Diffusion in polymers as investigated by two-dimensional correlation spectroscopy: The H2O/PCL system

• Time-resolved FTIR spectroscopy as a tool for studying polymer/penetrant systems.
• 2D-COS analysis to reveal fine details on the kinetic behaviour of the system.
• Verifying Noda’s sequential rules for sorption/desorption processes.
• Detection and quantitative analysis of two distinct water species.

Time-resolved FTIR spectra collected in situ during a sorption-desorption cycle constituted the experimental basis for a molecular level characterization of the H2O/PCL system. The data were analysed by a number of techniques (difference spectroscopy, least-squared curve-fitting (LSCF) and 2D-FTIR correlation spectroscopy (2D-COS) which provided relevant and complementary information on the diffusion mechanism and the molecular interactions formed (H-bonding). Particular attention was paid to the application of 2D-COS spectroscopy to diffusion studies and to the potential of this approach to disclose subtle details on the dynamic behaviour of the molecular specie involved. Its reliability was confirmed by comparison with simulation studies mimicking the system’s evolution during a sorption/desorption cycle. The conclusions drawn by 2D-COS were also supported by an analysis of the ν(OH) band of sorbed water with least-squared-curve-fitting techniques. The study has revealed two spectroscopically discernible water species – a first-shell, and a second-shell layer – which were quantified. It was concluded that on sorption, the concentration build-up of the first-shell layer is accelerated with respect to that of the second-shell, while the opposite occurs on desorption.

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