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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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Deep eutectic solvents, considered ionic liquid (IL) analogues, show promise for many material science and engineering applications over typical ILs because they are readily available and relatively inexpensive. Atomistic molecular dynamics simulations have been performed over a range of temperatures on one eutectic mixture, 1:2 choline chloride/urea, using different force field modifications. Good agreement was achieved between simulated density, volume expansion coefficient, heat capacity, and diffusion coefficients and experimental values in order to validate the best performing force field. Atom-atom and center-of-mass radial distribution functions are discussed in order to understand the atomistic interactions involved in this eutectic mixture. Experimental infrared (IR) spectra are also reported for choline chloride-urea mixtures, and band assignments are discussed. The distribution of hydrogen-bond interactions from molecular simulations is correlated to curve-resolved bands from the IR spectra. This work suggests that there is a strong interaction between the NH2 of urea and the chlorine anion where the system wants to maximize the number of hydrogen bonds to the anion. Additionally, the disappearance of free carbonyl groups upon increasing concentrations of urea suggests that at low urea concentrations, urea will preferentially interact with the anion through the NH2 groups. As this concentration increases, the complex remains but with additional interactions that remove the free carbonyl band from the spectra. The results from both molecular simulations and experimental IR spectroscopy support the idea that key interactions between the moieties in the eutectic mixture interrupt the main interactions within the parent substances and are responsible for the decrease in freezing point.

The effect of hydrogen bonding on the phase behavior of a chemically similar polyurethane and polyurea and their blends with poly(ethylene glycol) is examined. The polyurethane and polyurea were synthesized from the same diisocyanate, 1,5-diisocyanato-2-methylpentane, using an aromatic diol and aromatic diamine, respectively. Fourier transform infrared spectroscopy was used to characterize the distribution of hydrogen bonds in these polymers and their blends. The distribution of hydrogen bonds in the polyurethane homopolymer was found to be quite similar to that found in an amorphous polyurethane studied previously in this laboratory. However, upon annealing, some sort of ordered structure was detected spectroscopically. The polyurea formed an equivalent ordered structure much more readily at room temperature. Ordered hydrogen-bonded domains were also detected in the spectra of the blends, either after an extended time at room temperature or after annealing at an elevated temperature. The formation of ordered structures occurred at temperatures well below the thermally measured glass transition. Melting endotherms could not be detected in most of these samples. It is suggested that the order present in these blends might be largely two-dimensional. The hydrogen bonds between adjacent urethane (or urea) units can align to form a sheet, with the methyl group that is part of each segment essentially laying in a plane that is perpendicular to this structure. Because the methyl group is asymmetrically placed in the diisocyanate used to synthesize these polymers, packing between sheets would be imperfect at best.

The results of a Fourier transform infrared study of poly(ethylene-co-methacrylic acid) (EMAA) copolymer blends with poly(2-vinylpyridine) (P2VP) and a copolymer of styrene and 2-vinylpyridine are presented. EMAA copolymers are strongly self-associated at ambient temperatures through the formation of intermolecular carboxylic acid dimers. P2VP, a polymer that is inherently weakly self-associated, forms a strong association with EMAA by forming intermolecular hydrogen bonds between the carboxylic acid and pyridine groups. The fraction of interacting sites in these blends plays a major role in determining the solution and film forming properties of the mixtures and ultimately the degree of molecular mixing of the two polymers. Quantitative measurements of the fraction of pyridine groups that are hydrogen bonded to carboxylic acid groups have been obtained, and the results are discussed in terms of competing equilibria.