THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. 9. Patent Office)
VOLUME66
(0Copyright, 1962, by the American Chemical Society)
NUMBER1
JANUARY 24, 1962
STUDY OF MOLECULAR MOTION IN SOME ISOPRENES AND BUTADIENES‘ BY RAMDAS P. GUPTA~ Department of Physics, The Pennsylvania State Univwsity, Universaly Park, Pennsylvania Received December $3, 1860
Experiments on nuclear magnetic resonance and dynamic mechanical measurements have been conducted with samples of cis-l,4polybutadiene, trans-lI4polybutadiene, cis-l,4poiyisoprene and trans-l,4polyiso rene, to study the molecular motion. The transition in cis-l,4polybutadiene is observed a t 170°K.; for trans-lI4-poly!utadiene transitions are a t 235 and 370°K. Transition for cis-l14polyisopreneis a t 200”K., and for trans-l,4-isoprene it is a t 235 and 340°K. The main transition in sample B is observed at higher temperature than for sample D and comparatively it is less sharp. The high temperature transition in sample B appears a t a higher temperature. The low temperature transition, for all the samples, corresponds to glass transition when the substance changes from solid to rubber. This transition, therefore, probably results from onset of segmental motion. The high temperature transitions for B and D which occur at 370 and 340”K., respectively, may be associated with melting of the crystallites. For samples A and C the onset of segmental motion and the melting of crystallites occur a t temperatures very close together; therefore, we observe only one transition. One might be tempted to think that the high temperature transition which arises from melting of the crystallites probably is associated with loosening of the inter-chain-links, but sufficient evidence is needed to be sure of this.
Introduction Experiments with samples of cis-1,4-polybutadiene, trans-114-polybutadiene,cis-l14-polyisoprene and trans-l14-polyisoprene have been conducted by both nuclear magnetic resonance and dynamic mechanical methods to study molecular motions in these polymers. To my knowledge no one has reported any experimental data on the dynamic mechanical properties of these materials. h’uclear magnetic resonance experiments with natural rubber have been reported by Gutowsky8 in a temperature range up to 300OK. He has reported two transitions: one at about 150OK. and the other between 200 and 250OK. As he has reported, the temperatures for transitions depend on the curetime for the sample. Study of rubber samples also has been done by some other worker^.^-^ The aim of the present experiments has been to study the dynamic mechanical properties and to compare these with the n.m.r. results. I n certain cases dynamic
mechanical measurements have proven easier to detect molecular motions because of the suitability of the frequency range. By comparing dynamic mechanical properties with those of n.m.r., we expect to get a more correct picture of the processes responsible for different transitions and the associated molecular motions.
Experimental N.m.r. experiments have been performed using a dual purpose 40 Mc. Varian model spectrometer having a 12-in. magnet and a variable temperature probc. An account for the modulation broadening has been taken for the second moment correction. Damping (Q-l) was obtained from the ratio of the width of the resonance curve a t the half power points to fo. Q-l = Af/fo = half width/resonant frequency. For dynamic mechanical measurements the apparatus used by Kline9 has been employed. It is a transverse beam apparatus. The specimen rod was suspended by threads inside a Dewar covered with a brass plate. Lead balls were placed inside the Dewar to keep the temperature gradient inside as small as possible. A hot plate was put inside the Dewar in which the flow of current could be monitored to attain a desired temperature. T h e low temperature runs for the sample were made by allowing the temperature to drift up from liquid nitrogen temperature. The samples for this experiment were supplied by Dr. A. E. Woodward. The molding of the sample for dynamic measurements was done with great difficulty, using the principle of compression molding under vacuum. The sample used in this experiment has been in the form of a rod of a suitable length and diameter. A very slow and steady drift of temperature was maintained during the course of this ex-
(1) This work wae conducted in part with the financial help of the
U. S. Atomic Energy Commission. (2) Institut fur Elektrowerkstoffe der Fraunhofer-Gesellschaft, Freiburg, Germany. (3) H. S. Gutowaky and L. H. Meyer, J . Chem. P h ~ s . ,21, 2122 (1953). (4) P. J. Flory. “Principles of Polymer Chemistry,” ( 5 ) A. A. Morton and L. D. Taylor, J . Polymer Sci., 38, 7 (1959). (6) R . E. Cunningham, ibid., 42, 571 (1960). (7)A. A. Morton and E. J. Lampher, ibid., 44, 233 (1960). (8) G.9. Triok, ibid., 41, 213 (1959).
(9) D. E. Kline, ibid., 82, 449 (1956).
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RAMDAS P. GUPTA
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Vol. 66
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100 150 200 Temp., OK. Fig. 3.-cis-l,4-Polyisoprene.
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Fig. l.-cis-l,4-Polybutadiene.
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200 250 300 350 Temp., OK. Fig. 2.--trans-l,4-Polybutadiene.
periment to be sure that there was no gradient of temperature along the length and breadth of the sample rod. Under the present setup the error in temperature would not be more than a degree.
Results I n Fig. 1,damping (Q-l) and frequency have been plotted against temperature for cis-1,4-polybutadiene in a temperature range of 80 to 2OOOK. We find that damping decreases in the beginning of the temperature range and theii it begins to increase at about 110OK. until about 2OO0K., after which the sample becomes too soft to be used for further experiment. The rise in damping is very abrupt; on the other hand, frequency begins to decrease slowly in the beginning of the temperature range but the decrease becomes abrupt a t about 17OOK. It shows that the transition at about 170OK. is very much pronounced. I n Fig. 2, damping and frequency plots against
150 200 250 300 Temp., OK. Fig. 4.--trans-l,4-Polyisoprene. 100
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temperature have been shown for trans-l,4-polybutadiene. In the frequency plot the main transition appears to about 230 and 35OOK. There is one more transition at about 160°K., but it is so small that it may be due t'o scattering itself. For damping plots transition peaks appear a t 170, 235, 320 and 37OoK., but the peaks at 235 and 37OoK. are quite predominant whereas the other peaks may be due to scattering. The maximum (Q-l) value for the peak at 235OK. is about 0.13. Damping and frequency plots for cis-1,4-polyisoprene are shown in Fig. 3. Transition peaks at 140 and 200OK. are observed, but the peak at 200OK. is the maiii peak. I n Fig. 4, frequency and damping variation against teniperature for the sample of trans-1,4polyisoprene has been shown. The frequency plot shows peaks a t 225 and 335OK., whereas the damping plot shows peaks at about 235, 340 and 170OK. The peaks at 235 and 34OOIi. are the
Jan., 1962
MOLECULAR MOTIOX IN ISOPREXE AKD BUTADIENES
main peaks. The peak a t 170OK. has been neglected. The central peak a t 235'K. is not a discontinuous peak, rather the upper points fall above the graph on this scale and have not been plotted because it was considered for comparison purposes to be convenient to have the same scale for all ithe figures. The maximum &-l value for this peak is about 0.28. Figure 5 shows a n.m.r. second-moment plot against temperature for trans-1,4-polybutadiene and cis-l,4-polybutadiene. For trans-polybutadiene two transitions are seen, one a t about 225OK. and the other at about 350°K. The transition a t 225°K. is predominant but the transition a t 35OOK. is not so obvious and it easily might have been missed. For cis-l,4-polybutadiene there is oiily one transition, which appears a t about 175OK. I n Fig. 6, for the samples of trans-1,4-polyisoprene and cis-1,4-polyisoprene, 1i.m.r. second moment has been plotted against temperature. I n the caso of cis-1,4-polyisoprene the only transition which ir.,observed is at about 175OK., but in the case of trans-isoprene two transitions are observed; one at about 220OK. and the other a t about 33OOK. The transition at 220'K. is quite sharp, but the transition a t 330'K. is not so predominant. The maximum value of damping (Q-l) for the central peak of trans-1,4-isoprene is about double that of the maximum damping value for the corresponding peak of trans-1,4-polybutadieiie (shown in Figs. 2 and 4). Consequently that transition in trans -isoprene is sharper than the corresponding transition in trans-butadiene. The n.m.r. second-moment value for the cis samples is lower than the second moment for the trans samples. The second moment for cis1,4-poljrbutadiene a t 1OOOK. is about 18 gauss2 whereas for the trans-l,4-polybutadieiie it is about 20 gauss2 a t the same temperature. Similarly in the case of trans- and cis-isoprene a difference in their second moments has been observed. Discussion Fromi the above results one could infer that the low temperature transition. which is at about 235OK. in the case of trans-114-polyisoprene and trans-1,4polybui,adiene, corresponds to glass transition as the substance changes from a solid to a rubber state. This transition is due to the onset of segmental motion. The high temperature transition which appears at about 340OK. in the case of trans-1,4polyisoprene and at about 370OK. for trans-1,4polybut adiene probably is ascribable to melting of the crystallites. One might be tempted to associate the high temperature transition with melting of the crystallites associated with loosening of inter-chain links, but sufficient evidence is needed to be sure of this. There is only one transition in the case of the cis-1,4-polyisoprene and cis-114-polybutadiene samples. The onset of segmental motion and the melting of crystallites both take place at almost the same temperature in the case of cis samples,
100
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200 250 300 350 Temp., "K. Fig. 5.--trans-1,4- and cis-1,4-polybutadienes. 150
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200 250 300 350 Temp., "K. Fig. 6.-truns-1,4- and cis-1,4-polyisoprene. 100
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and therefore we observe only one peak corresponding to both these processes. The maximum value of the damping peak (Q-l) for trans-1,4-polyisopreiie is about 0.28, whereas for the trans-l,4-polybutadiene it is about 0.13 (about half the other one). I n the case of polyisoprene the transition js also very sharp. This shows that at the corresponding temperatures there is more extensive motion in the case of trans-l,4-polyisoprene than for trans-1,4-polybutadiene. The middle transition peaks are a t about the same temperatures in both trans samples, but the high temperature transition in the case of trans-l,+polybutadiene is a t a higher temperature than for the trans-l,4-polyisopreiie sample, because the melting of crystallites takes place a t a correspondingly higher temperature in the first case. Both n.m.r. and dynamic mechanical data show only one peak for cis samples, and two main peaks for trans samples, though the high temperature transition in the case of trans samples is more easily detectable by dynamic mechanical methods than by n.m.r. The processes responsible for these transitions are supported both by dynamic mechanical and n.m.r. methods, as discussed above. Acknowledgment.-The author is very thankful to Dr. A. E. Roodward for supplying the samples for this experiment and also for taking an interest in this work. Thanks also are due to Miss A. Zweng for helpful discussions.
