(2512) Zinser, hI., and Baumgaertel, C., Arch. Pharm. 297 (3), 158 (1964); C.A. 60, 13095f(1964). (2513) Zivanov, D., Blagojevic, Z., and Pllladenovic, M., Arh ‘v Farm. (Belgrade) 14 (5), 273 (1964); C.~1.62,2671h(1965). (2514) Zollner-Ivan, IC., Acta Pharm. Huna. 34 (2). 49 (1964): C.A. 61. 29119 (1964).” (2515) Zbid., (4), 145 (1964); C.A. 61, 15937h (1964). (2516) Zommer, S., and Lipiec, T., Chemia Anal. (R’arstcw) 9, 871 (1964); C.A. 62, 46058 (1965).
(2517) Zorin, E. B., Sb. Nauchn. Tr., Tsentr. Aptechn. A’auchn.-Zssled. Znst. 3 , 129 (1962); C.A. 61 , 1710h (1964). (2518) Zugaza, A., and Hidalgo, A., Rev. Real Acad. Cienc. Exact., Fis. iYat. Madrid 59, 221 (1965); C.A. 64, 6450g (1966). (2519) Zurkowska. J.. Budzvnska. &I.. and Ozarowski, A., Acta Pdon. Pharm: 20 (2), 109 (1963); C.A. 62, 1513e (1965). (2520) Zurkowska, J., Lukaszewski, IT., and Oearowski, A,, Ibid., 115 (1963); C.A. 62, l513e (1965).
(2521) Zurkowska, J., and Ozarowski, A., Zbid., 22 (I), 83 (1965); C.A. 63, 1 1 6 8 3 (1965). (2522) Zurkowska, J., and Ozarowski, A., PlantaMed. 12 ( 2 ) , 222 (1964); C.A. 61, 14469e (1964). (2523) Zurkowski, P., Clin. Chem. 10, 451 11964). (2524)‘Zw&-zchowski, Z., Acta Polan. Pharm. 20 ( 5 ) , 383 (1963); C.A. 62, l518d (1965). (2525) Zwimpfer, G., and Buechi, J., Pharm. Acta Helv. 39 (4/5), 327 (1964); C.A. 61, 415lb (1964).
Rubber Coe W. Wadelin and Gordon S. Trick, Research Division, The Goodyear Tire and Rubber Co., Akron, Ohio
T
covers characterization and chemical analysis of rubber. While many methods, such as pyrolysis followed by gas chromatography, are appropriate for both rubbers and plastics, they are includei here only when they have been applied to rubber. There are a few exceptions to this rule where the references are particularly interesting or may be obviously extended to rubber. hlethods for the identification or determination of rubber and materials in rubber ,are included, but methods for the analysis of rubber chemicals before they are added to rubber are omitted. The literature on chemical analysis which became availahle to the authors between September 1064, the end of the period covered by the last review in this series (222), and September 1966, is covered. In a few cmes, articles cited in the previous review have been repeated because they have now been translated into Englisi (29, 70, 7 8 ) . In the section on polymer characterization by physical methods, a longer period of time is covered. Literature searching was done by the mechanized information retrieval center operated by the University of Akron under contract with the Division of Rubber Chemistry (of the American Chemical Society (67’) The abbreviations recommended in ASThl Designation D1418-65 have again been used (9). They are listed in Table I. HIS REVIEW
GENERAL 1NFC)RMATION
Working Group A of Technical Committee 45 of the International Organization for Standardization is compiling methods for analysis of rubber and rubber products which are involved in international trade. Recent progress toward agreement on methods for determination of extrwt, ash, carbon
black, zinc oxide, copper, manganese, and iron was reported (167). il survey of polymer analysis was published in “Encyclopedia of Polymer Science and Technology” (84). Information on analysis can also be found in the article about the polymer of interest. Haslam and J17illisJbook deals with analysis of plastics but much of it is also applicable to rubber (95). h compilation of physical constants of rubbers has been published (254). POLYMER IDENTIFICATION
Pyrolysis-Gas Chromatography. Differences of opinion about the most desirable conditions for pyrolysis continue to be reported. A study of pyrolysis techniques indicated that the fragments recombine if they are permitted to cool before being injected into the gas chromatograph (79). The method that seems to be least sensitive to experimental conditions is pyrolysis of a small sample a t high temperature so that very rapid fragmentation to monomer occurs. Some workers found that temperatures above 500’ C cause monomers to crack further, thereby destroying the characteristic featurks of the gas chromatogram (128). However, other investigators found that monomers are stable a t 1050’ C and concluded that it is the best temperature (65). It was pointed out that rapid removal of the initial breakdown products from the pyrolysis zone is necessary to prevent secondary fragmentation (203). Differences in this feature may be the reason for different viewpoints on optimum temperature. Most of the pyrolysis is done thermally. I n addition to pyrolysis, there were preliminary reports on two other methods, electric discharge (205) and oxidative degradation (193). Neither of these has been thoroughly explored. If the column effluent is split and then simultaneously examined by flame ioni-
443 7 6
zation and electron capture detectors, some surprising identifications can be made (55). For instance, emulsion SBR can be distinguished from solution SBR and IR can be distinguished from KR. Rubbers containing chlorine, nitrogen, sulfur, or oxygen have much more electron capture response than hydrocarbon rubbers. Gas chroniatography is more sensitive than infrared for detecting small amounts. This is useful for I I R (79). Any method which uses pyrolysis as the initial step is unable to distinguish cis-trans structures in the original sample (55, 7‘9). Therefore pyrolysisgas chromatography cannot completely supplant infrared examination of the unpyrolyzed sample. The ability to distinguish mixtures of polystyrene and polyacrylonitrile from styrene-acrylonitrile copolymers was reported (138). The chromatograms of many polymers were reported (33, 144). The pyrolysis products of SBR, BR, IR, and EPM were identified (239). Infrared. The long used Dinsmore and Smith method for degrading polymers by refluxing in lJ2-dichloro-
Table I. Abbreviations Recommended by ASTM (9) BR Butadiene rubber IR Isoprene rubber, synthetic CR Chloroprene rubber KR Isoprene rubber, natural IIR Isobutylene-isoprene rubber NBR Nitrile- butadiene rubber SBR Styrene-butadiene rrtbber T8i Silicone elastomers having both methyl and vinyl containing groups on the polymer chain 111 Polyisobutylene EPM Ethylene-propylene copolymer EPDM Terpolymer of ethylene, propylene, and a diene with the residual unsaturated portion of the diene in the side chain CSM
Chloro-sulfonyl-polyethylene
VOL. 39, NO. 5, APRIL 1967
239 R
benzene (66) was modified by bubbling air through during digestion (194). This makes the dissolution of polymer more rapid. The use of a n airbrush to spray polymer solutions onto sodium chloride plates was described (72). The solutions are very dilute so the technique is applicable to polymers with limited solubility. The film can be built up to any desired thickness by repeated treatments. Solvent removal is complete under a heat lamp in 15 minutes. To save the cost of rock salt plates, films were cast on KBr disks (100). Samples were pyrolyzed and the products were trapped in CC14 or CS2. Infrared spectra were used to identify NR, SBR, S U R , BR, CR, and blends (206). Other Methods. A new approach to rapid polymer identification is examination of the ultraviolet spectrum of the gaseous pyrolysis products (188). No preliminary extraction is required and the main products are monomers, many of which have characteristic spectra in the 180-260 mp region. As little as 5% KR or 3% SBR can be identified. Polysulfide rubber gives CS2 and CSAI gives SO*. With spectrophotometers capable of going below 200 nip now available, this method seems to be very interesting. Some simple qualitative tests for rubber were described (169). CR is decomposed by boiling HSO, while polyvinyl chloride and CShI are not. The analyst's task of distinguishing products of different manufacture can be made simpler by incorporating 0.02 to 0.05% of a fluorescent dye in a black stock (103). The dye is carried to the surface by wax and fluoresces under ultraviolet light. The only drawback is that the cost is about a cent per pound of rubber product. Glass transition temperature can distinguish random, random-block, and stereo-block copolymers of ethylene and propylene (145). POLYMER CHARACTERIZATION BY PHYSICAL METHODS
General. By design, the previous review article in this series did not dwell upon physical methods for the Characterization of rubbers in terms of molecular veight, molecular weight distribution, transition behavior, etc. (225'). The present review will cover some of these subject areas. The presentation is by no means comprehensive but is intended to point out some of the important problems in polymer characterization and the techniques which may be used to solve these problems. Since some of these topics have not been covered in recent articles in this series, the literature reviewed often covers a period of greater than the last ts-o years. I n general, the analytical techniques to be considered are aimed a t measure240 R
ANALYTICAL CHEMISTRY
ments on rubbers in the uncured state. The characterization of the properties of elastomers in the cured state is not considered. I n a number of cases, the experimental techniques cited have been primarily aimed at the characterization of nonrubbery polymers, but such papers have been included when it is considered that the experimental techniques can be usefully applied to rubbers. Among recently published books containing information of use in the physical analysis of rubber are those dealing with the molecular weight of polymers (55), molecular weight distribution ( I @ ) , and light scattering (139). Other books discuss structuie and property relationships in terms of molecular weight, microstructure, elasticity, and transition behavior (146) and recent advances in ebulliometry and fractionation (184). Still other books cover a variety of physical methods for polymer characterization including thermal analysis, fractionation, centrifugation, and elastoosmometry (118), thermal analyses of polymers ( Z O l ) , and instrumental techniques as applied to plastics and rubbers (232). An extensive compilation of the pioperties of polymers has gathered into one source much useful information (32). Of particular interest are sections dealing with solid-state properties, solution properties, fractionation, and tables giving physical constants of UR, NR, SBR, IIR, and CR. Another recent book includes useful information on molecular weights, molecular weight distribution, branching, and transition behavior (153). General reviews on elastomers have been published ( 4 , 5 , 6 8 ) ,including an extensive survey of nitrile rubber (106). Other review articles contain useful information on the characterization of polymers (11, 54). I n more specific areas, publications are available on symposia dealing with thermal analysis of high polymers (117), analysis and fractionation of polymers (156),and the structure and properties of polymers (216)* Molecular Weights. During the period, a number of general papers have appeared dealing with various aspects of the established methods for determining the molecular weights and molecular weight distributions of polymers ($4, 26, 137, 160). Major advances have been made in instrunientation of automatic membrane osmometers (160). The coniniercially available instruments are the Nechrolab design (147) and the Shell design (185). Yariations of the latter instrument are produced by Hallikainen (89),Dohrmann (68), and Stabin (202). Each of these instruments has its advantages and disadvantages but in terms of speed, reproducibility, and validity of results, the automatic instruments represent a major improvement over previous designs.
Their principal disadvantage is the necessity for using semipermeable membranes which may tend to be variable in their diffusion characteristics. However, the designs are such that the presence of a n unsuitable membrane can readily be detected. Papers have also been published dealing with general aspects of osmometer design (64) and methods by which osmometers with semipermeable membranes can be employed to determine molecular weights of the order of magnitude of that of the solvent (105). Dynamic osmometry has been reported to be useful for polymers lvith molecular weights of 1200 and up (76). The preparation of very tight membranes useful down to molecular weights of 500 has been described (221). I n general, little nex work has been reported on the use of cryoscopic and ebulliometric techniques to determine number average molecular weights. Despite the experimental difficulties, some advances have been made in ebulliometric techniques (258) with the useful upper limit of measurement extended to molecular weights of 170,000 (87). A cryoscopic method has been published showing measurements in the 4000 to 30,000 range (166). An instrument reported to be useful for molecular weights up to 20,000 has also been described and is based on the sensitive measurement of temperature differences resulting from vapor transport from polymer solutions (171). Some applications of this instrument hare been reported (46, 168). Other papers dealing with the molecular u eights of polymers include reports on sedimentation analysis of SBR (107) and cis-1,4-polybutadiene (1). The determination of molecular weights from sedimentation velocity ( I % ) , fluorescence spectrometry (18), and light scattering (14) has been reported. Richardson has illustrated the usefulness of the electron microscope for molecular weights of greater than one million (183). ;1. review of the viscometric determination of molecular weight, including extensive experimental results, has appeared (148). For synthetic polyisoprene, a rather surprising dependency of density upon molecular weight has been found (37). Molecular Weight Distribution. The last fen- years have produced major advances in methods for determining the molecular weight distribution of polymers. These advances have dealt with both experimental techniques and methods of validly treating the resultant experimental data. The high level of activity in these areas arises from a growing appreciation that molecular weight distribution is of considerable importance in governing the processing behavior of polymers and, to some extent, the physical characteristics of the finished product. I n addition to the
analysis of the molecular weight distribution of polymers, techniques have been improved to allow fractionation of polymers on a large scale in order to prepare a series of samples of different molecular xeights. A very useful review has been published by Schneider coiering a variety of experimental techniqucs, together with a discussion of the associated theoretical aspects (191). A brief summary of the status of molecular w i g h t distribution methods has also appeared with particular emphasis on applications in the rubber industry (52). I n general, the classical methods of fractional precipitation from dilute solution or fractional elution of polymers have been replaced by more refined techniques s o r e of which, however, depend upon the same scientific principles. One paper using the classical approach deals with the fractionation of Seoprene W and the, changes brought about by mastication (61). Apparatus by which the molecular weight distribution of relatively mcnodisperse polymers may be determined by semiautomatic solvent extraction has been described (92). The fractionation of 111 at high pressures has also been reported (187).
A considerable advance in the ease of determining molecular weight distribution has arisen from i,he use (15) of a chromatographic column from which the polymer is eluted throL gh a temperature gradient by a liquid of gradually increasing solvent power. X number of papers have appeared discussing the principles of operation of the techniques and the effect of experimental conditions on the results obtained (42, 115, 127, 173, 1S0, 192, 195). The role played by the thermal gradient in increasing effectiveness of fractionation has been examined in detail (192). The application of the technique to a number of rubbers has been shown, including 111 (170),CR (Seoprene K R T ) and polyisoprene (high 3,4) (17'7), and polybutadienes of various structures (59, 112, 177, 217). Improved experimental techniques for use with the experimental system include a programmed solvent dispenser which allows delivery of a binary solvent mixture of predetermined composition (230). Other improvc>mentsin instrumentation of solvent {gradient and flow control have been proposed (86). I n addition, experimental methods have been presented which allow a scale-up of the fractionation syste n so it may serve as both a preparative m d an analytical tool (42, 119,176,223). It appears that this experimental approach may have general applicability t o rubbers but in each application the effect of experimental variables on the results must be examined, in particular the possibility of rubber instability during fractionation (69,217)*
The experimental technique of gel per-
meation chromatography (GPC) promises to bring about a major improvement in the analytical methods available for characterizing molecular weight distribution. The possibility of using gel particles for the separation of polymer molecules according to size has been appreciated for a number of years (178) but, in general, the applications have been concerned with water soluble materials (75). It remained for Moore to point out the usefulness of hydrophobic gels when incorporated in a highly stable chromatographic system with a differential refractometer t o monitor the eluent (159). The details of the principle of operation have not been w-ell worked out but it appears that as the polymer solution moves through the column, the lower molecular weight polymer enters the pores of the gel while the higher molecular weights are excluded. Consequently, the high molecular weights follow a shorter path and are eluted first. A commercial instrument based on these principles is available (140) although other designs have been envisioned ( 6 ) . Instrumentation and applications of GPC have recently been thoroughly reviewed (46, 47). The published applications of GPC to rubbers include measurements on butyl rubbers where the results did not agree well with those from precipitation chromatography (83). Adams and con-orkers have made a thorough investigation of the GPC of polybutadiene including a discussion of the effect, of experimental conditions upon the results obtained ( 2 ) . Harmon also compared results from the GPC of cis-l,4-polybutadiene with those obtained by a coacervation technique (91). An extensive evaluation of the usefulness of GPC as applied to polyisobutenes has been made (43). In addition, many papers have been presented dealing ith a variety of techniques and applications (227, 228). These seminars serve the very useful purpose of allon ing early informal communication of progress by the various workers in the field. Reports on methods of treatment of data of this type have appeared (19 ) ,including a computer program for handling the experimental results (174). Papers have also appeared dealing with corrections for the limited resolving power of GPC, one of the major problems when attempts are made to characterize samples of relatively narrow molecular weight distribution (101, 213-215). The principle of operation of the system has also been discussed (98, 149, 229), including information on the effect of polymer concentration upon the experimental results (44). In addition to its usefulness as an analytical tool, GPC shows promise to be useful as a preparative unit but no information is available on the experimental performance of such a scaled-up version (226). The work being reported on GPC re-
sults is expanding too rapidly for any adequate summary to be made in a short review of this type. However, the preceding discussion gives a general picture of progress to date. It is to be hoped that future work published will include further investigations on the calibration of the instrument and the quantitative significance of the experimental results obtained. Such information will serve to define the limits of usefulness to which the technique can be extended. Progress in a number of other techniques for the determination of molecular weight distribution has been described. Results obtained on cis-l,4polybutadiene and SBR with an ultracentrifuge have been found, after corrections for the effect of centrifugal pressure and solution concentration, to be in good agreement with elution chromatography results (97). Other applications of sedimentation analysis have been described (1, 27, 107'). Density gradient centrifugation also appears to be useful for measurement of molecular weight distributions (73, 74). Polymer fractionation in a thermal diffusion column (208) and by zone refining (136) has been described. Another useful technique involving turbidity measurements has been published (210) together with improved instrumentation (209) and results obtained on an ethylene-propylene copolymer (84), with turbidity being automatically obtained as a function of temperature. Measurement of decrease in osmotic pressure by polymer diffusion in an automatic osmometer s h o w promise of being a useful tool for the measurement of distribution in low molecular &-eightpolymers (110). In the area of treatment of molecular weight distribution data, a numerical method has been proposed for the calculation of the distribution curve and distribution moments (88). A critical analysis of the significance of molecular weight distributions derived from fractionation data has been made including the effect o f low and high molecular weight cut-off (121-123). Branching. Another characteristic of rubbers which probably is of considerable importance in governing mechanical behavior is the frequency of branch points and the length of the side branches. The branching of synthetic polyisoprenes has been discussed (175), and reports have described the intrinsic viscosity (166, 162) and light scattering (189) behavior of branched molecules. Theoretical papers deal with the evaluation of the extent of branching and the niolecular weight distributions of polydisperse polymers with random branching (20, 200).
Transition Behavior. Extensive work on the transition behavior of polymers continues to be reported but much of this effort is directed VOL. 39, NO. 5, APRIL 1967
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toward detailed examination of the morphology of highly crystalline materials. However, work has appeared c n the first- and second-order transition behavior of rubbers and, in some cases, such measurement can serve as a sensitive ana!ytical tool. An extensive review has reported on the transition behavior of polymers as related to chemical structure (31). The effect of pressure on the glass transition has been measured (22). A number of investigations have been made of the transition behavior of l14-polybutadienes, including a detailed dilatometric investigation of the melting behavior of cis-1,4-polybutadiene (154). Differential thermal analysis (DTA) measurements on cis-1,4-polybutadiene have shown the crystallization and melting temperature to be dependent upon the rate of cooling and heating (56). Under certain experimental conditions, two crystalline melting points could be detected. A transition in cis-1,4-polybutadiene a t about 60” C. has been detected by measurements of internal pressure (23) and coefficient of expansion (172) and may be related to the processing characteristics of this polymer. By x-ray measurements, the firstorder transition behavior of ll4-polybutadiene with trans contents from 71% to 94y0has been reported (21). Transition measurements on 1,4-polybutadienes with a range of high cis contents have also been made (142, 211). For ethylene-propylene copolymers, Jlaurer has presented new data and summarized existing data on the glass transition as a function of composition, including the effect of nonrandom microstructure (145). Further work on ethylene-propylene rubbers has been reported combining length-temperature, volume-temperature, and penetration techniques (85). With DTA, the first-order transition of ethylene-propylene copolymers has been examined and related to copolymer composition and degree of randomness (16). The glass transition behavior of various block copolymers has been reported, including the effect of overall composition and block size upon the torsion pendulum results (10). It has been shown that measurements of melting temperature and degree of crystallinity can be used to determine the average stereosequence length of copolymers ( 3 ) . Among new experimental techniques for measuring transition behavior of rubbers are a thermo-mechanical method (35),a rolling ball spectrometer (61), and a new dilatometric apparatus (141). Sequence Distribution. The wide range of copolymers that can be prepared with new catalyst systems has resulted in some detailed investigations of such copolymers, particu242 R
ANALYTICAL CHEMISTRY
larly with regard to possible nonrandom nature. Dilatometric measurements have proved useful for poly(propy1ene oxide) (9). The possibility of using solubility techniques to investigate irregularities in chemical composition has been discussed (80). Fractional crystallization shows promise for separating polymers which are heterogeneous with respect to ease of crystallization (116). This technique, combined with turbidimetric measurement of light transmission, has been applied to ethylene copolymers (114). The effects of molecular weight and molecular weight distribution were small compared to the effects of structural distribution. The latter were found to depend upon catalyst and experimental conditions. The possibility of using sedimentation velocity or density gradient centrifugation to detect differences in chemical structures has been reviewed (99). The chemical composition distribution of ethylene-propylene rubber has been discussed (164). A review has appeared on a number of specific applications of physical techniques such as adsorption spectra, NJfR, thermodynamics of polymerization, and phase transition behavior, to the analysis of sequence distribution in copolymers (94). Polymer Blends. Another area of increased activity involves the detailed analysis of polymer blends containing one or more rubber components. Of particular interest are measurements of polymer compatibility and, in the case of incompatible systems, the characterization of the heterogeneous structure. The glass transition behavior of rubber blends has been used as one criterion of compatibility, and results are presented for a number of systems ( I 7’) , including a discussion of compatibility related to the solubility parameters of the components (60).
A series of photomicrographs resulting from electron microscope studies has been presented showing results for a number of rubber blends of potential technical importance (120). Phase contrast techniques and electron microscope studies of fracture surfaces have been applied to a number of rubber blends and the results discussed in terms of the properties of the separate components and the method of blending (2%). A combination of swelling techniques and electron microscopy has also proved useful in studying rubber blends (149). Morris has applied rate of crystallization and glass transition measurements to an investigation of blends of cis-l14-polybutadiene with NR and with SBR (161). Other. Density gradient columns (138) offer a rapid and sensitive method of measuring polymer density, of particular use in the detection of small amounts of crystallinity.
POLYMER CHARACTERIZATION BY CHEMICAL AND SPECTROSCOPIC METHODS
General Information. Harwood reviewed the study of sequence distribution of copolymers by chemical degradation (93). SBR. If styrene content is known from other measurements, refractive index can be used to find the 1,2- and 1,4-butadiene content (206). The results are in agreement with data determined by reaction with benzoyl peroxide. BR. The absorption band at 911 cm-l was used for 1,2-structure, 967 cm-1 for trans-1,4-, and 1658 cm-1 for cis-1,4-. The sum of the unsaturations agreed with chemical measurements (237). Binder extended his previous work by assigning the 740 cm-1 band to the hydrogen out-of-plane vibration of cis-1,4-structure (26). The number average molecular weight of polybutadiene in the range 800 to 1800 was determined by combining spectroscopic techniques (‘71). NMR counts the number of terminal groups and, near infrared absorption a t 1.636 microns, determines their concentration. Molecular weight is calculated from the two. EPM. The “blockiness” was studied by examining the products of pyrolysis carried out in a hydrogen atmosphere and then passed over a hydrogenation catalyst (218). The mixture of saturated hydrocarbons is less complex than the original pyrolyzate. The skeletal configuration is related to polymer structure-e.g., propylene sequences yield 2,4-dimethylheptane. Alternating ethylene and propylene sequences give n-butane. Tail to tail propylene gives 2,5-dimethyl hexane, 3-methyl heptane, and 4-methyl octane. The position of the infrared band in the 930- to 975-cm-1 region is related to the number of consecutive methylene groups on either side of a side methyl group (165). EPDM. D C P D (dicyclopentadiene) is incorporated into the polymer through its 9,lO-double bond, leaving the 1,2-double bond available for sulfur curing (67‘). The 3060-cm-l band of the 9,lO-double bond disappears during polymerization while the 3045-cm-’ band of the 1,2-double bond is preserved (65). CR. Ozonolysis produced succinic acid. Absence of the 980-cm-l vinyl and 910-cm-1 terminal methylene bands show that the structure is predominantly 1,4- (12). This was further studied by NMR measurements which can distinguish head to tail, head to head, and tail to tail addition (77). Polyurethanes. Cured samples were solubilized by heating with dimethyl acetamide and n-butylamine (163). NMR was then used to measure
the sum of biuret ,rtnd allophanate. One linkage per 10,000 molecular weight can be determined with accuracy of 10 to 20%. In prepolymerri the two types of linkage can be differen.tiated. DETERMINATION OF POLYMERS IN POLYMER MIXTURES AND CO r(STITUENTS IN COPOLYMERS
EPM. The absorbance ratio of the 2.28- and 2.31-micron bands is proportional to propylene content in the range 60 t o 100% propylene, regardless of whether the sample is a random copolymer, block copolymer, or a mixture of homopolymers (28). Ratios of the 720- tcl 968-cm-I bands and the 720- to 1 1 6 3 - ~ m -bands ~ are proportional to the propylene content only for block copolymers or homopolymer mixtures (28). The methyl band at 1155 cm-I is independent of block or random copolymer cortfiguration as long as there is a t IeasL one methylene group on each side of the point of branching (165).
The use of the 1 3 8 0 - ~ m -methyl ~ band has previously been limited to absorbance ratios with other bands or to polymers soluble in CC1,. The intensity of the infrared absorpt on requires film samples to be very thin and there has been no practical way to measure film thickness. Now @-ray absorption has been used to make the thickness measurement (219). Accuracy is 1%. Block copolymers c m be analyzed to &lo% in the range 10 to 40% ethylene by proton magnetic resonance (179). The polymers are dissolved in diphenyl ether and the spectra are measured a t 200' C. The method is also applicable to mixtures of homopcllymers but not to random copolymers. Glass transition temperature cannot be used for determins,tion of ethylenepropylene ratio unless the microstructure is known (85, 146). If E P M is pyrolyzed in a stream of hydrogen and the products are passed over a hydrogenation catalyst,, all the fragments are hydrogenated. The composition was determined by passing the hydrogenated fragments into a gas chromatograph (218). Propylene content is proportional to the ratio of peak height of n-heptane t o the sum of the peak heights of 2-methyl heptane and 4-methyl heptane. The calibration curve is independent of the sequence of monomers in the copolymer. EPDM. One of the hydrogenated pyrolysis products from E P D M containing DCPD is cydopentane (218). The ratio of the height of its gas chromatographic peak to that of wbutane is proportional to the amount of DCPD present. If the pyrolyzate is not hydrogenated, the heighi, of the cyclopentene peak is linearly related to DCPD content.
The pyrolyzate may be caught in benzene to give a yellow solution whose absorbance at 436 mp is a measure of DCPD (90). The 3045-cm-1 band in the spectra of thin films was also used to determine DCPD (66). Results agree with values found by IC1 addition. NBR. The acrylonitrile content of N B R was measured by pyrolysis-gas chromatography using a thermal conductivity detector (66). NR/SBR. Pyrolysis followed by gas chromatography gave linear calibration curves under conditions where monomers were produced (128, 235). The accuracy was rt2% (235). Accuracy of =k5% was achieved when the pyrolysis products were analyzed by means of the ratio of the infrared bands at 698 and 1380 cm-l. NR/BR. Pyrolysis-gas chromatography was also used for mixtures of these polymers (55, 236), in one case with accuracy of =!=3%. Pyrolysis-infrared is accurate to &5% using the ratio of the 965-cm-' vinyl band of BR and the 1380-cm-' methyl band of K R (205). As 1,2-addition, which leads to the vinyl structure, is only a minor part of BR, one must be sure to calibrate with BR made by the same method as that in the samples. NR/NBR. Pyrolysis-infrared using the ratio of the 2260-cm-' cyanide band and the 1380-cm-' methyl band is accurate to *5% (205). NR/CR. The ratio of the 1220cm-I band from CR and the 1380-cm+ methyl band from N R was used (606). Different calibration curves were required for cured and uncured samples. SBR/BR. After degrading the sample in refluxing 1,2-dichlorobensene, a film was cast. The ratio of the 737- and 1316-cm-I bands is proportional to the BR content (194). The absorbance a t 1316 cm-I is proportional to film thickness. After pyrolysis, the ratio of the 698- and 1380-cm-l bands was used with accuracy of =k5% (206). Pyrolysis followed by gas chromatography measurement of styrene and butadiene monomer gave results accurate to &2% (235). SBR/NBR. Pyrolysis-gas chromatography with thermal conductivity detection was used (65). CR/NBR. Pyrolysis-gas chromatography with flame detection was used (56). CR/ESM. After pyrolysis, the effluent from a gas chromatographic column was split 1:1 between flame ionization and electron capture detectors (65). The ratio of the heights of a flame ionization peak and an electron capture peak was used. CR/IIR. The ratio of flame ionization and electron capture peaks is reproducibly related to polymer composition (65).
NR Grafts. The Weber test (231) used to identify N R has been put on a quantitative basis (196). The sample is treated with IBr, then phenol, and the resulting color is measured. The method was applied to mixtures and grafts with poly(methy1 methacrylate) with accuracy of *3%. Ungrafted N R was extracted by petroleum ether, poly(methyl methacrylate) by acetone, and polystyrene by ethyl acetate (81). The graft copolymer was taken as the material remaining after subtracting the homopolymers found. Accuracy was *l% at the 40% level. DETERMINATION OF RUBBER
Various rubbers can be determined by loss of weight under pyrolytic conditions (104). The cured, filled sample is acetone extracted, then heated a t 520' C under nitrogen. The polymer determined by loss in weight is accurate to *2% at the 56% level. The method is not applicable to CR which gives off HC1 leaving a residue of carbon, which causes low results. UNSATURATION
All of the new work reported in determination of unsaturation is concerned with EPDM, especially where the diene component is DCPD. The addition of IC1 was found to be quantitative on EPDM made with DCPD tagged with carbon-14 (90). The amount of substitution is very small and can be neglected. EPDM containing 1,4-hexadiene and 1,7-octadiene also gave good results by IC1 addition without correction for substitution (216). Where the diene is cyclooctadiene, there is considerable splitting out of H I , and a correction must be made. The amount of IC1 taken up by EPM was found to be very low (134). The amount of unsaturation in DCPD polymers found by infrared agrees with that found by IC1 addition and Brz addition (67). The 3045cm-l band was used to determine DCPD (66) while others prefer the ratio of the absorbances a t 1600 and 4300 cm-' (90). IC1 addition was used to establish standards for calibration. HALOGENS
Chlorine. Viscous I M was diluted in isooctane and burned in a Wickbold apparatus (186). Traces of chlorine from the samples were absorbed in Hg(CNS)Z. The thiocyanate which was displaced formed a complex with ferric iron which was measured colorimetrically. Precision was rt0.l ppm in the range 3 to 10 ppm. Combustion products from a furnace or oxygen flask were absorbed in 6% HzOz and titrated with Hg(NO& to the diphenylcarbazole endpoint (30). Accuracy is 3=0.5% in the 18 to 50% range. VOL 39, NO. 5, APRIL 1967
243 R
Fluorine. Polymers containing fluorine were mixed with diethylene glycol and Na20p and decomposed in a Wurzschmitt bomb (152). The fluoride was converted to H F by passing the dissolved combustion products through an ion exchange resin in the H-form. The H F was then titrated with NaOH. The relative error was 2% when the sample contained 50% fluorine. SULFUR AND SULFIDES
Samples were decomposed for determination of total sulfur by burning in a combustion furnace or oxygen Aask (80). The oxides of sulfur were absorbed in 6% H202which converts them to sulfate. The sulfate was then titrated with Ba(N03)zto the carboxyarsenazo endpoint. If Zn is present, it is masked by sodium 2,3-dimercaptopropanesulfonate. Free sulfur was determined by the method of Brock and Osborne (36). It was found that only elemental sulfur reacts with triphenylphosphine and that polysulfide crosslinks do not react a t 56" C (225). The copper spiral method for free sulfur agreed well with the polarographic determination (41). Other workers found that triphenylphosphine removes sulfur from polysulfide crosslinks a t 80" C, converting them to monosulfides (158). Sodium di-n-butyl phosphite reacts with polysulfides and disulfides, forming mercaptans or sulfides (158). Mercaptans were determined by swelling the sample in acetic acid, adding a measured excess of hgNO3, and backtitrating with KC1 to an amperometric end point (49). The relative standard deviation is 5 to 7%. Polysulfide crosslinks are reduced by LiAlH4according to : R-S,-R
+
2RSH
+ (X - 2)s-2
The reaction mixture is treated with acid and HzS is swept into cadmium acetate solution by a stream of nitrogen (60). The sulfide is measured by iodometric titration. The mixture remaining after removal of H2Sis filtered, and RSH is determined by amperometric titration of the filtrate with AgS03. In a slightly different approach, t,he acetone extract was titrated with KCN to determine free sulfur (230). Inorganic sulfide was released by treatment with HC1 in ether. The H2S was collected in cadmium acetate solution, where it was titrated potentiometrically with KI03. The extracted rubber sample was then treated with LiAIHa, and the mixture was titrated potentiometrically with AgN03 to determine both sulfide and mercaptan from the two breaks in the curse. Monosulfide crosslinks were determined from the difference between total sulfur and the sum of all of the above. 244 R
ANALYTICAL CHEMISTRY
The infrared band a t 1600 cm-1 increased in intensity up to 20% sulfur when NR was cured with sulfur, with or without accelerators (133).
CURING AGENTS
Identification. Papers were published dealing with separation by paper (199, 233) or thin layer (12.4) chromatography. The early work on paper chromatography was reviewed ( I S ) . One scheme was described for separating the acetone extract by column chromatography (180). In general, identification after chromatographic separation is based on R values and color tests. If a positive identification is required, use of infrared or ultraviolet spectra and other physical properties is preferred (13). Once the material is separated the hard work is done, and examination of the spectra of the fractions is well worth the small extra effort required. Determination. Sulfenamides and reaction products formed during vulcanization have been determined (40, 41). The benzene extract was separated on a silica gel column. Where possible the separated materials were converted to MBT (2-mercaptobenzothiazole) which was measured by means of its ultraviolet absorption. MBT is unique in that is has a band a t 329 mp with absorptivity of about 160 liters/ gram cm. There are few materials with bands a t this long wavelength and also few with such intense absorption. Where conversion to MBT was not feasible, polarographic measurement was used. Another method for determining sulfenamides and their reaction products in vulcanizates is based on reduction to l l B T by refluxing with SnClz and HCl 48). The 31BT is then determined by amperometric titration with &SO3. Mixtures of sulfenamides with MBT, the zinc salt of MBT, or MBTS (benzothiazyl disulfide) were analyzed with a relative error of +2%. Methods for determination of MBT, MBTS, and sulfenamides, each one taken by itself, were compared (108). These workers concluded that iodimetric titration of MBT is preferable to conductometric titration with AgNOa or NaOH. However, they did not consider application to commercial stocks where other oxidizable materials, such as age resisters, might be present. The iodine produced when MBTS, sulfenamide, or benzimidazolyl disulfide is treated with KI and acid can be titrated with thiosulfate to determine the curing agent. Adaptations of iodimetric and iodometric titrations were used to analyze mixtures of MBT and MBTS, sulfenamide and MBT, or sulfenamide and MBTS (109).
MBT and its derivatives were determined by measuring the ultraviolet absorption of a thin film of the sample (111). This is applicable only to rubber without fillers. Polymer-bound materials were determined by analyzing extracted samples. The dibenzoate ester of p-quinone dioxime in I I R was determined by measuring the absorption a t 1754 cm-l (38). Diphenylguanidine was determined in the presence of p-phenylenediamine-type age resisters by a differentiating nonaqueous titration (125). If there are no complications due to the presence of extender oils, this is a simple method. AGE RESISTERS
Identification. Separation by paper (13, 2SS), thin layer (124), or column
(180) chromatography prior to the use of color tests or absorption spectra has been described for age resisters. Here, as for curing agents, the use of spectra is to be preferred. Determination. Phenyl-2-naphthylamine and 2,6-di-t-butyl-p-cresol can be determined in uncured rubber or latex by gas chromatography (96). Solid rubbers are dissolved in CHCl3 and latex is diluted with water t o give solutions of low viscosity which can be injected into the chromatograph. The internal standard method is used. Relative standard deviation is 5% but recoveries are poor. The method of Lorenz and Parks (136) was extended to p-phenylenediamine (125). The presence of extending oil or other basic materials interferes. N-2-propyl-N'-phenyl-p-phenylenediamine was determined by a similar method (63). RESIDUAL MONOMERS
The strong ultraviolet absorption band a t 250 mp was used to determine styrene or a-methyl styrene in SBR latex (70). The sample was shaken with heptane to extract the monomer and as little as 0.1% was determined to
*5%.
Copolymers of styrene with butadiene and acrylonitrile were dissolved in dimethyl formamide. After adding toluene as internal standard, gas chromatography was used to determine 0.01 to 1% monomer (198). CARBON BLACK
The possibility of saving time has prompted several investigations of pyrolysis methods. Carrying out the pyrolysis in a vacuum was reported to eliminate the need for corrections from concurrent runs on known samples (181), as required by ASTM method D1416 for SBR black masterbatches (8). The vacuum method has a standard deviation of 0.17% and accuracy of
&0.2%. The use of COZ as inert a b mosphere was recommended for pyrolysis a t 550" C (78). Flelative error was ~ 1 % .At 800" to 900" C, COZ can react with carbon blac i to form CO and cause low results (104). Therefore nitrogen was recommended for high temperature pyrolysis with errors of i:1% a t the 25y0 level. Fu-nace black sometimes contains 1% volatiles, channel black as much as 6%, causing low results. I M was decomposed with HNOI and xylene according to Kress's method (126). The organic matter was then extracted and the residue was dried. The weight loss upon ignition of the residue is the carbon black content (29). Fluorine-containing polymers were decomposed with tetrachloroethane, HN03, and dirtethyl formamide before separating a residue of carbon black and ash which was finished as above (152). When clay is present, errors can occur if the pyrolysis is doni? at 50O0-6OO0 C, and the final ignitior a t 800"-900" C because of incomplete dehydration of the clay a t the lower temperature. I n this case, it is better to measure the COZ evolved during ignition of the carbon black than t o use the loss in weight (104). METALS
Identification. Thin films of vulcanizates were cut m ith a microtome a t -50" C. Infrared spectra of the sections were used to identify silica, clay, CaC03, and hfg(203 (204). Destruction of Organic Matter. Concentrated Hi304 and 50% H202 rapidly decompose or 5anic samples for subsequent determin:ttion of metals. Fe, Pb, Cd, and Zn, wkich tend to be lost in dry ashing due to the volatility of some of their compounds, are retained, and the blanks from reagents are low (207). Total Ash. Ash content can be calculated from the weight of residue remaining after igniticn of carbon black (78, 104). Porcelain crucibles must be avoided in ashing fluorine-containing polymers due to the volatility of SiF4, which would cause low results (152). EDTA Titration. I n a rapid method for Zn, titration of ti solution of the ash is carried out a t p H 4.5. Ca, Mg, Al, Fe, and Ti are mttsked by fluoride This and 2,4-pentanediono (113). method is superior to titration with ferrocyanide. Determination of Ca and M g in the ash of hevea brasiliensis leaves was compared in four laboratories (151). One used a novel method of precipitating Ca with molybdate and tungstate (160). Mg can then be titrated directly in the filtrate. Ca can be titrated directly at p H 12 or calculated by difference from
the titration of Ca plus Mg at p H 10. The other three laboratories did the latter two titrations, taking Mg by difference. The direct titration of Mg gave results significantly higher than the indirect determination. Colorimetric Methods. The ASTM methods for copper in raw natural rubber have been revised ( 7 ) . Zinc dibenzyldithiocarbamate is the reagent in the new referee method. It has the advantage that the copper complex forms in acid solution, thus neutralizing with ammonia is not necessary. Because the solution can be kept acidic, iron does not have to be masked with citrate. Interferences from other metals are low. Extraction is also more rapid. The new alternative method, which is being considered by ISO, uses zinc diethyldithiocarbamate as reagent. It, too, permits rapid extraction. I n an interlaboratory test program, Mn in hevea brasiliensis leaves was determined (151). There was no significant difference between oxidation to permanganate by potassium periodate or by sodium bismuthate. X-Ray Diffraction. ZnO in vulcanizates is determined quantitatively and distinguished from zinc stearate (225)* Flame Photometry. The determination of Ca in leaves of hevea brasiliensis by EDTA titration and flame photometry was found to be in agreement (62). Flame photometry is preferred because i t is faster. Emission Spectrometry. Rubber is ashed in a carbon electrode. The electrode is placed in an i i C arc to transfer Ti t o the upper electrode. The upper electrode is then used for emission spectrometric determination of Ti (82). Standard deviation is i:5% in the range 16 to 50% TiOz. Radiochemical Methods. The Li in poly(butadieny1) lithium is exchanged with tritium from tritiunilabeled alcohol. The sample is then reduced to a mixture of tritium and tritiated methane which is counted (39). The results are accurate to i:5% and agree with analysis by carbonation and acid number. OTHER M E T H O D S
The error of considering that all of the weight lost on heating or milling is due to water was pointed out. A preferred alternative is to pass dry nitrogen over the sample and to use a moisture monitor to measure the water picked up by the nitrogen (129). Precision is 209;b a t the 0.4y0 level. The Baker-hIullen cell was modified by placing the trays in a cylinder made of screen (167). This prevents the gel from migrating. Extender oil in E P D M was determined from the absorbance ratio of the
1620- and 4350-cm-l bands of a polymer film (69). The accuracy is =t2% of the amount of oil present if a sample of the oil is available for empirical calibration. The sensitivity depends on the type of oil being used as the 1620-cm-' band is due to aromatic structure. The analysis of leaves of hevea brasiliensis for nitrogen and phosphorus was compared in four laboratories (151). Nitrogen was determined by Kjeldahl digestion with titration or colorimetric finish while phosphorus was measured colorimetrically as molybdovanadophosphoric acid or as heteropoly blue. Starch was extracted from NR or SBR foam and hydrolyzed to glucose by HC1. Anthrone and H2S04were used in the colorimetric finish (131). Vinyl side groups on VSi were treated with PZOSand HZOto liberate ethylene which was measured by gas chromatography (102). The acetone extract of NR was separated by column chromatography, and the yellow pigment was identified as p-carotene (220). Castor oil in hevea crumb was determined by gas chromatography (130). LITERATURE CITED
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0
ANALYTICAL CHEMISTRY
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CONTRIBUTION No. 374 from the Research Division, The Goodyear Tlre and Rubber Co., Akron, Ohio 44316.
VOL. 39, NO. 5, APRIL 1967
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