Showing papers by "Peter R. Griffiths published in 2003"

Deformation and Relaxation of Polymers Studied by Ultrarapid Scanning FT-IR Spectrometry

Abstract: A recently developed ultrarapid scanning Fourier transform infrared (URS-FTIR) spec- trometer has been used to study the irreversible deformation of polymer films with a millisecond time resolution for the first time. The evolution of molecular orientation as a function of draw ratio and relaxation time was studied for films of amorphous poly(ethylene terephthalate) (PET) stretched above its glass transition temperature (Tg). Very good agreement was obtained between the results obtained by URS-FTIR and polarization modulation infrared linear dichroism (PM-IRLD) spectrometry. Reversible gauche-to-trans conversions were observed, indicating that the PET chains remain amorphous. The orientation and relaxation of polystyrene (PS) in films of pure PS and of blends of PS with poly(vinyl methyl ether) (PVME) were also studied above Tg. A method allowing the determination of the orientation function of PS using a single p-polarized spectrum is described. Results reveal a significant decrease in PS orientation during the first second following the end of deformation, emphasizing the importance of the experimental time resolution in the characterization of the relaxation behavior of polymers.

Accurate Wavelength Measurements of a Putative Standard for Near-Infrared Diffuse Reflection Spectrometry

Abstract: The diffuse reflection (DR) spectrum of a sample consisting of a mixture of rare earth oxides and talc was measured at 2 cm-1 resolution, using five different accessories installed on five different Fourier transform near-infrared (FT-NIR) spectrometers from four manufacturers. Peak positions for 37 peaks were determined using two peak-picking algorithms: center-of-mass and polynomial fitting. The wavenumber of the band center reported by either of these techniques was sensitive to the slope of the baseline, and so the baseline of the spectra was corrected using either a polynomial fit or conversion to the second derivative. Significantly different results were obtained with one combination of spectrometer and accessory than the others. Apparently, the beam path through the interferometer and DR accessory was different for this accessory than for any of the other measurements, causing a severe degradation of the resolution. Spectra measured on this instrument were removed as outliers. For measurements made on FT-NIR spectrometers, it is shown that it is important to check the resolution at which the spectrum has been measured using lines in the vibration-rotation spectrum of atmospheric water vapor and to specify the peak-picking and baseline-correction algorithms that are used to process the measured spectra. The variance between the results given by the four different methods of peak-picking and baseline correction was substantially larger than the variance between the remaining five measurements. Certain bands were found to be more suitable than others for use as wavelength standards. A band at 5943.13 cm-1 (1682.62 nm) was found to be the most stable band between the four methods and the six measurements. A band at 5177.04 cm-1 (1931.61 nm) has the highest precision between different measurements when polynomial baseline correction and polynomial peak-picking algorithms are used.

Determination of the resolution of a multichannel Raman spectrometer using Fourier transform Raman spectra.

Abstract: The resolution of a grating polychromator for Raman spectroscopy has been simulated by measuring spectra on a Fourier transform (FT) Raman spectrometer and selecting the FT of the apodization function so that the instrument line shape function mimics the triangular spectral slit function of the polychromator. To this end, FT-Raman spectra measured with a nominal resolution of 0.5 cm-1 were modified through the application of sinc2 apodization functions of various widths to simulate spectra measured on a polychromator at lower resolution. The success of this approach was validated using the 1085 cm-1 band of calcite. When the modified FT-Raman spectra were compared with spectra measured on a grating polychromator equipped with slits of widths 100 and 150 microns, the resolution of the polychromator was estimated to be 6.3 and 7.8 cm-1, respectively. This conclusion was verified experimentally by measuring the separation of two bands in the Raman spectrum of BaSO4 at approximately 460 cm-1.

Effect of resolution on the wavelength determination of a putative standard to be used for near infrared diffuse reflection spectra measured on grating spectrometers

Abstract: Diffuse reflection (DR) Fourier transform near infrared spectra of a powdered mixture of Er 2 O 3 , Dy 2 O 3 , Ho 2 O 3 and talc were measured at a constant resolution of at least 2 cm - 1 on four different combinations of spectrometers and sampling accessories. The wavenumber scale of each of these spectra was corrected with lines in the vibration-rotation spectrum of water vapour so that the accuracy was better than 0.02 cm - 1 . DR spectra of the powdered sample were then calculated at constant-wavelength resolution to simulate spectra that would have been measured on a grating monochromator. The precision and accuracy of the bands in the spectrum of this sample were then estimated. It is believed that the accuracy of the reported positions of the bands in the spectrum of this putative wavelength standard is about ten times better than the corresponding values in the DR spectrum of a mixture of Er 2 O 3 , Dy 2 O 3 and HO 2 O 3 that had previously been reported by the US National Institute of Standards and Technology.

Effect of resolution on the wavenumber determination of a putative standard to be used for near infrared diffuse reflection spectra measured on Fourier transform near infrared spectrometers

Abstract: A mixture of powdered Er2O3, Dy2O3, Ho2O3 and talc has been proposed as a wavenumber standard for near infrared (NIR) diffuse reflection (DR) measurements. Previous measurements were made at a resolution of 2 cm−1 which is significantly less than the full-width at half-height of the peaks in the spectrum. Since many NIR DR spectra are measured at a much lower resolution, some of the peaks are observed to fuse and/or shift. In this paper, the behaviour of the band positions is studied as a function of resolution. The factors that allow some of the peaks to be useful as wavenumber standards for NIR DR measurements while others should not be used are discussed. The instrument should first be calibrated with certain lines in the vibration–rotation spectrum of water vapour. Because most of the lines in the water spectrum are located less than 1 cm−1 from their nearest neighbours, only two of the 3768 lines in the region between 5750 and 7990 cm−1 can be used for calibration and only then if the spectrum is con...

Axicon as a retroreflector in open-path Fourier transform infrared spectrometry

Abstract: Ray-tracing analysis of a particular reflecting axicon, a right circular cone, for use as a retroreflector in active open-path Fourier- transform IR (OP/FT-IR) spectrometry, and the results of testing a 0.305-m aperture right circular cone using a commercial active OP/FT-IR spectrometer are presented. The ray-tracing model is based on the op- tical characteristics of a commercial single-telescope monostatic OP/ FT-IR spectrometer and models the off-axis behavior normally encoun- tered under practical field conditions during field use. Of practical concern are the trends for the diameter of the beam reflected from the retroreflector as a function of the path length between spectrometer and retroreflector and misalignment of the retroreflector with respect to the transmitted beam. Construction and the results of field-testing two smaller epoxy-composite axicons replicated from the 0.305-m master are presented. © 2003 Society of Photo-Optical Instrumentation Engineers.