Fourier transform infrared temperature studies of an amorphous polyamide are presented. The results strongly suggest that prior interpretations of the changes occurring in the N-H stretching region of the spectra of polyamides and polyurethanes with temperature were greatly oversimplified. In essence, these spectral changes were interpreted to be solely due to hydrogen-bonded N-H groups transforming to "free" N-H groups. Subsequent use of these data to obtain thermodynamic parameters associated with hydrogen bond dissociation must now be considered erroneous.The primary factor not taken into account concerns the very strong dependence of the absorption coefficient with hydrogen bond strength. With increasing temperature, the average strength of the hydrogen bonds decreases, which is observed in the infrared spectrum by a shift to higher frequency. Concurrently, the absorption coefficient decreases, leading to a reduction in the absolute intensity of the hydrogen-bonded N-H band. In this study we present experimental results in the N-H stretching and amide I, II, and V regions of the infrared spectrum of an amorphous polyamide. In addition, we present a model, justified by theoretical considerations, which we believe advances our understanding of the strong dependence of absorption coefficient with the strength of the hydrogen bonds. The ramifications of this work to hydrogen-bonded polymers are discussed.
Choline-chloride
based deep eutectic solvents (DES) have been used
for several different applications (e.g., solubility, electrochemistry,
and purifications) due to their relative inexpensive and readily available
nature. In this work, three choline chloride-based DESs are simulated
using molecular dynamics to study the hydrogen bonding interactions
of the system. Three hydrogen bond donors (HBD) are studied in order
to determine the changes in the hydrogen bonding interactions when
the HBD is different in the DES. One dicarboxylic acid and two polyols
(with different number of OH groups) were chosen as the HBDs of interest.
First, the simulations are validated by comparing simulated and experimental
thermodynamic and transport properties, when possible. Then, for maline
(choline chloride/malonic acid), the more anomalous system studied
here, molecular simulations complement results obtained from an FTIR
spectroscopic study in order to further understand this unique system.
Good agreement with experimental values was obtained for simulated
density, heat capacity, and transport properties. A high relative
percent of hydrogen bonding is observed for interactions between the
anion and the HBD for the three main systems studied here, consistent
with the nature of how these moieties interact in DESs. Comparison
is also done with a previous DES studied in our group. From the infrared
spectroscopic study conducted on maline films, band assignments were
discussed highlighting a “free” carbonyl group of the
carboxylic acid group in the eutectic mixture when the OH group is
hydrogen bonded to something else. Additionally, a band is assigned
to a hydrogen bonded carbonyl group. These band assignments are consistent
with findings in the molecular simulations and highlight the predominant
interactions of the system.
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