Problem 84 - 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 key molecular structural features causing high-density polyethylene (HDPE) to be denser than low-density polyethylene (LDPE) are the differences in their polymer chain structures. HDPE has a linear molecular structure, resulting in closely packed and well-aligned polymer chains with strong intermolecular forces, leading to a higher density. In contrast, LDPE has a branched molecular structure, causing irregular packing and misaligned polymer chains with weaker intermolecular forces, resulting in a lower density.

Step 1: Understanding HDPE and LDPE Polymers

High-density polyethylene (HDPE) and low-density polyethylene (LDPE) are both thermoplastic polymers made from the monomer ethylene (C2H4). They have different densities due to the variations in their molecular structures, which is a result of the polymerization process. HDPE has a linear molecular structure, while LDPE has a branched molecular structure.

Step 2: Linear Molecular Structure of HDPE

HDPE is produced using a low-pressure process and contains little to no branching in its molecular structure. This means that the polymer chains are closely packed and aligned, leading to stronger intermolecular forces and crystallinity. As a result, HDPE has a higher density compared to LDPE. The density of HDPE typically ranges between 0.941 to 0.965 g/cm³.

Step 3: Branched Molecular Structure of LDPE

LDPE, on the other hand, is produced using a high-pressure process, which results in a highly branched molecular structure. The branching in LDPE leads to irregular packing and alignment of the polymer chains, causing weaker intermolecular forces and lower crystallinity. Consequently, LDPE has a lower density compared to HDPE. The density of LDPE typically ranges between 0.910 to 0.940 g/cm³.

Step 4: Summary

The molecular structural features causing high-density polyethylene (HDPE) to be denser than low-density polyethylene (LDPE) lies in the differences in their polymer chain structures: 1. HDPE has a linear molecular structure, leading to closely packed and well-aligned polymer chains with strong intermolecular forces. This results in a higher density. 2. LDPE has a branched molecular structure, leading to irregular packing and misaligned polymer chains with weaker intermolecular forces. This results in a lower density.