N e w Differential Thermal Analysis Technique for Measuring Isothermal Crystallization Rates of High Polymers JEN CHlU Plastics Department,
E. I. du Pont de Nemours & Co., Wilmington, Del.
b A double-decker cell assembly is accommodated to an ordinary differential thermal analysis (DTA) unit for measuring isothermal crystallization rates of high polymers from their melts. The sample can be conditioned to destroy its thermal history prior to crystallization rate measurement. The amount of crystallization, as represented by the exothermic peak area on the thermogram, is followed as a function of time. The method features smafl sample size, rapid measurement, conditioning of the sample in situ, variable atmosphere, and general applicability to most polymers. The crystallization of isotactic polypropylene was used to illustrate the utility of this technique. A precision of 1% in half-time and 7% in peak area measurements was obtained.
C
has been focused on the crystallization kinetics of high polymers from the molten state because of its importance in both morphological studies and processing of the polymers. Previous techniques used to measure crystallization rates of high polymers are based on following the change in some physical property of the polymer, such as specific volume ( I ) , intensity of the crystallinity bands in the infrared spectrum (4), torsional modulus ( I @ , elasticity of the compressed specimen (3) growth of spherulites as observed under the microscope ( 1 5 ) , or increase in depolarization of plane-polarized light (6, 1%'). Each technique offers unique features of its own and serves useful purposes. Differential thermal analysis (DTA) has been used extensively to study physical transitions and to measure the crystallinity of high polymers (9). Inoue ( 7 ) used this technique to study the crystallization behavior of several polymers, particularly the effects of melting temperature, molecular weight, and nucleating agents on the crystallization thermograms of nylon 6 and polyoxymethylene. He also studied the crystallization behavior of melt blends of several nylons and polyolefins (8). OKSIDERARLE ATTENTION
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2058
ANALYTICAL CHEMISTRY
Wyckoff (18) showed by D T I that early melting and recrystallization occurs in polypropylene crystallized isothermally from the melt. More recently, Donald, Humes, and White (5) devised a DT.4 method to measure the crystallization rates of polypropylene by follo\ving the change in crystallinity as measured by the freezing peak area after holding t,he sample for a certain time a t a certain temperature. This work presents a differential thermal technique for measuring isothermal crystallization rates of high polymers from the melt by following the heat generated during crystallization. A double-decker cell assembly is used. The sample is first conditioned in a top block a t a predetermined temperature for a certain length of time, and t'hen pulled down into the bottom block kept a t the crystallization temperature. The temperature differential between the sample and an inert reference material is rapidly scanned and plotted as a function of time by an X-I' recorder. By proper modification, this apparatus should be able to study reaction kinetics in general, including quenching as a conditioning step. EXPERIMENTAL
Apparatus. T h e apparatus consists of a double-decker cell assembly, a preamplifier, and a n X - Y recorder with both a potential and time X-asis as shown in Figure 1. A temperature programmer is included in the circuit and can be readily switched into use to make a conventional DTX run.
The temperatures of the top conditioning block and the bottom crystallization block are separately controlled by two temperature controllers, t h e types of which depend on how good a control is needed. A Hallikainen Thermotrol Model 1053 A (Hallikainen Instruments, Berkeley, Calif.) was satisfactory in controlling the crystallization temperature to 1 0 . 0 5 " C . Another Thermotrol or an ordinary Brown Protect-0-vane (Minneapolis-Honeywell Regulator Co., Philadelphia, Pa.) was adequate in controlling the conditioning temperature. A schematic diagram of the doubledecker cell assembly is shown in Figure 2. Two aluminum blocks, A and F , (0.875 X 1.500 inches) one atop of the other, are separated by a Marinite insulator, D. Each block has a center cavity to retain a 55-watt cartridge heater, R and P , (0.375 X 1.125 inches, Du Pont 900 DTA Part No. 900175) which can be operated up to 500" C. A knockout access tunnel is provided underneath each heater. The top block, -4, has a 0.070- X 0.625-inch hole to retain a 28-gauge, glass-insulated Chromel-;\lumel thermocouple, S,as a feed-back device to the temperature controller. The bottom block, F , has a similar thermocouple, E , to control the temperature programming if an ordinary DTA run is performed. However, a miniature platinum resistance thermometer, Q (0.136 X 0.375 inch, Minco Products No. S 31, Minneapolis, Minn.), in conjunction with a 100-ohm resistance is connected to the Hallikainen Thermotrol to control the crystallization temperature of the bottom block, F . Two holes (0.136 x 1.500 inches) used to accommodate the
I COND.,
BLOCK
I
POWERSTAT
THERMOTRO
t.T 110 V.A.C. CRYST. BLOCK
AMPLIFIER
POWERSTAT
THERMOTROL
I
II I' PROGRAMMER
Figure 1 .
Block diagram of DTA apparatus for measuring crystallization rates
Figure 2. DTA cell assembly measuring crystallization rates
for
A.
Top conditioning block Sample thermocouple C. Sample tube D. Marinite insulator E. Programming thermocouple F. Bottom crystallization block G. Reference tube H. Marinite seat 1. Supporting tripod 1. Copper tubing as gas inlet K. Electrical leads to recarding system 1. Multiple-pin feedthrough connector or rubber stopper M. Neoprene O-ring N. Supporting aluminum plate 0. Reference thermocouplle R. Cartridge heoterr Q. Platinum resistance thermometer S. Temperature-controlling thermocouple Tension springs U. Vacuum-jacketed borosilicate glass bell jar
8.
P
r.
sample tube, C, and the reference tube, G, are drilled through the bottom block, F , and placed a t a 120" angle and symmetrical in position to the center heater. A similar hole is drilled in the top block, A , to allow the sample tube, C, to move in and out of this block. The two b1ock.s are held in position in a Marinite seat, H (Du Pont 900 DTA Part No. 900032) and fastened to the aluminum supporting plate, N (5.750 X 0.375 inch), by two tension springs, T . The cell assembly is enclosed by a vacuum-jacketed borosilicate glass bell jar, C , 3.250-inch i.d., and sealed by a neoprene O-ring X. The bell jar can be evacuated through the top outlet and filled with a n inert gas through the ','d-inch 0.d. copper tubing, .J, attached to the bottom plate, N, by standard fittings. Electrical connections of the thermocouples and the heaters are made through taper pins to a multiple-pin feedthrough connector, L , or simply a rubber stopper. The sample tube, C, is made
from a short piece of quartz tubing (3-mm. 0.d. X 13 mm. long) sleeved tightly on a ceramic thermocouple insulation tube (round, double-bore, 3/32inch 0.d. X 4 inches long with 0.025inch bores, LlcDanel Industrial Ceramics Part No. 21'025332, Beaver Falls, Pa.). The sample is placed on top of the thermocouple bead in the quartz tube as shown by the dotted area. The sample tube goes through a silicone rubber seal in a standard tubing adaptor to enable adjustment of its position. The reference tube, G, is similar in construction to the sample tube, except the ceramic tube is only 1.500 inches long and sits on the Marinite seat, H . Both the srtmple and reference thermocouples, B and 0, are made from 28gauge Chromel-.Uumel wires, and are led to the recording system, K , along with other electrical leads. The whole assembly is supported by a tripod, I . The circuit diagram for the thermocouple and heater junctions is shown in Figure 3. All electrical connections are arranged on a numbered terminal strip for easy identification. The heater of the bottom block can be switched to either a temperature programmer for an ordinary D T l i run or the Hallikainen Thermotrol for measuring crystallization rates. Powerstats are used in conjunction with the temperature controllers to obtain better temperature control. A Moseley Model 4s X-Y recorder is used. The temperature of the top conditioning block is measured by having the sample tube positioned in this block and by connecting the X-axis of the recorder to position 2 as shown in Figure 3. The temperature of the bottom block is best measured by the reference thermocouple which is not affected by the temperature changes in the sample. I n so doing, the X-axis of the recorder is connected to position 1. The X-axis should be calibrated and expanded in scale with the aid of a precision potentiometer to achieve best results. A water triple-point cell (17) is used as the reference junction for all temperature measurements. The X-axis of the recorder can be readily switched to time basis with reciprocal scanning rates of 50. 10, 5, 1, and 0.5 seconds per inch. The differential temperature sig-
nal, AT, between the sample and the reference is amplified with a Leeds & Northrup low level d.c. preamplifier KO. 9835-A, and fed to the Y-axis of the recorder. The sensitivity in AT measurement is 2.5 U.V. Der inch, The noise level is below 0:2 pv. or 0.006' C. Procedure. A conventional D T A is first performed by having both t h e sample a n d the reference tubes in the bottom block, a n d t h e bottom block heater connected to t h e temperature programmer. Both melting and freezing cycles are recorded with the X-axis on a temperature basis, From the thermogram obtained as shown in Figure 4, a crystallization temperature can be chosen in the vicinity of the freezing peak, T,. The bottom block is then maintained at the desired crystallization temperature and the top block a t a selected conditioning temperature by means of the two temperature controllers. The temperatures of the two blocks are measured from the recorder as described in the Apparatus section. For convenience, the Y-input can be disconnected during temperature measurement. I n the present system, a 10-mg. sample is conditioned in the top block for 5 minutes to destroy all crystallinity and the previous thermal history. The inside of the bell jar should be evacuated and flushed by an inert gas to avoid degrading the polymer during conditioning. When the polymer melt is ready for crystallization, the X-axis of the recorder is switched to time basis. I n
Figure 4. A conventional differential thermogram V O L 36, NO. 1 1 , OCTOBER 1964
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0
0
c
,20-
Q,
I
1 0 X
ISOTACLF
100N
2
80-
d W
%
IQ "M-/
c
t=o - P
60-
Y
a
W P
40-
7
Q)
t , sec. Figure 5. A typical differential thermogram isothermal crystaliization of high polymers
showing 0
20
40
60
80
100
120
140
160
TIME, sec.
most cases, a reciprocal scanning rate of 10 seconds per inch is recommended. Recording is started as soon as the sample tube is pulled down into the bottom block. h typical trace, PQRST, is shown in Figure a. At the beginning, the differential temperature signal is off-scale because of the large unbalance between sample and reference temperatures. In the present design, 30 seconds are required for the sample temperature to drop from 260 C. to 120" C. After that, the t,hermogram tends to come back to normal! developing into an exothermic peak as crj,stallization proceeds. This initial off-balance can be reduced by using a device to move simultaneously both the sample and reference tubes into and out of the t'no blocks. The scan is continued, and reset if necessary, until the peak area is no longer measurable. Each measurement normally requires approximately 10 minutes, including sample preparation. RESULTS A N D DISCUSSION
General. I n a n ordinary differential thermogram a$ shown in Figure 4, the differential temperature, A T , is plotted us. the sample temperature, T . Previous techniques utilizing DT.1 to study kinetics are essentially based on this dynamic thermogram, either by
Table I. Precision of Crystallization Rate Measurement for Isotactic Polypropylene Temperature melt. 260" f 1" C , c q s talliiation, 122 T o =k 0 1" C
Sample
tl 2,
1 2 3 4 Average U
Precision
2060
sec
82 84 85 84 81 0
8 0 2 6 2 8
1C;
T I 2,
sec
42 6 43 2 42 6 42 6 42 8 0 3 IC;
A , sq crn 112 6 118 4 103 2 123 8 114 5
ANALYTICAL CHEMISTRY
7 6 7 c*
Figure 6. Exothermic peak area-time curve for isotactic polypropylene
measuring the change in transition or reaction temperature with the change in heating rate (10, {I), or by evaluating the rate constant from selected points along the peak by approximated mathematical equations (2) Because the crystallization exotherm obtained by continuous cooling is usually sharp, and the peak temperature does not vary significantly with cooling rate, these two methods do not give precise rate measurements for crystallization of high polymers. I n the author's laboratory, the crystallization temperature, T,, as determined from the differential thermogram, has been used successfully as a crit,erion for comparing crystallization rates of closely related samples. .i polymer with a faster crystallization rate often crystallizes a t a higher temperature under controlled condit,ions. One example will be given a t the end of this section. Xlthough this method is rapid and usually reliable, it does not give any absolute rate data. In the present method, the heat of cryst,allization is followed as a function of time a t a constant temperature. As shown in Figure 5 , peak area, a, or r'RW represents the heat of crystallization up to time, t . The fraction of sample crystallized a t time, t , is simply the ratio a,I>-l,where A is the total peak area, I'RS. The initial part of the peak is not obtainable because of the temperature equilibration of the sample. An image of the last part of the peak can be assumed for the initial part if the peak is symmetrical. Otherwise, the triangular method has to be used. Two t,angents are draxn along the linear portions on both sides of the peak, and the total area is then represented by I'ZX. Half time, t l , ? , is defined as the time from t = O , when the sample is pulled down into the bottom block, to time t , corresponding to the I
peak minimum. Half time, T ~ is~ extrapolated from the initial maximum rate as shown by VI'. ;ipparently, t l 2 includes the induction t,ime for crystallization, t i as estimated from T C , and the time for temperature equilibration. For comparative purposes, either t l 2 or T I 2 or their reciprocals can be used as rate parameters. An experimental isotactic polypropylene was mainly used to illustrate the utility of the technique. The sample was highly isotactic and crystalline, and had a melt index of 6.96. I t showed a symmetrical crystallization peak similar to the one shown in Figure 5 . .I plot of peak area tis. time for this sample is shown in Figure 6. The polymer was conditioned a t 260" C. for 5 minutes and crystallized at 122.T" C. The broken curve was obtained by using the b 11-shaped peak area, I'RS, and the solid curve by using the triangular area, V Z X . The two curves do not seem to differ much in maximum rate measurements. However! the broken curve should be used whenever possible for mechanism studies. Precision of Technique. h precision test was made by making four separate runs with t h e above sample. The results are tabulated in Table I. ;Ipparently, the measurement of the half times showed a precision of lyO, and the area measurement was reproducible within 7yc for different determinations. This is comparable to most inst r u m en t a I techniques . Effect of Melt Temperature. -1 considerably higher temperature than the melting point is often necessary to destroy all the crystallites of a polymer. Vsually the crystallites remaining in the polymer melt accelerate the rate of crystallization just like a nucleating agent. This fact can be visualized by the present
~
,
T, = 119.6"
/'
/'
/
40
-
M.P. =161°
y T,=115.1"
i
L
140
160
180
220
260
300
MELT TEMP., 'C
Figure 7. Effect of melt temperature on crystallization rate of isotactic polypropylene
creasing temperature. T h e weightaverage molecular weight, GuJof these fractions \%asdetermined from viscosity measurements. Conventional D T A runs were made on these fractions to determine their melting points, T,, and freezing points, T,. -iprogrammed heating or cooling rate of 15" C. per minute was used. Isothermal crystallization rates of several fractions a t 116.5" C. were compared by their half times, tl 2 . The results are tabulated in Table 11, and plotted in Figure 8.
Table 11.
technique. For instance, the above polypropylene sample was conditioned in the top block for 5 minutes at temperatures ranging from 170" to 290" C., and then it crystallized a t two constant t,emperatures, 119.6" and 115.1" C., in the bottom block. Ha1.f time, t l ! ? , was determined. The results are shown in Figure 7 . The half time for the 170" treatment, could not' be measured precisely because it was shorter than 40 seconds and was included in the temperature equilibration region. Obviously the crystallization rate decreased as the melt 1:emperature increased. For this srtmple, a melt temperature above 240" C. seemed necessary to de.5troy all the crystallites. Xajer (13) found by #dilatometry the temperature necessary to produce a polypropylene melt free from nuclei to be 235" to 240" (1. Yoshizaki, Ishibashi, and Kagai (19) found that the rate of crystallization of polypropylene decreased gradually n-ith the melting temperature increasing from 185' to 215" C. In addition, the temperature to produ8:.e a complete mrlt varied with the molecular weight distribution, increasing :is the molecular weight increaied ( I d ) . Molecular Weight Effect. The effect of molec.ulnr eight on cryetallization rate of isotactic polypropylene was studied by this twhnique. N o lecular weight fraction< were obtained from commercial Hercules "Profax" 6512-E with a melt index of 0.39 by column elution with progressively in-
Crystallization Rates of Polypropylene Fractions
T,, "C. 158.0 158.0 159.2 160.4 162.5 160.6 163.0 164.2 162.6
A%fw
48,000 95,000 130.000 245;OOO 320,000 400,000 500,000 600,000 850,000
T j , "C. tls2) sec. 112.9 103 112.9 113.8 80 115.9 60 116.5 119.5 44 117.5 118.7
