By Felipe Cordero, Alberto Ortín. (Polymer Char)

Carbonyl Degradation Tracking Along the Molecular Weight Distribution: Enhanced Detection with GPC-IR

Introduction

Polyolefin degradation has become a critical issue in recycled plastics, long-term durability studies, sterilized medical devices, cable insulation, geomembranes and advanced formulations. Oxidative degradation generates oxygen-containing functionalities, particularly carbonyl groups, which are among the most widely accepted markers of polymer ageing and degradation. However, bulk FTIR only provides average information and cannot determine where these degradation products are located across the molecular weight distribution. This limitation becomes increasingly important because degradation may proceed through different competing mechanisms.

One of the most common degradation pathways is chain scission. Thermal, photo-oxidative and radiation-induced degradation generate radicals that eventually form carbonyl-containing species such as ketones, aldehydes, carboxylic acids and anhydrides.

As oxidation progresses, polymer chains break into shorter segments, leading to:

• lower molecular weight
• broader MWD
• increased carbonyl concentration

This relationship between carbonyl formation and molecular weight reduction has been widely reported in polyethylene and polypropylene degradation studies (Gugumus et al, Rabello et al., Celina et al).

In other systems, degradation can induce crosslinking reactions. Instead of producing shorter chains, radical recombination forms highly interconnected structures.

These materials progressively lose solubility and may become partially or completely insoluble in conventional HT-GPC solvents such as TCB or oDCB.

Characterization Tools for Degraded Materials

Consequently, molecular characterization becomes increasingly difficult precisely when degradation information is most needed.

Several approaches have been proposed:

• FTIR carbonyl index
• DSC oxidation induction time
• Mechanical testing
• Bulk spectroscopy

These techniques are extremely valuable and widely used (Rapp., Celina, Rabello at al). However, they only provide average information and cannot establish how degradation products are distributed throughout the molecular weight distribution.

High-temperature GPC coupled with infrared detection appeared as a promising solution because it combines molecular weight information with chemical composition. The introduction of IR6 enabled carbonyl detection around 1740 cm⁻¹ and opened new possibilities for EVA and other carbonyl-containing polymers.

Nevertheless, many degraded polyolefins generate carbonyl species whose strongest absorptions occur at higher wavenumbers, typically between 1780 cm⁻¹ and 1790 cm⁻¹. Moreover, solvent transparency limitations further restrict the applicability of conventional configurations.

New carbonyl detection band for degradation studies in polyolefins

To address this challenge, Polymer Char has developed a new IR6 filter centered at 1780 cm⁻¹.

The new filter enables:

• Improved detection of degradation-related carbonyl species
• Analysis in TCB mobile phase
• Enhanced signal-to-noise ratio
• Qualitative tracking of carbonyl distribution across the molecular weight distribution

This capability provides a direct link between degradation chemistry and molecular architecture, creating a new analytical tool for degradation studies.

The objective of this work is to demonstrate the analytical capabilities of the new 1780 cm⁻¹ IR6 filter through several representative applications involving degraded polyolefins, maleic-anhydride-modified polyolefins and EVA copolymers.

Application #1 – Carbonyl Tracking During Chain Scission in Degraded Polypropylene

Oxidative degradation of polypropylene frequently proceeds through chain scission reactions, generating oxygen-containing functionalities while simultaneously reducing molecular weight. The ability to measure carbonyl concentration as a function of molecular weight therefore provides direct insight into the degradation mechanism (Celina et al). In this example, the new 1780 cm⁻¹ filter reveals carbonyl groups concentrated in the lower molecular weight fractions, consistent with chain scission-driven degradation.

For the first time, degradation can be tracked along the molecular weight distribution

Application #2: Degraded PP/PE samples

Degraded PP/PE samples may contain different CO groups such as carbonyl or carboxyl groups caused during the degradation process. The presence of these kinds of groups can now be better addressed thanks to the possibility of using two different filters (1740 cm⁻¹or 1780 cm⁻¹).

Example of degraded PP that generate different peaks in both bands (band of 1780 and TCB in pink color, and band of 1740 and oDCB in green color).

Application #3: Analysis of PE/PP – MAH samples:

PE/PP samples containing maleic anhydride can now be analyzed using the new filter located at band of 1780 cm⁻¹ and in TCB as solvent. The analysis of this type of samples by oDCB had not been possible so far due to potential unrecoverable retention in the GPC columns due to the interaction with the oDCB.

Now, with the new filter, TCB was used instead as samples can travel through the columns normally and the new filter allows the measurement of the carbonyl groups, which are an invaluable complement to the MMD and short chain branching data.

Carbonyl groups concentration in band of 1780 for 3 different PE/PP – MAH samples analyzed in TCB

Application #4: Analysis of EVA

Measurement of EVA samples had been possible by the initial IR6 detector and its initial filter located in 1740 cm⁻¹ band and using oDCB. This capability had been a success allowing quantifying the amount of vinyl acetate accoss the MMD distribution. These samples have now been tested again using the new filter located in 1780 cm⁻¹ band and TCB as solvent, obtaining superior results both in sensitivity and also in S/N ratio compared to the initial studies with the filter located in 1740 cm⁻¹ band and oDCB as solvent.

Comparison of the conventional 1740 cm⁻¹ and the new 1780 cm⁻¹ IR filters under same chromatographic conditions (mobile phase TCB).

6. References

1. Gugumus et al, (1995)., Thermooxidative Degradation of Polyolefins in the Solid State: Part 1. Experimental Kinetics of Functional Group Formation. Polymer Degradation and Stability, 52: 131-144.

2. Rabello & White, (1997)., Crystallization and Melting Behaviour of Photodegraded Polypropylene– 1. Chemi-Crystallization. Polymer Degradation and Stability, Vol. 38 No.26, pp.6379-6387.

3. Celina et al, (2013), Review of Polymer Oxidation and Its Relationship with Materials Performance and Lifetime Prediction. Polymer Degradation and Stability, 1-11.

4. Geraldine Rapp et al (2019).. Influence of the physical state of a polymer blend on thermal ageing. Polymer Degradation and Stability, 163, pp.161-173. ⟨10.1016/j.polymdegradstab.2019.03.006⟩.

5. Scott 2002. Degradable Polymers: Principles and Applications (2nd Edition) DOI 10.1007/978-94-017-1217-