Assessing PET Grade Through Molecular Weight Analysis Using the BeSEC

Abstract: Polyethylene terephthalate (PET) grades are primarily defined by molecular weight, which directly influences mechanical performance and processing behavior. In this study, size-exclusion chromatography coupled with static light scattering and refractive index detection was applied to determine the absolute molecular weight and molecular weight distribution of PET samples, enabling accurate grade assessment beyond intrinsic viscosity measurements.

Keywords: Polyethylene terephthalate (PET), Absolute molecular weight, Molecular weight distribution, Polymer grade assessment, Size-exclusion chromatography (SEC)

 Introduction 

Polyethylene terephthalate (PET) is a versatile polyester widely used in fibers, films, bottles, and engineering materials. PET grades differ primarily by molecular weight.

Textile grade PET has a lower molecular weight, with intrinsic viscosity (IV) in the range of 0.64 to 0.68 dL/g, corresponding to approximately 20-30 kDa.

Bottle or chip grade PET has a higher molecular weight, with intrinsic viscosity (IV) between 0.7 and 1.0 dL/g and weight average molecular weight (Mw) typically 30 to 60 kDa or higher.

Molecular weight directly impacts both mechanical strength and processing behavior. While intrinsic viscosity provides an average value, it does not reveal the full molecular weight distribution. Combining Size Exclusion Chromatography (SEC) with light scattering detection enables direct measurement of absolute molecular weight and detailed distribution analysis, critical for process control and product optimization.

 Experimental Section 

This study employed a SEC system with refractive index (RI) and light scattering (LS) detectors. The light scattering detector equipped in the BeSEC LS2 from Bettersize Instruments includes 90° and 7° angles. The BeSEC workstation integrates light scattering with RI or UV signals to calculate molecular weight averages (Mn, Mw and Mz) and distributions.

 

System Configuration:

• Detectors: Light Scattering (LS) + Refractive Index (RI)
• Column: Shodex GPC KF-806M
• Mobile phase: Hexafluoroisopropanol (HFIP)
• Flow rate: 0.7 mL/min
• Injection volume: 100 μL
• Column temperature: 40 ℃
• dn/dc: 0.296 mL/g

 

Sample Preparation:

Three PET samples were analyzed. Each powder was accurately weighed and dissolved in HFIP to a concentration of 2 to 7 mg/mL, stirred until clear, filtered through a 0.22 μm PTFE syringe filter, and transferred into vials for autosampler injection.

 

 Results and Discussion 

 

Figure 1. Elution profiles of the multi-detector signals for Sample A

 

Figure 2. Elution profile of the molecular weight for Sample A

 

Figure 3. Molecular weight distribution of Sample A

 

Figure 4. Elution profiles of the multi-detector signals for Sample B

 

Figure 5. Elution profile of the molecular weight for Sample B

 

Figure 6. Molecular weight distribution of Sample B

 

Figure 7. Elution profiles of the multi-detector signals for Sample C

 

Figure 8. Elution profile of the molecular weight for Sample C

 

Figure 9. Molecular weight distribution of Sample C

 

Figures 1 to 9 present chromatograms, molecular weight elution profiles, and distribution plots for the three PET samples. In Figures 1, 4 and 7, the blue trace is the RI signal, the green trace is the right-angle light scattering (RALS) signal and the red trace is the low-angle light scattering (LALS) signal. Figures 2, 5 and 8 present chromatograms with molecular weight profiles, where the blue line shows the molecular weight as a function of elution time. Figures 3, 6 and 9 display the differential and cumulative molecular weight distributions.

 

The chromatograms exhibit clean baselines with minimal noise, and strong signal-to-noise ratios. Molecular weight profiles decrease steadily with elution volume, consistent with SEC principles, larger chains elute first, followed by smaller ones. Scattering peaks show no tailing and molecular weight curves remain stable at the end, confirming effective size-based separation.

 

No.	Mn (Da)	Mw (Da)	Mz （Da)	Mw/Mn
Sample A	27,594	42,463	59,631	1.53
Sample B	21,274	41,191	62,620	1.93
Sample C	26,112	42,411	68,615	1.62

 

Table 1 summarizes the molecular weight data. All three samples exhibit weight-average molecular weights above 40 kDa, meeting bottle-grade PET requirements. Notably, Sample B displays a broader distribution (Mw/Mn ≈ 2) indicating a higher proportion of low molecular weight species.

 

 Conclusion 

 

The BeSEC LS2 with light scattering detection provides accurate molecular weight characterization for PET samples. All three samples meet bottle-grade specifications, with clean, stable chromatographic profiles and well-aligned detector signals. The ability to resolve molecular weight distribution offers valuable insight for quality control and grade selection.

 

 

About the Authors

Zhibin Guo Application Research Lab, Bettersize Instruments Ltd.

 

BeSEC Advanced Light Scattering Detector ● For proteins: molecular weight, oligomer state, and aggregate characterization ● For polymers and polysaccharides: molecular weight distribution and size ● No column calibration required since Mw is independent of elution volume ● Low-angle detection enables accurate molecular weight without model assumptions or angle fitting ● Compatible with any GPC or SEC system   Learn more