Chem 5105: Advanced Polymer Chemistry (MS)

Advanced Polymer Chemistry (Chem 5105):

 4. Rheology & Mechanical Properties:

Rheology is derived from the greek word ‘rio’ which means flow and ‘lagea’ which means the study of, so Rheology is the study of a materials flow behavior under applied deformation forces or stress now.

Rheology plays an essential role in formulating everything from cosmetics to food to inks and coatings. 

It impacts all stages of material use across multiple industries including production pumping, storage, transportation, stability, and use application spreading and even product performance.

The essential elements that affect the materials flows or deformation process are:

(i)             Materials inner structure: how is the material built or what is its molecular makeup,

(ii)           Morphology: when molecules associate and make bond with each other to form particle and their shape and size. Shape could be small needle-like structures or bulky cotton-like structures or rod shapes or flower or ball shape etc.,

(iii)         Outside forces that stress the material which causes it to deform. Their flow will behave differently under different stress,

(iv)           Ambient conditions: Temperature

We can simply divide all materials into liquids and solids, but that's not really a good scientific approach.

In the real world materials are more complex from salad oil glue and shampoo to facial creams jelly and car tires. These materials should not be defined simply by two words (solid & liquid).

We define these materials better well with Rheology. Most materials in the world are visco elastic- nearly every material is made with a viscous portion and an elastic portion.

If a material is more viscous than it's a liquid, if a material is more elastic it's a solid.

Materials with the highest viscous portion are called viscous liquids or Newtonian liquids. At constant ambient conditions Newtonian liquids will always show the same viscosity no matter how they are stressed. Examples include water, oil etc.

Solid oil is viscoelastic liquids having liquids with an elastic portion. These materials exhibit a certain level of stiffness when they are stressed during flow.

Also when viscoelastic solids are deformed by an outside force that is not too large, the inner structure of these materials sticks together and tries to retain the materials original form.

Elastic solids always show the same level of stiffness as long as their structure is not destroyed.

Elasticity is the property of solid materials to return to their original shape and size after the forces deforming them have been removed. 

Strain is the change in some spatial dimension (length, angle, or volume) compared to its original value and stress refers to the cause of the change (a force applied to a surface).

Stiffness is the extent to which an object resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.

INTERPRETATION:

 

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. 

 

Stress/force for loading and unloading is not same (loading>unloading). This small unbalanced force is converted to heat energy during loading/elongation.

 

Elastomer: An Elastomer is a polymer with viscoelasticity (i.e., both viscosity and elasticity) and has very weak intermolecular forces. IUPAC defines the term "elastomer" by "Polymer that displays rubber-like elasticity."

 

The glass transition temperature is the temperature range where the polymer substrate changes from a rigid glassy material to a soft (not melted) material, and is usually measured in terms of the stiffness.

 

cross-link is a bond that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural polymers (such as proteins).

 

 

Amorphous: an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal.

 

 

Resilience of a rubber compound is a measurement of how elastic it is when exposed to various stresses. 

 

Resilience is the ratio of energy released in deformation recovery to the energy that caused the deformation.

 

 

 

Stress-Strain Behavior of Polymers

 

One of the most important characteristics of polymers is their inherent toughness (Toughness is the ability of a material to absorb energy before fracture or failure) and resistance to fracture (crack propagation). 

The mechanical properties of plastic materials depend on both the strain (change of length per unit length) and temperature. 

At low strain, the deformation of most solids is elastic, that is, the deformation is homogeneous and after removal of the deforming load the plastic returns to its original size and shape. 

In this regime, the stress (σ) [force/area] is proportional to the strain (ε):

                                Stress = Constant x Strain

                                    or, σ = E ε  

where, E is the tensile (or Young's) modulus of the plastic which is a measure of the stiffness of the material. 

  

This relationship is known has Hooke's law. It means, when a plastic specimen is pulled at a (constant) strain rate the applied stress (or load) is directly proportional to the observed strain (or elongation).

 

The maximum stress up to which the stress and strain remain proportional is called the proportional limit. 

 

If a plastic material is loaded beyond its elastic limit, it does not return to its original shape and size, i.e. a permanent deformation occurs. 

 

With increasing load a point is eventually reached at which the material starts yielding. This point is known as the yield point. A further increase in strain occurs without an increase in stress.

 

The maximum stress up to which the stress and strain remain proportional is called the proportional limit. 

 

If a plastic material is loaded beyond its elastic limit, it does not return to its original shape and size, i.e. a permanent deformation occurs. 

 

With increasing load a point is eventually reached at which the material starts yielding. This point is known as the yield point. A further increase in strain occurs without an increase in stress.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

https://www.youtube.com/watch?v=zc3b6LdDFtY&t=243s

 

 

 

 

 

Viscoelasticity is a type of deformation which consists of both elastic and viscous deformation.

Elastic deformation is a recoverable deformation which means the sample or the material can return to its original shape, once the applied force is removed.

Whereas viscous deformation is a non-recoverable deformation, which means applied force leads to a permanent deformation of the sample.

The example of elastic deformation is a stretching of rubber bands and example of viscous deformation is flow of fluids like water or honey.

 

Viscoelastic mechanical model:

For simplification, polymer is divided into an elastic component and a viscous component.

The deformation of the polymer can be described by combination of Hook’s and Newton’s law.

The linear elastic behavior is given by Hook’s law as,

            σ = Ee ………………………..………………………….(1)

or,       dσ/dt = E(de/dt) …………………………………………(2) , E is he elastic modulus, e is strain and σ is stress.

Newton’s law describes the linear viscous behavior through the equation:

            σ = η(de/dt) ……………………………………………..(3) 

η is the viscosity and de/dt is the strain rate.

A particularly useful method of formulating the combination of elastic and viscous behavior is through the use of mechanical models.

The two basic components used in the models are an elastic spring of modulus E and which obeys Hook’s law (Eq.1) and a viscous dashpot of viscosity, η, which obeys Newton’s law.

The various models that have been proposed involve different combinations of these two basic elements.

 

Standard linear solid

• Itbeen demonstrated that the Maxwell model describes the stress relaxation of a polymer to a first approximation whereas the Voigt model similarly describes creep.

·         A logical step forward is therefore to find some combination of these two basic models which can account for both phenomena.

A simple model which does this is known as the standard linear solid, one example of which is shown in Fig. 5.7(c). 

·         In this case a Maxwell element and spring are in parallel.

·         The presence of the second spring will stop the tendency of the Maxwell element undergoing simple viscous flow during creep loading, but will still allow the stress relaxation to occur.

·         There have been many attempts at devising more complex models which can give a better representation of the viscoelastic behaviour of polymers.

·         As the number of elements increases the mathematics becomes more models only give a on a much complex.

·         It must be stressed that the mechanical models only give a mathematical representation of the mechanical behaviour and as such do not give much help in interpreting the viscoelastic properties on a given molecular level.

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