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Montmorillonite–phenolic resin nanocomposites prepared by one-step in-situ intercalative polymerisation
Applied Clay Science 101 (2014) 484–489
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Montmorillonite–phenolic resin nanocomposites prepared by one-step in-situ intercalative polymerisation Miroslav Huskić a,b,⁎, Alojz Anžlovar a, Majda Žigon b,c a b c
National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia Polymer Technology College, Ozare 19, 2380 Slovenj Gradec, Slovenia Centre of Excellence for Polymer Materials and Technologies, Hajdrihova 19, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 9 September 2014 Accepted 10 September 2014 Available online 26 September 2014 Keywords: Clay Montmorillonite Nanocomposite Novolac Resol Glass transition
a b s t r a c t Montmorillonite (Mt) resol, and Mt–novolac nanocomposites were prepared by one-step in-situ intercalative polymerisation involving the simultaneous modification of the Mt with quaternary ammonium salts (QAS), polymerisation and polymer intercalation. The Mt–polymer nanocomposites were prepared using two types of QAS — with one long (C16) alkyl chain (cetyl trimethylammonium bromide) or with three medium-sized (C8) alkyl chains (tricaprylyl methylammonium chloride) — as well as without a QAS. Intercalated Mt–resol, and Mt– novolac nanocomposites were formed, as confirmed by XRD and STEM. The extent of the intercalation was higher in the Mt–resol nanocomposites and with the cetyl trimethylammonium bromide modified Mt. The presence of the Mt influenced the polymerisation process, which resulted in changes in the molecular structure of the novolac resin, an increased content of p–p linkages and a lower glass transition temperature of the resin. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Although the phenolic resins are the oldest commercially manufactured synthetic polymers their production is still growing and new fields of applications are arising. In many areas they are almost irreplaceable with other polymers, especially in fireproofing, electronic and thermal insulation. Nevertheless, better properties are still required, which might be obtained by the addition of nanoparticles such as clay minerals. Literature reports related to clay phenolic resin nanocomposites are rather scarce, especially compared to other clay duromer nanocomposites, i.e., clay epoxy nanocomposites. Like other Mt polymer nanocomposites, the Mt-phenolic resin nanocomposites were also prepared by either melt intercalation (Choi et al., 2000), solution intercalation (Zhang et al., 2010) or intercalative polymerisation (Jiang et al., 2006). The Mt–resol nanocomposites were synthesized by the intercalative polymerisation of phenol and formaldehyde in the presence of modified Mt (López et al., 2007) or prepared as a blend of modified Mt and pre-synthesized resin (Manfredi et al., 2007). Dispersion polymerisation has also been performed to produce clay polymer nanocomposites (CPN) from both resol and novolac. X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations showed that the Mt layers were more easily exfoliated or intercalated in ⁎ Corresponding author at: National Institute of Chemistry, POB 660, 1001 Ljubljana, Slovenia. Tel.: +386 1 4760 206; fax: +386 1 4760 420. E-mail address: [email protected] (M. Huskić).
http://dx.doi.org/10.1016/j.clay.2014.09.011 0169-1317/© 2014 Elsevier B.V. All rights reserved.
novolac than in resol. This was explained by the linear structure of the novolac, which enables its intercalation between the Mt layers (Wu et al., 2002). The type of Mt modifier also has a significant impact on the intercalation and thermal stability of Mt–phenolic resin nanocomposites. The best exfoliation was observed when the Mt was modified with octadecylamine, while the thermal stability was increased the most when the modifier was of an aromatic nature, e.g., benzyldimethylhexadecylammonium chloride, benzyltriethylammonium chloride and benzyldimethylphenylammonium chloride (Jiang et al., 2006). Increased thermal stability was also observed in Mt–phenolic nanocomposites prepared with modified Mt (López et al., 2007; Manfredi et al., 2008) or unmodified Na+-Mt (Manfredi et al., 2007, 2008). On the other hand, some authors report that neither Na+-Mt nor modified Mt improved the thermal stability (Zhang et al., 2010). The morphology of Mt–resol nanocomposites depends on various production parameters (different liquid resol-type phenolic resins, curing methods, modifying cations in Mt, etc.) (Kaynak and Tasan, 2006; López et al., 2012). The CPN prepared with modified Mt exhibited better mechanical properties than those with Na+-Mt. It was observed that the micro-void formation was the main problem when water-based resol resins were used. The highest strength and toughness were obtained in samples with only 0.5% Mt (Kaynak and Tasan, 2006), while the highest values of the tensile and flexural modules with only a very small concentration (b 1%) of Mt were confirmed in other reports (Tasan and Kaynak, 2009; Kaushik et al., 2010,).
M. Huskić et al. / Applied Clay Science 101 (2014) 484–489
Wang et al. (2002 and 2004) synthesized Mt–resol, and Mt–novolac nanocomposites by using protonated Mt (H+-Mt) as an acid catalyst and nanofiller. The H+-Mt-resol nanocomposites exhibited higher glass-transition temperatures and impact strengths compared with the pure resol resin. No such data were provided for novolac. Generally, the use of modified Mt ensures the good intercalation of polymers between the aluminosilicate layers and changes their properties. On the other hand, the price of the final product is increased since the modified Mt is relatively expensive. Therefore, an alternative preparation method, such as a one-step, in situ intercalative polymerisation has been proposed for preparation of CPN with various polymers: polyurethane (Kiersnowski et al., 2006), poly(methyl methacrylate) (Huskić and Žigon, 2007), and polystyrene (Weickmann et al., 2010). A similar method has already been used to prepare Mt–novolac resin nanocomposites with dodecylamine and oxalic acid as the modifier (Pappas et al., 2005). However, this method cannot be used to prepare Mt–resol nanocomposites. In this article, the preparation of Mt–resol and Mt–novolac nanocomposites using one-step in situ intercalative polymerisation is described. This method involves the simultaneous modification of the Mt with a quaternary ammonium salt (QAS) and phenolic resin intercalation during the polymerisation. Two types of QAS were used: (i) with one long alkyl chain (cetyl trimethylammonium bromide — CTM) and (ii) with three medium-sized alkyl chains (tricaprylyl methylammonium chloride — TC). In addition, some nanocomposites were prepared without QAS and some by pre-swelling the Mt in a water/QAS mixture for 24 h. The catalytic effect of Mt on the polymerisation of phenol and formaldehyde as well as a Mt influence on the structure of phenolic resin has been reported for the first time. The characterization of CPN was performed using nuclear magnetic resonance (NMR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning transmission electron microscopy (STEM).
2. Experimental 2.1. Materials The phenol was kindly donated by Fenolit d.d., Slovenia. The Mt Nanofil 757 was kindly donated by Rockwood Additives. The formaldehyde (37%) and oxalic acid (p.a.) were purchased from Merck. The hexamethylenetetramine (99%) (HMTA), cetyl trimethylammonium bromide (N 98%) and tricaprylyl methylammonium chloride were purchased from Sigma-Aldrich.
2.3. Curing of phenolic resins: All the samples were pressed into pellets and cured in an oven at 180 °C for 5 min. Mt–novolac nanocomposites were mixed with hexamethylenetetramine in the ratio of 9:1 prior to curing. 2.4. Characterization: The XRD experiments were performed on an X-ray powder diffractometer PANalytical X'Pert PRO MPD, (CuKα1 radiation = 1.5406 Å) in 0.033° steps from 1.5° to 15°. The dispersion of Mt in the phenolic resin matrix was studied by scanning transmission electron microscopy (STEM) using a Zeiss Supra 35 VP microscope at an accelerating voltage of 15 kV. The samples were sectioned to a thickness of between 100 and 250 nm on the Ultramicrotome Leica Ultracut prior to the microscopy. The degradation was studied by simultaneous thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) on a Mettler Toledo TGA/DSC1 StarSystem, operating in an oxygen or nitrogen atmosphere, at a heating rate of 5 K min−1, with a sample size of ~15 mg and a temperature range between 40 and 1000 °C. The glass transition was determined on a Mettler Toledo DSC1 at a heating/cooling rate of 10 K min−1, with a sample size of ~15 mg and a temperature range between −50 and 100 °C. The results were obtained from the second cycle. 13 C NMR spectra were recorded using a Varian VXR-300 spectrometer in the solvent methanol-d4 at 25 °C with an acquisition time of 1.5 s and a delay time of 2 s. Typically, 16,000 scans were recorded. 3. Results and discussion The Mt was modified with QAS to determine the basal spacing of the modified Mt, which was later used for a comparison with the Mt– phenolic resin nanocomposites. The XRD spectra are shown in Fig. 1. The basal spacing of the Mt modified with tricaprylyl methylammonium chloride (TC Mt) is 1.73 nm, which is slightly higher than that of the Mt modified with cetyl trimethylammonium bromide (CTM Mt) (1.68 nm). 3.1. Mt–resol nanocomposites In the XRD diffractograms of the Mt–resol nanocomposites prepared with or without the QAS there are one to three Bragg diffractions of the first, second and third orders, indicating a layered structure (Fig. 2).
2.2. Synthesis Mt-novolac nanocomoposites were synthesized with a formaldehyde/ phenol (F/P) mole ratio of 0.9. First, 0.225 mol of formaldehyde (37% aqueous solution), 0.25 mol of phenol (90% aqueous solution) and 4 ml of oxalic acid (0.5 M) were placed in a round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The Mt was added in quantities of 1%, 3%, and 5%, according to the mass of the pure reactants (without water). The required quantity of QAS was calculated from the quantity and cation exchange capacity (CEC) of the Mt, which was 85 meq/100 g, and added to the solution. The reaction mixture was heated to reflux and held there for 3 h. The resins or CPN were separated by pouring the mixture into 250 ml of ice-cold demineralised water. The product was washed several times with water and dried in a vacuum at room temperature for 24 h. Finally, it was ground to a powder and dried for another 48 h. Mt–resol nanocomposites were synthesized with F/P mole ratios of 1.7 and 2.2 using the same procedure as described above, except that 4 ml of concentrated NaOH instead of oxalic acid was used as a catalyst.
485
Fig. 1. XRD diffractograms of Mt modified with CTM and TC.
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Fig. 2. XRD diffractograms of the Mt–resol nanocomposites prepared: (A) without QAS, (B) with TC, and (C) with CTM.
Fig. 3. XRD diffractograms of TC Mt–resol nanocomposites prepared at F/P ratios of 1.7 and 2.2.
The positions of the diffractions depend on the method of preparation. In the XRD diffractogram of Mt–resol nanocomposite, prepared with Mt but without a QAS, the diffraction is at 2θ = 5.7°, which corresponds to a basal spacing of 1.55 nm. The result agrees well with previous reports (Choi et al., 2000; Kaynak and Tasan, 2006). The increase can be explained by the intercalation of polar resin between the Mt layers (Alexandre and Dubois, 2000). The diffractions of the CPN prepared with CTM Mt are at a lower angle than the diffractions of the CPN prepared with TC Mt, which is the opposite of the pure modified Mt (Fig. 1, Table 1). The same was observed for a polymethylmethacrylate (PMMA) based nanocomposites prepared using one-step, intercalative polymerisation (Huskić and Žigon, 2007). It appears that three shorter alkyl chains occupy most of the free volume between the aluminosilicate layers, which hinders the intercalation of the phenolic resin, resulting in a smaller basal spacing. The increase in the basal spacing compared to the CTM Mt is 1.75 nm and to the TC Mt is 0.92 nm (Table 1). The concentration of the Mt influences the intensities of the diffractions but not the position of the diffractions. The mole ratio between the formaldehyde (F) and phenol (P) influences the structure of the phenolic resin. When increasing the F/P ratio, the number of hydroxymethyl groups (\CH2OH) and therefore the polarity of the resin also increase. This increased polarity could be beneficial for enhanced interactions with Mt and consequently, a greater extent of intercalation. On the other hand, due to the condensation of \CH2OH groups during synthesis, the molecules can become larger, which could hinder the intercalation. Comparison of the XRD diffractograms of the CPN prepared using the F/P ratios 1.7 and 2.2 shows some differences in the intensity of the diffractions but no influence on the position and, consequently, on the intercalation was observed. The XRD diffractograms of the CPN prepared with the TC Mt are shown as an example in Fig. 3.
Pre-swelling the Mt in a water/QAS mixture for 24 h also did not influence the intercalation. This is an important feature showing that this method enables fast and straightforward production of Mt–phenolic resin nanocomposites. After curing, the XRD diffractions of the Mt–resol CPN were shifted slightly towards higher 2θ values. The basal spacings of the nanocomposites decreased for approximately 0.2 nm after curing. This is usually explained by a small deintercalation (Choi et al., 2000; Jiang et al., 2006), but it might be also a consequence of mass loss during the curing reaction. The results are summarised in Table 1. The TGA of Mt–resol nanocomposites was performed in a nitrogen and oxygen atmosphere. The mass loss below 300 °C, where curing reaction proceeds, is not significantly influenced by the quantity or type of Mt. The decomposition in air, above 300 °C, is also not influenced by the presence of modified or pristine Mt. Differences are observed in nitrogen atmosphere. Mass loss in CPN prepared with pristine Mt is only slightly smaller than in pure resol, indicating a catalytic effect of Mt. The results are shown in Fig. 4. For clarity, only the TGA curves of resol synthesized without Mt and with 5% Mt are shown. A strong catalytic effect, leading to higher mass loss than in pure resol is
Table 1 The basal spacing of pristine and modified Mt, and nanocomposites with resol, before and after curing. QAS
QAS Mt d (nm)
QAS Mt–resol d (nm)
Cured QAS Mt–resol d (nm)
Δd (nm)
/ CTM TC
0.97 1.68 1.73
1.55 3.43 2.65
1.55 3.22 2.45
0.0 −0.21 −0.20
Fig. 4. TGA curves of resol and CPN synthesized with 5% Mt, in oxygen and nitrogen atmosphere.
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Table 3 Glass transition temperatures (Tg) of novolac and Mt–novolac nanocomposites with respect to the type of QAS and Mt concentration. Concentration of Mt (%)a
Tg (°C) Without QAS
0 1 3 5 a b
Fig. 5. TGA curves of resol and CPN synthesized with 5% TC Mt, in oxygen and nitrogen atmosphere.
observed in CPN with modified Mt. TGA curves of nanocomposites with TC Mt are shown as examples in Fig. 5. The catalytic effect of Mt on various chemical reactions has been known and exploited in industry for decades (Adams and McCabe, 2006), while in decomposition of CPN it has been rarely mentioned (Witkowski et al., 2012; Hadj-Hamou et al., 2014). 3.2. Mt–novolac nanocomposites The novolac resin precipitates from the reaction mixture during synthesis. Without adding a QAS, the Mt remains dispersed in the aqueous phase. Consequently, novolac with a very small Mt content was obtained, as confirmed by the TGA in an oxygen atmosphere (≈ 0.1%). In addition, there were no diffractions of Mt in the XRD diffractogram of the product. CPN only form when the QAS is added to the reaction mixture. The interlayer spacings of the Mt layers in the Mt–novolac nanocomposites (Table 2) were slightly smaller than in the Mt–resol nanocomposites (Table 1). This can be explained either by the lower polarity of the novolac and consequently less intensive intercalation between the Mt layers, or by the linear structure of the macromolecules, which occupy less space than the branched resol. After curing the Mt–novolac nanocomposites, a reduction of the basal spacing is observed; however, the change is smaller than for the Mt–resol nanocomposites. Since there are no reactive hydroxymethyl groups in novolac intercalated between the Mt layers, the only explanation is a small deintercalation of novolac due to curing. HMTA, added to CPN as a curing agent, is only present outside the Mt layers, meaning that the curing also proceeds mainly outside the layers. Due to the high temperature, well above Tg, the novolac molecules are highly mobile and, at least in the beginning of curing, can slip out of the layers, while the cured ones are too big to enter. On the other hand, the decomposition products of HMTA are gasses and can also enter the Mt interlayer spaces and cure the novolac there.
24 18 14 3
CTM 21 20 19 15
b
TC 22b 17 19 17
Plus appropriate quantity of QAS. Polymerisation in the presence of 1% of QAS.
The thermal characterization of the prepared novolac and its CPN shows that the Mt decreases the Tg of the novolac resin. To the best of our knowledge, this effect has not yet been mentioned in the literature. The Tg of the novolac resin decreases with the increasing concentration of neat Mt in the reaction mixture. The decrease is reduced by the addition of the quaternary ammonium salt (Table 3). It seems that the reactive sites on the Mt, which influence the polymerisation, are screened by the QAS. The results are in contrast to those obtained for radical polymerisation, where the QAS, not the Mt, is responsible for the changes in the molecular structure of the synthesized PMMA (Huskić et al., 2012). The thermal stability of novolac synthesized in the presence of Mt is also reduced. The TGA was performed in a nitrogen and oxygen atmosphere and in both cases, thermal stability above 250 °C decreases with increasing the Mt content in the reaction mixture. The results are shown in Fig. 6. For clarity, only the TGA curves of novolac synthesized without Mt and with 5% Mt are shown. The results can be correlated to the Tg measurements. A small difference in thermal stability is observed between resin synthesized without Mt and that synthesized with 1% and 3% Mt, and a big difference when 5% of Mt is added to the reaction mixture. The TGA curves of CPN prepared with CTM Mt and TC Mt are similar to the curves in Fig. 6, except that the difference is smaller and the curves are slightly overlapping. Figures are shown in electronic supplementary information. The changes in Tg and thermal stability are ascribed to differences in the molecular structure of the novolac resin, which can be observed in their 13C NMR spectra. The phenolic rings in the novolac resin are bound by \CH2\ groups in the ortho–ortho, ortho–para or para–para position, whose chemical shifts are well separated at 31 ppm, 36 ppm, and 41 ppm, respectively (Sojka et al., 1979). The intensity ratio of those signals changed from 0.99: 1.96: 1.0 in the novolac synthesized without Mt, to 0.71: 1.62: 1.0, when 5% Mt was added to the reaction
Table 2 The influence of the type of QAS on the interlayer spacing of the modified Mt in Mt– novolac nanocomposites. QAS
QAS Mt d (nm)
QAS Mt–novolac d (nm)
Cured QAS Mt–novolac d (nm)
Δd (nm)
CTM TC
1.68 1.73
3.34 2.45
3.31 2.37
−0.03 −0.08
Fig. 6. TGA curves of novolac synthesized without Mt and in the presence of 5% Mt.
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Fig. 7. 13C NMR spectra of novolac synthesized without Mt and in the presence of Mt.
mixture (Fig. 7). The results show that the addition of Mt to the reaction mixture favours p–p novolac methylene bridge formation and lowers the content of o–o and especially o–p linkages. 3.3. The morphology of the CPN Although intercalation is observed by XRD in Mt–resol nanocomposites prepared with and without the addition of QAS, a different morphology of both CPN was confirmed by STEM. A STEM micrograph of a CPN with pristine Mt shows holes within the polymer matrix, indicating that there is practically no interaction between the polymer matrix and the Mt. Therefore, the Mt particles fell out of the polymer matrix during the electron microscopy measurement (Fig. 8, A and B). In contrast to this, when the QAS was added during synthesis, the particles were nicely embedded in the phenolic matrix (Fig. 8, C), for the resol as well as the novolac. The separated, smaller stacks of Mt can be clearly observed, indicating the partial intercalation of the phenolic matrix in the Mt structure as a consequence of an enhanced interaction between these two components. 4. Conclusions One-step in-situ intercalative polymerisation is a simple way to prepare Mt–phenolic resin nanocomposites based on either resol or novolac. The modification of the Mt, the intercalation and polymerisation proceed simultaneously, which leads to the formation of intercalated CPN. The variations in the mole ratio of phenol to formaldehyde (1:1.7
and 1:2.2) did not influence the extent of intercalation of the Mt–resol nanocomposites. Two types of quaternary ammonium salts were used as Mt modifiers: one with a single long C16 alkyl chain (cetyl trimethylammonium bromide — CTM) and one with three medium-sized C8 alkyl chains (tricaprylyl methylammonium chloride — TC). A better intercalation was observed in the CPN prepared with CTM, which exhibited a larger interlayer spacing, as determined by the XRD. The more polar nature of the resol also favours the intercalation. On the other hand, a decrease in the basal spacing was observed during the curing of both the resol and the novolac resins. A small increase in the interlayer spacing of the Mt in the resol was also observed when no quaternary ammonium salts (QAS) were used. A STEM analysis showed that in this case there were almost no interactions between the pristine Mt particles and the phenolic resin. Contrary to that, the QAS enhance the compatibility between the Mt and the phenolic resin, and the formation of the intercalated CPN was confirmed by STEM. Mt influences the mechanism of the polymerisation reaction in an acid medium, producing novolac with increased p–p linking and a lowered glass transition temperature. Acknowledgements This research was supported by the Slovenian Research Agency (Program P2-0145). The authors acknowledge the support from CE PoliMaT (financially supported by the Ministry of Education, Science
Fig. 8. STEM micrograph of cured resol resin with pristine Mt (A, B) and with CTM Mt (C).
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and Sport of the Republic of Slovenia through the contract No. 3211-10000057 (Centre of Excellence for Polymer Materials and Technologies)) where DSC and TGA measurements were performed. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2014.09.011. References Adams, J.M., McCabe, R.W., 2006. Clay minerals as catalysts. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science, Developments in Clay Science Vol. 1. Elsevier Ltd., Oxford, pp. 541–581. Alexandre, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. R 28, 1–63. Choi, M.H., Chung, J., Lee, J.D., 2000. Morphology and curing behaviors of phenolic resinlayered silicate nanocomposites prepared by melt intercalation. Chem. Mater. 12, 2977–2983. Hadj-Hamou, A.S., Matassi, S., Abderrahmane, H., Yahiaoui, F., 2014. Effect of cloisite 30B on the thermal and tensile behavior of poly(butylene adipate-co-terephthalate)/ poly(vinyl chloride) nanoblends. Polym. Bull. 71, 1483–1503. Huskić, M., Žigon, M., 2007. PMMA/Mt nanocomposites prepared by one-step in situ intercalative solution polymerization. Eur. Polym. J. 43, 4891–4897. Huskić, M., Žagar, E., Žigon, M., 2012. The influence of a quaternary ammonium salt and Mt on the in situ intercalative polymerization of PMMA. Eur. Polym. J. 48, 1555–1560. Jiang, W., Chen, S.H., Chen, Y., 2006. Nanocomposites from phenolic resin and various organo-modified montmorillonites: preparation and thermal stability. J. Appl. Polym. Sci. 102, 5336–5343. Kaushik, A., Singh, P., Verma, G., Rekha, 2010. Morphology, X-ray diffraction and mechanical properties of resol–montmorillonite clay composites. J. Thermoplast. Compos. Mater. 23, 79–97. Kaynak, C., Tasan, C.C., 2006. Effects of production parameters on the structure of resol type phenolic resin/layered silicate nanocomposites. Eur. Polym. J. 42, 1908–1921. Kiersnowski, A., Trelinska-Wlazlak, M., Dolega, J., Piglowski, J., 2006. One-pot synthesis of PMMA/montmorillonite nanocomposites. e-Polymers 072.
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Research paper
Montmorillonite–phenolic resin nanocomposites prepared by one-step in-situ intercalative polymerisation Miroslav Huskić a,b,⁎, Alojz Anžlovar a, Majda Žigon b,c a b c
National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia Polymer Technology College, Ozare 19, 2380 Slovenj Gradec, Slovenia Centre of Excellence for Polymer Materials and Technologies, Hajdrihova 19, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 9 September 2014 Accepted 10 September 2014 Available online 26 September 2014 Keywords: Clay Montmorillonite Nanocomposite Novolac Resol Glass transition
a b s t r a c t Montmorillonite (Mt) resol, and Mt–novolac nanocomposites were prepared by one-step in-situ intercalative polymerisation involving the simultaneous modification of the Mt with quaternary ammonium salts (QAS), polymerisation and polymer intercalation. The Mt–polymer nanocomposites were prepared using two types of QAS — with one long (C16) alkyl chain (cetyl trimethylammonium bromide) or with three medium-sized (C8) alkyl chains (tricaprylyl methylammonium chloride) — as well as without a QAS. Intercalated Mt–resol, and Mt– novolac nanocomposites were formed, as confirmed by XRD and STEM. The extent of the intercalation was higher in the Mt–resol nanocomposites and with the cetyl trimethylammonium bromide modified Mt. The presence of the Mt influenced the polymerisation process, which resulted in changes in the molecular structure of the novolac resin, an increased content of p–p linkages and a lower glass transition temperature of the resin. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Although the phenolic resins are the oldest commercially manufactured synthetic polymers their production is still growing and new fields of applications are arising. In many areas they are almost irreplaceable with other polymers, especially in fireproofing, electronic and thermal insulation. Nevertheless, better properties are still required, which might be obtained by the addition of nanoparticles such as clay minerals. Literature reports related to clay phenolic resin nanocomposites are rather scarce, especially compared to other clay duromer nanocomposites, i.e., clay epoxy nanocomposites. Like other Mt polymer nanocomposites, the Mt-phenolic resin nanocomposites were also prepared by either melt intercalation (Choi et al., 2000), solution intercalation (Zhang et al., 2010) or intercalative polymerisation (Jiang et al., 2006). The Mt–resol nanocomposites were synthesized by the intercalative polymerisation of phenol and formaldehyde in the presence of modified Mt (López et al., 2007) or prepared as a blend of modified Mt and pre-synthesized resin (Manfredi et al., 2007). Dispersion polymerisation has also been performed to produce clay polymer nanocomposites (CPN) from both resol and novolac. X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations showed that the Mt layers were more easily exfoliated or intercalated in ⁎ Corresponding author at: National Institute of Chemistry, POB 660, 1001 Ljubljana, Slovenia. Tel.: +386 1 4760 206; fax: +386 1 4760 420. E-mail address: [email protected] (M. Huskić).
http://dx.doi.org/10.1016/j.clay.2014.09.011 0169-1317/© 2014 Elsevier B.V. All rights reserved.
novolac than in resol. This was explained by the linear structure of the novolac, which enables its intercalation between the Mt layers (Wu et al., 2002). The type of Mt modifier also has a significant impact on the intercalation and thermal stability of Mt–phenolic resin nanocomposites. The best exfoliation was observed when the Mt was modified with octadecylamine, while the thermal stability was increased the most when the modifier was of an aromatic nature, e.g., benzyldimethylhexadecylammonium chloride, benzyltriethylammonium chloride and benzyldimethylphenylammonium chloride (Jiang et al., 2006). Increased thermal stability was also observed in Mt–phenolic nanocomposites prepared with modified Mt (López et al., 2007; Manfredi et al., 2008) or unmodified Na+-Mt (Manfredi et al., 2007, 2008). On the other hand, some authors report that neither Na+-Mt nor modified Mt improved the thermal stability (Zhang et al., 2010). The morphology of Mt–resol nanocomposites depends on various production parameters (different liquid resol-type phenolic resins, curing methods, modifying cations in Mt, etc.) (Kaynak and Tasan, 2006; López et al., 2012). The CPN prepared with modified Mt exhibited better mechanical properties than those with Na+-Mt. It was observed that the micro-void formation was the main problem when water-based resol resins were used. The highest strength and toughness were obtained in samples with only 0.5% Mt (Kaynak and Tasan, 2006), while the highest values of the tensile and flexural modules with only a very small concentration (b 1%) of Mt were confirmed in other reports (Tasan and Kaynak, 2009; Kaushik et al., 2010,).
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Wang et al. (2002 and 2004) synthesized Mt–resol, and Mt–novolac nanocomposites by using protonated Mt (H+-Mt) as an acid catalyst and nanofiller. The H+-Mt-resol nanocomposites exhibited higher glass-transition temperatures and impact strengths compared with the pure resol resin. No such data were provided for novolac. Generally, the use of modified Mt ensures the good intercalation of polymers between the aluminosilicate layers and changes their properties. On the other hand, the price of the final product is increased since the modified Mt is relatively expensive. Therefore, an alternative preparation method, such as a one-step, in situ intercalative polymerisation has been proposed for preparation of CPN with various polymers: polyurethane (Kiersnowski et al., 2006), poly(methyl methacrylate) (Huskić and Žigon, 2007), and polystyrene (Weickmann et al., 2010). A similar method has already been used to prepare Mt–novolac resin nanocomposites with dodecylamine and oxalic acid as the modifier (Pappas et al., 2005). However, this method cannot be used to prepare Mt–resol nanocomposites. In this article, the preparation of Mt–resol and Mt–novolac nanocomposites using one-step in situ intercalative polymerisation is described. This method involves the simultaneous modification of the Mt with a quaternary ammonium salt (QAS) and phenolic resin intercalation during the polymerisation. Two types of QAS were used: (i) with one long alkyl chain (cetyl trimethylammonium bromide — CTM) and (ii) with three medium-sized alkyl chains (tricaprylyl methylammonium chloride — TC). In addition, some nanocomposites were prepared without QAS and some by pre-swelling the Mt in a water/QAS mixture for 24 h. The catalytic effect of Mt on the polymerisation of phenol and formaldehyde as well as a Mt influence on the structure of phenolic resin has been reported for the first time. The characterization of CPN was performed using nuclear magnetic resonance (NMR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning transmission electron microscopy (STEM).
2. Experimental 2.1. Materials The phenol was kindly donated by Fenolit d.d., Slovenia. The Mt Nanofil 757 was kindly donated by Rockwood Additives. The formaldehyde (37%) and oxalic acid (p.a.) were purchased from Merck. The hexamethylenetetramine (99%) (HMTA), cetyl trimethylammonium bromide (N 98%) and tricaprylyl methylammonium chloride were purchased from Sigma-Aldrich.
2.3. Curing of phenolic resins: All the samples were pressed into pellets and cured in an oven at 180 °C for 5 min. Mt–novolac nanocomposites were mixed with hexamethylenetetramine in the ratio of 9:1 prior to curing. 2.4. Characterization: The XRD experiments were performed on an X-ray powder diffractometer PANalytical X'Pert PRO MPD, (CuKα1 radiation = 1.5406 Å) in 0.033° steps from 1.5° to 15°. The dispersion of Mt in the phenolic resin matrix was studied by scanning transmission electron microscopy (STEM) using a Zeiss Supra 35 VP microscope at an accelerating voltage of 15 kV. The samples were sectioned to a thickness of between 100 and 250 nm on the Ultramicrotome Leica Ultracut prior to the microscopy. The degradation was studied by simultaneous thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) on a Mettler Toledo TGA/DSC1 StarSystem, operating in an oxygen or nitrogen atmosphere, at a heating rate of 5 K min−1, with a sample size of ~15 mg and a temperature range between 40 and 1000 °C. The glass transition was determined on a Mettler Toledo DSC1 at a heating/cooling rate of 10 K min−1, with a sample size of ~15 mg and a temperature range between −50 and 100 °C. The results were obtained from the second cycle. 13 C NMR spectra were recorded using a Varian VXR-300 spectrometer in the solvent methanol-d4 at 25 °C with an acquisition time of 1.5 s and a delay time of 2 s. Typically, 16,000 scans were recorded. 3. Results and discussion The Mt was modified with QAS to determine the basal spacing of the modified Mt, which was later used for a comparison with the Mt– phenolic resin nanocomposites. The XRD spectra are shown in Fig. 1. The basal spacing of the Mt modified with tricaprylyl methylammonium chloride (TC Mt) is 1.73 nm, which is slightly higher than that of the Mt modified with cetyl trimethylammonium bromide (CTM Mt) (1.68 nm). 3.1. Mt–resol nanocomposites In the XRD diffractograms of the Mt–resol nanocomposites prepared with or without the QAS there are one to three Bragg diffractions of the first, second and third orders, indicating a layered structure (Fig. 2).
2.2. Synthesis Mt-novolac nanocomoposites were synthesized with a formaldehyde/ phenol (F/P) mole ratio of 0.9. First, 0.225 mol of formaldehyde (37% aqueous solution), 0.25 mol of phenol (90% aqueous solution) and 4 ml of oxalic acid (0.5 M) were placed in a round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The Mt was added in quantities of 1%, 3%, and 5%, according to the mass of the pure reactants (without water). The required quantity of QAS was calculated from the quantity and cation exchange capacity (CEC) of the Mt, which was 85 meq/100 g, and added to the solution. The reaction mixture was heated to reflux and held there for 3 h. The resins or CPN were separated by pouring the mixture into 250 ml of ice-cold demineralised water. The product was washed several times with water and dried in a vacuum at room temperature for 24 h. Finally, it was ground to a powder and dried for another 48 h. Mt–resol nanocomposites were synthesized with F/P mole ratios of 1.7 and 2.2 using the same procedure as described above, except that 4 ml of concentrated NaOH instead of oxalic acid was used as a catalyst.
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Fig. 1. XRD diffractograms of Mt modified with CTM and TC.
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Fig. 2. XRD diffractograms of the Mt–resol nanocomposites prepared: (A) without QAS, (B) with TC, and (C) with CTM.
Fig. 3. XRD diffractograms of TC Mt–resol nanocomposites prepared at F/P ratios of 1.7 and 2.2.
The positions of the diffractions depend on the method of preparation. In the XRD diffractogram of Mt–resol nanocomposite, prepared with Mt but without a QAS, the diffraction is at 2θ = 5.7°, which corresponds to a basal spacing of 1.55 nm. The result agrees well with previous reports (Choi et al., 2000; Kaynak and Tasan, 2006). The increase can be explained by the intercalation of polar resin between the Mt layers (Alexandre and Dubois, 2000). The diffractions of the CPN prepared with CTM Mt are at a lower angle than the diffractions of the CPN prepared with TC Mt, which is the opposite of the pure modified Mt (Fig. 1, Table 1). The same was observed for a polymethylmethacrylate (PMMA) based nanocomposites prepared using one-step, intercalative polymerisation (Huskić and Žigon, 2007). It appears that three shorter alkyl chains occupy most of the free volume between the aluminosilicate layers, which hinders the intercalation of the phenolic resin, resulting in a smaller basal spacing. The increase in the basal spacing compared to the CTM Mt is 1.75 nm and to the TC Mt is 0.92 nm (Table 1). The concentration of the Mt influences the intensities of the diffractions but not the position of the diffractions. The mole ratio between the formaldehyde (F) and phenol (P) influences the structure of the phenolic resin. When increasing the F/P ratio, the number of hydroxymethyl groups (\CH2OH) and therefore the polarity of the resin also increase. This increased polarity could be beneficial for enhanced interactions with Mt and consequently, a greater extent of intercalation. On the other hand, due to the condensation of \CH2OH groups during synthesis, the molecules can become larger, which could hinder the intercalation. Comparison of the XRD diffractograms of the CPN prepared using the F/P ratios 1.7 and 2.2 shows some differences in the intensity of the diffractions but no influence on the position and, consequently, on the intercalation was observed. The XRD diffractograms of the CPN prepared with the TC Mt are shown as an example in Fig. 3.
Pre-swelling the Mt in a water/QAS mixture for 24 h also did not influence the intercalation. This is an important feature showing that this method enables fast and straightforward production of Mt–phenolic resin nanocomposites. After curing, the XRD diffractions of the Mt–resol CPN were shifted slightly towards higher 2θ values. The basal spacings of the nanocomposites decreased for approximately 0.2 nm after curing. This is usually explained by a small deintercalation (Choi et al., 2000; Jiang et al., 2006), but it might be also a consequence of mass loss during the curing reaction. The results are summarised in Table 1. The TGA of Mt–resol nanocomposites was performed in a nitrogen and oxygen atmosphere. The mass loss below 300 °C, where curing reaction proceeds, is not significantly influenced by the quantity or type of Mt. The decomposition in air, above 300 °C, is also not influenced by the presence of modified or pristine Mt. Differences are observed in nitrogen atmosphere. Mass loss in CPN prepared with pristine Mt is only slightly smaller than in pure resol, indicating a catalytic effect of Mt. The results are shown in Fig. 4. For clarity, only the TGA curves of resol synthesized without Mt and with 5% Mt are shown. A strong catalytic effect, leading to higher mass loss than in pure resol is
Table 1 The basal spacing of pristine and modified Mt, and nanocomposites with resol, before and after curing. QAS
QAS Mt d (nm)
QAS Mt–resol d (nm)
Cured QAS Mt–resol d (nm)
Δd (nm)
/ CTM TC
0.97 1.68 1.73
1.55 3.43 2.65
1.55 3.22 2.45
0.0 −0.21 −0.20
Fig. 4. TGA curves of resol and CPN synthesized with 5% Mt, in oxygen and nitrogen atmosphere.
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Table 3 Glass transition temperatures (Tg) of novolac and Mt–novolac nanocomposites with respect to the type of QAS and Mt concentration. Concentration of Mt (%)a
Tg (°C) Without QAS
0 1 3 5 a b
Fig. 5. TGA curves of resol and CPN synthesized with 5% TC Mt, in oxygen and nitrogen atmosphere.
observed in CPN with modified Mt. TGA curves of nanocomposites with TC Mt are shown as examples in Fig. 5. The catalytic effect of Mt on various chemical reactions has been known and exploited in industry for decades (Adams and McCabe, 2006), while in decomposition of CPN it has been rarely mentioned (Witkowski et al., 2012; Hadj-Hamou et al., 2014). 3.2. Mt–novolac nanocomposites The novolac resin precipitates from the reaction mixture during synthesis. Without adding a QAS, the Mt remains dispersed in the aqueous phase. Consequently, novolac with a very small Mt content was obtained, as confirmed by the TGA in an oxygen atmosphere (≈ 0.1%). In addition, there were no diffractions of Mt in the XRD diffractogram of the product. CPN only form when the QAS is added to the reaction mixture. The interlayer spacings of the Mt layers in the Mt–novolac nanocomposites (Table 2) were slightly smaller than in the Mt–resol nanocomposites (Table 1). This can be explained either by the lower polarity of the novolac and consequently less intensive intercalation between the Mt layers, or by the linear structure of the macromolecules, which occupy less space than the branched resol. After curing the Mt–novolac nanocomposites, a reduction of the basal spacing is observed; however, the change is smaller than for the Mt–resol nanocomposites. Since there are no reactive hydroxymethyl groups in novolac intercalated between the Mt layers, the only explanation is a small deintercalation of novolac due to curing. HMTA, added to CPN as a curing agent, is only present outside the Mt layers, meaning that the curing also proceeds mainly outside the layers. Due to the high temperature, well above Tg, the novolac molecules are highly mobile and, at least in the beginning of curing, can slip out of the layers, while the cured ones are too big to enter. On the other hand, the decomposition products of HMTA are gasses and can also enter the Mt interlayer spaces and cure the novolac there.
24 18 14 3
CTM 21 20 19 15
b
TC 22b 17 19 17
Plus appropriate quantity of QAS. Polymerisation in the presence of 1% of QAS.
The thermal characterization of the prepared novolac and its CPN shows that the Mt decreases the Tg of the novolac resin. To the best of our knowledge, this effect has not yet been mentioned in the literature. The Tg of the novolac resin decreases with the increasing concentration of neat Mt in the reaction mixture. The decrease is reduced by the addition of the quaternary ammonium salt (Table 3). It seems that the reactive sites on the Mt, which influence the polymerisation, are screened by the QAS. The results are in contrast to those obtained for radical polymerisation, where the QAS, not the Mt, is responsible for the changes in the molecular structure of the synthesized PMMA (Huskić et al., 2012). The thermal stability of novolac synthesized in the presence of Mt is also reduced. The TGA was performed in a nitrogen and oxygen atmosphere and in both cases, thermal stability above 250 °C decreases with increasing the Mt content in the reaction mixture. The results are shown in Fig. 6. For clarity, only the TGA curves of novolac synthesized without Mt and with 5% Mt are shown. The results can be correlated to the Tg measurements. A small difference in thermal stability is observed between resin synthesized without Mt and that synthesized with 1% and 3% Mt, and a big difference when 5% of Mt is added to the reaction mixture. The TGA curves of CPN prepared with CTM Mt and TC Mt are similar to the curves in Fig. 6, except that the difference is smaller and the curves are slightly overlapping. Figures are shown in electronic supplementary information. The changes in Tg and thermal stability are ascribed to differences in the molecular structure of the novolac resin, which can be observed in their 13C NMR spectra. The phenolic rings in the novolac resin are bound by \CH2\ groups in the ortho–ortho, ortho–para or para–para position, whose chemical shifts are well separated at 31 ppm, 36 ppm, and 41 ppm, respectively (Sojka et al., 1979). The intensity ratio of those signals changed from 0.99: 1.96: 1.0 in the novolac synthesized without Mt, to 0.71: 1.62: 1.0, when 5% Mt was added to the reaction
Table 2 The influence of the type of QAS on the interlayer spacing of the modified Mt in Mt– novolac nanocomposites. QAS
QAS Mt d (nm)
QAS Mt–novolac d (nm)
Cured QAS Mt–novolac d (nm)
Δd (nm)
CTM TC
1.68 1.73
3.34 2.45
3.31 2.37
−0.03 −0.08
Fig. 6. TGA curves of novolac synthesized without Mt and in the presence of 5% Mt.
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Fig. 7. 13C NMR spectra of novolac synthesized without Mt and in the presence of Mt.
mixture (Fig. 7). The results show that the addition of Mt to the reaction mixture favours p–p novolac methylene bridge formation and lowers the content of o–o and especially o–p linkages. 3.3. The morphology of the CPN Although intercalation is observed by XRD in Mt–resol nanocomposites prepared with and without the addition of QAS, a different morphology of both CPN was confirmed by STEM. A STEM micrograph of a CPN with pristine Mt shows holes within the polymer matrix, indicating that there is practically no interaction between the polymer matrix and the Mt. Therefore, the Mt particles fell out of the polymer matrix during the electron microscopy measurement (Fig. 8, A and B). In contrast to this, when the QAS was added during synthesis, the particles were nicely embedded in the phenolic matrix (Fig. 8, C), for the resol as well as the novolac. The separated, smaller stacks of Mt can be clearly observed, indicating the partial intercalation of the phenolic matrix in the Mt structure as a consequence of an enhanced interaction between these two components. 4. Conclusions One-step in-situ intercalative polymerisation is a simple way to prepare Mt–phenolic resin nanocomposites based on either resol or novolac. The modification of the Mt, the intercalation and polymerisation proceed simultaneously, which leads to the formation of intercalated CPN. The variations in the mole ratio of phenol to formaldehyde (1:1.7
and 1:2.2) did not influence the extent of intercalation of the Mt–resol nanocomposites. Two types of quaternary ammonium salts were used as Mt modifiers: one with a single long C16 alkyl chain (cetyl trimethylammonium bromide — CTM) and one with three medium-sized C8 alkyl chains (tricaprylyl methylammonium chloride — TC). A better intercalation was observed in the CPN prepared with CTM, which exhibited a larger interlayer spacing, as determined by the XRD. The more polar nature of the resol also favours the intercalation. On the other hand, a decrease in the basal spacing was observed during the curing of both the resol and the novolac resins. A small increase in the interlayer spacing of the Mt in the resol was also observed when no quaternary ammonium salts (QAS) were used. A STEM analysis showed that in this case there were almost no interactions between the pristine Mt particles and the phenolic resin. Contrary to that, the QAS enhance the compatibility between the Mt and the phenolic resin, and the formation of the intercalated CPN was confirmed by STEM. Mt influences the mechanism of the polymerisation reaction in an acid medium, producing novolac with increased p–p linking and a lowered glass transition temperature. Acknowledgements This research was supported by the Slovenian Research Agency (Program P2-0145). The authors acknowledge the support from CE PoliMaT (financially supported by the Ministry of Education, Science
Fig. 8. STEM micrograph of cured resol resin with pristine Mt (A, B) and with CTM Mt (C).
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