Synthesis and properties of poly(4-vinylpyridine)/ montmorillonite nanocomposites

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1 e-polymers 2003, no ISSN Short communication: Synthesis and properties of poly(4-vinylpyridine)/ montmorillonite nanocomposites Sinan Sen *, Nihan Nugay, Turgut Nugay Department of Chemistry and Polymer Research Centre, Bogaziçi University, Bebek, Istanbul, Turkey; Fax ; (Received: August 6, 2003; published: October 2, 2003) Abstract: 4-Vinylpyridine monomer was mixed with organically modified montmorillonite (MMT) and polymerized in the presence of 2,2 -azoisobutyronitrile as radical initiator. Organophilic montmorillonite was obtained by using a block copolymer of poly(methyl methacrylate) and quaternized poly(4-vinylpyridine) (P4VP) in different compositions. X-ray diffraction (XRD) and thermogravimetric analysis confirmed that the block copolymer is inserted between MMT layers while the interlayer distance is expanded. The P4VP nanocomposites obtained from the block copolymer with the longer P4VP block exhibited no XRD peak, suggesting an exfoliated structure. These composites showed increased storage modulus and thermal stability at a very low loading of 1-2 wt.-%, compared to neat P4VP. Scanning electron microscopy and atomic force microscopy analyses were also conducted for selected nanocomposites. Introduction Polymer-clay nanocomposites exhibit improved mechanical, thermal, and optical properties compared to pure polymer or microscale composites. The polymer/layered silicate nanocomposites have been prepared in three different routes, in situ polymerization, solution polymerization and melt intercalation. Among them, in situ polymerization was the first method used to synthesize polymer-clay nanocomposites based on polyamide 6 [1] that showed enhanced mechanical properties such as increased moduli, strength, heat resistance and decreased gas permeability [2-3]. Polymer-clay nanocomposites based on epoxy [4], polyurethanes [5-6] and poly- (ethylene terephthalate) [7] obtained via the in situ method have been found to increase tensile strength and modulus. Additionally, increased thermal stability and flame resistance have been reported for several nanocomposite systems [8-10]. There are two types of layered clay nanocomposite structures, intercalates where polymer chains intercalate between the layers, and exfoliates whose silicate layers are completely delaminated in the polymer matrix [11]. Since improvements in many properties depend on the degree of dispersion of the nanoparticles, exfoliated nanocomposites are generally the target of many nanocomposite studies. The layered clay is treated with mostly quaternized alkylammonium ions to obtain organophilic clay being compatible with organic polymers. These polymers may then be able to intercalate between the clay layers. By exchanging with various organic 1

2 cations, montmorillonite clay can be compatibilized with organic matrix polymers. There are quite limited studies in which a polymer or block copolymer as an intercalant can be introduced into the silicate layers for nanocomposite synthesis [12-13]. The choice of a block copolymer as intercalant can be useful for tuning gallery spacing between the clay layers depending on the selective block lengths. In this study, using montmorillonite clay, we report on the in situ synthesis of poly(4- vinylpyridine) (P4VP) nanocomposites which are also good candidates for biomedical applications due to their compatibility to body tissues [14-15]. Block copolymers of poly(methyl methacrylate) (PMMA) and quaternized P4VP in different compositions were used as intercalants for the clay. Analysis, characterization and mechanical and thermal properties of the nanocomposites are also reported. Experimental part Materials 4-Vinylpyridine (4VP) was purchased from Aldrich and was purified by vacuum distillation over CaH 2 (Aldrich) under nitrogen. Montmorillonite (MMT) was kindly donated by Süd-Chemie (Nanofil 1080, cationic (Na + ) exchange capacity of 100 meq / 100 g). 2,2 -Azoisobutyronitrile (AIBN) was obtained from Merck and dried in vacuum at room temperature. Block copolymers of PMMA and P4VP, used for the organic modification of montmorillonite in different compositions, whose characteristics are depicted in Tab. 1, were used as synthesized by Nugay and coworkers [16]. Tab. 1. Block copolymers used for the modification of MMT PMMA-P4VP block copolymer B 1 a B 2 a 10-3 M n b M w /M n b PMMA content c in wt.-% a Synthesized via anionic polymerization and then quaternized with methyl iodide. b Based on the evaluation of gel permeation chromatograms using PMMA standards. c Calculated from 1 H NMR analysis. Preparation of organically modified MMT Intercalation of block copolymers in different block lengths into MMT layers was achieved through an ion exchange reaction (Scheme 1). 1 g MMT was dispersed in 100 ml deionized water at 80 C and a separate solution of quaternized block copolymer in 150 ml deionized water was heated and mixed at 80 C for 1 h. Then the block copolymer solution was added to the clay solution slowly and mixed vigorously while keeping the temperature of the solution at 80 C. After mixing, the total volume is brought up to 400 ml and stirred for 5 h. The organically modified MMT was recovered by filtering the solution followed by repeated washings of the filter cake with 15 ml deionized water twice to remove excess ions. The final product was dried at 30 C in a vacuum oven for 12 h. 2

3 CH 3 ( CH 2 CH ) n ( CH 2 CH ) m C=O OCH 3 N + CH 3 I _ Na + Na + MMT Clay Na + Na + Scheme 1. Schematic representation of a possible ion exchange reaction occurring between the polycation and the cations in MMT Preparation of the nanocomposites 4VP and modified MMT were mixed at 40 C for 5 h. The AIBN initiator (1 wt.-% of monomer) was added to the mixture and dissolved. Then the temperature was increased up to 60 C and mixed for about 40 min to prepare a prepolymer. After that, the viscous solution was poured into a mold and polymerized for 24 h at 65 C. The obtained nanocomposites together with the clay content are listed in Tab. 2. Tab. 2. Poly(4-vinylpyridine) nanocomposites Polymer nanocomposites 1B 1 M-C 2B 1 M-C 1B 2 M-C 2B 2 M-C wt.-% Organo-modified MMT 1% B 1 MMT 2% B 1 MMT 1% B 2 MMT 2% B 2 MMT Analysis and characterization X-ray diffraction (XRD) measurements were conducted on a Philips XL30 diffractometer with CuK α radiation (λ =1.54 Å), operating at 40 kv and 40 ma. The diffraction patterns were collected between 1 and 15 with a scanning rate of 2 /min. Basal spacing of MMT was obtained from the peak position of the d 001 reflection in the XRD pattern. Thermogravimetric analysis (TGA) was performed using a Netzsch STA1500H under nitrogen flow and an STA449C under air flow. Nanocomposite samples were heated to 800 C with a heating rate of 10 C/min. The dynamic mechanical thermal analysis (DMTA) of the resultant nanocomposites was done using a Polymer Laboratories dynamic mechanical thermal analyzer. Samples were scanned at a frequency of 1 Hz with a heating rate of 5 C/min under nitrogen atmosphere. Fracture surfaces of the composites were investigated via scanning electron microscopy (SEM) using an SEM-FEG & ADAX instrument. 3

4 Atomic force microscopy (AFM) (Nanoscope IIIa, Digital Instruments) imaging was carried out in tapping mode in air with oxide sharpened Si tips. AFM images were recorded in the phase mode. An E-type scanner was employed with a probing area of 17 x 17 mm 2. Results and discussion Synthesis and properties of organically modified MMT Block copolymers of PMMA and quaternized P4VP in different block lengths (Tab. 1) were used to obtain organo-modified MMT via ion exchange reaction. Fig. 1 shows XRD diffractograms of MMT and modified clays, B 1 MMT and B 2 MMT. XRD analysis showed that the intercalation of B 1 and B 2 block copolymers was successful since the d-spacing between MMT layers increased by approximately 2.78 Å from Å in unmodified MMT to Å in B 1 MMT. B 2 block copolymer having a short block of quaternized P4VP caused a 2.44 Å increase in d-spacing. Fig. 1. XRD diffractograms of MMT and modified MMT clays The increment of the basal spacing of MMT with modification was also confirmed by thermogravimetric analysis. Fig. 2 shows TGA thermograms of MMT and modified MMT. Pure MMT has only 10% total weight loss indicating water removal. On the other hand, after block copolymer intercalation, this amount reaches almost 53% at higher temperatures, resulting from the degradation of intercalated block copolymer. 4

5 Synthesis and properties of P4VP-clay nanocomposites The nanocomposites were prepared by in situ free radical bulk polymerization of 4- vinylpyridine monomer in the presence of organo-modified clay at 65 C for 24 h. The nature of the nanocomposites, such as intercalation or exfoliation, was investigated using XRD (Fig. 3). In the XRD curves, the nanocomposites 1B 1 M-C, 2B 1 M-C and 1B 2 M-C exhibited no d 001 reflection peak in the relevant angle region. Fig. 2. TGA thermograms of MMT and modified MMT, B 2 MMT, under nitrogen flow 2B 2 M-C d (A ) = B 1 M-C 1B 1 M-C 1B 2 M-C Fig. 3. X-ray diffraction curves of 1B 1 M-C, 2B 1 M-C, 1B 2 M-C and 2B 2 M-C 5

6 This result indicates that organophilic clay in those hybrids disperses homogeneously in the P4VP matrix, which is a clear indication of an exfoliated nanocomposite structure. The AFM image also confirms this observation (Fig. 4a). On the other hand, for the nanocomposite 2B 2 M-C, a small peak appeared in the XRD region, which may be due to less swelling of the B 2 MMT clay, having a shorter P4VP block, in 4VP monomer. This may cause the formation of micelles of some portion of the clay for 2 wt.-% loading in the matrix. In other words, in 2 wt.-% loading, 4VP has difficulties to penetrate into the B 2 MMT clay galleries compared to the B 1 MMT clay having a longer P4VP segment. Also, the increment of the d-spacing of the composite 2B 2 M-C was decreased most probably because of high electrostatic interactions between clay layers after aggregation, which is quite clear also in the AFM image of the related nanocomposite (Fig. 4b) a b Fig. 4. AFM images of the composites (a) 2B 1 M-C and (b) 2B 2 M-C Fig. 5. shows TGA thermograms of virgin P4VP, 2B 1 M-C and 2B 2 M-C taken under nitrogen and air flow. In both similar cases, the TGA curve of the hybrid 2B 1 M-C shows delayed decomposition compared to that of P4VP and 2B 2 M-C composite. From the TGA data, it is clear that 2B 1 M-C is more stable than 2B 2 M-C and P4VP at least up to 350 C, which may be attributed to the existence of much more interaction of P4VP polymer chains with B 1 MMT layers than with B 2 MMT layers. This result is also in agreement with the XRD data (Fig. 3). Fig. 6 shows a SEM examination of the fractured surfaces of the nanocomposites. It is apparent that 2B 1 M-C exhibits a smooth surface with a very fine dispersion of individual silicate layers in the form of bright regions or at least small clusters containing a few sheets in the polymer matrix. On the other hand, the SEM picture of 2B 2 M-C reveals the existence of most of the clay as aggregates with a rough surface. This aggregation may be due to a possible solid micelle formation between PMMA tails outside the MMT layers in 2 wt.-% loading for B 2 MMT clay. This result is well consistent with the XRD data that give a d 001 reflection peak and whose d-spacing decreased with increasing loading of the clay from 1 to 2 wt.-%. Moreover a good adhesion of the sheets during fracture of the composite material 2B 1 M-C was found to cause crack propagation along a rougher path. 6

7 (a) 100 2B 1 M-C (b) 80 P4VP Weight ( percent) B 2 M-C Temperature ( o C) Fig. 5. TGA thermograms of pure P4VP, 2B 1 M-C and 2B 2 M-C under (a) N 2 and (b) air flow The effect of clay reinforcement on mechanical properties of P4VP polymer was investigated by dynamic mechanical thermal analysis (DMTA). Two different parameters were determined as a function of temperature: The elastic or storage modulus (E ) represents the response of elastic material related to the potential energy stored by the material under deformation and is a measure of rigidity. The loss factor (tan δ) is one of the damping parameters of interest since it is a measure of the ability of a 7

8 polymer to convert mechanical energy into heat at a temperature or frequency of interest. tan δ Peak temperature also corresponds to the glass transition temperature of the material. At the glass transition, a polymer is more efficient in converting sound and mechanical vibration energy into heat; that results in absorption. So, a shift in the position of the tan δ peak to higher temperatures and magnitudes indicates improved thermomechanical properties at those conditions. a ) b c Fig. 6. Scanning electron micrographs of fracture surfaces of (a) P4VP, (b) 2B 1 M-C, and (c) 2B 2 M-C As can be seen from Fig. 7, storage modulus and tan δ peak temperature increased by the addition of B 1 MMT clay. These improvements can be ascribed to the intercalation of block copolymer B 1 in MMT clay galleries as well as fine dispersion of the organo-clay particles in the polymer matrix resulting in an exfoliation morphology, which increases the polymer-clay interactions making the entire surface of the layers available for the polymer. This maximized interaction leads to dramatic changes in mechanical and thermal properties by preventing segmental motions of the polymer chains [3]. Fig. 8 exhibits the influence of the block length in block copolymers, used for the modification of MMT, on DMTA data. For 2B 2 M-C, the reduced storage modulus and tan δ peak temperature may be attributed to a poor adhesion between P4VP matrix 8

9 Fig. 8. DMTA measurements: storage modulus vs. temperature and tan δ vs. temper- ature (insert) for a) P4VP, b) 1B 1 M-C, c) 2B 1 M-C, d) 1B 2 M-C, and e) 2B 2 M-C 9 and B 2 MMT clay particles in 2 wt.-% loading, which was also confirmed with a SEM picture (Fig. 6c) indicating a phase separated structure. Too big particles, as can be seen from the SEM image of 2B 2 M-C, may act as points of discontinuity in the compound, resulting in a plasticizer effect in the structure, which is also supported from a lower tan δ peak at lower temperature [17]. Fig. 7. DMTA measurements: storage modulus vs. temperature, and tan δ vs. temperature (insert) for a) P4VP, b) 1B 1 M-C, and c) 2B 1 M-C

10 Conclusion Block copolymers with different block lengths were successfully intercalated into MMT clay layers, which can be confirmed by both XRD and TGA analysis. From this organically modified clay, P4VP nanocomposites have been prepared by in situ polymerization. The stiffness of the nanocomposites, 1B 1 M-C and 2B 1 M-C, is significantly improved compared to neat P4VP, even at a B 1 MMT content as low as 2 wt.-%. The silicate layers were completely delaminated as evidenced by the absence of any diffraction peak in the XRD region. On the other hand, B 2 M-C having higher loading (2B 2 M-C) exhibited a reduced stiffness probably due to the agglomeration of some part of B 2 MMT clay for 2 wt.-% loading in the matrix leading to the switching of exfoliation to intercalation. In general, it is difficult to determine the degree of exfoliation or intercalation. From the results obtained, it can be concluded that intercalation/exfoliation can be achieved by variation and adjustment of the lengths of both blocks in the block copolymer. Acknowledgement: Support given by Boğaziçi University Research Foundation project no. 03B501D is gratefully acknowledged. [1] Okada, A.; Kawasumi, M.; Usuki, A; Kojima, Y.; Kurauchi, T.; Kamigaito, O.; Mater. Res. Soc. Proc. 1990, 171, 45. [2] Gilman, J. W.; Kashiwagi, T.; SAMPE J. 1997, 33, July/August, no. 4. [3] Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Karauchi, T.; Kamigaito, O.; J. Mater. Res. 1993, 6, [4] Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J.; Chem. Mater. 1995, 7, [5] Wang, Z.; Pinnavaia, T. J.; Chem. Mater. 1998, 10, [6] Zilg, C.; Thomann, R.; Mülhaupt, R.; Finter, J.; Adv. Mater. 1999, 11, 49. [7] Ke, Y.; Long, C.; Qi, Z.; J. Appl. Polym. Sci. 1999, 71, [8] Doh, J. G.; Cho, J.; Polym. Bull. 1998, 41, 511. [9] Gilman, J. W.; Kashiwagi, T.; Brown, J. E. T.; Lomakin, S.; SAMPE J. 1998, 43, [10] Gilman, J. W.; Appl. Clay Sci. 1999, 15, 31. [11] Alexandre, M.; Dubois, P.; Mater. Sci. Eng. 2000, 28, 1. [12] Hoffmann, B.; Dietrich, C.; Thomann, R.; Friedrich, C.; Mülhaupt, R.; Macromol. Rapid Commun. 2000, 21, 57. [13] Fischer, H. R.; Gielgens, L. H.; Koster, T. P. M.; Acta Polym. 1999, 50, 122. [14] Wang, J.; Tuzhi, P.; J. Electrochem. Soc. 1987, 134, 586. [15] Ruths, M.; Sukhishvili, S. A.; Granick, S.; J. Phys. Chem. B 2001, 105, [16] Nugay, N.; Hosette, C.; Nugay, T.; Riess, G.; Eur. Polym. J. 1994, 30, [17] Ferrigno, T. H.; in Handbook of Fillers for Plastics, Katz, H. S.; Milewski, J. V., editors; Van Nostrand Reinhold Inc, New York 1987, p