Miscibility, Phase Separation, and Interdiffusion in the Poly(methyl

Chapter 16

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 30, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch016

Miscibility, Phase Separation, and Interdiffusion in the Poly(methyl methacrylate)—Poly(vinylidene fluoride) System Solid-State NMR Study Werner E. Maas Bruker Instruments, Inc., 19 Fortune Drive, Billerica, MA 01821

The miscibility of PMMA and P V F is investigated using Solid State Nuclear Magnetic Resonance. Miscibility at the molecular level is detected usingfluorineto carbon cross polarization experiments andfluorineto proton to carbon double cross polarization experiments. The proton tofluorinecross depolarization technique is used to determine the amounts of PMMA and P V F that are intimately mixed. These NMR techniques probe miscibility in a range from 3 to 50 Å. The proton to fluorine cross depolarization technique is applied to the studies of phase separation and interdiffusion in the P M M A / P V F system. 2

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The occurrence of a single glass transition temperature (T^) is the generally accepted criterion for distinguishing between miscible and immiscible polymerpolymer systems. While this criterion is satisfactory for many applications of polymer blends, the observation of a single T does not provide insight on miscibility on a molecular scale. It is generally believed that a single T is observed if the dimensions of the domains in which the separate constituents of a polymer blend occur, are smaller than 150 Â. Distance-sensitive techniques such as X-ray scattering, neutron scattering and Nuclear Magnetic Resonance (NMR) provide information on polymer miscibility on a molecular scale. We have developed Solid State NMR techniques to characterize the molecular miscibility in blends of poly (methyl methacrylate) (PMMA) and poly (vinylidene fluoride) (PVF ). These techniques detect the magnetic dipole interaction between nuclear spins of PMMA and P V F and allow the determination of the distance between PMMA and P V F fragments, as well as quantify the degree g

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0097-6156/95/0598-0274$12.00/0 ©-1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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of mixing in these blends. The presented Solid State NMR techniques probe molecular miscibility in a range from 3 to 50 Â. In this paper we will briefly review the results of these experiments on blends of PMMA and P V F 2 and discuss the application of one of these techniques, the proton to fluorine cross depolarization technique, to the study of dynamic processes in P M M A / P V F 2 systems.


Experimental Experiments are performed on a modified Bruker CXP-300 spectrometer. In order to study P M M A / P V F blends an additionalfluorineRF channel is added to the spectrometer. In addition the standard double-tuned Cross Polarization Magic Angle Spinning (CPMAS) probe is triple-tuned to enable simultaneous C , F and H high-power excitation and decoupling. Figure 1 shows the C CPMAS spectra of pure PMMA, obtained with high power proton decoupling, and of pure P V F 2 , obtained with simultaneous high power proton and fluorine decoupling, together with the respective repeat units and the carbon resonance assignments. The improvement in resolution in the P V F carbon spectrum by simultaneous decoupling of thefluorineand proton spins may be appreciated by comparison with the spectra shown in Figure 1C and D, which are obtained under H-only decoupling and F-only decoupling, respectively. For a detailed experimental description the reader is referred to reference (1). 2

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NMR Techniques Detection of Molecular Miscibility. The magnetic dipole interaction between two nuclear spins is proportional to the product of the gyromagnetic ratios 7 of the nuclei and inversely proportional to the cube of the distance r between the nuclei. This r~ dependence causes the magnitude of the dipolar interaction to fall off rapidly with increasing distance, thus providing the NMR spectroscopist with a tool to obtain short range information (several Â). The existence of a dipolar interaction between spins can be detected for instance through magnetization transfer experiments. A frequently used experiment is the Hartmann-Hahn cross polarization experiment in which spin polarization of one type of nuclei is transferred to another type of spins with which they share a dipolar coupling. The magnetization transfer is enabled through the use of radio frequent magnetic fields with frequencies equal to the Larmor frequencies of the spins involved and amplitudes that are matched to the Hartmann-Hahn condition (2). In a F - C cross polarization (CP) experiment magnetization is transferred from fluorines of P V F 2 to carbon spins via the dipolar coupling. If subsequently the carbon spectrum is recorded, then the occurrence of PMMA C resonances proves that PMMA carbons have a dipolar coupling with P V F 2 F spins and thus are in close proximity to P V F fluorines. An example of a F - C CP spectrum of a P M M A / P V F blend is shown in Figure 2A, where in addition to the carbon res3

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Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

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Figure 1. CPMAS C spectra of (A) PMMA acquired with H high power decoupling, (B) P V F acquired with simultaneous H and F high power decoupling, together with the repeat units of PMMA and P V F 2 , and P V F spectra obtained with F decoupling only (C) and H decoupling only (D). X

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Figure 2. Magic angle spinning C spectra of P M M A / P V F 60/40, obtained with simultaneous H and F high power decoupling: A. cross polarized from F and B. doubly cross polarized from F to E to C . 2

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onances of P V F 2 resonances of PMMA carbons are observed. The pulse sequence of the F - C CP experiment is diagrammed in Figure 3A. In addition to the detection of molecular miscibility, an average Ρ V F fluorine to a nearby PMMA carbon distance can be determined from the rate of magne tization transfer, which can be obtained by measuring the carbon intensities as a function of the cross polarization time. The cross polarization rate is propor tional to the square of the dipolar coupling and therefore to r" , which limits the detection of the proximity of nuclear spins with the F - C CP experiment to approximately 10 Â. For the various weight ratios of the P M M A / P V F blend studied (1,3,4), the distance between afluorineof P V F and a carbon on a neighboring PMMA segment is found to be approximately 3 Â. Since a single F - C distance is not a good description for an amorphous blend, the carbon intensities as a function of the CP time were modeled with a distribution of distances(^). It was found that only a narrow distribution of F - C distances around a mean value of 3 Â could account for the experimental data. The intensities of the PMMA resonances observed in the F - C CP experiment (Figure 2A) are small, tempting one to conclude that only a small portion of the PMMA chains is in close proximity to P V F chains. However, one must take into account the relaxation processes which compete with the magnetization transfer. In particular for the P M M A / P V F blends, the build-up of carbon magnetization, when cross polarizing from P V F fluorines, is severely restrained due to the short fluorine rotating frame relaxation time Τχ («1 ms). In unfa vorable cases where the relaxation rate is larger than the cross polarization rate, the detection of molecular miscibility may be hindered or even prevented. An alternative technique which overcomes this problem is to transfer magnetization from fluorines to protons. Since the 7 of protons is four times larger than that of carbons, the F - H CP rate is sixteen times larger than the F - C CP rate. This results in a more efficient build-up of proton magnetization in the F - H CP experiment, as compared to the F - C CP experiment. However, due to strong homonuclear dipolar couplings, solid state proton spectra often exhibit poor resolution and in order to distinguish between P V F and PMMA proton magnetization, the proton magnetization is subsequently transferred to carbon via Ή - ^ Ο CP. Figure 2B shows a spectrum of the P M M A / P V F 60/40 blend, obtained via the F - H - C double cross polarization experiment (Figure 3B). In comparing this spectrum to the F - C CP spectrum of Figure 2A it is seen that the double cross polarization technique indeed results in much larger PMMA carbon intensities. Apart from the higher efficiency of the fluorine to proton to carbon magnetization transfer as compared to the fluorine to carbon transfer, this increase in PMMA carbon intensities is also due to the occurrence of proton spin diffusion, which causes proton magnetization from protons close to P V F fluorines to be transferred to protons farther removed from fluorines, thus en abling the detection of PMMA segments at larger distances from P V F fluorines than those detected in the direct fluorine to carbon magnetization transfer ex19

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Figure 3. Pulse schemes of the NMR experiments: A . F - C cross polariza tion; B . F - H - C double cross polarization; C. H- F cross depolarization followed by H - C cross polarization. 19

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periments. While the additional occurrence of proton spin diffusion hampers the determination of P V F to PMMA distances, the " F ^ H - ^ C double CP experiment provides qualitative proof of molecular miscibility up to approximately 15-20 Â. 2

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Quantification of the degree of miscibility. Although the F - C CP experiment and the F - H - C double CP experiments are suitable to detect molecular miscibility in P M M A / P V F blends, it is difficult to derive information from those experiments on the amounts of P V F and PMMA that are intimately mixed. In order to derive quantitative information we developed the H - F cross depolarization technique (#). Instead of using thefluorinesas a source of magnetization, we will now use the fluorine reservoir as a sink, through which proton magnetization can disappear. If, after creation of spin-locked proton magnetization in the whole sample, a fluorine RF field is turned on, its strength adjusted to the proton-fluorine Hartmann-Hahn condition, then all the protons that are dipolar coupled tofluorineswill lose their magnetization to the lattice, via the fluorines. In a second step the remaining proton magnetization is transferred to carbons via H - C cross polarization, and the carbon magnetization is detected (see Figure 3C). The observed PMMA carbon magnetization is then from parts of PMMA molecules, whose protons do not have a dipolar interaction with fluorine. In other words, only those parts of PMMA molecules that are remote from P V F molecules are observed. Through the additional occurrence of proton spin diffusion, PMMA proton magnetization from regions at larger distancesfromfluorineswill be transported to the fluorine sink, which enlarges the detection limit for miscibility in the P M M A / P V F blends to approximately 50 Â. Apart from losing magnetization tofluorines,proton magnetization also disappears to the lattice (via Τχ relaxation). This is corrected for by dividing the C intensities obtained as a function of the F - H transfer time by the intensities obtained from a blank experiment in which nofluorineRF field is used and where the loss of proton magnetization is thus entirely due to relaxation. The PMMA (S (t)) and P V F (S (t)) carbon intensities as a function of the Ή - ^ cross depolarization time t and corrected for H - T i relaxation, can thus be equated as: (1) (») = » +(ΐ-α) 19

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(2) where fj£ and f^ are the mixed PMMA and P V F fractions, respectively, (1-fJriJ and (l-f^riJ are the isolated fractions of PMMA and P V F , respec tively, and D(t) is a function which decays to zero and which describes the loss of proton magnetization to the sink through H - F cross depolarization and proton spin diffusion. The equations for PMMA and P V F differ in an extra term exp(—t/TnF) which arises from the fact that protons from isolated P V F x

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still lose their magnetization to thefluorinesink since all P V F 2 protons are close to fluorines. Figure 4 shows the result for the PMMA proton magnetization detected via the C=0 resonance, which levels off to a nearly constant value at 3.5 ms. This apparent constant level is attributed to the proton magnetization of the fraction 1-fmix of PMMA that is not closely mixed with P V F . In the absence of proton spin diffusion PMMA units close to and PMMA units remote from P V F can be distinguished by the distance aspect of H - F cross depolarization. Proton spin diffusion complicates this distinction but clearly, the ability of proton spin diffusion to transfer magnetization from PMMA protons to P V F protons can also be used to distinguish well-mixed PMMA from non-mixed PMMA. The proton tofluorinecross depolarization experiments with subsequent carbon detection, directly provide information on the fractions of PMMA and P V F that are intimately mixed. The isolated PMMA fractions are determined from the PMMA carbon intensities at longer cross depolarization times, whereas the isolated P V F fractions are obtained from the P V F carbon intensities at short depolarization times (the initial decay is mainly determined by the proton to fluorine transfer rate Tgp). In addition, based on a spin diffusion model, information is obtained on the dimensions of the miscible domains in the P M M A / P V F blends (see ref.(#) for details). The cross depolarization technique has been applied to quantify the miscibility of P M M A / P V F blends as a function of composition (#), and also to examine the effect of PMMA microstructure on mixing with P V F by studying blends of isotactic, atactic and syndiotactic PMMA with PVF (^). In the remainder of this paper we report on the detection of dynamic processes in the P M M A / P V F system. 2

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Phase separation in P M M A / P V F blends 2

The proton to fluorine cross depolarization technique, described above, can be employed to study the result of dynamic processes in P M M A / P V F blends. In this section we will summarize the results of a study on the phase separation that occurs upon annealing of a P M M A / P V F blend. Samples of a P M M A / P V F 60/40 blend (by weight ratio) are annealed at either 120°C or 140°C for a time t and subsequently quenched in liquid nitrogen. A cross depolarization experiment then yields the fractions of not-mixed PMMA and P V F , which are diagrammed in Figures 5A and 5B, as a function of the annealing time. Both the fractions of isolated PMMA and P V F increase sharply at short annealing times and the changes are slower at longer annealing times. In addition to the NMR experiments Differential Scanning Calorimetry (DSC) experiments are performed. The DSC experiments show a melting peak from P V F crystallites, indicating crystallization of P V F to be the origin of the phase separation in the blend upon annealing. The DSC measurements after annealing 2

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Figure 4. C intensities as a function of the proton to fluorine cross depolarization time, scaled with respect to the intensity at zero depolarization time, for the C=0 resonance of PMMA. The dashed line indicates the isolated PMMA fraction.


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Miscibility in the PMMA-PVF System 2


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at 120°C or 140°C allow the crystalline fractions to be determined as a function of the annealing time; these crystalline fractions are depicted in Figure 6. Similar to the isolated fractions of PMMA and P V F obtained from the NMR experiments, the crystalline fractions, observed with DSC, increase rapidly at short annealing times and slower at longer annealing times. A comparison between the data from DSC measurements and NMR experiments reveals that at least part of the phase separation, observed as an increase in the isolated P V F fraction upon annealing, can be attributed to crystallization of P V F in the blend. In addition, however, also an increase in the not-mixed fraction PMMA is revealed by the cross depolarization experiments. In a previous study (3) on P M M A / P V F blends with varying weight composition, it was found that the isolated PMMA fraction increases with higher PMMA content, while the fraction of not-mixed P V F decreases with higher P M M A / P V F ratio. The increase in the isolated PMMA fraction with annealing time thus indicates the formation of a PMMA-richer phase, due to the crystallization of P V F . Thisfindingis also supported by a measured increase in proton Tip (see Papaverine et al.(5)). This change in the P M M A / P V F ratio in the mixed phase also explains the observation that the growth of the isolated P V F fractions slows down at longer annealing times. Due to depletion of P V F in the amorphous regions surrounding the crystals, the crystal growth should continuously decrease, while leaving a PMMA-richer phase around the P V F crystals (6). This, however, does not explain the significant discrepancy between the fractions of crystalline P V F , determined with DSC, and the fractions of not-mixed P V F , obtained from the NMR experiments (see Figure 7). We believe this difference to be caused by a crystalline-amorphous PVF -interphase, as suggested by Hahn et al. (7). Such an interphase is believed to be caused by head-to-head and tail-to-tail defects in P V F and is expected to expel PMMA. The observation of a constant value of the difference between the NMR and DSC data after an initial annealing time also supports the existence of such an interphase, since this interphase is expected to have a constant thickness. In an additional experiment the blend, annealed at 140°C for 16.5 hours, and containing according to the DSC measurements approximately 30% crystalline P V F , is heated in an oven at 190°C, which is well above the melting temperature of the blend and the P V F crystals. NMR experiments performed after a certain time at 190°C revealed that after only 10 min. all the PMMA and P V F is mixed again, i.e. the fractions of isolated PMMA and P V F are again at the same values as before annealing. Melting at 190°C for longer times did not decrease these values further. This finding seems to suggest that no large P V F crystallites have developed during the annealing time. Since the remixing of P V F and PMMA after the melting of the P V F crystals is governed by diffusion and the diffusion coefficient of P V F chains is on the order of 10"" cm /s at 190°C (see next section) the mean displacement L=(2Dt)s of the polymer chains during the 10 minutes at 190°C is approximately 3.5/im. This places therefore an 2

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upper limit of a few micrometers on the size of the P V F 2 crystals. In the wellmixed P M M A / P V F system, the depletion of P V F in the regions surrounding the P V F 2 crystallites, may prevent the development of large crystals and lead to the formation of many small crystallites. 2

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Interdiffusion of PMMA and P V F

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The ability of the proton to fluorine cross depolarization experiment with sub sequent carbon detection to distinguish between PMMA and P V F chains that are in close proximity to each other, or mixed, and chains that are remote from each other, or isolated, is exploited to detect interdiffusion of PMMA and P V F at temperatures above the T of both components. In this study thin sheets of PMMA and P V F 2 (with an average thickness of approximately 90 μπι) are stacked alternately in a ceramic magic angle spinner. The spinner with its content is heated in an oven at 190 °C for a certain time after which it is quenched in liquid nitrogen. During the time at 190 °C chains of PMMA and P V F 2 diffuse across the boundaries between the sheets. PMMA and P V F 2 form a miscible system and the negative free energy of mixing (8-10) will be the main driving force for mixing. Before heating, at zero diffusion time, PMMA and P V F are separated on a macroscopic scale. The distances between PMMA chains and P V F 2 chains are too large to be bridged by proton spin diffusion and no dipolar interactions ex ist between PMMA protons and P V F fluorines. A H - F cross depolarization experiment before annealing reveals 100 % isolated PMMA and 100 % isolated P V F 2 . As the diffusion progresses PMMA and P V F 2 segments intermix, and near the boundaries between the sheets the distances between PMMA and P V F 2 will be small enough to be bridged by proton spin diffusion. This reveals itself in a decrease in the fractions of isolated PMMA and P V F 2 in the cross depolarization experiments with time at 190 °C. Some examples are shown in Figure 8 in which the PMMA O C H 3 carbon intensities are plotted as a function of the cross depo larization time for different diffusion times at 190 °C. The decrease in intensities at longer depolarization times for curves obtained at increasing diffusion times, indicates that as diffusion progresses more PMMA segments are close to P V F 2 segments, resulting in a decrease in the fraction of PMMA segments that are not close to P V F . Figure 9 shows the data for both the isolated PMMA and P V F 2 fractions as a function of the diffusion time at 190 °C. These data can be interpreted based on a diffusion model which involves three diffusion fluxes: one of PMMA chains, one of P V F chains and one vacancy flux due to the differences in mass and size between PMMA and P V F chains. This net vacancy flux causes a massflowof both PMMA and P V F 2 in the direction of the faster moving component. For details of this model the reader is referred to Wu et al. (11) and Maas et al. (12). The solid lines in Figure 9 are fits calculated with a diffusion equation based on this model and yield the interdiffusion coefficients of PMMA ( ( 7 ± 5 ) Ί 0 ~ cm /s) 2

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Figure 8. PMMA 0 C H carbon intensities as a function of the H - F cross depolarization time, obtained after different times at 190°C during which chain diffusion takes place. 3

Figure 9. Fractions of isolated PMMA and P V F as a function of the interdiffusion time. The solid lines are fits calculated with a diffusion equation, from which the interdiffusion coefficients are obtained. 2


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and P V F ((15 ± 5) · 10~ cm /s). Although the calculated curves approximate the experimental data reasonably well, we note that in the diffusion model the intrinsic diffusion coefficients are assumed concentration-independent. This assumption cannot, however, be exact since the driving force for the interdiffusion, the free energy of mixing, is known to be a function of composition (10,13). In previous methods of detecting polymer diffusion, inert marker particles are inserted at the boundary between the components before diffusion takes place (11,13). The particles will move as a result of the mass flow, and from the displacement as a function of the diffusion time one can then obtain the ratio of the interdiffusion coefficients. The individual diffusion coefficients can then only be estimated based on prior knowledge of one of the diffusion coefficients. In the NMR technique presented here, data axe acquired from both components involved in the diffusion, thereby enabling the determination of both diffusion coefficients simultaneously and without prior knowledge of either a single coefficient or their ratio.


2

Conclusions The foregoing sections show that with the presented NMR techniques a detailed picture on the miscibility of polymer blends can be obtained. In the P M M A / P V F blend, thefluorineto carbon cross polarization experiments and the fluorine to proton to carbon cross polarization experiments provide insight in miscibility on the molecular scale. The proton tofluorinecross depolarization experiments with subsequent carbon detection yield information on the amounts of PMMA and P V F that are intimately mixed. In addition this technique provided information on dynamic processes in the P M M A / P V F system, including phase separation and crystallization in the blend and the interdiffusion of PMMA and P V F . 2

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Acknowledgments. The author thanks prof. W.S. Veeman for helpful discussions and support, C. Klein Douwel, P. van der Heijden, A. Eikelenboom, T. Papavoine, G. Werumeus Buning and J. Vankan for theoretical and experimental contributions and S. Pochapsky for critically reading the manuscript.

Literature Cited (1) C.H. Klein Douwel, W.E.J.R. Maas, W.S. Veeman, G.H. Werumeus Buning, and J.M.J. Vankan. Macromolecules, 1990, 23, 406. (2) S.R. Hartmann and E.L. Hahn. Phys. Rev., 1962, 128, 2042. (3) W.E.J.R. Maas, W.A.C. van der Heijden, W.S. Veeman, J.M.J. Vankan, and G.H. Werumeus Buning. J. Chem. Phys., 1991, 95, 4698.


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(4) A. Eijkelenboom, W.E.J.R. Maas, W.S. Veeman, J.M.J. Vankan, and G.H. Werumeus Buning. Macromolecules, 1992, 25, 18. (5) C.H.M. Papavoine, W.E.J.R. Maas, W.S. Veeman, G.H. Werumeus Buning, and J.M.J. Vankan. Macromolecules, 1993, 26, 6611.


(6) B.S. Morra and R.S. Stein. Polym. Eng. Sci., 1984, 24, 311. (7) B.R. Harm, O. Hermann-Schonherr, and J.H. Wendorff. Polymer, 1987, 28, 201. (8) T. Nishi and T.T. Wang. Macromolecules, 1975, 8, 909. (9) E . Roerdink and G. Challa. Polymer, 1978, 19, 173. (10) J.H. Wendorff. J. Polym. Sci., Polym.Lett.Ed., 1980, 18, 439. (11) S Wu, H-K. Chuang, and C.D. Han. J. Polym. Sci: Polym. Phys. Ed., 1986, 24, 143. (12) W.E.J.R. Maas, C.H.M. Papavoine, W.S. Veeman, G.H. Werumeus Buning, and J.M.J. Vankan. J.Polym Sci.,part B: Polym. Phys., 1994, 32, 785. (13) E.J. Kramer, P.F. Green, and C.J. Palmstrom. Polymer, 1984, 25, 473.

RECEIVED February 2, 1995


Miscibility, Phase Separation, and Interdiffusion in the Poly(methyl

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