Rubber Coe W. Wadelin and Marion C. Morris R e s e a r c h Division, The Goodyear Tire a n d Rubber Co., Akron, Ohio 44316
This review covers chemical analysis of rubber and the characterization of rubber by physical, chemical, and spectrometric methods. Methods for thc kkz!:ficz: ion. characterization, and determination of' rubber nnd marerials in rubber are included but the analysis of rubber additives before they are put into a rubber compound is not included. Polymers other than rubber are covered in another review in this issue (190). The literature which became available to the authors between September 1970, the end of the period covered by the.last review in the series (272), and September 1972 is covered. Abbreviations recommended in ASTM Designation D1418-72a have been used (1 I). They are listed in Table I.
Table I. Abbreviations Recommended by ASTM ( 1 1 )
GENERAL INFORMATION The use of thin layer chromatography for a variety of purposes in rubber analysis was reviewed (151, 186). The need for running known samples in each laboratory was pointed out as R , factors and the colors produced by spray reagents are not closely reproducible from one laboratory to another. Classical physical and chemical methods (74) and NMR (nuclear magnetic resonance) spectroscopy (237) for the analysis of rubber were also reviewed.
SBR AU
POLYMER IDENTIFICATION Pyrolysis-Gas Chromatography. The work in this area was reviewed by several authors (39, 72, 137, 244, 2179). It is recognized that interlaboratory reproducibility is a problsm (39, 244). The opinion was expressed that laboratories should concentrate on apparatus that will give controlled, reproducible data for interlaboratory transmittal rather than gathering empirical data from their own unique apparatus (39). A catalog of chromatograms of 23 elastomers was compiled by pyrolysis for 12 sec a t 720 "C on a Pt-ir filament. NR was distinguished from IR by differences in peak heights in two parts of the chromatogram (62). Curie point pyrolysis of a 1-mg sample at 610 "C for 10 sec was preferable to pyrolysis on a wire coil at 550 "C (67). With the Curie point pyrolysis, fewer products were formed, the monomer yield was higher, and the products were separated better. The improved separation may be due to a smaller dead volume in the pyrolysis chamber. NR and SBR were detected in tire treads by means of the isoprene and the butadiene and styrene peaks, respectively. IIR was detected via the methyl propene peak. T R T (temperature rise time, the time required for the temperature of the sample to rise from ambient temperature to its equilibrium value) is important because pyrolysis starts to take place during temperature rise (26.3, 164). The T R T should be less than one-tenth of' the decomposition half time for good interlaboratory reproducibility. A new circuit for filaments gave a '7 to 12 msec TRT and the close temperature control of a Kelvin bridge, but a true pyrolysis temperature which is below the final equilibrium temperature for filaments (163). The T R T of Curie point pyrolyzers is slow (200 msec) thrnugh the last :iO 'C before reaching the Curie point temperature because oi the sharp
CSM
Chloro-sulfonyl-polyethylene
EPDM Terpolymer of ethylene, propylene, m d a diene with the EPM FKM
ABR
BR
CR IIR
IR NBR
NR
residual unsaturatedgortion of the diene in the side chain Copolymer of ethylene and propylene Fluoro rubber of the polymethylene type having substituent fluoro and perfluoroalkyl or perfluoroalkoxy groups on the polymer chain Acrylate-butadiene rubber Butadiene rubber Chloroprene rubber Isobutene-isoprene rubber Isoprene rubber, synthetic Nitrile-butadiene rubber Natural rubber Styrene-butadiene rubber Polyester urethane rubber
decrease in power consumption a t temperatures close to the Curie point (164). Laser pyrolysis offers ultrafast temperature rise time of 100 to 300 psec and expansion of the products into a plume where they are rapidly cooled, minimizing secondary reactions. Thermal equilibrium is not attained in this short time. Product distributions are similar to those from thermal degradations at 700 to 1200 "C (86). Absorption of the laser energy is accomplished by coating the sample with graphite or mixing it with carbon (90). Mixing is more reproducible; 5% carbon is the preferred amount. The sample can also be placed on a cobalt glass holder (85,86). Photolysis with a mercury arc was also used to decompose polymers for gas chromatography but required 5 to 30 min (138). The standard errors of peaks from a blend of NR and SBR were estimated to select the most reliable peaks (6). Metathesis. Treatment of BR and 2-butene with WCls, ethanol, and ethyl aluminum dichloride gives 2,6octadiene. SBR gives 5-phenyl-2,g-decadiene and 4-phenyl cyclohexene in addition (188). If the olefin is 2-hexene, a more complex mixture of products results (128). Metathesis is a good tool for studying polymer structure because it gives clean reaction products. Nuclear Magnetic Resonance. An excellent review of both principles and applications WE^ published (37). An older book was translated into English (238). In carbon-13 spectra, the peaks ale more widely spaced than in proton spectra. This makes it possible to distinguish cis- and trans-1,4-polybutadiene which cannot be done by proton spectra. An additional feature is that the spectra of solid samples as well as solutions can be determined (80). This means that cross-linked samples can be examined. Polyols, isocyanates, and chain-extenders in polyurethanes were identified (57). Infrared. The spectra of gaseous pyrolyzates were used for polymer identification by comparing with a library of reference spectra. The acetone-extracted samples were pyrolyzed at 515 "C in a nitrogen atmosphere (123).
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Attenuated total reflectance spectra were used for coating identification to avoid the necessity of separating the coating from its substrate (162). This overcomes the problems of finding a suitable solvent for casting a film and the radiation scattering of polymers in KBr pellets. Other Methods. Comparison of the Raman Elid infrared spectra of polymers shows the complementiicy nature of the two types of spectra. Use of the He-Ne laser as the Raman source overcomes the problem of the fluorescence which occurs when a Hg arc is used (42). DSC (differential scanning calorimetry) thermogrtims in nitrogen and oxygen from 0 to 500 “C were used to identify polymers. DSC is preferred over DTA (differentisl thermal analysis) because it is independent of particle size and sample packing (234). Polymer pyrolyzates obtained a t 700 “C were dissolved in dimethyl formamide and examined by polarography using tetramethyl iodide as supporting electrolyte. The half wave potential was used to identify the components, particularly styrene (232). Thin layer chromatography of polymer pyrolyzatf!s was also used for identification (255). AU was hydrolyzed with KOH and the components were separated by distillation and extraction. Tlw diamines were identified by gas chromatography and ultraviolet spectroscopy, the acids by thin layer chroma :ography, and the polyols by gas chromatography (166). A series of United States patents describes methods for identifying rubber by chemical tests which give chariicteristic colors (210-212). The claims made in these p e lents defy some of the fundamental principles of polymer science. For instance, extracting with acetone is su 3posedly used to “remove all traces of vulcanization,” after’ whjch “NBR is dissolved in 2,2,4-trimethyl pentane.” MOLECULAR WEIGHT AND MOLECULAR. WEIGHT DISTRIBUTIONS General. The literature surveyed in this portion of the review is primarily concerned with physical techniqurs of characterizatiop of uncured elastomeric materials by molecular weight and molecular weight distribution (M\!m) Because these materials are often molecularly heterog m e ous in structure and composition as well as in molewlar size, adequate characterization of the properties of tec linical importance often involves several techniques. Thi: inteqt of this review is to point out significant developments in these areas. The list of references is not to be coriiidered a complete bibliography of the field. Molecular Weight. The tendency of the last few yea] L to using Gel Permeation Chromatography (GPC) for mol x u lar weight analyses has continued. However, other m?thods have been studied and “Recent Trend in Determination of Molecular Weights,” was the topic for a joint symposium of Analytical Chemistry and Polymer Divisions of ACS in 1971 (8). On light scattering (149), topics covcled were instrumentation, use of lasers as light sources, automated systems, and low angle photometers. Calibrat on, correction factors, and standards were discussed as well as computer treatment of data and treatment of data ]*or polymer systems containing microgel. A paper was giTren on characterization of extremely high molecular wei ;ht polymers (236) and the special problems which are ?ncountered. Light scattering, sedimentation equilibria, 5 lid viscosity measurements were used. It was shown that 1 he same Mark-Houwink viscosity parameters did not hold in the lo7 molecular weight region. At the other extreme of molecular size, low molecular weight materials were ch i r acterized by vapor phase osmometry (VPO), and the r d 334 R
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vantages of using ca1ib;ation charts were shown (126). A review was also given a t this symposium concerning the requirement of a successful ebulliometric system for molecular weight determination as well as estimates of the precision and accuracy for routine use (98). Another paper covered the need for a n independent measure of for narrow molecular weight distribution polymers since GPC introduces errors due to peak broadening of the same magnitude as the polydispersity (29). Other papers a t this excellent symposium covered reference materials for polymer characterization by the National Bureau of Standards (273) and recent work on the use of thin layer methods of determining molecular weight distributions (208). Difficulties in characterization of BR and SBR due to microgel and microstructural variations were brought out (15). In another symposium, the use of lasers in polymer science for light scattering was discussed (10). The ACS Award Address in Polymer Chemistry by A’. V. Tobolsky was on Viscoelasticity and Molecular Weight Distributions and dealt with the molecular aspects of viscosity in concentrated solutions (256). Recent theoretical and experimental work on the refractive index increment used in light scattering and the effects of polydispersity of molecular weight and of composition were shown (173). Certain cellulose acetate membranes used in determination of molecular weights of polystyrenes showed no diffusion for molecular weights of 1500 (245). While membrane osmometry, vapor phase osmometry, and light scattering techniques were used in other routine polymer characterizations and in calibration of GPC, little effort was reported in further development of these methods. Determination of molecular weights and molecular weight distributions by direct imaging of molecules in a n electron microscope has been attempted. Difficulties were encountered with regard to sample preparation with g t i facts present that look’ similar to molecules (18). In one study, polystyrene molecules were deposited on a sub’strate and shadow was cast by vacuum deposition of platinum (19). Variations of end group analysis methqds for determination of molecular weights were reported using infrared analysis for acetate group concentration (75) and using pulsed NMR (69). Reviews and Equipment, GPC. Recent books on GPC (7, 213) are especially useful for those who are new in the field. Reviews with good general treatment are available (30, 135, 261, 278). Molecular size and calibration aspects are stressed in other reviews (23, 28, 268). Of particular interest to newcomers in the field are the definitions, symbols, and units for GPC recommended by a task force under ASTM committee D-20.70 (31). Fundamental relationships are also given as are recommended practices for publication. Current trends iq GPC theory, equipment, and methodology have been reviewed (33, 49). There have been numerous new developments in equipment and techniques for gel permeation chromatography (GPC) in the last two years. Apparatus and detectors have been the subject of a paper (20). Commercial developments have centered around low cost liquid chromatographs (58, 82, 274) and chromatographs capable of high pressure operation (58, 82, 103, 201, 266, 274). High pressures allow the use of longer columns for resolution and higher flow rates to keep the analysis time dowu. Recycle equipment is available making possible in effect the use of extremely long columns (26, 27, 34, 35, 274). The types and uses of GPC column packings have-been reviewed (111, 117). The properties of polystyrene gels for
NO. 5, APRIL 1973
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GPC are affected by diluent and.co-monomers (200, 277) used in preparation. Other organic polymer gels have been found useful for GPC separations (59, 114, 119), many based on vinyl acetate (115, 116, 118, 218). Porous glasses have been found useful for column packings since they resist collapsing under high pressures and are more inert chemically (63-65, 285). Scanning electron microscopy studies have aided in the understanding of the porosity of these packings (73, 99). GPC D a t a Treatment. Considerable effort has been spent on understanding the separation mechanism of GPC. The theory on separation by flow through small pores seems very successful (76, 107). Other work on separation mechanisms has been done (267, 281). Axial dispersion of solute is a closely related phenomenon and has been considered from a theoretical point of view connected with the separation mechanism (76, 107). Investigations concerning the shape of the chromatographic band for individual species have been made (110, 122, 141, 202) and a review has been published (142). On a qualitative basis, GPC separates polymers into components by elution according to molecular size. Precise quantitative use of GPC to calculate molecular weight averages requires that calibration and correction factors be applied to the raw data since the shape of the chromatogram depends on the separational ability of the columns. The two major factors are the character of the elution volume, molecular weight relationship and the degree OS band spreading. The first factor has generally been approached by calibration with narrow MWD polystyrene standards and indirectly related to the molecular weight calibration for the particular polymer type. The most widely used approaches depend on the universal calibration curve relating hydrodynamic volume of the molecules to elution volume (102). Additional verification of the concept has been made (77, 225) and successfully applied to molecular weight calibration for polymers where only polydisperse samples are available (40, 195, 276). Further simplification by use of a constant Mark-Houwink exponent has been reported (94). Methods making use of a universal calibration have the advantage of avoiding the use of narrow MWD standards of each polymer. The universal calibration curve is generally established for a given column set and solvent system by use of narrow MWD polystyrene standards. Calibration for other polymers may then be calculated once the Mark-Houwink parameters are’ known for the polymer-solvent system. Operational variables can have an important effect on the apparent molecular weight distribution. In particular, the sample viscosity has an effect on the elution characteristics (101) and the safe operating range in terms of concentration and viscosity has been described (194). Nonlinear fractionation effects with overloading and in relation to column properties have been observed (209). Concentration and viscosity effects as well as flow rate have been considered in setting up fast GPC (168, 169). Flow rates up to 35 mljmin were found useful with less loss of resolution than might be expected from low flow rate results. Corrections for instrument spreading are basically methods to solve Tung’s equation (260). This is the generalized integral equation for the GPC response taking into account the instrumental spreading function. Several mathematical methods exist for solving the equations, all requiring computation by computer. Methods involving linear algebraic equations have the advantage of allowing spreading as a function of elution volume (53, 223, 240, 264). Methods using Fourier analysis reduce analysis time
but have other shortcomings (224, 264, 269). Simplified corrections have been suggested (16). Broadening effects were examined directly by comparing GPC results to osmotic pressure and light scattering results for fractionated and unfractionated poly(methy1 methacrylate) (24). Experimental comparisons using polystyrene also exist (21 7, 223, 240) as well a5 experimental verification of the spreading correction for BR and poly(viny1 chloride) (217). Recycle GPC has been used to determine the width of narrow distribution samples (275). Very narrow MWD polystyrene standards of heterogeneity ratio less than 1.009 are now available (274). GPC on Copolymers and Blends. While GPC techniques are more difficult to apply to copolymers, they are necessarily an important practical pioblem. Compositional and structural heterogeneity affects detector response as does the concentration of solute. Multiple detectors combining differential refractometry, infrared, and ultraviolet analyses are commonly used to separate these variables. The composition and molecular weight of SBR copolymers were examined using ultrriviolet analysis coupled with refractometry and the presence of some low molecular weight polystyrene was detected ( I ) . In another investigation, infrared detection was used to examine the styrene content and the microstructure of the polybutadiene component (113) in SBR. A study of styrene-butadiene block copolymers was made with GPC comparing those made with dilithium us. monofunctional catalyst (183). Preparative GPC has also been used in the structural evaluation of SBR (17). Other copolymer systems examined are ethylene-vinyl acetate ( I 7), styrene-siloxane (70), polyisoprene-polystyrene, polyisoprene PMMA, alpha methyl styrene-butadiene and vinyl-toluene-butadiene block copolymers (183). Other work on monomer distribution (12) and molecular weight analysis of block copolymers has appeared (52). Separations by methods other than GPC are often valuable for blends (22). GPC and Branching. GPC has b e m used to examine the degree of branching in polymers by means of the effect of branching on the viscosity-molecular weight relationship. Several specific approaches exist and all depend on the fact that hydrodynamic volume controls the GPC separation. In some cases, model materiak such as star molecules are available and treatment is facilitated (66, 78). Where no model is available, another approach has been taken which relies on preparative fractionation and measurement of viscosities and molecular weights of the fractions (262). In this way BR and SBR which differ widely in method of synthesis and branching, have bee: examined (150). Studies of the effects of branching, on bulk polymer viscosities have been made for BR (134). GPC of Other Materials. Since po1,ystyrenes are used widely for calibration, it is important to have molecular weight-viscosity characterization under different conditions, especially for the standards. This information is helpful in setting up the universal calibration c u y e (195, 222). Characterization was accomplished for polystyrenes prepared by several techniques. The molecular weight distribution of polymer from radiation incluc,ed polymerization of styrene a t low temperatures gave Mw/MN ranging from 1.79 to 4.15 (125). Polystyrene prepared by inducing polymerization using a high electric field reveals both free radical and cationic polymerization (157). Characterization of polystyrene latices polymerize4 without surface active agents and showing a high degree of particle size monodispersity yielded a heterogeneity ratio, MwjMN, ranging from 2.1 to 4.4 (148). Recent work has been accomplished to determine the
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MWD of NR. Five clones were reported to show distinct bimodal distributions. However, results do not agree wiLh fractional precipitation results. Branching or difficulty in calibration are given as possible causes (246). GPC treatment has been described for BR f2t2). Branching in BR has been covered separately in this leview (134, 150) The MWD was found for BR catalyced with triisobutyl aluminum and titanium tetraiodide. 4t high Al/Ti ratios, the distribution curves exhibited tailing a t the low molecular weight end and became bimcdal (172). Anionic polymerization studies were made on polybutadiene prepolymers. Limited success was realize0 in preparing reproducible molecular weights and M b l l ’ s (219). Application of GPC was made to studies of functionality of carboxy and hydroxy terminated pol3 butadienes (160). Direct calibration has been made for low molecular weight polybutadiene using dual detectors (226).
PHASE STRUCTURE, BLOCK COPOLYMERllj AND POLYMER BLENDS Polymer Blocks. Block copolymers have been the subject of a great amount of research since they Tave shown promise of control over phase structure. Styrenebutadiene blocks have been studied from the standpoint of block length and the method of sample preparation. ABA block copolymers were found to be very sensitilie to the type of polymer treatment (196). Thermal anrlysis and torsion pendulum results show two distinct Filiase transitions (130). Changes in mixing states of blenlls of block copolymer with polystyrene with heat treat tilent suggest that blocks act as surfactant depending on Aock content (229). Low angle X-ray and stress-strain rrwlts have shown relationship of morphology to physical properties (112). Softening by remilling of blends of random copolymer, block copolymer, and polystyrene showed strong interaction by the block in controlling the state of dtnpersion (230). Styrene-isoprene-styrene ABA block copolymers have been studied and also show sensitivity to sample prcparation (196). The glass transitions of the block appear3 between those of polyisoprene and polystyrene 030). {nvestigation of the role of block and graft copolymers ol isoprene-styrene in resin-elastomer blends show that the block copolymers act as emulsifying agents for both polymers creating an oil-in-oil type emulsion permitting control of the degree of dispersion and assuring interfacilnl adhesion (221). The mechanism of domain formatioii has been studied in cast ternary blends of styrene-isoprene block with polystyrene and polyisoprene (132). Prc prints of a symposium on block copolymers are available covering many aspects of the subject (9). Blends of Rubbers. DTA analysis of BR anc SBR blends reveal that the number of phtses depends pi imarily on thermal pretreatment. Unvulcanized polybler (1s are essentially a two-phase system while vulcanized blends are essentially one phase (220). Glass transition te riperatures and thermal expansion coefficients have been determined for several blend systems of the polymers pdystyrene, BR, SBR, PMMA, and styrene-acrylonitrile (178). Discrepancies in the additivity of these propertic s, were noticed and discussed. Statistical treatment for small angle light scattering has been given for studying the phase structure of polymeric solids (197). Visc ,elastic data on rubber mixtures yielding glass temperatiiil-e and loss modulus have been obtained by an automatic 1,orsion pendulum (265). Work on blends as vibration dc.Lmpers shows some to have broad loss peaks (191). Bljnds of 336 R
EPDM and other elastomers were studied and found to have good resistance to ozone cracking (204). Rheological tests with a high pressure capillary viscometer have been. done on blends (258). The morphology of solution-mixed uncured binary blends has been studied under many conditions for several elastomers (95). Compatibility of rubbers has been studied by evaluating their mutual adhesion for 64 pairs of 11rubbers including NR, BR, SBR, and others (56). A similar study involved the use of vulcanized plied rubbers studied dynamically (206). A solvent swelling technique was used to study the ability of elastomers in binary blends to covulcanize (284). The effect of accelerator type on blend vulcanization has been approached by examining extracted unvulcanized rubber in blends containing IIR, SBR, BR, and IR (46). Certain effects in properties have been attributed to the migration of vulcanizing agent from one blend component to another (205). Interaction of Rubbers with Fillers. A considerable amount of work has been focused on the interactions of fillers with blends. Thus, it is found that the mixing sequence used in the preparation of filled blends has a marked influence on extrusion properties (258). Electron microscope studies on SBR, NR, IIR, BR, CR, and NBR showed prior thermal and/or mechanical treatment affects the transfer of fillers between components of elastomer blends (121). Phase microscopy of a triblend of NR, CR, and EPDM did not show transfer of carbon black between phases (181). Quantimet analysis of images has been made showing relative affinity of NR, EPDM, IIR, NBR, and SBR for carbon black (43). One study showed by electron microscopy that regardless of method of mixing filled BR-SBR blends were homogeneous while NRSBR blends were heterogeneous with carbon black concentrated in the SBR phase (241). Filler distribution is found to depend on elastomer viscosity as well as other factors (36). A solvent swelling technique was used to study the ability of elastomers in binary blends to covulcanize (284). Rubber for Impact Plastics. The use of a rubbery phase to toughen plastics is common practice. A review on the production properties and uses of toughened blends, polystyrene in particular, is available ( I 79). Comparisons have been made of impact and relaxation strength with the rubber damping peak (143), and to particle size and the weight of graft copolymer phase (13, 14) and to processing (15). Phase contrast microscopy has been applied to determining the shape and sizes of rubbery phase in such materials (283). CRYSTALLIZATION AND GLASS TRANSITION BEHAVIOR General. The principles and applications of Differential Thermal Analysis (DTA) have been well covered in a recent book (176). While a great deal of literature exists on DTA and Thermogravimetric Analysis (TGA) of plastics, there is relatively little for elastomers. However, an extensive review on the application of DTA and TGA to elastomers has been published (185). This review covers the crystallization and glass transition behavior of blocks, blends, and filled materials. A review of broad scope is available covering methods, equipment, and experimental problems as well as organizations and symposia of thermal analysis (199). Inverse phase gas chromatography using polymers supported on a substrate and with solvents as “molecular prohes” has been used to determine glass transistions and melting temperatures. The degree of crystallization can be
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determined by this method. Unlike the determination of crystallinity by calorimetry, one need not know the heat of fusion. Rates of crystallization are determined from the change in crystallinity with time (104, 239, 280). Dynamic mechanical properties are also used to study crystallinity and glans transition temperature (127). Other methods used for crystallization are X-ray diffraction (127) and stress-optical properties (83). Gas transport in polymers is closely related to free volume and the glass transition (233, 286) as is the diffusion of solvents (41). NR, 1R, a n d Balata. Fundamental studies continue on the properties of NR. Measurements of the thermal conductivity of soft NR have been made extending over a temperature range of below the glass transition to well above room temperature (215). Calorimetry of stretched and unstretched NR gave a glass transition temperature which was not affected by applied strains, Le., the heat capacity of stretched NR was indistinguishable from that of unstretched NR (136). Studies of‘ fusion of NR after isothermal crystallization reveal a low temperature transition dependent on crystallization temperature (145). The rate of crystallization and degree of crystallinity attained for strained NR has been obtained by a stretching calorimeter of high sensitivity (100). Other workers have followed the crystallization of strained NR and IR by dilatometry (253). The effects of microstructure, molecular weight, and mechanical history on bulk crystallization kinetics have been investigated for IR (71). Balata, natural trans-1,4-polyisoprene, was fractionated by preparative precipitation chromatography and molecular weights were determined by GPC. The effects of temperature and molecular weights on bulk crystallization rates were determined by dilatometry. A maximum rate of crystallization was found in the molecular weight range 60,000 to 80,000 (175). Spherulite growth rates and free energy of nucleations were measured and kinetics of crystal growth was studied (174). The annealing behavior of melt crystallized trans-1,4-polyisoprene was studied by X-ray diffraction. The decrease in long-spacing at the transition temperature corresponded to content of low melting form (182). nilatometric, calorimetric, and dissolution studies were made of two crystal modifications of trans-1,4-polyisoprene and equilibrium melting points determined (88). Crystallization of NR, IR, and blends have been examined in a capillary rheometer. The effects of temperature, molecular weight and structure were determined (91-93). Transitions in BR. The change in transition temperatures with varying amounts of cis and trans isomers has been determined (154). The effect of elongation and compressive deformations on the crystallization half-times for BR was examined by stress relaxation (180). DTA, creep, and gas permeation rates showed broad transition zones for NBR copolymers (97). Melting points and crystalcrystal transitions of bulk crystallized trans-1,4-polybutadiene was studied by X-ray diffraction, dilatometry, and calorimetry. Heats of fusion of the crystalline form were determined (25). Infrared spectra and epoxidation data have been used to examine the crystal and amorphous structure of trans-1,4-polybutadiene crystal mats grown from six solvents and solvent mixtures (120). SBR, E P D M a n d IIR. Thermal transition behavior of block copolymers and of blends are primarily covered under phase structure of blocks and blends since these areas overlap. The physical properties of hydrogenated polybutadiene and styrene-butadiene copolymers were related to crystallinity (81). Physical properties of block copolymers of butadiene and styrene were related to the
glass transition temperature (55). Thi? dependence of tread wear and skid resistance of tires on glass temperature was examined for twelve SBR rubbers having major variations in microstructure. A high degree of correlation exists between wear and glass transition temperature (144). Crystallinity was shown by X-ray diffraction in ethylene rich EPDM (21). The stress crystallization temperature of EPDM does not depend on stretching ratio and appears to be related to the ease of nucleation and microstructural homogeneity (89). The glass transition of IIR was affected by various oils and plasticizers and the effect on crystallinity shown (184). POLYMER CHARACTERIZATION BY SPECTROMETRIC METHODS Nuclear Magnetic Resonance. The minimum amount of trans-1,4 detectable in high cis-l,,l-IR is 2.0% a t 60 MHz but is decreased to 0.5% a t 220 R?Hz. The minimum 3,4-structure detectable a t 60 MHz is O..j% (48). Styrene sequences were measured in SBR (193). An analog computer was used to resolve the spectra so that short (2-3 units) sequences could be distinguished from long ( > 3 units) sequences (192). Spectra of EPM a t 220 MHz were used to study tacticity and blockiness (87, 252). Spectra of NBR were measured in 5% solution in CDC13 a t 100 MHz. The degree of alternation found agreed with that predicted from the reactivity ratio (139). Isocyanate endgroups of polyurethanes were converted to dimethyl ureas by dimethylamine. Hydroxyl endgroups were converted to methyl urethanes by phosgene and methyl amine. Number average molecular weights up to 50,000 were then calculated from the ratio of the absorption of the endgroup to that of interna’i structures (57). Carbon-13 NMR was reviewed (79.). I t is a very effective tool because of its widely spaced chemical shifts. For instance, cis-1,4 can be distinguished from trans-1,4 in BR while proton NMR does not differmtiate these isomeric forms (80). Tacticity of EPM was also more easily studied byI3CNMR(68). Infrared. The field was reviewed (79). In 1,2-polybutadiene, bands of atactic units had lower absorptivities than those of syndiotactic units. In IR, the absorptivity of the 840 cm-l band due to 1,4-content was dependent on the cis-trans ratio while the absorptivities at 889, 1476, and 1385 cm-’ were independent of isomer content (254). The bands a t 3003 and 3025 cm-I are preferred to those a t 743 to 967 cm-1 for ck-1,4 and trans-1,4 determination in BR due to the lower sensitivity of the stretching vibrations to neighboring groups (270). Raman. Applications to polymers were reviewed f 79). Cross-links having C=C, C-C, ( 2 - 4 , and S-S bonds were studied. The method is applicable to insoluble Samples but not to carbon black-loaded samples as the carbon black absorbs incident radiation (243). POLYMER CHARACTERIZATION BY CHEMICAL METHODIS Pyrolysis-Gas Chromatography. Dimer fractions from IR were examined to relate the slxuctures of the products to head-head, head-tail, tail-tail, 1,4-vinyl, and vinylvinyl sequences in the polymer ( 1OJ, 109). Pyrolysis of BR a t 700 to 770 “C was used to measure 1,2- content (4). Block structure in SBR was determined using poly (amethylstyrene) as internal standard. Calibration was done using polystyrene and polybutadirme (287).
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Metathesis. Treatment of BR with 2-butene gives 2,6octadiene from the 1,4-1,4 sequences. SBR and ?.-butene give 2,6-octadiene from l,4-butadiene-1,4-butadi8ene sequences while 5-phenyl-2,8-octadiene and 4-pheli:ylcyclohexene are formed from 1,4-butadiene-styrene-1,4-butadiene sequences (188). Ozonolysis. Gas chromatography of ozonolysis products gives quantitative date for head-head, head-tail, and tailtail structures (108). Extraction. Benzene was used to extract unvulcanized polymer from vulcanizates. After evaporating tlir? benzene, the uncured polymer was taken up in CS2 for NMR examination. The weight of the extract gives the amount of unvulcanized polymer while the NMR spectrurr; gives its composition (45).
DETERMINATION OF POLYMERS IN POLYMER MIXTURES AND CONSTITUENTS IN COPOLYMERS NR-SBR-BR. An acetone-extracted sample was pyrolyzed at 900 "C in a vacuum. The pyrolyzate was collxted on a KBr disk at -78 "C. The absorptivities of all three polymers were nearly the same a t 1450 cm-l, so this wavelength was chosen for a reference point. The rstio of absorbances at 699 and 1450 cm-1 was used for SBli and at 885 and 1450 cm-I for NR. BR was found by tlifference. A separate calibration is required for each lele1 of bound styrene in the SBR (189). RF induction heating was used to pyrolyze sample; for gas chromatography. The dipentene, styrene, and 4-1 inylcyclohexene peaks were used to calculate the compo: ition of the rubber (247). NR-SBR-EPDM. Pyrolysis of an extracted samph a t 700 "C was followed by gas chromatography. The isopiene peak was used for measurement of NR, butadienc for SBR, and 1-pentene for EPDM. The standard devi s I ion ' was 3% for NR, 3% for SBR, and 10% for EPDM (153). NR-SBR-BR-IIR. An extracted sample was heated in a vacuum from ambient temperature to 500 "C at 35 t:, 40 "C per minute. The pyrolyzate was collected in a :old trap, taken up in hexane, filtered, and the hexane was evaporated. A 10% solution of the residue was madv in CS2 and the infrared spectrum was recorded. The ratio of absorbances at 885 and 2300 cm-l was used to calcu rite the NR content, 700 and 2300 cm-1 for SBR (assuming 23.5% bound styrene), 910 and 2300 cm-l for BR, inid 1220 and 2300 cm-l for IIR. The NR results were accurnte to 2.8% and SBR to 1.9% (133). NR-SBR. The ratio of absorbances of the infrared spictrum of the pyrolyzate at 699 and 1450 cm-l or 885 aild 1450 cm-I was used (189). Pyrolysis at 900 "C (84) or 600 to 800 "C (60) and gas chromatography were also used. NR-BR. The ratio of the absorbance a t 885 cm-I to the sum o f t h e absorbances at 885 and 965 cm-1 from the i n frared spectrum ofthe pyrolyzate was used (189). SBR-BR. The ratio of the absorbances at 699 and 14:iO cm- was used f 189) SBK-CR. The styrene peak of a gas chromatogran made after pyrolysis at 600 to 800 "C was used to mea sui^: the SBR content provided the styrene content of the SBtZ was known (60). Use of the chlorine content is preferablc as it requires no prior knowledge of the S B R s composition and requires simpler equipment. NBR-ABR. The methacrylic acid content was found from the carbonyl absorption in the infrared spectrum (227). 338 R
SBR. The styrene content was determined via pyrolysis and gas chromatography using a variety of conditions. Pyrolysis a t 900 "C led to results accurate to 10% (84). Results accurate to 0.6% were obtained by pyrolyzing a t 520 "C (203). The range 10 to 40% styrene was covered by pyrolysis at 600 to 800 "C (60). When peak height measurements were used, a dependence on sample size in the range 2 to 8 mg was found. A dependence was also found on whether the copolymers were random or block (5). The composition was determined with good accuracy by NMR(193). Butadiene contents in the range 0 to 56% were measured with accuracy of 2.5%. Random and block copolymers were analyzed equally well. Oil did not interfere (235). NBR. The ratio of the infrared absorbances a t 2237 (nitrile) and 1460 cm-l (methylene) was used to measure the composition (282). EPM. One set of samples was made with carbon-14 ethylene, another with carbon-14 propylene. They were analyzed using the infrared bands a t 720, 1055, and 1378 cm-l, by NMR, by liquid scintillation counting, and by combustion and counting the C02. All results were consistent (96). The ratio of absorbances of the 1380 (methyl) and 1460 cm-l (methylene) bands was linearly related to propylene content a t concentrations above 70%. The accuracy was 7% (177). Infrared measurements were made on 1% solutions in CC14 in 1-mm cells. The absorbance a t 1378 cm-I was measured relative to a base line from 1325 to 1395 cm-l. Copolymers were made from carbon-14 for calibration. The scans were made automatically under the control of a computer which also collected the data and calculated the results. Precision was less than 1% propylene in the range 30 to 85% (38). ABR. The methacrylic acid content was determined to H % from the infrared absorption of the carbonyl group a t 1700 cm-l(259).
SULFUR AND SULFIDES CR was burned in an oxygen flask containing HzO2. The solution was treated with excess silver ion to precipitate chloride and then with a strong acid cation exchanger to remove the excess silver. The sulfate was then titrated in isopropyl alcohol with a barium-lead mixture to the Arsenazo I11 end point. Results are accurate to 0.12% in the range 1 to 2% (161). The sulfate can also be titrated with barium acetate in a mixture of water and alcohol using a high frequency apparatus to detect the end point (228). When curing agents tagged with sulfur-35 were studied, the vulcanizates were degraded in liquid scintillation counting vials by heating with tert-butyl hydroperoxide in p-xylene solution using os04 as catalyst. The accuracy obtained by counting compared favorably with methods in which the sample was burned and the combustion gas was counted (54). X-Ray fluorescence was used to measure sulfur content in studies of its solubility in rubber (2). Free sulfur was determined by extracting with aqueous acetone containing KCN to form KCNS. The CNS- was titrated with Ag+ (131). This approach has been reported before (231). Triethyl phosphite can be used in place of triphenyl phosphine to remove polysulfidic sulfur from crosslinks (129).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 5, APRIL 1973
CURING AGENTS As the literature of thin layer chromatography grows, the difficulty of reproducing R, values between laboratories becomes apparent (151, 186). Therefore, it is necessary for each laboratory to run its own standards, preferably alongside the unknown samples. Curing agents were removed from rubber by heating a 5-mg samgle to 320 “C in a stream of nitrogen which impinged directly on a thin layer chromatography plate. 2Mercaptobenzothiazole was detected (187). 2-Mercaptobenzothiazole was also volatilized from rubber by heating to 270 “C for 50 to 60 min in a vacuum for identification by infrared spectroscopy or thin layer chromatography (B9). A mixture of benzothiazyl displfide, 2-mercaptobenzothiazole, tetramethyl thiuram disulfide, and zinc dimethyldithiocarbamate was separated on a silica gel G plate using 1 to 1 CHCl3-benzene as developer. A spray of copper sulfate and ammonia was used to detect the spots (3). AGE RESISTERS The use of acetonitrile as extractant concentrates the age resisters from 1 part per 30 parts of extender oil in the rubber to 1 to 1 in the extract as the oil has very low solubility in this polar solvent. NMR, infrared, and mass spectrometry were then used to identify the extracted materials (47). Various developing solutions and detecting reagents were described for thin layer chromatography plates (50, 51, 140, 242, 255). The amounts were estimated from the size and intensity of the spots (51, 140) or by scraping the separated fractions from the plates, dissolving them in ethanol, and making colorimetric measurements (242). N-Phenyl-%naphthylamine, N-2-propyl-N‘-phenyl-pphenylene diamine, and poly(2,2;4-trimethyl-1,2-dihydroquinoline) were removed from rubber by heating in a nitrogen stream (187) or in a vacuum (249). The material removed was separated by thin layer chromatography. Phenolic age resisters removed from rubber used for food packaging were measured by ultraviolet spectrophotometry. Styrenated diphepylamine and N-phenyl-2naphtliylamine were coupled with 4-nitrobenzene diazonium fluoborate and measured spectrophotometrically in the visible region while 1,3,5-tri-2’-propyl benzene2ii,4if-polycarbodiimide’was measured by infrared (124). RESIDUAL MONOMER A solution of 0.1 gram of BR latex per 100 ml of solvent was made and 10 pl was injected iyto a gas chromatograph. The peak area of butadiene monomer was measured and its concentration was read from a calibration curve made with pentane in p-xylene. The limit of detection was 50 ppm ( 165). Acrylonitrile monomer was determined by gas chromatography’with a n error of 20% relative (216). CARBON BLACK Identification can be made from a reflectance measurement at 540 nm on a milled sheet whose surface is wet with a mixture of glycerol and alcohol. The method is applicable to samples containing only a single type of carbon black. The composition must be 25 to 35% carbon black, 40 to 60% polymer, and less than one-fifth as much white filler as carbon black. Calibrations are made in the same polymers that the samples contain (152). This method avoids the labor of separating the carbon black from the rubber.
Adsorption of Aerosol OT was used to measure particle size of carbon black isolated from rubber by pyrolysis in nitrogen at 950 “C. The results correlate better with electron microscope particle size than results by iodine absorption. The particle sizes were used to identify carbon blacks in NR, SBR, and SBR-BR vulcanizates (158). The reference to the detailed procedure is incorrectly stated in the article but is corrected in the “Literature Cited” a t the end of this review (159). Identification of carbon black recovered from vulcanizates was done easily by oil adsorption or nitrogen adsorption for large particle sizes but was more difficult for small particle sizes (62). Four methods for quantitative determination were compared. Pyrolysis in hydrogen was considered neither safe nor accurate. Pyrolysis in nitrogen i s preferred for NR, SBR, BR, IIR, and chlorobutyl ruhber. Degradation by tetrachloroethane and “ 0 3 was preferred for NBR, CR, CSM, and FKM. Relative error ranged from 0.8 to 4% (155). The residue from pyrolysis in nitrogen was subjected to combustion in oxygen. The weight loss was taken as carbon (123).
METALS The distribution of fillers as studied by electron microprobe. The measurements are on surface layers only 2 microns thick, so caution must be used if a bulk analysis is desired (156). Organosilicon and filler silica in cured silicones were distinguished by the fact that their Si K’ Si K values are different. Filler content of less than 50% was determined with error of less than 7% (198). Cu (105), Mn (105), and Zn (250) were determined by dissolving the ash of rubber samples in acid and measuring the metals by atomic absorption. The standard deviation for Cu was 0.088 ppm in the range 0.5 to 8 ppm (105); for Mn, 0.064 ppm in the range 0.5 to 20 ppm (105); and for Zn, less than 6% relative (250). If the Zn was accompanied by SiOz, a mixture of H F and H N 0 3 was used to dissolve the ash. Otherwise, HzS04 was used (251). Cu was determined colorimetrically using 2,2’-bicinchoninic acid. Al, Fe, Ca, Mg, Zn, and T i did not interfere (32). Zn was determined in halogenated IIR by colorimetric measurement with dithizone after combustion in an oxygen flask. Duplicates agreed to within 0.018% a t the 0.1 to 02% level and recovery was good. When this type of sample was dry ashed, 95% of the zinc was lost bewuse of the volatility of the zinc halides ( I 71). Zn is present in cured CR or halogenated IIR as ZnClz. It can be extracted with ethanol-toluene azeotrope, then measured polarographically with a relative error of about 5%. Three successive extractions of 24 hours each a t r m m temperature were preferred over extraction a t the boiling point of the solvent in order not to change the state of cure (207). Rubber was placed in a carbon electrode without preliminary ashing and arced in a COz atmosphere. The emission lines of Co were measured with a repeatibility of 10% relative. The standards were prepared by adding a n alcohol solution of Co(NO3)z to a toluene solution of rubber and then evaporating the solvents (146). Rubber samples were “semi-coked” by heating a t 380 “C for 20 min. The residue was then rnixed with charcoal and sprayed into the arc. Fe, Cu, Al, and Ca were determined (248). Samples were also completely ashed before mixing with carbon powder for emission spectroscopy.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 5, APRIL 1973
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Relative standard deviations of about 7% for Al, Si, l’rlg, .Ti, and Zn (214) and 20% for Al, Ti, Fe, Cu, and Zn (147) were achieved. Ash was determined by weighing the residue from determination of carbon black by pyrolyzing first in nitrogen, then in air (123). WATER The Karl Fischer titration was adapted for determination of 5 ppm moisture in IIR with relative error of :!O% (170). An instrument was developed which uses a probe containing LiCl(271). OTHER DETERMINATIONS Unsaturation was determined in copolymers of propylene oxide and unsaturated epoxides. A 30-mg sample was treated with an ethylene dichloride solution of Hg(OAc)2 and methanol-J4C which added to the double bonds. The excess reagents were volatilized, the residue was bui ned, and the I4CO2 in the combustion gas was counted. The results compared well with the pyridine sulfate dibro iiide method (44). Resorcinol resins in cured rubber were identified by pyrolysis and gas chromatography. Phenolic resins dic not interfere (167). The carboxyl and hydroxyl contents of functionall) terminated BR were determined by infrared. For carboxyl, the ratio of absorbances at 1708 and 1435 cm-1 or 1708 and 1638 cm-1 was measured on films. The 1708 c:in-l band was also measured in CC14 solutions. Results iigree with titration within 0.03% in the range 1.0 to 2.5% Hydroxyl was measured in CS2 solution. Results agreed with acetylation values within 0.03% in the range 0.5 to 1.7% (257). Emulsifiers were determined in latex by conductonLc:tric titration. Water, ethanol, and aqueous NH3 were added to the sample Spllowed by sufficient HC1 to make it a1:idic. Titration with KOH was then carried to the second riharp increase in conductance (106). C o n t r i b u t i o n No. 499 from T h e Goodyear T i r e & R u b b e r Cc., Re-, search Laboratory, Akron, O h i o 44316. LITERATURE CITED
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(19) Barnikol, I., Barnikol, W. R., Jovanovic, N., Schulz, G. V., ibid., p 123.
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