High Temperature Infrared Cells for Studies of Solid High Polymer Reactions W. A. BISHOP Copolymer Rubber & Chemical Corp., Bafon Rouge, La.
b Two thermostatically controlled infrared cells are described which have been found suitable for high temperature reaction studies of thin polymer film systems deposited on, or sandwiched between, pressed potassium chloride pellets. The use of these cells extends the versatility of the infrared spectrophotometer and increases the instrument's capacity for unattended data logging.
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ERRORS attendant with infrared studies of thin polymer films arise primarily from an inability to prepare these films completely free from surface irregularities. For this reason kinetic studies can be made with a higher degree of precision if the sample is held in the same position in the infrared beam throughout the course of the reaction. This is best accomplished with a heated sample cell which can be left in the infrared instrument during the reaction. High temperature cells of this type have been described by Longworth ( 2 ) and Olsen (3) for studies of materials in potassium bromide pellets. Lady, Adams, and Kesse ( I ) have described the use of a high temperature cell in the study of high polymer oxidations. A cell for investigating samples a t high temperatures in controlled atmospheres has been described by Vratny and Graves HE
(6)
SAMPLE PREPARATION
Polymer films were prepared by evaporating the solvent from measured volumes of toluene solutions of the polymers deposited on the potassium chloride windows. Regardless of the care taken in leveling the windows and the efforts to control evaporation rate, surface ripples or edge concentration frequently result in the dried film. These irregularities can result in as much BS 10% variation in total absorbance as the window is rotated in the infrared beam. Where the exclusion of air was desired, as in vulcanization studies, a second potassium chloride window was placed over the dried film and the film sandwich repressed in the die. These preparations yield excellent optical properties while giving complete protection from oxidation. INFRARED CELLS
*
I n this laboratory, solid polymer reactions a t high temperatures involving the kinetics and mechanism of the degradative oxidation of polymers by atmospheric oxygen and the effects of vulcanization on polymer microstructure were studied. The equipment and techniques developed for these studies are reported. POLYMER FILM SUPPORTS
Die-pressed potassium chloride windows were used as film supports instead of polished, crystal sodium chloride windows because of the difficulties encountered in removing oxidized films from the salt window supports. The pressed potassium chloride windows are inexpensive and can be discarded a t the end of the oxidation. When these windows are used in a high temperature application the traces of moisture usually tolerated in pressed salt techniques must be removed prior to heating. 456
This is accomplished by preheating the windows a t a temperature in excess of the anticipated reaction temperature. If this temperature is very high-i.e., 100' to 200' C.-the windows will become excessively turbid on the first heating. However, if the turbid window is returned to the die and repressed, a window is obtained having excellent optical properties. After this preheating procedure the windows can be heated to the reaction temperature without significant variations in their infrared transmission.
ANALYTICAL CHEMISTRY
Thermostatically Controlled Heated Type. Figure 1 shows a heated infrared cell which has proved satisfactory from the standpoint of con-
struction simplicity, ease of handling, and temperature control. The cell is designed for vertical positioning in the infrared beam of the Model 21 Perkin-Elmer spectrophotometer. Approximately 85% transmission can be obtained through the cell without attenuation of the reference beam. Although the cell was designed for solid polymer films, it can be adapted to a heated demountable or capillary cell for studies of melts. The cell bath, A , was machined from cold roll aluminum stock to hold the appropriate windows, heaters, and control equipment. The heater wells, E , were symmetrically spaced on either side of the light path to receive the two 100-watt, 2l/2 X 3/8 inch cartridgetype heaters, C. The sample holding insert, D, and spacer, E, are used to position a t 5 X 25 mm. sodium chloride window, F , in the center of the cell. This window serves as a heat exchanger to the thin potassium chloride windom-, G, containing the film sample. As the mass of the average sample is small compared to the temperature controlling surface, the heating work load is derived essentially from heat transfer to the instrument and the atmosphere. During oxidation studies the sample is exposed directly t o the atmosphere, which results in a significant thermal gradient between the heaters, bath, and sample. Since this thermal gradient is reasonably constant in an air-conditioned room, the control sensing element, H, is moved as close as physically possible to one of the heaters. This arrangement reduces to a minimum the tendency for a large temperature cycling band width which can result from thermal lag in metal baths of this type. Further refinement of tempera\\
Figure 1. Thermostatically controlled, heated infrared cell for polymer film studies
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a (3 'W
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Figure 2.
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Rotating infrared cell for monitoring spectra of 10 polymer films at controlled, elevated temperatures
ture control is obtaiwd by adjusting the power to the heaters with a rheostat to the point where the on-off cycle of the controller is 50360. h Yellow Springs thermistor controller was uscd for temperature control. The temperature a t the sample point was measured with a thermistor thermometer utilizing a hypodermic needlemounted thermistor probe, I . The temperature monitoring thermowell, J, was drilled in the top of the sodium chloride n indow to a point just over the light path. With the heat to the cell properly balanced, a temperature band width of less than 0.3" C. can be realized a t the sample window with the uninsulatcd cell in an operating range of 100' to 200" C. Under these conditions the mean temperature is held constant to a 0.5' C. range. If the cell is wrapped with an insulating tape, even better temperature control is possible. The thermistor controller and thermometer were calibrated in the cell by substituting an aluminum calibrating plug for the sodium chloride windoff and spacers. This plug fills the windom- compartment completely and extends through the front window port. The plug is drilled to a depth equal to the emersion point of a mercury thermometer with the mercury bulb positioned in the center of the cell bath. A thermistor thermowell is drilled in the plug perpendicular to, and just over the mercury bulb. With the temperature monitoring probe held in this thermowll a t the same depth as in the sodium chloride window, the temperature monitoring probe can then be accurately calibrated. After this calibration the windows and spacers are replaced and the controller is calibrated with the thermistor thermometer. It is not necessary to disassemble the cell to introduce the sample. The cell is completely preassembled to the cell backplate, K , with the exception of the heaters and probes which are removed for cell storage. The heaters and probes are easily slipped into place after the cell is located in the sample compartment of the instrument.
Samples are placed in the cell in one or two types of holders, both of which screw into the cell to hold the samplecontaining potassium chloride window tight against the sodium chloride heat exchanger. One sample holder, L (Figure 2 ) , is a threaded sleeve having a recess in one end to hold the samplecontaining window. The other end of this holder is slotted to engage an insertion tool. The second holder, M , is similar to the first with the exception of an added gas purging attachment and a front window, A', held t o the end of the holder with a threaded retaining cap, 0. This arrangement forms a gas cell over the sample when the holder and front window are in place. The second holder is used where the oxidations are fast compared to the four or five minute Tvarm-up period. A slight positive pressure of nitrogen is maintained on the cell during the warm-up period. When the reaction temperature is stabilized, the nitrogen purge is discontinued and the front window is r e m v e d . Convection quickly clears the inert atmosphere from the cell and the reaction starts. The first cell holder is used n-here the oxidation rate is slow or where protected samples are being studied. Multisample Rotating Type. Figure 2 shows a heated infrared cell which permits the simultaneous monitoring of the spectra of the 10 film samples in extended, unattended runs. The heated cell compartments are rotated about a n axis above the infrared beam in such a manner as t o position successively the samples in the infrared beam on a programmed time schedule. This cell is used primarily to monitor the time course changes in the absorption intensity a t a single wave length. However, it is possible to use the cell in conjunction with existing repetitive scan features of the spectrophotometer to scan the 10 samples consecutively. 115th the exception of the location of the heaters and control probe, the basic construction principles used in the single cell are employed in the design of the rotating cell. Five 50-watt 11/* x 3/*
inch cartridge-type heaters were symmetrically located in a 21/2-inch brass hub, P. The temperature control probe thermowell, &, was drilled diagonally through the front of the hub to a position in the center of the heating elements. The heat exchange sodium chloride windows, R, are held in the aluminum bath ring, S, by an aluminum face plate, 2'. The bath assembly is completed by an insulating fiber plate, U,and 10 machine screws used to hold the assembly together. Knurled headed cap screws, V , are used in the fiber plate to provide 100% transmission adjustment for the individual cells. The bath ring assembly is rotated about the hub with a small drive motor, W , through an internal gear arrangement, X . The drive motor support is insulated from the hub by a fiber spacer, Y , which also provides a cover for the heater connections. The drive motor support, 2, is hinged to permit the motor to be easily disengaged from the cell plug-in drive shaft. This feature is used where manual positioning of the cells is desired. The room temperature tolerance between the bath ring and hub is approximately 0.003 inch. The difference in the thermal expansion of the aluminum ring bath and the brass hub is such that the temperature gradients encountered in fast warm-up do not result in binding of the ring to the hub. A light coating of graphite is used to lubricate the bearing surfaces. The hub is secured to the cell backplate, A A , with machine screws. The hub is thermally insulated from the backplate with a fiber insulating spacer, BB. Typical program circuitry for the sample changing operation is so arranged that the timer, CC, energizes the drive motor a t three minute intervals for a period of 3 seconds. This sample changing command is just long enough for the cell positioning microswitch, DD, to ride out of the detent, EE, in the face plate. The cell positioning switch then sustains the cell changing operation until this switch falls into the next detent to position the corresponding cell in the infrared beam. The push button VOL. 33, NO. 3, MARCH 1961
457
film thickness
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TIME
0.51
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REACTION
TIME
44 62 (MINUTES TO
80 5.81
p
Figure 4. tions
98 BAND)
2.0 (MINUTES)
30
4.0
Use of heated infrared cell to follow fast reac-
Effect of film thickness upon styrene-butadiene rubber oxidation rote a t
Figure 3. Typical scans using heated infrared cell to follow small changes in a reacting polymer system
zoo0 c.
Changes in carbonyl bands with thermal decomposition of rtyrenebutadiene rubber oxidation intermediates a. 5.64 microns b. 5.93 microns C. 5.81 microns
override, F F , is used for unprogrammed operation. Khere full scans are desired, the sample changing command can be obtained from a microswitch tripped by the recorder drum to change samples during the drive-back period. Three of the cell compartments are drilled with temperature monitoring thermowells. The same methods of probe and controller calibration used for the single c ~ lare l used in calibrating the controller for the rotating ccll. The cell was calibrated in the static position using the thermowells in each of the three positions. No significant differences mere noted in the temperatures of the three cell compartments. -4fter controller calibration the temperature is not monitored during the use of the cell.
Sample holders of the type previously described were used to hold the sample containing potassiuni chloride window, GG, in the cell. Samples are identified by the sample crll numbers stamped on the ring bath. INFRARED DATA COLLECTION
Three procedures for data collection have been employed : Scanning Spectrum. The sample is placed in the heated single cell and the 2- t o 15-micron region, or a selected portion of the spectrum, 1s repeatedly scanned during the reaction. For kinetic studies the time a t which each scan was started is noted and the scanning rate used t o compute the additional reaction t i p e s
to the various bands of interest. These scans are superimposed on the same chart with each scan being identified by a numbered notation or, preferably, n ith varying colors of recorder ink. This procedure often shows obscure bands that may otherwise be lost in the overlap of the more prominent bands. Signal Recording on Strip Chart Recorder. Because of the design of the scanning mechanism of the PerkinElmer instrument, the scanning abscissa cannot be used for a criterion of reaction time n i t h a high degree of accuracy. For this reason the instrument was revised to permit signal recording on a strip chart potentionieter recorder. This revision utilized the transmitting potentiometer of the scale eypansion system in the standard hfodel 21 Perkin-Elmer spectrophotometer. -4 selector switch was installed to disconnect the a.c. excitation voltage to the transmitting potentiometer and replace it with an attenuated d.c. excitation voltage from a mercury battery. The transmitted d.c. signal \vas then fed to a standard 10-mv.
WASH
Figure 5.
PROCEDURE
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Typical record obtained using rotating infrared cell to monitor reaction rates of five samples in duplicate
Study of efficiency of various washing procedures in removing oxidation-catalyzing trace impurities from cis-l,4-polybutadiene
458
ANALYTICAL CHEMISTRY
potentiometer recorder. The excitation voltage to the potentiometer was made variable to provide continuous scale expansion up to tenfold. This arrangement was used in addition to the automatic repetitive scanning mechanism to rerord repeating scans on the strip chart. This technique was used ~5 here the time course variations in the intensities of readily identifiable bands R ere of interest. Monitoring of Single Wave Length during Reaction. This method was used with t h e single c ~ l and l the strip (,hart potentiometer recorder. \There this method is used n i t h the rotating ~ 1 1 ,the regular recording facilities can be uqed since the cycle of sample clhanges provides a criterion of reaction time. PERFORMANCE
One point of interest in the mechanism of styrene-butadiene rubber oxidation is the thermal decomposition of the intermediate hydroperoxides. To study this reaction the instrument was set to scan repetitively the 5.5- to 6.5micron region of the spectrum. The polymer film was then ovidized in the single cell to the point of maximum rate in the autocatalytic reaction stage. The reaction compartment was then purged with nitrogen and the com-
partment sealed off with the front window. K i t h a slight positive pressure of nitrogen held on the reaction compartment, the scans were continued until no additional changes in the spectrum were obvious. Selected scans from this record are shown in Figure 3. As the system was not perceptibly disturbed except for the exclusion of oxygen, fine details of this reaction are obtained which may have otherwise been lost in sampling errors. Figure 4 demonstrates the use of the single cell in studying very fast reactions. I n this case the single wave length position of the 5.81-micron carbonyl band was monitored on the strip chart m-hen styrene-butadiene rubber films of varying thickness Ivere oxidized a t high temperature. The calculated results from the scale expanded runs show diffusion control of this reaction rate with these very thin films. I n studies of this nature the limiting reaction rates n-hich may be follon.ed are limited only by the response speed of the instrument. The rotating multisample cell is especially useful for comparative studies. For example, it was of interest to compare several procedures of washing the ox
