Problem 92 - Chemistry The Central Science

Question

What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?

Step-by-Step Solution

Answer

The denser molecular structure of high-density polyethylene (HDPE) compared to low-density polyethylene (LDPE) is due to HDPE's near-linear configuration and minimal branching of polymer chains, which allows them to pack closely together, resulting in stronger intermolecular forces and higher density. In contrast, LDPE's more branched and irregular polyethylene chains interfere with close packing, leading to weaker intermolecular interactions and a less dense material. These molecular structural differences contribute to the distinct properties of HDPE and LDPE, such as strength, rigidity, and chemical resistance.

Step 1: Understand the basic structure of polyethylene

Polyethylene is a polymer made from the monomers of ethylene (C2H4). Each ethylene monomer consists of two carbon atoms connected by a double bond, with each carbon atom also bonded to two hydrogen atoms. During polymerization, the carbon atoms are linked together, forming long chains. Both HDPE and LDPE are made from ethylene but have different molecular structures that affect their densities.

Step 2: Analyze the molecular configuration of high-density polyethylene (HDPE)

HDPE is synthesized at relatively low pressures and temperatures using catalysts that allow for a more controlled polymerization process. This results in a near-linear structure, with the carbon atoms forming a straight chain and the hydrogen atoms bonded at approximately 120° angles. There is very minimal branching of the polymer chains, allowing them to pack closely together. This tight packing results in a high density and strong intermolecular forces, leading to higher strength and rigidity.

Step 3: Analyze the molecular configuration of low-density polyethylene (LDPE)

In contrast, LDPE is synthesized at much higher pressures and temperatures without the use of catalysts. The reaction is less controlled, which results in a significant number of branches and irregularities in the polyethylene chains. These branches interfere with the close packing of the polymer chains.

Step 4: Compare the density of HDPE and LDPE

The presence of branches in LDPE reduces the intermolecular interactions between the chains, leading to a less dense material. The less controlled process of LDPE synthesis also introduces more errors and irregularities, contributing to its lower density. On the other hand, HDPE's linear configuration allows for closer packing of the chains, stronger intermolecular forces, and consequently, a higher density.

Step 5: Relate the molecular structure to the overall properties of HDPE and LDPE

The differences in molecular structure and density between HDPE and LDPE lead to significant differences in their properties. HDPE is known for its strength, high tensile strength, resistance to chemicals and moisture, and rigidity, making it suitable for applications such as piping and containers. On the other hand, LDPE is more flexible, has lower tensile strength, and is less resistant to chemicals and moisture, making it an ideal material for applications such as plastic films and bags.