Quantitative Analysis of a Water-soluble Polymer Using the i-Raman EX Spectrometer

Significance of the Topic

Vibrational spectroscopy plays a crucial role in polymer analysis by delivering molecular fingerprints for both qualitative and quantitative characterization. Raman spectroscopy, in particular, offers selective insight into nonpolar functional groups, enabling precise determination of polymer composition and chain conformation even in aqueous environments. The ability to monitor the degree of functionalization of polystyrene to yield a water-soluble polymer underpins quality control in industrial production and supports rapid decision-making in multiuser laboratories.

Goals and Overview of the Study

The study aimed to establish a straightforward, robust Raman-based method to quantify the extent of polystyrene functionalization in its aqueous phase. Key objectives included:

• Developing a classification model to confirm sampling of the correct (aqueous) phase.
• Constructing a quantitative PLS1 model to determine percent functionalization with defined accuracy and precision.
• Implementing a user-friendly, push-button workflow suitable for plant QC laboratories.

Instrumentation Used

• Raman spectrometer: i-Raman EX with 1064 nm laser, 495 mW power, 180° backscattering geometry.
• Detector: Thermoelectrically cooled 512-element InGaAs array; spectral range 250–2500 cm⁻¹; resolution ~9.5 cm⁻¹.
• Sample handling: Enclosed cuvette/vial compartment with borosilicate vials (10–20 wt % solids).
• Data acquisition software: BWSpec v4.04 (dark subtraction, 500 ms exposure, 264 accumulations, 5 min total).
• Chemometrics software: BWIQ v3.0.6 for PCA-MD classification and PLS1 regression.

Methodology and Data Analysis

• Phase classification: PCA-MD model using Kennard-Stone sampling, mean centering, max-value normalization; spectral window 650–1700 cm⁻¹; three principal components achieved 100 % accuracy in distinguishing aqueous vs. organic phases.
• Quantitative regression: PLS1 calibration on 60 samples spanning 65–98 % functionalization (by ¹H NMR); data preprocessed by autoscaling and Savitzky-Golay second derivative (order 3, window 5) over 995–1200 cm⁻¹; six latent variables selected.
• Calibration set design: Included lab design-of-experiments, pilot‐scale batches, and production lots to capture process variability.

Main Results and Discussion

• Raman spectra exhibited strong, distinct aromatic bands at ~1002 and ~1132 cm⁻¹, enabling clear differentiation between starting and functionalized polymers. A minor band at 981 cm⁻¹ was attributed to inorganic impurities.
• A simple univariate band-ratio method (1126 / [1005 + 1126 cm⁻¹]) showed excellent linearity with ¹H NMR reference (R² > 0.95), confirming data quality for multivariate modeling.
• PLS1 model performance:
  • Calibration R² = 0.95, RMSEC = 1.22 %.
  • Validation R² = 0.87, RMSEP = 1.43 %.
• Precision study demonstrated single-day, single-lot repeatability of 0.49 % and interlot RMSE ~1.31–1.43 % across 16 production batches.

Benefits and Practical Applications

• Rapid, nondestructive quantitation of polymer functionalization in aqueous samples without elaborate sample preparation.
• High specificity in differentiating polymer species and robust performance in a multiuser QC environment.
• Push-button operation with built-in chemometric models that ensure consistent, reproducible results.

Future Trends and Potential Uses

• Integration with online process monitoring for real-time control of polymer functionalization reactions.
• Expansion of chemometric workflows to other polymer systems and complex mixtures using Raman, NIR, or LIBS.
• Application of advanced machine-learning algorithms for improved classification of subtle compositional changes.

Conclusion

The study demonstrates that the i-Raman EX spectrometer combined with tailored chemometric models provides a precise, accurate, and user-friendly solution for quantifying water-soluble polymer functionalization. The method’s robustness and simplicity make it ideally suited for routine QC in industrial settings.

References

• Everall NJ, Chalmers JM, Griffiths PR. Vibrational Spectroscopy of Polymers: Principles and Practice. John Wiley & Sons; 2007.
• Koenig JL. Spectroscopy of Polymers. American Chemical Society; 1991.
• Larkin PJ. Infrared and Raman Spectroscopy, Principles and Spectral Interpretation, 2nd Ed. Elsevier; 2018.
• Otto M. Chemometrics: Statistics and Computer Application in Analytical Chemistry. Wiley-VCH; 2017.