Rheological analysis, polymers viscoelasticity

Doufas A.K., Analysis of the rheotens experiment with viscoelastic constitutive equations for probing extensional rheology of polymer melts. J. Rheology, 50 749-769, (2006). 

Detailed analysis of the isothermal dynamic mechanical data obtained as a function of frequency on the Rheometrics apparatus lends strong support to the tentative conclusions outlined above. It is important to note that heterophase (21) polymer systems are now known to be thermo-rheologically complex (22,23,24,25), resulting in the inapplicability of traditional time-temperature superposition (26) to isothermal sets of viscoelastic data limitations on the time or frequency range of the data may lead to the appearance of successful superposition in some ranges of temperature (25), but the approximate shift factors (26) thus obtained show clearly the transfer viscoelastic response... 

Additionally, other features of polymer characterisation are discussed such as the determination of molecular weight, polymer fractionation techniques, chemical and thermal stability, resin cure, oxidative stability, photopolymers, glass and other transitions, crystallinity, viscoelasticity, rheological properties, thermal properties, flammability testing, particle size analysis and the measurement of the mechanical, electrical and optical properties of polymers. 

Measurement of viscoelastic and rheological properties of polymers are of increasing importance and recently developed instrumentation for measuring these properties discussed in Chapter 15 includes dynamic mechanical analysis, thermo-mechanical analysis... 

Some of the viscoelastic and rheological properties of polymers that can be measured by dynamic mechanical analysis are [90-92]... 

Study of the bulk viscoelastic properties of 11a are hampered by the crystallinity of the material, even though crystallization is slow. By introducing linkers with a mixed methyl substitution pattern, noncrystallizing supramolecular polymer 11b was obtained, which was studied using dynamic mechanical thermal analysis (DMTA), rheology, and dielectric relaxation spectroscopy [20]. 

Dynamic mechanical analysis in polymeric multiphase systems in solid state, as part of rheology, is associated with oscillatory tests that are employed to investigate all kinds of viscoelastic material from the point of view of flow and deformation behavior. In particular, it evaluates the molecular mobility in polymers, the pattern of which may be an indication of phase-separated systems. Although there are certain preferred tools for visual examination of phenomena for these kinds of systems, dynamic mechanical analysis has the advantage of examination in dynamic conditions and of the prediction of properties. 

Polymers are viscoelastic materials, whose mechanical behavior exhibits characteristics of both solids and liquids. Thermal analysts are frequently called on to measure the mechanical properties of polymers for a number of purposes. Of the different methods for viscoelastic property characterization, dynamic mechanical techniques are the most popular, since they are readily adapted for studies of both polymeric solids and liquids. They are often referred to collectively as dynamic mechanical analysis (DMA). Thermal analysts often refer to the DMA measurements on liquids as rheology measurements. 

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

Relaxation in Nanocomposites. At concentrations above the percolation threshold polymer/nanoparticle interactions dominate the viscoelastic terminal behaviour of polymer nanocomposites. As has been reported for phenoxy based nanocomposites [8], the analysis of tan 5 relaxation at low frequencies constitutes a reliable rheological method to investigate the strength of phenoxy/nanoclay interactions. Moreover, since coordinates ((o)Max (tan 5)Max)) reflect the blocking effect of nanoparticles on polymer chains, the dependence of (o)Max with nanoparticles volume fraction can be used in the percolation equation X=Xq (volume fraction threshold [Pg.69]

The term viscoelasticity combines viscous and elastic stress-strain flow characteristics. If materials behavior is dominated by viscous flow it is generally referred to as a fluid, whereas if the elastic properties dominate the mechanical properties of a material it is considered to be solid. Most adhesives are applied in a liquid or pasty condition to allow wetting and promote spreading and then are required to phase change into a solid. In the liquid state, rheology provides the methods to differentiate between elastic and viscous flow characteristics while, for example, dynamic mechanical analysis of cured adhesive polymers uses similar principles to access elastic and viscous parameters of the stress-strain response. 

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