Impulse-induced compression rheo-optics study of polymers using attenuated total reflection based step-scan Fourier transform infrared time-resolved spectroscopy

An impulse-induced attenuated total reflection (ATR) based dynamic compression step-scan time-resolved Fourier transform rheo-optical system has been developed. This system was used to observe different viscoelastic properties of poly(ethylene terephthalate) (PET), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx), and carbon-black-filled polyester-polyamide blend. In the case of PET, almost no viscoelastic response extending beyond 15 ms was observed in the dynamic absorbance difference time domain spectrum. In contrast, PHBHx showed apparently different viscoelastic responses in the dynamic absorbance difference spectrum, especially in the C=O stretching band region. A long relaxation tail of the 1723 cm(-1) band lasting about 2.7 milliseconds was clearly seen. The tail corresponds to the structural or morphological reorganization of a less ordered crystalline form (Type II) under compressive perturbation. The carbon-black-filled polyester-polyamide blend film also shows different viscoelastic response tails. In this case, the amide C=O stretching vibration band does not show distinct viscoelastic responses, suggesting that the polyamide component does not contribute much to the viscoelastic properties. The present method shows promise for characterizing a wide variety of viscoelastic materials, including polymer alloys, blends, composites, copolymers, and semicrystalline polymers.     

Detection of reversible nonlinear dynamic responses of polymer films by using time-resolved soft-pulse compression attenuated total reflection step-scan Fourier transform infrared spectroscopy

An improved time-resolved soft-pulse dynamic compression attenuated total reflection (ATR) step-scan Fourier transform rheo-optical system has been developed. This system was used to observe reversible dynamic responses of poly(ethyleneterephthalate) (PET) and poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) films. In the case of PET, reversible nonlinear dynamic responses were observed in the C=O stretching vibration. The nonlinear responses decreased with decreasing compressive strain from 0.045 to 0.018. For the C-O stretching bands associated with the backbone structure of the PET, the nonlinear responses were very small. Characteristic burst-like reversible nonlinear dynamic responses can be seen in the in-phase and out-of-phase C=O stretching vibrations of cyclic imides, and phenyl ring deformation bands in the PDA parts of the BPDA-PDA. The results suggest the presence of inter-molecular interaction between C=O of cyclic imides and the phenyl ring groups of the PDA parts. The present method shows promise for characterizing a wide variety of polymeric materials, including polymer alloys, blends, composites, and copolymers and semicrystalline polymers.     

Two-dimensional correlation analysis of polyimide films using attenuated total reflection-based dynamic compression modulation step-scan Fourier transform infrared spectroscopy

Attenuated total reflection (ATR)-based dynamic compression modulation two-dimensional (2D) correlation study of poly(p-phenylene biphenyltetracarboximide) film is carried out in combination with spectral simulation analysis by density functional theory (DFT). The dynamic 2D infrared (IR) correlation spectra in the region of imide I (C=O stretching mode) show three distinct correlation peaks located around 1777, 1725, and 1708 cm(-1). The band at 1708 cm(-1) is the lower wavenumber shift component of 1777 or 1735 cm(-1) peaks and is attributed to the results from intermolecular interactions, according to the DFT analysis. The 1708 cm(-1) band also shows the largest dynamic response, suggesting that these intermolecular interactions may enhance the dynamic response. The dynamic 2D IR correlation spectra in the region of imide II (C-N-C axial stretching mode) vibrations also show three correlation peaks located around 1335, 1355, and 1370 cm(-1), although the imide II band is shown to consist substantially of one component by the DFT analysis. These multiple peaks may be attributed to the compression-induced wavenumber shift of the band in the backbone structures. The sequential analysis of 2D correlation data show that, upon applying the dynamic compression, the response of the backbone regions (imide II) occurs first, followed by that of the side-chain regions (imide I, C=O).     

Characterization of infrared matrix-assisted laser desorption ionization samples by Fourier transform infrared attenuated total reflection spectroscopy

Fourier transform infrared attenuated total reflection (FT-IR ATR) spectroscopy was used to characterize thin films of succinic acid, a matrix compound commonly used with infrared matrix-assisted laser desorption ionization (IR-MALDI) mass spectrometry. IR spectra of succinic acid thin films deposited alone and in combination with the analyte biomolecules insulin and cytochrome c were obtained by FT-IR ATR spectroscopy. Spectra of analyte and matrix alone were similar to those obtained previously from KBr pellets, Nujol mull, or thin-film absorption, although the ATR spectra have significantly lower background interferences. Thin films deposited from mixtures of water and methanol have additional peaks compared to films deposited from a methanol solution. These additional peaks are attributed to carboxylate groups stabilized by residual water molecules. No evidence was found to suggest that residual water absorption contributes to absorption at wavelengths typically used for IR-MALDI. Absorption of energy by analyte vibrational modes with rapid energy transfer to the matrix is suggested as a contributor to desorption and ionization consistent with the FT-IR ATR results.     

Single-pass attenuated total reflection Fourier transform infrared spectroscopy for the prediction of protein secondary structure

Principal component regression (PCR) was applied to a spectral library of proteins in H2O solution acquired by single-pass attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy. PCR was used to predict the secondary structure content, principally alpha-helical and the beta-sheet content, of proteins within a spectral library. Quantitation of protein secondary structure content was performed as a proof of principle that use of single-pass ATR-FT-IR is an appropriate method for protein secondary structure analysis. The ATR-FT-IR method permits acquisition of the entire spectral range from 700 to 3900 cm(-1) without significant interference from water bands. An "inside model space" bootstrap and a genetic algorithm (GA) were used to improve prediction results. Specifically, the bootstrap was utilized to increase the number of replicates for adequate training and validation of the PCR model. The GA was used to optimize PCR parameters, particularly wavenumber selection. The use of the bootstrap allowed for adequate representation of variability in the amide A, amide B, and C-H stretching regions due to differing levels of sample hydration. Implementation of the bootstrap improved the robustness of the PCR models significantly; however, the use of a GA only slightly improved prediction results. Two spectral libraries are presented where one was better suited for beta-sheet content prediction and the other for alpha-helix content prediction. The GA-optimized PCR method for alpha-helix content prediction utilized 120 wavenumbers within the amide I, II, A, B, and IV and the C-H stretching regions and 18 factors. For beta-sheet content predictions, 580 wavenumbers within the amide I, II, A, and B and the C-H stretching regions and 18 factors were used. The validation results using these two methods yielded an average absolute error of 1.7% for alpha-helix content prediction and an average absolute error of 2.3% for beta-sheet content prediction. After the PCR models were developed and validated, they were used to predict the alpha-helix and beta-sheet content of two unknowns, casein and immunoglobulin G.     

Fourier transform infrared attenuated total reflection and transmission spectra studied by dispersion analysis

Fourier transform infrared transmission (FT-IR) and attenuated total reflection (ATR) spectra of water-ethanol mixtures are recorded and reconstructed thanks to a causal dispersion analysis technique. As expected, the Beer's law technique is an empirical approximate method that cannot account for complex spectral features. On the other hand, a rigorous analysis performed by using the theoretical optical paths for both experimental techniques and Gaussian dispersion analysis (GDA) allows the dielectric functions of the pure liquids to be calculated. Simulations of the whole mid-infrared spectra in the range 500-4000 cm(-1) match the experimental data very well, whatever the water-ethanol mixtures. This method is a powerful tool to quantify such model mixtures and more generally could be the first step toward software for assistance to the FT-IR spectrum analysis.     

Discrimination of soil-borne fungi using fourier transform infrared attenuated total reflection spectroscopy

Fourier transform infrared (FT-IR) attenuated total reflection (ATR) spectroscopy was used to discriminate five commonly encountered soil-borne fungi that cause severe economic damage to agriculture: Colletotrichum, Fusarium, Pythium, Rhizoctonia, and Verticillium. Contrary to previous studies related to microorganism discrimination using FT-IR-ATR spectroscopy, the pathogen samples were not dried on the ATR crystal, which is a time-consuming operation. Rather, after removing some pathogen filaments from the solution using tweezers, these were placed directly on a flat ATR crystal and pressure was applied using a pressure clamp. Following water subtraction, baseline correction, and normalization of the spectra, principal component analysis was used as a data-reduction step and canonical variate analysis was used for discrimination. Discrimination was performed at the genus level and at the strain level for Colletotrichum. For discrimination between the five fungi at the genus level, the success rate for the validation samples ranged from 75% to 89%. For discrimination between the two Colletotrichum strains, the success rate was 78%. Comparison with spectra of similar fungi dried on the ATR crystal showed that both types of spectra were very similar, indicating that drying the samples on the ATR crystal is not required and can be replaced by mathematical post-processing of the spectra. For routine analyses that involve rapid screening of very large amounts of samples, this approach allows for increasing significantly the number of samples that can be analyzed daily.     

Single-pass attenuated total reflection Fourier transform infrared spectroscopy for the analysis of proteins in H2O solution

The application of single-pass attenuated total reflection Fourier transform infrared (ATR-FT-IR) microscopy was investigated for secondary structure analysis of 15 representative proteins in H2O solution. This is the first reported application of single-pass ATR-FT-IR for protein analysis; thus, the method was validated using transmission FT-IR and multipass ATR-FT-IR as referee methods. The single-pass ATR-FT-IR technique was advantageous since the single-pass geometry permits rapid secondary structure analysis on small volumes of protein in H2O solution without the use of demountable thin path length sample cells. Moreover, the fact that H2O backgrounds were small allowed the simultaneous observation of the amide I-III, A, and B regions without having to perform H2O subtraction. A comparison of replicate protein spectra indicated that the single-pass ATR-FT-IR method yields more reproducible data than those acquired by transmission FT-IR. The observed trends for the amide I-III and A bands obtained by single-pass ATR-FT-IR agreed with those in the literature for conventional transmission FT-IR.     

Two-dimensional correlation study of uniaxially drawn poly(ethylene terephthalate) films by using attenuated total reflection based dynamic compression modulation step-scan fourier transform infrared in combination with spectral simulation analysis by density functional theory

Attenuated total reflection (ATR) based dynamic compression modulation two-dimensional (2D) correlation studies of uniaxially drawn poly(ethylene terephthalate) (PET) films have been performed in combination with spectral simulation analysis by density functional theory (DFT). The dynamic 2D infrared (IR) correlation spectra in the region of the CCO stretching mode vibrations show four distinct correlation peaks located around 1290, 1265, 1248, and 1234 cm(-1). These bands can be clearly assigned to the combination bands or coupling modes of the CH in-plane bend of the benzene ring or the CH(2) deformation of the ethylene glycol unit, as well as CCO stretching vibrations, which are gauche conformer's characteristic bands, by DFT analysis. The sequential analysis of 2D correlation data shows that, upon applying the dynamic compression, the response of the side chain regions (ester groups) occurs first, followed by that of the backbone regions (benzene rings). The ATR based dynamic compression modulation 2D correlation spectroscopy in combination with DFT analysis can be a powerful tool for various polymer characterizations.     

Analysis of fiber blends using horizontal attenuated total reflection Fourier transform infrared and discriminant analysis

We have investigated the utility of a horizontal attenuated total reflection Fourier transform infrared spectrometer (HATR/FT-IR) for the analysis of fiber and textile blends. The identification of a blended textile can be accomplished by subtracting a reference spectrum of the textile's most abundant component, leading to a difference spectrum that infers the identity of the second constituent of the blended textile. Mathematical post-processing of the spectra employing discriminant analysis provided a useful statistical tool to confirm the fiber blend components.     

Thickness measurement of nanoscale polymer layer on polymer substrates by attenuated total reflection infrared spectroscopy

For the first time, attenuated total reflection (ATR)-Fourier transform infrared (FT-IR) spectroscopy was utilized to measure the thickness (d0) of a nanoscale polymer layer on polymer substrate with significant credibility. First, a mathematical formula, A/A0 = 1 - 2d0/ d(p), was derived based on a self-defining subsection function (where d(p) was defined as depth of penetration of ATR and A and A0 were defined as the absorption band area of the characteristic functional group only contained in bulk substrate with a thin polymer layer attachment and the same group in blank substrate, respectively). On the mathematical model, through changing incidence angles, a series of values of A (A0) and corresponding d(p) were obtained, and when plotting A/A0 versus 2/d(p), d0 was obtained as the slope. With polystyrene (coating)/olypropylene (substrate) as a model system, we obtained the relevant values (d0). Comparing the results with the values of practical coating thickness (calculation and TEM observation), we found that this method was able to characterize well the thickness of a thin polymer layer on a polymer substrate in the range from 10 to 110 nm. Errors in the measurement were given and analyzed. Furthermore, this method was well applied in the thickness measurement of a polyacrylamide graft layer on a polypropylene film surface. The effect of pressure in the ATR technique on the coating thickness measurement was also discussed. In comparison with other methods such as XPS, SEM, TEM, and AFM, this approach based on a universal ATR technique was very convenient and fast. This method is expected to widen the application of the ATR-FT-IR technique and stimulate the further development of many fields such as surface self-assembly and surface functionlization.     

Attenuated total reflection Fourier transform infrared spectroscopy analysis of human hair fiber structure

The structure of human hair was studied by attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy. The use of Ge, ZnSe, and Si internal reflection elements, various incident light angles, and difference spectra allowed detailed analysis of the cuticle, cortex, and cuticle-cortex intercellular regions without physically or mechanically removing the cuticle of the hair fiber. The ATR-FT-IR data showed the cuticle to be composed of protein having predominantly beta-sheet and disorder and beta-turn configurations. In contrast, the cortex spectra showed alpha-helical structures due to the presence of intermediate filaments of alpha keratin plus beta-sheet, beta-turn and disorder structures. In the cuticle-cortex interface region the protein structures were primarily disorder and beta-turn with small amounts of beta-sheet configurations. The spectral analyses are consistent with the general information on hair fiber structure proposed in the literature.     

Novel attenuated total reflection Fourier transform infrared microscopy using a gem quality diamond as an internal reflection element

A novel technique for attenuated total reflection Fourier transform infrared (ATR FT-IR) spectral acquisition by an infrared microscope with a gem-quality faceted diamond as an internal reflection element (IRE) is introduced. Unlike conventional IREs, the novel diamond IRE has a sharp tip configuration instead of a flat tip configuration. Light at normal incidence was coupled into the diamond while the transflected radiation from the diamond was collected through the table facet by the built-in 15x Cassegrainian objective. The number of reflections in the novel diamond IRE equals two. The evanescent field generated under total internal reflection at the pavilion facet was exploited for ATR spectral acquisition of materials attached to the IRE. The observed ATR spectra were compared to those obtained via a traditional zinc selenide IRE.     

Rapid nondestructive on-site screening of methylamphetamine seizures by attenuated total reflection fourier transform infrared spectroscopy

The identification and quantification of illicit substances in the field is often desirable. Fourier transform infrared spectroscopy (FT-IR) has both qualitative and quantitative capabilities and field portable instruments are commercially available. Transmission infrared spectra of mixtures containing ephedrine hydrochloride, glucose, and caffeine and attenuated total reflection (ATR) infrared spectra of mixtures composed of methylamphetamine hydrochloride, glucose, and caffeine were used to develop principal component regression (PCR) calibration models. The root mean sum of errors of predictions (RMSEP) of all individual components in a mixture from a single measurement was <6% w/w, which reduced to approximately 3% w/w when triplicates were averaged. Sample mixing and grinding are essential to minimize the effect of heterogeneity, as deviations of up to 20% w/w were observed for single measurements of unground samples. Poor predictions of the components in a mixture occurred when samples were "contaminated" with substances not present in the calibration set, as would be expected. When only a single analyte (drug) was targeted, using a calibration set that contained both contaminated and uncontaminated samples, an RMSEP of approximately 4% w/w was achieved. The results demonstrate that ATR-FT-IR has the potential to quantify methylamphetamine samples, and possibly other licit or illicit substances, in at-seizure and on-site scenarios.     

Use of attenuated total reflectance Fourier transform infrared spectroscopy to monitor the development of lipid aggregate structures

Attenuated total reflectance Fourier transform infrared spectroscopy is used to monitor the adsorption of 100 nm 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipid vesicles to the surfaces of Ge, electrolessly deposited Au, and a well formed self-assembled monolayer of 1-octadecanethiol. The interaction of DPPC vesicles in solution with these different surfaces yields distinctly different surface structures: intact DPPC vesicles on Ge, a supported phospholipid bilayer on an electrolessly deposited Au surface, and a phospholipid monolayer onto the hydrophobic self-assembled monolayer. IR peak position, bandwidth, and intensity are used to confirm structure formation and quantitation of the amount of lipid that desorbs during film formation.     

Characterization of historic silk by polarized attenuated total reflectance Fourier transform infrared spectroscopy for informed conservation

When assessing historic textiles and considering appropriate conservation, display, and storage strategies, characterizing the physical condition of the textiles is essential. Our work has concentrated on developing nondestructive or micro-destructive methodologies that will permit this. Previously, we have demonstrated a correlation between the physical deterioration of unweighted and "pink" tin (IV) chloride weighted silk and certain measurable spectroscopic and chromatographic signatures, derived from polarized Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy (Pol-ATR) and high-performance liquid chromatography (HPLC) microsampling analyses. The application of the Pol-ATR technique to aged silk characterization has now been extended to include a more comprehensive range of weighting methods and aging regimes. This was intended to replicate the full spectrum of states of deterioration observed in silk textiles, from pristine to heavily degraded. Breaking strength was employed as a measure of the physical integrity of the fibers, and, as expected, decreased with aging. An orientational crystallinity parameter, reflecting the microstructural ordering of the fibroin polymer within the fibers, was derived from the Pol-ATR spectra. A good correlation was observed between the breaking strength of the variety of fibers and this parameter. This suggests that the physical state of historic silk fabrics might be adequately characterized for conservation purposes by such indirect micromethodology.     

Monitoring 2-ethylhexyl-4-methoxycinnamate photoisomerization on skin using attenuated total reflection fourier transform infrared spectroscopy

Photoisomerization and photodimerization of a widely used UVB filter, 2-ethylhexy-4-methoxycinnamate (EHMC) on a ZnSe surface and baby mouse (Mus musculus Linn.) skin were monitored using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Differentiation between the E- and the Z-EHMC could be achieved by examining the infrared (IR) peak at 981 cm(-1) (b peak), which corresponds to the CH rocking deformation vibration of Ph-CH=CH- detected only in the E configuration. By plotting the ratios of the peak area of the b peak and an internal standard peak (1060-998 cm(-1)) against mole percentage of Z-isomer in the E-Z mixtures, a linear calibration plot was obtained. Thus, a simple estimation of the mole percentage of each configuration in a sample was obtained. At the same UVB exposure, photostationary equilibrium of the E/Z isomerization on the surface varied with the applied amounts of EHMC. Photoisomerizations on ZnSe and on baby mouse skin were comparable. Less than 10% of E-EHMC changed configuration when the mouse skins applied with 1.0-4.0 mg/cm(2) E-EHMC were exposed to sunlight for 60 min (UVB radiant exposure of approximately 0.30 J/cm(2)). This corresponded to less than 5% loss in UV filtering efficiency. However, at a typical EHMC skin coverage ( approximately 0.2 mg/cm(2)), 0.30 J/cm(2) UVB exposure induced approximately 50% photoisomerization resulting in 25% loss of UV filtering efficiency. No photodimerization was detected even at the extreme EHMC coverage of 4.0 mg/cm(2) after a UVB exposure of 0.90 J/cm(2).