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volume cell could conceivably provide a factor of 1000 improvement in this minimum detectable quantity. Amplitude measurements afford the possibility of measuring molecular weights directly at the output of the GC column, since amplitude response is a function of molecular weight. Improved sensitivity, especially for high molecular weight compounds, could easily rank the SAW device with thermal conductivity and flame ionization type GC detectors. The prospects for improving the amplitude measurement scheme are quite good when one considers that the system noise level is presently about four orders of magnitude greater than the theoretical noise level generated by the detector itself. Compared to existing GC detectors, the lithium niobate SAW device is not presently competitive in terms of sensitivity, linearity, or dynamic range. A conventional thermal conductivity cell could probably do a factor of 10 better with little difficulty (3). In addition a thermal conductivity detector has a linear range of about 10' compared to about 10' for the SAW device. The sensitivity of the SAW device could be improved by operation a t higher frequencies. The linearity could be improved slightly with an electronic oscillator that was less amplitude sensitive.
The SAW device GC detector does have some properties that continue to make it interesting. It is nondestructive and can theoretically be used with any carrier gas. The cost of the detector and electronics is low and it is possible to make the detector specific by the application of a selective coating.
ACKNOWLEDGMENT The authors thank H. M. McNair for advice and consultation on the chromatographic aspects of this research.
LITERATURE CITED (1) Famell, G. W. "SAW Propagationm Piezoe!ecVic %W', Wave€k3mks, 1976, 2, 15. (2) King, W. H. "Using Quartz Crystals as Sorption Detectors . . . Parts 1 & 2 " , Res.lDev. 1969, 20(4 8 5), 28. (3) Hartmann, C. H. "Gas Chromatography Detectors," Anal. Chem. 1971, 4 3 ( 2 ) , 113A.
RECEIVED for review January 22,1979. Accepted May 1,1979. The authors thank the Gillette Charitable and Educational Foundation whose funds helped support this research, and Bendix Corpoation for the loan of a Model 2200 Gas Chromatograph. All plots were made on a Benson Lehner Plotter, a gift from Corning.
Surface Acoustic Wave Probes for Chemical Analysis. 111. Thsrmomechanical Polymer Analyzer Henry Wohltjen' and Raymond Dessy" Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1
A surface acoustic wave device was developed to perform thermomechanical analysis of polymer films. Amplitude measurements of TBfor polycarbonate, polysulfone, and single and twephase copolymers of the above agree with Rheovibron measurements. Low order transitions (at 24 "C) have been measured in Teflon. Although the T, measurements using SAW devices were made at 30 MHz, the poor coupling with the surface made by normal disk samples resulted in the excellent agreement observed with classical low frequency methods. On the other hand, cast films, which have good surface contact with the substrate, show the shifts in T , predicted by the time-temperature principle.
T h e attenuation of the amplitude of a Rayleigh wave propagating in a quartz SAU' delay line has been used to determine the glass transition temperature of polymer films clamped to the surface. Agreement with low frequency dynamic mechanical measurements on the same films is good. The attenuation is a result of changes in the surface contact between the polymer film and the SAW device which occur when the elastic modulus of the polymer decreases a t the glass transition temperature. Measurements obtained by this measurement are not reversible. A crystalline transition occurring in Teflon was easily detected in a specimen clamped t o the SAW device. Attenuation of the surface wave was a consequence of the surface Present address: IBM Watson Research Laboratory, Yorktoan Heights, N.Y. 0003-2700/79/0351-1470$01 OO/O
contact increasing as the sample expanded. The transition is characterized by a significant change in the coefficient of linear expansion of Teflon. Observations of this transition are reversible. A cast film is mechanically coupled to the surface wave. Under these conditions, the measured glass transition temperature, T g ,is observed to be much higher than that found in experiments conducted a t low frequencies, as theory predicts. Measurements of the cast film properties are reversible. The utility of the SAW device in photoresist investigations has been demonstrated by monitoring the effects of solvent evaporation and photo induced cross-linking of the resist. The device affords a significant advantage in studies of this kind because it can monitor films of the same thickness used in industrial applications. The SAW device will undoubtedly find many other applications in polymer analysis where its high sensitivity and ability to handle small samples are essential.
EXPERIMENTAL A quartz SAW device was employed in all studies of the thermal
behavior of polymer films in contact with the surface. The previously described temperature test apparatus ( I ) was used to provide temperature control over the range of 0 to 200 "C. The polymer films required a clamping mechanism to provide a reproducible contact force. The system shown in Figure 1 was the most effective. Owing to the very large attenuation of the surface wave amplitude which was produced by a polymer film a t its glass transition temperature, a modified amplitude measurement system was used. The sensitive balanced bridge system described previously was replaced by a much simpler apparatus shown in Figure 2 . The size of the polymer samples was kept small to F 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979 RELEASE
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prevent excessive attenuation of the wave. Samples were '/s-in. in diameter and were obtained by using an ordinary paper punch. The polymers were pressed into films about 10 mils thick. After mounting the 1/8-in.diameter film on the surface of the SAW device, the rf power level and rf amplifier output tuning controls were adjusted to provide about 9.5 V of dc signal as measured at the output of the detector amplifier. The temperature scans in all experiments were done in increments of 3 "C at a rate of 1.5 OC/min. The bisphenol A polycarbonate homopolymer film was 10 mils thick and had a number average molecular weight (M,) of about 15000. An oriented sample was obtained by straining it 50% in a tensile test apparatus. The thickness of this oriented bisphenol A polycarbonate sample was 7 mils. The polysulfone homopolymer film was originally 10 mils thick and had a molecular weight (M,) about 28 000. An oriented polysulfone sample had a thickness of 8 mils and was obtained by straining it 25% in a tensile tester. Single- and two-phase block copolymers of the polysulfone and polycarbonate were also tested. The single-phase = 10 000) and block copolymer had blocks of polycarbonate (M, polysulfone (M, = 10000) combined to give a molecular weight (M,) of about 36 000. The single-phase block copolymer films were 14 mils thick. A two-phase block copolymer was tested which was made from blocks of polycarbonate ( M , = 22 000) and polysulfone ( M , = 26 000) to yield a product with a molecular weight (M,) of about 40000. Films of the two-phase copolymer were 14 mils thick. The classical glass transition temperatures (T,) were measured on a Rheovibron dynamic mechanical tester operating a t 3.5 Hz. An attempt was made to detect a well known crystalline transition occurring in poly(tetrafluoroethy1ene) around room temperature. A 25-mil thick sample of Teflon sheet was obtained and a '/8-in. diameter sample was clamped onto the surface and cycled between 0 and 60 "C in search of the transition. An experiment was conducted to see if the mechanical behavior of very thin polymer films cast upon the surface could be monitored. A 0.2-g piece of poly(methy1 methacrylate) was dissolved in 20 mL of acetone. A single drop of this solution was placed on the SAW device and the solvent was permitted to evaporate. The temperature of the film was scanned from 60 to 180 "C. Current interest in polymer photoresists and their behavior inspired a series of experiments to ascertain if the rate of solvent evaporation and rate of photo irradiation induced cross-linking could be followed using the SAW detector. Kodak KPR photoresist was mixed 1 part to 2 with Kodak KPR Thinner. A drop
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Figure 4. Bisphenol A polycarbonate (unstrained) glass transkion profile
of this solution was applied, under subdued lighting conditions, to the surface of the SAW device and the amplitude response was monitored at room temperature. When the solvent was completely evaporated, a small UV lamp was used to illuminate the photoresist. The response of the SAW device amplitude was recorded.
RESULTS A 10-mil thick film of polyethylene terephthalate produced the curve shown in Figure 3. As the temperature approached the glass transition, the sample began to soften and adhere to the SAW device, thus attenuating the wave. At 75 "C the wave was almost completely attenuated. This is the reported glass transition temperature of polyethylene terephthalate. Thus, points of discontinuity in the amplitude vs. temperature profiles provide a clear indication of the glass transition temperature of polymer specimens clamped t o the SAW device. Figure 4 presents the results of a n unoriented bisphenol A polycarbonate sample. A glass transition temperature of 156 "C is apparent. Another polycarbonate sample was strained by 50% and the resulting profile appears in Figure 5. The results are very similar with the strained sample, indicating a T , of 152 "C. Figures 6 and 7 show the amplitude vs. temperature response of an unoriented polysulfone film and one strained by 25%. Once again, the results are quite similar with the T , of the unoriented sample occurring a t 186 "C and that of the
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Table I. Comparison of Tg Measurement Methods Glass Transition Temperature, 'C
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Figure 7. Polysulfone (strained 25 % ) glass transition profile
strained sample occurring at 189 "C. The glass transition for a single-phase polycarbonatepolysulfone copolymer is presented in Figure 8. A single T , of 174 "C is indicated. Figure 9 illustrates the results obtained from a two-phase polycarbonate-polysulfone block copolymer.
SAW
polymer
Rheovibron
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polycarbonate polysulfone polycarb-polysulfone, single-phase copolymer polycarb-polysulfone, two-phase copolymer
150 190
175
156 186 174
Tgl= 1 6 0 ' C Tp,= 1 8 1 ^ C
171 195
Two discontinuities appear in the profile at 171 and 195 "C which correspond to the Tg'sof the two domains present in the film. Interpretation of the data at temperatures above 100 "C was made difficult by the temperature dependence of the transducer efficiency. A large base-line shift is actually superimposed on the profiles in Figure 4 through 9. It should be possible to design interdigital transducers for the SAW device which minimize this base-line shift with temperature. Comparison T , measurements were performed on a Rheovibron dynamic mechanical analyzer operating a t 3.5 Hz. Table I lists the results obtained by measuring tan delta peaks on the Rheovibron and results obtained from the SAW device. An experiment was performed to see if the subtle crystalline transition of Teflon could be detected around room tem-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979 0
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Figure 12. Solvent evaporation in KPR photoresist
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Flgure 11. Glass transition profile of a PMMA cast film
perature. The results are presented in Figure 10. A substantial signal was obtained with maximum attenuation of the surface wave occurring at a temperature of 27 O C , in agreement with the literature. All of the studies conducted up to this point utilized polymer film specimens about 10 mils thick. A very thin film of poly(methy1 methacrylate) (PMMA) was cast as described in the Experimental section. This film contained about 200 gg of the polymer and covered a circular area on the SAW device which was about 9 mm in diameter. The thin film had a thickness of approximately 0.1 mil. No clamp was used on this specimen. The amplitude response vs. temperature profile for the PMMA film is displayed in Figure 11. A very strong attenuation of the surface wave was observed with the peak attenuation occurring a t a temperature of 150 "C. Figure 12 illustrates the response of the SAW device as solvent evaporates from a spot of KPR photoresist present on the surface. The effect of then irradiating this photoresist film is shown in Figure 13. The sudden shift in the base line after 200 s is caused by electronic interference when the UV lamp is turned on. A steady decrease in the attenuation is observed until approximately 300 s has elapsed, a t which time the irradiation ceases to have a significant effect on the film.
DISCUSSION The interesting mechanical properties of polymers are a direct consequence of the fact that they are viscoelastic
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
perimentally cannot be explained so simply. First of all, the loss modulus for a linearly viscoelastic material experiences a maximum at or near the glass transition temperature (2). The Tgp.rofiles obtained from the SAW device do not exhibit any maxima (or minima) when the sample is clamped to the SAW device. Instead, complete attenuation of the surface wave persists long after the glass transition temperature has been passed. An explanation for the glass transition behavior of thick film (i.e., 10 mils) polymer specimens mechanically clamped to the surface can be realized if one considers the following: First, the loss modulus of the sample is large enough to cause substantial attenuation of the surface wave even when the sample is nowhere near the glass transition temperature. Second, the only factor preventing complete attenuation of the wave prior to reaching the glass transition temperature is the relatively poor adhesion of the polymer in the SAW device. The films used were not optically flat and this would be essential if good adhesion to the surface was to be obtained. Third, as the glass transition temperature was reached, the polymer would soften and the clamp would press it into more intimate contact with the SAW device. Thus, the adhesion would be increased and the wave would be greatly attenuated. There is additional evidence to support this hypothesis. T h e T , profiles of polymer films clamped to the surface were not reversible. T h a t is, if the sample was cooled down far below the glass transition temperature, the large surface wave attenuation would remain. (Cooling the film would not make the surface rough again.) Further support of the hypothesis can be obtained by considering the time-temperature superposition principle (3). Essentially, this principle states that the temperature at which a polymer transition occurs is frequency dependent. Dynamic mechanical test systems usually excite a periodically varying strain in the polymer and monitor the stress produced. The frequency of the strain commonly falls in the range of to lo3 Hz. Glass transition temperatures which are measured dynamically exhibit a frequency dependence. The SAW device was exciting strain oscillations at a frequency of approximately 3 X lo7 Hz. The time-temperature superposition principle predicts very large shifts in the observed TB'sfor the polymers studied. But the glass transition temperature measurements experimentally obtained show very good agreement with low frequency dynamic measurements. This is proof that the adhesion of the mechanically clamped polymer was not adequate for coupling to the surface wave to occur, otherwise the experimentally observed T i s would have occurred at substantially higher temperatures. The SAW device is not making a dynamic mechanical measurement on polymer films clamped to the surface. Rather, the SAW device is monitoring changes in surface contact. As the glass transition temperature is approached, the storage modulus (G,) decreases permitting better surface contact owing to the reduction in polymer elasticity. The increased surface contact leads to a large attenuation of the surface wave from compressional wave energy loss into the polymer film. The comparison illustrated in Table I shows relatively small differences between the Rheovibron and the SAW device. It should be understood that a number of methods exist for determining polymer glass transition temperatures. These include DTA, DSC, TMA, dilatometry, and, of course, dynamic mechanical methods ( 4 ) . Each method requires interpretation to determine the glass transition temperature. For this reason, exact agreement between measurement techniques is frequently not obtained. There are several factors which distinguish the SAW device as a useful monitor of polymer glass transitions. The device is very sensitive and this permits very small samples to be used. Sample preparations and mounting are absolutely
trivial. The device is quite rugged and possesses a small thermal mass which permits fairly rapid-temperature scans to be made. While T , measurements are very useful, the ability of the SAW device to detect more subtle transitions would greatly increase the spectrum of potential applications. Most polymers experience relaxation transitions a t temperatures far below the glass transition. These relaxations typically involve movements of small chain segments or side groups and exhibit modulus changes substantially less than those seen at the T,. Poly(tetrafluoroethy1ene) (Teflon) undergoes a first-order transition around room temperature which is a result of order-disorder effects in the crystalline regions of the polymer (5). The response obtained from the SAW device with a Teflon film on the surface (Figure 10) agrees very closely with data describing the linear expansion coefficient of Teflon (6). Between 10 and 27 "C, the Teflon linear exto 2.1 X pansion coefficiencincreases from about 1.25 X reciprocal degrees centigrade. This would cause the film to swell slightly on the surface, thus increasing the surface contact which results in greater attenuation of the surface wave. The phenomenon was reversible and reproducible, as one would expect. Thus, the SAW detector seems very well suited as a sensitive monitor of expansion coefficient changes associated with subtle polymer mechanical transitions. The results of casting a poly(methy1 methacrylate) film on the surface were very revealing. It was surprising that the presence of such a small amount of polymer could produce such a large interaction with the surface wave. The thickness of the film (about 0.1 mil) was significantly less than the wavelength of the surface wave (4 mils). This demonstrates that the surface wave interaction does not extend very far into the adjacent layer. The most striking result, however, was the temperature at which the PMMA film exhibited its glass transition. Conventional, low frequency measurements indicate that the T, of PMMA is in the vicinity of 100 OC. The SAW device indicated a T , of 150 "C. This large shift is consistent with the time-temperature superposition principle. It, therefore, appears that the cast film was indeed coupled to the propagating surface wave. An additional significant feature of the profile shown in Figure 11 is that a minimum occurs. This represents a maximum in the energy loss caused by the modulus ( 2 ) . Experiments with the PMMA cast film indicated that the phenomenon was reversible and reproducible. It is obvious then, that the dynamic mechanical measurements can be performed on very small polymer films cast on the surface of a SAW device. The final application of the SAW device was in the area of photoresist materials. Solvent evaporation rate during the baking of the photoresist can have a significant effect on the quality of the film ( 7 ) . The use of a SAW device permits investigations of this rate to be conducted with films that are the same thickness as those used in practice. Presently available techniques which employ microbalance monitoring of bulk photo resists do not provide direct information about film behavior (8). The effect of placing a drop of KPR photo-resist on the surface is shown in Figure 12. Wild oscillations experienced shortly after application of the liquid photoresist are caused by interference effects as the film thickness changes owing to solvent loss. A compressional wave is launched into the photoresist solution and partially reflected back into the surface from the photoresist/air boundary. This causes constructive and destructive interference effects to be observed as the boundary distance changed. The visual observation of the photoresist being dry coincides with the end of the wild oscillations. Other effects persist in the sample after the gross drying has occurred. Attenuation of the wave slowly increases until the trend reverses itself at about 1200
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
s. The activity observed in the film levels off after about 5000 s has elapsed. Further research will be necessary before the nature of the observed behavior can be completely understood. The presence of this unexposed photoresist on the surface of the SAW device provided a convenient opportunity to study the effects of photo cross-linking of the resist. When polymers are cross-linked the effect is to raise the Tg(Le., make the polymer more brittle). This is seen in Figure 13. After illumination with UV light, the attenuation of the wave is reduced which is precisely the response expected. The signal-to-noise ratio obtained was poor. I t should be noted that there was no attempt made to filter the signal either by analog or digital means in both experiments. The important fact is that it was possible to observe the effect of UV induced cross-linking in a very small photoresist film. Improvements in instrumentation could provide significantly greater signal-to-noise ratios. ACKNOWLEDGMENT The authors thank A. J. Wnuk and T. C. Ward for the
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polymer film samples and valuable discussions.
LITERATURE CITED Wohltjen, H., Dessy, R . E. Anal. Chem., preceding paper in this issue. Ward, I.M. "Mechanical hoperties of Soli Polymers", Wiley Interscience: New York, 1971; p 98. Rosen, S.L. "Fundamental hinciples of Polymeric Materialsfor Practicing Engineers"; Barnes & Noble, New York, 1971; p 229. Wendlandt, W. W. "Thermal Methods of Analysis", 2nd ed., Wiley Interscience: New York, 1974. Haldon, R . A,; Schell, W. J.; Simha, R. "Transitions in Glasses at Low Temperatures," "Cryogenic Propertiesof Polymers", Koenig and Serafini, Ed., Marcel Dekker: New York, 1968; p 152. Haldon. R . A.; Schell, W. J.; Simha, R. Ref. 5 , p 148. Dill, F. H.; Shaw, J. M. "Thermal Effects on the Photoresist AZ1350J," IBM J . Res. Dev. 1977, 21, 210. Hatzakis, M., Thomas J. Watson Research Laboratories, P.O. Box 218, Yorktown Heights, N.Y. 10598, private communication, April 25, 1978.
RECEIVED for review January 22,1979. Accepted May 1,1979. The authors thank the Gillette Charitable and Educational Foundation whose funds helped support this research. All plots were made on a Benson Lehner Plotter, a gift from Corning.
Detection of Explos ves with a Coated Piezoelectr c Quartz Crystal Yutaka Tomita, Mat H. Ho, and George G. Guilbault" Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 722
A coated pierolelectric quartz crystal, which has potential use as a simple device for assay of explosives, is used for the detection of mononitrotoluenes (MNT). The detector can indicate the presence of trinitrotoluene, the less volatile parent molecule. Carbowax 1000 was found to be useful as a coating for the Sensitive and selective detection. With the coating, MNT vapor in the ppb-ppm range can be detected without serious interferences. The response time observed was only 10 s and a complete reversibility of response was obtained in less than 50 s. Some parameters that affect the efficiency of the detector (amount of coating, interferences, flow rate, temperature) were also investigated.
A highly sensitive and selective method for detection of explosives is in great need. For example, airport surveillance is only able to check for the presence of metallic materials but cannot detect explosives strapped to an individual's body. Some of the recent skyjackings have been caused by this threat. A number of techniques for detection of explosives have been reported using gas chromatography ( I , Z ) , mass spectrometry (3),NMR ( 4 ) ,plasma chromatography ( 5 , 61, thin-layer chromatography (71, and visible spectrometry (8). While some of these laboratory techniques are capable of ppb detection and may be satisfactory for a specific purpose, the detection systems need elaborate techniques for operation and are usually not portable and simple, thus are not useful for field use. In recent years there has been a growing interest in coated piezoelectric quartz crystals, not only as a highly sensitive and selective detector of various air pollutants (9) but also as a simple, inexpensive, and portable device which is even small enough to be carried in a worker's pocket (10). King ( I I ) , 0003-2700/79/0351-1475$01.00/0
developed a sensitive piezoelectric-sorption detector for monitoring hydrocarbons in the atmosphere. Janghorbani and Freund (12) have described the use of these crystals as digital sensors for sulfur compounds commonly found in pulp mill effluents. Frechette and Fasching (13)have proposed their use in a static system for the detection of sulfur dioxide. Cheney et al. (14) have described several coatings suitable for sulfur dioxide detection. Guilbault et al. (15-21) developed a sensitive crystal cell design and coatings for organophosphorus pesticides, and inorganic gases such as SO2,NO2, NH3, HCl, and H2S in the atmosphere. The principle of the detector is that the frequency of vibration of an oscillating crystal is decreased by the adsorption of a foreign material on its surface. A gaseous pollutant is selectively adsorbed by a coating on the crystal surface, thereby increasing the weight on the crystal and decreasing the frequency of vibration. The decrease in the frequency in proportional to the increase in weight due to the presence of gas adsorbed on the coating, according to the following equation (20).
AF = K-LC
(1)
where AF is the frequency change (Hz), K is a constant which refers to the basic frequency of the quartz plate, area coated, and a factor to convert the weight of injected gas (9) into concentration (ppm), and I C is concentration (ppm), of sample gas. Previous works (22, 23) have shown that the theoretical limit of detection for a coated crystal is about g and that a commercially available 9-MHz crystal would have a mass sensitivity of about 400 Hz/pg. We examined many commercially available materials as coatings for the detection of mononitrotoluene (MNT), which is a volatile substance that can serve as a reliable indicator for the presence of its less volatile parent, trinitrotoluene C 1979 American Chemical Society
