/heat conduction calorimetry: Measurement of the sorption enthalpy of hydrogen in palladium

A sensitive new measurement technology is described which combines calorimetry, gravimetry, and rheology applied to chemical reactions in thin films: quartz crystal microbalance/heat conduction calorimetry (QCM/HCC). The quartz crystal microbalance/heat conduction calorimeters constructed so far simultaneously measure heat generation, mass uptake or release, and viscoelastic property changes in the same, sub-milligram solid film sample when gases interact with the film in an isothermal surrounding. It is possible to measure the energetics of formation of a single layer of adsorbed molecules on a gold surface with this technique. The principles of operation of both the mass and the heat flow sensor are described, and one implementation of the combined sensor and apparatus and its electronics is presented. Methods for calibration and the preparation of thin sample films are summarized. As an illustrative example, the determination of the sorption enthalpy of hydrogen in a 25 °C palladium film of 140 nm thickness is discussed in detail. Other examples of the operation of the QCM/HCC are tabulated.

Long-term ovariectomy decreases ovine compact bone viscoelasticity

Changes in bone mineral density associated with estrogen depletion in humans do not account for all of the associated change in fracture risk, and it is possible that some of this variation may lie in changes of other aspects of bone quality. The purpose of this study was to investigate changes in viscoelastic behavior of compact bone that may be associated with estrogen depletion. Changes in compact bone viscoelastic properties associated with three years of ovariectomy were investigated with dynamic mechanical analysis (low-amplitude 3-point bending at frequencies of 1–20 Hz) using beams milled from the diaphysis of the ovine radius. The viscoelastic storage modulus was significantly (5.2%) lower at the higher frequencies for the ovariectomized animals. The general anatomic variation in storage modulus, in which cranial sectors had higher values than caudal sectors, did not change with ovariectomy. The loss tangent (tan δ, a measure of damping) was also greatly decreased (up to 83%) at high frequencies in the ovariectomized animals. Anatomic variation in tan δ at low (6–12 Hz) frequencies (cranial and caudal sectors having higher values than lateral or medial sectors) was enhanced with ovariectomy. Changes in viscoelastic properties associated with long-term estrogen depletion could be responsible for a significant reduction in the toughness or strength of a bone without concomitant changes in screening modalities used to evaluate bone quality (e.g., DXA, QCT, QUA).

Quasi-linear viscoelastic properties of fibrotic neck tissues obtained from ultrasound indentation tests in vivo

Background. Hand palpation is a conventional way to assess and document soft tissue fibrosis. But it is semi-quantitative and subjective, so there is a need to develop quantitative and objective methods for this purpose. Methods. 105 patients with different degrees of radiation-induced fibrosis of soft tissue of the neck were assessed using an ultrasound indentation method. The force response was reconstructed from the indentation history using a quasi-linear viscoelastic model with four material parameters. The parameters which best curve-fitted the force response with respect to the experimentally measured one, were selected as the viscoelastic properties of the tested soft tissue. These parameters were compared among patient subgroups with different degrees of fibrosis as scored by hand palpation, and also compared with those of a control group of healthy, non-irradiated subjects. Their relation to the rotation range of the neck and the effective Young’s modulus, were also assessed. Findings. Soft tissue with a more severe degree of fibrosis was associated with a larger initial stiffness and a more rapid increase in stiffness under loading. Viscoelasticity parameters could discriminate soft tissue with different degrees of clinical fibrosis and had significant correlation with clinical parameters of fibrosis. Interpretation. Change of viscoelastic properties is reflection of pathological modifications of components in fibrotic soft tissues. Measurement of viscoelasticity parameters of soft tissue provides a quantitative and objective approach for the researcher and clinician to quantify soft tissue fibrosis. Relevance Measurement of the change of viscoelastic properties of soft tissue provides a quantitative and objective approach for researchers and clinicians to quantify soft tissue fibrosis which is one of the most common late effects of radiotherapy.

Influence of the Ripening Time on the Viscoelastic Behaviour of Tetilla Cheese

Controlled shear stress tests were used to monitor the changes of viscoelastic properties in industrial samples of Tetilla cheese with the ripening time. The linear viscoelastic range was determined from both stress sweep and creep-and-recovery tests conducted at 20ºC. One-week-old samples were found to be more rigid and elastic, and to exhibit broader linear ranges, than older samples. Mechanical spectra recorded at the same temperature confirmed this trend since viscoelastic moduli G’ and G" decreased markedly during the first week of ripening, indicating a loss of structure during this period. Besides, physico-chemical information (pH, water content, protein content, nitrogen fractions, and s1 and β casein fractions) was obtained by analytical methods. Quite good correlation between rheological and chemical results was found. For each ripening time, an additional study of mechanical spectra as a function of temperature was performed. A marked increase of the power law exponents as temperature is raised is observed after the second week. Crosslink breakdown and the resulting structural weakening of the micellar network through casein hydrolysis during ripening can explain this result. The whole study provided similar results to those found for other type of cheeses and, also, allowed the classification of samples into two groups of markedly different behaviour, namely, one-week-old samples and older samples. In conclusion more than one week of ripening is needed to reach the adequate texture standardisation of Tetilla cheese.

Time and temperature dependence of the viscoelastic properties of CFRP by dynamic mechanical analysis

Understanding viscoelastic properties of composite materials is essential for the design and analysis of many advanced structures. Viscoelastic material characterization is critical in designs that incorporate specified levels of damping, and to the understanding of processing problems. However, experimental viscoelastic characterization of anisotropic materials can be complicated because of the number of independent parameters to be evaluated. Recently, an approach leading to the 3-D viscoelastic characterization of transversely isotropic materials using a reduced number of measured parameters has been developed. Based on this recently developed model, the present work evaluates time and temperature effects on the viscoelastic properties of unidirectional carbon fiber reinforced laminae and cross-ply laminates. The experimental investigation is conducted on subscale specimens loaded in flexure, using dynamic mechanical analysis (DMA) equipment. The results presented demonstrate that DMA equipment offers good potential to study changes in viscoelastic properties of transversely isotropic laminae and laminates, related to temperature and frequency. This work provides a means for the study of viscoelastic properties of fiber-reinforced composites, and, therefore, constitutes a valuable contribution to the understanding of time and temperature dependence of these mechanical properties.

Aggregation and degradation of plasticized wheat gluten during thermo-mechanical treatments, as monitored by rheological and biochemical changes

Different thermo-mechanical treatments were carried out on plasticized gluten samples using the rubber process analyser (RPA 2000) which allows to record changes in viscoelastic properties during sample treatment. Temperatures ranging from 90 to 150 °C were tested at two oscillatory strains (7%: linear domain; 70%: high deformation domain). Biochemical changes of gluten proteins during treatments were followed by SE-HPLC. Rheological and biochemical changes were found to be in good accordance and allow us to characterize two major phenomena: aggregation and degradation of wheat gluten proteins upon heating. A kinetic approach of both phenomena was carried out in order to determine the temperature dependency of both reactions with respect to applied strain. Strain was found to decrease the temperature dependency of modulus values, to moderately accelerate the gluten protein aggregation reaction, and to greatly accelerate the gluten thermal degradation.

Impedance analysis of the thickness shear mode resonator for organic vapour sensing

The impedance analysis of thickness shear mode (TSM) resonator coated with calixarene films was successfully exploited for the registration of organic solvent vapours of relatively high (pre-explosive) concentrations. The impedance analysis consisted of fitting the experimental admittance spectra of TSM resonators to the equivalent circuit model using a highly accurate and reliable least squares algorithm. This approach allowed simultaneous monitoring of the changes in mass and viscoelastic properties of the sensitive membrane caused by adsorption of organic vapours and therefore enables both quantification and discrimination between the vapours of different types of organic solvents, such as hexane and toluene.

Using Oscillatory Squeeze Flow to Monitor the Change in Viscoelastic Properties of Curing Materials

Small amplitude oscillatory squeeze flow was found to provide a convenient means of monitoring the changes in the viscoelastic properties of two types of adhesive materials as they cure, right up to their final set state. The high stiffness of the instrument, the use of band-limited pseudorandom noise as the input signal and the use of disposable disks enabling easy sample removal represent the main advantages of this technique over conventional rheometers to monitor the changes in the viscoelasticity of these materials.

The Mechanical Properties of Thin Polymer Films from Nanoindentation

ABSTRACTThe thermomechanical properties of thermoset polymeric materials are shown to be suitable for detecting relevant degradative processes in the matrix of a fibre composite. In space environments, the surface of the material will be modified first because of the diffusion of the active molecules. Nanoindentation has been shown to be capable of determining changes in viscoelastic properties of thin polymer films and at surfaces of bulk material. The Hysitron Triboscope has been used to characterise an epoxy resin and polycyanurate in dynamic and static scanning modes. Dynamic nanoindentation has been shown to provide a full description of the viscoelastic response. Progress has been made in analysing static indentation for time dependence.

Change of properties during storage of a UDMA/TEGDMA dental resin.

The aim of this study was to evaluate the changes in viscoelastic properties of a UDMA-based dental resin as a function of time after initial light exposure. Specimens of a UDMA/TEGDMA (70:30 wt%) resin were irradiated by a visible-light-curing unit. Immediately after the irradiation, the light-cured specimen was stored in the dark for different times from 1 to 120 h at 37 degrees C, and characterized by means of DMA, DSC, and FTIR spectroscopy. The irradiated specimen exhibited a bimodal shape in the form of two rapid declines in log E' corresponding to glass transition with a plateau between the two declines. Two distinct peaks were seen in tan delta versus temperature. The thermal reaction of the incompletely cured sample with residual groups trapped by the fast reaction during irradiation is responsible for the plateau. After storage, significant changes were observed in dynamic mechanical parameters, DSC exotherm, and degree of conversion. Storage modulus continued to increase during the 4 h of storage and leveled off thereafter. Peak heights of tan delta versus temperature were also influenced by storage. Degree of conversion increased from 75 +/- 2% immediately after irradiation to 87 +/- 3% after 120 h storage. The changes of the properties of this dental resin system when stored at 37 degrees C after irradiation are clinically important in terms of stability, durability, and performance after initial polymerization.

Surface mechanical properties of pHEMA contact lenses: viscoelastic and adhesive property changes on exposure to controlled humidity.

The surface mechanical properties of poly(hydroxyethyl)methacrylate (pHEMA)-based contact lenses were monitored as a function of humidity by atomic force microscopy (AFM). Surface viscoelastic and adhesion values were extracted from AFM force versus distance interaction curves and were found to be strongly dependent on the bulk water content of the lens and on the relative humidity. At low relative humidity, 40-50%, the dehydration rate from the surface is faster than the hydration rate from the bulk, leading to a rigid surface region that has mechanical properties similar to those measured on totally dehydrated lenses. At relative humidity values > 60%, the dehydration rate from the lens surface rapidly decreases, leading to higher surface water content and a softer surface region. The results indicate that, in an ocular environment, although the bulk of the pHEMA contact lens is hydrated, the surface region may be in a transition between a dehydrated glassy state and a hydrated rubbery state.

Changes in viscoelastic properties of the myocardium and major arteries in sportsmen of high qualification with mild arterial hypertension

Changes in viscoelastic properties of the myocardium and major arteries in sportsmen of high qualification with mild arterial hypertension

Cell spreading on quartz crystal microbalance elicits positive frequency shifts indicative of viscosity changes.

Cell attachment and spreading on solid surfaces was investigated with a home-made quartz crystal microbalance (QCM), which measures the frequency, the transient decay time constant and the maximal oscillation amplitude. Initial interactions of the adsorbing cells with the QCM mainly induced a decrease of the frequency, coincident with mass adsorption. After about 80 min, the frequency increased continuously and after several hours exceeded the initial frequency measured before cell adsorption. Phase contrast and fluorescence microscopy indicated that the cells were firmly attached to the quartz surface during the frequency increase. The measurements of the maximal oscillation amplitude and the transient decay time constant revealed changes of viscoelastic properties at the QCM surface. An important fraction of these changes was likely due to alterations of cytosolic viscosity, as suggested by treatments of the attached cells with agents affecting the actin and microtubule cytoskeleton. Our results show that viscosity variations of cells can affect the resonance frequency of QCM in the absence of apparent cell desorption. The simultaneous measurements of the maximal oscillation amplitude, the transient decay time constant and the resonance frequency allow an analysis of cell adsorption to solid substratum in real time and complement cell biological methods.