Polymer Rheology(Properties study of polymer)

Basic Behavior Polymer
Polymer Engineering

1. Fundamentals of Polymer Engineering Polymer Rheology 4/19/2019 1 Course Code: PE-2102 Credits : 3-0-3 Teacher: Engr. Asra Tariq Email Id: [email protected]

2. What is Rheology Rheology is the study of the flow of matter, primarily in a liquid state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. It is a branch of physics which deals with the deformation and flow of materials, both solids and liquids. The rheology of the polymer melt is sensitive to small changes in the polymer structure. Rheology is considered as the most convenient method to characterize polymers. Even small amounts of a high molecular weight polymer can affect the processing behaviour and so does the melt rheology. 4/19/2019 2

3. What is Rheology Clearly industrial flows are complex, not only because the geometries are complex, but also because the constituents are not usually simple. We have several components in a shampoo, performing various actions. Molecular weight distribution of a polymer is another level of complexity, as polymers are rarely synthesised in a sharp monodisperse population. 4/19/2019 3

4. Viscosity 4/19/2019 4 The  viscosity  of a fluid is a measure of its resistance to deformation at a given rate. One is often interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. These are called viscous stresses. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation (the strain rate). 

5. Absolute Viscosity 4/19/2019 5 The tangential force per unit area of two parallel planes at unit distance apart when the space between them is filled with a fluid and one plane moves with unit velocity in its own plane relative to the other. Also known as coefficient of viscosity. Considering two parallel plane of area A [m 2 ] at distance y [m], one fixed and one moving under a Force F [N], the moving plane will move at velocity u [m/s]. In an ideal laminar flow, at low speed, the velocity variation would be linear and the shear stress τ will be proportional to distance.

6. Absolute Viscosity 4/19/2019 6 Considering that the conditions described above are difficult to obtain a more realistic condition would be that the velocity would not increase linearly. The measuring unit of dynamic viscosity in SI is the pascal per second [Pa s], that is equal to a poiseuille [PI], often in lubricant industry the submultiple millipascal per second [mPa s] is used or in alternative the centipoise [ cP ] (CGS system) that is equal to 1 mPa s. 1 Pa s = 1 PI 1 cP = 1 mPa.s In order to understand better the measuring unit used in viscosity: The shear stress is defined as

7. Viscoelasticity Classical theories of elasticity and viscosity of a body assume steady state stress, strain and strain rate. Viscoelasticity is, the study of the response of polymers (or other materials) which exhibit some of the features of both elastic and viscous behaviour. Elastic materials deform instantaneously when a load is applied, and “remember” their original configuration, returning there instantaneously when the load is removed. In solids, the relaxation of the structure at the molecular level is extremely low and, therefore, their response is essentially elastic. On the other hand, viscous materials do not show such characteristics, but instead exhibit a time-dependent behaviour. While under a constant stress, a viscous body strains at a constant rate, and when this load is removed, the material has “forgotten” its original configuration, remaining in the deformed state. 4/19/2019 7

8. Viscoelastic Behavior Elastic Response Viscous Response Visco -elastic Response 4/19/2019 8

9. Importance of Rheology Studies Rheology (particularly in the study of the viscous flow of polymers) is very important because it gives a relationship between the properties, structure and processing of the materials. Rheology helps describe the mechanical behaviour of materials as a function of stresses (shear rate), strain , temperature and pressure in order to develop materials with the correct processing behaviour based on their viscosity, elasticity and time. These parameters can be calculated using a dynamic mechanical testing method at melt temperatures. 4/19/2019 9

10. Types of Fluids - Ideal fluid The viscosity is zero, this is used for the analysis of potential flow problems such as found in aerodynamics - Newtonian fluid A good viscosity model for gases and simple liquids such as water - Non Newtonian fluid The viscosity of the fluid changes with shear rate. For some fluids, it increases with shear rate (dilatant fluids) while for others it decreases with shear rate (pseudoplastic behaviour). This pseudoplastic (or shear thinning) behaviour is what we observe for polymers. Factors affecting viscosity - pressure - molecular weight - temperature - shear rate - Time 4/19/2019 10

11. Rheology 4/19/2019 11 Shearing Stress

12. Measurement of Rheological properties Rheometer 4/19/2019 12 The use of rheometers or viscometers to determine the rheological properties of materials with the measured values based on force, length and time. Rheometers are devices use to determine both the viscous and viscoelastic properties of materials depending on the force exerted in both the rotational and oscillatory test. While on the other hand viscometers are devices use to determine the viscosity depending on the rotation, temperature and time.

13. Measurement of Rheological properties Rotational Rheometer 4/19/2019 13 In a rotational rheometer the sample is sheared between two plates or a cone and plate geometry. The viscosity is calculated as the ratio of the applied stress and the applied deformation rate (rotation speed). This instrument in the case of polymers is use to extract information about the average molecular weight and molecular weight distribution via measurement of the viscoelastic properties and thus determine how this affects the processing characteristics of the material.

14. Measurement of Rheological properties Melt Flow Indexer 4/19/2019 14 Rheological properties can be measured using either Melt Flow Indexer: Actually this instrument is mostly use for quality control. It does not compute a material property. Its mainly to compare the relative flow behaviour of different materials based on the viscosity.

15. Measurement of Rheological properties Capillary Rheometer 4/19/2019 15 Capillary rheometers are mostly used to examine processing behaviour, rather than just determining the rheological parameters. In a capillary rheometer the material is forced through a slit or round die. From the pressure drop and the volume flow rate, a steady state viscosity as a function of the applied rate (piston speed) or stress (applied pressure) is measured. Advantages: (1) Can operate at high shear rates (2) May be closer to real processing situation than a rotational rheometer Disadvantages: (1) Shear rate is not uniform (2) Wall slip (3) Melt fracture (4) Difficult to clean In addition we also have gravity driven capillary rheometers also called viscometers but mostly function for viscous Newtonian fluids.