Exploring the Dynamics of Bound Water in Nylon Polymers with Terahertz Spectroscopy

Abstract

Exploring the Dynamics of Bound Water in Nylon Polymers with Terahertz Spectroscopy. ChemRxiv. Preprint. Terahertz (THz) spectroscopy was used to observe adsorbed water structure and dynamics within polymer films, ultimately providing a strong rationale for the observed rates of water desorption. The THz absorption spectra of nylon-6 films undergoes drastic changes during the hydration and drying process. Additionally, the structural change from g to a crystals, induced by the hydration, was observed by the characteristic band of a-nylon-6 at 6.5 THz. Importantly, the THz spectra of adsorbed water, as well as deuterated water, within in the nylon films were observed by the continuous measurement of a-nylon during dehydration. The differential spectra clearly show three absorption bands of water molecules named Peak I, II and III, which behaved differently between the H 2 O and D 2 O materials. The spectra were assigned using a combination of ab initio molecular dynamics simulations and solid-state density functional theory calculations, and were compared to previous spectral assignments of bulk water. The results show that the inclusion of H 2 O and D 2 O into polymer films results in a distinct set of spectral features that, while similar in frequencies to the dynamics of bulk water, represent significantly different motions owing to the unique chemical environment within the material. These results highlight the significant utility of using THz spectroscopy to study the hydration dynamics and spectral signature of bound water in this important class of materials. File list (4) download file view on ChemRxiv 20191018_Nylon_final.docx (1.02 MiB) download file view on ChemRxiv Peak3.gif (16.32 MiB) download file view on ChemRxiv Peak2.gif (16.78 MiB) download file view on ChemRxiv Peak1.gif (18.27 MiB)

Differences in Intermolecular Interactions and Flexibility between Poly(ethylene terephthalate) and Poly(butylene terephthalate) Studied by Far-Infrared/Terahertz and Low-Frequency Raman Spectroscopy

“…29 Our previous studies have performed the band assignments of the vibrational modes due to hydrogen bonding in polymers with weak hydrogen bonding in their crystal structures, such as PHB, 19,22,23,26,32−36 PGA, 17,20,39 PCL, 15 and PBS, 29 and in those with strong hydrogen bonding, such as nylon 6. 16,21 In our previous studies, comparison of experimental FIR/ THz and low-frequency Raman spectra of PHB 22,23,26 with the QCCs results 19 allowed us to assign an FIR/THz band at around 82 cm −1 and a Raman band at 79 cm −1 to weak intermolecular hydrogen bonding between the CO and CH 3 groups of two adjacent PHB chains. These bands exhibited a shift to lower frequency with increasing temperature, suggesting weakening of the inter-chain hydrogen bonding.…”

“…14−31 In the case of polymeric materials, FIR/THz and low-frequency Raman spectra can be used to investigate directly the skeletal motions, higher-order structure, thermal expansion, and interand intramolecular interactions, including hydrogen bonding and cross-linked structure dynamics. [15][16][17][19][20][21][22][23][24]26,29 The higher-order structure of polymers is closely related to their physical properties, and hydrogen bonding is one of the important factors that determine the higher-order structure by stabilizing the crystal structure. 15,17,[19][20][21][22][23]26,29 Low-frequency vibrational spectroscopy has also been used to explore the dynamics of bound water in polymers.…”

“…15,17,[19][20][21][22][23]26,29 Low-frequency vibrational spectroscopy has also been used to explore the dynamics of bound water in polymers. 16,24 Therefore, vibrational spectroscopies in the low-frequency region are very useful and unique techniques for exploring polymer structures.…”

“…Measurement of spectra in the low-frequency region has become easier in the last decade or so with the development of far-infrared (FIR) − and terahertz time-domain spectroscopy (THz-TDS). − Low-frequency Raman spectroscopy has a long history; however, the measurement of Raman spectra in the low-frequency region has become much easier recently because of remarkable improvements in notch-filter and edge-filter technologies and in the stability and wavelength specificity of diode lasers. − ,− …”

On the influence of water on THz vibrational spectral features of molecular crystals

“…The fact that THz spectra include information of both water of crystallization and the host crystal structure makes THz-TDS very sensitive to dehydration. This is shown by many studies on the structure and dynamics of bound water in glucose 44 and polymers 45,46 , the dehydration of crystalline proteins 47 , and the observation of the critical effect of hydration on the resonant signatures of biomolecules 48 . During dehydration, THz spectroscopy will precisely monitor both the loss of water of crystallization and the eventual molecular rearrangement.…”