Variations In Viscoelastic Properties Research Articles

Q -sensitive quartz crystal microbalance as a fast probe for viscoelasticity in polymeric film

A new quartz crystal microbalance (QCM) with high time resolution was developed to study photoinduced variation of viscolastic properties in polymeric films. We present a full characterization of the mechanical properties (elasticity and viscosity) of the film deposited on the electrode, measuring both the frequency shift and the quality factor Q of the resonator in real time, with millisecond time resolution. This allows us to monitor the fast dynamics of mechanical properties and we present a study of the variation of viscoelastic properties under illumination in an azobenzene side-chain poly-meta-acrylate. Materials of this family are interesting because both optical and mechanical properties can be externally controlled by illumination, with the possibility to induced photo-triggered isothermal phase transitions and optical quenching.

Biomechanics of single zonal chondrocytes

Articular cartilage has a distinct zonal architecture, and previous work has shown that chondrocytes from different zones exhibit variations in gene expression and biosynthesis. In this study, the material properties of single chondrocytes from the superficial and middle/deep zones of bovine distal metatarsal articular cartilage were determined using unconfined compression and digital videocapture. To determine the viscoelastic properties of zonal chondrocytes, unconfined creep compression experiments were performed and the resulting creep curves of individual cells were fit using a standard linear viscoelastic solid model. In the model, a fixed value of the Poisson's ratio was used, determined optically from direct compression of middle/deep chondrocytes. The two approaches used in this study yielded the following average material properties of single chondrocytes: Poisson's ratio of 0.26±0.08, instantaneous modulus of 1.06±0.82 kPa, relaxed modulus of 0.78±0.58 kPa, and apparent viscosity of 4.08±7.20 kPa s. Superficial zone chondrocytes were found to be significantly stiffer than middle/deep zone chondrocytes. Attachment time did not affect the stiffness of the cells. The zonal variation in viscoelastic properties may result from the distinct mechanical environments experienced by the cells in vivo. Identifying intrinsic differences in the biomechanics of superficial and middle/deep zone chondrocytes is an important component in understanding how biomechanics influence articular cartilage health and disease.

Spatially resolved microrheology through a liquid/liquid interface

We present a light-scattering technique for the measurement of the microrheological properties of viscoelastic liquids in small volumes over a large frequency range (on the order of eight decades). The accuracy of the method for model viscoelastic liquids (polyethylene oxide solutions in water) is demonstrated by comparing the results with conventional mechanical measurements of the loss and storage moduli. Then we show that the method can be used to measure variations in viscoelastic properties in a heterogeneous system by measuring the variation in the moduli with position (and time) across a liquid/liquid interface between a viscoelastic polymer solution and a Newtonian liquid.

Material’s Volumetric-Flow Rate (MVR) as a Unification Parameter in Asphalt Rheology and Quality Control / Quality Assurance Tool for High Temperature Performance Grading

Abstract Rheological data of unmodified and polymer-modified asphalts are conventionally obtained from dynamic mechanical characterization and expressed in terms of sets of curves showing the variation of viscoelastic properties with frequency. Using the conventional melt flow indexer, the material’s volumetric-flow rate MVR (in cm3 / 10 minutes) through a predefined die under conditions of constant temperature and stress when obtained for the same asphalts, shows a direct relationship with the dynamic data. The MVR value helps in unifying the sets of dynamic data curves of |G*|, G” and |G* |/sin δ versus frequency in the case of unmodified asphalts, polymer-modified asphalts and asphalt mastics. The unification technique has a sound theoretical basis and the unified curves have far-reaching implications. Since MVR is so simple to determine quite accurately on a relatively inexpensive, easy-to-use flow measurement device (FMD), this parameter can be generated on paving sites or at refineries, if needed, rather than in research laboratories as is the case with the fundamental rheological parameters. The MVR can then be used as an excellent indicator of the fundamental rheological parameters through the use of the unified curves. The MVR can be utilized to accurately determine the currently used high temperature performance grade specification of paving asphalt. On account of the simplicity in obtaining this specification value from the MVR, it may be routinely used for quality control / quality assurance purposes. It can also be used as a rapid product development / formulation tool.

Odd-even effect in the viscoelastic properties of main-chain liquid crystal polymer-low-molar-mass nematogen mixtures

The viscoelastic properties of dilute nematic mixtures containing 4'-(pentyloxy)-4-cyanobiphenyl (5OCB) and the main-chain liquid crystal polymer [OPhPhCH 2 CH(CH 3 )PhO(CH 2 ) m ] n (TBP-X) were determined using dynamic light scattering and Freedericksz transition measurements. An odd-even variation in viscoelastic properties with spacer length of TPB-X was found. Specifically, TPB-2n/5OCB mixtures have substantially lower splay and twist decay rates and slightly higher bend decay rate than those of TPB-2n+1/5OCB mixtures. These differences are primarily due to changes in the associated viscosity coefficients

Viscoelasticity of cellulose polymers and mucociliary transport on frog palates

The viscoelastic propertied of model polymer, hydroxypropylmethylcellulose (HPMC), in ternary solvent mixtures and the variation of viscoelastic properties under dilution with water were investigated. HPMC was dispersed in mixtures of glycerol formal (GF) or ethyl alcohol, propylene glycol (PG), and water. Each polymeric sample was oscillated at different frequencies with fixed stress amplitude and the changes in elastic modulus, viscous modulus, and apparent viscosity were measured at 30°C. The moisture absorption rate of HPMC in PG:GF solvent mixtures in a 94% relative humidified environment was also studied. A linear relationship between the elastic modulus or viscous modulus and polymer concentration was observed for the HPMC samples. The relative mucociliary transport rate tested on the non-depleted frog palate model revealed a curvilinear correlation with the loss tangent (a ratio of the viscous modulus to the elastic modulus) of the polymeric solution. Using GF as a solvent for HPMC resulted in a formation of a rigid gel with the highest elastic modulus and viscous modulus among solvents selected. A higher affinity for water was found for HPMC in the PG:GF 90:10 mixture compared with HPMC in the PG:GF 70:30 mixture. However, the elastic modulus and viscous modulus were much higher for HPMC in the PG:GF 70:30 mixture.