Chapter 16
Damping in Polydienes 1
C. Michael Roland and Craig A. Trask
Naval Research Laboratory, Washington, DC 20375-5000
Cis 1,4-polyisoprene and 1,2-polybutadiene form thermodynamically miscible, nearly ideal mixtures; consequently, the respective chain subunits are statistically distributed in space apart from the constraints of chain connectivity. This segmental mixing and the resulting uniformity of access of the segments to the free volume, however, does not result in the onset of liquid like mobility for all segments at precisely the same temperature or frequency. The differing free volume requirements to make the transition from glass to liquid behavior for chain units within the blends cause the transition to occur over a very broad region in certain compositions. Since the glass transition is associated with a high level of damping, such damping can be obtained therefore over a broad range of frequency or temperature with these materials.
Recent studies of blends of polyisoprene (PIP) with polybutadiene (PBD) have revealed a number of remarkable features [1-5]. Non-polar hydrocarbon polymers such as PIP and PBD are not expected to exhibit miscibility given the absence of specific interactions. When the polybutadiene is high in 1,2 microstructure, however, it has a remarkable degree of miscibility with PIP. This miscibility is the consequence of a close similarity in both the polarizability and the expansivity of the two polymers [3,4]. Their mixtures represent a very unusual instance of miscibility between chemically distinct, non-reacting homopolymers. As its 1,4- content increases, both the polarizability and the thermal expansivity of the PBD diverge from that of PIP, resulting in a reduced degree of miscibility. This effect of PBD microstructure on miscibility with PIP can be seen in the data in Table I [3]. The critical degree of polymerization N*, above which phase separation is expected for a mixture containing equal volume fractions of components of equal N, increases from 830 to over 10,000 as the 1,2- content of the PBD increases from 8 to 97%. Actually, phase separation has never been observed in blends of PIP with high 1,2-PBD, with 10,000 representing a lower limit for N* [4]. Only the miscible region of the phase diagram has been accessible for this system. Current address: Allied-Signal Inc., P.O. Box 31, Petersburg, VA 23804 This chapter not subject to U.S. copyright Published 1990 American Chemical Society
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SOUND AND VIBRATION DAMPING WITH POLYMERS
It is often desirable that a material intended for the attenuation of mechanical or sonic energy exhibit high damping over a wide frequency regime. Since the glass to liquid transition region is associated with such energy dissipation, a material in which this transition transpires over a broad range will be an attractive candidate for certain damping applications. The glass transition temperature of 1,2-PBD is about 0°C and that of high cis 1,4-polyisoprene roughly -63°C. Their miscible mixtures exhibit intermediate transition temperatures, of course, since the homogeneous morphology implies share equivalent free volumes for the components. The research described herein was directed toward probing the transition behavior of mixtures of 1,2-PBD and PIP in order to explore their suitability as damping materials.
3
Table I. Mixtures of PIP with PBD [3,4]
% 1,4 units in PBD
b
a
X
N*
c
d
PBD
92 74 59 3
a
b
3
1.8xl0" 1.6xl0" 1.5xl0" 9.2xl0
3 3
-4
830 1200 2900 >10000
-3
2.4xl0 1.7xl0 0.7xl0"
0.10
0.18
J
1
Figure 5 - Small angle neutron differential scattering cross section (ooo) measured from a sample consisting of sheets of PIP (N = 23000) and deuterated 1,2-PBD (N = 3200) which were in contact for 162 hours at 52°C. The scattering contrast significantly exceeds the incoherent background (—) determined from measurements on the individual polymers, evidencing the thermodynamic miscibility of the blend. w
0.01
-108
I
I
-88
I
I
-68
i
l
w
i
l
-48 -28 TEMPERATURE (°C)
I
I
-8
I
I
12
Figure 6 - Calorimetry results for three blends of 1,2-PBD with 1,4-PBD, in which it can be seen that reducing X* from 6.4 x 10" (sample III A) to 2.4 x 10" (sample IIIC) effects phase separation. A broad T is only observed in the sample (IIIB) with a high concentration of 1,2-PBD, despite the fact that this is a more thermodynamically stable composition than IIIA. 3
3
16. ROLAND AND TRASK
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Damping in Polydienes
1 3
Recent C solid state N.M.R. experiments have revealed that, despite compositional homogeneity that extends to the segmental level, upon heating from below T the vinyl carbons of the PIP begin to manifest liquid-like motions at temperatures for which the vinyl carbons on the 1,2-PBD remain in the glassy state [18]. Since the blend is miscible, the chain units of the two polymer species experience the same free volume, both on average and with respect to its fluctuations. This, of course, is why only a single T , intermediate between the glass transition temperatures of the pure polymers, is measured for the blends. Nevertheless, a greater local free volume is evidently required for liquid like mobility in the vinyl carbons of the PBD. Although the mixtures are homogeneous and thus the components share the same free volume, differences exist in their dynamical response in reflection of a divergence in the free volume requirements for segmental mobility. The bulk T measurements are broadened because of these differences in the temperature at which local motion is engendered in the various moieties. Consequently, morphological homogeneity can be accompanied by dynamical heterogeneity [18]. In both calorimetry and expansivity experiments, the anomalously broad transitions were only seen in blends with a high concentration of PBD that was high in 1,2microstructure. It is not clear why the transition anomaly appears to require both a PBD which is high in 1,2- units, and a relative abundance of the 1,2-PBD itself. Segmental motions in the mixtures are undoubtedly inter-related, particularly with respect to the competition (between deformation induced local motion and fluctuations in the available free volume) governing the glass transition. The details of this interdependence, along with the reasons that manifestations of it become apparent only in certain compositions, remain to be investigated. g
g
Mechanical Damping. For damping purposes, the relevant glass transition behavior is that exhibited during mechanical deformation. The dynamic moduli of a 97% 1,2-PBD, PIP, and their blends were measured in tension over a range of frequencies at a series of temperatures spanning the glass transition region. These low temperature data were expressed as a single function of frequency by time-temperature superpositioning [19]. Previously time-temperature master curves of the dynamic moduli of 1,2-PBD/PIP blends were obtained from higher temperature measurements, encompassing the rubbery and terminal zones of the viscoelastic spectrum [2,5]. A breakdown of superpositioning due to thermal effects on the chemistry of the blend (which could have a different temperature dependence than the friction coefficient) is not of concern with strictly van der Waals mixtures. In miscible blends of poly(ethylene oxide) and poly(methacrylate), a failure of time-temperature superposition has been reported [20]. Consistent with tracer diffusion measurements in polystyrene/poly(2,6-dimethyl 1,4-phenylene oxide) blends [21], this failure suggests a difference in the friction factor for the components of the blends [20]. Nevertheless, the results for PIP/1,2-PBD mixtures [2,5] demonstrate unambiguously the validity of time temperature superposition for this system. For example, distinct local maxima were observed for the loss modulus in the terminal region of the viscoelastic spectrum [2,5], providing a sensitive test of time-temperature shifting. A apparent difference in the friction constant of components of a miscible blend may arise when the extent of chain entanglement (as defined by the ratio of the molecular weight and the critical value of molecular weight) differs significantly for the components [22]. This is the situation in blends described in references 19 and 20, but not for the 1,2-PBD/PIP blends [2,5]. Such an effect would not be operative at frequencies and temperatures for which glassy behavior is approached. The only fundamental requirement for the superpositioning of viscoelastic data is that the shift factors be a single, smooth function of temperature, as is seen for the present mixtures in Figure 7. The master curves for the storage modulus for a blend containing 75% of the 1,2-PBD, along with the corresponding E' measured for the pure PIP and 1,2-PBD, are presented in Figure 8. Different reference temperatures were used in depicting the curves in order to display the very gradual approach of the blend to a glassy plateau modulus. Its transition to the full glassy state occurs over a very broad
310
SOUND AND VIBRATION DAMPING WITH POLYMERS
-80
-60
-40
-20
0
20
-60
-40
-20
0
20
TEMPERATURE (t) Figure 7 - Empirically determined time-temperature shift factors for two blends of 97% 1,2-PBD with PIP. The concentration of the PIP is as indicated.
BLEND
s -1 -6
-3
0
LOG
3
6
12
FREQUENCY
Figure 8 - Superpositioned storage moduli for pure 97% 1,2-PBD, pure PIP, and a mixture containing 25% by volume of the PIP. Different reference temperatures (+5°C and -50°C for the pure 1,2-PBD and PIP respectively and -20°C for the blend) were employed to allow depiction of the data on an expanded scale. The transition temperatures of all blends were, of course, intermediate to the pure component T ' s (-63°C and 0°C for the PIP and 1,2-PBD respectively).
16. ROLAND AND TRASK
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Damping in Polydienes
range (more than fifteen decades of frequency). The loss moduli for these same materials is shown in Figure 9. The pure 1,2-PBD has a significantly broader mechanical glass transition than does pure PIP, an effect not noticeable in the heat capacity and expansivity data. The 25% PIP blend, however, exhibits an extremely broad transition, with virtually no maximum in evidence in Figure 9. In the blend the peak value of the loss modulus, and perhaps also its integrated area, is lower than for the pure components. It is expected that when a transition is very broad the magnitude of the damping will be decreased since at a given temperature or frequency fewer dissipative mechanisms are active. The loss tangents for the two pure materials and the 25% PIP blend are shown in Figure 10. Interestingly the width of the damping peak at the half maxima points is almost equivalent for the pure 1,2-PBD and the blend. The pure PIP has a significantly sharper loss tangent peak, which may indicate something about the dynamical free volume requirements for the respective chain constituents. The only manifestation in Figure 10 of a broadened glass transition in the blend is the high frequency tail of its loss tangent, which can be seen to persist many decades of frequency in the glassy region of the spectrum. As described above, anomalously broad transitions occurred in calorimetry and expansivity measurements only in blends with a high concentration of PBD that was moreover high in 1,2- microstructure. The equivalent effect is found in the mechanical spectra. For example, a blend containing 50% PIP, although having a broader transition region than the pure polymers, exhibits a sharper glass transition than a blend with higher 1,2-PBD content (Figure 11). Similarly an absence of the anomalous broadening was found in the dynamic mechanical spectra of blends having a high concentration of PBD, but in which the latter had only an 85% 1,2-content.
-1
1
1
1
1
1
1
1
1
1
1
1
1
— — — — —'—•— —'— —'—'—'—'—'— — — —'— — -6 -3 0 3 6 9 12
LOG
FREQUENCY
Figure 9 - The loss moduli corresponding to the data in Figure 8. Note that different reference temperatures were chosen in order to allow the curves to be displayed on an expanded scale.
312
SOUND AND VIBRATION DAMPING WITH POLYMERS —I
1
1
1
1—
I— z
LU 2 h CD 1. 2 - P B D ' ;*
BLEND
CO
PIP
o
-6
-3
0
3
LOG
6
FREQUENCY
Figure 10 - The loss tangents corresponding to the data in Figure 8.
i
i i
CO CL
Z
2
LU CD O
_ 1—,—.—.—•—.—.—.—.—.—.—.—.—.—.—.—.—.—.—.—I 1
-6
-3
0
LOG
3
6
9
12
FREQUENCY
Figure 11. Superpositioned storage moduli for two blends of PBD that had 97% 1,2- units with 50% PIP and 25% PIP. The reference temperature is -20 °C in both cases, with the shift factors given in Figure 7.
16. ROLAND AND TRASK
Damping in Polydienes
313
Conclusions Mixtures of cis 1,4-polyisoprene and 1,2-polybutadiene make apparent that the structural homogeneity conferred by thermodynamic miscibility is not always accompanied by equivalence in the dynamics of the components. This heterogeneity of the chain motions gives rise to very broad glass transitions in blends with a high proportion of polybutadiene that is high in 1,2- content. The current absence of any detailed understanding of the phenomenon need not inhibit the exploitation of these blends as novel damping materials.
Literature Cited 1. Roland,C.M.;Macromolecules 1987, 20, 2557. 2. Roland,C.M.;J.Polym.Sci.Polym.Phys.Ed. 1988, 26, 839. 3. Trask, C.A.; Roland,C.M.;Polym.Comm. 1988, 29, 322. 4. Trask, C.A.; Roland,C.M.;Macromolecules 1989, 22, 256. 5. Roland,C.M.;Trask, C.A.; Rub. Chem. Tech. 1988, 61, 866. 6. Bank, M.; Leffingwell, J.; Thies,C.;J.Polym.Sci.: A 1972, 10, 1097. 7. Roland,C.M.In Handbook of Elastomers - New Developments and Technology, Bhowmick, A.K.; Stephens, H.L., Eds.; Marcel Dekker: New York, 1988; chapter 6. 8. Roland,C.M.Rubber Chem. Technol. 1989, 62, 456. 9. DeGennes, P.G.; Scaling Concepts in Polymer Physics, Cornell University Press: Ithaca, N.Y., 1979. 10. Wignall, G.D.; Encyclopedia of Polymer Science and Engineering 1987, 10, 112. 11. Kramer, E.J.; Green, P.F.; Palmstrom, C.J. Polymer 1984, 25, 473. 12. Green, P.F.; Doyle, B.L. Macromolecules 1987, 20, 2471. 13. Bartels, C.R.; Crist, B.; Graessley, W.W.; Macromolecules 1984, 17, 2702. 14. Roland,C.M.;Bohm, G.G.A.; Macromolecules 1985, 18, 1310. 15. Suess, M.; Kressler, J.; Kammer, H.W.; Polymer 1987, 28, 957. 16. Alexanderovich, P.; Karasz, F.E.; MacKnight, W.J.; Polymer 1977, 18, 1022. 17. Prest, W.M.; Roberts, F.J.; Proc. 28th IUPAC Macromol.Symp.1982, p. 664. 18. Miller, J.B.; McGrath, K.J.; Roland,C.M.;Trask, C.A.; to be published. 19. Ferry, J.D.; Viscoelastic Properties of Polymers, 3 Edition, Wiley: New York, 1980. 20. Colby, R.H.; Polymer 1989, 30, 1275. 21. Composto, R.J.; Kramer, E.J.; White, D.M.; Macromolecules 1988, 21, 2580. 22. K.L. Ngai, private communication. rd
RECEIVED January 24, 1990
