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Macromolecules 2001, 34, 4466-4475
Component Dynamics in Polyisoprene/Polyvinylethylene Blends Well above Tg Bumchan Min,† XiaoHua Qiu, and M. D. Ediger* Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706
Marinos Pitsikalis and Nikos Hadjichristidis Department of Chemistry, University of Athens, Panepistimiopolis Zografou 15771 Athens, Greece Received October 25, 2000
ABSTRACT: 13C and 2H NMR relaxation time measurements were performed on low molecular weight blends of polyisoprene (Mn ) 1350 g/mol) and deuterated polyvinylethylene (1,2-polybutadiene; Mn ) 2350 g/mol) in order to extract the segmental correlation times for each component. A wide range of temperatures (295-405 K) and several compositions (PI/dPVE: 100/0, 70/30, 50/50, 30/70, and 0/100) were investigated. At high temperatures, dPVE dynamics are 6 times slower than PI dynamics even after considering self-concentration effects. This intrinsic mobility difference is in quantitative agreement with measurements on each chain individually as a dilute component in a common solvent, suggesting a purely intramolecular origin. The reported correlation times at various compositions fit together smoothly with those obtained by Kornfield and co-workers near Tg, thus providing a unified data set for testing models. One model based on local composition fluctuations captures some qualitative features of the experimental data while failing to describe others.
Introduction In the next generation of materials, polymers will be utilized increasingly as part of multicomponent systems. Examples of these components include solid substrates, fillers, other synthetic polymers or copolymers, and biological structures. In such an environment, the dynamics of segments of one type are influenced by the presence of other segment types or nearby surfaces, and these changes in dynamics will, in turn, influence transport and mechanical properties critical to the function of these composite structures. Miscible polymer blends are an important test of our ability to understand dynamics in multicomponent systems for two reasons. First, because of intimate mixing, each component is strongly influenced by the presence of the other. Second, polymer blends are technologically important,1 and we can be optimistic that an improved understanding of dynamics in such systems will lead to strategies for dealing with the complex rheological features which these systems exhibit. Indeed, significant progress has already been made along these lines.2,3 For a comprehensive understanding of dynamics in miscible polymer blends, it is essential to have data on the segmental dynamics of both components over a wide temperature range. Polyisoprene/polyvinylethylene (PI/PVE) is probably the best studied blend system,4-18 with a slightly negative χ making it miscible at all molecular weights and temperatures.4-6 Various techniques utilized include thermal analysis, mechanical measurements,5,12 solid-state 1H NMR, 13C NMR,7 2H 2D-NMR,9,11 quasi-elastic neutron scattering,16,17 and dielectric spectroscopy.10,13,16 Dynamics in other blend systems have also been studied extensively.19-25 Despite these important contributions, we do not yet have data for any blend system on the dynamics of both compo* To whom correspondence should be addressed. † Present address: Samyang Central Research Institute, Daejoen 305-348, Korea.
nents at multiple compositions over a wide temperature range. Such data are essential for testing models15,26-29 or simulations17 of blend dynamics. NMR methods based upon isotopic labeling provide a clean separation of component dynamics, and the combination of solid-state and relaxation time measurements provides access to a wide range of time scales (1-10-10 s) corresponding to a temperature range from Tg to Tg + 200 K. Solid-state measurements have already been performed on PI/PVE blends by Kornfield and co-workers.9,11 In this study, we complement this previous work with relaxation time measurements on the PI/PVE system. We have studied the blend of hydrogenous PI with deuterated PVE, using 13C and 2H NMR respectively to obtain 13C T1 and NOE and 2H T1. A wide range of temperatures (295-405 K) and several compositions (PI/dPVE: 100/0, 70/30, 50/50, 30/70, and 0/100) were investigated. For each composition and component, correlation times for segmental dynamics were extracted by fitting T1 (and NOE) values to the modified Kohlrausch-Williams-Watts correlation function and a VTF temperature dependence. Direct superposition of the experimental data at different compositions gave information about changes in dynamics which was consistent with the fitting procedure. The major conclusions of our study are as follows. First, at sufficiently high temperature, the segmental dynamics of dPVE are a factor of 6 slower than the PI dynamics while the dynamics of both components are essentially independent of composition. This can only be explained by an intrinsic mobility difference between the two chains. Interestingly, this intrinsic mobility difference can be quantitatively explained on the basis of the segmental dynamics of PI and PVE in dilute solution, suggesting that the observed mobility difference is completely intramolecular in origin. Second, our results at high temperature fit together smoothly with those reported by Kornfield and co-workers near Tg,
10.1021/ma0018345 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/24/2001
Macromolecules, Vol. 34, No. 13, 2001
thus providing a unified data set for testing various models. Finally, when compared with the ratio of segmental relaxation times for PVE and PI from the combined data set, the recent model of Kumar et al.15 does not quantitatively describe the experimental data but does capture some qualitative features of the results. The experiments reported here were performed with a narrow molecular weight distribution and relatively low molecular weight chains (Mn ) 1350, 2350 g/mol) in order to allow quantitative comparisons with molecular dynamics computer simulations. We anticipate that such comparisons will result in improved simulations and microscopic insight into dynamics in blend systems. Based on comparisons made below, these chains are long enough to exhibit the segmental dynamics of high molecular weight chains in the pure melt state, once differences in Tg are taken into account. Samples and Methods Materials. The solvents (benzene and cyclohexane) and isoprene were purified according to standard methods for anionic polymerization.30 Deuterated butadiene was obtained by the thermal decomposition of the corresponding sulfone at 400 K. The monomer gas was transferred through KOH 20% solutions to remove the SO2 and condensed in a flask containing CaH2 at 77 K. Deuterated butadiene was subsequently purified by standing over n-butyllithium at 263 K for 30 min twice.31 sec-Butyllithium was prepared under vacuum by the reaction of the corresponding chloride with lithium dispersion. Dipiperidinoethane (dipip) was purified over CaH2, Na mirror, and Na/K alloy, and the characteristic blue color was obtained. Synthesis. The synthesis was performed in vacuo in allglass reactors with break-seals and constrictions. The polymerization of isoprene was conducted in benzene at room temperature. Deuterated butadiene was polymerized in cyclohexane at 273 K using dipip as a microstructure modifier ([dipip]/[CLi] ) 4/1).32 The living polymers were terminated with degassed methanol. Because of the low molecular weight of the samples, a considerable amount of lithium salt was produced by the termination reaction. Therefore, the samples were dissolved in toluene, and the salts were extracted several times with distilled water. The organic phase was dried over anhydrous sodium sulfate, and then the polymers were precipitated in methanol and dried under vacuum. Transparent samples were obtained. Sample Characterization. SEC experiments were conducted at 313 K using a modular instrument consisting of a Waters model 510 pump, a Waters model U6K sample injector, a Waters model 401 differential refractometer, and a set of 4 m Styragel columns with a continuous porosity range from 106 to 500 Å. The columns were housed in an oven thermostated at 313 K. THF was the carrier solvent at a flow rate of 1 mL/ min. The number-average molecular weights Mn were obtained using a Jupiter model 833 vapor pressure osmometer (VPO) operating at 323 K. Toluene refluxed over CaH2 was used as the solvent. The microstructure of the samples was analyzed by 1H and 13C NMR spectroscopy. Differential scanning calorimetry experiments were conducted using a Seiko 220C DSC (Seiko Instruments) coupled with a Seiko DSC 5200 data station. The samples were cooled at a rate of 10K/min to about Tg - 60 K held for 2 min and heated for 10 K/min to about Tg + 30 K. Tg was determined using the midpoint convention. Sample characterization is given in Table 1. Preparation of Blend Samples. Five different NMR samples (PI/dPVE: 100/0, 70/30, 50/50, 30/70, and 0/100 in wt %) were prepared as follows. First, PI and dPVE were dissolved in cyclopentane to make 10% solutions and mixed in the appropriate proportions for each blend sample. The mixed solution was transferred to an NMR tube, and about 3/ of the solvent was removed under vacuum. The resulting 4 concentrated solution was physically coated onto the walls of the NMR tube to make a layer less than 1 mm thick. A vacuum
Polyisoprene/Polyvinylethylene Blends
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Table 1. Characterization of PI and dPVE Samples samples properties
PI
dPVE
Mn (g/mol)a Mw/Mnb microstructurec
1350 1.11 cis-1,4: 61% trans-1,4: 24% 3,4: 15%
2350 1.05 1,2: 94% 1,4: 6%
a Number-average molecular weight by VPO in toluene at 323 K. b Molecular weight distribution by SEC in THF at 313 K. c From 13C NMR.
Table 2. Glass Transition Temperatures of Pure and Blend Samples samples Tg (K)
pure PI 201
70/30 207
50/50 212
30/70 223
pure dPVE 260
was then applied while holding the tube horizontally. After 72 h, the tubes were sealed under vacuum. From inspection of the 13C NMR spectrum, no solvent remained at a level higher than 0.5%. Samples were refrigerated except during sample preparation and measurement to prevent degradation (e.g., cross-linking of dPVE). The measured Tg values of blend samples are presented in Table 2. Several tests were performed to ensure that no significant degradation took place during experimental measurements (typically several hours at each temperature). NMR Relaxation Measurements. Three different relaxation measurements were conducted so that the dynamics of both components could be determined in each sample. All of these NMR observables are sensitive to segmental dynamics as they measure the reorientation of C-H or C-D bond vectors. 13C NMR experiments tell us only about the dynamics of the polyisoprene chains while 2H experiments tell us only about the dynamics of polyvinylethylene. T1 measurements determine the length of time required for the spin population to return to equilibrium after the spin population is inverted. This time is not directly the time scale for segmental dynamics, but under the conditions of our experiments, T1 is controlled by segmental dynamics as shown by eqs 1-9. As a qualitative illustration, in the high-temperature region of our experiments, T1 gets an order of magnitude longer as the segmental dynamics get an order of magnitude faster. For PI, 13C T1 and NOE were measured for the three backbone carbons bonded to hydrogens (carbons 1, 3, and 4 in Figure 1) from 295 to 405 K. 13C Larmor frequencies of 25.2 and 125.9 MHz were utilized with Bruker AC-100 and Varian Unity-500 NMR spectrometers. The standard inversion recovery method was used for T1 with proton decoupling. A total of 12-16 delay times were used to determine the magnetization curves, which were well described by single exponentials. The relaxation delay between each scan was 5 times the longest relaxation time of the carbons of interest. For NOE measurements, the intensity of the spectrum with continuous decoupling was compared with that from inverse gated decoupling. Here the relaxation delay between successive scans was 10 times the longest relaxation time. To investigate dPVE dynamics, 2H T1 values were measured at 2H frequencies of 15.4 and 76.8 MHz from 320 to 405 K. 2H T1 values were also measured by the inversion-recovery method. At lower temperatures and at the lower field, substantial overlap was observed among the four resonance lines. To interpret the data consistently, all 2H NMR spectra were processed by imposing 500 Hz line broadening to generate a single line. The evolution of magnetization was fitted by four exponential functions. Though we could not obtain meaningful individual 2H T1s from this procedure, reasonable and reproducible rate-averaged T1 values could be obtained; based on various tests, the measured rate-average T1 is considered accurate within 10%. The rate-average T1 is used because this quantity is linear in the spectral density function (see eq 6). For all measurements, temperature was calibrated using an ethylene glycol sample (accuracy of (2 K) and controlled to (1 K.
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Min et al.
Macromolecules, Vol. 34, No. 13, 2001 Deuterium relaxation is dominated by the electric quadrupole coupling of deuterium nuclei and has the following relationship to the reorientation of a C-D bond.37
3 1 ) π2(e2qQ/h)2[J(ωD) + 4J(2ωD)] T1 10
(6)
Here ωD/2π is the Larmor frequency of deuterium. The 2H quadrupole coupling constant e2qQ/h was taken as 172 and 190 kHz for backbone and side group deuterons, respectively.37 In all above expressions, J(ω) is the spectral density function and is the Fourier transform of the orientation autocorrelation function, G(t), for the C-H or C-D bond of interest:
J(ω) ) 1/2
∫
∞
-∞
G(t) exp(-iωt) dt
(7)
G(t) is the object of our interest because it describes the dynamics of individual C-H and C-D bonds:
G(t) ) 3/2〈cos2 θ(t)〉 - 1/2
Figure 1. 13C NMR relaxation time measurements for backbone carbons in pure polyisoprene (carbons 1, 3, and 4). NMR Relaxation Equations. The spin of the 13C nucleus in a 13C-{1H} bond has two potential relaxation mechanisms in the investigated systems. Dipole-dipole (DD) coupling is the only significant relaxation process for sp3 carbons and PI carbons 1 and 4. For sp2 carbons, the chemical shift anisotropy (CSA) relaxation mechanism plays a minor role (
