Investigation of Water Interaction with Polymer Matrices by Near-Infrared (NIR) Spectroscopy

Abstract

The interaction of water with polymers is an intensively studied topic. Vibrational spectroscopy techniques, mid-infrared (MIR) and Raman, were often used to investigate the properties of water-polymer systems. On the other hand, relatively little attention has been given to the potential of using near-infrared (NIR) spectroscopy (12,500-4000 cm^-1; 800-2500 nm) for exploring this problem. NIR spectroscopy delivers exclusive opportunities for the investigation of molecular structure and interactions. This technique derives information from overtones and combination bands, which provide unique insights into molecular interactions. It is also very well suited for the investigation of aqueous systems, as both the bands of water and the polymer can be reliably acquired in a range of concentrations in a more straightforward manner than it is possible with MIR spectroscopy. In this study, we applied NIR spectroscopy to investigate interactions of water with polymers of varying hydrophobicity: polytetrafluoroethylene (PTFE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), polyoxymethylene (POM), polyamide 6 (PA), lignin (Lig), chitin (Chi) and cellulose (Cell). Polymer-water mixtures in the concentration range of water between 1-10%(w/w) were investigated. Spectra analysis and interpretation were performed with the use of difference spectroscopy, Principal Component Analysis (PCA), Median Linkage Clustering (MLC), Partial Least Squares Regression (PLSR), Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) and Two-Dimensional Correlation Spectroscopy (2D-COS). Additionally, from the obtained data, aquagrams were constructed and interpreted with aid of the conclusions drawn from the conventional approaches. We deepened insights into the problem of water bands obscuring compound-specific signals in the NIR spectrum, which is often a limiting factor in analytical applications. The study unveiled clearly visible trends in NIR spectra associated with the chemical nature of the polymer and its increasing hydrophilicity. We demonstrated that changes in the NIR spectrum of water are manifested even in the case of interaction with highly hydrophobic polymers (e.g., PTFE). Furthermore, the unveiled spectral patterns of water in the presence of different polymers were found to be dissimilar between the two major water bands in NIR spectrum (ν_s + ν_as and ν_as + δ).

Keywords: NIR; chemometrics; data analysis; hydrophilic; hydrophobic; near-infrared spectroscopy; polymer; polymer-water interaction; water.

Figure 1

Averaged NIR absorbance spectra of the polymer–water mixtures after linear offset correction, in the range of 1–10% (w/w) water and the pure water spectra (dark blue) for comparison. The polymers are ordered according to increasing hydrophilicity, with the least hydrophilic polymer, polypropylene (upper left corner), to the most hydrophilic polymer, cellulose (lower right corner).

Figure 2

PCA scores (left) and MLC dendrogram (right) for the pure polymers after linear offset correction. The PCA scores and MLC dendrograms for the entire concentration range of water (1–10%) added to the polymer (w/w) are displayed in the Supplementary Materials (Figures S1 and S2, respectively).

Figure 3

PLSR scores (top), regression coefficients (middle) and predicted vs. reference (bottom) for polypropylene (left) and cellulose (right) polymer–water mixtures, after linear offset correction. The scores, regression coefficients and predicted vs. reference of all polymers in comparison are displayed in the Supplementary Materials (Figures S3–S5).

Figure 4

Water difference spectra of the polymer–water mixtures after linear offset correction and subtraction of the polymer spectra, in the range of 1–10% (w/w), with, respectively, the pure water (dark blue) and polymer (red) spectra for comparison, of PP (left) and cellulose (right). The water difference spectra of all investigated polymers are displayed in the Supplementary Materials (Figure S6).

Figure 5

Polymer difference spectra of the 10% polymer–water mixtures after linear offset correction and subtraction of the water spectrum, with, respectively, the pure water (dark blue) and polymer (red) spectra for comparison of PTFE (left) and cellulose (right).

Figure 6

MCR-ALS polymer (orange) and water spectra (light blue) of polypropylene (left) and cellulose (right), additionally the NIR absorbance spectra of the pure polymer (red) and pure water (dark blue) are shown. The reference spectra as well as the resolved curves were normalized using an SNV transformation. The MCR-ALS polymer and water spectra of the remaining six polymers are displayed in the Supplementary Materials (Figure S7).

Figure 7

Synchronous (left) and asynchronous (right) 2D-COS spectra of the polymer–water mixtures after linear offset correction, in the ranges from 0–10% water (w/w), of PP (left) and cellulose (right). Note, the intensity scale of the synchronous 2D-COS spectra is the same for all polymers. The remaining 2D-COS spectra are displayed in the Supplementary Materials (Figure S8).

Figure 8

Aquagrams of both water regions for the polymer-water mixtures after SNV and standardization, in the ranges from 0–10% water (w/w), of PP (left) and cellulose (right). Aquagrams of all investigated polymers are displayed in the Supplementary Materials (Figure S9).

Figure 9

Approximate order of hydrophilicity of the polymers used in this study. From the most hydrophobic (left) to the most hydrophilic (right) polymers: Polytetrafluoroethylene, polypropylene, polystyrene, polyvinylchloride, polyoxymethylene, polyamide 6, lignin, chitin and cellulose.