Langmuir 2004, 20, 2039-2041
Solid-State NMR Study of Poly(E-caprolactone)/ Clay Nanocomposites
2039 Chart 1
Cedric Calberg,† Robert Je´roˆme,† and Jean Grandjean*,‡ University of Liege, Institute of Chemistry B6a, COSM and CERM, Sart Tilman, B-4000 Liege, Belgium Received October 6, 2003. In Final Form: December 2, 2003
Introduction Composites of polymers with smectite clay minerals,1 such as montmorillonite, have received significant attention because of improvements in mechanic, thermal, and barrier properties that can result from synergistic effects of the polymers and the lamellar solid.2,3 NMR is known to be a powerful technique for probing molecular structure, conformation, and dynamics of molecules at interfaces,4,5 and particularly solid-state NMR techniques have been used to characterize modified clays6,7 and polymer/clay nanocomposites.8-19 Natural clays, montmorillonites in particular, are often used as raw materials for preparation of polymer nanocomposites. Such minerals usually contain Fe(III) atoms substituting for Al(III) in the octahedral layer,20 and even in small amounts, these paramagnetic species can overshadow nuclear interaction, perturbing the NMR parameters. In two recent papers,11,12 using a previous theoretical approach,14 the proton relaxation rate in the laboratory frame has been correlated to the clay dispersion in nanocomposites from industry. In this study, we have * To whom correspondence should be addressed. E-mail:
[email protected]. † CERM. ‡ COSM. (1) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; J. Wiley: New York, 1974; Chapter 1. (2) Alexandre, M.; Dubois, P. Mater. Sci. Eng., R 2000, 98, 1-63. (3) Gianellis, E. P.; Krishnamoorti, R.; Manias, E. Adv. Polym. Sci. 1999, 138, 107-147. (4) Grandjean, J. Annu. Rep. NMR Spectrosc. 1998, 35, 216-260. (5) Grandjean, J. Nuclear magnetic resonance spectroscopy of molecules and ions at clay surfaces. In Encyclopedia of Surface and Colloid Science; Hubbard, A. T., Ed.; Marcel Dekker: New York, 2002; pp 3700-3712. (6) Wang, L.-Q.; Liu, J.; Exharos, G. J.; Flanigan, K. J.; Bordia, R. J. Phys. Chem. B 2000, 104, 2810-2816. (7) Kubies, D.; Je´roˆme, R.; Grandjean, J. Langmuir 2002, 18, 61596163. (8) Mathias, L. J.; Davis, R. D.; Jarrett, W. L. Macromolecules 1999, 32, 7958-7960. (9) VanderHart, D. L.; Asano, A.; Gilman, J. W. Chem. Mater. 2001, 13, 3781-3795. (10) Davis, R. D.; Jarrett, W. L.; Mathias, L. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2001, 42, 55-56. (11) VanderHart, D. L.; Asano, A.; Gilman, J. W. Chem. Mater. 2001, 13, 3796-3809. (12) VanderHart, D. L.; Asano, A.; Gilman, J. W. Macromolecules 2001, 34, 3819-3822. (13) Forte, C.; Geppi, M.; Giamberini, S.; Ruggeri, G.; Veracini, C. A.; Mendez, B. Polymer 1998, 39, 2651-2656. (14) Yang, D.-K.; Zax, D. B. J. Chem. Phys. 1999, 110, 5325-5336. (15) Wong, S.; Zax, D. B. Electrochim. Acta 1997, 42, 3513-3518. (16) Wong, S.; Vaia, R. A.; Gianellis, E. P.; Zax, D. B. Solid State Ionics 1996, 86-8, 547-557. (17) Harris, D. J.; Bonagamba, T. J.; Schmidt-Rohr, K. Macromolecules 1999, 32, 6718-6724. (18) Hou, S. S.; Beyer, F. L.; Schmidt-Rohr, K. Solid-State NMR 2002, 22, 110-127. (19) Goddard, Y. A.; Vold, R. L.; Hoatson, G. L. Macromolecules 2003, 36, 1162-1169. (20) Vantelon, D.; Montarges-Pelletier, E.; Michot, L. J.; Briois, V.; Pelletier, M.; Thomas, F. Phys. Chem. Miner. 2003, 30, 44-53.
considered NMR relaxation parameters of poly(-caprolactone)/clay nanocomposites prepared by melt intercalation21 and in situ polymerization,22 varying the nature and concentration of the modified clay and the polymer molecular weight. These well-characterized materials are used to define the abilities and limits of NMR relaxation parameters to study paramagnetic nanocomposites. Experimental Section Poly(-caprolactone) of a number-average molar mass of 49 000 (CAPA Solvay Chemicals sector SBU) and 20 000 were used as reference polymers or to obtain clay nanocomposites by melt intercalation.21 Poly(-caprolactone)/clay nanocomposites were also prepared by in situ polymerization catalyzed by dibutyltin dimethoxide following a published procedure.22 We have used two organo-modified montmorillonites, Cloisite 25A (Chart 1A) and Cloisite 30B (Chart 1B) from Southern Clay Products. Their ferric content, expressed by the percentage of Fe2O3, is 3.41 and 3.51%, respectively. These nanocomposites were analyzed as described previously.21,22 13C CP MAS NMR spectra were recorded with 4 mm zirconia rotors spinning at 4 kHz on a Bruker Avance DSX 400WB spectrometer (B0 ) 9.04 T) working at the Larmor frequency of 100.62 MHz. Cross polarization (CP) experiments were performed under high-power proton decoupling (83 kHz) with a delay time of 1-4 ms and a contact time of 2 ms. The relaxation times were determined by using well-known pulse sequences.7
Theory In the used clays, paramagnetic Fe(III) atoms substitute partly for Al(III) in the octahedral layer, giving rise to an electron-nuclear dipolar interaction which is the dominant relaxation process. Adapting the equation of the electron-nuclear dipolar Hamiltonian14 to the 13C nucleus (eq 1) leads to an interaction frequency as large as 65 kHz.
HD ) -γCγepIzSz,n(3 cos2 βn - 1)/rn3
(1)
where Iz is the spin quantum number of the 13C nucleus, Sz,n is the corresponding quantum number of the nth electron spin, rn is the distance between the two, and βn is the angle between the external applied magnetic field B0 and the vector rn. Rapid averaging of the electron spin orientation, brought by electron spin-electron spin coupling of different Fe(III) (21) Lepoittevin, B.; Devalckenaere, M.; Pantousier, N.; Alexandre, M.; Kubies, D.; Calberg, C.; Je´roˆme, R.; Dubois, Ph. Polymer 2002, 43, 4017-4023. (22) Lepoittevin, B.; Pantousier, N.; Devalckenaere, M.; Alexandre, M.; Kubies, D.; Calberg, C.; Je´roˆme, R.; Dubois, Ph. Macromolecules 2002, 35, 8385-8390.
10.1021/la030369c CCC: $27.50 © 2004 American Chemical Society Published on Web 02/05/2004
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Langmuir, Vol. 20, No. 5, 2004 Chart 2
atoms (flip-flop term),14 strongly reduces this interaction as indicated by smaller line widths of the 13C NMR spectrum. Fe(III) atom distribution in the octahedral layer depends drastically on the montmorillonite sample, as shown by iron extended X-ray absorption fine structure (EXAFS) spectroscopy.20 Most investigated clays exhibit a slight or a strong tendency to iron clustering. Thus, the proximity between the paramagnetic sites increases the spin exchange interaction, decreasing the corresponding electronic correlation time in such an environment. By contrast, Wyoming montmorillonites used here and in the literature studies9,11 do not show iron-iron pairs.20 The shortest Fe-Fe distance is ca. 6.2 Å, and assuming a random distribution of Fe(III), an average distance between two species is calculated to be ca. 11.4 Å.14 For a monolayer of organic molecules intercalated in montmorillonite, the polymer protons and carbons can be as close as 5-10 Å from the paramagnetic species which are at the midplane of the 10 Å thick layer. Fe sites in both bracketing clay layers contribute to the dipolar field at the investigated nucleus. Thus, the paramagnetic effect more strongly reflects statistical variations in local Fe concentration within a pair of layers rather than variations in distance from the clay midplanes.9 On the other hand, for exfoliated clay (isolated platelets), the effect comes mainly from a single clay layer, the next layer being too distant. Results and Discussion The dynamics of the poly(-caprolactone) (Chart 2) chain is rather complex. Simulation of chemical shift anisotropy spectra indicates the nucleus C1 is almost rigid or undergoes a small amplitude (δ < 30°) jump motion around the molecular chain axis whereas the nuclei C2,6 are characterized by 60°-90° jump motions in the kHz range. More complex molecular motion is found for the nuclei C3,4.24 The 13C relaxation time in the rotating frame T1F(C), used here, is sensitive to mobility in the kHz range. To select the relaxation component which is mainly associated with molecular dynamics,25 the spin-locking field should be greater than 75 kHz26 (83 kHz was used here). The relaxation decay of C1 is correctly fitted by a monoexponential law. The relaxation process of the other nuclei is clearly biexponential, and the intensity of the short component increases as C6 < C2 < C5 < C3,4, showing probably an additional increasing effect of the polymer segmental motion (Table 1). The relaxation time values of the other component follow the sequence C1 > C6 > C3,4 > C2,5. As the plot of the relaxation time versus the correlation time shows a minimum in the kHz range, this suggests that the T1F(C) values of the polymer span the minimum (C2,5). Accordingly, faster mobility at the midchain (C3,4) and slower mobility (C1,6) are characterized by longer relaxation times (Table 1). Similar data are obtained for polymers (49 and 20 K). (23) Grandjean, J.; Bujda´k, J.; Komadel, P. Clay Miner. 2003, 38, 369-375. (24) Kaji, H.; Horii, F. Macromolecules 1997, 30, 5791-5798. (25) Laupreˆte, F. Prog. Polym. Sci. 1990, 15, 425-474. (26) Ganapathy, S.; Chacko, V. P.; Bryant, R. G. Macromolecules 1986, 19, 1021-1032.
Notes Table 1. 13C Spin-Lattice Relaxation Times in the Rotating Frame T1G(C) (ms) of the Poly(E-caprolactone) (49 K) and the Nanocomposite with the Highest Clay Content number, δ(ppm)
T1F(C) (PCL)
T1F(C) (NC-MB-50)
C1, 172 C2, 32.8 C3,4, 24.8 C5, 27.9 C6, 64.2
393 (mono) 8.53(0.12); 213(0.39) 12.5(0.29); 295(0.72) 12.5(0.22); 224(0.42) 13.0(0.06); 305(0.39)
237 (mono) 6.56(0.14); 157(0.47) 9.61(0.28); 225(0.72) 10.3(0.24); 193(0.52) 8.87(0.11); 208(0.34)
Results shown in Table 1 deal with nanocomposites (NC) prepared by melt blending (M) with 50% of Cloisite 30B (B-50), but a similar variation of the 13C relaxation time in the rotating frame was also observed at lower clay contents with Cloisite 30B and 25A or with NCs prepared by in situ polymerization (data not shown). When the mineral amount is greater than 30%, the relaxation times become significantly smaller. The shortening of this parameter value is consistent with higher mobility induced by the increasing amount of the amorphous phase.27,28 This conclusion is also supported by X-ray diffraction (XRD) data.21,22 Any significant paramagnetic effect should level off the variation of the relaxation time with the carbon position. Therefore, this relaxation time describes correctly polymer chain dynamics, at least until a clay content of 50%. In contrast to 13C NMR at natural abundance in which spin-diffusion does not occur, the paramagnetic effect can be widely distributed through the proton network (spindiffusion) in 1H NMR of solids. The covered distance is given by the formula
〈L2〉 ) 6Dτ where D is the self-diffusion coefficient (4-6 × 102 nm2 s-1 in rigid poly(-caprolactone)),29 and τ is the diffusion time. A proton spin-lattice relaxation time in the laboratory frame T1(H) of ca. 1 s gives rise to magnetization propagation on a distance of ca. 25 nm. Thus, clay platelets separated by ca. 50 nm can influence the T1(H) values of polymer protons. From several measurements on different samples, we have shown that the 13C-detected proton relaxation times in the laboratory frame T1(H) do not depend on the nucleus position, and the average value is used. Thus, the paramagnetic effect levels off the proton values as a result of spin-diffusion. The paramagnetic contribution rules the relaxation process as indicated from line widths of ca. 180023 and 340 Hz7 obtained for octadecylammonium ions intercalated in the studied paramagnetic montmorillonite and in the nonparamagnetic Laponite, respectively. Whatever the clay content could be, the ratio of the line widths (relaxation rates) remains similar, and nonparamagnetic interaction accounts for only ca. 20% of the relaxation rate. Variation of the paramagnetic contribution to the relaxation rate (R1p(H) ≈ R1obs - R1polymer) as a function of the clay content is shown in Figure 1 for NC-MA and NC-MB. Significant differences of the paramagnetic relaxation rate (R1p(H)) are shown even at a clay content as low as 1%. These values stand in the same range as those obtained with nylon-6/polymer nanocomposites.11 Formation of microcomposites at high clay content should reduce the variation of the paramagnetic (27) Sefcik, M. D.; Schaefer, J.; Stejskal, E. O.; McKay, R. A. Macromolecules 1980, 13, 1132-1137. (28) Schaefer, J.; Sefcik, M. D.; Stejkal, E. O.; McKay, R. A. Macromolecules 1984, 17, 1118-1132. (29) Wang, J.; Cheung, M. K.; Mi, Y. Polymer 2002, 43, 1357-1364.
Notes
Langmuir, Vol. 20, No. 5, 2004 2041
Figure 1. Paramagnetic contribution R1p(H) (s-1) (NC-M) as a function of clay content (9, Cloisite 25A; 2, Cloisite 30B). Table 2. Paramagnetic Contribution R1p (s-1) to the Proton Relaxation Rate Determined for In Situ Polymerized Nanocomposites at Different Clay Contents NC-IB
R1p
NC-IA
R1p
1 5
0.25 0.67
1 10
0.09 0.19
relaxation rate as an effect of clay platelets stacking. The opposite is observed, indicating no further clay aggregation until a mineral content of 50%. The amount of Fe(III) in Cloisite 25A is smaller by 2.8% than that of Cloisite 30B. Accordingly, the upper curve concerns the nanocomposite NC-MB. Poly(-caprolactone)/clay nanocomposites NC-IA and NC-IB have been prepared by in situ polymerization catalyzed by dibutyltin dimethoxide with the two clays. The interlayer distance of the silicate is shifted from 1.86 nm for A to 2.68 nm for NC-IA-3, indicating the formation of an intercalated structure22 similar to that of NC-MA and NC-MB.21 By contrast, the XRD profile of nanocomposites NC-IB-3 does not show any diffraction peak, consistent with an exfoliated structure, as supported by TEM (transmission electron microscopy) micrographs.22 Typical results of the paramagnetic contribution R1p to the proton spin-lattice relaxation rates are shown in Table 2. The value of the nanocomposite NC-IA increases smoothly with the clay content. By contrast, a greater effect is observed with NC-IB (Table 2). At the same clay content, exfoliated nanocomposites induce a stronger paramagnetic effect than intercalated systems as reported earlier.11 Indeed, isolated clay platelets maximize the spin-
diffusion process. For in situ preparations, smaller polymer molecular weights are usually measured.22 However, this does not influence the above data since identical T1(H) values (0.842 ( 0.009 and 0.844 ( 0.017 s) have been determined for NC-IB-3 containing polymer molecules with molecular weights Mn of 18 000 and 30 000, respectively. Thus, in agreement with a previous proposition indicating a correlation between variations in T1(H) and the quality of clay dispersion in the nanocomposite,11 the paramagnetic contribution is greater with the exfoliated system. On the other hand, NC-MA, NC-MB, and NC-IA have been characterized as intercalated systems from X-ray or TEM data,21,22 but the last material exhibits a much smaller paramagnetic effect. Furthermore, the relaxation time value T1(H) of 0.679 s ((0.021) determined for the intercalated system NC-MB-5 is identical to that of the exfoliated material NC-IB-5 (0.686 ( 0.029 s) with the same molecular weight. No correlation between nanocomposites prepared by in situ polymerization and melt blending is found. [Note added in proof: Data published after the receipt of this manuscript, demonstrating that the dispersion of the clay layers is strongly dependent on the synthetic route, support this observation. (Viville, P.; Lazzaroni, R.; Pollet, E.; Alexandre, M.; Dubois, P.; Borcia, G.; Pireaux, J.-J. Langmuir 2003, 19, 9425-9433.)] Conclusions The 13C relaxation time T1F(C) remains a useful probe to study polymer dynamics in these paramagnetic nanocomposites. The 13C-detected relaxation time T1(H), proposed previously to characterize clay dispersion in nylon-6/montmorillonite nanocomposites,11 remains appropriate for the studied systems as far as the same preparation procedure is concerned. Acknowledgment. J.G. and R.J. are grateful to the FNRS (Bruxelles) for a grant to purchase the solid-state NMR spectrometer and to support this study (No. 2.4503.02). C.C. is indebted to the Walloon Government for financial support. LA030369C
