Morphology, Crystalline Structure and Isothermal Crystallization Kinetics of Polybutylene Terephthalate/ Montmorillonite Nanocomposites

Transcription

1 Morphology, Crystalline Structure and Isothermal Crystallization Kinetics of Polybutylene Terephthalate/ Montmorillonite Nanocomposites Defeng Wu, Chixing Zhou a, Xie Fan, Dalian Mao and Zhang Bian 1 School of Chemistry & Chemical Technology, Shanghai Jiaotong University, Shanghai , China 2 NanTong XinChen Synthetic Material Co. Ltd., Jiangsu , China Received: 13 April 2004 Accepted: 29 June 2004 SUMMARY The melt intercalation method was employed to prepare poly(butylene terepathalate)/ montmorillonite nanocomposites, and their microstructure was characterized by wide angle X-ray diffraction and transmission electron microscopy. The XRD results showed that the crystalline plane such as (010), (111), (100) was smaller than that of pristine PBT, which indicates that the crystallite size of PBT in the nanocomposites could be diminished by adding clay. Moreover, the isothermal crystallization kinetics of PBT and PBT/MMT nanocomposites was investigated by differential scanning calorimetry (DSC). During isothermal crystallization, the development of crystallinity with time was analysed by the Avrami equation. The results show that very small amounts of clay dramatically increased the rate of crystallization and high clay concentrations reduced the rate of crystallization at the low crystallization temperatures. At low concentrations of clay, the distance between dispersed platelets was large so it was relatively easy for the additional nucleation sites to incorporate surrounding polymer, and the crystal nucleus was formatted easily. However, at high concentrations of clay, the diffusion of polymer chains to the growing crystallites was hindered by large clay particles, despite the formation of additional nucleation sites by the clay layers. At the higher crystallization temperature, the crystallization of the nanocomposites was slower than that of the pure PBT under the experimental conditions, which means that with the increase in chains mobility at the high crystallization temperature, the crystal nuclei are harder to format, and the hindering effect of clay particles on the polymer chains was stronger than the nucleating effect of the layers. In addition, the activation energies of crystallization for PBT and its nanocomposites were calculated by the Arrhenius relationship, and the results showed that the nanocomposites with a low clay content had the lower activation energy values than PBT, while high amounts of clay increased the activation energy of PBT. 1. INTRODUCTION Poly(butylene terephthalate) (PBT), is a typical semicrystalline polymer and an engineering plastic with excellent mechanical properties, which has found wide application in fibres and mouldings. Many research studies have concentrated on blending PBT with another polymer or with one of a variety of fillers to obtain new polymeric materials with desirable properties 1-5. The use of layered aluminosilicates as fillers in polymers has received considerable attention in a Corresponding author, Tel: , Fax: , address: cxzhou@.sjtu.edu.cn recent years. Studies have repeatedly shown that dispersing individual high aspect ratio silicate platelets leads to dramatic property enhancements, e.g. increased stiffness and strength, improved barrier properties, and better dimensional stability at very low filler concentrations. A large number of polymers with varying degrees of polarity and chain rigidity have been used as matrices for polymer/ clay nancomposites, including polystyrene 6, polyamides 7 9, polyimides 10, epoxy resins 11, polyurethanes 12, poly(ethylene terephthalate) 13, polypropylene 14, and so on. There are only a few papers dealing with PBT/MMT nanocomposites 15. Those studies usually focused 61

2 Defeng Wu, Chixing Zhou, Xie Fan, Dalian Mao and Zhang Bian on the preparation of PBT/MMT nanocomposites and the characterization of their structure and mechanical properties. It is well known that the physical and mechanical properties of crystalline polymers depend on their morphology, crystalline structure and degree of crystallization. No report has been found by the authors on the effect of MMT content on the crystallization behaviour of PBT nanocomposites until now. Since PBT is a semicrystalline polymer, and the microstructure of MMT and the matrix crystallite may have remarkable effects on the properties of the nanocomposites, it is important to study the influence of MMT on the crystallization process. In this paper, the microstructure and the crystalline structure of PBT/MMT hybrids were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Then the isothermal crystallization of PBT/MMT hybrids was investigated using differential scanning calorimetry (DSC). 2. EXPERIMENTAL 2.1. Material Preparation The poly(butylene terephthalate) (PBT, 1097A, M n =23200) used in this study was a commercial product of Nantong XinChen Synthetic Material Co. Ltd., P. R. China. The commercial organic montmorillonite (MMT), 10A, modified with dimethyl, benzyl, hydrogenated tallow, quaternary ammonium (2MBHT) was provided by the Southern Clay Co., USA. PBT/MMT nanocomposite was prepared by direct melt compounding of MMT10A with PBT in a Rheomix-600 mixer (Haake Rheocord 900, Germany) at 230 C and 50 rpm for 10 min, and the clay loadings were 1 wt%, 3 wt%, 6 wt%, and 9 wt% respectively Microstructure Characterization The degree of swelling and the interlayer spacing of the clay in PBT/MMT were determined by XRD. The experiments were performed using a Rigaku DmaxrC diffractometer with a Cu target and a rotating anode generator operated at 40 kv and 100 ma. The scanning rate was 2 o /min from 1 o to 10 o. The film sample for XRD measurement was prepared by compression moulding at 230 o C and 10 MPa. The transmission electron micrographs were taken from 80 to 100 nm thick, microtomed sections using a transmission electron microscope (HITACHI H-860, Japan) with 100 kv accelerating voltage. 2.3 Isothermal Crystallization Processes Isothermal crystallization from the melt was performed in the sample pan of the differential scanning calorimeter as follows. The samples were melted at 230 o C for 10 min to eliminate any previous thermal history; then they were rapidly cooled at -100 o C/min to the predetermined crystallization temperatures, Tc, in the range o C, and maintained at Tc for whatever time was required for isothermal crystallization. The exothermal curves of heat flow as a function of time were recorded. 3. RESULTS AND DISCUSSION 3.1 Microstructure and Morphology The microstructure and morphology of polymer/clay nanocomposites is typically elucidated using XRD and TEM. In XRD patterns the interlayer spacing of clay can be determined by the site of the peak corresponding to the {001} basal reflection of MMT (hitherto referred to as d 001 peak). Figure 1 shows the XRD patterns of MMT10A and PBT/MMT nanocomposites with different MMT contents. The d 001 peak of the 10A powder was observed at 2θ =4.36o, which means that the interlayer distance of the original clay was 1.98 nm. Compared with that of the clay powder, the d 001 peaks of 10A were dispersed and shifted to lower angles in the PBT/ MMT nanocomposites. The interlayer spacing of the MMT in those nanocomposites with 3, 6, and 9 wt% 10A content increased to 3.10, 3.91 and 3.20 nm respectively, which means that PBT chains in the nanocomposites diffused into the galleries of the clay, that is, the spacing of the silicate layers increased and some of them were exfoliated. A TEM study was carried out to confirm the dispersion of the clay in the matrix. Figure 2 presents the TEM image of the nanocomposite, which shows a distinct intercalated and partly-exfoliated structure. The silicate crystallites, or tactoids (dark lines), composed of up to ten silicate layers, no thicker than nm, were dispersed in the matrix (white field) in samples with the low MMT contents (1 and 3 wt%), and some were bent in the shear field, as shown in Figure 2 (a) and (b). With increasing loadings of MMT, the average size of the multi-layers gradually reached 100 nm or more, and a distinct intercalated structure was apparent. Meanwhile, many fragments of silicate layers were observed with a few single layers exfoliated from the silicate crystallites as a result of the high shear stresses during the moulding process. The TEM analysis was consistent with the XRD measurements. 62

3 Figure 1. XRD patterns for clay and PBT/MMT samples: (a) 1wt%, (b) 3wt%, (c) 6wt%, (d) 9wt% Figure 2. TEM images of PBT/MMT samples: (a) 1wt%, (b) 3wt%, (c) 6wt%, at a magnification of 100,000 and (d) 9wt% at a magnification of 50,000 (a) (b) (c) (d) 63

4 Defeng Wu, Chixing Zhou, Xie Fan, Dalian Mao and Zhang Bian 3.2 Crystalline Structure of PBT and PBT/ MMT Nanocomposite According to Yokouchi et al. 16, PBT can crystallize in both the α and the β forms, the latter being obtained by mechanical deformation. The X-ray diffraction spectra of the PBT and hybrids are shown in Figure 3. The data from the diffraction lines in the patterns were compared with the crystallographic data reported in the literature 17. The PBT crystals obtained in this study are believed to be in the α form. From Figure 3, it is evident that the diffraction patterns of those nancomposites with 1 to 9 wt.% of MMT were very similar to those of pure PBT, without the appearance of any other characteristic peak, suggesting that the crystalline structure of the PBT in the blends was not degraded in the presence of MMT. However, with the addition of MMT, the peak position shifted to a slightly larger angle, as shown in Table 1, which indicates that the crystallite dimensions of the hybrids were smaller than those of PBT. Since it is well known that the silicate layers of MMT are active substrates for heterogeneous nucleation in the nanocomposites, the size of the PBT crystallites formed must be related to the number of nuclei. To estimate the crystallite size from the broadening of the diffraction pattern, a method based on the Scherrer equation is generally used 18. The crystallite dimension, L hkl can be calculated by: L hkl = Kλ β hkl cosθ hkl (1) Figure 3. XRD patterns of PBT and PBT/MMT nanocomposites Table 1. The position of the diffraction peaks of PBT and PBT/MMT hkl PBT (Deg.) PBT/MMT 1wt%(Deg.) PBT/MMT 3wt%(Deg.) PBT/MMT 6wt%(Deg.) PBT/MMT 9wt%(Deg.)

5 where L hkl is the crystallite dimension, or coherence length, perpendicular to the (hkl) plane, K is the Scherrer constant, λ is the wavelength of the X-rays and u is the Bragg angle. When β hkl is the diffraction half-width, K takes a value of 0.9. The results are summarized in Table 2. It is obvious that the crystallite dimensions of those samples containing clay were smaller than those of neat PBT. In contrast to the slight decreases in the (011) and (100) planes, remarkable decreases in L hkl corresponding to the (010) refraction, which is known to be the preferred growth plane of a lamellae growth direction 19, as well as (111), can be found for the nanocomposites, compared with the neat polymer. The crystallite dimensions of a semicrystalline polymer are the functions of crystallization temperature and the density of nucleation. The larger crystallite size seems to suggest that fewer nuclei were present in the blends during crystallization. The results above indicate that the silicate layers have a nucleating effect on the crystallization of PBT. 3.3 Isothermal Crystallization Behaviour Analysis DSC curves for PBT and the hybrids in Figure 4 were recorded by heating the samples from room temperature to 250 o C, about 30 o C above T m, at 20 o C /min, holding them there for 10 min in order to eliminate any previous thermal history, and then cooling rapidly (-100 o C /min) to the various crystallization temperatures (Tc). As shown in Figure 4, the exothermic crystallization peaks of hybrids became flatter, and the time to reach the maximum degree of crystallization increased, as the crystallization temperature increased, which was the same trend as that shown by PBT. The above demonstrates that the crystallization temperature significantly affected the crystallization behaviour of both PBT and the hybrids. To make a proper comparison between PBT and the nanocomposites, Figure 5 shows the dependence of heat flow on crystallization time for PBT and various loadings of clay in the PBT/MMT nanocomposites at 198 o C, Figure 4. The curves of heat flow vs. time during isothermal crystallization at different crystallization temperatures by DSC. (a) PBT (b) PBT/MMT 1wt% Table 2. Crystallite dimensions of PBT and PBT/MMT nanocomposites Samples L 011 (nm) L 010 (nm) L 110 (nm) L 100 (nm) L 111 (nm) PBT PBT/MMT1wt% PBT/MMT3wt% PBT/MMT6wt% PBT/MMT9wt%

6 Defeng Wu, Chixing Zhou, Xie Fan, Dalian Mao and Zhang Bian 199 o C and 200 o C respectively. The addition of small amounts of clay (1 wt%) to the PBT matrix resulted in a significant reduction in crystallization time; but any further addition of clay reversed this trend, which was most obvious at 198 o C, as seen in Figure 5(a). At 3 wt% MMT and above, the peak location of each exotherm occurred at times comparable to, and in some cases longer than that of pure PBT. It is well known that the layers exfoliated from silicate crystals have a nucleating effect on the crystallization of the polymer 8,9,14. As seen in Figure 2, the size of the layers shown in (a) and (b) were smaller than those shown in (c) and (d), which means that the extent of exfoliation in nanocomposites diminished with addition of clay. At low clay concentrations, the distance between the dispersed platelets was large, so it was relatively easy for the additional nucleation sites to incorporate surrounding polymer and the crystal nuclei were formatted easily, that is, the exothermic peaks of the nanocomposites were sharper than those of the matrix, especially at lower crystallization temperature. However, at high concentrations of clay, the diffusion of polymer chains to the growing crystallites was hindered by the larger clay particles, despite the formation of some additional nucleation sites by the clay layers. As a result, the crystallization exothermic peaks associated with the crystallization of the hybrids became flatter, which means that the crystallization rate declined. Different behavior was observed at 199 o C and 200 o C, as can been seen in Figure 5(b) and (c). With increasing crystallization temperature, even the peak for the nanocomposites having lower clay contents (1 wt%) shifted to longer times and the crystallization time exceeded that of the PBT matrix. At higher temperatures, with the increase in chain mobility, the crystal nuclei Figure 5. The curves of heat flow vs. time of nanocomposites with different contents of MMT at identical crystallization temperatures by DSC. (a) 198 o C (b) 199 o C (c) 200 o C 66

7 became hard to format, and the hindering effect of clay particles on polymer chains was stronger than the nucleating effect of the layers. Thus, even a small amount of clay retard the crystallization of PBT. The combination of a larger number of nucleation sites and limited crystal growth would be expected to produce crystals of fine grain size. In other words, the presence of high concentrations of dispersed MMT platelets prevented large crystalline domains from forming due to limited space and restrictions imposed on the polymer chains by a large number of silicate platelets; this led to smaller crystallite structures and more defect ridden crystalline lamellae, which is consistent with the conclusions from XRD. Clay layers do indeed disrupt crystallite formation and lead to less ordered crystals. Such imperfections in crystalline structure may also explain the lower melting points observed for the nanocomposites. 3.4 Isothermal Crystallization Kinetics Analysis The isothermal kinetics of crystallization can be better visualized by evaluating the degree of crystalline conversion as a function of time at a constant temperature. The relative crystallinity at different crystallization times, X(t), can be calculated according to the equation X t t ()= Q t / Q = dh / dt 0 ( )dt / ( dh / dt)dt (2) where Q t and Q was the heat generated at time t and infinite time, respectively, and dh/dt was the rate of heat evolution. The relative amount of crystallization has been plotted in Figure 6 for different crystallization temperatures. These curves in Figure 6(a) reiterate 0 Figure 6. Relative crystallinity X(t) vs. time t of nanocomposites with different contents of MMT at identical crystallization temperatures. (a) 198 o C (b) 199 o C (c) 200 o C 67

8 Defeng Wu, Chixing Zhou, Xie Fan, Dalian Mao and Zhang Bian that very small amounts of clay (1 wt%) dramatically increased the rate of crystallization; however, high clay concentrations reduced the rate of crystallization at a lower crystallization temperature, namely 198 o C. At 199 o C (see Figure 6(b)), the crystallization rate of the nanocomposite with a clay content of 1 wt% was almost equal to that of the matrix. However, at a higher crystallization temperature, such as 200 o C, the crystallization of the nanocomposites was slower than that of pure PBT, regardless of the clay loading, as can be seen in Figure 6(c). Similar effects have been observed for nanocomposites in other matrices like poly(1-caprolactone) 20 and in polyamides/clay nanocomposites 21. As mentioned above, introducing particles of clay hinders the chain mobility and thus, retards crystal growth process; moreover, at higher temperatures, the crystal nucleus form hardly, which accords with the results from Figure 5. The development of relative crystallinity can be analyzed using the Avrami equation 22,23 : X(t) = 1 exp(-kt n ) (3) log[-ln(1 X(t))] = n logt = logk (4) where n is a constant whose value depends on the mechanism of nucleation and on the form of crystal growth, K is a rate constant containing the nucleation and growth parameters. From a graphic representation of log[-ln(1 X(t))] vs. logt, the Avrami exponent n (slope of the straight line) and the crystallization kinetic constant K (intersection with the y-axis) could be obtained. Plots of log[-ln(1 X(t))] vs. logt are shown in Figure 7(a), in which a pretty good linear relationship for the melt crystallization of PBT holds from about 5% to 90% relative crystallinity. In comparison with the matrix, the nanocomposites showed a slightly extensive linear relationship between relative crystallinity and amount of added clay, which indicates that the secondary crystallization of PBT was retarded by the silicate layers under the experimental conditions, as seen in Figure 7(a) and (b). This demonstrates that the crystallization behaviour of nanocomposites was different from that of PBT, which usually has a secondary crystallization at a longer crystallization time. Fitting the linear line of log[-ln(1 X(t))] vs. logt allows us to determine n and K. The values obtained are listed in Table 3. A narrow spread of n values for PBT and the nanocomposites centered at 4.0 could be obtained. The result indicates that the crystals in a pure PBT melt showed mostly polyhederal (spherulitic) growth, and the nucleation process was homogeneous and simultaneous under the experimental conditions 24. With addition of clay, n values decreased slightly, which indicates that the silicate layers had moderate heterogeneous nucleating effect on PBT. The crystallization rate parameter K values showed that crystallization of the hybrid with 1 wt% content of clay was faster than that of PBT. Moreover, the hybrids containing 3 wt% clay and above had slower crystallization process under the experimental conditions, which is in accordance with the results from Figure 5 and Figure 6. Crystallization half-time, t 1/2, is defined as the time at which the extent of crystallization reaches 50%. It is also regarded as a very important crystallization kinetic parameter. Usually t 1/2 or the reciprocal of t 1/2, namely τ 1/2, was employed to characterize the rate of crystallization directly. The greater the value of t 1/2, the lower the rate of the crystallization. Furthermore, the crystallization rate parameter K could be derived from t 1/2 according to Eq. (5): n K = ln 2 / t 1/2 (5) As listed in Table 3, the values of K were in agreement with those obtained from Figure 7, which suggests the Avrami analysis works very well in describing the crystallization from the melt of PBT and its nanocomposites. In addition, the time to reach the maximum rate of crystallization (t max ) can also be used to describe the rate of crystallization. It is very easy to find t max from Figure 4, and the values of various crystallization temperatures are listed in Table 3. YangChuan Ke, etc. 8 studied PET/MMT nanocomposites and found that the nanocomposites had a dramatically effect, increasing the crystallization rate considerably with additional amounts of clay. At the same crystallization temperature, the value of t 1/2 of PET was three times than that of PBT. This shows that the overall process of PET chains diffusing to the growing crystallites was slower, and so the nucleating effect brought about by the silicate layers was stronger than the retarding effect. Accordingly, 68

9 Figure 7. Plots of log[-ln(1 X(t))] vs. log t for isothermal crystallization of (a) PBT (b) PBT/MMT1wt% (c) PBT/ MMT3wt% a high concentration of clay can accelerate the crystallization rate of PET/MMT hybrids, which is different from the crystallization behaviour of PBT/MMT nanocomposites. Additionally, the Avrami parameter K is assumed to be thermally activated and can be used to determine the energy for crystallization. Thus, the crystallization rate parameter K can be described by an Arrhenius relation 25 as follows: K l/n = K 0 exp(δe/rt c ) (6) where K 0 is a temperature independent preexponential factor, ΔE is an activation energy, R is the gas constant, and Tc is the crystallization temperature. ΔE is determined by the linear regression of the experimental data of K 1/n against 1/T, as plotted in Figure 8. The crystallization activation energies of PBT and various hybrids are listed in Table 3. As can be seen, the value of ΔE for PBT is 316 kj/mol. The crystallization activation energy value of the nanocomposite with low addition of clay is less than that of pure PBT. The high clay content makes the value of ΔE increase, however. 3. CONCLUSIONS The XRD patterns and The TEM analysis show that PBT/MMT is a nanocomposite in which montmorillonite was partly exfoliated. With the addition of clay, the crystalline plane such as (010), (111), (100) is smaller than that of pristine PBT, which indicates that the crystallite size of PBT could be diminished and the silicate layers were having a nucleating effect on crystallization. The exothermal curves show that very small amounts of 69

10 Defeng Wu, Chixing Zhou, Xie Fan, Dalian Mao and Zhang Bian Table 3. Parameters of isothermal crystallization Samples Temp. ( C) n -logk a -logk b t 1/2 (min) t max (min) τ 1/2 (min -1 ) ΔE (kj/mol) PBT MMT1wt% MMT3wt% MMT6wt% MMT9wt% a Determined from Figure 7 b Calculated from Eq. (5) Figure 8. (1/n) lnk vs. 1/Tc for evaluating the activation energy of isothermal crystallization 70

11 clay effectively increase the rate of crystallization whereas high clay concentrations reduce the rate of crystallization at the low crystallization temperature. At low concentrations of clay, the distance between the dispersed platelets is large, so it is relatively easy for additional nucleation sites to incorporate the surrounding polymer, and the crystal nuclei are formatted easily. However, at high concentrations of clay, the diffusion of polymer chains to the growing crystallites is hindered by significant amounts of the large clay particles, despite the formation of additional nucleation sites by clay layers. At the higher crystallization temperature, the crystallization of the nanocomposites is slower than that of the pure PBT under the experimental conditions, which means that with the increase in chain mobility at the higher crystallization temperature, the crystal nucleus was hard to form, and the hindering effect of the clay particles on the polymer chains was stronger than the nucleating effect of the layers. The calculated Avrami exponent n shows that the crystals of pure PBT showed mostly polyhedral (spherulitic) growth, and that with the addition of clay, the n values decreased slightly, which suggests that the silicate layers had a heterogeneous nucleating effect on the PBT in some extent. The calculated crystallization activation energies were 316kJ/mol for PBT. The activation energies of the nanocomposites with low amounts of clay were less than pure PBT. The high clay content increased the ΔE values. ACKNOWLEDGEMENTS This work was supported by the research grants from the National Natural Science Foundation of China (No and No ) and from the Natural Science Foundation of Jiangsu Province, China (No. BE ). REFERENCES 1. J.K. Kim and H.Y. Lee, Polymer, 37, 2 (1996) K. Qi and K. Nakayama, J. Mater. Sci., 36, (2001) L. Finelli, N. Lotti and A. Munari, J. Europ. Polym., 37, (2001) J. Jang and J. Won, Polymer, 39, 18 (1998) M. Okamoto and T. Inoue, Polymer, 36, 14 (1995) R.A. Vaia, K.D. Jandt, E.J. Kramer and E.P. Giannelis, Chem. Mater., 8, (1996) Y. Kojima, A.M. Usuki and A. Kawasumi, J. Polym. Sci., Part A: Polym. Chem., 31, (1993) Z.G. Wu, C.X. Zhou, R.R. Qi and H.B. Zhang, J. Appl. Polym. Sci., 83, (2002) Z.G. Wu and C.X. Zhou, Polym. Test., 21, 4 (2002) H.L. Tyan, Y.C. Liu and K.H. Wei, Polymer, 40, 17 (1999) T. Lan, P.D. Kaviratna and T.J. Pinnavaia, J. Phys. Chem. Solids, 57, (1996) J. Ma, S. Zhang and Z.N. Qi, J. Appl. Polym. Sci., 86, 10 (2001) Y.C. Ke, C.F. Long and Z.N. Qi, J. Appl. Polym. Sci., 73, 10 (1999) J. Li, C.X. Zhou and G. Wang, Polym. Test., 21, 3 (2002) X.C. Li, T.K. Kang and W.J. Cho, Macro. Rapid Commu., 22, (2001) M. Yokouchi, Y. Sakakibara, Y. Chatani, H. Tadokoro, T. Tanaka, and K. Yoda, Macromolecules, 9, 2 (1976) M. Desper, M. Kimura and R.S. Porters, J Polym. Sci., Phys Ed, 22, (1984) M. Kakudo and N. Kasai, X-ray diffraction by polymers. Tokyo: Kodansha, P.H. Geil, Polymer single crystals. New York: Interscience, G. Jimenez, N. Ogata, H. Kawai and T. Ogihara, J. Appl. Polym. Sci., 64, 11 (1997) T.D. Fornes and D.R. Paul, Polymer, 44, (2003) M. Avami, J. Chem. Phys., 7, (1939) M. Avami, J. Chem. Phys., 9, (1941) K. Bhawna, K.G.. Anup and M. Ashok, Polymer, 44, 16 (2003) P. Cebe and S.D. Hong, Polymer, 27, 1 (1986)