Shear modification

Planar extension is the most difficult to generate, but two techniques for doing so have been described [ 160,162 ]. No techniques for biaxial or planar extensional rheometry that are suitable for routine use in industry are known at present. 

The practical application of shear modification in a plastics forming operation, for example to improve the surface of blown film, is not practical because of the substantial energy required to perform the shear modification. 

Constitutive equations can now be derived from the tube-based theories. Since the parameters in these equations are related to basic molecular parameters, these models should be truly predictive, but their ability to make quantitative predictions is limited because of the simplifying assumptions necessary to derive them. These equations and their strengths and weaknesses are presented in detail in Chapter 11. 

Turning to the behavior of typical melts, it is found that the damping function is not nearly as sensitive to molecular structure as are the linear viscoelastic properties, e.g. the storage and loss moduli. The rubberlike liquid, as well as the tube model, predict that the ratio of the first normal stress difference to the shear stress in step shear should be equal to the strain at all strains, and this is in fact observed. The other quantity measured in simple shear experiments is the second normal stress difference, but this is difficult to measure and few data are available. Of the shear histories other than step strain than have been used to study nonlinear viscoelasticity, start-up of steady simple shear has been the most used. If the shear rate is sufficiently large, some degree of chain stretch can be generated in the early stages. 

Extensional flows yield information about rheological behavior that cannot be inferred from shear flow data. The test most widely used is start-up of steady, uniaxial extension. It is common practice to compare the transient tensile stress with the response predicted by the Boltzmann superposition principle using the linear relaxation spectrum a nonlinear response should approach this curve at short times or low strain rates. A transient response that rises significantly above this curve is said to reflect strain-hardening behavior, while a material whose stress falls 

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For start-up of shear flow at constant rate, the transient viscosity grows in a power law with time. This might be utilized for detecting GP. The total strain must be kept small because, near GP, stress relaxation is infinitely slow and shear modification cannot be avoided even at extremely low rates of deformation. 

LCB polymers normally show a distinct dependence on their flow histories, a behaviour that is used for shear modification of LDPE. 

Leblans, R J. R., Bastiaansen, C. Shear modification of low-density polyethylene Its origin and its effect on the basic rheological functions of the melt. MacromoL (1989) 22, pp. 3312-3317... 

Bourrigaud, S., Marin, G., Poitou, A. Shear modification of long-chain branched polymers A theoretical approach using the pom-pom model. MacromoL (2003) 36, pp. 1388-1394... 

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