FLASHCARDS-Rheological properties of materials (polymers)-

Introduction to Rheological Properties

  • Rheology: The study of how materials deform and flow, particularly the flow properties of complex fluids like polymers.

Definition of Rheology

  • Materials under stress: Molecules in fluids lack fixed positions, allowing them to flow when shear stress is applied.

  • Viscosity: Resistance to flow caused by internal friction between adjacent fluid layers.

Types of Fluids

  • Newtonian Fluids: Constant viscosity independent of shear rate.

  • Non-Newtonian Fluids: Viscosity changes with shear rate. Includes three types:

    • Bingham Plastics: Require a yield stress to begin flowing.

    • Pseudo-plastic (Shear-thinning): Viscosity decreases with increasing shear rate.

    • Dilatant (Shear-thickening): Viscosity increases with increasing shear rate.

Key Rheological Concepts

  • Power Law Equation: Models shear stress (τ) and shear rate (γ). Common for expressing non-Newtonian behavior.

    • Pseudoplastic Behavior: Many polymer melts and solutions.

Viscosity Calculations

  • Apparent Viscosity (ηa): Measured under specific conditions; not constant for non-Newtonian fluids.

  • Apparent Shear Stress: Expressed by different definitions depending on the type of fluid (Newtonian vs. Non-Newtonian).

Effects on Viscosity

Temperature Handling

  • Viscosity decreases with increasing temperature.

  • WLF Equation: Describes the temperature dependence of viscosity.

Molecular Parameters

  • Molecular Weight: Higher molecular weights lead to increased apparent viscosity at low shear rates.

  • Molecular Weight Distribution: Polymers with broad distributions exhibit more significant shear thinning than those with narrow distributions.

  • Branching: Influences viscosity; high branching typically results in higher viscosity at lower shear rates compared to linear polymers.

Additives in Polymers

  • Fillers: Influence the overall viscosity.
    - Large fillers increase viscosity while potentially leading to lower die swell.

  • Processing Aids: Can lower viscosity and improve processing behavior, particularly in dynamic flow conditions.

Rheological Behavior of Polymer Melts

  • Shear Thinning: Characterized by decreased viscosity under shear stress. Example: Common in creams and paints.

  • Elasticity: Polymer melts exhibit viscoelastic behavior, showing both viscous and elastic responses.

    • Die Swell: Associated with elastic recovery upon exiting a die. Key in polymer processing (e.g. extrusion).

Experimental Techniques for Rheology

  • Capillary Rheometry: Measures viscosity as a sample flows through a capillary, corrected for flow discrepancies using the Bagley correction for better accuracy.

  • Rotational Rheometers: Used to analyze viscosity at different speeds and shear rates, but often limited in the shear rate range compared to practical processing conditions.

    • Conical and Cylindrical: Common configurations where shear rates are calculated based on the geometry and rotation rates.

  • Extensional Rheometer: Evaluates elongational viscosity behavior of fluids; particularly relevant in foaming processes as it inspects how materials behave under tension.

Conclusion on Processing

  • Understanding rheological properties is vital for optimizing polymer processes like extrusion, injection molding, and blow molding, owing to their complex flow behaviors and the need to manage viscosity accurately under varying conditions.