Two-dimensional infrared correlation spectroscopy as a probe of sequential events in the diffusion process of water in poly(ε-caprolactone)

Abstract

In this study, the diffusion process of water molecules in poly(ε-caprolactone) (PCL) has been investigated using in-situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. In our system, the original broad water OH bands in 1D IR spectra can be effectively differentiated into four bands, located at 3641, 3593, 3410, and 3203 cm−1, respectively, using 2D correlation analysis. The bands at 3641 cm−1 (antisymmetric) and 3593 cm−1 (symmetric) are assigned to the OH stretching vibration of water partially hydrogen bonded with hydrophilic carbonyl group (C=O) of PCL, while the other band pair at 3410 cm−1 (antisymmetric) and 3203 cm−1 (symmetric) could be attributed to the stretching vibration of water fully hydrogen bonded with another water molecule. According to the result of the asynchronous correlation, it was concluded that the water molecules at first diffuse into free volume (microvoids) or are molecularly dispersed into the PCL matrix and then form hydrogen bond with the C=O group of the polymer during the process of water diffusion. In addition, the diffusion coefficient was estimated using nonlinear curve fitting of OH band areas in the range of 3800–3000 cm−1.