Rheology and viscoelasticity

Rheology is the science that deals with the deformation and flow of matter under various conditions. The rheology of plastics, particularly of TPs, is complex but understandable and manageable. These materials exhibit properties that combine those of an ideal viscous liquid (with pure shear deformations) with those of an ideal elastic solid (with pure elastic deformation). Thus, plastics are said to be viscoelastic. 

The mechanical behavior of plastics is dominated by such viscoelastic phenomena as tensile strength, elongation at breaks, stiffness, and rupture energy, which are often the controlling factors in a design. The viscous attributes of plastic melt flow are also important considerations in the fabrication of plastic products. (Chapter 8, INFLUENCE ON PERFORMANCE, Viscoelasticity). 

The viscoelasticity is a combination of viscous and elastic properties in a plastic with 

The viscoelastic nature of the material requires not merely the use of data sheet information for calculation purposes, but also the actual long-term performance experience gained that can be used as a guide. The allowable working stress is important for determining dimensions of the stressed area and 

For those not familiar with this type information recognize that the viscoelastic behavior of plastics shows that their deformations are dependent on such factors as the time under load and temperature conditions. Therefore, when structural (load bearing) plastic products are to be designed, it must be remembered that the standard equations that have been historically available for designing steel springs, beams, plates, cylinders, etc. have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

They are a phenomenon of time-dependent in addition to elastic and deformation (or recovery) in response to load. This property possessed by all plastics (primarily thermoplastics) to some degree, highlights that 

With plastics there are two types of deformation or flow viscous, in which the energy causing the deformation is dissipated, and elastic, in which that energy is stored. The combination produces viscoelastic 

There is a different flow behavior of plastic when compared to water. The volume of a so-called Newtonian fluid, such as water, when pushed through an opening is directly proportional to the pressure applied following a straight line (flow vs. pressure). The flow rate of a non-Newtonian fluid such as plastics when pushed through an opening increases more rapidly than the applied pressure resulting in a curved line. Different plastics have their own flow rates so that their non-Newtonian curves are different. 

This property of viscoelasticity is possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics, they also have liquid-like characteristics. This mechanical behavior is important to understand. It is the mechanical behavior in which the relationships between stress and strain are time dependent for plastic, as opposed to the classical elastic behavior of steel in which deformation and recovery both occur instantaneously on application and removal of stress. 

As discussed iu Section 2.16, dynamic mechanical analysis offers an enhanced means of evaluating the performance of polymeric systems at elevated temperatures. It provides a complete profile of modulus versus temperatures, as well as measurement of mechanical damping. Operating in the creep mode and coupled with the careful use of time-temperature superpositioning, projections can be made regarding the long-term time-dependent behavior under constant load. This provides a much more realistic evaluation of the short- and long-term capabilities of a resin system. 

Rheology is concerned with the flow and deformation of matter. Viscoelastic properties are more concerned with the flow and elasticity of matter. 

Numerous references to rheology and viscoelasticity occur. Thus, 250 references have been identified in the 5-year period between 2004 and 2008. 

In addition to various moduli (storage, loss, loss shear, plateau), the following rheological properties of polymers can be determined by a range of techniques  

Stress relaxation behavior Order-disorder transitions Shear rate dynamic viscosity Molecular entanglement Creep strain 

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Since 1953, a great deal of work has been directed to the estimation of LCB in LDPE the more important studies are listed in Table 10.1 in roughly chronological order. There have also been many studies of the effects of LCB on the properties of LDPE, including melt rheology and viscoelasticity these are discussed in Section 10.5. 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

While movement of cells in response to chemical factors has been a popular research topic, only a few studies have evaluated the effects on cell motility of mechanical stimuli. In one important study [27], neutrophil morphology, rheology, and viscoelasticity were monitored when cells were seeded in microchannels of smaller dimensions than cell diameter (Fig. 5a). An important observation was made in that mechanical... 

Measurement of Rheological and Viscoelastic Properties of Unreinforced Polymers... 

Khoshravan and Bathias [13] used dynamic mechanical analysis to measure rheological and viscoelastic properties of glass fiber-reinforced polyether ether ketone. 

Thus the conformational statistics of chain molecules are now increasingly able to provide a basis for estimating the rheological and viscoelastic behavior of linear amorphous polymers above their glass transition temperatures. 

Royer, J. R., DeSimone, J. M. Khan, S. A. (2001). High-pressure rheology and viscoelastic scaling predictions of polymer melts containing liquid and supercritical carbon dioxide. Journal ( Polymer Science Part B-Polymer Physics 39(23) 3055-3066. 

Rheology and Viscoelastic Properties of Polymer Composite Materials... 

DPD simulation has been applied to predict the rheological and viscoelastic behaviors of nanopartide-polymer nanocomposites and to examine the effects of particle shape, particle-particle interaction, and partide dispersion states of such behaviors. It was found that partide-particle interaction has a distinct effect on the dynamic shear modulus. Havet and Isayev [39,40] proposed a rheological model to predict the dependence of dynamic properties of highly interactive filler-polymer mixtures on strain and the dependence of shear stress on shear rate. 

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