The effect of microscopic inhomogeneities in acetone/methanol binary liquid mixtures observed through the Raman spectroscopic noncoincidence effect

Raman spectroscopic insights into the glass transition of poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) is a very versatile polymer which is used as a glass substitute or as an economical alternative to polycarbonate for many types of important applications, due to its particular physical properties. In this study we deal with the Raman spectroscopic characterization of the glass transition of PMMA, the value of the glass transition temperature being generally a decisive parameter for determining the application of polymers. The information obtained by two-dimensional correlation spectroscopy (2DCOS) analysis and perturbation-correlation moving-windows spectroscopy (PCMW2D) analysis of the temperature dependent depolarized Raman spectra enabled us to recognize that the glass transition of PMMA is ruled by intermolecular interactions which influence the vibrational modes of the molecular groups associated with ν(C[double bond, length as m-dash]O), δa(C-H) of α-CH3 and/or O-CH3, ν(C-O-C), ν(C-COO), and ν(C-C-O). This information was employed for the temperature dependent study of the Raman shift and of the full width at half maximum of the Raman peaks obtained through anisotropic and isotropic Raman spectra, of the depolarization ratio, of the Raman spectroscopic noncoincidence effect, and of the Raman peak intensities represented by Arrhenius-type plots, all results supporting the outcomes of this work. The comparison with results obtained by differential scanning calorimetry and with published results in molecular dynamics studies was also part of this work. As the main result, one can highlight the peak associated with the ν(C-O-C) stretching mode at around 812 cm-1 as the one which presents the better outcome for explaining the glass transition from the molecular point of view.

Investigation of the Brill transition in nylon 6,6 by Raman, THz-Raman, and two-dimensional correlation spectroscopy

The Brill transition is a phase transition process in polyamides related with structural changes between the hydrogen bonds of the lateral functional groups (CO) and (NH). In this study, we have used the potential of Raman spectroscopy for exploring this phase transition in polyamide 6,6 (nylon 6,6), due to the sensitivity of this spectroscopic technique to small intermolecular changes affecting vibrational properties of relevant functional groups. During a step by step heating and cooling process of the sample we collected Raman spectra allowing us from two-dimensional Raman correlation spectroscopy to identify which spectral regions suffered the largest influence during the Brill transition, and from Terahertz Stokes and anti-Stokes Raman spectroscopy to obtain complementary information, e.g. on the temperature of the sample. This allowed us to grasp signatures of the Brill transition from peak parameters of vibrational modes associated with (CC) skeletal stretches and (CNH) bending, and to verify the Brill transition temperature at around 160°C, as well as the reversibility of this phase transition.

Microheterogeneity in binary mixtures of methanol with aliphatic alcohols: ATR-IR/NIR spectroscopic, chemometrics and DFT studies

This paper reports ATR-IR and NIR spectroscopic studies on microheterogeneity in binary mixtures of methanol with five short chain aliphatic alcohols: ethanol, 1-propanol, 2-propanol,tert-butanol and cyclopentanol.

Molecular Solution Behaviour of an Intermediate Biofuel Feedstock: Acetone-Butanol-Ethanol (ABE)

Mixtures of acetone, butanol, and ethanol (ABE) are common intermediate products in the production of biofuels via biomass fermentation. Their separation to yield, for example, bio-butanol, is still difficult due to the lack of a fundamental understanding of these mixtures at the molecular level. In order to bridge this gap, a detailed analysis of characteristic features of the vibrational spectrum is carried out. A systematic study of the binary solutions of acetone with ethanol and butanol does not only reveal a universal behaviour at the molecular level when acetone is mixed with short-chain alcohols, it also shows that the phenomena at a length scale between the molecules and in the macroscopic solution need to be taken into account to understand the structure-property relationships. The size of self-associated molecule clusters seems to determine whether or not a system exhibits an azeotrope. When a second alcohol is added to an acetone/alcohol solution, no additional non-idealities are induced, which is advantageous for modelling ternary ABE mixtures and for improving their processing in the production of biofuels.

Raman bandshape analysis on CH and CSC stretching modes of dimethyl sulfoxide in liquid binary mixture: comparative study with quantum-chemical calculations

The interacting nature of dimethyl sulfoxide (DMSO) in binary mixtures has been carried out on CH and CSC stretching modes of vibration using chloroform (CLF), chloroform-d (CLFd), acetonitrile (ACN) and acetonitrile-d3 (ACNd) solvents. Peak frequencies of both the stretching modes show blue shift with the increase in solvent concentration. Variation of Raman bandwidth with the solvent concentration was discussed using different mechanisms. Ab initio calculation for geometry optimization and vibrational wavenumber calculation have been performed on monomer and dimer structures of DMSO to explain the experimentally observed Raman spectra. Theoretically calculated values are found in good agreement with the experimental results. Vibrational and reorientational relaxation times have been studied corresponding to solvent concentrations to elucidate the interacting mechanisms of binary mixtures.

Solvent dependent frequency shift and Raman noncoincidence effect of S=O stretching mode of Dimethyl sulfoxide in liquid binary mixtures

The isotropic and anisotropic Raman peak frequencies of S=O stretching mode of Dimethyl sulfoxide (DMSO) have been discussed in different chemical and isotopic solvent molecules using different mechanisms. The shifting of peak frequency in further dilution of DMSO with solvent molecule is observed for all solvents. Transition dipole - transition dipole interaction and hydrogen bonding may play a major role in shifting of peak frequencies. The non-coincidence effect (NCE) of DMSO was determined for all the solvents and compared with four theoretical models such as McHale's model, Mirone's modification of McHale's model, Logan's model and Onsager-Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data.

Solvent dependent Raman bandshape analysis on CO containing molecules: vibrational relaxation study

Solvent dependent Raman band studies of some carbonyl containing molecules have been carried out in chlorobenzene (CLB) and chloroform (CLF) solvents. The variation of bandwidth with the solvent concentration was discussed using different mechanisms. The vibrational and reorientational correlation times were studied corresponding to solvent concentrations. The reorientational correlation time follows with the liquid viscosity in molecules under study. It reflects about the nature of solvent molecules hindering to the orientational motion of solute in liquid solution.