Polymer Analysis and Nanoscale IR: a New Chapter in a Decade-Long Story

Abstract

The story of infrared (IR) spectroscopy is in a way also the story of polymer science, with first experiments being reported in the 1930s. The advent of FTIR in the 1960s brought about a tremendous improvement in the signal- to-noise ratio and measurement times which firmly established IR spectroscopy as one of the standard polymer characterization techniques. In the last century, synthetic polymers have become ubiquitous in day-to-day life due to their advantages over traditional materials, such as lower price, lighter weight, versatility, and durability. This success led to large amounts of polymer waste being released into nature every year where they persist for decades or even centuries. Polymer recycling can be used to both reduce the production of new polymers and the percentage of already existing polymers that become waste. In contrast to recycling of inorganic materials polymer recycling poses some additional challenges: there is a limit to the amount of recycling cycles a polymer can go through before it becomes waste, and the presence of contaminants can lead to undesirable blends. While some of the crucial parameters in recycling (e.g. composition, purity) are routinely monitored with FTIR, the key to further improvements to the recycling process lies at the nanoscale. This scale is inaccessible to conventional IR techniques that are diffraction limited to the micrometre range. This is where new, scanning probe-based nanoscale IR techniques like AFM-IR make a difference. AFM-IR, also known as PTIR, combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to overcome the diffraction limit and provide IR spectra at spatial resolutions in the 10 nm range. Since the signals obtained through PTIR and FTIR are both proportional to the wavelength-dependent absorption coefficient of the sample, the long-established spectra-structure correlations of FTIR can also be applied in the interpretation of PTIR spectra. Hence, PTIR is able to rely on almost a century worth of experience in polymer IR spectroscopy to address the challenges of polymer recycling. However, like many nanoscale analysis techniques, PTIR suffers a trade-off between representative sampling and high resolution. Care needs to be taken to not only record pretty images, but also provide relevant data. In this work, we introduce a protocol to specifically address the issue of going from unknown polymer samples to nanoscale chemical distribution maps and demonstrate this approach on industrially relevant polymers (blends and recycled blends).