Influence of Miscibility on Viscoelasticity, Structure, and Intercalation of

The nature and extent of intercalation of the oligomer strongly depend on the nature of the interactions between the oligomer and the organically modi...
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Langmuir 2003, 19, 5502-5510

Influence of Miscibility on Viscoelasticity, Structure, and Intercalation of Oligo-poly(caprolactone)/Layered Silicate Nanocomposites Pralay Maiti* Department of Material Science and Engineering, Cornell University, Bard Hall, Ithaca, New York 14853-1501 Received January 24, 2003. In Final Form: April 16, 2003 Nanocomposites of oligo-poly(caprolactone) (o-PCL) with layered silicates having a variety of organic modifiers have been prepared by simple mechanical mixing. The effect of organic modifier type on the miscibility with o-PCL has been studied. The results indicate that miscibility increases with increasing alkyl chain length in the silicate modifier. All possible dispersion states (immiscible, intercalated, and exfoliated) were observed in the o-PCL nanocomposites produced in this study, based on different kinds of organic modifiers and various aspect ratios of the layered silicates. Viscoelastic properties strongly depend on the miscibility factor in the same way as the nanostructure of a nanocomposite determines the ultimate properties. Observed non-Newtonian behavior of a highly miscible nanocomposite was explained by the formation of network structure under a force field. The nature and extent of intercalation of the oligomer strongly depend on the nature of the interactions between the oligomer and the organically modified silicate. The intercalation mechanism has been given on the basis of the observed results, with the experimental results corresponding well with thermodynamic considerations.

Introduction Fillers have a significant influence on the viscoelastic, mechanical, gas barrier, crystallization, and thermal properties of polymer-based composites, which is made even more significant by their widespread industrial use.1 The use of oligomers during processing of high molecular weight polymers is also common, as a means to reduce the viscosity of the whole system or to decrease the processing temperature. In some cases, both filler and oligomer are used in order to achieve the desired properties under moderate conditions. The behavior of fillers, in particular, nanosized fillers, toward oligomers is different compared to their behavior toward high molecular weight species. In the past decade, the preparation and materials properties of polymer/layered silicate nanocomposites haven been a subject of much attention, because of properties enhancements that may be realized in these systems.2-6 There is still a lack of understanding of the molecular origin of the enhanced properties of these materials, as well as the thermodynamic issues involving intercalation and/or exfoliation. Mainly two types of polymer nanocomposite exist, namely intercalated and exfoliated, however. Nanocomposite structure can be varied depending on the nature of silicate and the organic modifier(s) used in nanocomposite preparation. The molecular weight of the pristine polymer also plays an important role, with respect to intercalation and/or exfoliation.7 Fornes et al.8

show the molecular weight dependence of the extent of intercalation and the rheological properties within the 16K to 29K range of molecular weights. They find that nanocomposites with higher molecular weights tend to exhibit better properties. Now, the obvious question is how the very low molecular weight, oligomeric species influence the nature of intercalation and thereby the structural, rheological and thermal properties of nanocomposites. In the previous publications, successful syntheses of high molecular weight, intercalated polylactide (PLA) nanocomposites9,10 were reported, along with a mechanism of intercalation. To focus on the mechanism of silicate dispersion in the system of interest, the analogous hydroxy terminated oligo-poly(caprolactone) (o-PCL) has been selected, due to its ability to strongly interact with silicates and/or different organic modifiers, and layered/silicate nanocomposites have been prepared, to gain insight into the nature of the intercalated species and the formation mechanism of the intercalated nanostructure. Aqueous clay suspensions have traditionally been described in accordance with the Bingham theory11 of plastic flow, but depending on the intriguing nature of clay/solvent interactions, they may exhibit either thixotropy12,13 or rheopexy14 behavior. All of the structures developed under an applied force field have been explained by face-to-face and edge-to-face interactions15-17 of the clay platelets in various solvents, with changes in rheological

* E-mail: [email protected]. Fax: 607-255-2365. (1) Enikolopyan, N. S.; Fridman, M. L.; Stalnova, O.; Popov, V. L. Adv. Polym. Sci. 1990, 96, 1. (2) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. Vaia, R. A.; Vasudevan, S.; Krawice, W.; Scanlon, L. G.; Giannelis, E. P. Adv. Mater. 1995, 7, 154. (3) Giannelis, E. P. Adv. Mater. 1996, 8, 29. (4) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaiti, O. J. Mater. Res. 1993, 8, 1179. Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Macromolecules 1997, 30, 6333. Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. J. Appl. Polym. Sci. 1998, 67, 87. (5) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 573. Shi, H.; Lan, T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 1584. (6) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1.

(7) Koo, C. M.; Kim, J.; Choi, M. H.; Kim, S. O.; Chung, I. J. Hwahak Konghak 2001, 39, 635. (8) Fornes, T. D.; Yoon, P. J.; Keskkula, H.; Paul, D. R. Polymer 2001, 42, 9929. (9) Maiti, P.; Yamada, K.; Okamoto, M.; Ueda, K.; Okamoto, K. Chem. Mater. 2002, 14, 4654. (10) SinhaRay, S.; Maiti, P.; Okamoto, M.; Yamada, K.; Ueda, K. Macromolecules 2002, 35, 3104. (11) Bingham, E. C. Fluidity and Plasticity; McGraw-Hill: New York, 1992. (12) Van Olphen, V. An Introduction to clay colloid chemistry; Interscience: New York, 1963. (13) Van Olphen, H. J. Colloid Interface Sci. 1964, 19, 313. (14) Okamoto, M.; Taguchi, H.; Sato, H.; Kotaka, T.; Tateyama, H. Langmuir 2000, 16, 4055.

10.1021/la0341308 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/23/2003

Oligo-poly(caprolactone)/Layered Silicate Nanocomposites Table 1. Name, Chemical Formula, and Designation of the Organic Modifiers Used name and formula [(C4H9)3P(C8H17)]+

Br-

n-octyltri-n-butylphosphonium bromide [(C4H9)3P(C12H25)]+ Brn-dodecyltri-n-butylphosphonium bromide [(C4H9)3P(C16H33)]+ Brn-hexadecyltri-n-butylphosphonium bromide [(C6H5)3 P (CH3)]+ Brmethyltriphenylphosphonium bromide

Langmuir, Vol. 19, No. 13, 2003 5503 Table 2. Characteristics of the Clays Used (Natural, Organically Modified, and in Nanocomposites)

designated as C8 C12 C16 CPh

and microstructural behavior in accordance with these ideas. The rheological properties and dynamics of nanoscopically confined high molecular weight polymer layered silicate have also been reported in the literature.18-25 Only a few reports,26-28 however, have involved poly(-caprolactone)/layered silicate nanocomposites, and all are based on high molecular weight materials and emphasize preparation and properties. So far, there have been no reports regarding oligomeric nanocomposites whose fine structure might change drastically the properties of the whole system when mixed with high molecular weight materials. The microstructural and rheological study of an oligomeric nanocomposite can shed light onto the complex behavior of high molecular weight polymer nanocomposite systems and how the presence of oligomeric species might alter their properties. In the present study, first, the influence of organic modifier on the miscibility with o-PCL and its ability to intercalate into different types of layered silicates with varying aspect ratios has been described. Second, attention has been paid to the effects of miscibility and nanocomposite structure on the rheological and thermal behavior. Finally, the mechanism of intercalation and the related thermodynamic issues, which control the structure of nanocomposites, have been elucidated. Experimental Section Materials and Preparation. Hydroxy terminated oligopolycaprolactone (o-PCL) (Mw ) 2000, KOH value ≈ 56 mg/g) used in this study was supplied by Unitika Co. Ltd., Japan. Four different types of alkylammonium modifiers, having different chemical structures and alkyl chain lengths, were used in this work. Their names, chemical formulas, and designations (as written in the text) are presented in Table 1. Henceforth, we will term the methyltriphenylphosphonium bromide, n-octyltri-nbutylphosphonium bromide, n-dodecyltri-n-butylphosphonium bromide, and n-hexadecyltri-n-butylphosphonium bromide as CPh, C8, C12, and C16, respectively. Three different layered silicates were used: synthetic fluoromicas (SOMASIF, UniCoOp Japan), (15) Bradenburg, U.; Lagaly, G. Appl. Clay. Sci. 1988, 3, 263. Lagaly, G. Appl. Clay. Sci. 1989, 4, 105. (16) Callaghan, I. C.; Ottewill, R. H. Faraday Discuss. Chem. Soc. 1974, 57, 110. (17) Duran, J. D. G.; Ramos-Tejada, M. M.; Arroyo, F. J.; GonzalezCaballero, F. J. Colloid Interface Sci. 2000, 229, 107. (18) Manias, E.; Bitsanis, G.; Hadziioannou, ten Brinke, G. Europhys. Lett. 1996, 33, 371. (19) Huh, J.; Balazs, A. C. J. Chem. Phys. 2000, 113, 2025. (20) Granick, S. MRS Bull. 1996, 21, 33. (21) Kotsilkova, R. Mech. Time-Depend. Mater. 2002, 6, 283. (22) Krishnamoorti, R.; Ren, J.; Silva, A. S. J. Chem. Phys. 2001, 114, 4968. (23) Ren, J.; Silva, A. S.; Krishnamoorti, R. Macromolecules 2000, 33, 3739. (24) Krishnamoorti, R.; Giannelis, E. P. Macromolecules 1997, 30, 4097. (25) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728. (26) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064. (27) Morin, A.; Dufresne, A. Macromolecules 2002, 35, 2190. (28) Lepoittevin, B.; Pantoustier, N.; Alexandre, M.; Calberg, C.; Jerome, R.; Dubois, P. Macromol. Symp. 2002, 183, 95.

sizea/nm CEC/mequiv per 100 g d001(clay with Na+)b/nm d001(organoclay with C16 salt)b/nm d001 in nanocomposites with C16 clay (15 wt %)b/nm crystallite size of organoclay with C16 saltc/nm crystallite size in nanocomposites with 15 wt % clayc/nm

hectorite

smectite

mica

50-60 113 1.2 2.15

100-120 87 1.2 1.87 1.72

400-500 120 1.25 2.44 3.24

3.8

7

20

7

12

a Data supplied by the manufacturer. b Calculated from wideangle X-ray diffraction data. c Calculated from the Scherrer equation Dhkl ) kλ/(β cos θ), where k ) constant, λ ) wavelength, β ) half width, and θ ) peak angle.

synthetic smectites (as called by the supplier, COOP chemicals, Japan), and hectorite (Hoechst Celanese). Organic modifications of the synthetic smectite with all of the aforementioned modifiers were prepared via aqueous ion exchange, while organically modified synthetic fluoromicas and hectorites were prepared only with the C16 modifier. Some characteristics of the unmodified and modified layered silicates are reported in Table 2. Unless otherwise mentioned, all nanocomposites were based on the synthetic smectite material. Nanocomposites of o-PCL were prepared via direct mixing of o-PCL with different organically modified layered silicates. The mixtures were sonicated for several hours while being cooled with ice in order to ensure mixing without thermal damage. The molecular weight, melting point, and crystallinity of o-PCL were the same before and after sonication. The amount of organically modified layered silicate used was varied in order to vary the silicate content in the nanocomposite. Nanocomposites were characterized by using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), polarized optical microscopy (POM) observation, and dynamic stress rheometery, to study structural, thermal, miscibility, dispersion, and rheological behavior. Blends of o-PCL and the organic modifiers alone (60: 40 by weight) were prepared via direct mixing, and their miscibility was checked via DSC. Optical Microscopy. The microscale dispersion of different layered silicates in o-PCL was determined via optical microscopy (Nikon OPTI-PHOTO2-POL). A thin, semisolid layer of ∼40 µm thickness was used for this study. WAXD. X-ray diffraction experiments were performed using an MXlabo diffractometer (MAC Science Co.) with Cu KR radiation and a graphite monochromator. The generator was operated at 40 kV and 20 mA. The semisolid samples were placed inside the aluminum sample holder at room temperature and were scanned from 2θ ) 1° to 10° at the rate 0.5 °C/min. DSC. The samples were characterized by using a temperaturemodulated differential scanning calorimeter, operated in the conventional DSC mode (TMDSC, TA2920, TA Instruments) at a heating rate of 5.0 °C/min, to determine the miscibility, heat of fusion, ∆H, and melting temperature, Tm, of pure o-PCL and nanocomposites. The differential scanning calorimeter was calibrated with indium before use. Dynamic Mechanical Characterization. A dynamic stress rheometer (Rheometrics DSR200) operating in the oscillatory shear mode was used to determine the rheological properties of o-PCL and o-PCL nanocomposites, using a parallel plate geometry with a 25 mm plate diameter over a frequency range of 0.01 to 100 rad/s. To determine the linear viscoelastic range, the storage modulus, G′, and loss modulus, G′′, were measured as functions of strain amplitude at a constant frequency of 0.628 rad/s. Time dependent viscosity, step stress (creep) default tests were carried out for dense samples using the same rheometer but with a coneplate geometry, using a cone angle of 0.04 rad and a plate diameter of 25 mm, at 25 °C with a constant stress of 1.0 Pa.

Results and Discussion Miscibility: A Function of Chain Length and Chemical Structure. Figure 1 shows the dispersion of

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Figure 2. DSC thermograms of pure o-PCL and its nanocomposite with synthetic smectites with different phosphonium modifiers, as indicated in the figure. The arrows indicate the melting temperatures of the corresponding organic modifiers.

Figure 1. Optical micrographs of organically modified layered silicates in o-PCL: (a) 15 wt % CPh clay; (b) 15 wt % C16 clay. Scale bar ) 100 µm.

different organoclays in o-PCL. CPh clay/o-PCL exhibits clusters of particles, indicating bad microscale dispersion probably arising from the immiscibility of the organoclay and o-PCL. The C16 modified silicate, however, shows good microscale dispersion in o-PCL, as do both the C8 and C12 modified silicates. Figure 2 shows the DSC thermograms of o-PCL and 15 wt % of the various modified silicates, revealing the chain length dependence of miscibility. OligoPCL and CPh show only melting peaks, at 3.5 °C, while C8 and C12 modified silicates show two melting peaks, corresponding to o-PCL and the organic modifier. The melting peak of the organic modifier in the CPh modified silicate is very high (233 °C) and, therefore, cannot be observed in the temperature range studied here. The arrows indicate the melting peaks corresponding to the pure organically modified silicates. The high temperature peak gradually shifts toward lower temperatures with increasing alkyl chain length in the alkyl phosphonium modifier and vanishes for the C16 modified silicate, indicating that the solubility of the silicate in the o-PCL increases with higher alkyl chain lengths. Hence, the C16 modified silicate is completely miscible with o-PCL, while the C8 modified silicate is slightly miscible and the C12 modified silicate is only partially miscible with o-PCL. This is also reflected in the respective optical images. The enthalpy of fusion, ∆H, gradually decreases with increasing chain length (∆HPCL ) 22, ∆H15%CPh ) 21.3, ∆H15%C8 ) 19.2, ∆H15%C12 ) 18, and ∆H15%C16 ) 17.0 J/g), also indicating that miscibility increases with chain length. The average size of clay particle aggregates gradually decreases from

the C8 modified silicate to the C16 modified silicate. OligoPCL is immiscible with the CPh modified silicate, as the mixture shows two distinct melting peaks corresponding to the pure o-PCL and CPh salt. From studies of microscale dispersion, shifts in melting temperature, and heat of fusion measurements, therefore, it is clear that o-PCL is immiscible with the CPh modified silicates and that miscibility increases with chain length from the C8 to the C16 organic modifiers. Miscibility Dependent Viscoelasticity. Viscoelastic properties of o-PCL nanocomposites with different types and concentrations of organically modified layered silicates are presented in Figure 3. It is apparent from Figure 3 that the nature of the organic modifier has a dramatic effect on the nanocomposite properties. CPh modified silicate, immiscible with o-PCL, shows a very slight increase in storage modulus, G′(ω), with increasing clay content, behaving like a conventional composite. With increasing silicate miscibility with o-PCL, however, G′(ω) increases significantly with organoclay content, as seen in the C8, C12, and C16 modified silicate/o-PCL systems. C16 modified silicate/o-PCL gives a value of G′(ω) 2 orders of magnitude higher than that of pure o-PCL. Pristine o-PCL exhibits a typical terminal regime G′(ω) ∼ ω showing semisolid like behavior, and likewise, there is no change of terminal slope value for CPh/o-PCL nanocomposites even for high clay content (15 wt %), indicating that no structural change has occurred in these immiscible nanocomposites. The terminal slope value decreases monotonically with organoclay loading according to G′(ω) ∼ ω1.3, G′(ω) ∼ ω0.2, and G′(ω) ∼ ω0.1, respectively, for pure o-PCL and 4 wt % and 15 wt % C8 modified silicate/o-PCL systems, however. For higher clay content C16 modified silicate/o-PCL nanocomposites (Figure 3c), the terminal behavior almost completely disappears, especially in the lower frequency range, which suggests that strain relaxation in the nanocomposites is somehow restricted in the presence of the silicate layers. Here, it must be remembered that the effects of entanglement completely disappear for such low molecular weight oligomers and hence the formation of some kind of structure is only responsible for this type of behavior at long time periods (equivalent to low frequencies). Interestingly, at high oscillation frequencies, the effect of clay loading is relatively strong for the miscible system only. These results suggest that

Oligo-poly(caprolactone)/Layered Silicate Nanocomposites

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Figure 4. (a) Plots of storage modulus, G′(ω), and loss modulus, G′′(ω), of pure o-PCL and a nanocomposite containing 15 wt % of the indicated silicate vs frequency. (b) Complex viscosity, η*, vs frequency of pristine o-PCL and 15 wt % indicated organoclay.

Figure 3. Frequency dependence of the storage modulus, G′, of pure o-PCL and nanocomposites with varying silicate content in the (A) CPh, (B) C8, and (C) C16 systems.

the influence of organoclay on strain relaxation dynamics is comparatively weak in miscible systems but that, as the extent of miscibility increases, the influence becomes stronger. For the immiscible system, CPh modified silicate/ o-PCL, we observe the same value of G′(ω) at higher oscillatory frequencies, indicating faster relaxation dynamics, similar to the case of pristine o-PCL. Slower relaxation dynamics of the miscible system might be related to the highly interacting nature of the layered silicates and the o-PCL and the restricted motion of the polymer molecules in the proximity of the highly anisotropic silicate layers, resulting in the formation of a solidlike phase. Such a phase is categorically absent in immiscible systems, giving rise to faster dynamics similar to what is observed in pristine o-PCL. Figure 4a shows the liquidlike behavior of o-PCL, with G′′(ω) being larger than G′(ω), but with the increase in miscibility from CPh to C16 clay, the difference between G′′(ω) and G′(ω) gradually decreases, indicating more solidlike behavior in the C16 modified silicate system. The complex viscosity (η*) shows a perfect plateau for o-PCL

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and the immiscible CPh modified silicate system (Figure 4b), while, with increasing miscibility, η* starts to increase at long times (or equivalently at low frequencies). This effect is most pronounced in the C16 modified silicate, suggesting mesostructure development even under oscillatory shear. Previously, Ren et al.23 demonstrated the pseudosolidlike behavior of diblock copolymer nanocomposites, and posited that oriented anisotropic silicate stacks form a percolated network structure incapable of complete relaxation. Here, it is observed that the formation of a mesoscopic network structure is a function of the miscibility between the organoclay and the o-PCL. As a result, a perfect plateau is observed for pristine polymer and the immiscible system, while with increasing miscibility the viscosity gradually increases at long times (low frequencies). The subsequent results and discussion emphasize the orientation of silicate layers more clearly in steady flow. Time Dependent Viscosity. Figure 5 shows the time dependent shear viscosity of o-PCL and different types of nanocomposites. CPh modified silicate/o-PCL nanocomposites, being immiscible, behave like conventional composites, exhibiting almost the same viscosity as o-PCL and with no time dependent phenomena even at high shear. For a partially miscible, C8 modified silicate system (Figure 5b), the viscosity increases around 1 order of magnitude upon addition of 15 wt % modified silicate, and very slight rheopexy is observed at high shear, while, for the C16 modified silicate system, the viscosity increases more than 1 order of magnitude and stronger rheopexy is simultaneously observed. In any case, there is a percolation threshold of increasing viscosity of the miscible system in the range 4-15 wt % modified silicate. Okamoto et al.14 showed up to 3 orders of magnitude increment in the steady shear viscosity in lipophilized-smectite/toluene suspensions as a result of gel-like structure formation. In the o-PCL/silicate system, however, gelation is not occurring; rather, the orientation of the tactoids normal to the direction of flow is perhaps the primary explanation for the observed rheopexy. This perpendicular orientation comes from the strong interactions between the matrix and modified silicate, which we previously reported in polypropylene nanocomposites as a house-of-cards structure.29 Another possibility for network formation is through the face-to-edge electrostatic interaction of silicates in slightly acidic media such as hydroxy terminated o-PCL, as described by van Olphen some 25 years ago,30 which enhances the viscosity of the system. Parallel orientation of clay tactoids is also reported in the literature22-25 and is indicated to cause shear thinning, perhaps due to the weaker interaction of organoclay and matrix polymer. In the present study, being immiscible, CPh modified silicate nanocomposites also exhibit slight shear thinning behavior at high shear. The stronger interaction of the C16 modified silicate and the o-PCL may restrict alignment in the flow direction and instead result in alignment perpendicular to the applied force, to keep the geometry of interaction/contact more favorable via house-of-cards structure formation through the face-toedge interaction in a slightly acidic medium. In any case, the higher viscosity of the o-PCL arises from the mesoscopic networking, and the rheopexy, though small, appears due to the perpendicular orientation of silicate layers with respect to the applied force. (29) Okamoto, M.; Nam, P. H.; Maiti, P.; Kotaka, T.; Hasegawa, N.; Usuki, A. Nano Lett. 2001, 1, 295. (30) Van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1977.

Figure 5. Time dependent viscosity under constant shear for pure o-PCL and nanocomposites with varying silicate content in the (a) CPh, (b) C8, and (c) C16 systems.

Oligo-poly(caprolactone)/Layered Silicate Nanocomposites

Silicate and Nanocomposite Nanostructure. From the WAXD analysis, the nanostructures of organically modified silicates were determined for different types of silicates (aspect ratio and cation exchange capacity) having the same C16 modifier. The gallery spacings, d001, of the organically modified silicates with the C16 modifier are in the order smectite (1.87 nm) < hectorite (2.15 nm) < mica (2.44 nm). For a single modifier, the interlayer spacing increases with increasing cation exchange capacity (CEC) of the unmodified clay (Table 2). From the supplier’s data, it is believed that Na+ ion present in the clay is fully replaced by phosphonium (P+) ion during the ion exchange process. So, from the CEC data, the amount of P+ (modifier) present in the silicate galleries should be in the order smectite < hectorite < mica. In mica, having larger lateral dimensions and more organic modifiers due to a higher CEC, organic modifiers present at the core have restricted conformations due to physical hindrance.9 This physical hindrance is less of a problem in the case of the synthetic smectite, due to lower CEC and lateral dimensions, but it is very pronounced in the case of hectorite, as the CEC is high but the lateral dimensions are the smallest of all of the silicates. Another important factor is that the coherency of the organoclay increases with increasing lateral dimension of clay. This can be explained by the mechanical healing effect, the disordered nature of silicates with smaller lateral dimensions, or the physical hindrance of the organic modifiers due to a high CEC. The crystallite size, calculated from the Scherrer equation, Dhkl ) kλ/(β cos θ), of different types of organoclay is presented in Table 2. Again, the gallery spacing, d001, is 1.69, 1.78, and 1.87 nm for C8, C12, and C16 modified synthetic smectites, respectively. For any kind of organic modifier, the gallery spacing of modified silicate should increase in the order smectite < hectorite < mica. It is obvious from these results that gallery spacing increases with chain length as well as with the CEC of the clay. It is believed that, out of these two factors, CEC and lateral dimensions, the density of organomodification (CEC) plays the more important role determining the d-spacing of silicate layers while the coherency is governed by the dimension of silicate layers. Figure 6 shows the WAXD patterns of different types of nanocomposite made from different organoclays. The CPh modified silicate/o-PCL system exhibits the same peak positions as are observed in the silicate alone; that is, the o-PCL does not intercalate into the CPh modified silicate, due to the immiscibility between the organic modifier and the o-PCL. The C8 modified silicate nanocomposite shows intercalation, and surprisingly, with increasing organoclay content, the (001) peak gradually shifts toward lower angles, meaning that the interlayer spacing is increasing with increasing clay content. In our previous report, we found that, by decreasing the clay content in polypropylene nanocomposites, an increased degree of intercalation was observed.31 For the oligomeric systems, however, the reverse appears to be true, probably due to the stronger interactions between the hydroxy terminated o-PCL and the silicates, coupled with the absence of entanglements. The d001 spacing of every clay system used has been shown in Figure 7. Interestingly, the (001) peak for the C16 modified silicate nanocomposite gradually shifts toward higher angle with increasing silicate content (Figure 6c), indicating the collapse of the galleries following nanocomposite formation. The C12 system is the intermediate case; initially the gallery spacing increases, but beyond 4 wt % (31) Maiti, P.; Nam, P. H.; Okamoto, M.; Hasegawa, N.; Usuki, A. Macromolecules 2002, 35, 2042.

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Figure 6. WAXD patterns of synthetic smectite nanocomposites with organic modifiers (showing chemical structure and chain length dependency of intercalation): (a) CPh; (b) C8; (c) C16. The broken lines represent the peak positions of the corresponding organically modified smectites alone.

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Figure 7. Dependence of the interlayer spacing of CPh, C8, C12, and C16 modified silicate nanocomposites on silicate concentration. The horizontal lines indicate the gallery spacing of the organically modified layered silicate in the absence of o-PCL. The best-fit lines are also shown in the figure.

silicate, it then decreases with silicate content. Somehow, with the increase of organic modifier chain length (and therefore miscibility), the gallery spacing begins to follow a decreasing trend with increased silicate content following nanocomposite formation. Both the “diffuse-in” mechanism, involving polymer molecules inserting themselves into the silicate galleries, and the “diffuse-out” mechanism, involving organic modifiers aligned toward the exterior of the silicate galleries, are operating in o-PCL nanocomposites, and they will be discussed further in the next section. Miscibility Considerations. From the rheological, thermal, and microstructural studies, it is clear that the miscibility of the modified silicate and the oligomer has a significant impact on the formation and properties of the relevant nanocomposites. It should be noted, however, that the miscibility might be either between the silicate and the polymer or between the modifier and the polymer. To find the origins of this miscibility, then, DSC analyses have been carried out for 60:40 (weight %) blends of o-PCL and the various bromide salts of the CPh, C8, C12, and C16 organomodifiers. Oligo-PCL/CPh shows a melting peak at the same position as that of pure o-PCL, shown by the dotted line in Figure 8, clearly indicating an immiscible system. The melting peak of the CPh salt (Tm ≈ 233 °C) was not observed in the temperature range studied here. Both the C8/o-PCL and C12/o-PCL systems show a considerable change in melting peak position, leading to the conclusion that they are both partially miscible with o-PCL, while for C16 there is only one melting peak observed, suggesting good miscibility of the C16 salt with o-PCL. The arrows indicate the melting peaks for the corresponding pure bromide salts. These results clarify that the origin of the miscibility is through interactions of the organic modifier with the o-PCL and that miscibility increases with modifier chain length. Otherwise, if miscibility were to occur through interactions between the silicate layers and o-PCL, an intercalated structure

Maiti

Figure 8. DSC thermograms of blends (60:40 wt %) of o-PCL with the different phosphonium salts indicated in the figure. The arrows indicate the melting temperature of the corresponding salt. The vertical dotted line represents the melting temperature of pristine o-PCL. Scheme 1. Schematic Representation of Organoclay and Nanocomposite Formation with Different Modifiers

should be observed in the CPh modified silicate nanocomposite as well. Here, it should be mentioned that there is no significant difference in the interlayer spacing of the CPh (d001 ≈ 1.60 nm) and C8 (d001 ≈ 1.68 nm) modified silicate. Miscibility, not insufficient space in the interlayer, is therefore mainly responsible for intercalation. Mechanism of Intercalation. Oligo-PCL is a very interesting system in that, depending on the organic modifier, it may be immiscible, it may intercalate into silicate galleries as is usual in polymer intercalation, or the organic modifier may diffuse out and be solubilized in the o-PCL. When o-PCL is immiscible with a certain organic modifier, it cannot intercalate into the silicate gallery, while, for a short chain miscible modifier, o-PCL intercalates and, in the case of a long chain modifier, the modifier orients itself away from the silicate surface and is solubilized into the o-PCL phase, resulting in the collapse of the silicate gallery. The situation of the probable orientation of organic modifier, smectite silicate layers, and oligomers is shown in Scheme 1. Considering the thermodynamics of intercalation, it has been posited that, when polymer molecules intercalate into the silicate gallery, there is a decrease in entropy, which is compensated either by the increase in entropy in the organic modifiers due to the higher gallery spacing or the enthalpic term arising from the interaction of polymer and organic

Oligo-poly(caprolactone)/Layered Silicate Nanocomposites

Langmuir, Vol. 19, No. 13, 2003 5509 Scheme 2. Schematic Representation of PCL Nanocomposite in Different Clays

Figure 9. WAXD patterns of various C16 modified silicates and their nanocomposites at the indicated clay concentrations for (a) synthetic mica and (b) hectorite.

modifier/silicate layers.32-34 When the modifier is sufficiently long, as in the C16 modified silicate case, interactions may occur outside of the silicate gallery, with the modifiers orienting accordingly (Scheme 1) in order to increase their entropy in the liquid o-PCL. Short miscible modifiers, such as in the C8 modified silicate system, cannot do this, and therefore, the o-PCL intercalates by a diffuse-in mechanism, due to favorable interactions, and increases the silicate gallery spacing. The C12 modified silicate system is of an intermediate type, as low silicate content nanocomposites are intercalated, but with increasing amount of silicate, the extent of interaction increases and the organic modifier diffuses out. The difference between polymer and oligomer intercalation is that there is a huge loss of entropy in oligomers when (32) Vaia, R. A.; Jandt, K. D.; Kramer, E. J.; Giannelis, E. P. Macromolecules 1995, 28, 8080. (33) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 7990. (34) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000.

they intercalate into the silicate galleries, due to their high configurational entropy in a liquid state. As a result, when the chain length of the organic modifier is sufficient, it can align toward the outside of the silicate galleries and interact with material there, ultimately making a thermodynamically more stable system. The mechanism of the high molecular weight polymer is a bit different, in that the entropy of the polymer is not so high before intercalation, as opposed to the liquid o-PCL case, and the ultimate change in entropy is almost zero after intercalation, as explained and calculated by Vaia et al..33,34 In the C16 modified silicate nanocomposite system, the total entropy change is slightly negative, as the gallery begins to collapse at high silicate loadings, but the stronger interaction with the organic modifier, as shown in Figure 8, and hence the enthalpic term is the main driving force for diffuse-out type of mechanism. For shorter chain length organic modifiers, which have moderate interactions with o-PCL and display small changes in total entropy as the silicate galleries expand, oligomers must intercalate inside, as the short chains cannot access material outside the silicate galleries; this behavior is ultimately manifested in a diffuse-in mechanism. Previously, we reported the diffuse-in type of intercalation in the case of a molten polypropylene nanocomposite, where it was found that the amount of time spent in a molten state dictated the gallery spacing.31 Now, the question arises as to whether the chain length factor is associated with this particular intercalation mechanism. For that purpose, we have changed the lateral dimensions of the silicate layers in order to observe changes in intercalation behavior. Up to now, the effects of different organic modifiers on intercalation behavior have been studied using a single synthetic smectite material with an aspect ratio of 100150, and it has been shown that, depending on the chain length of organic modifier, either the oligomer will intercalate or the organic modifier will align toward the outside of the gallery. To give further evidence of these diffuse-in and diffuse-out mechanisms, the aspect ratio of clay has been changed from 50 to 500. C16 modified synthetic fluoromica, aspect ratio ≈ 500, shows strong intercalation, like a polymeric system, even at higher clay loadings (Figure 9a), while C16 modified hectorite, aspect ratio ≈ 50, exhibits an almost exfoliated structure (Figure 9b). The nature of intercalation involving the C16 modifier and different aspect ratios of layered silicates has been shown in Scheme 2. For both the healing effect and the disordered nature of the layers due to mechanical distortion and the greater ability of the organic modifiers to orient toward the outside of the silicate galleries for the purposes of interacting with o-PCL, the exfoliated struc-

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ture is explained well by the lower aspect ratio of the hectorite used here, combined with its higher CEC and greater propensity for hindrance of the organic modifiers, which in turn encourages the diffuse-out mechanism. The WAXD peak of C16 hectorite organoclay is very broad, and its weak coherency and lower crystallite size (Table 2) also suggest its disordered nature after organic modification. On the contrary, for higher aspect ratio silicates, even though the extent of interaction is the same, the larger lateral dimensions of the silicate layers ensure that much less of the organic modifier is in a position to access areas outside of the silicate gallery, and the o-PCL must intercalate instead. From thermodynamic considerations, then, both the lateral dimensions and the CEC determine the nanocomposite structure of o-PCL. The nature and mechanism of intercalation in oligomeric systems are somehow different from those of polymer intercalation, and depending on the nature of organic modifier, all possible combinations of structures of immiscibility, intercalation, and exfoliation can be formed with oligomers. This, in turn, sheds light on the expected structure and properties of polymer/clay nanocomposites prepared in the presence of oligomers.

Maiti

Conclusion The miscibility of organic modifiers with oligomers and polymers plays an important role in the intercalation/ exfoliation of silicate layers. Immiscible, intercalated, and exfoliated nanostructures have all been observed in o-PCL nanocomposites, dependent on the nature of the organic modifier and the aspect ratio of the silicate layers. Viscoelastic properties change depending on the structure of nanocomposites. The higher viscosity, non-Newtonian behavior, and rheopexy of miscible nanocomposites indicate the formation of a network structure under a force field. The extent of miscibility between the silicate and o-PCL strongly influences the intercalation mechanism. Thermodynamic explanations have been given for the different kinds of structures formed in o-PCL/layered silicate nanocomposites. Acknowledgment. The author thanks Prof. T. Kotaka and Prof. M. Okamoto of Toyota Technological Institute, Japan, for their kind help and providing instrumental support. LA0341308