J. L. Koenig

Least-Squares Curve-Fitting of Fourier Transform Infrared Spectra with Applications to Polymer Systems

A method is presented for least-squares curve-fitting of Fourier transform infrared spectra. A demonstration of the determination of xylene solution compositions illustrates the accuracy of the method. Least-squares coefficients are shown to be valuable for the analysis of several polymer systems by digital subtraction of spectra.

Structure of silane coupling agents adsorbed on silicon powder

The molecular structure of γ-aminopropyltriethoxysilane (γ-APS) coupling agent was studied on silicon powder by diffuse reflectance Fourier Transform Infrared (FT-IR) spectroscopy. The coupling agents are chemically bound to the surface in two ways; (1) the Si-OH groups forming Si-O-Si bonds and (2) hydrogen bonding through the organofunctional groups of the coupling agent with surface S-OH groups. There are structural differences between the chemisorbed silane molecules (interacting directly with the substrate) and the physisorbed silane molecules. These differences can be summarized as follows: (1) the chemisorbed silanes are bound to the surface through Si-O-SiO 2 bonds, while the physisorbed layers are bound to each other through Si-O-Si bonds; (2) the chemisorbed layers are less condensed than the physisorbed layers; and (3) the amine groups of the chemisorbed layer are hydrogen-bonded to the surface in the NH 3 + form while in the physisorbed layers it is free NH 2 or in an aminebicarbonate salt form NH 3 + (HCO 3 ) − . The formation of the Si-O-SiO 2 bond is responsible for the hydrothermal stability of the γ-APS coupling agent.

FT-IR characterization of the reaction at the silane/matrix resin interphase of composite materials

Abstract FT—IR Spectroscopy was used to characterize the chemical reactions at the silane/matrix resin interphase of a composite material. The system studied was γ-aminopropyltriethoxysilane (γ-APS), a difunctional epoxy resin and E-glass fiber composite. The reactions were studied with and without the glass fiber. The reactivity of the silane coupling agent with the epoxy resin was found to vary with the drying conditions of the silane coupling agent. The reactions between the coupling agent and the epoxy resin were minimized when the silane interphase was most condensed. A bicarbonate salt, which forms with the primary amines of the coupling agent, slightly decreases the reactivity of the coupling agent with the epoxy resin. It is necessary to keep the reaction temperature under 115°C in air to prevent the oxidation of the primary amines to imine groups under atmospheric conditions. The reactivity of the resin with the coupling agent is mainly a function of optimizing their contact or interpenetration. Indirect evidence was also found to support the interpenetrating network theory in that the siloxane linkages are needed to improve the chemical resistance of the crosslinked interphase region of the composite to solvent attack.

Optical Constant Determination of Thin Polymer Films in the Infrared

Thin films of poly(vinyl chloride), poly(methyl methacrylate), and poly(styrene) were analyzed by Fourier transform infrared spectroscopy. The interference fringes present in the transmission spectra of these samples were used to determine film thickness and average refractive index. Subsequent Kramers-Kronig analysis of these transmission spectra provided the dispersion of the refractive index and the absorption index across the entire mid-infrared region. Interference fringes were absent in the optical constant spectra, and good agreement was obtained between our optical constant spectra and those of other authors.

Applications of Diffuse Reflectance FT-IR to the Characterization of an E-Glass Fiber/γ-APS Coupling Agent System:

The utility of diffuse reflectance FT-IR (DRIFT) spectroscopy is explored to study the surfaces of clean E-glass fibers and E-glass/coupling agent systems. The ability to use a nondestructive sampling procedure is an important benefit for the study of these systems. However, the nature of the fiber samples leads to several experimental difficulties. We solve these problems through the use of a KBr overlayer. Using this sample-preparation procedure, we increase the information derivable from the spectra. The inherent surface sensitivity of the DRIFT technique is demonstrated by the study of small amounts of the silane coupling agent γ-aminopropyltriethoxysilane (γ-APS) on the surface of E-glass fibers.

Fast FTIR imaging: A new tool for the study of semicrystalline polymer morphology

The distribution of chemical species and the degree of orientation in semicrystalline polymer systems have been studied using fast Fourier transform infrared (FTIR) imaging. A variety of poly(ethylene glycol) systems, including pure polymer, high and low molecular weight blends, and blends with amorphous polymers, were studied. It is shown that fast FTIR imaging can be used to determine the distribution of species with different molecular weights and can be used to determine the degree of segregation of different components in blends with amorphous polymers. Additionally, by employing an infrared polarizer, the degree of orientation was determined in these systems by the generation of spatially-resolved dichroic ratio images.

Noise Reduction via Factor Analysis in FT-IR Spectra

The application of factor analysis to reduce random noise in FT-IR spectra is described. By utilizing the total information content of all the spectra, it is possible to reduce the amount of random noise present in individual spectrum.

CHARACTERIZING THE PERFORMANCE OF A FAST FT-IR IMAGING SPECTROMETER

The noise sources present in fast FT-IR imaging instrumentation are analyzed in order to ascertain the limits of the quantitative ability of this technique. Methods to increase the quality of spectral and spatial information are presented. It is shown that a technique comparable to the coaddition of multiple mirror scans in standard FT-IR spectroscopy is able to increase the signal-to-noise ratio of the spectral data obtained, and that the expected square root behavior is obeyed. A method of distinguishing real image features from artifacts is also presented.

NMR Imaging of Solvent Diffusion in Polymers

Nuclear magnetic resonance (NMR) imaging is sensitive to the mobile protons of solvent molecules and, as such, is well suited for studying the solvent diffusion in polymers. The fundamentals of NMR imaging are described, providing a platform for application of NMR imaging to diffusion processes in polymers. The constraints on the imaging experiment imposed by the diffusion process are discussed with respect to sample geometry and the rate of diffusion. A fast imaging scheme known as FLASH is described as an alternative method which reduces the influence of the diffusion rate and NMR relaxation parameters. Example images are given with regard to each of the constraints. An example of the successful application of NMR imaging to the study of Case II diffusion in PMMA is also given. The advantages and disadvantages of NMR imaging as applied to diffusion in polymers, as well as examples of the diffusion process that NMR imaging is uniquely qualified to study, are given in the conclusion.

Studying anomalous diffusion in a liquid crystal/ polymer system using fast FTIR imaging

The diffusion of liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) into a poly(butyl methacrylate) (PBMA) matrix has been studied using fast FTIR imaging. The concentration profiles were obtained as a function of time at several temperatures above and below the nematic to isotropic transition temperature (TNI) of 5CB and the Tg of PBMA. The time-dependent progression of the diffusion front position, when fitted to a power law model, exhibited exponential values between 0.40 and 0.52. This, along with the observed progression of a sharp 5CB-diffusion front into the PBMA matrix, indicates the presence of an anomalous diffusion process. It was shown that fast FTIR was able to correctly identify the diffusion process as anomalous, whereas a simple mass uptake analysis would have led to the conclusion that the process proceeded according to Ficks second law.