Motional Heterogeneity of Intercalated Species in Modified Clays and

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Langmuir 2006, 22, 4818-4824

Motional Heterogeneity of Intercalated Species in Modified Clays and Poly(E-caprolactone)/Clay Nanocomposites L. Urbanczyk,† J. Hrobarikova,‡ C. Calberg,† R. Je´roˆme,† and J. Grandjean*,‡ COSM and CERM, Institute of Chemistry B6a, UniVersity of Liege, Sart Tilman, B-4000 Liege, Belgium ReceiVed January 5, 2006. In Final Form: March 3, 2006 Modified laponites and synthetic saponites are used as precursors for the preparation of poly(-caprolactone) (PCL)/clay nanocomposites. The structure and dynamics of species intercalated in the modified clays and the corresponding nanocomposites are characterized by X-ray diffraction and magic-angle spinning NMR. The influence of the headgroup, the hydrocarbon chain length, and the loading of the surfactant on the nanocomposite formation are discussed. The yield of PCL intercalation is related to the probability of direct polymer-clay interactions and to the size of the clay platelets. Relaxation times in the laboratory and rotating frames that allow characterization of fast and slow molecular dynamics in these systems are discussed, showing a motional hetereogeneity of the intercalated species.

Introduction minerals1

Composites of polymers with smectite clay have received significant attention because of improvements in mechanical, thermal, and barrier properties that can result from synergistic effects of the polymers and the lamellar solid.2-4 To ensure the dispersion of the negatively charged mineral within the polymer matrix, cationic surfactants are ion-exchanged with interlamellar cations. Such hybrid materials which contain typically 5% of the mineral are often characterized by macroscopic techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and thermal analysis (TGA). The understanding of the particular properties of the nanocomposites compared to the bulk polymer requires the estimation of the relevant interactions and the characterization of the structure and dynamics of the species near the clay surface. On one hand, theoretical calculations can provide appropriate information.5-9 On the other hand, nuclear magnetic resonance (NMR) is a powerful and versatile tool for elucidating the structure and dynamics of species near a clay surface.10 Intercalated poly(ethylene oxide) has been studied in particular and compared to the bulk by solid-state NMR,11-14 but other nanocomposites have been investigated too.15 * To whom correspondence [email protected]. † CERM. ‡ COSM.

should

be

addressed.

E-mail:

(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) Fischer, H. Mater. Sci. Eng. 2003, C23, 763-772. (5) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 7990-7999. (6) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000-8009. (7) Balazs, A. C.; Singh, C.; Zhulina, E.; Lyatskaya, Y. Acc. Chem. Res. 1999, 8, 651-657. (8) Lee, J. Y.; Baljon, A. R. C.; Loring, R. F. J. Chem. Phys. 1999, 111, 9754-9760. (9) Gardebien, F.; Gaudel-Siri, A.; Bredas, J.-L.; Lazzaroni, R. J. Phys. Chem. B 2004, 108, 10678-10686. (10) Grandjean, J. Nuclear magnetic resonance spectroscopy of molecules and ions at clay surfaces. In Interface Science and Technology; Wypych, F., Satyanarayana, K. G., Eds.; Elsevier: Amsterdam, 2004; Vol. 1, pp 216-246. (11) Kwiatkowski, J.; Whittaker, A. K. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1678-1685. (12) Hou, S. S.; Beyer, F. L.; Schmidt-Rohr, K. Solid State NMR 2002, 22, 110-127. (13) Hou, S. S.; Bonagamba, T. J.; Beyer, F. L.; Madison, P. H.; SchmidtRohr, K. Macromolecules 2003, 36, 2769-2776.

In a recent paper, we prepared poly(-caprolactone) (PCL)/ clay nanocomposites by in situ polymerization using synthetic saponites modified by a 1/1 exchange between the sodium clay counterions and halides of quaternary hexadecylammonium.16 These data deal with materials containing low and moderate amounts of polymers to characterize mainly the properties of species near the clay surface. Recently, the preparation of a few laponite-based nanocomposites has been reported.17,18 Therefore, we prepared PCL nanocomposites using modified laponites as precursors and compared their properties to those obtained with modified saponites. For better understanding of the nanocomposite formation, saponites modified by organic cations with a shorter alkyl chain (C12 and C14 instead of C16) and different polar headgroups (trimethylphosphonium or bis(2-hydroxyethyl)methylammonium instead of trimethylammonium) were also formed and used to prepare PCL nanocomposites. The alcoholic polar group, which can interfere in the bulk polymerization process, is particularly interesting to study. Thus, we have investigated how polymer intercalation is affected by the structure and loading of the surfactant. In the second part of this paper, the molecular dynamics of the intercalated species are characterized by different NMR relaxation parameters, showing chain motional heterogeneity in the modified clays and the nanocomposites. These latter investigations mainly deal with the nanocomposites formed with the highest charge saponites, leading to better NMR spectrum resolution. Experimental Section Synthetic sodium saponites with a charge per half unit cell in the 0.35-0.80 range (SAP0.35-SAP0.80) were prepared19 and characterized as described previously.19-21 Laponite RD (LAP0.28) is a commercial synthetic hectorite (Laporte Industries Ltd.) with wellknown properties. These clays were exchanged with the halides of bis(2-hydroxyethyl)methylhexadecylammonium (DHEMHA), hexa(14) Reinholdt. M. X.; Kirkpatrick, R. J.; Pinnavaia, T. J. J. Phys. Chem. B 2005, 109, 16296-16303. (15) Grandjean, J. Clay Miner., in press. (16) Hrobarikova, J.; Robert, J.-L.; Calberg, C.; Je´roˆme, R.; Grandjean, J. Langmuir 2004, 20, 9828-9833. (17) Dundigalla, A.; Lin-Gibson, S.; Ferreiro, V.; Malwitz, M.; Schmidt, G. Macromol. Rapid Commun. 2005, 26, 143-149. (18) Hou, W.; Zhao, W.; Li, D. Chin. J. Polym. Sci. 2004, 22, 459-462. (19) Bergaoui, L.; Lambert, J. F.; Frank, R.; Suquet, H.; Robert, J.-L. J. Chem. Soc., Faraday Trans. 1995, 91, 2229-2239. (20) Michot, L. J.; Villie´ras, F. Clay Miner. 2002, 37, 39-57. (21) Delevoye, L.; Robert, J.-L.; Grandjean, J. Clay Miner. 2003, 38, 63-69.

10.1021/la060041u CCC: $33.50 © 2006 American Chemical Society Published on Web 04/06/2006

Motional Heterogeneity of Species in Clays decyltrimethylphosphonium (HDTP), hexadecyltrimethylammonium (HDTA), tetradecyltrimethylammonium (TEDTA), and dodecyltrimethylammonium (DDTA) as described previously, adjusting the salt concentration for modulation of the surfactant loading.22,23 The number after the acronym refers to the organic cation content with respect to the cation exchange capacity (CEC) of the original clay. Thus, HDTA1.0 means a 1/1 cation exchange. The clay/PCL nanocomposites were prepared by in situ polymerization catalyzed by dibutyltin dimethoxide, at 55 °C for 48 h, using clay/monomer and monomer/dibutyl dimethoxide ratios of 3/2 (w/w) and 7/2 (v/v),16 respectively. These conditions were modified slightly when SAP0.80 was used: 0.5 g of modified clay, 0.5 mL of caprolactone, and 0.3 mL of Bu2Sn(OMe)2 (0.098 M) (toluene) were allowed to proceed at 55 °C for 2 days (HDTA) or 3 days (DHEMHA), and smaller amounts of organic matter were used to decrease the polymer content (indicated after the PCL acronym) of the nanocomposite. These materials were characterized by thermogravimetric analysis (Thermal Analyst 2100, TA Instruments; heating range of 20-800 °C at a heating rate of 10 °C/min, under N2 flow) and X-ray diffraction (XRD) measurement (Siemens D 5000 powder diffractometer; Cu KR radiation (λ ) 1.54 Å), Ni filter, 25 °C). 13C and 29Si cross-polarization (CP) magic-angle spinning (MAS) and 31P MAS NMR spectra were recorded with 4 mm zirconia rotors spinning at 7 kHz on a Bruker Avance DSX 400WB spectrometer (B0 ) 9.04 T) working at Larmor frequencies of 100.62, 79.50, and 161.9 MHz, respectively. The 13C, 29Si, and 31P chemical shifts were referenced relative to the peaks for TMS, TMS, and H3PO4 (85%), respectively. The CP NMR spectra were performed under highpower proton decoupling (83 kHz) with a delay time of 4 s and a contact time of 2 ms. Quantitative data and the time constants TCH were determined from the plots of line intensity versus contact time in the CP MAS experiments. These experiments (16 delays) were run with 3000 scans. Such plots were also used to estimate the conformational ratio of the surfactant alkyl chain in the modified clays and nanocomposites.16,22 The 13C NMR longitudinal relaxation times in the laboratory frame, T1(C), and in the rotating frame, T1F(C), were determined by application of the usual CP pulse sequences (13C; 90° pulse of 5.5 µs; relaxation delays of 6 and 2 s, respectively, 15 delays, 4000 scans for surfactant nuclei; relaxation delay of 300 s, 6-8 delays, 128 scans for PCL nuclei). The silicon-detected proton relaxation times in the rotating frame, T1F(H), were determined by varying the duration of the 1H spin-lock period with high-power decoupling (83 kHz). The experiments (15 delays) were run with 300 scans and a relaxation delay of 5 s. The 31P longitudinal relaxation times in the laboratory frame, T1(P) (15 delays, 100 scans), were measured according to a well-known procedure, using a 90° pulse of 4 µs and a relaxation delay of 20 s.

Results and Discussion First, we briefly discuss the characteristics of a few modified clays used to prepare PCL nanocomposites. Substitution of the surfactant ammonium by phosphonium (HDTP) does not alter significantly the characteristics of the modified saponites. In particular, these data corroborate the progressive increase of the basal spacing values d001 with the layer charge of saponite and variation of the all-trans conformation of the surfactant alkyl chain, which reaches a minimum value for a saponite charge of ca. 0.60.22 Similar observations were also reported later following increases in the surfactant loading of a montmorillonite.24 Both effects can be explained similarly, resulting from the interplay between the repulsive silicate surface-hydrocarbon chain interaction and the interchain van der Waals interactions between (22) Mu¨ller, R.; Hrobarikova, J.; Calberg, C.; Je´roˆme, R.; Grandjean, J. Langmuir 2004, 20, 2982-2985. (23) Kubies, D.; Je´roˆme, R.; Grandjean, J. Langmuir 2002, 18, 6159-6163. (24) He, H.; Frost, R. L.; Deng, F.; Zhu, J.; Wen, X.; Yuan, P. Clays Clay Miner. 2004, 52, 350-356.

Langmuir, Vol. 22, No. 10, 2006 4819 Table 1. Main Characteristics of Typical Modified Low-Charge Clays/PCL Nanocomposites

sample

d001 (Å)

HDTP1.2LAP0.28/PCL6.0 HDTP2.0LAP0.28/PCL7.0 DDTA0.6SAP0.50/PCL12 TEDTA0.6SAP0.50/PCL12 TEDTA1.1SAP0.50/PCL17

34 (16) 34 23.5 (16.5) 28.0 32.5

all-trans conformer PCL content (%) of the content surfactant chain (%) 40 35 50 40 47

6.0 7.0 12 12 17

the hydrogen atoms that increase with the surfactant concentration, favoring the all-trans conformation of the alkyl chains. Small surfactant loadings (low-charge saponites) give rise to weak interchain interaction, promoting the formation of the gauche conformation. Until near the lowest all-trans conformer population, the alkyl chains remain approximately parallel to the silicate surface, whereas they radiate away from the clay surface for higher surfactant loadings (high-charge saponites), forming paraffin-type monolayers and paraffin-type bilayers for the highest clay contents.15,22,24 Increasing the surfactant content in saponite from 0.6 to ca. 1.0 also increases the all-trans conformation, in line with the above results. Like HDTALAP,23 HDTP-exchanged laponite keeps similar properties, intercalating a monolayer of surfactant with the hydrocarbon chain lying down at the clay surface. The ca. 1/1 exchange of interlamellar inorganic cations of laponite by HDTP (HDTA) cations does not lead to a high yield of PCL intercalation (Table 1). The two broad diffraction peaks observed for HDTP1.2 (surfactant content) LAP0.28 (clay charge)/PCL6.0 (polymer content) indicate only partial polymer intercalation since the diffraction peak characteristic of the modified clay remains important. Intuitively, higher surfactant loading is expected to provide a better environment for polymerization. Surfactant incorporation between the clay platelets can exceed the CEC of the original clay, and the excess of the cationic surfactant enters the interlayer with its counterions through hydrophobic bonding, leading to a continuous increase of the gallery space.25,26 However, HDTP loading corresponding to twice the CEC of laponite (HDTP2.0LAP0.28) does not increase significantly the d001 value (14.7 instead of 14.0 Å), significantly less than the value of surfactant cations forming a bilayer structure with the long chains lying down on the clay surface (lateral bilayer, d001 ) 18-20 Å).22 By contrast, the 1/1 cationic exchange in the investigated natural clays25,26 gives rise to such a bilayer arrangement of the intercalated surfactant, favoring insertion of a sandwiched third layer by interchain interaction. Such a bilayer structure is also observed for the modified saponites, and the excess of cationic surfactant leads to an increase in the d001 values (for instance, HDTP1.0SAP0.80, d001 ) 31.1 Å; HDTP2.0SAP0.80, d001 ) 40.0 Å) corresponding to its insertion in the gallery space. The lateral monolayer structure of the surfactant-exchanged laponite22,23 does not provide such an intercalation pathway. The repulsion between the negative charges of the clay and the halide ions probably prevents (reduces) surfactant intercalation from exceeding the laponite CEC. The small size of the laponite platelets results in a poorly ordered structure as shown by broad diffraction peaks. Therefore, long surfactant alkyl chains may partly reside outside the interlayer space, giving rise to interaction with the extra surfactant ions (not intercalated). Thus, the external polymerization process could be favored, but the polymer is not significantly intercalated since (25) Zhao, Z.; Tang, T.; Qin, Y.; Huang, B. Langmuir 2003, 19, 7157-7159. (26) Kwolek, T.; Hodorowicz, M.; Stadnicka, K.; Czapkiewicz, J. J. Colloid Interface Sci. 2003, 264, 14-19.

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Table 2. 29Si-Detected Proton Relaxation Times in the Rotating Frame, T1G(H) (ms), of Modified Clays and the Corresponding PCL Nanocomposites sample

T1F(H)

DDTA0.6SAP0.50 TEDTA0.6SAP0.50 HDTP2.0LAP0.28

15.4 14.3 10.2

sample

T1F(H)

DDTA0.6SAP0.50/PCL13 11.2 TEDTA0.6SAP0.50/PCL12 8.11 HDTP2.0LAP-NANO/PCL7.0 8.48

its interaction with the chains of the surfactant ions exceeds the laponite CEC and reduces the polymer-silicate contacts. Increasing the surfactant loading (HDTP2.0LAP0.28/PCL7.0) leads to a better defined d001 peak with a value characteristic of intercalated nanocomposites.16 However, the PCL content in the nanocomposite was not significantly increased (Table 1), and the nonintercalated polymer was probably eliminated during the purification procedure used to obtain the nanocomposites. The 13C CP MAS NMR spectrum of HDTP2.0LAP0.28/ PCL7.0 shows mainly the signals of the intercalated surfactant. The dipolar times TCH (eq 1) of these resonances increase upon nanocomposite formation, indicating higher surfactant mobility. Such behavior was observed with HDTA1.0SAP/PCL as precursor.16 Accordingly, a shorter 31P relaxation time in the laboratory frame (T1(P) ) 1.71 s) for HDTP2.0LAP0.28/PCL7.0 than for the modified laponite (T1(P) ) 2.30 s) supports this conclusion (slow-motion regime, ωτ > 1), but the opposite was observed with HDTA1.0LAP0.28/PCL with smaller amounts of intercalated PCL.16 The bilayer lateral structure of the modified saponites provides an intercalation pathway with much smaller polymer-silicate contacts, improving PCL intercalation (Table 1). The small shortening of the surfactant alkyl chain shows no significant effect since the yields of the intercalated nanocomposite obtained either with TEDTA1.1SAP0.50 and HDTA1.1SAP0.5016 samples or with DDTA0.6SAP0.50 and TEDTA0.6SAP0.50 are equal (Table 1). The polymer content decreases with the surfactant loading (TEDTA0.6SAP0.50). Furthermore, with the smaller chain surfactant (DDTA0.6SAP0.50), a diffraction peak at 16.5 Å, characteristic of the precursor clay, remains present and partial PCL intercalation occurs (Table 1).Thus, the expected increase of direct contacts between the polymer chains and the clay silicate layer leads again to poorer polymer intercalation. The all-trans conformer population of the surfactant alkyl chain is smaller in the nanocomposites (Table 1) than in the modified clay, as noted previously.16 The 29Si-detected proton relaxation times T1F(H) are governed by the 1H-1H interactions responsible for the spin-diffusion process. Therefore, these parameters are influenced by the proton environment of the silica nuclei and provide another tool for characterizing the nanocomposites. We measured the 1H relaxation times in the rotating frame that monitor spin diffusion on the rather small scale surrounding silicon nuclei. The 29Si MAS NMR spectrum of SAP0.50 shows two signals at -93.5 and -89.9 ppm assigned to the Si nuclei at the center of a SiO4 tetrahedron bound to either three SiO4 tetrahedra or two SiO4 tetrahedra and one AlO4 tetrahedron, respectively.21 Laponite is characterized by a main peak at -94.1 ppm.21 Better intercalation of PCL is expected to increase proton density near the Si nuclei, leading to a decrease of the T1F(H) values (Table 2). Mobility variation, which could also change the dipolar interaction, does not play a significant role here. Although the polar ester function of PCL may interact with the silicate clay layer,9 direct inorganic/ organic contact, as provided with the surfactant lateral monolayer of HDTP(HDTA)2.0LAP, slightly decreases T1F(H) corresponding to weak polymer intercalation (Table 2). At similar surfactant loading, the surfactant bilayer arrangement in HDTA16- or

Table 3. Main Characteristics of the Modified High-Charge Saponites and the Corresponding PCL Nanocomposites sample HDTA1.0SAP0.8 HDTA1.0SAP0.8/PCL43 DHEMHA1.0SAP0.8 DHEMHA1.0SAP0.8/PCL53 DHEMHA1.0SAP0.8/PCL23 DHEMHA0.76SAP0.8 DHEMHA0.76SAP0.8/PCL27

surfactant content (CEC)

d001 (Å)

1.0

28.0 33.1 30,4 35.1 31.7 29.7 34.2

1.0 0.76

polymer content (%) 43 53 23 27

TEDTA-exchanged saponites (Table 1) favors polymer intercalation by surfactant-polymer chain interactions. The precursors DDTA0.6SAP0.50 and TEDTA0.6SAP0.50 give rise to better PCL intercalation (Table 1), and accordingly show a greater shortening of T1F(H) (Table 2). Molecular dynamics simulations of PCL intercalated in montmorillonite (MONT) have been reported in the literature.9 For MONT modified by intercalation of dimethyl(2-ethylhexyl)n-octadecylammonium ions, it has been calculated that van der Waals interactions account for the major part of the polymersurfactant interactions (90%) whereas the van der Waals and electrostatic terms contribute to ca. 60% and 40% of the polymerclay interaction, respectively.9 That study shows that direct polymer-clay contacts in the nanocomposite should be taken into account even in montmorillonite-based nanocomposite formation9 in which the clay precursor forms a lateral bilayer. Better ordering of the intercalated species is observed with the highest charge saponites (larger clay platelets20), leading to better resolved NMR spectra. Now, we report our study on such saponites modified by HDTA and DHEMHA, and their corresponding nanocomposites. Under the usual experimental conditions, polymerization is activated by dibutyltin dimethoxide, acting as initiator of the ring-opening polymerization of lactones.27 The addition of an alcohol function leads to reaction with the tin and temporarily converts the propagating species (metal oxide) into dormant species.28 Accordingly, the number-average molecular weight Mn of PCL in exfoliated nanocomposites decreases with increasing content of DHEMHA-modified montmorillonite.29 The main characteristics of typical nanocomposites are shown in Table 3. Using SAP0.80 with a lower DHEMHA loading (CEC ) 0.76 instead of 1.0) leads to greater d001 values in the 34-39 Å range, in agreement with the previous observation indicating better polymer intercalation with lower alcohol content. Such an effect could also be consistent with another report showing better clay dispersion in a polypropylene derivative matrix when montmorillonite inorganic ions were only partially exchanged with the cationic surfactant.30 However, such an observation was not observed for our systems as illustrated with the TEDTA0.6SAP0.50-based nanocomposite showing a lower PCL yield and smaller d001 value than those observed with TEDTA1.1SAP0.50 (Table 1). This supports the role of the hydroxyl groups of DHEMHA on the polymerization process of our systems. According to the basal spacing values, the polymer chains are intercalated between the modified clay platelets whereas no diffraction peak results from exfoliated nanocomposites.29 Saponite platelets are larger for charge higher than 0.6,20 leading to more ordered systems.22 Furthermore, surfactant intercalation (27) Lo¨fgren, A.; Albertsson, A.-C.; Dubois, P.; Je´roˆme, R. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1995, C35, 379-418. (28) Penczek, S.; Biela, T.; Duda, M. Macromol. Rapid Commun. 2000, 21, 941. (29) Lepoittevin, B.; Pantousier, N.; Devalckenaere, M.; Alexandre, M.; Kubies, D.; Calberg, C.; Je´roˆme, R.; Dubois, P. Macromolecules 2002, 35, 8385-8390. (30) Zao, Z.; Tang, T.; Qin, Y.; Huang, B. Langmuir 2003, 19, 7157-7159.

Motional Heterogeneity of Species in Clays

Figure 1.

13C

Langmuir, Vol. 22, No. 10, 2006 4821

CP MAS NMR spectra of HDTA1.0SAP0.75PCL25 (top) and HDTA1.0SAP0.80PCL43 (bottom).

in the highest charge saponites gives rise to a higher all-trans/ gauche conformational ratio of the long hydrocarbon chain and higher basal spacing values, forming the so-called paraffin complex. This results in higher PCL intercalation in the nanocomposites under the same experimental conditions as shown previously16 and confirmed with TEDTA0.6SAP0.75, providing 22% of intercalated PCL instead of 12% with TEDTA0.6SAP0.50 (Table 1). Then, we adapted the experimental conditions to obtain nanocomposites with different PCL contents to characterize variation of the material properties as a function of the clay content (Table 3). A typical 13C CP MAS NMR spectrum is shown for HDTA1.0SAP0.75/PCL25 (Figure 1, top) and HDTA1.0SAP0.80/ PCL43 (Figure 1, bottom). After extraction from the last composite, the polymer characteristics (Mn ) 4000 and Mw/Mn ) 1.85) are similar to those previously reported, in particular for HDTA1.0SAP0.75/PCL25.16 The peak intensities of the intercalated surfactant are too weak in the nanocomposite with the high polymer content to be useful in the following dynamic studies (Figure 1, bottom). Quantitative spectra are obtained by 13C CP experiments by varying the contact time. During the pulse sequence, the 13C magnetization M(t) increases exponentially with the dipolar relaxation time TCH whereas the proton magnetization is governed by the relaxation in the rotating frame, T1F(H), decreasing M(t). The variation of the carbon magnetization is given by the equation

M(t) ) M0[exp(-t/T1F(H) - exp(-t/TCH)]/ {1 - TCH[T1F(H)]-1} (1) where t is the contact time and M0 is the equilibrium magnetization. Assumptions in applying this equation31 are usually fulfilled for our systems, and the three parameters M0, TCH, and T1F(H) can be obtained from the plot of line intensity versus contact time. The calculated M0 values of HDTA1.0SAP0.75/PCL25 at ca. 55 ppm (NCH3 of HDTA) and ca. 66 ppm (C6, PCL) are 1.18 and 0.82, respectively, and their ratio (1.44) is clearly different from that measured from the spectrum taken with one contact time value (Figure 1). The polymer was recovered from the nanocomposite by clay extraction, and the molecular weight deter(31) Kolodziejski, W.; Klinowski, J. Chem. ReV. 2002, 102, 613-628.

mination was performed by size exclusion chromatography according to the literature.29 The above ratio of 0.17 obtained by NMR indicates an efficient polymer extraction. Such ratios vary also in parallel with the polymer content of the nanocomposite deduced from TGA analysis (negligible for DHEMHA1.0SAP0.80/PCL53, 0.06 for HDTA1.0SAP0.80/ PCL43, and 0.31 for DHEMHA1.0SAP0.80/PCL23). Quantitative information can also be obtained from one-pulse experimental spectra.15 The overall curve analysis was also used to estimate the all-trans conformer population of the surfactant hydrocarbon chain intercalated in saponite or in PCL nanocomposites with low polymer content (Table 1), typically 5-15%.16,22 In a previous paper,16 the molecular dynamics were based mainly on dipolar relaxation times TCH (eq 1). Other NMR experiments are used here to probe the dynamics of interfacial species in the previously prepared and new modified clays. The T1 spin-lattice relaxation times in the laboratory frame are sensitive to molecular motions in the megahertz frequency range, while the T1F relaxation times in the rotating frame and the protonproton dipolar line shapes, which can be measured by using the 2D WISE (wide-line separation) NMR experiment, are sensitive to molecular motions on the kilohertz frequency scale. Motional heterogeneity of the intercalated surfactant was sometimes described by different means in the literature.32-35 The 13C longitudinal relaxation time T1(C) of HDTA1.0LAP0.28 increases from the polar headgroup to the terminal methyl of the long alkyl chain23 as typically observed in the fast-motion limit (ωτc , 1). The negative charges of saponite that result from cation isomorphous substitution in the tetrahedral layer (instead of the octahedral layer) are directly in contact with the organic cations. Furthermore, the arrangement of the surfactant alkyl chain intercalated in saponites, changing with the saponite charge, is different from that in laponite.22,23 Therefore, similar relaxation measurements were performed with HDTASAP of variable interlamellar charge (Table 4). In all cases, the relaxation time (32) Ishimaru, S.; Yamauchi, M.; Ikeda, R. Z. Naturforsch. 1998, 53a, 903908. (33) Yamauchi, M.; Ishimaru, S.; Ikeda, R. Mol. Cryst. Liq. Cryst. 2000, 341, 315-320. (34) Wang, L.-Q.; Liu, J.; Exharos, J.; Flanigan, K. Y.; Bordia, R. J. Phys. Chem. B 2000, 12, 2810-2814. (35) Mirau, P. A.; Vaia, R. A.; Garber, J. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2005, 46, 440-441.

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Table 4. Typical 13C Spin-Lattice Relaxation Times in the Laboratory Frame, T1(C) (ms), of Surfactant-Exchanged Saponites sample

CH2(2)

CH2(3)

(CH2)4-14

CH3

TEDTA1.1SAP0.50 HDTA1.1SAP0.50 HDTA1.0SAP0.80 DHEMHA1.0SAP0.80

533 529 514 189

816 663 600 363

937 770 594 385

>1000 >1000 619 714

Table 5. Typical Values of the Relaxation Times T1G(C) (ms) in the Rotating Frame of HDTA (Indicated with an Asterisk) and PCL Nuclei in the Nanocomposites HDTA1.0SAP0.80PCL43 and HDTA1.0SAP0.75PCL25 and of PCL Nuclei after Extraction from HDTA1.0SAP0.75PCL25

δ (ppm) 174.5 (C1) 66.9 (C6) 55.5* (NCH3) 34.6 (C2) 34.1* (C4-14(t)) 32.0* (C4-14(g)) 30.0 (C5) 26.7 (C3,4) 26.2 (C3,4) 24.9* (C15) 16.5* (CH3)

T1F(C), HDTA1.0SAP0.80PCL43

T1F(C), HDTA1.0SAP0.75PCL25

(249) 69.4 13.7 51.2

(282) 84.9 29.5 39.7 36.0 11.4 61.4 52.1 52.2 30.8 35.2

53.8 42.7 42.4

T1F(C)extr (368) 85.1 67.7 73.2 58.7 59.5

T1(C) increases from C2 to C16. Thus, mobility increases progressively from the polar headgroup to the terminal methyl group as observed for HDTA1.0LAP0.28.23 With the highest charge saponites, the smaller T1(C) values and their slowest variation (nearer to the curve minimum) indicate less mobility, as could be assumed for the more ordered paraffin complex. The 13C relaxation times in the rotating frame, T1F(C), that do not show averaging by the spin-diffusion relaxation mechanism of T1F(H) are sensitive to mobility in the kilohertz range. However, information on dynamics is obtained only if the fluctuations induced by the flip-flop terms of the proton-proton dipolar interaction have lower frequency than that of the detected molecular motion.36 High mobility found for the intercalated species is in line with such behavior. Furthermore, the intensity of the spin-lock field higher than 75 kHz (83 kHz was used here) is expected to select the relaxation component associated with molecular dynamics.37 The T1F(C) values were calculated by assuming a purely exponential decay of the signal intensity as a function of the delay time (Table 5). Signal assignments were obtained from the literature.23,38 Thus, molecular motions on the kilohertz frequency scale are also present for the surfactant hydrocarbon chain, supporting the presence of motional heterogeneity in our systems. The plot of T1F(C) as a function of the correlation time is typically described by a curve with a minimum. For HDTA1.0SAP0.75 and HDTA1.0SAP0.80 (data not shown), intermediate mobility is associated with the smallest T1F(C) values (C4-14) while slower (C1) and faster (C15,16) motions give rise to greater relaxation times. Such behavior was also observed with HDTA1.0SAP0.75/ PCL25, but the signals of C2 (PCL) and C4-14 (all-trans-HDTA) are only partly resolved, and each relaxation time value is probably perturbed by the evolution of the other peak. The PCL T1F(C) values of the two nanocomposites and those of the extracted polymer change similarly, with the smallest values for C3,4. It (36) Laupreˆte, F. Prog. Polym. Sci. 1990, 15, 425-474. (37) Ganapathy, S.; Chacko, V. P.; Bryant, R. G. Macromolecules 1986, 19, 1021-1032. (38) Kaji, H.; Horii, F. Macromolecules 1997, 30, 5791-5798.

should be noted that the position and the broadness of the curve minimum depends on the assumed dynamic model. Therefore, that minimum is probably different for the surfactant and the polymer chains, and our deductions are only qualitative. The dynamics of the poly(-caprolactone) 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 kilohertz range. More complex molecular motion is found for the nuclei C3,4,38 and the highest mobility is expected for the C3,4 nuclei at the midchain of the monomer unit.38,39 Assuming the same theoretical curve, higher relaxation time values mean slower motion and the intercalated polymer chain shows higher mobility than the extracted polymer (Table 5), supporting data based on the dipolar relaxation time TCH.16 That observation probably results from the loss of polymer crystallinity upon nanocomposite formation.29 The PCL 13C spin-lattice relaxation in the rotating frame was better described by a biexponential decay for the PCL proton bearing carbon nuclei both in the bulk phase and in montmorillonite-based nanocomposites.39 The 13C nuclei of the extracted polymer do not show any significant nonexponential behavior, but its molecular weight and molecular weight distribution16 are quite different from those previously studied.39 The biexponential analysis of the nanocomposite data leads to conclusions similar to those previously found39 supporting the mobility sequence C3,4 > C2,5 > C6. The 13C CP MAS NMR spectra of DHEMHA1.0SAP0.80 and DHEMHA1.0SAP0.80/PCL23 show smaller (broader) signals for the modified saponite than for the nanocomposite (Figure 2). Although the number of scans is lower by a factor of 2, the surfactant headgroup peaks are not detected on the first spectrum. This indicates higher mobility and/or better ordering of the surfactant in the nanocomposite than in the modified clay. The T1F(C) values of the HDTA(TEDTA) saponites are an order of magnitude greater than those of DHEMHA1.0SAP0.80, which are an order of magnitude smaller than those of the intercalated polymer. Therefore, chain motion in DHEMHA1.0SAP0.80 is more efficient (less mobility) than that in HDTA1.0SAP0.80 for the relaxation of 13C nuclei in the rotating frame, supporting previous conclusions from the TCH values of the headgroup nuclei of DHEMHA.22 On the basis of the previous assumptions, the minimum of the theoretical curve occurs near C15 of the surfactant hydrocarbon chain with mobility enhancement from C2 to C16 (motion in the kilohertz range). The PCL T1F(C) values, which are in the same range as those of the two previous nanocomposites (Table 5), decrease with an increase of the basal spacing (Table 6), corresponding to higher polymer chain motion as also observed with the HDTA (TEDTA)-based nanocomposites (data not shown). A few other nanocomposites were prepared with DHEMHA1.0SAP0.80, varying the polymerization conditions to obtain different polymer contents. However, the d001 values do not change progressively with the polymer content (data not shown), probably indicating different PCL molecular weight distributions and/or arrangements within the nanocomposite. This can be explained by a change in the polymerization rate that appears from the dihydroxyl groups of DHEMHA whose influence depends on the varying experimental conditions, alcohol and monomer concentration, solvent, and temperature, changing the efficiency of the dormant species.28 Intercalation of PCL between the saponite platelets might also change its dynamic behavior, leading to more heterogeneous (39) Calberg, C.; Je´roˆme, R.; Grandjean, J. Langmuir 2004, 20, 2039-2041.

Motional Heterogeneity of Species in Clays

Figure 2.

13C

Langmuir, Vol. 22, No. 10, 2006 4823

CP MAS NMR spectra of DHEMHA1.0SAP0.80 (top) and DHEMHA1.0SAP0.80/PCL23 (bottom).

Table 6. Typical Values of the Relaxation Times T1G(C) (ms) in the Rotating Frame of DHEMHA (Indicated with an Asterisk) and PCL Nuclei in DHEMHA1.0SAP0.80(/PCL23) and of PCL in Two Corresponding Nanocomposites T1F(C), DHEMHA1.0SAP0.80

δ (ppm) 66.9 (C6) 62.2* (+NCH2CH2OH) 58.7* (+NCH2CH2OH) 54.1* (NCH3) 34.7 (C2) 34.1* (C4-14(t)) 32.0* (C4-14(g)) 30.0 (C5) 29.9* (C2) 26.7 (C3,4) 26.4* (C3) 26.2 (C3,4) 24.8* (C15) 16.5* (CH3)

4.65 5.03

T1F(C), DHEMHA1.0SAP0.80PCL23

T1F(C), DHEMHA1.0SAP0.80PCL53

69.7 7.06 5.91 9.07 40.3

62.6

41.6

13.8 57.4

47.0

47.7

39.7

47.3 27.0 35.1

39.7

5.53 5.72 4.32 18.4

mobility. The polymer motions above are studied on the kilohertz frequency scale. Could polymer intercalation increase polymer mobility by disentangling polymer chains? Therefore, we measured the 13C relaxation times T1(C) in the laboratory frame, which are sensitive to fast molecular motions. The values of the extracted polymer discussed previously are in the 200-300 s range, in agreement with the literature.38 The values of the intercalated polymer of the nanocomposites vary between 30 and 90 s. A decrease in the PCL crystallinity within the nanocomposites compared to the bulk phase was reported previously.29 This accounts for the decrease of the T1(C) values upon polymer intercalation since the amorphous polymer is known to be characterized by smaller such values,38 but fast motion remains of rather low efficiency for polymer nuclei relaxation even in nanocomposites.

Conclusions The yields of PCL intercalation were weaker with surfactantexchanged laponites than with similarly modified synthetic saponites, as a result of unavoidable polymer-clay interactions that appear in the first system. The negative effect of such interactions for PCL intercalation was also indicated with saponite-based nanocomposites. The length of the surfactant hydrocarbon chain (C12-C16) and the saponite charge, lower or

equal to 0.6 per half unit cell, do not seem to affect significantly the intercalative polymerization process. Larger saponite platelets of the higher charge saponites increase the PCL content of the nanocomposite. The in situ polymerization process was also studied with alcoholic functions introduced in the headgroup of the intercalated surfactant. These hydroxyl groups reduce the polymerization efficiency. 13C CP MAS NMR was also used to estimate the surfactant/polymer ratio in the nanocomposites. In surfactant-exchanged saponites, 13C relaxation times less than 1 s, increasing from the polar headgroup to the terminal methyl of the long hydrocarbon chain, indicate fast molecular motion of the intercalated surfactants. In the highest charge saponite, the high all-trans conformation population of the hydrocarbon chain results in a slightly smaller mobility. However, slower motions in the kilohertz range, detected by relaxation measurements in the rotating frame, are also present, showing motional heterogeneity. Surfactant mobility is higher in the laponite-based nanocomposite (HDTP2.0LAP0.28) than in the original clay as observed with the saponite systems. On the other hand, the mobility of polymer chains intercalated between the clay layers occurs mainly on the kilohertz frequency scale. Nanocomposite formation leads to an increase of the polymer chain motions as a result of a decrease of the polymer crystallinity. Along the chain, mobility decreases as C3,4 > C2,5 > C6.

4824 Langmuir, Vol. 22, No. 10, 2006

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. J.H. thanks the FNRS for a postdoctoral fellowship (Grant 2.4503.02). J. L. Robert (Orle´ans) kindly supplied us with the synthetic saponite samples. We are very much indebted to the Belgian Science Policy for financial support in the frame of the Interuniversity Attraction Poles

Urbanczyk et al.

Programme (PAI V/03)sSupramolecular Chemistry and Supramolecular Catalysis. L.U. and C.C. are grateful to the Re´gion Wallonne for its financial support in the frame of the TECMAVER and PROCOMO projects. LA060041U