Practically all UV absorption spectroscopy is done in solution, and examples of its use in examination of neat (one component) solid materials are relatively few. In the IR on the other hand, the examination of thin films and microtomed sections of elastomers and plastics is a normal procedure. The reasons for this are fairly obvious: scatter by optically imperfect films is much more serious in the UV, many polymers give no specific UV absorption, and those that do absorb often give such intense absorption that impracticably thin films would be required. The main value of UV is in examination of trace amounts of UV-absorbing materials in those polymers which are more or less transparent, the best example being the study of crosslinking of elastomers. This work cannot be done in solution because the cross-linked polymers are insoluble in all solvents. A great deal of the work on the chemistry of vulcanization has therefore been done on model compounds of low molecular weight, most notably at the Natural Rubber Producers' Research Association. Although the use of model olefins has yielded valuable information, investigations on polymers are required because the different conditions, particularly in steric factors, may appreciably alter the balance of reactions.
It appeared that an investigation of the reactions between peroxides and deactivators of rubber degradation, such as mercaptobenzimidazole, would help to explain the action of these sulfur compounds. Le Bras, who proposed the use of these compounds, considered that they function by deactivating the peroxides formed by autoxidation. It was found that mercaptobenzimidazole and simpler related compounds such as thiourea reacted with alkyl hydroperoxides with the formation of hydroxyl or alkoxy radicals and this led to attack on the solvent or on other molecules present, in a manner resembling the action of ferrous salt-hydrogen peroxide. This suggested that rubber would also be attacked and probably degraded. It was found that mercaptobenzimidazole with excess of peroxide gave rapid degradation of rubber in solution if access of oxygen was permitted. Thiourea, thioacetamide, some dithiocarbonates, and aromatic thiols were also effective, but aliphatic thiols and ethylmercaptobenzimidazole were ineffective. Softened or “depolymerized” rubber was prepared by addition of the reagents to rubber solutions or to latex. The use of these sulfur compounds in the presence of peroxides provides a means of obtaining a controlled amount of breakdown of rubber, and products ranging from normal plasticized rubber to a viscous liquid can be prepared. Under different conditions, mercaptobenzimidazole may either promote degradation (as shown in this paper) or retard it (as stated by Le Bras), and a possible explanation for this reversal of behavior is suggested.
An ATR method is given for the examination of pharmaceuticals in the solid state as a layer of crystals on adhesive tape. Representative spectra are chosen to compare with, and to indicate advantages over, disc and mull techniques. Application of the method to pharmaceutical formulations is illustrated by the direct spectral identification of amodiaquine hydrochloride in tablet formulation.
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