A Modified Differential Scanning Calorimeter Technique Utilizing a Quenched Polymer Reference Material Carl R. Foltz and Paul V. McKinneyl Armstrong Cork Company, Lancaster, Pa. I7604
A MODIFIED DSC PROCEDURE has been developed (1) in the course of making a quantitative examination of the effect of annealing of poly(viny1 chloride) near the glass transition temperature (2). In this modified procedure a quenched specimen of the polymer which is under investigation has been used as the reference material replacing the usual inert reference material. In the DSC two sample holders are provided, one for the reference material and one for the sample under investigation. Both are subjected to a linear temperature program and the heat flow rate into each is continuously monitored. The difference in power required to maintain the programmed temperature in the two cells is charted. By using a quenched sample of the polymer as reference, the instrument charts directly differences in energy requirements between it and the sample specimen which previously had received special conditioning. Thus the difference in energy between the two physical states of the sample is measured directly. With this technique it is possible t o measure small energy differences which in the usual procedure appear as minor inflections on the slope of the T , curve or as irregularities prior to the start of the glass transition. EXPERIMENTAL Apparatus. The instrument used in this investigation was
the Perkin-Elnicr Differential Scanning Calorimeter (DSC) described in detail 17) Watson et al. (3) and O'Neill ( 4 ) . Procedure. The following procedure was adopted. Equal weights (20 mg) of the polymer sample were prepared in matched aluminum containers and placed in the reference and sample holders. They were then heated to a temperature above the glass transition temperature in order to eliminate previous thermal history effects. Quenching to room temperature by application of a direct air stream completed the effort to attain identically conditioned sample and reference specimens prior to the annealing process. The capsule containing the quenched reference specimen was then removed from the DSC and maintained at room temperature to preserve the quenched state. The test specimen remaining in the DSC sample holder was rapidly brought to a selected annealing temperature and maintained for the selected period of time. Following this annealing period, the test specimen wasquenched to room temperature. The quenched reference specimen was then returned to the DSC. Final scanning provided the thermogram displaying the energy difference between the quenched and annealed polymer specimens. All curves were obtained by scanning at a rate of 20 "C/min and the DSC instrument was calibrated for peak area using the recrystallization transition of ammonium thiocyanate (5). 'Present address, 620 Great Road, Princeton, N.J. (1) C. R. Foltz, M.S. Thesis (Physics), Franklin and Marshall
College, Lancaster, Pa., 1968. (2) P. V. McKinney and C. R. Foltz, J. Appl. Polym. Sci., 11, 1189 (1967). (3) E. S. Watson, M. J. O'Neill, J. Justin, and N. Brenner, AKAL. CHEM., 36, 1233 (1964). (4) M. J. O'Neill, ibid., 36, 1238 (1964). ( 5 ) J. J. Keavney and E. C. Eberlin, J . Appl. Polym. Sci., 3, 50 (1960).
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TEMPERATURE, "K Figure 1. Normal DSC thermogram Quenched (------)and annealed (-) copolymer (Exon 487) 20-mg sample
curves superimposed. PVC
RESULTS AND DISCUSSION
An example of conventional DSC thermograms using an inert reference material is shown in Figure 1. The dashed line is the usual T , curve for the amorphous polymer after heating well above T , and quenching. The solid line is for a specimen of the same polymer aged (annealed) for a long period in storage. The curves are superimposed to illustrate the difference produced at the To by aging. Both curves depict the abrupt change in specific heat typical of the glass transition while the aged specimen displays an added peak. The results obtained for the same polymer by the modified DSC procedure, Figure 2, display the same difference in energy at the glass transition but without the complicating shift associated with the specific heat increase. The peak area for a 20-mg specimen by conventional DSC procedure is 13.5 m-cal and by the modified procedure is 14.1 m-cal. These values are in satisfactory agreement for such a small transition energy change. If desired, greater accuracy in defining peak area can be achieved by superimposing the thermogram representing the quenched state of both reference and sample specimens (quenching both from above T , to room temperature and rescanning after obtaining the initial thermogram) rather than employing the ruled line technique. However, the difference in defined peak area for the two techniques is small when both reference and sample polymer specimens are weighed to an accuracy of 10.01 mg and the DSC differential VOL. 41, NO. 4, APRIL 1969
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TRANSITION ENERGY
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TEMPERATURE, "K Figure 2. Modified DSC technique Thermogram of annealed specimen vs. quenched specimen as reference. PVC copolymer (Exon 487) 20-mg sample. ------ Ruled line defining peak area temperature calibration is initially adjusted to produce essentially a straight line in the scan of quenched polymer reference cs. quenched polymer sample. The data in Figure 3 illustrate the value of this modified D S C procedure in detecting small differences. Four measurements on the PVC homopolymer Opalon 630 annealed at 68 "C (ten degrees below its quenchedstate TB)for, respectively, 15 min, 1 hr, 2 hr, and 64 hr are shown. In the conventional DSC procedure, the values a t 15 minutes and 1 hour, if noted at all, would be difficult t o estimate because no peak appears on such a thermogram, but only a n inflection on the slope of the T , curve. The slight displacement of the initial portion of the two curves in Figure 1 is real and represents a difference in specific heat between annealed and quenched specimens, the specific heat of the annealed specimen being less than that of the quenched specimen. The slight exothermal rise before the peak in Figures 2 and 3 results from the difference in temperature at which the transition starts in the two forms of the polymer. In Figure 1 it is evident that the quenched polymer begins absorbing additional energy a few degrees in temperature before the annealed specimen. Because it is placed in the reference cell in the DSC, the increase in specific heat of the quenched specimen is endothermal for the reference cell but appears as a n exothermal energy change of the sample specimen up to the temperature that the two curves in Figure 1 cross. These data for PVC can be compared with similar differences in specific heat between annealed and quenched
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ANALYTICAL CHEMISTRY
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Figure 3. Annealing study of PVC homopolymer (Opalon 630) employing modified DSC technique Twenty-milligram sample polymer specimens below T , which have been reported by Karasz et af. (6) for poly(2,6-dimethyl-l,4-phenyleneether). In the case of PVC small changes in physical properties in addition to specific heat can be expected on annealing and such changes have been reported for rigid PVC by Phillips (7). A theoretical consideration and analysis of the time factor in measurement of the glass transition by scanning procedure can be found in the study of polystyrene by Wunderlich and associates (8). ACKNOWLEDGMENT The authors thank the assisting faculty of Franklin and Marshall College for their kind cooperation in helping t o organize the original thesis from which this article has been prepared.
RECEIVED for review December 2, 1968. Accepted January 22, 1969. Permission to publish and financial support granted by Armstrong Cork Company. (6) F. E. Karasz, J. M. O'Reilly, H. E. Bair, and R. A. Kluge,
Polymer Preprints, Vol. 9,155th National Meeting of the American Chemical Society, San Francisco, April 1968, p 822. ( 7 ) R. Phillips, R. L. Cox, and C. A. Heilberger, Preprints, 26th Antec, Society of Plastics Engineers, May 1968, p 216. (8) B. Wunderlich, D. M. Bodily, and M. A. Kaplan, J. Appl. Phys., 35, 95 (1964).
