Question

Which of the following properties of polyethylene increases as the number of monomer units per molecule of the polymer increases? (a) melting point; (b) viscosity; (c) density; (d) C:H ratio; (e) fuel value

Short answer

Answer: The melting point and viscosity of polyethylene increase as the number of monomer units per molecule increases.

Step-by-step solution

Examine Property (a): Melting Point

The melting point of a polymer depends on the intermolecular forces of attraction. As the molecular weight increases, these forces become stronger, making it more difficult to separate the molecules. Therefore, the melting point of polyethylene increases as the number of monomers per molecule increases.

Examine Property (b): Viscosity

Viscosity is a measure of resistance to flow. In polyethylene, increased molecular weight often leads to an increase in viscosity because more massive molecules are more entangled and difficult to move around. Thus, as the number of monomer units increases in polyethylene, the viscosity of the polymer also increases.

Examine Property (c): Density

The density of polyethylene is mainly determined by the packing of molecular chains in its structure. When the number of monomers per molecule increases, it doesn't significantly influence the density of the polymer. The density remains relatively constant as the molecular weight increases.

Examine Property (d): C:H Ratio

The chemical formula for the monomer units in polyethylene is (C2H4)_n, where n is the number of monomers per molecule. The C:H ratio is constant in all monomers (1:2), regardless of the molecular weight. So, the C:H ratio does not change as the number of monomers per molecule increases.

Examine Property (e): Fuel Value

The heat released during the combustion of polyethylene depends on the amount of carbon and hydrogen present. Since the C:H ratio remains constant as the molecular weight increases, the fuel value of polyethylene does not change as the number of monomers per molecule increases.

Conclusion

Based on our analysis of the properties of polyethylene, we can conclude that both the melting point (a) and viscosity (b) increase as the number of monomer units per molecule of the polymer increases. The density (c), C:H ratio (d), and fuel value (e) do not change with an increase in molecular weight.

Key concepts

Melting Point of Polyethylene

Polyethylene is a versatile polymer, and understanding its melting point is key to utilizing its properties. The melting point is the temperature at which a solid becomes a liquid. This property is crucial for processes like molding or extrusion.

In polyethylene, the melting point increases with the number of monomer units. This is because larger molecules with more monomers have stronger intermolecular forces. These forces arise due to the larger surface area that allows molecules to attract each other more effectively. As a result, more energy is required to overcome these forces, leading to a higher melting point.

• More monomers = stronger intermolecular forces.
• Higher energy needed to break bonds = higher melting point.

Thus, as the molecular weight of polyethylene increases, so does its melting point. This relationship is important for industries that need precise temperature management, like plastics manufacturing.

Viscosity of Polyethylene

Viscosity is an intrinsic property of substances that describes the resistance to flow. In simple terms, it's how 'thick' a liquid or polymer is. High viscosity means the fluid is thick and flows slowly, like honey, while low viscosity means it's thin, like water.

For polyethylene, viscosity increases as more monomer units are added. This occurs because larger polyethylene molecules get more tangled up with each other, making it harder for them to slide past one another. This entanglement results in additional resistance to flow, thereby increasing viscosity.

• More monomers = more entanglement.
• More entanglement = higher viscosity.

This increased viscosity is significant when processing polyethylene, as it affects how the polymer can be shaped and formed in industrial processes.

Molecular Weight and Polyethylene

Molecular weight refers to the total mass of a molecule, calculated by adding up the atomic masses of all atoms in the molecule. For polyethylene, molecular weight is dictated by how many monomer units of ethylene (C₂H₄) are incorporated into the polymer chain.

An increase in molecular weight means more monomer units, which in turn affects various properties of the polymer, such as melting point and viscosity, as discussed earlier. However, not all properties change with molecular weight — for instance, polyethylene's density and C:H ratio remain largely unaffected, as seen from the initial exercise. But, the molecular weight significantly impacts the mechanical properties, such as strength and toughness, making it a crucial factor in material selection.

• Molecular weight = number of monomer units.
• Affects properties like melting point and viscosity.
• Unaffects density and C:H ratio.

Thus, understanding molecular weight helps determine how polyethylene will perform in different applications, guiding engineers and scientists in its implementation across various industries.