As found from previous studies, a Gaussian curve describes the swelling, by various liquids, of a series of nitrile rubbers containing from 0 to 50% bound acrylonitrile. If only dispersion forces are involved, it is found that the value of maximum swelling is inversely proportional to the molar volume of the liquid causing the swelling. As a result, knowing the molar volume and the polarity index of a liquid, it is possible to draw curves relating the volume increase with the bound acrylonitrile level of the rubber. This relation also ap plies to associated liquids, such as acetone, methyl acetate, and alcohols, if the degree of association is known. For those liquids involved in acid-base interac tions with the rubbers, the swelling is the sum of two Gaussian equations. One equation relates to the swelling due to dispersion forces while the other gives the swelling due to acid-base interactions. Only the former equation can be pre dicted with any accuracy. Nitrile rubbers are involved not only in acid-base in teractions with acidic liquids, such as certain chlorinated hydrocarbons but also with basic liquids such as toluene, aniline, tetrahydrofuran, and cyclohex anone. Because nitrile rubbers interact with hard acids and soft bases, they are considered to be hard bases and soft acids.

We have demonstrated that good quality Raman spectra can be obtained from vulcanizates prepared from cis-l,4-polybutadiene. Furthermore, significant differences are seen in the spectra of extracted vulcanizates prepared from various recipes. Tentative assignments have been made for many of the following Raman lines observed in the spectra. 1. The 1633, 1187, 734, and 720 cm−1 lines are thought to be associated with dialkenyl sulfide crosslinks. 2. Lines occurring at 440 and 505 cm−1 are assigned to polysulfidic and disulfidic structures respectively. 3. The 635, 690, and 708 cm−1 lines are thought to be associated with cyclic sulfides. In particular, the 635 cm−1 line with six membered thioalkenes, the 690 cm−1 line with five membered thioalkanes and the 708 cm−1 line with five membered thioalkenes. 4. Pendent side groups derived from TMTD are thought to have lines occurring at 1142 and 577 cm−1. Other lines occurring in the Raman spectra of the vulcanizates examined cannot be assigned with the same degree of confidence. The line occurring at 1606 cm−1 may be associated with conjugation but further work is necessary to verify it. The recipes chosen do not necessarily reflect those used currently in industry but were designed to accentuate certain structural features in the network. Consequently, the weak band at 1587 cm−1, observed only in systems with large amounts of TMTD and associated with vinyl thioether type structures, should not be overemphasized. Lines occurring at 558, 852, and 872 cm−1 are unassigned at present. Although the initial results are encouraging, it must be stated that many problems exist. Due to difficulties associated with fluorescence and degradation, Raman spectra of many important rubber systems will be difficult to obtain. Low levels of fluorescence cause serious problems when weak lines are present and when quantitative measurements of line intensities are made.

Acrylic elastomers have the ASTM designation ACM for polymers of ethyl acrylate and other acrylates, and ANM for copolymers of ethyl or other acrylates with acrylonitrile. In both cases, the M indicates a polymer having a saturated chain of the polymethylene type. The combination of a saturated backbone with polar side chains results in a class of polymers with very good resistance to heat and oil, including oils containing hypoid additives. Acrylic elastomers also have good resistance to sunlight and ozone. Ethylene–acrylic elastomers are discussed in a separate article. The first acrylic elastomers were homopolymers of either ethyl acrylate or methyl acrylate. Because these had limited utility, particularly for vulcanized applications, various copolymer modifications were developed to improve performance, and there evolved a division of monomers into two types: backbone monomers , which comprise the principal proportion of the monomers and determine the physical and chemical properties of the polymer, and cure‐site monomers , which are incorporated to the extent of 1–5% to introduce reactive sites for subsequent cross‐linking reactions. Emulsion and suspension polymerization are the important methods for preparing the elastomers. Their rheological characteristics require processing that addresses contamination more than for other types. Fairly rigid processing is required. The monomers from which these elastomers are prepared have varying degrees of toxicity. Because of the wide property range, the acylic elastomers have many application, eg, coatings, textiles, automotive products, adhesives, paper, and agriculture.

Immersion tests in various polar and nonpolar solvents have been undertaken with a range of butadiene-acrylonitrile copolymers. For a particular liquid, it would appear that the volume change can be related to the acrylonitrile content of the rubber' by a Gaussian type curve which is described by three constants α, β and γ. The constant a corresponds to the nitrile content of the rubber which has maximum swelling in the liquid and would appear to be related in some way to the dielectric constant of that liquid. The constants β and γ control the value of maximum swelling. Investigation into the nature of the three constants tends to suggest that α and β are governed by molecular attraction due to the “dispersion forces”. On the other hand, γ seems to be connected with the electrostatic forces of attraction and is probably not a constant over the full range of nitrile content, but should be replaced by an expression governed by two other constants. Therefore, to describe the solvent power of a nonpolar solvent, two parameters are needed while four are required for a polar solvent.

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