M. J. D. Low

I.R. studies of carbons. VII: The pyrolysis of a phenol-formaldehyde resin

Abstract Infrared spectra were recorded of the chars produced by pyrolyzing a Novolac phenol-formaldehyde resin in vacuum and in nitrogen, using photothermal beam deflection spectroscopy. The two pyrolysis techniques led to the same results. Thermal branching and cross-linking occurred near 350°C, with the formation of diphenyl ether structures. These reactions continued at higher temperatures when aryl-aryl ethers were formed. Autooxidation is not an important degradation pathway. Although some changes occur, the polymer network remains essentially intact until 500°C, the aromatic systems being held apart and stabilized by aliphatic bridges. In the 500–560°C range, however, drastic changes occur in that the network collapses, aliphatic bridges are destroyed, hydrocarbonaceous residues are eliminated and those remaining are altered, polyaromatic domains form, and the resulting char is much like other intermediate temperature chars.

Infrared studies of carbons. XII The formation of chars from a polycarbonate

Abstract Series of infrared spectra were recorded of chars produced by heating a pure bisphenol-A polycarbonate resin (PC) in vacua at successively increasing temperatures, up to 750°C. FT-IR photothermal beam deflection spectroscopy was used. PC, after remaining essentially intact over a wide range of temperatures, underwent massive and dramatic changes upon heating in a narrow temperature interval (440–490°C). Thermal branching and cross-linking occurred near 440°C with the formation of diaryl ether, ester and unsaturated hydrocarbonaceous bridges. Small, still discrete aromatic entities were formed on pyrolysis above 490°C. Fusion of the aromatic rings into larger polyaromatic structures, however, occurred to a significant extent only at 590°C, when the polymeric network collapsed completely; all aliphatic bridges were destroyed and the oxygen bridges were incorporated in the polyaromatic domains which were formed. The resulting char was similar to other intermediate temperature chars. The continuum of the carbon absorption extends throughout the spectral region at 750°C.

IR studies of carbons—V Effects of nacl on cellulose pyrolysis and char oxidation∗

The pyrolysis in vacuum of cellulose containing 5 wt.% of NaCl (NCC) was followed by i.r. photothermal beam deflection spectroscopy, and compared observations made with pure cellulose (PC) [Carbon21, 238 (1983)]. Although the overall aspects of NCC and PC pyrolysis were similar, NaCl accelerated the cellulose decomposition, the relative amounts of aliphatic and aromatic residues were changed, and i.r.-detectable species disappeared about 100°C lower than with PC, all implying changes in the decomposition and charring mechanisms. The reactions of NCC chars with O2 was also followed and compared with those of PC chars. NaCl caused the ratios of oxidation products to change (some band assignments are discussed) but the behavior of NCC chars produced at 650°C, when NaCl evaporated, was similar to high-temperature PC chars.

IR studies of carbons—IV The vacuum pyrolysis of oxidized cellulose and the characterization of the chars

Abstract The pyrolysis of an NO2-oxydized cellulose (NOC) in vacuum from 22 to 700°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra were recorded at various stages of pyrolysis and compared with and contrasted to similar data previously obtained with cellulose. Also, NOC-derived chars were oxidized, and the effects of treating NOC and NOC char with KOH were observed. The detailed IR spectroscopic data indicate that NOC degrades more easily than cellulose and yields chars that possess different functional groups than cellulose chars, up to charring temperatures of near 500°C. Above that temperature the chars have similar chemical and physical properties.

The Assignment of the 1600 cm−1 Mystery Band of Carbons

Abstract The assignment of a band near 1600 cm−1 in IR spectra of carbons has been controversial for four decades. However, many different carbons have been studied: effectively, a single band assignment was sought for an absorption appearing with three different classes of carbon. As these differ in over-all structure, not one but three explanations are needed. These are discussed. However, undue emphasis has been placed on a single absorption; attention should also be paid to other absorptions accompanying the 1600 cm−1 band.

Infrared studies of carbons. XIII, The oxidation of polycarbonate chars

Abstract IR photothermal beam deflection spectroscopy was used to record IR spectra of chars, produced by the in vacuo pyrolysis of a polycarbonate resin (PC), at several stages of oxidation. The high-temperature chars seem to be more susceptible to oxidation than the low-temperature PC chars. This behavior is quite unlike that of coals, cellulose based chars, and other carbons. Oxidation of the low temperature chars proceeds mainly through attack on the hydrocarbonaceous side chains and the elimination of carbonate functionality, accompanied by an extensive formation of benzophenone, lactones, and acidic carbonylic species. However, the chars obtained at temperatures higher than 590°C, when polyaromatic domains first form to a significant extent, exhibit the normal oxidation behavior found with chars derived from several types of precursors.

Photothermal Beam Deflection Spectroscopy of Complex Systems: Infrared Fourier Transform Band Shapes of Infrared-Absorbing Gases in Presence of Infrared-Absorbing Solids

Absfracf-Fourier transform infrared (FTIR) photothermal beam deflection (PBD) spectra were recorded over the 3900-4500 cm-’ range of solids absorbing weakly, medium-strongly, strongly, and very strongly in various spectral regions, as well as of carbons that absorb IR radiation very strongly over the entire spectral region, in the presence of IR-absorbing gases such as SOz or NO2, which exhibit absorption bands of different intensities at various wavenumbers. The gas over the solid absorbs IR radiation, and the extent of that absorption affects the intensity of the PED signal of the solid (which is itself a function of the solid’s texture) and also of the gas immediately above the solid’s surface, so that the net results which are observed are very complex indeed. It would be prudent to avoid such systems, but as they are frequently encountered in studies of the chemistry of reactions mcurring on solid surfaces, they must be examined. In view of the many variables, which include not merely the gas partial pressure and the relative absorption coefficients of gas and solid in specific wavenumber regions, but also the photothermal response of the solid as function of texture as well as the refractive indices and thermal properties of the gas, a phenomenological description of the IR PBD spectra of such IRabsorbing gas-solid samples is made using numerous FTIR PBD spectra of gas-solid systems.

Unusual Bands in the Infrared Spectra of Some Chars

Abstract When organic materials are charred at low or medium temperatures (up to about 450–500°C), their infrared (IR) spectra show a plethora of bands below about 2000 cm−1, and there are additional bands in the OH and CH stretching regions, above about 2600 cm−1. The in-between region, from about 2600 to 200 cm−1, is quite “empty” (except for an occasional atmospheric CO2 band caused by instrument imbalance). The reason for this emptiness is, simply, that there are very few species that have fundamentals in that region, as is well known from group frequency tables; and those that do absorb, such as metal hydrides, are quite unlikely to exist in organic precursors. Some overtones or combinations may appear, but these are usually very weak. We have, however, observed some bands in the empty region on several occasions.

A relation between the spectral profile and the infrared continuum of spectra of medium-temperature carbons

Abstract It is frequently found that when various organic materials are pyrolyzed at temperatures in excess of about 550°C, the infrared spectra of the chars assume a similar, almost identical profile. That would indicate that the chars had become almost or completely spectroscopically and chemically indistinguishable and unrelatable to their precursors. This topic and the possible mechanisms involved are described in detail elsewhere [1].

Infrared dispersive mirage spectra of solids submerged in water

Abstract Some time ago Fournier et al1 showed that the “mirage effect”2 [the deflection of a light beam by refractive index (n) gradients in a fluid above a heated surface] was two orders of magnitude greater when the fluid was a liquid than when it was a gas. The larger beam deflections arose because of the larger dn/dT changes found with liquids than with gases. Advantage has been taken of this higher sensitivity in a variety of nonspectroscopic and spectroscopic studies (in the visible region of the spectrum) of solids submerged in liquids, and recently Palmer and Smith3, 4 and also Varlashkin and Low5 have extended the spectral measurements into the infrared (IR) range; IR Fourier transform (FT) spectra of solids submerged in CC14 were obtained.