An improved (“thermocouple-feedback”) pyrolysis-GLC technique and its application to study polyacrylonitrile degradation kinetics

Eur. Polym. J. Vol. 18, pp. 443 to 461, 1982

0014-3057/82/050443-19503.00/0 Copyright © 1982 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

AN IMPROVED ("THERMOCOUPLE-FEEDBACK") PYROLYSIS-GLC TECHNIQUE AND ITS APPLICATION TO STUDY POLYACRYLONITRILE DEGRADATION KINETICS RoY S. LEHRLE, JAMES C. ROBB and JOHN R. SUGGATE* Department of Chemistry, University of Birmingham, Birmingham Bt5 2TT, England (Received 26 October 1981)

improved pyrolysis-GLC unit has been designed in which a micro-thermocouple is spotwelded to the pyrolysis filament. The thermocouple output is used as a feedback signal to control the power supply to the filament. Fast temperature rise-times (0.02 0.1 s) and stable filament temperatures (better than + 1°) have been achieved in this way. The system has been used to study the pyrolysis of polyacrylonitrile throughout the temperature range 300 800 °. It was found that for samples of the order of 1 #m thickness (2.5 #g total mass) the degradation behaviour was independent of sample thickness. Total available yields of the six principal products and two uncharacterized products were measured as a function of temperature. Conversion curves and logarithmic plots permitted first-order rate constants to be evaluated at several temperatures, and Arrhenius parameters have been calculated from the results. Various mechanisms consistent with the results have been proposed. Abstract--An

INTRODUCTION In a previous publication [1], the first kinetic studies of the thermal degradation of polyacrylonitrile (PAN) by pyrolysis-GLC were presented. Kinetic measurements were made on the individual degradation products obtained from pyrolyses of sub-microgram samples of P A N throughout the range 200-850 °. F r o m the results of this work various degradation mechanisms were proposed. Further measurements have now been undertaken using a pyrolysis G L C apparatus of improved design. The present paper provides information about the new design and reports results of studies with it. In the improved apparatus, a micro-thermocouple has been spot-welded to the pyrolysis filament. This thermocouple not only provides continuous accurate monitoring of the filament temperature, but also is used as a feedback signal to control the power supply to the filament. Fast temperature rise-times (0.02-0.1 s) and stable filament temperatures (better than + 1°) have been achieved in this way. The improved kinetic measurements in the present work have been facilitated not only by the improved temperature control afforded by the new pyrolyser, but also by the improved gas chromatographic resolution which has been achieved with the use of a new stationary liquid phase. This has permitted quantitative measurements to be made on some additional volatile products. THE PYROLYSIS--GLC APPARATUS

The technique of pyrolysis G L C has proved valuable for both characterization and quantitative analysis of polymeric and other materials [-2]. With suit* Present address: BXL Plastics Ltd, Flexible Packaging Division, Huddersfield Road, Darton, Barnsley, Yorks $75 5NA, England.

able refinements [3] the technique is well suited to study the kinetics of polymer degradation processes [-4, 5] over a temperature range which exceeds that of many other techniques. The method essentially is to pyrolyse the sample in the carrier-gas stream at the inlet to the G L C column; the degradation products are swept away, with minimum opportunity for interaction, and analysed gas chromatographically. For precise kinetic work it is not only necessary to control the size and the mounting of the sample (thin samples mounted in the middle of a ribbon filament must be used), but also to define the temperature-time profile followed by the pyrolysis filament. The requirement is to heat up the sample very quickly from ambient to the specified pyrolysis temperature, to maintain the sample at this temperature for the desired time, and then to cool the sample rapidly in order to ~freeze" the reaction. The problem is thus one of ensuring that the temperature time profile followed by the sample is as close to rectangular as possible. In an attempt to achieve this, a boosted filament technique was originally developed [-3] in these laboratories. This involved supplying a controlled high current to the pyrolyser filament for the first second of a pyrolysis to boost the filament to the desired temperature: the much lower pyrolysis current was then supplied for the duration of the pyrolysis (usually 5 15 s). The switching of the boost and change-over to pyrolysis current was achieved by a system of microswitches operated by cams on the shaft of a synchronous clock motor. The apparatus has been used in this form over a number of years for kinetic studies of polymer degradation [-1 7], during which it became clear that further development of the technique might be considered in order to improve: (i) the temperature rise time of the filament, (ii) the filament temperature measurement,

443

444

R.S. LEHRLEet al.

(iii) the filament current reproducibility, (iv) the filament current stability, i.e. reduction of temperature drifts. Various developments of the pyrolysis-GLC technique have been reported. Fast temperature rise times have been claimed by Levy et al. [8], who used an initial boost supplied by a rapid discharge from a capacitor. Various filament resistance sensitising devices have been reported [8-10] for controlling the filament current supply. The induction heating (Curiepoint) technique [11] was originally claimed to provide many of the idealized requirements for pyrolysisGLC but, after assessing a commercial apparatus of this type 1-6], and also constructing and assessing apparatus [12] according to the design of the original authors, the present authors can vouch for the fact that the performance of the technique falls badly short of the original expectations. Other workers have reported that the performance is very sensitive to experimental parameters and apparatus design [8, 13, 14]. Moreover, quoted Curie temperatures are often unreliable; they depend upon the physical state of the metal or alloy, and are difficult to measure with precision. The technique is therefore unsuitable for kinetic studies. The thermocouple filament pyrolyser

In an attempt to achieve the objectives listed in the previous section, an apparatus based on thermocoupie temperature measurements [13, 15] has now been developed. The filament unit is shown in Fig. 1. The nichrome pyrolysis filament (25 x 0.8 x 0.08 mm ribbon, nominal resistance 0.5 ~) is spot-welded to 1-mm dia tungsten supporting rods via nickel intermediate. The chromel-alumel thermocouple is carefully spotwelded to the middle of the filament on the side reverse to that on which the samples are coated. The thermocouple is constructed from wires of only 0.025-mm dia; wire as thin as this causes negligible heat drain from the filament and thus does not upset the temperature distribution along the filament length. More robust wires are used for the parts of the thermocouple circuit not in the proximity of the filament. Initial experiments with the thermocouple revealed that there was an electrical leakage from the filament heating current into the thermocouple circuit such that the observed thermal voltage Vo was the sum

Vo= V, + V~, where Vt is the true thermal voltage and Vs the superimposed voltage due to leakage. It was found that this superimposed voltage arose because the thermocoupie, being of finite size, was sampling the potential drop along the portion of the filament to which it was attached. The voltage Vs is therefore directly related to the filament current I so that Vs = IR where R is a constant. Hence Vo= V, + IR. The observed voltage will be higher or lower than the true thermal voltage depending on the direction of the filament current. This can be utilized to obtain a value for the constant R, by measuring the difference in observed voltage for "forward" and "reverse" cur-

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thermocouple. rents. This is done by passing a known current through the filament until a stable observed thermocouple reading V~; is obtained, and then quickly reversing the current with a reversing switch and noting the new observed thermocouple reading V~. Then

v?, - v d ' = 21R and

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This constant R is independent of filament current, and has the units mV A- 1. Its value remains constant for a given thermocouple and depends on the exact shape of the spot-weld of the thermocouple to the filament. Knowing R and the filament current, the thermal voltage is readily calculated from the observed voltage V, = V o - IR. The method estimates the thermal voltage to +0.02 mV, which is equivalent to +0.5 °. Temperatures measured in this way were found to correspond (within + 1°) with measurements using microscopic observation of the melting points of standards deposited on the filament. (Note: the use of an a.c. supply to the filament would eliminate the need to correct for Vs. However, it was essential to use a d.c. filament power supply for the control unit described below.)

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Thermocouple.feedback control of the filament power supply A filament power supply unit incorporating the thermocouple output as a feedback signal has been designed and constructed. The system supplies a very large initial current to the filament until the thermocouple senses that the desired temperature has been almost attained; the feedback signal then automatically adjusts the filament current to a lower value to maintain the desired temperature. Throughout the pyrolysis the feedback continuously controls the filament supply so that the filament temperature remains stable. Circuit. The circuit and electronic details of this system are given in the Appendix. The general principles of the system, its operation, and its performance are given here. A block diagram of the circuit is shown in Fig. 2. The power source for the filament current (obtained from 12 + 6 V heavy-duty lead acid batteries) is controlled by the current amplification stage. The desired filament temperature is selected by adjustment of the temperature reference voltage. When the system is "fired", the current ampli.fication stage is initially in saturation, so that the filament unit (resistance 0.5 0.6 ff~) receives most of the 18 V supply, resulting in an initial current of about 30 A. As the filament temperature rises, the amplified thermocouple signal approaches the level of the temperature reference voltage, and the current amplification stage comes out of saturation. Current regulation controlled by the thermocouple signal is then in operation. The gain of the system is very high, so that the stability of the thermocouple signal at equilibrium is very good (+_0.01mV--- _+0.25 ). By increasing the temperature reference voltage, the system attains its equilibrium state at a higher thermocouple signal. The timer circuit controls the duration of the pyrolysis over the range 0.1 50 s. Longer pulses were obtained using a manual control and a stopwatch. The 1/~ volta qe compensation is included in order to subtract the superimposed voltage from the thermo-

couple signal to give a true measure of the temperature on the oscilloscope. When the filament was operating at equilibrium temperature, this superimposed voltage represented only a small fraction (about 4~10°,o depending on the particular thermocouple) of the total thermocouple signal, but during the boosting period the superimposed voltage was considerably larger, because of the much higher filament current flowing. Because of this large initial V~ voltage, it was necessary for the system to be operated with the filament current flowing in the "forward" direction, i.e. so that the observed thermocouple signal was given by

V(~= V, + I R. If the filament was connected to the output current in the opposite way, the control system brought the current amplification stage out of saturation far too late, causing overshoot and blowing the 3-A fuse connected into the output circuit to protect the filament. The Vs voltage compensation circuit subtracts a voltage proportional to the instantaneous filament current from the amplified thermocouple signal. The proportionality factor had to be reset whenever the thermocouple was replaced (because of a change in the constant R); the procedure for this is given in the Appendix. The output from this circuit gives a voltage proportional to the filament temperature (c. 3.4 mV per degree) and this was normally monitored on an oscilloscope (Telequipment model $54A, fitted with a P%-long persistence tube) so that the t e m p e r a t u r e time profile of the filament could be observed during a pyrolysis. The output on the oscilloscope was also used to observe the presence of "inductive coupling", which appeared as high-frequency oscillations on the oscilloscope traces. Inductive coupling between the input leads (thermocouple) and the output leads (current) occurred when these leads were incorrectly positioned. It gave rise to spurious behaviour, manifesting itself as a fall in filament current on the ammeter, and an unstable thermocouple voltage. Careful positioning of these leads was necessary to eliminate it. The thermocouple signal was measured using a

446

R. S. LEHRLEet al. Table 1. Filament temperature rise times Temperature rise-time to 95~o

Filament 1. R = 0.78 mV A- ~ Filament 2. R = 0.49 Filament 3. R = 0.12

digital voltmeter (Bradley Electronics, Model 173 B) capable of measuring voltages up to 100 mV with an accuracy of ___0.01 mV. (It was not possible to use a potentiometric method to measure the thermocouple signal since this would upset the signal fed into the feedback control unit.) The digital voltmeter had an input impedance greater than 109 ~, so that negligible current was taken from the thermocouple by the voltmeter. An accurate measure Ol the filament temperature at equilibrium was calculated from the observed thermocouple signal and the filament current, using the relationship Ii, = Fo - IR. In this way V, was obtained accurate to +0.02 mV, which is equivalent to +0.5 ° assuming no error in the thermocouple voltage-temperature reference tables (B.S. 1827: 1952). Filament temperatures thus measured were checked against melting point measurements and were found to give excellent agreement. PerJormance. The apparatus can be used for pyrolysis at temperatures up to 900 °. Temperature measurements are accurate to better than +0.5 °. (The reproducibility of the equilibrium thermocouple signal between runs was never worse than +0.01 mV, which corresponds to +0.25°.) Temperature drifts during a pyrolysis are typically 0.25 ° and at worst 1° (for long pyrolyses at high temperature using filaments having a high R value). Temperature rise times for three different filaments at different temperatures are given in Table 1. Clearly the temperature rise time is dependent on the Vs voltage component in the thermocouple signal. To prevent overshooting the preset temperature, the system was designed to come out of saturation and begin current regulation when the filament temperature was 50 ° below the preset temperature. However, because of the V5 voltage and its large value during the boosting period, the actual temperature of the filament is lower than this when the thermocouple signal registers the equivalent of 50 ° below the set temperature. The larger the value of R, the earlier the system comes out of saturation, and therefore the slower the rise time to plateau temperature. In theory, the best temperature rise obtainable with a 30-A initial boost current should be c. 0.01 s, and filament 3 displays a value close to this. However, thermocouples with values of R as low as this were rarely obtained and they lasted only a short time (a few days to a week) before they became detached from the filament. Filaments 1 and 2 were more typical, and these thermocouples would last for about a month. With the latter filaments, the temperature rise time decreased with increasing temperature; this is because with these larger values of R, the initial current at the lower temperatures, is determined by the V~ voltage. For example, with the system set to balance 16mV

300°

500°

800°

140 ms 75 10

100 ms 60 15

80 ms 50 23

(about 400 °) at equilibrium and with a Vs voltage of 0 . 8 m V A -1, it is obvious that an initial current greater than 20 A cannot flow. Even when the Vs voltage is small enough to permit an initial current of 30 A, at the lower temperatures the system comes out of saturation very early. Oscillograms of the temperature-time profiles are shown in Fig. 3. The trace in Fig. 3(a), from a pyrolysis at 800 °, shows a temperature rise time of c. 0.02s (here R = 0.12 mVA-1). A more typical profile is shown in Fig. 3(b) for a pyrolysis at 300 °, where a temperature rise time of 0.1 s is obtained (here R = 0 . 5 7 m V A - I , a typical value). Since the apparatus is operated with VS positive with respect to the thermal voltage, when Vs is large it brings the amplification stage out of saturation earlier and consequently increases the temperature rise time. (It is not possible to operate with V5 negative as this does not produce a stabilizing feedback condition and burns out the filament.) One- and 10-s pyrolysis traces are shown in Figs 3(c) and (d). These illustrate the temperature decay of the filament after pyrolysis. Any reduction in the decay time is very difficult to achieve practically, and this must now be regarded as the experimental limitation on obtaining rectangular temperature-time profiles. Comparison with other pyrolyser systems. Some comparative performance figures are given in Table 2, which shows that the thermocouple feedback method is the technique most suitable for kinetic work. In particular, the great advantage is that the temperature is controlled at the pyrolysis zone. Although filament temperature control has been obtained successfully by filament resistive devices, these measure only an average filament temperature and not the true pyrolysis temperature at the centre of the filament. This new pyrolyser provides precise control at the pyrolysis zone so that the degradation temperature is not influenced by factors such as the development of temperature gradients along the filament, or by varying heat losses caused by the degrading sample, walls of the degradation chamber, or carrier gas flow rate. APPLICATION TO THE STUDY OF POLYACRYLONITR1LE DEGRADATION

The polymer sample The PAN sample was synthesized by free radical polymerization in 20~ (v/v) solution in dimethyl formamide under high vacuum at 60 °. A 0.10-M concentration of azobisisobutyronitrile initiator was used, and conversion was taken to 12~o. The polymer was precipitated in "Analar" methanol, filtered, repeatedly washed in "Analar" methanol, and dried under vacuum at 60 ° to give a white polymer. This polymer had an intrinsic viscosity of 0.95 + 0.03 d i g -1 in

An improved Cthermocouple-feedback") pyrolysis-GLC technique

(a)

447

(b)

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(c) (d) Fig. 3. Filament temperature-time profiles. (a) Plateau temperature 800, R = 0.12mV A ~. Ordinate units 200 °, abscissa units 10ms. (b) Plateau temperature 300 °, R = 0.57mVA 1. Ordinate units 100, abscissa units 50 ms. (c) Plateau temperature 500 °, R = 0.57 mV A 1. Ordinate units 100, abscissa units 0.2 s. (d) Plateau temperature 300 °, R = 0.57 mV A 1. Ordinate units 100, abscissa units 2.0s. (The ripple on the waveform is 100-Hz noise, which was probably picked up from the rectified power supply.)

dimethyl formamide at 25 ° corresponding 1-16] to a number-average molecular weight of 32,000.

GLC apparatus, products and their resolution G L C apparatus with a flame ionization detector was used. The capillary columns were of stainless steel (30m, i.d. 0.5 mm); one was coated with tricresyl phosphate (TCP) and the other with fl,ff-oxydipropionitrile (OXY). Examples of pyrograms obtained with each of these columns are shown in Fig. 4. The

products identified are ammonia, hydrogen cyanide, acetonitrile (CH3CN), acrylonitrile monomer, methacrylonitrile [CHz=C(CH3)--CN], propionitrile (CH3~CHz--CN), isobutyronitrile [(CH3)2CH-CN], and vinyl acetonitrile ( C H 2 = C H - - C H 2 - - C N ) . There were also two peaks corresponding to products of high volatility which were not identified. The general separation obtained with the T C P column was superior to that with the OXY column. However, the latter gave better resolution of the acetonitrile and

Table 2. Comparison of pyrolysis techniques

Technique Simple boosting [1, 3-5] Capacitor discharge boosting [8, 13] Induction heating [6, 11, 12, 14] Filament resistance regulation I-9, 10] Thermocouple feedback control

{

30 W

Temperature rise-time (s)

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1

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0.04-0.2 0.08 0.02-0. l

No Yes (Overall temperature) Yes (Pyrolysis zone temperature)

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Temperature measurement Melting points ( + 5 ) Thermocouple {_+0.5') Melting point or thermocouple Curie temperature ( + 5~ at best) Melting points Thermocouple I _+0.5 ) directly attached to pyrolysis zone

R.S. LEHRLEet al.

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Fig. 4. Typical chromatograms from degradation of polyacrylonitrile. (a) Stationary liquid phase: TCP at 0°. (b) Stationary liquid phase: OXY at 20°. In both cases a 30 m x 0.5 mm capillary column was used, and the N2 carrier gas flow rate was 10 cm3 rain-1. propionitrile, and was used where better separation of these products was required.

Sample deposition After experimentation, the following sample deposition procedure was adopted. The filament was first cleaned by heating at 650 ° (dull red heat) in air for 1 rain. The PAN was deposited over the central 3-mm portion of the filament from a dilute dimethylformamide solution using a microlitre syringe, while the filament was maintained at 60°. One minute or so was allowed for the solvent to evaporate before the filament unit was inserted into the degradation chamber which was maintained at 100°. The pyrolysis was effected after a stable baseline had been obtained on the chromatograph.

Sample pyrolysis and its reproducibility The long-term temperature reproducibility of the thermocouple feedback control apparatus was found to be dependent on ambient temperature because of slight drifts in the characteristics of the electronic components with temperature. Therefore to obtain maximum reproducibility, it was necessary to turn on supplies to the apparatus at least an hour before use, so that the system could warm up to a stable temperature. The apparatus was therefore normally left on overnight during a working week, to reduce drifts in behaviour at the beginning of the day. When the GLC apparatus was ready, and with the system preset for the appropriate temperature and pyrolysis time, the degradation was performed by pushing the "fire" button. During the run the thermocouple signal and filament current were noted (to calculate the degradation temperature) and the temperature-time profile was observed on the oscilloscope to check both that the boosting was correct and that there was no inductive coupling.

Under these conditions the product peaks were reproducible to between + 5 and 10~o (standard deviation) except for the NH3 and the two unknown peaks which were reproducible to about +30~. The HCN and NH3 peaks also exhibited some day-to-day inconsistencies.

Sample size dependence It is important in any study of polymer degradation to ensure that the degradation behaviour is independent of the thickness of the polymer sample, otherwise the primary products, yields and rates could be influenced by physical effects such as heat transfer and possible secondary reactions. In the present work the degradation of samples of thickness 0.00840 #m has been investigated at 350, 400, and 500°, and the thickness dependence of the products (monomer, methacrylonitrile, acetonitrile, and hydrogen cyanide) has been examined. Total yields and rates of degradation have been measured. The results obtained for monomer and methacrylonitrile are shown in Fig. 5; the plots for acetonitrile and HCN were similar to those for monomer. If there is no anomalous sample size dependence, a straight line through the origin would be obtained. This behaviour is observed over a wide range of sample size, but with samples thicker than about 2pm (5 #g total mass) lower rates of degradation and in some cases, changes in product yields are observed. These are consistent with thick samples attaining lower temperatures due to restricted heat transfer. However, for the smallest samples used, a small systematic effect leads to an apparent positive intercept. Considerable effort was devoted to the investigation of this effect; investigations of detector response, of possible errors in peak area measurements, and of possible catalytic effects showed that these factors were not responsible, and no real expla-

An improved ("thermocouple-feedback") pyrolysis GLC technique nation can be offered at present. Studies were therefore confined to films of c. 1-/~m thickness, since both rates and limiting product yields are safely independent of sample size in this region.

449

Temperature-dependence of total available yields of products The total available yield of each product was measured at temperatures in the range 300-800 °, and

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acetonitrile was observed only at high temperatures. The NH3 results were too inconsistent to give a reliable plot, and the molar yields plotted for HCN are very approximate due to the difficulty of calibrating

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An improved ("thermocouple-feedback') pyrolysis GLC technique

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Fig. 6. Dependence of total available yield of products on temperature. (a) Monomer, acetonitrile. (b) Monomer, acetonitrile, vinylacetonitrile, methacrylonitrile. (c) Propionitrile. (d) Hydrogen cyanide. (e) Uncharacterized peaks A and B. the detector response to HCN with precision. The following characteristics may be noted: (a) The monomer and acetonitrile yields show similar trends with temperature, and it is therefore possible that they arise as products from a common mechanism. (b) Methacrylonitrile and propionitrile also show similarities in that their yields decrease with increasing temperature from 300 to 500 ° and then increase at high temperatures.

(c) The yield of hydrogen cyanide increases rapidly with temperature up to 400 °, and then appreciably flattens off. Conversion curves and rate measurements

Conversion curves were obtained by plotting the results from a series of identical samples (3/~g; 1.2-/~m thickness) degraded for different times at the chosen temperature (300, 340, 380 and 420°). Curves were obtained for monomer, methacrylonitrile, acetonitrile,

452

R. S. LEHRLEet al.

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propionitrile, and hydrogen cyanide; examples are shown in Fig. 7. The plots shown in Figs 8 and 9 show that the conversion curves follow first-order growth to limiting yields. First-order rate constants are listed in Table 3 and Arrhenius plots are shown in Fig. 10. The Arrhenius parameters are listed in Table 4. A significant induction period was observed in the yield of products at 300 c. This is shown in Fig. 9 for monomer and methacrylonitrile; the value lies

1,0

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Fig. 7. Conversion curves (product yield vs time of pyrolysis). (a) Monomer formation at 380°. (b) Methacrylonitrile formation at 380°.

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Fig. 9. First order plots; pyrolyses at 300, for monomer evolution (a), for methacrylonitrile evolution (b). between 12 and 15 s for all five products, and experiments showed that the effect was independent of film thickness. A related anomaly is displayed in the 300 ° results on the Arrhenius plot (Fig. 10) which are clearly inconsistent with the results at high temperature. The 300 ° behaviour is probably explained by the fact that the crystalline melting point of PAN is at about 320 ° [17, 18]. We are consequently observing the differences between the degradation of a solid sample at 300 ° and a molten sample at higher temperatures. Induction periods have been observed by other workers when studying PAN degradation at lower temperatures [17, 19, 20]. The temperature range studied in the present work to obtain activation energies is higher than that used by most other workers who have used temperatures below 300 °. Moreover, the literature values quoted rarely refer to specific products; thus thermogravimetric results have given values of the overall activation energy of 130 and 255 kJ mol-1 (refs [21] and [19] respectively). Arrhenius plots of the induction period have given values of 125 and 185 kJ mol-1 (refs 1-19] and [17] respectively). None of these is directly comparable with the present results quoted in Table 4. The value 63 kJ m o l - 1 obtained for the activation energy for H C N formation over the range 280~450 °, is only about half the value obtained in the present study, but the uncertainty in the literature value is difficult to assess.

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DISCUSSION _c i

0,5

Stabilization versus volatilization I

I 2

I 3

Time I s

Fig. 8. First order plots; pyrolysis at 380°, for monomer evolution (a) and for methacrylonitrile evolution (b),

It has been demonstrated [1] that although at low temperatures (200 ° and below) there are no volatile products evolved from polyacrylonitrile, in this temperature range, structural modifications occur and lead to sections of the polymer becoming inert to

An improved Cthermocouple-feedback') pyrolysis GLC technique

453

Table 3. First-order rate constants for PAN degradation (s- ~) 300 ° AN MAN

0.060 0.065 0.049 0.047 0.73

C2HsCN

CH3CN HCN

± ± ± ± ±

340 °

0.008 0.008 0.011 0.010 0.023

0.134 ± 0.138 ± 0.059 ± 0.065 ± 0.084 ±

380 ~

0.015 0.078 0.008 0.005 0.030

0.67 0.41 0.26 0.37 0.45

420 c

+ 0.10 ± 0.06 ± 0.04 ± 0.05 ±" 0.09

3.0 _+ 0.8 0.72 _+ 0.10 0.71 ± 0.18 1.15 + 0.18 1.7 ± 0.45

Uncertainties are 90~o confidence limits. degradation at high temperatures. Up to c. 50,°o of the polymer can be stabilized in this way, and the unstabilized parts of the polymer degrade at higher temperatures to yield the same products, at the same specific rates, as polymer which has not been preheated at 200 c. It is considered that this stabilization reaction is associated with the thermal coloration process, which is believed to be due to the cyclization reactions involving adjacent nitrile groups to form short conjugated sequences of

I

C

I

C

N N in a ladder structure [23 29]. The length of these ladder sequences is thought to not exceed five or six monomer units [28, 29]. An exothermic process, evident between 250 and 300% has been attributed [30] to this cyclization reaction, and at these higher temperatures the rate of coloration is much greater. Careful studies have shown that significant weight loss does not occur until after the exotherm reaches its peak [31-33]. Recent work [34] has attempted to relate the characteristics of the exotherm to specific structural aspects of the polymer. At the temperatures used in the present work, this stabilization process must be competing against degradation processes leading to volatile formation. The observed increase in the available yields of volatiles at the higher temperatures shows that fragmentation processes are more able to compete with cyclization in this region. Possible further implications of the stabilization process are discussed below in the sections on monomer and propionitrile and methacrylonitrile.

material, have concluded that the chains are initiated at the ends of the molecules, though they also suggest that chain scission occurs at structural defects in the chain. If monomer is formed by a chain depropagation reaction, the existence of stable cyclized sequencies would severely restrict the kinetic chain length unless chain transfer occurs. The occurrence of chain transfer provides plausible mechanisms for the production of several other pyrolysis products [1,35], discussed below. It seems likely that the presence of c~-H atoms in the P A N chain could facilitate such transfer processes, and it is significant that in the pyrolysis of polymethacrylonitrile, which of course does not possess these ~-H atoms, the corresponding products are not observed [1]. The increase in the total available yield of monomer with temperature (Fig. 6a) indicates that the overall rate of monomer formation, relative to that of stabilization, must steadily increase with temperature.

Acetonitrile Acetonitrile may be formed as a result of the occurrence of either intermolecular or intramolecular chain transfer [1]. If intermolecular transfer occurs during depolymerization /'v'~C~CH2--CH"

+ X~H'-'--~

I CN

~

AN MAN C2H5CN CH3CN HCN

340 420 300~-20 340~1.20 340 420 340420

Activation energy (kJ mol- 1) 140 70 110 128 132

± ± ± ± ±

15 10 15 17 23

Uncertainties are 900(i confidence limits. i.PJ. 1~ ~

[

I

(l) I

Monomer is one of the major products in the pyrolysis of polyacrylonitrile; it seems very probable that the essential process responsible is a chain depropagation ("unzip") reaction. Galin and le Roy [35], from a study of the way in which the yield of monomer depends on the molecular weight of the starting

Temperature range (C)

CH 2 +X"

CN

Monomer

Table 4. Arrhenius parameters

CH 2 __

o

Acrylonitrile

Q

Methacrylonit rile

•

Propionitrile

O

Acetonitrile

• Hydrogen cyanide

-I

logao A I1.0 _+ 1.2 5.3 _+ 0.7 8.2 ± 1.1 9.8 ± 1.3 10.2 ± 1.8

-2 O. 0014

I 0.0015

I 0.0016

I 0.0017

K/T

Fig. 10. Arrhenius plots. The anomalous results at the highest value of lIT (corresponding to 300 °) are discussed in the text. The line of smallest gradient is associated with the methacrylonitrile points.

R.S. LEHRLEet al.

454

and the molecule I subsequently unzips from the left, the final product is a cyanomethyl radical, which forms acetonitrile by abstraction of an H atom: oCH2CN + X--H---* CHaCN + X o. If intramolecular transfer occurs during depolymerization, subsequent scission and unzip again leads to a cyanomethyl radical:

@ ~

~

v

~ CN

H b CN

isms for the chain reactions, forming HCN. The more labile initiation centres are all consumed at temperatures below 400 °, whereas above that temperature a second mechanism (or initiation mechanism) becomes increasingly evident. The activation energy (132 ___23 kJ mol- 1) observed for HCN evolution gives no clue about the mechanism; its similarity to the activation energies for monomer and acetonitrile

H;

H

H

CN

CN

•~ t r a n s f e r CN

CN

(I1) scission

H

I I CN

•C~

CN

/CH~

H/ICN unzip

monomer +

• CH2CN

I

X--H

CH3CN + X• The alternative scission of radical II gives CH 2 - - C H e

I CN

+ell2---- C~CH

I CN

but the trimeric molecule (III) would be too involatile to be within the range of the present study. The extent to which the activation energies for monomer formation and acetonitrile formation are similar (140 + 15 and 128 + 17 k J m o l - t , respectively) may reflect the importance of transfer reactions in allowing the depropagation chain to proceed beyond cyclized stable sections of the polymer.

Hydrogen cyanide It was previously suggested [1] that hydrogen cyanide is evolved by a chain reaction involving its random elimination from the chain. Furthermore, it was inferred that the yield of HCN was limited in some way, either by the existence of only a small number of initiating centres or by limitation of the monomer units capable of yielding their HCN. It was considered that the limited number of head-to-head sites might be responsible for the restriction. The variation of total available yield with temperature observed in the present work (Fig. 6d) indicates that there could be two mechanisms, or at least two initiation mechan-

2~CH~cH

I CN

2-

CH2

I CN

(lID

formation is believed to be coincidental. Galin and le Roy [35] have suggested that the elimination of HCN from the chains is not entirely a random process; traces of benzene detected from pyrolyses at temperatures in excess of 600 ° show that HCN evolution may result in the formation of short sequences of conjugated double bonds. The results of Galin and le Roy are, however, in accord with the present work in that they find the molar yield of HCN to be significantly smaller than that of monomer. This conflicts with the findings of Tsuchiya and Sumi [36]. The latter authors studied the pyrolysis of polyacrylonitrile in both air and nitrogen over the range 400-800 °, and concluded that HCN was the main product under all conditions studied (the ammonia yield was not estimated). However, it should be pointed out that they assessed the HCN by trapping it in a 0.4% NaOH solution and then estimating it both electrometrically and by titration; all other products were analysed gas chromatographically using a flame ionization detector. It is an open question whether this approach gives more reliable yields of HCN.

An improved Cthermocouple-feedback") pyrolysis-GLC technique

Methacrylonitrile and propionitrile Although vinyl acetonitrile was reported by some earlier workers as one of the main products, the component was almost certainly methacrylonitrile, wrongly characterized because of the similarity of its mass spectrum. Gas chromatographic analysis [1] provides good evidence that the component is methacrylonitrile, and this conclusion has recently been confirmed [36]. The variation of limiting yield with temperature for both methacrylonitrile and propionitrile displays a minimum at intermediate temperatures (Figs 6b and c): it therefore seems reasonable to consider whether their mechanisms are related. This could arise because they are both derived from the radical (IV) resulting from a main-chain scission:

We must now question why the limiting yields of methacrylonitrile and propionitrile decrease over the temperature range 300-500 °. Although it might be suggested that the processes yielding these products might fail to compete with the stabilization reaction as the temperature increases over this range, it is hard to see how this could be reconciled with the above mechanism based on initial scission to give radical IV. It is more plausible to suggest that radical IV shows an increasing tendency to undergo intermolecular transfer at higher temperatures to give a dead polymer molecule (Vll): H

J

IV + X - - H - - * " , ' C H 2 - - C - - C H 3 + X • CN H

polymer

=

~,~/X.~r'~.r~ CH2~

The first stage of depolymerization of radical IV would yield a transient diradical (V)

I I CN

C~

~ C H 2 - - C *

IV

I I CN

CH2* + oC

,CH 2 (IV)

The final increase in limiting yields of methacrylonitrile and propionitrile with temperature suggests that H

f

• CH 2 - -

~ --

CH 2 •

CN

CN and V could rearrange to methacrylonitrile by internal transfer of one hydrogen atom [1]. If, on the other hand, IV undergoes intramolecular transfer, then subsequent scission yields a radical which may abstract a

(VII)

H

H

[ I

455

(V)

other mechanisms become important at higher temperatures. It is quite likely that end-structures such as VI would undergo scission ("end-initiation") and lead to the formation of methacrylonitrile: H

H

~ - ~ C H 2 •

CN

+

•

CH2__ c~CH

P

CN

2

I

CN

CN X--H

CH 3 ~

C

I

" CH 2

CN

hydrogen atom to become propionitrile:

H

H

IV

-

~

CH2~/~/ CN

/' i

CN

CH a scission _~ ~ "

CH2NC//CH2 I + *C--CH ( ( CN

CN X--H

(Vl) CH3CH2CN

3

456

R.S. LEHRLEet al.

End-structures such as VI are formed by any transfer reactions to polymer involving abstraction of an ~t-H, followed by chain scission. When it is recalled that this process is involved in a mechanism for acetonitrile formation, and that high yields of acetonitrile are observed at high temperatures, a significant number of such end-structures may well then be present. It is possible that the increased yields of propionitrile observed at higher temperatures could arise from decomposition of structure VII. Vinyl acetonitrile

Vinyl acetonitrile is formed in detectable yield only at higher temperatures, as shown in Fig. 6(b). A possible mechanism for this was presented previously l-l] and involves the presence of double bonds in the polymer chain. If it is assumed that intramolecular transfer occurs when the depolymerizing radical reaches such a double bond, subsequent scission and hydrogen abstraction gives vinyl acetonitrile:

sis. Further work on ammonia evolution, and on the two uncharacterized peaks from very volatile products is clearly desirable. Isobutyronitrile was formed in such low yields that it could conceivably have been formed from end-groups arising from the polymerization initiator. Acknowledgements--The authors thank the Science Research Council for a research studentship to JRS. They are also grateful to Mr B. L. Jones of the Department of Electronic and Electrical Engineering of the University of Birmingham for co-operation in the design and construction of the thermocouple feedback control unit. REFERENCES

1. F. A. Bell, R. S. Lehrle and J. C. Robb, Polymer 12, 579 (1971). 2. A. Barlow, R. S. Lehrle and J. C. Robb, Polymer 2, 27 (1961).

H

~ CN

C

H CN

CN

2 CN

( VIII ) H CH 2

\c//

CH2

I

+

I I CN

*C~

CH

CH 2

CN X--H

CH 2

CH

---

CH 2

I

CN An alternative mechanism involves intermolecular transfer of radical VIII to give H

~

C

H

H CN

2 CN

which could either subsequently degrade by depolymerization from the left, or undergo an "end-initiation" type scission to give the radical CHE~---CH--'CH--CN which would abstract an H atom to give vinyl acetonitrile. It may well be the case that sufficient double bonds to give rise to significant yields of vinyl acetonitrile are not available until significant loss of H C N has occurred by the high temperature reaction. Other products

The present work throws no further light on ammonia formation; the principal problem here is that a flame ionization detector is unsuitable for NH3 analy-

3. A. Barlow, R. S. Lehrle, J. C. Robb and D. Sunderland, Polymer 8, 523 (1967). 4. A. Barlow, R. S. Lehrle, J. C. Robb and D. Sunderland, Polymer 8, 537 (1967). 5. G. Bagby, R. S. Lehrle and J. C. Robb, Polymer 10, 683 (1969). 6. R. S. Lehrle, Lab. Pract. 17, 696 (1968). 7. R. S. Lehrle and J. C. Robb, J. Gas Chromato#raphy 5, 89 (1967). 8. R. L. Levy, D. L. Fanter and C. J. Wolf, Analyt. Chem. 44, 38 (1972). 9. M. Krejci and M. Deml, Collect. Czech. Chem. Commun. 30, 3071 (1965). 10. R. Nunnikoven and G. Bostbarge, Chim. Suppl. 257 (1970). 11. W. Simon and H. Giacobbo, An#ew. Chem., Int. Ed. 4, 938 (1965). 12. G. Bagby, Ph.D. thesis, University of Birmingham (1968). 13. R. L. Levy and D. L. Fanter, Anlyt. Chem. 41, 1465 (1969). 14. C. Buhler and W. Simon, J. chromatogr. Sci. 8, 323 (1970).

An improved ("thermocouple-feedback")pyrolysis-GLC technique

457

PE.=~

I,eq

~

u

"=E

~ e~

°

i

~

rI3

~o ~,"

d

.

~3~ ~.~-

~e~-

,~,

~'~

I N

~

e~ •

r----n

r-----I

o3

~ '

----

I1~ I~" _~ II~: !~:

~ ['- :i

-E

I

~,

~.

~[

x~:

'-'< ~ "Sdx

d ~-

~__L I m

O II1

i

~o

= c= .=~.~ .-~= 4

E

458

R.S. LEHRLE et al.

15. J. M. Stuart and D. A. Smith, J. appl. Polym. Sci. 9, 3195 (1965). 16. P. F. Onyon, J. Polym. Sci. 22, 13 (1956). 17. P. Dunn and B. C. Ennis, J. appl. Polym. Sci. 14, 1795 (1970). 18. I. A. Litvinov and V. A. Kargin, Chem. Abstr. 78, 84969 (1973). 19. B. E. Davydov, M. A. Gerderikh and B. A. Krentsel, Izv. Akad. Nauk. SSSR Khim. 4, 636 (1965). 20. J. N. Hay, J. Polym. Sci. Al, 6, 212 (1968). 21. S. L. Madorsky and S. Straus, J. Res. natn. Bur. Stand. 63A, 261 (1959). 22. A. R. Monahan, J. Polym. Sci. A1, 4, 2391 (1966). 23. W. J. Burlant and J. C. Parsons, J. Polym. Sci. 22, 249 (1956). 24. N. Grassie and J. N. Hay, J. Polym. Sci. 56, 189 (1962). 25. T. Takata, I. Hiroi and M. Taniyama, J. Polym. Sci. A, 2, 1567 (1964). 26. L. H. Peebles, J. Polym. Sci. A-l, 5, 2637 (1967). 27. V. D. Braum and I. A. A. El Sayed, Angew. Makromol. Chem. 6, 136 (1969). 28. I. Noh and H. Yu, J. Polym. Sci. B, 4, 721 (1966). 29. V. F. Gachkovskii, Vysokomolek. Soedin. Al3, 2207 (1971). 30. L. Reich, Macromol. Rev. 3, 49 (1968). 31. B. Kaesche-Krischer, Chem.-Intr. Tech. 37, 944 (1965). 32. W. N. Turner and F. L. Johnson, J. appl. Polym. Sci. 13, 2073 (1969). 33. N. Grassie and R. McGuchan, Eur. Polym. J. 6, 1277 (1970). 34. M. Minagawa and T. Iwamatsu, J. Polym. Sci. C, 18, 481 (1980). 35. M. Galin and M. Le Roy, Eur. Polym. J. 12, 25 (1976). 36. Y. Tsuchiya and K. Sumi, J. appl. Polym. Sci. 21, 975 (1977).

Table 5 (continued). R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 R38

1.5 k (½ W) 18k 16k 18k 68 k 680 1.2k 68 k 500 k (10-turn helipot) 56k 21k llOk 12k 24 k 220 (2 W)

Transistors T1 BFY 76 T2 ZTX 300 T3 BFY 76 T4 ZTX 300 T5 ZTX 300 T6 ZTX 300 T7 ZTX 500 T8 ZTX 500 T9 2N 5195 T10 2N 2156 Zenner diodes (½ W) Z1 4.7V 1N 5230 B Z2 4.7 V 1N 5230 Z3 2.7 V 1N 5223 Z4 2.7 V 1N 5223 Z5 4.7 V 1N 5230

APPENDIX

A block diagram for this apparatus is given in Fig. 2 and the general principles and performance of the system have been discussed in the text. The electronic details of the apparatus are discussed in this appendix. The main circuit diagram is shown in Fig. 11, and the component list is given in Table 5. The operational amplifiers A1, A2 and A3 are run off + 1 7 V and - 1 7 V lines

Diode D1

OA 95

Integrated circuits AI ~ A2 f A3

Texas Instruments SN 52709 A N

Table 5. Component values Resistors (f~) ¼W, 5~o unless specified R1 3.3 k R2 3.3 k R3 270 k R4 1.5 k R5 5 M (log) R6 56 R7 3.6k R8 75 k R9 360 k R10 ll0k RI1 560 RI2 500 (10-turn helipot) R13 ll0k R14 ll0k R15 220 k R16 1.5 k R17 1M R18 1.5 k R19 56 R20 56 R21 8.2k R22 3.6 k R23 3.6 k

Capacitors C1 390 pF C2 18 pF C3 5 nF C4 39 pF C5 1 nF C6 100 pF C7 27 pF C8 470 pF C9 10 pF (25 V) C I0 50 #F (25 V) C11 250/tF (25 V) C12 1 nF Switches SI $2 $3 $4 $5 $6

3 A S.P.S.T. 0.5 A S.P.D.T. miniature 0.5 A S.P.D.T. miniature 2-pole-3-way switch miniature push-make button Heavy-duty D.P.D.T.

Ammeter 0-5 A 1% meter

An i m p r o v e d ( " t h e r m o c o u p l e - f e e d b a c k " ) p y r o l y s i s - G L C t e c h n i q u e

459

> i~-

>

--

0

--

,,6 o e~

bee-

t~

,EZZ3

w

u

=()

O e-~

--ql ~

E

___q ) ~II ;i

._11 °II

i

0"1

8 [,-4 <

<

e~ N

e,,

;

N

N

N

i

% t-

k~M2J

k 2 ) k2,/ N

511 O

i..,

.o

en

T~ e.~ g O O eq

h2

0d

460

R.S. LEHRLE et al. Table 6. Component list for power supplies circuit Resistors (f2)

½w, 5% R39 R40 R41 R42 R43 R44 R45 R46 R47 R48

220 220 1.6 k 1.6 k 39 56 10k 10 k 100 100

Diodes D2 and D3 Rectifier bridges of 0A 200 diodes Zenner diodes Z6, 7, 8 and 9 9 V 1 N 5339, 500mW Transistors TI1 BFY 51 T12 BFX 29 Signal lamps LlandL2 18V, ¼ W l a m p s

Capacitors CI1 25,uF 50V C12 25/iF 50 V C13 25/~F 50V C14 100#F 25 V C15 100/~F 25V C 16 500 #F 25 V C17 500#F 25 V C18 1.5/~F C19 1.5/~F C20 50/~F 25 V C21 50/IF 25 V

which float on the thermocouple negative wire as zero. The circuit diagram for this power supply is in Fig. 12 and the component list is given in Table 6. Both supply lines can give about 50mA before appreciable lack of regulation occurs and the ripple peak to peak is less than 0.3~o of the supply voltage. Amplifier A 1 Amplifier A1 uses a non-inverting configuration which gives a high input impedance, and thus negligible current ( < 1 #A) is taken from the thermocouple. The gain of this amplifier stage is 83, thus bringing the thermocouple signal up from 0.041 to about 3.4 mV per degree. The compensation components were chosen to obtain a flat response, i.e. no overshoot on a step input, which gives a bandwidth of about 200 kHz. The temperature reference and differential amplifier A Zener diode (Z1) of 4.7 V is used to provide the reference voltage. A 10-turn potentiometer R12 was chosen to give the required range and accuracy of the variable reference voltage. Amplifier A2 is used as a differential amplifier and has a gain of 10. Besides the usual frequency compensation elements, the capacitor C4 was included to stabilize the closed loop system by cutting down the bandwidth. Without this, oscillations (at about 30 kHz) occur due to the unavoidable inductive coupling between output and input at the filament. R23 and C8 form a phase-lead network, inserted to remove instability in the closed loop system. V ~ voltage compensation The amplifier A3 serves to subtract a voltage proportional to the filament current (taken between the end of the filament and the thermocouple attachment point) from the amplified thermocouple voltage supplied by A1. This gives a voltage output from A3 which is directly proportional to the true thermal e.m.f, from the thermocouple and which can be readily monitored on an oscilloscope. The proportionality factor between the subtracted voltage and the filament current is controlled by R5 and this has to be reset whenever a different thermocouple is used.

During the temperature fall-off after a pulse, there is no compensation because there is no current flowing, and therefore a true temperature signal is being monitored. Therefore the procedure adopted to correctly set R5 is to observe the output on an oscilloscope while repeatedly "firing" the system for a short pulse, and to adjust R5 until there is no discontinuity in the trace when the current switches off. The type of trace observed for correct compensation is shown in Fig. 13(a), and the traces observed for overcompensation and undercompensation are shown in Fig. 13(b) and (c), respectively. The isolation stage The amplifiers A1, A2 and A3 are all earthed on the thermocouple negative wire, and their potential relative to the filament supply can vary by up to 8 V when large currents are flowing through the filament because most of the supply voltage then appears across the filament. The isolation stage is a differential amplifier designed to reject this voltage change and give an output with respect to the filament supply. A long tail pair amplifier design is used and since the potential of the bases of T1 and T3 can approach the zero of the filament supply when smaller filament currents are flowing, an additional - 6 V rail was required to give reasonable voltage across T2 in these circumstances. A 6 V dry battery was originally used to supply this voltage but since it had to be replaced fairly frequently it was eventually replaced by a 6 V lead-acid battery. It was important that the current taken Irom the operational amplifier circuit is very small since this current must return via the thermocouple wires which have appreciable resistance and will thus cause errors in the thermal e.m.f. The current down the "tail" of the amplifier is 250 #A and high current gain, small signal transistors were used so that less than 2/~A is taken at the bases. The gain of this stage is about 13, and the common mode rejection ratio was better than one in seven hundred (57 dB). Power output stages A double Darlington Pair Configuration was used with four transistors in cascade for the power output stage. The system is purely to give current gain and four transistors were necessary since little current can be accepted by the

An improved ("thermocouple-feedback") pyrolysis G L C technique ¢0

)

:I::

l

c u m b e r s o m e and the system was not intended for continuous operation, a heat sink that will dissipate 12 W was fitted. Since this takes appreciable time to heat up, the system can be operated at full power for 1 rain in every 5 min.

o

ID L.

u

u

I I

-,i ~"

(')

f

(o) I I

I I

I

(b) (0. 0

O.

I I

$

E0

Y

461

I I

Timer circuit The timer circuit was based on a monostable circuit. The switch $3 connected the circuit. Transistor T4 is normally held on by the monostable and so keeps the output voltage of the isolation stage up, thereby holding the power output stage in the "off" position. When the circuit is fired by a trigger pulse from $5, the monostable circuit drops the base voltage of T4 by 6 V below the top rail, ensuring that T4 is cut off" and that the system operates in the normal manner. The time it remains in this "on" state is determined by setting of R32 and C9, C10 or C l l . In the original design R32 was a logarithmic potentiometer and only C10 was present, which gave logarithmically spaced times from 0.1 to 10s. This has been modified to improve the precision and range of time settings. R32 was changed to a 10-turn potentiometer (linear), and C9 and C l l were included. This gave the following timer settings:

C9

(c)

CIO Cll

I I

Ti m e

Fig. 13. Oscilloscope traces for V~ voltage compensation. (a) Correct compensation. (b) Over-compensation. (c) Under-compensation. isolation stage. The transconductance of this output stage is - 1 6 A V 1. The power dissipation in T10 is quite considerable, 55 W when the filament is run at 900 C'. Since a heat sink to remove this power would have been quite

0.10(+0.01) - 2.0(_+0.05)s 1.0(_+0.05)- lO.O(+O,1)s 5.0(_+0.2) - 5 0 ( + 0 , 5 ) s

External current supply The system includes provision for an external d.c. supply to be connected to the filament. This is necessary because a d.c. supply is required for the V~ voltage calibration. In addition, this external supply has been used for such purposes as cleaning the filament and driving-off residual solvent in the film after polymer deposition. It was desirable to use the external supply for these functions so that the feedback control unit could be left at its set temperature, since repeated resetting of the degradation temperature decreases reproducibility.