Why Polyethylene’s IR Spectrum Splits: HDPE, LDPE, and LLDPE Explained

This video is inspired by and builds directly on the excellent article “The Infrared Spectra of Polymers II: Polyethylene” by Brian C. Smith, published in Spectroscopy. The foundation for this video and demonstrates how something as deceptively simple as polyethylene actually comes in a family of structural varieties—each with its own spectral fingerprint.

Polyethylene might seem like one of the simplest polymers around, but a closer look through the lens of infrared (IR) spectroscopy reveals a surprisingly rich structural story. This explainer video takes viewers step by step through the mystery of the split peak in the CH₂ rocking vibration of high-density polyethylene (HDPE)—a feature that should, in theory, appear as a single band in the IR spectrum but instead shows up as two. The investigation begins with this puzzling observation and unfolds into a fascinating exploration of molecular order, polymer architecture, and the powerful insights spectroscopy provides.

At the center of the explanation is the concept of crystallinity. HDPE chains, with their long, unbranched backbones, are able to line up neatly in parallel, forming crystalline regions. Within these regions, neighboring CH₂ groups can vibrate either in sync (in-phase) or out of sync (out-of-phase). These two modes lead to slightly different force constants, and thus two distinct absorptions in the IR spectrum—a phenomenon known as crystalline splitting. By contrast, low-density polyethylene (LDPE), with its bulky long side chains, cannot achieve the same orderly packing, so it produces just a single CH₂ rocking peak.

But the story doesn’t end there. Linear low-density polyethylene (LLDPE) adds nuance: although it has side chains like LDPE, they are short enough (such as ethyl groups) that they don’t block crystallization. As a result, LLDPE shows crystalline splitting similar to HDPE. Even more impressively, the precise position of the CH₂ rocking band shifts depending on side chain length—ethyl groups resonate near 780 cm⁻¹, propyl groups near 740 cm⁻¹, and longer groups closer to the classic 720 cm⁻¹ region. This makes IR spectroscopy not just a tool for distinguishing polymer types, but also for probing fine structural details of side-chain length and arrangement.

This video was created using NotebookLM and Gamma.