Properties of a Polymer: Polymers can be defined as any class of natural ( Polymers may be naturally found in plants and animals) or (Polymers that are man-made) synthetic substances built from significantly large molecules, called macromolecules that are manifolds of simpler chemical units known as monomers. The term polymer is derived from Greek, which means ‘many parts’, and the process of formation of polymers by repeated bonding of its monomer units is known as polymerisation.

Polymers can be found all around us. Most of the products we use in our daily lives are polymers, such as synthetic clothes, plastic bags, Teflon, cookwares, rubber products, and more. Embibe provides all the features and the complexities of a polymer in an easy manner. Continue reading the article to learn more about polymers like polypropylene melting point, polyethene structure, and more.

What are Polymers?

Have you ever noticed a necklace made up of several beads? If yes, then you can well understand the meaning of polymers! Just as a necklace, polymers are giant molecules made up of repeating monomer units called monomers. Scientifically, we can define polymers as large organic molecules assembled by small repeating units known as monomers. Examples of polymers are rubber, plastics, and nylon.

Properties of a Polymer

The properties of a polymer are affected by the structure, type of monomer units from which polymers are formed, and other factors. Polymers have different physical and chemical properties, which are listed below:

Physical Properties of Polymer

1. Tensile Strength – The strength of a polymer to elongate without breaking is its tensile strength. Physical strength and durability depend on this property of polymers.

2. Melting Point and Boiling Point – Polymers have a high melting point and boiling point. Greater the intermolecular forces, longer the chains, and hence higher the melting point and boiling point.

3. Hardness – Hard polymers resist the penetration of hard substances into them. They withstand wear and tear, scratches and are used in the manufacturing of constructing devices.

4. Density- Polymers are classified into high-density polymers and low-density polymers based on the density differences.

5. Heat Capacity / Heat Conductivity – This decides the extent to which a polymer acts as an insulator of heat. The stiffness of molecules decides whether a polymer is a good conductor of heat.

6. Thermal Expansion – The extent to which a polymer expands or contracts when subjected to heat or cold is measured by this property.

7. Crystallinity – Polymers with less crystallinity are more useful as they are brittle. This property is based on the type of arrangement of polymeric chains.

8. Elasticity– Polymers with weak intermolecular bonds stretch to a greater extent and are more elastic.

9. Permeability – This is the tendency of particles to pass through the polymers. For example, low density polyethylene is less permeable to air, so it is used to pack food items.

10. Refractive Index– The extent to which the light bends as it passes the polymer is measured as its refractive index. Polymers use this property in spectroscopy.

11. Resistance to Electric current – Most of the polymers are bad conductors of electricity. Nowadays, conductive polymers are used in semiconductor devices. Their conductivity arises due to conjugated carbon-carbon double bonds.

Chemical Properties of a Polymer

1. Bonding and reactivity – The strong covalent bond and other weak forces such as hydrogen bonding between the particles of polymers determine its property like reactivity. Generally, polymers are resistant to chemicals due to their low reactivity.

2. Interaction between the reactive groups – Intermolecular forces among the monomers is decided by their dipole. The carbonyl group (amide group) present at the side chains of the monomers is responsible for the formation of the hydrogen bond.

3. Adhesion of polymers on the surface, its interaction with coating, and the external environment also affects their quality, like paints.

4. Biodegradability– Polymers can degrade by the action of decomposers. Natural polymers like rubber are biodegradable, while synthetic polymers are non-biodegradable.

Mechanical Properties of a Polymer

The ability of a polymer to resist or withstand physical stress is known as its mechanical property. It is one of the critical factors to decide where a particular kind of polymer should be used. Some of these properties are:

1. Strength – The minimum force or stress required to break the sample of a given polymer is the strength of a polymer. Various types of strength of polymers are tensile strength, torsional strength, compressibility, and flexibility. The increasing orders of different polymers’ strength are: linear < branched < cross-linked < network. Factors that affect its strength are:

a. Molecular Weight: The tensile strength of the polymer arises with an increase in molecular weight and reaches a level of saturation after a specific value. The equation that relates the tensile strength with the molecular weight is:

\({\rm{\sigma  = }}{{\rm{\sigma }}_\infty }{\rm{ – }}\frac{{\rm{A}}}{{\rm{M}}}\)

Where \({\rm{A}}\) is some constant and \({\rm{M}}\) is the molecular weight.

b. Cross-linking: The motion of the chains in polymers is restricted by cross-linking and increases the strength of the polymer.

c. Crystallinity: In the crystalline phase, the intermolecular bonding is more strong and significant. Hence, the crystallinity of the polymer increases its strength.

2. Percentage Elongation of a polymer to break is the measure of elongation of a polymer without deforming it.

3. Young’s Modulus– It is the ratio of tensile stress and tensile strain. It determines how easily a polymer can stretch and deform. In short, it is the measure of the stiffness of a polymer. \({\rm{E = }}\frac{{{\rm{Tensile\;strength\;}}\left( {\rm{\alpha }} \right)}}{{{\rm{Tensile\;strain\;}}\left( {\rm{\varepsilon }} \right)}}\)

4. Toughness– The toughness of a polymer is determined by the area under the stress-strain curve. Toughness \({\rm{ = }}\int {{\rm{\sigma d\varepsilon }}} \)

5. Viscoelasticity – Viscoelasticity measures elasticity as well as viscosity. It is caused due to temporary connections between the particles of fibres. Due to this property, polymers return to their original shape when released after stretching.

Molecular Mass of Polymers

Polymer properties are closely related to their molecular mass, size and structure. The growth of the polymer chain during their synthesis is dependent upon the availability of the monomers in the reaction mixture. Thus, the polymer sample contains chains of varying lengths and hence its molecular mass is always expressed as an average.

There are two ways of expressing molecular mass of a polymer:

1. Number Average molecular mass 
2. Weight average molecular mass

Number Average molecular mass

Osmotic pressure method is used to determine Number average molecular mass.
It is obtained by the expression:

\({\overline {\mathbf{M}} _{\text{n}}} = \frac{{\sum {{{\mathbf{N}}_{\text{i}}}} {{\mathbf{M}}_{\text{i}}}}}{{\sum {\left( {{{\mathbf{N}}_{\text{i}}}} \right)} }}\)
Or
\({\overline {\text{M}} _{\text{n}}} = \frac{{{{\text{N}}_1}{{\text{M}}_1} + {{\text{N}}_2}{{\text{M}}_2} + \ldots \ldots \ldots }}{{{{\text{N}}_1} + {{\text{N}}_2}}}\)
Where \({{\text{N}}_{\text{i}}} = \) no. of molecules and \({{\text{M}}_1} = \) molecular mass

Weight average molecular mass

It is determined by light scattering and ultra centrifugation methods. It is obtained by the expression:

\({\overline {\text{M}} _{\text{w}}} = \frac{{\sum {{{\text{N}}_{\text{i}}}} {\text{M}}_1^2}}{{\sum {{{\text{N}}_{\text{i}}}} {{\text{M}}_{\text{i}}}}}\)
Where \({{\text{N}}_{\text{i}}} = \) no. of monomer molecules and \({{\text{M}}_1} = \) molecular mass

Poly Dispersity Index

\({\text{PDI}} = \frac{{{{\overline {\text{M}} }_{\text{w}}}}}{{{{\overline {\text{M}} }_{\text{n}}}}}\)
The ratio of weight average molecular mass and number average molecular mass is called “Poly dispersity index” \(\left( {{\text{PDI}}} \right)\).
For natural polymers \({\text{PDI}} = 1\)
\(\therefore {\overline {\text{M}} _{\text{w}}} = {\overline {\text{M}} _{\text{n}}}\) And polymer is monodisperse.
For Synthetic polymers \({\text{PDI > }}1\)
\(\therefore {\overline {\text{M}} _{\text{w}}} > {\overline {\text{M}} _{\text{n}}}\) and polymer is polydisperse.

Factors Contributing to Polymer Properties

Properties of a polymer depend mainly on three factors:

1. Chemicals from which polymers are formed.
2. Polymerisation conditions such as chain length, type of bonds present between polymers, and nature of the functional group present at the end of the monomers.
3. The type of monomer units polymerised to form the repeating units in polymers is one of the critical factors that decide the properties of polymers.

Factors that Affect Properties of a Polymer

Properties of Polymer depend on several factors. Some of them are discussed below:

1. Temperature – Polymers are sensitive to temperature: the flexibility and compressive strength decrease as the temperature increases. The kinetic energy of the molecules increases with increasing temperature, and Young’s modulus decreases.
2. Chain Length – It can be inferred that as the chain length of polymers increases, their strength also increases.
3. Branching – As branching increases, the mechanical strength of polymers also increases.  For example, the degree of crystallinity is high density polyethylene and has relatively poor mechanical properties. Thus branching makes polymers stiffer, harder and stronger.
4. Cross-linking: When polymer chains are cross-linked extensively by strong covalent bonds, their strength increases, making it difficult to melt them.
5. Nature of Side Groups – The presence of polar side-groups increases the strength of attraction between the polymeric chains, making them stronger due to hydrogen bonding and other attractive forces.

Summary

In a nutshell, we can say that polymers are macromolecules formed by the covalent bonding of monomer units. Polymers are used in almost every sphere of life, such as clothing, manufacturing plastic items, industrial uses, medicines, dentistry, and many more. Based on the physical and chemical properties, polymers are used in various fields, such as strong ropes are made to carry weights in factories, construction sites. Properties of polymers such as high tensile strength, strain, elasticity, toughness determine their uses.

More complex polymers form strong bonds, and hence, they are strong enough to withstand high force and pressure. Electrically conductive polymers are used in making electrical devices. Their conductivity arises due to conjugated carbon-carbon double bonds. Plastics are the most common polymers used in our daily life. However, we should avoid using them because they are mostly synthetic and non-biodegradable, posing a threat to our environment. Many research works are going on to develop organic polymers such as organic plastics that are biodegradable.

FAQs on Properties of Polymers

Q.1: What are polymers and their properties?
Ans: The polymer can be defined as any class of natural or synthetic substances built from very large molecules, called macromolecules that are manifolds of simpler chemical units known as monomers. Polymers have different properties based on their structures. For example, polymers with relatively high molecular weight and long-chain lengths result in entanglement, and insufficiency of covalent intermolecular bonds facilitates the mobility and flexibility of polymer chains.

Q.2: What are the chemical properties of polymers?
Ans: The various chemical properties of polymers include the bonding and reactivity of polymers, the formation of the strong covalent bond, and other weak forces such as hydrogen bonding between the particles of polymers that determine its reactivity. Generally, polymers are resistant to chemicals due to their low reactivity. The presence of functional groups at the side chain, such as the carbonyl group, amide group of the monomers, is responsible for forming the hydrogen bond. Adhesion of polymers on the surface, its interaction with coating, and the external environment also affects their quality, like paints. The ability of polymers to degrade by the action of decomposers is its biodegradability. Natural polymers like rubber are biodegradable, while synthetic polymers are non-biodegradable.

Q.3: What are the physical properties of polymers?
Ans: The various physical properties of polymers are their tensile strength, melting point, boiling point, hardness, heat conductivity, electrical conductivity, refractive index, elasticity, crystallinity, permeability, etc.

Q.4: What are the three properties of polymers?
Ans: The three main properties of polymers are mechanical strength, elasticity, and thermal expansion.

Q.5: What are the uses of polymer?
Ans: Polymers are used in almost every sphere of life, such as clothing, manufacturing plastic items, industrial uses, medicines, dentistry, manufacturing electrical conducting devices, cooking utensils and their handles, and many more.

Q.6: What are the properties of natural polymers?
Ans: Natural polymers are non-toxic, biodegradable, biocompatible, and stable. Examples of natural polymers include rubber, carbohydrates (polysaccharides), and proteins (polyamides).