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ViscoelasticityCHEE 39023.1 Introduction to Viscoelasticity Polymers display VISCOELASTIC properties All viscous liquids deform continuously under the.

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Presentation on theme: "ViscoelasticityCHEE 39023.1 Introduction to Viscoelasticity Polymers display VISCOELASTIC properties All viscous liquids deform continuously under the."— Presentation transcript:

1 ViscoelasticityCHEE Introduction to Viscoelasticity Polymers display VISCOELASTIC properties All viscous liquids deform continuously under the influence of an applied stress – They exhibit viscous behavior. Solids deform under an applied stress, but soon reach a position of equilibrium, in which further deformation ceases. If the stress is removed they recover their original shape – They exhibit elastic behavior. Viscoelastic fluids can exhibit both viscosity and elasticity, depending on the conditions. Viscous fluid Viscoelastic fluid Elastic solid

2 ViscoelasticityCHEE Introduction to Viscoelasticity The response of polymeric liquids, such as melts and solutions, to an imposed stress may resemble the behavior of a solid or a liquid, depending on the situation.

3 ViscoelasticityCHEE Linear Viscoelasticity Viscoelasticity is observed in polymer melts and solutions irrespective of the magnitude and rate of the applied deformation. –LINEAR viscoelastic behaviour is restricted to mild deformations such that polymer chains are disturbed from their equilibrium configuration and entanglements to a negligible extent. –Under these conditions, stress is LINEARLY proportional to strain, or  (t) = G(t)  o where  is the stress, G is the modulus and  is the strain. –Since polymer processing operations very often involve severe deformations, studies of linear viscoelasticity have little practical utility. –The value of data acquired under linear viscoelastic conditions lies in its amenability to fundamental analysis, which leads to useful inferences on polymer structure (molecular weight distribution, branching) and valuable insight into the characteristics of polymer flow (relaxation phenomena, elastic recovery).

4 ViscoelasticityCHEE Dynamic (Oscillatory) Rheometry Linear viscoelasticity in polymer melts is examined by dynamic measurements –Examine the dynamic elasticity as a function of temperature and/or frequency. –Impose a small, sinusoidal shear or tensile strain (linear  region) and measure the resulting stress (or vice versa) Stress Strain

5 ViscoelasticityCHEE Dynamic (Oscillatory) Rheometry A. The ideal elastic solid A rigid solid incapable of viscous dissipation of energy follows Hooke’s Law, wherein stress and strain are proportional (  =E . Therefore, the imposed strain function:   sin  t) generates the stress response  G   sin  t)   sin  t) and the phase angle, , equals zero. B. The ideal viscous liquid A viscous liquid is incapable of storing inputted energy, the result being that the stress is 90 degrees out of phase with the strain. An input of:   sin  t) generates the stress response   sin  t  and the phase angle, , .

6 ViscoelasticityCHEE Dynamic (Oscillatory) Rheometry Being viscoelastic materials, the dynamic behaviour of polymers is intermediate between purely elastic and viscous materials. –We can resolve the response of our material into a component that is in- phase with the applied strain, and a component which is 90° out-of-phase with the applied strain, as shown below:

7 ViscoelasticityCHEE Dynamic (Oscillatory) Rheometry The dynamic analysis of viscoelastic polymers the static Young’s modulus is replaced by the complex dynamic modulus: G* = G’ + i G” –The storage (in-phase) modulus, G’, reflects the elastic component of the polymer’s response to the applied strain. Reflects the portion of the material’s stress-strain response that is elastic (stored). –The loss (out-of-phase) modulus, G”, reflects the viscous component of the response. Reflects the proportion of the material’s stress-strain response that is viscous (dissipated as heat). The ratio of the two quantities is the loss tangent, tan  = G”/G’, which is a function of temperature, frequency and polymer structure.

8 ViscoelasticityCHEE Logarithmic plots of G’ and G” against angular frequency for uncrosslinked poly(n-octyl methacrylate) at 100°C (above Tg), molecular weight 3.6x10 6. Dynamic (Oscillatory) Rheometry Zones relevant to polymer melt processing

9 ViscoelasticityCHEE Dynamic (Oscillatory) Rheometry: HDPE

10 ViscoelasticityCHEE Simple models of Viscoelasticity Liquid-like behavior can be described by a Newtonian model, which can be represented by using a “dashpot” mechanical analog: The simplest elastic solid model is the Hookean model, which can be represented by a “spring” mechanical analog.

11 ViscoelasticityCHEE Maxwell Model A simple model of a viscoelastic fluid requires at least two components, one to describe the elastic component and the other viscous behavior. One such model is the Maxwell model: which responds with a stress, , when deformed by a strain,  :

12 ViscoelasticityCHEE Maxwell Model The deformation rate of the Maxwell model is equal to the sum of the individual deformation rates:  G (s) is called the relaxation time If the mechanical model is suddenly extended to a position and held there (  =const.,  =0):. Exponential decay in stress – Stress Relaxation

13 ViscoelasticityCHEE Viscoelasticity and Stress Relaxation Stress relaxation can be measured by shearing the polymer melt in a viscometer (for example cone-and-plate or parallel plate). If the rotation is suddenly stopped, ie.  =0, the measured stress will not fall to zero instantaneously, but will decay in an exponential manner.. Relaxation is slower for Polymer B than for Polymer A, as a result of greater elasticity. These differences may arise from polymer microstructure (molecular weight, branching).

14 ViscoelasticityCHEE n Viscoelasticity and Stress Relaxation The Maxwell model is conceptually reasonable, but it does not fit real data very well. Instead, we can use the generalized Maxwell model

15 ViscoelasticityCHEE n Viscoelasticity and Stress Relaxation The relaxation of every element is: The relaxation of the generalized model is: where G i is a weighting constant or “relaxation strength” and i the “relaxation time”

16 ViscoelasticityCHEE Viscoelasticity and Dynamic Rheology The Generalized Maxwell model can also be used to analyze dynamic oscillatory measurements, by fitting G’ and G” with an appropriate number of elements, each having a unique relaxation strength (G i ) and relaxation time ( i ):

17 ViscoelasticityCHEE Viscoelasticity and Dynamic Rheology i (s) 8.04x x x x10 -1 G i (Pa) 3.00x x x x10 2 This example illustrates the storage and loss modulus of uncrosslinked polybutadiene, plotted as a function of oscillation frequency. An adequate representation of G’ and G” as a function of frequency required four elements, whose G i and i are tabulated.

18 ViscoelasticityCHEE Dynamic and Stress Relaxation Testing Recall stress relaxation data from page and dynamic rheology from G(t) vs t G ’ (  ) vs  A is monodisperse with M >M c and C is polydisperse The information contained in a stress relaxation plot is complementary to that acquired in a dynamic measurement. Stress relaxation measurements are used when very low frequencies are needed to characterize terminal flow behaviour.


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