Variable-temperature Infrared Spectroscopy Research Articles

Thermally and mechanically robust self-healing supramolecular polyurethanes featuring aliphatic amide end caps

A series of supramolecular polyurethanes (SPUs) were designed and synthesised with synergetic multifunctional hydrogen bonding aliphatic amide end-caps. Hydrogen bonding between the urethane, urea, and amide motifs in the polymers afford strong dynamic association between polymer chains in the solid state. Phase separation of the apolar and polar components of the polyurethanes also serves to reinforce their thermal and mechanical properties. The supramolecular polyurethane with bisamide-morpholine end caps associates via multiple hydrogen bonds and exhibits enhanced tensile and thermal properties when compared to the other materials. Variable-temperature infrared spectroscopy (VT-IR) and atomic force microscopy (AFM), were carried out to study the phase morphology of the polymers and revealed a correlation between increased phase separation and the introduction of amide motifs in the end-caps. These SPUs also exhibit excellent healing abilities, requiring temperatures >200 °C to recover their physical properties.

Structure and Stability of Gas Adsorption Complexes in Periodic Porous Solids as Studied by VTIR Spectroscopy: An Overview

Variable-temperature infrared (VTIR) spectroscopy is an instrumental technique that enables structural characterization of gas-solid adsorption complexes by analysis of meaningful vibrational modes, and simultaneous determination of the standard enthalpy change (ΔH0) involved in the gas adsorption process, which allows one to quantify the stability of the corresponding complex. This is achieved by a van’t Hoff analysis of a set of IR spectra recorded over a sufficiently large temperature range. Herein, the use of this versatile spectroscopic technique is demonstrated by reviewing its application to the study of carbon monoxide, carbon dioxide and dinitrogen adsorption on several (alkaline) zeolites, which can be regarded as the archetype of periodic porous solids.

Non-covalently crosslinked anion exchange membranes: Effect of urea hydrogen-bonding group position

To investigate the effects of noncovalent intermolecular interaction on the performance of anion exchange membranes (AEMs), two types of side chain urea-containing poly(phenylene oxide)-based AEMs, namely, benzyl urea-linkaged terminal quaternary ammonium (QA)-containing AEMs (bU-AEMs) and benzyl QA-linkaged urea-containing AEMs (sU-AEMs), were synthesized and comparatively studied. The analyses by solubility test and variable-temperature infrared spectroscopy showed that the urea-based hydrogen-bonding strength in bU-AEMs was higher than that of sU-AEMs. The bU-AEMs were characterized with a stable water uptake (WU), as well as high swelling resistance relative to that of the sU-AEM counterparts, as the temperature increases. A high chloride conductivity of 50.3 mS cm−1 was achieved for bU-AEM-0.45 at 90 °C, while its WU maintained a moderate value (80.6 wt%). The H2-O2 single fuel cell performance using the bU-AEM-0.35 membrane as a separator reached a peak power density of 68 mW cm−2, which was higher than that of the sU-AEM-0.35 sample with 48 mW cm−2. More importantly, there was less than 10% conductivity loss for the bU-AEMs after incubation in 1 M NaOH at 60 °C after 7 days, while approximately 30% loss was observed for the sU-AEMs. The results indicated that the hydrogen-bonding crosslinking strategy, as well as the configuration of H-bonding groups, provides a platform to tune the properties of AEMs suitable for fuel cell applications.

Intermolecular Interactions of a Chiral Amine Borane Adduct Revealed by VCD Spectroscopy.

Amine boranes feature strong hydrogen bonding acceptor and donor moieties in close proximity, leading, for instance, to dihydrogen bonding driven self-aggregation. In this work, the infrared (IR) and vibrational circular dichroism (VCD) spectra of the bulky bis(α-phenylethyl)amine borane 1 in chloroform and acetonitrile solution are reported. By comparison with calculated spectra, the VCD spectral features observed in chloroform solution can clearly be associated with the presence of monomeric species. A shift of the conformational preferences occurs when changing the solvent to acetonitrile, which can only be deduced from the VCD spectral signatures but not from the IR spectrum. Using variable-temperature IR and VCD spectroscopy, the dihydrogen bonded dimeric species is characterized experimentally at -50 °C and theoretically by means of density functional theory calculations.

One-step hydrothermal conversion of VO2(B) into W-doped VO2(M) and its phase transition and optical switching properties

W-doped monoclinic vanadium dioxide VO2(M) nanobelts have been transformed from VO2(B) nanobelts in the presence of tungstic acid by a facile one-step hydrothermal route for the first time. The as-prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and variable-temperature infrared spectroscopy (IR). The results show that W atoms are doped into the crystal lattice of VO2(M) matrix, and the morphology and size of the as-obtained VO2(M) nanobelts are dependent on that of the precursor VO2(B). The phase transition temperature (Tc) of W-doped VO2(M) can be tuned by W doping. The variable-temperature IR reveals that the as-synthesized W-doped VO2(M) nanobelts have outstanding thermochromic characters and optical switching properties.

Thermodynamic Features of the Reaction of Ammonia with the Acidic Proton of H-ZSM-5 As Studied by Variable-Temperature IR Spectroscopy

The interaction of ammonia with H-ZSM-5 zeolite (Si/Al = 11.5) was studied by means of variable-temperature infra-red spectroscopy, in the temperature range of 570−680 K. The interaction consists mainly in the reaction of ammonia with the zeolite Brønsted acid OH groups (absorbing at ca. 3610 cm−1) to form ammonium species that show a bending mode at ca. 1410 cm−1. From the changes in intensity of these IR absorption bands at 3610 and 1410 cm−1, with both temperature and NH3 equilibrium pressure, the standard enthalpy of reaction was calculated to be ΔH0 = −128 (±5) kJ mol−1. The corresponding entropy change is ΔS0 = −184 (±10) J mol−1 K−1, a figure suggesting that the ammonium cation interacting with the negative framework has little residual degrees of freedom. These results are discussed in the broader context of relevant data reported in the literature.

Impact of Stoichiometry on the Hydrogen Storage Properties of LiNH2−LiBH4−MgH2 Ternary Composites

We recently reported (Yang, J.; et al. Angew. Chem., Int. Ed. 2008, 47, 882) a novel hydrogen storage composite involving a 2:1:1 LiNH2:LiBH4:MgH2 ratio. On the basis of in-depth experimental and computational analysis, this composite was found to release hydrogen via a complex multistep reaction cascade, which seeded the products of a subsequent reversible hydrogen storage reaction. This so-called autocatalytic reaction sequence was found to result in favorable kinetics, ammonia attenuation, and partial low-temperature reversibility. Here, we extend our original study by examining the effects of reactant stoichiometry on the ensuing hydrogen storage desorption pathway and properties. In particular, we examine four (LiNH2)X−(LiBH4)Y−(MgH2)Z composites, where X:Y:Z = 2:1:2, 1:1:1, 2:0.5:1, and 2:1:1 (original stoichiometry). For each sample, we characterize the postmilled mixtures using powder X-ray diffraction (PXRD) and infrared spectroscopy (IR) analyses and observe differences in the relative extent of two spontaneous milling-induced reactions. Variable-temperature hydrogen desorption data subsequently reveal that all composites exhibit a hydrogen release event at rather low temperature, liberating between 2.3 (1:1:1) and 3.6 (2:0.5:1) wt % by 200 °C. At higher temperatures (200−370 °C), the hydrogen release profiles differ considerably between composites and release a total of 5.7 (1:1:1) to 8.6 (2:0.5:1) wt %. Utilizing variable-temperature IR and PXRD data coupled with first-principles calculations, we propose a reaction pathway that is consistent with the observed phase progression and hydrogen desorption properties. From these data, we conclude that premilled reactant stoichiometry has a profound impact on reaction kinetics and high-temperature reaction evolution because of reactant availability. From this enhanced understanding of the desorption process, we recommend and test a stoichiometrically optimal ratio (3:1:1.5) which releases a total of 9.1 wt % hydrogen. Finally, we assess the reversibility (at 180 °C) of the four primary composites over two desorption cycles and find that only the 2:1:1 and 2:0.5:1 are reversible (3.5 wt % for 2:0.5:1).

Vibrational and thermodynamic properties of Ar, N2, O2, H2 and CO adsorbed and condensed into (H,Na)-Y zeolite cages as studied by variable temperature IR spectroscopy

The adsorption of Ar, H2, O2, N2 and CO on (H,Na)-Y zeolite (Si/Al = 2.9, H+/Na+ approximately 5) has been studied at variable-temperature (90-20 K) and sub-atmospheric pressure (0-40 mbar) by FTIR spectroscopy. Unprecedented filling conditions of the zeolite cavities were attained, which allowed the investigation of very weakly adsorbed species and of condensed, liquid-like or solid-like, phases. Two pressure regimes were singled out, characterized by: (i) specific interaction at low pressure of the probe molecules (P) with the internal Brønsted and Lewis sites, and (ii) multilayer adsorption at higher pressure. In the case of CO the perturbation of the protonic sites located inside the sodalite cages was also observed. As the molecule is too large to penetrate the sodalite cage, the perturbation is thought to involve a proton jump tunneling mechanism. The adsorption energy for the (HF)OH...P (P = Ar, H2, O2, N2 and CO) specific interaction involving the high frequency Brønsted acid sites exposed in the supercages was derived following the VTIR (variable temperature infrared spectroscopy) method described by E. Garrone and C. Otero Areán (Chem. Soc. Rev., 2005, 34, 846).

Synthesis and crystallization behavior of nylon 12 14. II: Chain‐folded lamellar crystals and crystalline transformation behavior of nylon 12 14

AbstractChain‐folded lamellar crystals of nylon 12 14 have been grown from a dilute 1,4‐butanediol solution with the “self‐seeding” technique. The morphology and structure of nylon 12 14 lamellar crystals were studied by both transmission electron microscopy (TEM) and wide‐angle X‐ray diffraction (WAXD). Two kinds of electron diffraction patterns were detected when different areas were selected for diffraction, which indicates that the α crystal phase and the β crystal phase coexist for nylon 12 14 under the present crystallization conditions. The WAXD diffractograms of the crystal mats confirm the results obtained from electron diffraction (ED). In addition, the changes of the crystal structure as a function of temperature for melt‐crystallized and dilute solution‐crystallized nylon 12 14 were monitored by variable‐temperature WAXD and variable‐temperature infrared spectroscopy (IR). It was found that the melt‐crystallized sample undergoes a Brill transformation at 80°C–90°C, but no Brill temperature can be observed for the dilute solution‐crystallized nylon 12 14.

Heteromeric liquid-crystalline association chain polymers: structure and properties

Liquid-crystalline association chain polymers involving hydrogen-bond connectivities between pyridyls and carboxylic acids have been synthesized and characterized by optical, thermal, infra-red, solid-state nuclear magnetic resonance and rheological analysis. For one system a chemical reaction occurs during its preparation, which alters its primary (covalent) structure. The liquid-crystalline thermal stability is increased by this modification, wherein it has greatly diminished crystallinity. Variable-temperature infra-red spectroscopy was used to follow the extent of hydrogen bonding as a function of temperature. At the melting transition a discontinuous decrease in hydrogen bonding is seen. Rheological data on complexes with varying degree of polymerization show similar values of G′, with gradually increasing G′ on cooling through the mesophase, as would be expected for increased hydrogen bonding at lower temperatures.