Spectroscopy crystallinity, determination

Using proton NMR of solutions, the composition of polymers can be analyzed.47 Carbon-13 NMR spectroscopy is a useful tool for studying the sequence length of segments in copolymers and thereby determining the blockiness of the copolymer. With solid-state NMR, the mobility of chain segments can be studied and the crystallinity determined. 

Both vibrational spectroscopies are valuable tools in the characterization of crystalline polymers. The degree of crystallinity is calculated from the ratio of isolated vibrational modes, specific to the crystalline regions, and a mode whose intensity is not influenced by degree of crystallinity and serves as internal standard. A significant number of studies have used both types of spectroscopy for quantitative crystallinity determination in the polyethylenes [38,74-82] and other semi-crystalline polymers such as polyfethylene terephthalate) [83-85], isotactic poly(propylene) [86,87], polyfaryl ether ether ketone) [88], polyftetra-fluoroethylene) [89,90] and bisphenol A polycarbonate [91]. 

The decomposition of the ZSM-5 lattice at high temperatures can also be followed by monitoring the structure sensitive infrared bands. Figure 1 shows a set of spectra from the recent paper of Tallon and Buckley (ref. 11) in which the kinetics of ZSM-5 decomposition were investigated by using FTIR spectroscopy to determine the crystallinity of samples subjected to isothermal annealing. 

The spectrum of sucrose in aqueous solution has been studied to examine further the potential of F.t.i.r. spectroscopy for determining carbohydrate configurations. While bands characteristic of an a- rather than 6-linked D-glucopyranosyl unit were present, no criteria for discerning the stereochemistry of the fructose unit were found. Some bands in the hydroxyl stretching region of crystalline carbohydrates have been identified through a... 

Either, one uses an independent method for the crystallinity determination, such as dilatometry. X-ray diffraction or infrared spectroscopy for the determination of Wc [4], or one tries to determine the amorphous fraction, Wa, from the measured increase of the heat capacity at the glass transition temperature, ACp, and the same quantity for the fully amorphous sample, ACf... 

This section provides a broad overview of the application of NMR spectroscopy to crystallinity determination in polymers. The approaches illustrated are certainly not exhaustive of those available for the powerful NMR technique and the interested reader is encouraged to consult one or more of the reference books on NMR spectroscopic investigations of polymeric materials (51-53) (see NUCLEAR Magnetic Resonance). 

Molecular weight of recycled HDPE as measured by size-exclusion chromatography can be correlated with diffuse reflectance near- and mid-infrared spectroscopy (NIR/MIR). The RMSEP obtained with MIR data was 360 and with NIR data was 470. The degree of crystallinity determined by DSC can also be correlated to the spectra. The prediction ability of this model was poor in the 1100 to 1900 nm region, however the 2000 to 2500 nm region yielded better results. Four PC factors explained the most of the variance in the spectra and the RMSEP was 0.4% wt. [51]. 

Various authors have developed methods based on Raman spectroscopy for determining crystallinity. In some polymers, specific vibrational bands have been attributed to the crystalline phase. In others, the ratio of the intensities of trans conformation vibrations to gauche conformation vibrations has been used. Still others have used the width of a particular vibrational band to indicate crystallinity. The last method is based on the effect of the local environment on the frequency of the band. In a random environment, many different local environments exist and the band is broadened by these variations. In a crystal, the local environment is the same for all of the chain segments. Then all of these segments have (nearly) the same frequency and the band is narrow. For a band that is sensitive to the local environment, a narrow band indicates high crystallinity and a broad band indicates low crystallinity. 

Process monitoring using Raman spectroscopy (mainly in its NIR Fourier transform variant) is proposed for QA/QC purposes, on-line polymer analysis, in situ cure kinetics, emulsion polymerisation, non-invasive analysis of physical parameters (in situ crystallinity determination, etc.) and reactor compositions, real-time measurements, molecular interactions, and components in aqueous solutions. 

It has been noted [11] that when unique vibrational bands can be associated with each phase, it is not necessary in principle to calibrate against other standards, although it is often desirable to do this. All that is required is a set of samples of widely varying composition for which the relative band intensities can be determined and extrapolated to the 100% and 0% crystallinity levels. Raman measurement of crystallinity in polyethylene illustrates this approach [229], as does the IR measurement of crystallinity of nylon [224]. Extrapolation of the spectroscopic calibration has sometimes been used to determine the amorphous and crystalline densities of a polymer [224, 225], thereby adding to the information available from the primary calibration standard. However, one must note that vibrational spectroscopy, DSC, X-ray and density actually measure different physical parameters and so a crystallinity determined solely by vibrational spectroscopy may well differ from that obtained by other techniques [230]. Also, with anisotropic materials, molecular orientation can alter band intensities and invalidate calibrations developed using isotropic standards [48, 227, 231]. 

Polarised confocal Raman microscopy was used to measure molecular orientation in uniaxially drawn PETP films, prepared with draw ratios from 1 to 3.5. The orientation of both polarised Raman microscopy and polarised attenuated total reflection IR spectroscopy. Crystallinity profiles were measured through the thickness of the film samples and compared with the orientation gradients existing in the films. This procedure was to determine whether the intuitive assumption that orientation and crystallinity would be positively correlated actually holds true on the microscopic scale for these samples. 18 refs. 

Whenever degrees of crystallinity are considered, it must always be kept in mind that values depend to some extent upon the manner in which they were obtained. Each method measures different material properties that are related to the sample morphology in different ways. Those methods that assume a two-phase model of morphology include the interface with either the crystalline or disordered region. A full comparison of the above techniques, applied to a common group of samples, is not available. Figures 20-23 compare the results of crystallinity determination from density, differential scanning calorimehy, and Raman spectroscopy for a variety of commercial and experimental resins [86-88]. In each plot a broken line is added to indicate the relationship that would be expected if the results were equivalent. 

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

The geometries of oxiranes have been determined mainly by X-ray diffraction on crystalline natural products, the oxirane ring being widespread in nature (Section 5.05.5.3). However, the detailed structure of the parent compound (Figure 1) has been secured by microwave spectroscopy and electron diffraction studies (64HC(l9-l)l). The strain in this... 

As with other diffraction techniques (X-ray and electron), neutron diffraction is a nondestructive technique that can be used to determine the positions of atoms in crystalline materials. Other uses are phase identification and quantitation, residual stress measurements, and average particle-size estimations for crystalline materials. Since neutrons possess a magnetic moment, neutron diffraction is sensitive to the ordering of magnetically active atoms. It differs from many site-specific analyses, such as nuclear magnetic resonance, vibrational, and X-ray absorption spectroscopies, in that neutron diffraction provides detailed structural information averaged over thousands of A. It will be seen that the major differences between neutron diffraction and other diffiaction techniques, namely the extraordinarily... 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

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