Characterizing the Average Composition and Molar Mass Distributions of a Copolymer by SEC-MALS-dRI-UV

Significance of the Topic

The chemical composition and molecular weight distribution of copolymers critically influence their physical properties and performance in applications ranging from controlled release pharmaceuticals to advanced carbon capture materials. Accurate characterization of these parameters drives copolymer development and quality control.

Objectives and Study Overview

This application note describes a rapid method to obtain both absolute molecular weight and detailed composition profiles of UV-absorbing random copolymers by combining size exclusion chromatography with multi-angle light scattering, differential refractive index, and UV detection (SEC-MALS-dRI-UV). Styrene–acrylic acid copolymer standards were analyzed and results were compared to 1H NMR spectroscopy.

Methodology

The approach uses on-line SEC separation followed by three detectors in series: multi-angle light scattering for absolute molar mass determination, differential refractive index for concentration measurement, and UV absorbance at 254 nm to quantify copolymer composition in real time. ASTRA software processes detector signals to calculate molecular weight, refractive index increment (dn/dc), and monomer fractions across elution volumes.

Used Instrumentation

• DAWN multi-angle light scattering detector
• Optilab differential refractive index detector
• UV/Vis detector (ABI 785A, 254 nm)
• Size exclusion chromatography system
• ASTRA software for data analysis

Main Results and Discussion

System validation with a 30 kDa polystyrene standard confirmed uniform molecular weight across the chromatographic peak and accurate extinction coefficient determination. Analysis of poly(styrene–co-acrylic acid) samples revealed that the mass fraction of styrene increases with elution time, indicating lower molecular weight regions are enriched in styrene. Comparisons with 1H NMR measurements showed good agreement for both molecular weight and styrene content across three standards.

Benefits and Practical Applications

• Absolute molar mass measurement without external calibration
• Continuous compositional profiling across the molecular weight distribution
• Reduced analysis time by eliminating fraction collection and NMR
• Enhanced insight for copolymer design and quality control

Future Trends and Potential Applications

• Integration with alternative separation techniques such as HPLC and field-flow fractionation
• Extension to diverse copolymer systems containing UV-active monomers
• High-throughput automation for industrial QA/QC workflows
• Combination with advanced detection methods (e.g., on-line FTIR or NMR) for richer structural information

Conclusion

The SEC-MALS-dRI-UV method, coupled with ASTRA software, provides a robust, efficient approach for simultaneous determination of molecular weight and compositional distribution in UV-absorbing copolymers, matching NMR accuracy while streamlining analytical workflows.

References

• Wei Gao, Ph.D.; Michelle Chen, Ph.D. AN1002: Characterizing the Average Composition and Molar Mass Distributions of a Copolymer by SEC-MALS-dRI-UV. Wyatt Technology Corporation.