Effect of ionizing radiation on the chemical composition,crystalline content and structure,and flow properties of polytetrafluoroethylene

This article describes a study of ionizing radiation-induced changes in the chemical composition, crystalline content and structure, and flow properties in polytetrafluoroethylene (PTFE). Irradiatins conducted in the presence of oxygen cause acid fluoride end groups to be formed, which on exposure to water vapor hydrolyze to form carboxylic acid end groups. Analyses by infrared (IR) spectroscopy indicate that when irradiated in a vacuum PTFE exhibits defect absorption bands which have been attributed to branch and crosslink formation. The crystalline content of PTFE which increases after exposure to radiation was monitored by IR spectroscopy, density, x-ray diffraction on unoriented samples, and differential scanning calorimetry (DSC) as the measurement probes. The melt viscosity of PTFE exposed to various radiation doses in air decreases dramatically after irradiation. Between 2.5 and 5 Mrd an increase in viscosity is attributed to the formation of branches and crosslinks. The effects of preirradiation crystallinity and postirradiation heat treatment were studied. A model is presented to explain the mechanism of the observed radiation effects.     

Infrared and Raman spectra of alkali metal salts of TCNQ

The infrared and Raman spectra of the series of alkali metal salts of the monoanion radical of tetracyanoquinodimethane MTCNQ (M = Li, Na and K) are observed and analysed both in the low-frequency lattice vibration region and in the intramolecular vibration region. The vibrational assignments are made, by comparing the spectra of MTCNQ with one another and investigating the difference between the powder and solution spectra and the temperature dependence of the spectra. The monomer—dimer phase transition in NaTCNQ and KTCNQ of the columnar stacking of TCNQ⁻ does not cause a marked change in the low-frequency spectra due to the lattice vibrations. The vibronically activated bands of the totally symmetric modes of the TCNQ⁻ anion are observed in the infrared spectra, which are associated with the charge-transfer intermolecular interaction. A lattice dynamical analysis for the optically active crystal vibrations of NaTCNQ and KTCNQ is performed, and the result of this calculation also verifies the vibrational assignments of the low-frequency infrared and Raman bands attributed experimentally to the translational and rotational lattice modes.     

Low frequency vibrations in crystalline biphenyl: Model calculations and raman and neutron spectra

Vibrational frequencies of crystalline biphenyl have been calculated in the rigid phenyl approximation using the crystal structure at 110 K. The calculated results explain successfully the experimental infrared and Raman frequencies as well as the polarization data of the Raman bands at 80 K that have been observed in the present study for the first time. The frequency dispersion curves calculated in the b^* direction show that the coupling between the intramolecular CC torsion and the translational lattice vibrations changes drastically as the wave vector changes. The existence of a minimum frequency point located away from the Brillouin zone boundary indicates that the pulse transition near 40 K is a commensurate—incommensurate one in accordance with the results of neutron diffraction that have recently been reported. Incoherent inelastic neutron scattering spectra have also been investigated above and below the phase transition temperature.     

Das Schwingungsspektrum von Magnesium-orthovanadat

The infrared and laser-Raman spectra of crystalline Mg₃(VO₄)₂ are reported and a factor group analysis for the full lattice is presented to support the discussion. The 1,000 to 600 cm^?1 region of the spectrum is essentially related to internal (stretching) modes of the greatly distorted VO₄ tetrahedra, whereas the bands in the low frequency region must be assigned to complex vibrations originating in the interaction of the bending vibrations of the tetrahedral groups with the external modes.     

Application of fourier transform infrared spectroscopy to the study of semicrystalline polymers: Poly(ethylene terephthalate)

An infrared absorbance subtraction technique has been used to “isolate” bands in the composite spectrum of semicrystalline polymers according to their crystalline or amorphous character. Amorphous and crystalline spectra for annealed, melt-quenched, and solution-cast poly(ethylene terephthalate) have been separated. The spectra of the amorphous component show an increased intensity of bands associated with the trans configuration of oxygen about the C? C bond when the polymer is annealed. This increased “trans” band intensity reflects the increased proportion of trans structures as a result of annealing. The amorphous trans bands are shifted approximately 1–3 cm^?1 from their positions in the crystalline “trans” spectrum. The frequency shift of these bands can be attributed to the differences in chain interactions that exist in the amorphous phase and the crystalline lattice. We have also found that under identical anealing conditions the amorphous phase of the melt-quenched polymer contains an increased intensity of conformational trans bands compared to the sample cast from solution.     

Far infrared spectrum of crystalline carbon disulphide

The far infrared spectrum of crystalline carbon disulphide has been measured at about 1.8 K. Two absorption bands are observed, with maxima at 66.5 and 68.2 cm^?1, respectively, and with nearly equal intensities. They are considered to be due to the two infrared active translational modes expected by lattice dynamics.     

Study by Polarization Modulation Infrared Spectroscopy of an Oriented PTFE Film and Its Effect on the Organization of a Thin Chiral Smectic Adsorbed Layer

PTFE deposited by friction at 300°C is known to align crystals. We have used infrared modulation linear dichroism (MLD) and polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) to study the PTFE layer itself and coated with a very thin layer of a chiral smectic liquid crystal. From the experimental and simulation results of a spectra we deduce that the PTFE chains are perfectly parallel to the deposition direction. Whatever the thickness of liquid crystal (0.1–100 nm), the organization is very well defined and reproducible. We give the angles of the main vibrations of the liquid crystalline molecules with the PTFE chains and with the plane of the substrate. The liquid crystal can be considered as quasi-epitaxied on the PTFE-oriented chains.     

Infrared spectroscopic characterization of oriented polyamide 66: Band assignment and crystallinity measurement

Having found much ambiguity in the infrared band assignments for polyamide 66 (PA66), we revisited some of these assignments before using infrared spectroscopy to assess microstructure changes resulting from multiple thermal treatments. We discovered that earlier assignments of the 1144 and 1180 cm⁻¹ bands to the amorphous (noncrystalline) phase were incorrect, whereas the bands at 924 and 1136 cm⁻¹ can be attributed unambiguously to the noncrystalline phase. We also confirmed that PA66 bands at 936 and 1200 cm⁻¹ are crystalline bands. The normalized absorbance of the 1224‐cm⁻¹ fold band increases in proportion to crystallinity, indicating that chain folding is the predominant mechanism of thermal crystallization in PA66. We demonstrated that infrared spectroscopy can be used to estimate the degree of crystallinity of PA66, and two methods were explored. One is a calibration method in which the band ratio of 1200 and 1630 cm⁻¹ is plotted against crystallinity measured by density. The other is an independent infrared method based on the assumption that PA66 satisfies a two‐phase structure model. The crystallinity determined by the independent infrared method showed good agreement with the crystallinity obtained from density measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 516–524, 2000     

Structural studies on linear aliphatic polyesters by electron diffraction,infrared, and Raman spectroscopy

Infrared and Raman spectra of linear aliphatic polyesters synthesized by polycondensation of α,ω-diols and α,ω-dicarboxylic acids have been measured in the crystalline and amorphous states to study the crystallinity and conformation of the polyester chains.The repeating unit of crystalline polyesters with an even number of methylene groups in the alcoholic or acidic part and with a planar conformation of the chain exhibits a centre of inversion. In this special case the spectroscopic rule of mutual exclusion is followed: vibrations that are infrared-active are Raman-inactive, and vice versa.Comparison of the infrared and Raman spectra of a large number of polyesters showed that this rule is applicable in all cases but one. Presently, the only exception is polyester-2, 4 (ethanediol-succinic acid) where for almost all infrared bands a Raman line can be found. Consequently we have to assume that the chain of this polymer in some way deviates from planarity. Since typical infrared progression bands appeared in the region of the methylene wagging (1400-1200cm^?1) and rocking vibrations (910-720 cm^?1) the methylene sequences must have an essentially planar zigzag conformation. Thus, non-planarity should be due to the plane of the ester groups between methylene sequences having an angle other than zero with respect to the planes of adjacent methylene sequences. Some of the polyesters were also studied by electron diffraction. The results of these examinations support the assumption of a planar conformation of the chains of almost all the polyesters studied.     

Infrared absorption spectra of carbon monoxide in rare gas matrices

Medium and high resolution infrared spectra of CO trapped in solid Ne, Ar, Kr and Xe are presented. Spectra of low optical density samples are identical to those previously observed but a different assignment is proposed. Monomeric absorption is clearly identified by double doping experiments and by sample deposition through a nitrogen cold trap. In addition other broad weak bands are observed on the spectra of high optical density samples. These bands are assigned to combinations between internal vibration, libration and lattice vibrations. The barrier hindering rotation due to site distortion is estimated to 30–50 cm^?1. Librational motion is strongly perturbed by coupling with lattice motion. Carbon monoxide is also an efficient probe to study the dynamical properties of rare gas lattices.     

Molecular Design of Fluorocarbon Film Architecture by Pulsed Plasma Enhanced and Pyrolytic Chemical Vapor Deposition

Pulsed plasma enhanced chemical vapor deposition (pulsed PECVD) and pyrolytic chemical vapor deposition (pyrolyric CVD) of fluorocarbon films from hexafluoropropylene oxide (HFPO) have demonstrated the ability to molecularly design film architecture. Film structures ranging from highly amorphous crosslinked matrices to linear perfluoroalkyl chain crystallites can be established by reducing the modulation frequency of plasma discharge in plasma activated deposition and by eventually shifting mechanistically from an electrically activated to a thermally activated process. X-ray photoelectron spectroscopy (XPS) showed CF₂ content increasing from 39–65 mol%. Fourier transform infrared spectroscopy (FTIR) showed an increasing resolution between the symmetric and asymmetric CF₂ stretches, and a reduction in the intensity of the amorphous PTFE and CF₃ bands. High-resolution solid-state ¹⁹F nuclear magnetic resonance spectroscopy (NMR) revealed an increasing CF₂CF₂CF₂ character, with the pyrolytic CVD film much like bulk poly(tetrafluoroethylene) (PTFE). X-ray diffraction (XRD) patterns evidenced an increase in crystallinity, with the pyrolytic CVD film showing a characteristic peak at 2 = 18° representing the (100) plane of the hexagonal structure of crystalline PTFE above 19°C.     

Terahertz time-domain and Raman spectroscopy of the sulfur-containing peptide dimers: Low-frequency markers of disulfide bridges

The terahertz time-domain and Raman spectra of sulfur-containing cystein-based peptides in the region of the low-frequency infrared vibrations have been measured at room temperature. The low-frequency bands that can be assigned to the S–S bridges are observed. The vibrational modes found in molecular crystalline materials should be described as phonon modes with strong coupling to the intra molecular vibrations.     

Far-infrared spectra of some β-hydroquinone clathrates

Far-infrared spectra (400-30 cm^?1) of Nujol mulls of the β-hydroquinone clathrates containing the following guest molecules were investigated: formic acid, formic acid-d₂, methanol, methanol-d₄, acetonitrile, acetonitrile-d₃, sulphur dioxide and also both of methanol and sulphur dioxide. The observed infrared bands of the mulls in the region of 4000-30 cm^?1 were classified into those due to the host lattice and those due to the guest molecules. On the basis of the comparison of the spectra, some bands were assigned to the translational or the rotational vibrations of guest molecules. Appearance of those bands suggested that some guest molecules are considerably bound in the cavities of the host lattice. Effect of temperature change on the bands was also measured.     

Fourier transform infrared investigation of the effects of irradiation on the 19 and 30°C phase transitions in polytetrafluoroethylene

Fourier transform infrared spectroscopy has been used in conjunction with differential scanning calorimetric measurements to investigate the nature of molecular degradation and its effect on the phase transition temperatures in irradiated polytetrafluoroethylene (PTFE). Both the 19 and 30°C transitions are observed to exhibit similar shifts to low temperatures upon irradiation. Infrared absorbance subtraction data from irradiated PTFE indicate a continual decrease in sample crystallinity accompanied by an increase in the number of free and bonded ? COOH groups with increasing dose consistent with molecular degradation by chain scission. By comparing infrared band intensities on a number of irradiated PTFE samples with those from short chain perfluoro n-alkanes, it was determined that the overall reduction in chain length caused by irradiation was primarily responsible for the observed reduction in both phase transition temperatures.     

Microscopically-viewed structural changes in solvent-induced phase transitions of synthetic polymers

Structural changes occurring in the solvent-induced phase transition have been investigated by carrying out the time-resolved measurements of X-ray diffraction, infrared spectra and Raman spectra. First example is about the solvent-induced crystallization of syndiotactic polystyrene glass. By comparing the time evolution of the various infrared and Raman bands and the X-ray reflections, the process of nucleation, growth, and aggregation of regular helical sequences in the crystalline lattice could be traced concretely. It was also found experimentally that the amorphous chain segments started an active motion immediately after absorbing solvent molecules and became a trigger to induce the local regularization of random coils into regular helical segments. The second example is the reversible solid-state phase transition of poly(ethylene imine) between the anhydrate of doubly-stranded helices and the hydrates of planar zigzag chains. By carrying out the time-resolved infrared spectral measurements in water vapor atmosphere (H₂O or D₂O), the characteristic bands could be identified for these crystalline phases and the structural transformation in the hydration process could be clarified in detail.     

Far- and mid-infrared of crystalline 2,2'-bithiophene: ab initio analysis and comparison with infrared response

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electron-phonon interactions play an important role.     

Evolution of crystallinity in photodegraded polyethylene films studied by ft-raman spectroscopy

FT-Raman spectroscopic studies of photodegraded polyethylene films have enabled the evolution of the crystallinity process to be measured. Commercial polyethylene films of M_w=90 000 were exposed in a weathering UV-chamber under known conditions of exposure time and radiant energy. The spectral profiles were modelled using Fourier methods. The relative amounts of the orthorrombic crystalline phase, α_c, the amorphous phase, α_a and the interphase, α_b, were calculated using Raman bands at 1416 cm⁻¹ characteristic of the crystalline phase and the bands at 1080, 1305 cm⁻¹, characteristic of the amorphous phase. The interphase content can be calculated from the relationship αb= 1-(α_c+α_a). It was found that the weathering process affects only the relative intensities of the bands attributed to crystalline and amorphous fractions; the crystalline content increases at the expenses of the amorphous fraction. These results are discussed in terms of the changes in the intermolecular forces caused by radiation exposure.     

A re-investigation of the far-infrared spectrum of isotactic polystyrene in the 7–2400 cm−1 region at ambient temperature

Fourier-transformed interferometric techniques have been utilized to obtain new data for the far-infrared spectrum of isotactic polystyrene. The region in which rotational and translational crystalline bands are observed has been investigated and 21 absorptions associated with infrared active crystal lattice modes have been observed in the 7–400 cm^?1 region. Possible assignments of these bands have been attempted, and a comparison with the previously reported data for polypropylene has been made.     

Spectroscopic characterization of the rotational conformations in the disordered phase of poly(ethylene terephthalate)

Recently developed data processing techniques have been applied to determine the complete spectra of the trans and gauche isomers of PET and to use these spectra to determine the conformational composition of PET samples isothermally crystallized from the glassy state between 75 and 240°C. A linear correlation was found between normalized absorbance of conformational bands and density measurements, with the trans bands showing a positive slope and the gauche bands a negative slope. Extrapolation of the results from trans bands to zero percent of trans yields the density of the pure gauche isomer, 1.326 ± 0.002 g/cm³. Similarly, from the gauche bands the density of 100% crystalline trans is found to be 1.510 ± 0.014 g/cm³. Meanwhile, the density of the amorphous trans is calculated to be 1.430 ± 0.003 g/cm³. From these values and the combined density and infrared measurements, the content of trans isomers in the amorphous phase is calculated. It is found that the content of amorphous trans is a function of both annealing temperature and time. It is observed that the amorphous trans content decreases as the crystalline trans content increases. At high crystallinity, the amorphous trans content approaches zero. These amorphous trans isomers are associated with the extended units making up the interlamellar links. The loss of these extended trans interlamellar links is reflected in the mechanical properties of PET.     

Analyse par spectrométrie d'absorption infrarouge jusqu'à - 180 °C,de la structure cristalline des mélanges chloroforme-acétonitrile et chloroforme-propionitrile

A study, by infrared spectroscopy between 5000 and 200 cm^?1, in the liquid and solid states down to ? 180 °C, confirms the results obtained by thermal analysis: there is a complex in the case of ethyl cyanide-chloroform mixtures, whereas in the case of methyl cyanide-chloroform mixtures a eutectic is obtained. Furthermore we show, by the analysis of vibrations of the molecular lattice obtained, that, in this eutectic, the chloroform influences the crystalline structure of methyl cyanide.