Introduction. Seung-Yeop Kwak,* 1 Kwang Sei Oh 2

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1 Macromol. Mater. Eng. 2003, 288, Full Paper: Poly(e-caprolactone) (PCL) nanocomposites were prepared using two different types of organically modified nanosilicates by melt intercalation with an internal mixer. Dynamic mechanical analysis revealed possible structural changes in the nanocomposites even during the small deformation occurring during shear oscillatory measurements, as evidenced by a V-shaped modulus change in the plot of the dynamic storage modulus as a function of stepwise increased temperature. X-ray diffraction patterns were recorded at different simulated temperatures during the various stages of dynamic measurements. The X-ray data indicate that the structural changes can be ascribed to a further intercalation of the PCL matrix chains into the silicate layers. This further intercalation is a consequence of the heat treatment during the dynamic mechanical measurements. Furthermore, there is a considerable vertical shift in addition to the horizontal shift in the higher temperature regime, which allows the mapping of a master curve through the application of the time-temperature superposition principle to the dynamic storage and the loss modulus data obtained at various isothermal temperatures. The present study is also concerned with the relative molecular mobility of both PCL nanocomposites in the given experimental conditions considering the Williams-Landel- Ferry (WLF) equation and the Arrhenius relationship between the horizontal shift factor and the activation energy of flow. Moreover, the extent of the vertical shift as a function of temperature made it possible to determine the apparent activation energy of the further intercalation of PCL into the silicate layers. This intercalation is caused by the additional exposure to heat during the dynamic mechanical measurements after mixing, which led to a comparison of the relative diffusivity of the PCL matrix in the two nanocomposites. Dynamic shear storage moduli G 0 of PCLOC25A and PCL- OC30B as a function of temperature with increase increments of 20 8C from 60 to 260 8C. The G 0 data were obtained from isothermal frequency sweep G 0 (o) data at o ¼ 1 rad s 1 at the corresponding temperatures. Effect of Thermal History on Structural Changes in Melt-Intercalated Poly(e-caprolactone)/Organoclay Nanocomposites Investigated by Dynamic Viscoelastic Relaxation Measurements Seung-Yeop Kwak,* 1 Kwang Sei Oh 2 1 Hyperstructured Organic Materials Research Center (HOMRC), and School of Materials Science and Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul , Korea sykwak@snu.ac.kr 2 Research Institute of Advanced Materials (RIAM) and School of Materials Science and Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul , Korea Keywords: activation energy; clay; modulus; nanocomposites; poly(e-caprolactone) Introduction Layered silicate-based polymer nanocomposite spheres have attracted considerable attention because of their possible technological application in addition to their use as an alternative way to overcome the drawbacks of conventional micro and macrocomposites. [1 8] Technological applications of these nanocomposites are due to the drastic improvement of the physical, thermal, and mechanical properties of polymer-based materials with a minimal increase in Macromol. Mater. Eng. 2003, 288, No. 6 ß WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, /2003/ $17.50þ.50/0

2 504 S.-Y. Kwak, K. S. Oh the density, as the degree of loading with the inexpensive silicates is low. [6 8] Therefore, preparation of these nanocomposites involving organically modified layered silicates (organoclays) and their characterization have been studied thoroughly over the last decade. [9] From a practical point of view, a better understanding of the interaction between the matrix polymer in addition to the structural changes above the softening or melting temperature of the polymer as melt intercalation proceeds is required. Among the methods used to analyze nanocomposites, dynamic mechanical analysis is a suitable means of examining the overall three-dimensional phase structure in conjunction with the intercalated and/or exfoliated silicate layers. [10] Recently, Giannelis et al. [3] and Krishnamoorti et al. [10,11] investigated the dynamic viscoelastic properties of a series of polymer nanocomposites based on layered silicates. However, these studies employed dynamic mechanical analysis and did not show if the phase structure obtained after melt intercalation had changed during the additional oscillatory experiments performed at a range of higher temperatures. Therefore, if no structural changes occur during the experiments, the time-temperature superposition principle, which is applied to viscoelastic functions such as the dynamic storage modulus G 0 (o) and loss modulus G 00 (o), holds. It should then be possible to construct a master curve by considering the horizontal shift a T only. [12] This suggests some structural stability in the temperature range given. In contrast, if the time-temperature superposition fails and an additional correction along the modulus axis, that is the vertical shift b T, is needed to obtain a master curve, the nanocomposite undergoes some structural change and is therefore structurally unstable at the given experimental temperatures. [12] The aim of this study was to determine the effect of the additional heat imposition, which accompanies post-dynamic mechanical measurements, on the structural development and stability of melt-intercalated poly(e-caprolactone)/organoclay nanocomposites by their linear viscoelastic behavior. In addition, this study elucidates the role of the organoclay s polarity on the melt intercalation process, and hence the structural stability after compounding. Experimental Part Materials Figure 1. Chemical structures of the intercalants of the organoclays: a) dimethyl hydrogenated tallow-2-ethylhexyl (2MHTL8), OC25A and b) methyl tallow-bis(2-hydroxyethyl) (MT2EtOH), OC30B. Poly(e-caprolactone) (PCL, melting temperature of ca. 61 8C and glass-transition temperature of ca. 608) with a numberaverage molecular weight, M n of g mol 1, was purchased from Aldrich. Organically modified sodium montmorillonites (organoclays, MMT), which originally possessed a cation exchange capacity (CEC) of 90 mequiv./100 g were supplied from Southern Clay Products, Inc., USA, under the trade name of Cloisite 1 25A (OC25A) and Cloisite 1 30B (OC30B). According to the supplier, the organophilic intercalants (Figure 1) were (dimethyl hydrogenated-tallow-2-ethylhexyl (2MHTL8) and methyl-tallow-bis(2-hydroxyethyl) (MT2EtOH) quaternary ammonium, respectively. The tallow was composed of 65 wt.-% C 18, 30 wt.-% C 16, and 5 wt.-% C 14. The CEC values of the two organoclays were 95 mequiv./100 g for OC25A and 90 mequiv./100 g for OC30B. The interlayer gallery heights were 1.14 and 0.94 nm, respectively. Compounding and Preparation of Nanocomposites The PCL and organoclay were mixed mechanically with an internal mixer (Haake Rheocord 300P) with roller blades at a rate of 100 rpm at 230 8C for 450 s. The compositions used to prepare the PCL/organoclay nanocomposites are given in Table 1. In the following, the PCL nanocomposite with organoclay OC25A is named PCLOC25A, whereas that with organoclay OC30B is denoted PCLOC30B. After mechanical mixing, the nanocomposites were cooled to room temperature and then broken into small pieces to facilitate press-molding into samples for the subsequent dynamic mechanical measurements. Dynamic Mechanical Measurements Dynamic oscillatory shear measurements were performed on a Rheometrics Mechanical Spectrometer Model 800 (RMS 800) with a torque transducer working over the range between 2 Table 1. Composition of the PCL/organoclay nanocomposites. Sample Composition wt.-% Poly(e-caprolactone) Organoclay Intercalant Montmorillonite PCLOC25A PCLOC30B

3 Effect of Thermal History on Structural Changes in Melt-Intercalated and g cm. Dynamic isothermal frequency sweeps were performed using a parallel geometry of plates 25 mm in diameter at different temperatures ranging between 60 and 260 8C. The angular frequency was rad s 1. For each sample, the strain regime, which was regarded as the linear region of the viscoelastic data, was determined from strain sweep experiments at selected temperatures. X-Ray Diffraction The gallery height and the tactoid size of the organoclays in the nanocomposites were measured by X-ray diffraction (XRD) using a MAC Science MXP 18A-HF diffractometer. CuK a radiation (l ¼ nm) was generated at 40 kv and 100 ma. The XRD patterns were obtained over the 2y range between 1.5 and 128, and the diffraction angle was scanned starting from 1.58 at a rate of 38 min 1. The basal lattice spacing of the layered silicates was estimated from the position of the (001) peak. Results and Discussion Figure 2 shows the dynamic shear storage moduli G 0 of PCLOC25A and PCLOC30B obtained from the isothermal frequency swept G 0 (o) data at a rate of o ¼ 1 rad s 1, and plotted as a function of temperature. As the temperature is increased, the G 0 values of PCLOC25A and PCLOC30B reach a minimum at 180 and 160 8C, respectively, and then increase upon continuous heating to 260 8C. The former decrease in G 0 for both hybrids results from the timetemperature correspondence due to the dominant PCL matrix responses, whereas the latter must be a consequence of structural changes in the respective temperature regimes. Recognizing that the thermal degradation temperature of PCL is higher than 300 8C, and considering other known properties of neat PCL, the only possible structural change in the system is further penetration of the PCL chains into the silicate interlayers. Giannelis et al. [13] determined thermodynamically stable equilibrium states of the nanocomposites, such as immiscible, intercalated and exfoliated systems, which are a consequence of an interplay of various entropic and enthalpic factors. However, if the total entropy change in the system is small, the change of internal enthalpic factors, such as intermolecular interaction, will determine if intercalation or exfoliation are thermodynamically possible. For the enthalpic factor, favorable polar interactions have to be maximized, whereas unfavorable apolar interactions have to be minimized in order to facilitate further penetration of the silicate interlayer. [13] OC30B shows not only a polymer-surface polar interaction but also favorable polymer-intercalant polar interaction. On the other hand, OC25A shows unfavorable polymer-intercalant apolar interaction in addition to favorable polymer-silicate polar interaction. These considerations imply that the total change in the Helmholtz free energy of PCLOC30B with polar intercalants is smaller than that of PCLOC25A with apolar intercalants. Therefore, penetration of PCL into OC30B is more favorable than that into OC25A. Furthermore, additional heat imposed on both nanocomposites, PCLOC25A and PCLOC30B, made the low Helmholtz free energy, and PCLOC30B with lower Helmholtz free energy than PCLOC25A showed increase of G 0 from lower temperature than PCLOC25A. Evidence of the structural changes is provided by XRD analysis, as shown in Figure 3 and 4. The XRD peak in both PCLOC25A and PCLOC30B is still Figure 2. Dynamic shear storage moduli G 0 of PCLOC25A and PCLOC30B as a function of temperature with increase increments of 20 8C from 60 to 260 8C. The G 0 data were obtained from isothermal frequency sweep G 0 (o) data at o ¼ 1 rad s 1 at the corresponding temperatures. Figure 3. XRD traces of a) the organoclay OC25A, b) the PCLOC25A nanocomposite after mixing, and of the nanocomposite after mixing and a further dynamic frequency sweep up to c) 160, d) 200, and e) 240 8C.

4 506 S.-Y. Kwak, K. S. Oh Figure 4. XRD traces of a) organoclay OC30B, b) the PCLOC30B nanocomposite after mixing, and of the nanocomposite after mixing and a further dynamic frequency sweep up to c) 160, d) 200, and e) 240 8C. detectable after mixing, which suggests that these systems remain intercalated but are not yet exfoliated. However, the XRD curves of both nanocomposites obtained at the corresponding temperatures after further dynamic isothermal frequency sweeps show a gradual decrease in intensity of the d 001 silicate peak, although a slight extent of inverse exfoliation (reorientation of the silicate platelets back into stacks) is detected for PCLOC25A. Finally, neither nanocomposite gives rise to a d 001 silicate peak after the dynamic frequency sweep at 260 8C. This indicates that the PCL chains have penetrated further into the silicate interlayers. Therefore, the individual systems become more intercalated, that is tend more towards complete exfoliation, due to the additional heat treatment during the dynamic mechanical measurements. If the above structural changes in the nanocomposites are a consequence of the dynamic mechanical measurements, the resulting linear viscoelastic data of storage and loss modulus at different temperatures cannot be superimposed along the frequency axis, and a master curve cannot be found. This means that the time-temperature superposition is no longer applicable by considering the horizontal shift factor a T only, but becomes valid if the additional vertical (modulus) shift b T is taken into account, as shown by Equation (1): [12] G 0 ðt 1 ; a T1 oþ b T1 ¼ G0 ðt 2 ; a T2 oþ b T2 ð1þ The master curves of G 0 (o) as a function of the reduced frequency a T oare shown in Figure 5 for pure PCL, Figure 6 for PCLOC25A (Figure 6a), and PCLOC30B (Figure 6b). The curves were generated by applying the time-temperature Figure 5. Dynamic shear moduli G 0 (o) and G 00 (o) of PCL as a function of the extended frequency, reduced to 120 8C, by the horizontal shift factor a T. superposition on G 0 (o) data over an extended frequency scale at a given reference temperature by either neglecting b T for PCL or by considering a T and b T for both nanocomposites. The a T values for PCL and the nanocomposites at the experimental temperature range are shown in Figure 7 as a function of temperature. The solid lines show the results of the nonlinear curve fitting by the Williams-Landel-Ferry (WLF) equation (Equation (2)) as follows: [12] log a T ¼ C 1ðT T 0 Þ ð2þ C 2 þðt T 0 Þ Both C 1 and C 2 are constants, and T 0 is the reference temperature, in this case 120 8C. The resulting C 1 and C 2 values are summarized in Table 2. Subsequently, the apparent activation energy of flow E a,h can be determined using Equation (3), where the WLF equation is substituted into the horizontal shift factor in the Arrhenius equation, which links the activation energy with the horizontal shift factor: E a;h ¼ R d ln a " # T C 1 C 2 T 2 ¼ 2:303R dð1=tþ ðc 2 þ T T 0 Þ 2 ð3þ The activation energies become thus dependent on the temperature. Their values are shown in Table 2. The apparent activation energy of flow is a measure of molecular mobility and the energy barrier that must be overcome. Therefore, judging from the lower E a,h values as a function of temperature, the molecular motion of PCLOC30B appears to be more favored than that in PCLOC25A. The introduction of b T required for temperatures above 160 8C for the nanocomposites must be associated with a decrease of structural stability, which originates from further exfoliation of the silicate layers within the PCL matrix as shown by XRD. Further detailed information on

5 Effect of Thermal History on Structural Changes in Melt-Intercalated Table 2. WLF constants and apparent activation energies for pure PCL and PCL/organoclay nanocomposites. Sample C 1 C 2 T E a,h 8C kj mol 1 PCL PCLOC25A PCLOC30B Figure 6. Dynamic shear moduli G 0 (o) and G 00 (o) of a) PCLO- C25A and b) PCLOC30B as a function of the extended frequency, reduced to 160 8C by both the horizontal shift factor a T and the vertical shift factor b T. the structural changes from the dynamic viscoelastic relaxation measurements was obtained through analysis of the vertical shift factors b T as a function of temperature. The variation of b T and hence the shift in the G 0 (o) data along the modulus axis become predominant at the temperature ranges between 160 and 260 8C for both nanocomposites (see Figure 8), which is in good agreement with the definite modulus increase shown in Figure 2. According to the method of reduced variables and the theory of rubber elasticity, b T is equal to rt/r 0 T 0, where r 0 and r are the densities at T 0 and T, respectively. [12] The b T values of both nanocomposites, particularly in the temperature range above 160 8C change dramatically with increasing temperature. This suggests that b T cannot be simply explained in terms of inherent entropic nature of elasticity (density effect). However, this might be largely related to a diffusion phenomenon due to the additional intercalation of the polymer chains into the silicate interlayers. Therefore, the modulus change correction may be governed by the diffusion rate constants, which generally results in the form of an Arrhenius relation as associated with the vertical shift. This can be expressed in Figure 7. Temperature dependence of the horizontal shift factors a T, chosen empirically and used in the time-temperature superposition of PCL, PCLOC25A, and PCLOC30B. The solid lines correspond to the calculation by Equation (2). Figure 8. Temperature dependence of the vertical shift factors b T, chosen empirically and used in the time-temperature superposition of PCLOC25A and PCLOC30B.

6 508 S.-Y. Kwak, K. S. Oh Equation (4): [14,15] b T ¼ exp E a;v 1 R T 1 ð4þ T 0 where E a,v is the apparent activation energy for diffusion and R is the gas constant. Therefore, the activation energies E a,v for PCLOC25A and PCLOC30B are estimated to be 10.6 and 10.9 kj mol 1, respectively. Recognizing that the activation energy in this case is the energy barrier to diffusion, the diffusivity of both nanocomposites is similar irrespective of the nature, that is polarity, of the organoclay. Despite the similar E a,v values, the final phase structure of PCLOC30B, as compared with that of PCLOC25A, might become more effectively intercalated and hence change towards exfoliation due to its lower E a,h value. This aspect is subject of further research. Finally, the structural changes and further development of the morphology towards exfoliation in the melt-intercalted nanocomposites were reported as a consequence of different thermal history after static heat treatment. This result may be of practical significance in fabrication and application of these nanocomposites to achieve optimized processing conditions and a controlled morphology. Conclusions Poly(e-caprolactone) (PCL) compounds containing two different types of organically modified nanosilicates (OC25A and OC30B), namely PCLOC25A and PCLOC30B, were prepared by melt intercalation using an internal mixer at 230 8C for 450 seconds. A definite increase in the dynamic storage modulus of both nanocomposites as a function of temperature was detected. The smallest modulus value was measured in the higher temperature region. This suggests structural changes occur due to additionally subjecting the samples to heat during the dynamic mechanical measurements after mixing. The structural changes were verified by in-situ XRD studies at the various stages of the dynamic mechanical measurements, which show a gradual disappearance of the d 001 silicate peak up to 260 8C. This suggests that the PCL chains diffuse further into the silicate interlayers. Therefore both nanocomposites show extended exfoliation caused by the additional heat treatment during the dynamic mechanical measurements. The application of the time-temperature superposition principle on the dynamic storage and loss indicates that in addition to the horizontal shift, a vertical shift along the modulus axis is needed to construct a master curve for both nanocomposites. The introduction of the vertical (modulus) shift factor b T, particularly at the higher temperature region of dynamic mechanical measurements, confirms the onset of further exfoliation of the silicate layers within the PCL matrix. From the dependence of the horizontal shift factor a T and the vertical shift factor b T on the temperature, the apparent activation energies E a,h of flow and of further diffusion of the PCL matrix into the organo-modified silicate layers were determined by considering the Arrhenius relationship between the shift factor and the activation energy. This relationship provides unique information on the relative mobility and diffusivity within the PCL nanocomposites. Therefore, a comparison of both E a,h and E a,v values for both nanocomposites indicated that molecular motion was relatively faster in PCLOC30B (polar organoclay) than that in PCLOC25A (nonpolar organoclay), whereas the diffusivity for a further PCL intercalation was almost identical for both nanocomposites. Acknowledgement: This study was supported by the Korea Research Foundation (KRF) through Grant No. E Received: October 8, 2002 Revised: February 14, 2003 Accepted: March 4, 2003 [1] E. P. Giannelis, Adv. Mater. 1996, 8, 29. [2] P. B. Messersmith, E. P. Giannelis, Chem. Mater. 1993, 5, [3] P. B. Messersmith, E. P. Giannelis, J. Polym. Sci., Part A: Polym. Chem. 1995, 33, [4] R. Krishnamoorti, R. A. Vaia, E. P. Giannelis, Chem. Mater. 1996, 8, [5] E. P. Giannelis, R. Krishnamoorti, E. Manias, Adv. Polym. Sci. 1999, 138, 107. [6] K. Yano, A. Usuki, A. Okada, T. Kurauchi, O. Kamigaito, J. Polym. Sci., Part A: Polym. Chem. 1993, 31, [7] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, J. Mater. Res. 1993, 8, [8] A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, J. Mater. Res. 1993, 8, [9] J. S. Bergman, H. Chen, E. P. Giannelis, M. G. Thomas, G. W. Coates, Chem. Commun. 1999, [10] R. Krishnamoorti, E. P. Giannelis, Macromolecules 1997, 30, [11] J. Ren, A. S. Silva, R. Krishnamoorti, Macromolecules 2000, 33, [12] J. D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York [13] R. A. Vaia, E. P. Giannelis, Macromolecules 1997, 30, [14] H. Mavrids, R. N. Shroff, Polym. Eng. Sci. 1992, 32, [15] J.-D. Nam, J. C. Seferis, J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 907.