Thermal Analysis - ACS Publications

thermal analysis (DTA), toexplosives. (166) also has been the subject of reviews. Although equipment for TGA con- tinues to be evolved (122, 152),most...
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Thermal Analysis C. 6 .

Murphy, Xerox Corp., Webster, N. Y

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HIS review covers the major trends in thermal analysis throughout the period covered from the previous review (218) to October 1965. Significant strides have been made in this field during the cited period, and more particularly in 1965. Symposia on thermal analysis were held in Toronto in February (189) and in London in April. The symposium on thermal analysis of synthetic fibers arid fiberforming polymers was held in September in Atlantic City under the auspices of the ACS. The First International Conference on Thermal Analysis (259) was held in Aberdeen in September. At the last meeting, the first formal international organization devoted to thermal analysis was organized. Two new books on the subject have been published: one by Rendlandt (336) and the other by Garn (104). It is obvious that such activity is indicative of the prominence that thermal analysis has gained in recent years.

THERMOGRAVIMETRY

Excellent reviews on thermogravimetry have been written by Coats and Redfern (58)and Kewkirk (226). The application of the technique to minerals (317) and, together with differential thermal analysis (DTA), to explosives (166) also has been the subject of reviews. Although equipment for TGA continues to be evolved (122, 162), most of the recent developments have been concerned with instrumentation for special applications. Steed and his associates (301) have described an instrument for use with radioactive materials. At low temperatures, corrosive atmospheres can be accommodated in equipment described by Vast (323), while Sorenson (298) has studied the chlorination of plutonium oxide with phosgene in his apparatus. Mettler’s multipurpose equipment (324, 345) and its application in special gas atmospheres under various pressures have been described (151). Hurd (138) has converted a conventional null-type recording balance for TGA application with controlled pressure or controlled atmosphere. Balesdent (10) has developed microequipment operable in a sealed tube. The Derivatograph has been described again (89) and the Netzsch (327) and Stanton (53) thermobalances have been modified to provide simultaneous DTA-TGA measurements. Khristianov and Korovyatnikov (159)

also have described equipment for simultaneous DTA-TGA. A spring microbalance has been devised (216) for magnetic susceptibility, adsorption, and TGA studies. Simultaneous TGA and gas analysis have been applied to calcium oxalate decomposition (47) using a modified Ugine-Eyraud thennobalance. Simultaneous pressure and weight changes have been used (11, 91) for thermal investigations, and the name “thermobarogravimetric analysis ’’ (TBGA) was coined for this technique (11). Application of the method to CaC204.H20, Cd(BrO&.2H20, and LiBrOa (11) showed excellent correspondence between the coupled techniques. Baker (9) has applied high pressure equipment to the study of the BaO-CO2 system. Duval (81) has reviewed the precautions to be observed in operation of thermobalances, and Lukaszewski (187) has continued his treatment of accuracy in TGA. The influence of a number of operating parameters on TGA traces obtained with CaC204HzO has been shown in an excellent paper by Simons and Newkirk (293). The effects of aerodynamic forces (46) and molecular lift (96) on TGA also have been discussed. The effects of thermal gradients on precision two-pan balances (223), pertinent in TGA, and the positioning of furnaces in TGA (250) have been considered. Manche and Carroll (199) have incorporated a uni-junction transistor oscillator, with a thermistor as the resistive part, and power supply as part of the balance suspension to provide better temperature measurements in TGA. The acquisition of kinetic data from TGA has been the subject of many publications, especially in polymeric applications (4, 48, 77, 99, 101, 136, 197, 262, 263). Magnuson (197) has provided details for the calculation of the procedural rate constant (76) for dimethylsiloxane polymer. Reich (262) has presented a method for rapid estimation of activation energy from two thermograms of the same material a t different heating rates; he obtained reasonable agreement with more conventional methods with epoxy resins (less than 10% deviation). Kinetic parameters calculated by the differential and integral methods have been compared by Carroll and Manche (48), who have stated that good agreement is obtained in simple processes, but, for complex reactions-that is, volatilization of poly(tetrafluoroethy1ene)-the e

differential method is preferred. A method not requiring approximations, transformation to a time scale, or pointwise differentiation has been presented by Fuoss and his associates (101). Reich et al. (263) have compared several methods of obtaining kinetic parameters with dynamic TGA of Teflon 7 . Kinetic data also have been obtained for the decomposition of many inorganic materials: the nitrites of Ca, Sr, and Ba (252); polyhalite, 2CaS04 MgS04 . K2SO4.H20 (108); aluminum and chromium alums (353); and CaHPOl 2H20 (79). Gas-solid reaction kinetics also have been determined by TGA: the reaction of H2S with NiClz (63) and powdered Ni and Cu (61); oxidation of NiSz (62); and reduction of several sulfates with hydrogen (236). Ingraham and Marier (147) have plotted the logarithm of the weight loss of CaC03 per unit area us. 1/T, obtaining a modified Arrhenius plot, permitting calculation of the activation energy. I n addition to these specific inorganic applications, Ingraham (146) has reviewed the determination of reaction kinetics of heterogeneous inorganic reactions by TGA. The thermal stability of many polymer systems has been determined by TGA. Among the many materials investigated have been styrene-acrylonitrile copolymer (141), poly(oxymethylene) (142),poly(propy1ene) (205), brominated epoxy resins (41), rigid urethane foams ( 8 ) , phenolic condensation polymers (178), poly( a-methylstyrene) (%), fluorine-containing polymers (68), and polyimide polymers (44, 67). In addition, several polymers employed as insulation (318) and new high-temperature polymers (100) have been examined by TGA. Inorganic polymers based on 8-quinolinol-that is, 8-quinolinol formaldehyde (74) and bis(8-quinolinol)-Schiff base coordination polymers (135),and zinc derivatives of bis-chelating agents (269)-have been subjected to TGA. Polymeric phosphorus compounds such as 1,3,2,4 diazadiphosphetidine 2,4-dioxide compounds (239) and linear double-bridged chromium(II1) phosphinate polymers (276) also have been examined by this technique. TGA and differential TGA have been applied (143) to show decreased thermal stability of irradiated poly(ethy1ene). Irradiation also has been shown (326) to enhance the rate of volatilization from several fluoropolymers. Isothermal weight loss experiments with polye

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(339), while the corresponding ammine chloride complex, [Cr(NH3)4(Hz0)2]C13. 2Hz0, underwent stepwise dehydration. Such dehydration studies often lead to the discovery of new hydrates-e.g., disodium ditungstate pentahydrate (292), tetrasodium decaborate heptahydrate (213), and various tungstate / of the carborane group ( C I B ~ ~ H into ~ ~ ) hydrates (35). Materials have been equilibrated \ over drying agents (321, 352) as well as silicone polymers has been shown to rein dynamic atmospheres containing sult in stability in excess of 400" C. (119). controlled partial pressures of water As one might suspect, polymeric vapor (188), in an effort to understand systems could be volatilized from inbetter the phenomena associated with organic fillers, leaving residues of the the thermal dehydration process. latter and permitting analysis of filler Dehydroxylation, as well as dehydracontent. This technique has been tion, studies also are possible, and have applied to the determination of silica in permitted differentiation between bound poly(tetrafluoroethy1ene) (183) and in and sorbed water in montmorillonites polyesters (270). In the poly(tetra(190), and have shown that the nonfluoroethylene), it was necessary to use stoichiometric ratios of water bound in an inert atmosphere, oxygen contributzeolites decrease with increasing ionic ing to the total volatilization of the radius of the cation in zeolites (319). sample. Distinction between the various Derivatography has been applied hydroxides of aluminum (329), and (311) to a number of organic chromatoestimation of free calcium hydroxide in graphic sorbents to determine the maxihydrated cements (64), are possible by mum temperatures of applicability. TGA. Typical of the temperatures cited are The temperature stability of other 280" for Apiezon L, 150' for Carbowax covalently bound materials also is 4000, and 100' for dibutyl phthalate. possible, and is exemplified by work on One of the major applications of inthe amine complexes of europium (52) organic TGX involved determination and osmium (346). Among other of the thermal stability of analytical complex compounds studied by TGA reagents (82, 88). iln investigation of are: hexammine cobalt (111) chloride the thermal stability of 26 ammonium (314); dilaturates of alkali metals salts (87) also has been reported. (115), alkaline earths ( 1 1 4 ) , and rare Among the many hydrates examined by earths (116); complexes of 6-chloro-2this technique have been the oxalates methoxy-9-thioacridine (113) ; ethylof plutonium (150), scandium (130), enediamine-oxalate complexes (128) ; zinc (237),europium (110), and calcium metal chelates of N-benzoyl-N-phenyl(293, 328). The investigation of several hydroxylamine (210); salicylaldehyde hydrated rare earth oxalates (214) has (61, 334), salicylaldimine (337), and permitted classification of these salicylaldehyde-ethylenediamine (337); materials through the number of deand oximates of aluminum (155) and hydration steps: one stage, La, Ce, and palladium (288). Iodine trioxide-sulfur Pr; two stages, Y, Ho, Er, Tm, Yb, and trioxide complexes (70) and dinitrogen Lu; and three stages, Nd, Sm, Eu, Gd, tetroxide adducts of anhydrous cobalt Tb, and Dy. Among other inorganic nitrate ( 2 ) have been studied by TGA. hydrates studied by TGA have been Low-temperature TGA has been uranyl sulfate (17 9 ) ,manganese selenate applied by Comer (64) over the range (356), basic copper sulfates (235), basic -45' to 150' C. for the analysis of hydracopper carbonates (268), and a number zine-ammonia mixtures with errors of of rare earth bromides (207). Wendthe order of 0.3%. Zavitsanos (354) landt and his associates (296, 303, has attached a collector to a micro341) have continued their dehydration balance to capture the effluent from a studies of complex compounds. In a Knudsen cell, and has applied the recent study (50),it was found that the technique to measurement of the vapor thermal decomposition of the isomeric pressure of silver. This fast method chromium (111) chloride 6hydrates, gave very good agreement with data [Cr(H~0)61C4,[Cr(Hz0)51C12.HzO, and obtained more conventionally. [Cr(Hz0)4C1z]C1HzO, followed identical Atmospheres have a pronounced decomposition patterns. It was coneffect on TGA results, a fact that can be cluded that the three isomers are used to study adsorption-that is, separate entities only in the solid state. rehydration of alumina (112). More Fusion with Amax a t 90" to 100" C. by important, however, have been the gasDTA results in an equilibrium mixture solid reactions such as carbon monoxide which subsequently follows the same reduction of ammonium molybdate course of decomposition. The chro(131) ; chlorination of magnesium oxide mium ethylenediamine halide comby phosgene, carbon tetrachloride, plexes, [Cr(en)z(HzO)~]X3. 2HzO(X = hydrogen chloride, and chlorine with C1, Br), lost all the water in one step

(propylene) (313) have shown more extensive weight loss with aged, and, more particularly, stress-cracked material. A number of diolefin cyclopolymers have been examined (5) and shown to be more stable thermally than poly(ethy1ene). The introduction

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carbon and with carbon monoxide (158);oxidation of manganese-vanadium spinel with oxygen ( 3 ) ; and reduction of vanadium sulfate by hydrogen sulfide (320). DIFFERENTIAL THERMAL ANALYSIS

Equipment for DTA has been described by Schulz (282), Veronis (325), Pedersen (24W), Hopkins (134), and Mason and his associates (13). Reed (260) has modified a FisherJohns apparatus to serve as an inexpensive DTA instrument. New commercial equipment is being marketed by the Fisher Scientific Co. Special-purpose equipment also has been described for high pressure operation (73, 126, 198), with controlled atmospheres for examination of corrosive materials (%@), for accommodation of semimicrosamples (208, SO+$), semimicroequipment, without reference material, for use in atmospheres of UFe, Fz, and C1F3 (226), using selfgenerated atmospheres a t sub- or supraatmospheric pressures (102), and for low-temperature applications (200). Among the multipurpose equipment, a subject reviewed by Wendlandt (335), instrumentation capable of simultaneous DTA and x-ray examination (332),and simultaneous TGA, DTA, and derivative TGA (279) has been described. Barrall and his associates (15) have described thermal expansion apparatus suitable for use with DTA, which has been applied (280) to the study of asphalt transitions. The dynamic scanning calorimeter (DSC) is being marketed by PerkinElmer. This instrument produces thermograms similar to those obtained from DTA. However, the DSC plots are directly related to the energy and simplify analytical or thermodynamic measurements. The analysis of this instrument (232) and its application to quantitative analysis (330) have been discussed. A modular calorimeter for attachment to the Du Pont DTA also has appeared (80) on the market. DTA results are highly dependent upon operational conditions. Wunderlich and Hellmuth (350) have considered superheating of solids in DTA, and, in the case of glucose, examined a t 0.5', 5", and 50" per minute, have shown that the thermogram peak temperature can vary from 150° to 160", to 173", respectively. The onset of the peak temperature, associated with the initiation of the reaction, becomes more difficult to assess with the higher heating rates, and indicates the value of differential DTA for deteimination of this point. Garn (103) has reviewed the merits of metal us. ceramic sample holders, and considers the distinctions that have been made in this aspect of DTA to have been overemphasized. Impurities, however, can exert con-

siderable influence on thermograms. This has been demonstrated in investigations of additives (220) and moisture (219) on thermograms of NH4N03. In an investigation of sugars (286), it was shown that boron introduced from borosilicate glass sample holders could significantly alter thermograms of these materials. Dynamic gas systems have been shown to have advantages in the study of catalytic reactions (156, 215). The use of dynamic dry air and of water vapor has permitted the essential differences between dehydration and decomposition in DTA of uranyl nitrates to be elucidated (185). The question of particle size on thermograms continues to be an unresolved problem. Gregoire and his associates (120) have shown that the particle size affects the polymorphic transitions of uranium. Bayliss (21) has examined calcite of different mesh sizes and subjected 0.2 gram to DTA a t 15' per minute in an atmosphere of CO2 without detecting any variation in the thermograms. I n a subsequent paper (22), it was stated that the recorded transformation temperature is not affected by any particle size range greater than 1 micron. This is in contrast to the report on the organic hydrates (195), where it was reported best not to grind hydrates in order to obtain sharp reproducible peaks. Grinding has been shown to result in decomposition of CaC03 (815 ), and reaction of CaC03 with S :O2 to form the CaS:03 (257) and effect the transformation of anatase to brookite (162). I n a DTA investigation of the oxidation of pyrites (248), particle sizes of 200 to 150,150 to 75,75 to 53,lO to 2, and 2 microns were used. Between 200 and 53 microns the effect of size was small, but for the 10- to 2- and 2-micron fractions the peak temperature was lowered ca. 100' and the peak area decreased by 3ooj,. The lower temperature was attributed to the higher surface, and the decreased area to low temperature oxidation without peaks. The application of the ClausiusClapeyron equation to DTA has been the subject of two investigations. Sarasohn (277) has studied the boiling endotherm of water as a function of pressure, with peaks a t 100.0", 66.1°, and 49.8' C. occurring a t 766 f 1,206 f 2, and 98 f 2 minutes, respectively. Using the conventional expression, A H v = 1.987(T2T1/T2 - T,)In P2/P1, the heat of vaporization was calculated to be 541 and 545 cal. per gram between 766 and 206, and 766 and 98 mm., respectively. I n high pressure studies, Majumdar and Roy (198) used the equation in the form A H c = d p X AV x Tldt to study the heat of transition as a function of pressure, and found reasonable agreement with data obtained for CsCl, NHIC1, and NH4NOa.

However, the result with Na3hlF6 was much higher than that obtained by more conventional methods. Microboiling points have been determined a t 30 to 760 torr by DTA (16) for a number of hydrocarbons. Dobovisek and Paulin (75) have employed DTA to determine the evaporation diagrams for several binary metal systems, as well as heats of evaporation as a function of composition, Bohon (33) has reviewed the quantitative aspects of DTA. Against Carborundum, cubic AgI showed an endotherm a t 140.6' C. with the heat of transition, calculated from the area under the peak, of -1780 f 90 cal. per mole, while the hexagonal AgI, transforming endothermally a t 143.0' C., involved a heat of transformation of -2000 100 cal. per mole (19). Applying DTA with the different forms of AgI in the opposing cells (19), the respective peaks were found to cross the base line essentially midway between the two cited transition temperatures, and the difference between the respective heats of reaction was determined by subtracting the smaller area from the larger to provide a comparative technique for measuring the heat of reaction. Equipment has been designed (14) with thermocouples silver-soldered to small copper cups. When applied to NH4N03, this equipment gave a fusion peak (168' C.) larger than the lower temperature transition (128' C.) ,in contrast to results obtained with more conventional equipment. The apparatus was applied to calorimetric measurement of transitions in inorganic salts, benzoic acid, and poly(ethy1ene)-poly(propy1ene) mixtures. David (72) has reviewed the mathematics of DTA as applied to determination of heats of reaction and specific heats, and, employing a similar sample holder system, determined the specific heat and heat of fusion for a number of compounds in excellent agreement with accepted data. Heats of some polymorphic metal sulfate transitions (148); fusion of InSb, I n h , GaAs, and I n P (267); fusion of poly(propylene) (160); vaporization of organic compounds (196) ; transformations in Bi203 (180); and transition for nematic mesophases (18) have been determined by DTA. Heats of reaction can be associated with a specific quantity of material undergoing reaction to provide quantitative results. This approach has been employed for the analysis of bauxite minerals (164), carbonate minerals (297), and pyrites (248). I n the last work, it was shown that a 3Oy0 relative error could result in semiquantitative determinations of 0.125 to 0.062% pyrites in clay minerals. Differential scanning calorimetry has been used to analyze AgC1-AgBr mixtures (82) with reasonable success. Nicolet and Vernet

*

(227) have subjected mixtures of calcite to dolomite to D T d and have presented the data in two ways: (1) plotting the ratio of the heights of the dolomite to calcite peaks, and (2) plotting the ratio of areas of the dolomite to calcite peaks. Neither technique produced a straightline relationship, but permitted analyses of mixtures containing 2 to 98% calcite without difficulty. A somewhat similar approach has been taken in studies of ethylene-propylene mixtures (17 ) ,where the ratio (A/A - B ) , where A is the ethylene peak height and B the propylene peak height, was plotted against composition to provide an analysis of the blend. A mathematical treatment of the kinetics of the glass transition has been developed (192) and applied to the glass transformation of glycerol. Enthalpy and entropy of activation values obtained by this technique are in reasonable agreement with data obtained by other methods. Randino and Andreotti (256) have developed a mathematical expression for obtaining kinetic data from thermograms of polymeric materials subjected to oxygen injection. Although the plotted data are displaced from those obtained by more conventional methods, activation energies having reasonable agreement with the conventional methods were obtained. I n a significant work published by Reed, Weber, and Gottfried (261), the kinetic theory of Borchardt and Daniels (36) was validated by both a rigorous mathematical treatment and re-examination of the decomposition of benzenediazonium chloride. Kissinger's (161) approach was shown to be in serious error by both application to benzenediazonium chloride decomposition and theoretical considerations. Inorganic materials have been the subject of the greatest activity. Investigations have considered the reactions of classes of compounds such as the derivatographic examination of cesium compounds (88) and thermal properties of perchlorates (201, 202, 302). Several fluorides and silicofluorides (95) also have been examined. DTA, together with TGA and differential TGA, has permitted classification of a number of zeolites (144). The polymorphism of a number of compounds has been investigated: Biz03 (181), KSCN (276), ThCz (Is$),and ZrOz (118). Among the hydrates that have been examined have been U(C204)2*6H20(SO), (NH4)3[(VO)Z(CZO~)~] .2H20 (278),and BeS04 4 H z 0 (245). I n the investigation of the last-named compound, it was shown that the compound BeSO4- H 2 0did not exist. Among the extensive work on complex compounds, Wendlandt and his associates continue to dominate the field. Typical investigations consist of

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determination of the forms of Co( C ~ H S N ) ~(334) C ~ ;~ [Cr(NH&IX3 complexes, where X is C1, Br, I, and NOs (338); [ C U ( N H ~ ) ~ ]where X ~ , X is C1, Br, I, and ‘/L so4 (295); and [Co(NH3)sX](N03)2,where X = F, C1, Br, I, and N O L (340). Uranyl nitrate adducts of the type U O Z ( N O ~.) ~N204, UOz(N03) 2MeCOsEf, and UO, (NO3)2 2MeCN also have been investigated ( I ) , as has a series of phthalocyanintin complexes (168). Phase equilibrium in the ZnO-CdOPZOs (431, Zn(POdrCd(P03)~-Mg(PO& (42), and Cd3;1sz-Zn3As2 (49) systems has been studied with the help of DTA. The study of the liquidgas equilibrium in the hydrazine-water system has proved amenable by the DTA technique (193). Fusion temperatures of sulfur (308) and selenium ($Or), a t high pressures, also have been determined by the DTA technique. Inorganic reactions are amenable to DTA investigation. Oxidation reactions, exemplified by oxidation of uranium phosphide (20), are easily studied in atmospheres of air or oxygen. Solid-state reactions, such as the formation of ferrimagnetic spinels (26) and rare earth-iron garnets ( 2 5 ) , take place through the reaction of oxides. The reaction of sulfur with copper oxides (203) has been studied by DT.4and TGA. A significant trend in DTA has been proposed by Melnick and his associates ( I $ ) , where DTX and effluent gas analysis are applied to analysis of nonmetallic inclusions in steel. In connection with this effort, thermograms were presented for a number of carbides and nitrides, in air, oxygen, and steam. Reviews on the applications of thermal analysis in the building industry (331) and for analysL of blast furnace slags (169) have been presented. Commercial glasses have been subjected to the technique by Yamamoto (351). Pedersen (242) has investigated the thermal properties of a number of organic materials used in the compounding of elastomers. This work has included such materials as tetramethylthiuram disulfide, morpholine disulfide, quinone dioxine, and p-nitroso-N-nitrosomethylaniline. Takashima and his associates have used DT.4 to determine the fusion characteristics of organic compounds (312), dehydration of organic hydrates (195), and boiling points and heats of vaporization of organic compounds (196). The liquidsolid phase diagram for (+) and (-) 54ecanolide has been presented (264). Thermograms of 2-nitrofluorine and phenylarsonic acid have been obtained with the modified Fisher-Johns apparatus (260). One of the most extensive investigations of hydrocarbons has been conducted by Lewis and Edstrom and their associates. This work (83, 84, 182) consists of the most

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systematic DTA investigation of essentially aromatic compounds ever conceived. Van Hook (322) has used DTA to measure the saturation temperature of sugar solutions. Applications to polymeric materials, which has been the subject of reviews (133, 212), has increased significantly. One of the possible reasons for this is the independence of reactions, a point vividly illustrated by Chiu (55), who presented a thermogram of a mixture of seven commercial polymers, each exhibiting its own characteristic peak permitting identification. Einhorn (85) has discussed the applicability of DT.4 to identification of thermoplastics. The classes of materials examined by DTA have been extended considerably, making such identification more practical. Polyurethanes (106, 255, 294), linear polyimides (228), aromatic polyamides, (251), linear polyesters (281),phenolic and epoxide resins (175), poly(hexaallylme1amine) and related polymers ( l o g ) , and poly(ethy1ene) blends (67, 140, 299) have been investigated by DTA. The techniqae also has been applied to synthetic fibers and fiber-forming materials (283, 285). I n addition, many individual polymers have been studied. Among these have been poly(ethy1ene terephthalate) (IS?’), bisphenol-A polycarbonate (177), poly(2,Gdimethyl phenylene ether) ( 1 5 4 , poly(propy1ene) (160), poly(3-methyl1-butane) (157), and poly(1-butene). The last-named material has been shown to exist in three solid forms (34, 105, 272), whose transitions have been investigated. Similarly, poly(& methyl-1-butene) (167) has been shown to exist in three forms. Determination of glass transitions in polymeric systems by DTA has continued to provoke interest. Wunderlich and Bodily (348) have shown a peak or shoulder a t 52’ =!c 4’ C. in poly(styrene) which was dependent upon the thermal history of the samples. This peak was attributed to the freezing of a rotational mode below the glass transition temperature. I n additional work on this material, the authors (349) have analyzed the glass transition temperature (103’ to 110’ c.) quantitatively. The differential scanning calorimeter has been applied to the detection of the glass transition in hisphenol-A polycarbonate (SSO), but the advantages of the equipment were not applied to the measurement of the change in specific heat at this transformation. Karasz and O’Reilly (154) have made such a measurement with poly(2,Gdimethyl phenylene ether). Haly and Dole (123) synthesized a DTA thermogram for poly(propy1ene) with the aid of a computer. The latter indicated DTA detection of glass transition temperatures to be higher than the absolute value. Poly (ethylene terephthalate) has been

examined in cold-drawn form (137),and the thermograms obtained were said not to reflect differences observed in previous x-ray studies. The 200’ C. endotherm in thermograms of this material was shown to be associated with an increase in specific volume (2 7 2 ) , which was attributed to the thickening of the folded chain crystal. I t also was shown (172) that quenched poly(ethylene terephthalate) as well as undrawn, nonheated filament, exhibited an exotherm a t ca. IZO’, not found in slowly cooled polymer, which probably is associated with crystallization. This latter peak was shown to be decreased as the draw ratio applied to the filament increased (171). Phase diagrams for polymer systems have been established. Pistorius (247) has established the pressure-temperature diagram for poly(tetrafluoroethylene), employing pressures above 8 kilobars. In this work, DTA was employed to detect the solid-liquid transition temperatures. I n a study of poly(ethy1ene) blends, Clampitt (57) has been able to construct the compositiontemperature diagram based on the detectable differences in peaks for high pressure (115’ C.), cocrystal (124’ C.), and linear (134’ C.) polyethylenes. Recent work by Wunderlich and Arakawa (347) has shown that linear, low-pressure poly(ethylene) crystallized from the melt under elevated pressure can melt as high as 140’ C.. It is obvious that sample history will have a significant effect on such work. DTA has been applied (343) to study cure rates of phenolic resins as a function of temperature, catalyst level, resin advancement, and water content. The technique has been used to determine the thermal properties of nickel peroxide (222), which was used as a styrene polymerization catalyst, and it was one of the tools employed to study radiationinduced solid-state polymerization (129). The products of graft polymerization-that is, reactions of sodium cellulosate and alkali cellulose with vinyl monomers , including acrylonitrile-have been examined by DTA (284), and it would appear that process analysis could be based on such an approach. I n a recent report by Swanson (309), DTA was applied to the detection of volatile products and moisture in nylon 6. As moisture adversely affects the molding of this material, DTA was suggested as a quality control tool for evaluating the condition of the molding material. DILATOMETRY

Wendlandt (333) has coined the name “thermodilatometric analysis” (TDA) for the detection of transformations in matter by dilatometry. Among the instruments reported for polymer applications are capacitance

types using low-expansion Invar 36 (206), and quartz-expansion types employing the linear variable differential transformer technique (60), as well as dial indicator direct reading (6). I n the last equipment, the specimen was surrounded by silicone oil to provide uniform temperature. Equipment developed by Otchenashenko et al. (234) employed such an oil bath as a working fluid, operating on transducers to measure cubic expansion. This same principle has been incorporated in equipment using mercury as a working fluid (40, 184, 271). More recently, a dilatometer employing gas as the working fluid has been developed (92) which can be used in conjunction with conventional stress-strain testing equipment. Mason (204) has employed a buoyant weighing technique to follow the thermal expansion of a-keratin. Equipment for inorganic applications also has been described. An inexpensive equipment, suitable for operation to 500'C., has been described by Wendlandt (3%). A highly sensitive, differential transformer-type, automatic dilatometer, operable over the temperature range 25' to 1700' C., has been described (139). Graphite heating elements have permitted development of equipment with upper operational temperature limits of approximately 2500" (232) and 2843' C. (211). Dilatometry is applied extensively for the determination of glass transition temperatures in polymers. In an excellent review on the relation of transition temperatures to chemical structure in high polymers, Boyer (37) has discussed criteria for such measurements. Quantitatively, the Simha-Boyer (291) relationships pertain : AaTo = K I and

CXLTG= Kz where a~ and a~ are the coefficients of cubic expansion above and below To, respectively; To is the glass transition temperature; A a = (YL - ac; and K1 and K z are constants. I n a recent investigation on poly(tetrafluoroethy1ene), Araki (6) has confirmed that the value of K 1 is approximately 0.11, and has demonstrated that Tc/TM = 0.66, where T M is the polymer melting point. Eisenberg (86) has expressed the relationship of TG and molecular weight mathematically :

To

= Ta"(0)

- A/Mo

where To"(0) is the glass transition temperature of the polymer of infinite molecular weight at zero pressure, M o is the molecular weight of the polymer having glass transition temperature TO,and A is a constant. TG has been measured for many polymers, including poly (styrene) (40, 198), methyl methacrylate styrene co-

polymers (138), poly(viny1 acetate) (289), poly(propy1ene) (206), cellulose 2,5acetate (69), and poly(tetrafluor0ethylene) ( 6 ) . The glass transition temperatures of natural rubber, as well as styrene-butadiene, butyl, and ethylene-propylene rubbers also have been determined dilatometrically (206). I n addition to these specific investigations, Watanabe and his associates (165) have dilatometrically determined the glass transition temperatures of 47 different polymers, and discussed their results in terms of side-chain flexibility, bulkiness, and polarity and in comparison with the isofree-volume theory of glass transition temperature set forth by Simha and Boyer (291). Dilatometry has been used to study crystallization in polymeric systems. A transition a t 65' c. in isotactic poly(1-butene), peculiar to the crystalline phase, has been detected by Danusso and Gianotti ( 7 f ) . As a result of a dilatometric investigation of poly(tetrafluoroethylene), Lanzavecchia and his associates (274) have indicated that transitions are due only to the crystalline phase of the polymer, depending on the internal order of crystallites and molecular weight, in addition to the degree of crystallinity. I n poly(butadienes), dilatometry has been used (24f) to show that the detected transition temperature increased with increasing quantities of cis-1,4butadiene. Polymerization reactions also have been studied by dilatometry (184, 240, 27f). The dilatometer, filled with monomer, is employed to volume changes resulting from polymerization. Such measurements can provide kinetic data on the polymerization process. However, as polymerization reactions usually are highly exothermic, heat is not usually dissipated and isothermal conditions are not maintained. Employing a technique similar to that of Noguchi and Yang (929) may provide for more adequate heat dissipation and still permit the sensitivity required. Dilatometry has been employed to study solid-solid phase transitions in KAsF6, KCH3S04, KC2HsS04, and Co(pyridine)2Clz,dehydration of BaClz 2Hz0, and fusion of acetanilide (333). Transition temperatures obtained by dilatometry were in good agreement with data previously obtained by DTA. The high temperature transformation of HfOp, per se, as well as affected by additions of ZrOz (300) and MgO and CaO (231),the 32' C. transition in NH4NO3 ( 2 3 4 , and the 2100" C. transition in Be0 (66) also have been studied by dilatometry. Xonlinear expansion coefficients, indicating phase transitions, have been reported a t 411" C. for PbZrOo (38) and SnTe (230). Electrical resistivities of the various forms of MnOz have been shown to be different (78).

I n a dilatometric study of Bsl-xLax (Feo.98Mn,,.02)03,it was shown (355) that with increasing concentrations of La the phase transition temperature was lowered and became more diffuse. In a recent investigation of tantalum (65), it was shown that impurities in the He gas employed caused a hysteresis effect in a plot of thermal expansion us. temperature. This was not observed with purified gas. Oxygen and nitrogen have been shown (167) to stabilize a-Hf, significantly affecting the a-P transition. From these observations, it is obvious that impurities can significantly affect dilatometric results. ELECTRICAL METHOD

The advent of Du Pont's electrical thermal analyzer, a plug-in module for its DTA equipment, should make programmed temperature resistivity measurements a more accepted thermoanalytical practice. Equipment has been designed for application to polymeric systems (23,124,265), while others have been constructed with high temperature capability to accommodate refractory materials (59,93,19Q). The equipment developed by Fenerty and Smith (93) is operable to 1500' C. Wynne-Jones and his associates (24) have developed equipment for making resistivity measurements on samples under pressures up to 1000 kg. per sq. cm. Apparatus has been developed for simultaneous determination of electrical conductivity curves and x-ray diffraction (28) which has been applied to the study of refractory oxide systems (27, 29) to 1500' C. Chiu (56)has described equipment for simultaneous electrothermal analysis (ETA) and DTA. Although major applications of the technique currently involve inorganic materials, its use with organic and polymeric materials is increasing. The sensitivity of ETA can be illustrated by Chiu's (56) work with Du Pont's SP polyimide polymer. The resistivity curve showed amarked break a t 280" C., while DTA depicted only a gradual endothermic trend initiated a t the same temperature. Transition temperatures often are difficult to assess because of the gradual onset of the thermal phenomena being observed. ETA applied to an experimental formaldehyde homopolymer (56) has shown a sharp break from 164' to 174' C., associated with the melting of this material. The DTA thermograms of the same material would not permit as sharp a definition of this transition. The technique has been used to study polymerization kinetics of bis(triethy1ene glycol monoacrylate ) (46), the log of the resistivity being proportional to the degree of conversion up to 80 to 85%. Transitions, as well as the eutectic, in the U-Cr system (255s) have been deterVOL. 38, NO. 5, APRIL 1966

* 447 R

Langer (117), and the technique was mined by the same technique. Elecshown to be complementary to the more trical measurements have been emconventional DTA and TGA. Wendployed for detection of solid-solid transilandt and Southern (342) have described tions in PbzCoWOe (94) and ZrOz (59). equipment for simultaneous gas evoluI n high pressure equipment, resistivity tion analysis and mass spectrometric measurements have been employed to analysis and have suggested coupling detect phase transitions in HgSe (249) of this equipment to DTA and TGA. and ZrOz (344). Resistivity measurePyrolysis has become a widely acments also have been employed in cepted technique for polymeric degradastudies of the columbium-hafnium (290) tion studies, and the work of Happ and and the MoTez-WTez and MoSez-WSez (266) systems. I n the Pu-0 system Maier (125) is a typical example of the value of such analyses. The mechanism ( 1 9 4 , transitions detected by resistivity of the thermal degradation of phenolic measurements have been shown to corcondensation polymers (176) and the respond well with the phase diagram distribution of monomers in ethyleneestablished by dilatometric methods. propylene copolymer (2.24) are typical Electrothermal measurements also of the thermal studies that can be have been employed to study chemical conducted with a mass spectrometer. reactions. Although the dehydroxylaFriedman has used a xenon lamp (97), tion of serpentine resulted in a rapid as well as an arc image furnace (98), as decrease in electrical resistance (221), means for pyrolysis. no change was detected with kaolinite. Infrared analysis also has been applied Dehydration of phosphoric acid also has to the analysis of polymer decomposition been studied by conductance measureproducts ( 7 ) . ments (274). Three distinctly different The potentialities and techniques of slopes were observed in the plot of elecpyrolysis-gas chromatography have trical conductivity us. 1/T for KaBr03 been reviewed by Perry (243), and a (254), which were attributed to one, or bibliography of the applications made all, of the following reactions: the of this technique over the period 1960 presence of free oxygen resulting from to 1963, covering 205 references, has NaBr03 decomposition, the presence and been presented by McKinney (191). motion of Br- ions, and the bromideThis method can be made quantitative bromate eutectic. ( g o ) , as has been illustrated by the Electrical measurements have been identification of microgram quantities of employed in glass systems (163, 186), zinc dialkyl dithiophosphates (244). and a quality control technique has been Typical of the application of the techproposed (273) based on such measurenique to polymers is the study of ments. Shibasaki and Kambe on the thermal Applying the ETA technique to glass degradation of poly(styrene) (153), and beads, Chiu (66) detected effects not on the influence of composition on degdetectable by DTA, which were attribradation in styrene-acrylonitrile copolyuted to desorption of gases. It would mer (287). appear that sorption studies could be Manometric methods also have been conducted by this approach. At 150 to 10+ mm. Hg of oxygen, CY-U~OB applied to thermal analysis. .4n automated apparatus for such studies has shows two linear portions, with a break been described (31) based on radiochema t 450' C., when the log of the conducical techniques. Ingraham (145) has tivity is plotted us. 1/T (107,). I n isoemployed much simpler equipment to thermal measurements (217 ) , u308 was study the Co-S-0 system over the shown to fix oxygen between 300' and temperature range 950' to 1200' K. 500' C., associated with a decrease in electrical conductivity. The transition MICROSCOPY in this temperature range has been associated with the p -+ a transformation Although many microscopic hot stages Of u308 (27). have been described, the application of Electrical methods are highly susthis technique to thermal analysis has ceptible to effects of impurities. Albeen limited. Swift (310) has described though this has been demonstrated a low-cost microheating unit prepared many times, recent investigations with from a Christmas tree lamp. Although TiOz illustrate this point. The resistOrdway (233) developed the concept of ance of TiOz is lowered by additions of using a thermocouple as a microfurnace, small amounts of NbzOs (149). Conhis equipment was complicated and ductivity measurements made on this expensive. Mercer and Miller (209) material a t constant temperature (127) presented simpler equipment based on have been shown to vary with time, this concept, and, recently, it has been presumably as a result of migration of described in complete detail (111). ionic impurities. High temperature equipment operating to 1800" (316) and 2150" C. (121) has PYROLYSIS been described. The small sample size required makes the technique very useThe applications of mass spectroful for examination of toxic materials metric thermal analysis, MTA (173), (305). A distinct disadvantage of the have been reviewed by Gohlke and 448 R

ANALYTICAL CHEMISTRY

technique is that the same element is both the heater and temperature-sensing device. Microscopy has constituted a means of establishing phase equilibria in many systems. The NaC1-Na2SO4 ( I l l ) , Na2C03-Li2C03 ( I l l ) , NaF-NaC1 (305), NaF-A1F3 (305), and KC1-NaCl (111, 305) systems typify this application. Metallurgical systems (170) and coal (258) have been examined by this technique. Wunderlich has applied microscopy extensively to the study of polymer systems. I n a recent publication with Sullivan (306), it was pointed out that the use of an incident microscope with the Nomarski interferometer increases the detail observable in such systems. LITERATURE CITED

( 1 ) Addison, C. C., Champ, H. A. J.,

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367 (1965).

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NO. 5 , APRIL 1966

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