Polymer viscoelastic behavior affected

This section presents the factors that affect polymer viscoelastic behavior. These factors include polymer concentration, salinity, surfactant, and temperature. The viscoelastic behavior in a typical Daqing solution is also presented. 

The attenuation and velocity of acoustic energy in polymers are very different from those in other materials due to their unique viscoelastic properties. The use of ultrasonic techniques, such as acoustic spectroscopy, for the characterization of polymers has been demonstrated [47,48]. For AW devices, the propagation of an acoustic wave in a substrate causes an oscillating displacement of particles on the substrate surface. For a medium in intimate contact with the substrate, the horizontal component of this motion produces a shearing force. In such cases, there can be sufficient interaction between the acoustic wave and the adjacent medium to perturb the properties of the wave. For polymeric materials, attenuation and velocity of the acoustic wave will be affected by changes in the viscoelastic behavior of the polymer. 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

The damping of the composite structure will be affected by the thicknesses of the various layers, stiffnesses of the base and top plates, and the viscoelastic properties of the constrained layer (12). In the present instance (13), it was desired to develop a a broad-band material to damp a model composite structure consisting of a 2.54 cm. base plate (H ), 0.079 cm. polymer layer (H ), and 0.159 cm. cover (Ho)fi oase and cover were composed oi brass with a modulus of 10 Pa. In this instance, the only variable was the viscoelastic behavior of the polymer layer. A temperature range from 0 to 20 degrees Centigrade and a frequency range from 100 Hz to 10 kHz were desired. 

We have approached the subject in such a way that the book will meet the requirements of the beginner in the study of viscoelastic properties of polymers as well as those of the experienced worker in other type of materials. With this in mind. Chapters 1 and 2 are introductory and discuss aspects related to chemical diversity, topology, molecular heterodispersity, and states of aggregation of polymers (glassy, crystalline, and rubbery states) to familiarize those who are not acquainted with polymers with molecular parameters that condition the marked viscoelastic behavior of these materials. Chapters 1 and 2 also discuss melting processes and glass transition, and factors affecting them. 

The change of viscoelastic behavior of ciosslinked elastomers on swelling depends on polymer physical network. This, in turn, depends on the concentration and nature of the plasticizer in the material expressed by parameter m describing the ability of liquid to affect physical network of elastomer. 

In these hydrogels, the number of crosslinks is determined by the ehemi-cal equilibrium among boric acid, borate ions, and diol sites on the polymer chains, which can be influenced by both temperature and pH. Therefore, pH and temperature can dramatically affect the viscoelastic behavior of the borate-polyol gels. The sol-gel transition typically occurs at pH 8-9, near the pAa of the boron compound. The formation of the crosslink is exothermic, between 1-2 kj mol so that gel formation can be opposed by heating. ... 

Summary. Both temperature and scan frequency affect the patterns created by movement of a nanoscopic tip in contact with a polymer surface. At experimental time scales which are faster than the relaxation time associated with the elongation of a polymer coil, the polymer s response to the tip-induced shearing forces is elastic. Experimental time scales slower than the characteristic relaxation time results in alignment of the polymer with the tip trajectory and net translational movement of polymer towards the center of the scan area. The time-temperature dependence of the patterns are well-described by the WLF equation which is typically used to describe viscoelastic behavior. Analysis of our data further suggests that the Tg of the polymer is elevated in the region confined between the tip and the substrate. 

One obstacle of the above experiments was the limited velocity range available in commercial FFM, typically 4 orders of magnitude. It is well known that both the time scale and the temperature affect the overall viscoelastic behavior of a polymer system (10). To date the limitations of piezoelectric scanners have restricted the range of temperatures examined to a narrow range near ambient conditions. With newly-designed force microscopes a broader range of temperature variation is now accessible. 

The authors also describe the effect of temperature on the Payne effect. With increasing temperature the amplitude of the Payne effect decreases significantly (Fig. 12). Very surprisingly, enhanced Payne-like behavior was observed for rubber vulcanizates at room temperature where filler-filler and filler-polymer interaction are not observed in comparison to the typically filled vulcanizates. The authors concluded that in addition to the contribution from the filler-filler network, there are many other factors that affect the nonlinear viscoelastic behavior. Nevertheless, the Payne effect is assumed to arise from the elementary mechanism consisting of adsorption-desorption of polymer chains from the surface of the particles [50]. Besides the experimental investigation, the authors have applied the Maier... 

The drastic changes in the physical properties of polymers, due to reinforcement, also lead to pronounced changes in their viscoelastic behavior. Viscoelastic properties and relaxation behavior of composites change as a result of the formation of surface layers at the pol5Tner-solid interface. The molecular mobility of polymeric chains is restricted in these layers, which affects mechanical properties. 

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