Surface-sensitive molecular information on polymers from UV-resonance Raman spectroscopy

Ultraviolet resonance Raman spectroscopy (UV-RRS) has been applied to a series of samples of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) and poly(ethylene terephthalate) (PET) of varying thickness. The spectra demonstrate that under resonance conditions, when absorption is very strong, only a very thin top layer of the sample is probed (hundreds of nanometers range). This allows probing molecular vibrational spectra of the top layer of the sample, with a surface-resolution at least an order of magnitude better than in the case of normal non-resonance Raman spectroscopy and using a microscope.     

Probing the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy

A rational approach to investigate the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy (GERS) is developed. A planar molecule, copper phthalocyanine (CuPc), is used as probe molecule. Annealing allows the CuPc molecule in a Langmuir-Blodgett film to change orientation from upstanding to lying down. The UV-visible absorption spectra prove the change of the molecular orientation, as well as the variation of the interaction between the CuPc molecule and graphene. The Raman spectra of the molecule in the two different orientations are compared and analyzed. The results show that chemical enhancement is highly sensitive to the molecular orientation. The different molecular orientations induce different magnitudes of the interaction between the molecule and graphene. The stronger the interaction, the more the Raman signal is enhanced. Furthermore, the sensitivity of GERS to molecular orientation is promising to determine the orientation of the molecule on graphene. Based on this molecular orientation sensitive Raman enhancement, quantitative calculation of the magnitude of the chemical enhancement is implemented for a series of Pc derivatives. It shows that the magnitude of the chemical enhancement can be used to evaluate the degree of interaction between the molecules and graphene. Moreover, an understanding of the chemical enhancement in GERS is promoted and the effect of molecular orientation on the intensity of chemical enhancement is explained.     

Orientation of carbon nanotubes in polymers and its detection by Raman spectroscopy

Small amounts of single-wall carbon nanotubes embedded in a polymer matrix were used to sense the mechanical response of the polymer using microRaman spectral data. A flow orientation method was applied to align the nanotubes in the matrix. The Raman spectra obtained for specimens cut both parallel and perpendicular to the flow direction were found to be significantly different, as a function of mechanical strain. Thus Raman spectroscopy combined with mechanical testing provides a way to probe the alignment of nanotubes in composites. The Raman shift-strain response for samples loaded perpendicular to the flow direction suggests that nanotube reorientation is achieved upon straining the polymer beyond its yield point. Our data suggest that the adhesion between the nanotubes and the polymer exceeds the shear yield strength of the matrix. We show that a stress–strain curve for the polymer may be produced directly by means of Raman spectroscopy.     

Determination of the molecular orientation of poly(propylene terephthalate) fibers using polarized Raman spectroscopy: a comparison of methods

For the first time, four different methods to determine the degree of molecular orientation from polarized Raman spectroscopy measurements are compared. The great influence of molecular orientation on the properties of polymers has driven the development of multiple experimental techniques and procedures. This study is based on the C(1)-C(4) ring stretching vibration of poly(propylene terephthalate) (PPT) at 1614 cm(-1). It is shown that simply ratioing the band intensity obtained with the polarization parallel and perpendicular to the unique axis of the sample provides a good qualitative method to observe the evolution of orientation in a series of similar samples. To quantitatively compare the degree of orientation one needs to utilize a more complex method yielding the second- and fourth-order parameters of the orientation distribution function (P(2) and P(4), respectively). To date, most studies have been based on the assumption of a cylindrically symmetric polarizibility tensor. It is shown that this assumption is highly questionable although this method has been used fairly successfully in the past. This method results in orientation parameters that are clearly different from those obtained with the two more complex procedures. The most complex method, both theoretically and experimentally, requires the most measurements per sample. Major problems have occurred when trying to calculate the desired parameters, in particular for samples with high birefringence. These problems are related to experimental complexities occurring for measurements when the samples are tilted with respect to the polarization direction of the incident light. These measurements are replaced by a simple determination of depolarization ratio in the third method. This method assumes that the depolarization ratio is independent of changes in molecular orientation and structure. It was found that this assumption is not correct. Thus, the most complex method is the method of choice to quantitatively determine the second- and fourth-order parameters of the orientation distribution function, unless one has knowledge of the depolarization ratio of each sample being studied. That knowledge permits the use of an experimentally simpler method to obtain the desired parameters.     

On confocal Raman spectroscopy of semicrystalline polymers: the effect of optical scattering

A series of semicrystalline polymers has been prepared through morphological control. Each of these has an identical refractive index but a different, well-defined, scattering behavior. From existing geometrical optical theories of confocal Raman spectroscopy, these materials should behave identically. Initially, the extent of scattering in each system was assessed quantitatively, from the near-infrared through the visible wavelength range, by UV/visible spectroscopy. The effect of optical scattering on the variation of intensity of the Raman scattered radiation with subsurface position was then examined in all four materials; the effect of surface roughness was also considered in the highest clarity system. Where surface effects are removed through careful sample preparation and the materials are interrogated using identical optical systems to mitigate against the impact of refractive index mismatch and other optical effects, the Raman response is strongly affected by the scattering characteristics of each material. A simple empirical relationship has been determined that adequately described all our specimens.     

Near-infrared spectroscopy for cocrystal screening. A comparative study with Raman spectroscopy

Near-infrared (NIR) spectroscopy is a well-established technique for solid-state analysis, providing fast, noninvasive measurements. The use of NIR spectroscopy for polymorph screening and the associated advantages have recently been demonstrated. The objective of this work was to evaluate the analytical potential of NIR spectroscopy for cocrystal screening using Raman spectroscopy as a comparative method. Indomethacin was used as the parent molecule, while saccharin and l-aspartic acid were chosen as guest molecules. Molar ratios of 1:1 for each system were subjected to two types of preparative methods. In the case of saccharin, liquid-assisted cogrinding as well as cocrystallization from solution resulted in a stable 1:1 cocrystalline phase termed IND-SAC cocrystal. For l-aspartic acid, the solution-based method resulted in a polymorphic transition of indomethacin into the metastable alpha form retained in a physical mixture with the guest molecule, while liquid-assisted cogrinding did not induce any changes in the crystal lattice. The good chemical peak selectivity of Raman spectroscopy allowed a straightforward interpretation of sample data by analyzing peak positions and comparing to those of pure references. In addition, Raman spectroscopy provided additional information on the crystal structure of the IND-SAC cocrystal. The broad spectral line shapes of NIR spectra make visual interpretation of the spectra difficult, and consequently, multivariate modeling by principal component analysis (PCA) was applied. Successful use of NIR/PCA was possible only through the inclusion of a set of reference mixtures of parent and guest molecules representing possible solid-state outcomes from the cocrystal screening. The practical hurdle related to the need for reference mixtures seems to restrict the applicability of NIR spectroscopy in cocrystal screening.     

Micro-Raman spectroscopy study of the process of microindentation in polymers

The residual deformation produced by Vickers microindentations in poly(3,3-dimethyl oxetane) (PDMO) was analysed by Raman microspectroscopy. Microstructural variations were observed as the Raman spectra were recorded at different positions along and out of the microindentation. A gradual distribution of the monoclinic and orthorhombic structures arises as the spectra were scanned from the centre to the edge of the impression area. This crystalline variation in PDMO, which reflects the degree of stress present on the surface of the sample, was utilized to obtain information on the distribution of pressure in and around the microindentation. In addition to the clear variation of pressure inside the permanently deformed region, no crystalline transformation was noted out of the impression area. Similar features were observed when the Raman spectra were recorded along one of the diagonals of the indentation.     

A combined molecular dynamics and Raman spectroscopy approach for designing ice-metal interfaces

Summary Understanding the structure and interatomic interactions of an ice-metal interface plays a fundamental role in the design of deicing coatings. This is demonstrated by a novel approach, combining vibrational results from laser Raman spectroscopy with molecular dynamics simulations to obtain insights into icing on solids which, in turn, lead to design criteria for minimizing adhesion. An atomistic model of ice-copper interaction is constructed based on electronic structure calculations and used to show that reasonable molecular geometry and binding energy at the interface can be obtained. Through molecular dynamics simulations we find that the ice layer adjacent to the copper surface is structurally more disordered than the layers further away, a result which is verified by the Raman spectra of vibrational frequencies. The primary adhesive bond is made by the adsorption of oxygen atoms at the lattice sites of the metal substrate. The information obtained by Raman spectroscopy and molecular dynamics is then exploited to arrive at specific recommendations for designing polymeric deicing coatings and materials.     

氧化锌薄膜的拉曼光谱研究

利用拉曼光谱结合X射线衍射分析对未掺杂和掺杂的ZnO薄膜，陶瓷薄膜进行了研究，ZnO薄膜及ZnO陶瓷薄膜均由sol-gel法制备，掺杂组份有Bi2O3,Sb2O3,MnO和Cr2O3等。结果表明，未掺杂的薄膜的ZnO主晶相均表现出显著的定向生长特征，其拉曼光谱特征谱峰为437cm^-1，谱峰强度随薄膜退火温度的提高略有增强，掺杂后ZnO的拉曼谱峰发生了红移，掺Bi2O3后ZnO的拉曼谱峰由347cm^-1移质移至434cm^-1，掺Sb2O3后ZnO的拉曼谱峰移至435cm^-1，而掺杂Bi2O3,Sb2O3,MnO和Cr2O3等组份的ZnO陶瓷薄膜的ZnO拉曼谱峰则移至434cm^-1，说明掺杂元素进入了ZnO晶格，引起了晶格的变化，ZnO薄膜性能不仅受次晶相组成的影响，而且受因掺杂元素进入而引起的ZnO晶格畸变的影响。     

Raman spectroscopy of crystals for stimulated Raman scattering

Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material.OCIS: 300.6450; 290.5910; 190.2640     

Simultaneous multianalyte identification of molecular species involved in terrorism using Raman spectroscopy

Raman spectroscopy is a form of vibrational spectroscopy that is well suited to the molecular identification of a variety of analytes, including both explosives and biological agents. The technique has been gaining more widespread interest due to improvements in instrumentation, sensitivity, and its ease of use, in comparison to other techniques. In this paper, we describe recent advances in Raman spectroscopy with respect to the detection of high-energy explosives and biological materials. In particular, emphasis is placed on the exploitation of enhancement factors that overcome traditional limitations on sensitivity, namely, surface enhancement and resonance enhancement, functionalization of target analytes, and the use of novel lab-on-a-chip technology.     

Fluorocarbon fiber-optic Raman probe for non-invasive Raman spectroscopy

We report the development of a novel fiber-optic Raman probe using a graded index fluorocarbon optical fiber. The fluorocarbon fiber has a simple Raman spectrum, a low fluorescence background, and generates a Raman signal that in turbid media serves as an intense reference Raman signal that corrects for albedo. The intensity of the reference signal can easily be varied as needed by scaling the length of the excitation fiber. Additionally, the fluorocarbon probe eliminates the broad silica Raman bands generated in conventional silica-core fiber without the need for filters.     

Uniplanar-axial orientation in hot-rolled polymers

The orientations produced by high temperature rolling of polyoxymethylene (126 C) and polytetrafluoroethylene (150 C) and nylon 66 are examined by pole figures. No evidence is found to support the theory of Akahane and Mochizuki that it is the plane of the zig-zag chains that orientates into the rolling plane in nylon 66. The classical rolling texture of Bunn and Garner describes the pole figures except for the (100) pole, which is observed at 28 to the transverse direction; in the texture of Bunn and Garner it should occur at 24 . In hot-rolled polyoxymethylene the texture observed corresponds to a unique orientation of the hexagonal unit cell: (10¯10) planes parallel to the rolling plane and c-axis parallel to rolling direction. The polyoxymethylene texture differs significantly from the pseudo-fibre textures observed in the cold-rolled polymer. The rolling texture of hot-rolled polytetrafluoroethylene is similar to that of hot-rolled polyoxymethylene. These results show the plastic slip system in polyoxymethylene and polytetrafluoroethylene to be (10¯10) [0001].     

Comparative spontaneous Raman spectroscopy of crystals for Raman lasers

A comparison of the spectroscopic parameters of Raman-active vibronic modes in various crystalline materials with a view to the use of these crystals for stimulated Raman scattering (SRS) is presented. It includes data on the Raman frequency shift, linewidth, integral, and peak Raman scattering cross sections. For steady-state SRS the highest Raman gain coefficient has been proved to be in barium nitrate and sodium nitrate crystals; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium tungstate and strontium tungstate are proposed as new highly efficient Raman materials for both SRS cases.     

Terahertz spectroscopy to study the orientation of glass fibres in reinforced plastics

We employ terahertz time-domain spectroscopy (THz TDS), a novel, non-destructive testing method, to study the fibre orientation and fibre content in reinforced plastics. The birefringent properties of plastics filled with differing amounts of short glass fibres are measured at frequencies from 100 GHz up to 1 THz. To predict the permittivity of the experimentally examined composite materials, we use an effective medium theory first introduced by Polder and van Santen. On the basis of the measured data and this model, we deduce the additive content ξ, the preferential orientation of the fibres φ and the fraction of orientated fibres a. Our findings agree well with corresponding mold flow simulations performed with commercially available software.     

Device for Raman difference spectroscopy

A new layout of a versatile Raman difference setup is presented. The new device combines the advantages of the rotating cell for exploitation of the resonance Raman enhancement and the high precision of Raman difference spectroscopy together with the multiplex advantages and very high quantum efficiency offered by a CCD detector. While Raman difference spectroscopy is the most accurate method for the detection of very small band shifts, the method requires the strict prevention of any environmental perturbation of one of the two spectra, which are used for the difference spectra calculation. The presented device satisfies this requirement by implementing a double-beam layout, where the simultaneously detected Raman signals of two sample cells are combined within a Y-fiber bundle and imaged together onto the CCD detector. The accuracy of the new apparatus in detecting frequency shifts and minor sample components is greatly increased compared to conventional Raman spectroscopy as shown by studying binary mixtures of CHCl3 and CCl4. Hereby it was possible to resolve a formerly undetected shift of <0.02 cm(-1) of the CCl4 band at 218 cm(-1). The new RDS setup has a very versatile design. The device can take advantage of the high sensitivity and selectivity of the resonance Raman enhancement applying excitation wavelengths from the UV to NIR and can be used for a variety of samples with only minor changes in the optical arrangement. The new device will be of utmost importance for a fast, gentle, sensitive, selective, and precise investigation of biomolecules and their interactions. Some first results are shown concerning the interaction of the antimalarial chloroquine with hematin in a hydrous environment.     

In vivo determination of the molecular composition of artery wall by intravascular Raman spectroscopy

Atherosclerotic plaque vulnerability is suggested to be determined by its chemical composition. However, at present there are no in vivo techniques available that can adequately type atherosclerotic plaques in terms of chemical composition. Previous in vitro experiments have shown that Raman spectroscopy can provide such information in great detail. Here we present the results of in vitro and in vivo intravascular Raman spectroscopic experiments, in which dedicated, miniaturized fiber-optic probes were used to illuminate the blood vessel wall and to collect Raman scattered light. The results make clear that an important hurdle to clinical application of Raman spectroscopy in atherosclerosis has been overcome, namely, the ability to obtain in vivo intravascular Raman spectra of high quality. Of equal importance is the finding that the in vivo intravascular Raman signal obtained from a blood vessel is a simple summation of signal contributions of the blood vessel wall and of blood. It means that detailed information about the chemical composition of a blood vessel wall can be obtained by adapting a multiple least-squares fitting method, which was developed previously for the analysis of in vitro spectra, to account for signal contributions of blood.