- Email: [email protected]
Elastomeric epoxy nanocomposites: Nanostructure and properties
Composites Science and Technology 72 (2012) 640–646
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Elastomeric epoxy nanocomposites: Nanostructure and properties M. Lipinska a, J.M. Hutchinson b,⇑ a b
Institute of Polymer and Dye Technology, Technical University of Lodz, Stefanowskiego 12/16, 90-924 Lodz, Poland Centre for Research in NanoEngineering (CRNE) and Departament de Màquines i Motors Tèrmics, ETSEIAT, Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
a r t i c l e
i n f o
Article history: Received 6 October 2011 Received in revised form 12 January 2012 Accepted 15 January 2012 Available online 24 January 2012 Keywords: A. Nanoclays A. Nanocomposites B. Mechanical properties Exfoliation D. Transmission electron microscopy (TEM)
a b s t r a c t In polymer layered silicate nanocomposites, significant differences have been reported between the effects of the nano-reinforcement on rigid and elastomeric nanocomposites. In this paper, we have studied elastomeric nanocomposites based upon DGEBA epoxy resin filled with montmorillonite (MMT) and cured with a long-chain polyoxypropylene diamine, for comparison with analogous rigid nanocomposites. Ultrasonic mixing was used to disperse the MMT in the matrix to improve homogeneity and decrease the agglomerate size. Two different methods of nanocomposite preparation were used in which the MMT was first swollen with either the curing agent or the epoxy before the addition of, respectively, DGEBA or diamine. A better dispersion of the nanoclay in the matrix and a greater amount of intercalation occurred when the MMT was first swollen with the diamine. The effect of MMT concentrations up to 8 wt.% on the mechanical behaviour of the epoxy/MMT nanocomposites was investigated. It was found that the addition of MMT increased the tensile strength and modulus, although SAXS and TEM indicated that a significant fraction of the clay layers were not exfoliated. Nevertheless, the addition of the clay resulted in changes in the fracture surfaces, as indicated by SEM, consistent with the tensile results and indicative of toughening. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polymer layered silicate (PLS) nanocomposites based on thermosetting polymers are of interest as a new class of materials characterised by improved mechanical, thermal and physico-chemical properties. Such nanocomposites are generally obtained by first mixing the resin monomer with the organically modified clay, in the form of stacks of lamellae of thickness about 1 nm and several hundred nanometres long and wide [1,2]. After curing, the final properties of the nanocomposite are significantly influenced by the dispersion of the clay particles in the polymer matrix and by the degree of exfoliation of the clay layers. In fully exfoliated PLS nanocomposites, an ideal nanostructure that is in general difficult to achieve, the clay lamellae would be completely separated and uniformly distributed throughout the polymer matrix, and the property enhancement would be optimised. For epoxy-based PLS nanocomposites, the structure of the three-dimensional network formed can be significantly different, depending on the type of resin, curing agent and curing conditions used, which has an influence on the properties and performance of the cured materials [3]. Furthermore, by curing the epoxy with cross-linking agents of increased chain length, it is possible to
⇑ Corresponding author. Tel.: +34 93 739 8123; fax: +34 93 739 8101. E-mail addresses: [email protected] (M. Lipinska), [email protected]. edu (J.M. Hutchinson). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2012.01.012
obtain nanocomposites with rubber-like rather than rigid properties [4–8]. It has become clear that, in the case of rigid epoxy PLS nanocomposites, the procedure involved in the nanocomposite preparation and the curing conditions strongly influence the exfoliation of the clay layers [9–12]. For significant property enhancement in the nanocomposite materials, it is essential to disperse the clay homogeneously and then exfoliate the individual clay platelets. Unfortunately, in general it still remains a problem to achieve well-dispersed MMT particles in the epoxy matrix. Numerous methods, such as mechanical, ultrasonic and solvent mixing have been applied by many authors in an attempt to obtain a homogeneous dispersion of MMT in the nanocomposite [13–15]. Besides the effect of these different mixing methods, the procedure used for the preparation of elastomeric epoxy/organoclay nanocomposites in particular also plays an important role. One procedure is to swell the clay in the epoxy resin before adding the cross-linking agent and curing the system, while another is to add the clay directly to the mixture of epoxy and cross-linking agent [4,5]. As a further alternative, Boukerrou et al. [7] proposed the swelling of the MMT in the curing agent before the addition of the desired amount of the epoxy resin. This method of preparation resulted in a better dispersion of the clay and a greater extent of intercalation (larger d-spacing) of the curing agent in the MMT compared with that obtained by swelling the clay in the resin. The montmorillonite concentration is another important parameter to consider. In the case of elastomeric epoxy/clay
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
nanocomposites [4–8], the modulus increases rather linearly with clay content, and the enhancement is much more pronounced in comparison with rigid epoxy nanocomposites. Thus the objective of nano-reinforcement is achieved in these elastomeric PLS nanocomposites while it is generally not achieved in their rigid epoxy counterparts. There are many factors which could be important here, a principal one being the dispersion of the nanoclay in the composite, which is directly influenced by the nanocomposite preparation method. The objective of the present study was therefore to examine the effect of the preparation method on the nanostructure and properties of elastomeric epoxy nanocomposites, with a view subsequently to applying an improved understanding of the preparation–property relationship to achieve an enhanced performance in rigid epoxy PLS nanocomposites. 2. Experimental 2.1. Materials The epoxy resin was DGEBA (DER331, Dow Chemical Company) and the curing agent was a polyetheramine (Jeffamine D-2000, Huntsman Corp.). A stoichiometric ratio of DGEBA to Jeffamine D-2000 was used to prepare the nanocomposites. The nanoclay was octadecyl onium ion modified montmorillonite (Nanomer I.30E, Nanocor Inc.), with a cation exchange capacity (CEC) of 92 meq/100 g [16]. Clay contents from 2 wt.% to 8 wt.% were used.
641
Advanced (Consulting de Imagen Digital, S.L., Micron Spain, S.A.) for the five images taken for every sample. In all cases, the particle size was quantified in terms of an equivalent diameter, Deq, being the diameter of a circular particle of the same area. 2.2.3. Preparation and curing of epoxy composites After mixing the clay with the resin or with the Jeffamine D2000, the appropriate amount of the second reagent for the stoichiometric ratio of DGEBA to Jeffamine D-2000 was added to the clay mixture. The clay–amine–epoxide complex was stirred for 2 min at room temperature and then degassed under vacuum for 10 min to eliminate air bubbles before curing. The bubble-free mixture was then cast into a PTFE mould of dimensions approximately 200 mm by 300 mm, placed in an air circulating oven and subjected to two curing stages, first curing at 75 °C for 3 h and then post-curing at 125 °C for an additional 3 h. 2.2.4. X-ray diffraction (XRD) The intercalation of the Jeffamine D-2000 and/or the epoxy resin into the clay galleries was measured by small angle X-ray scattering (SAXS). X-ray diagrams were recorded on film under vacuum using a modified Statton camera (W.R. Warhus, Wilmington, DE). For the cured nanocomposites, a Bruker model D8 Advanced diffractometer with Bragg–Brentano geometry in reflection mode was also used; the measurements were taken in a range of 2h = 1–10° and were made on small samples cut from cast sheets with copper Ka radiation and scanning at 0.05°/s.
2.2. Methods 2.2.1. Methods of preparation of clay mixtures Two different preparation methods were used: one in which the MMT was first mixed with the epoxy before the addition of the curing agent, and the other in which the MMT was first mixed with the curing agent before the addition of the epoxy resin. For these purposes, the amount of either resin or curing agent was mixed directly with the amount of clay necessary to obtain the desired clay content, which was selected as 2, 4, 6 or 8 wt.%. The mixing was done at room temperature, either by hand or using an ultrasonic bath (Branson DTH-3510) with a heating power of 205 W; in the latter case, cooling was applied and different periods were used depending on the clay content: 2 wt.% clay for 2 h, 4 wt.% clay for 3 h, 6 and 8 wt.% clay for 4 h. 2.2.2. Analysis of the dispersion The dispersion of the clay in the resin and in the Jeffamine D2000 following the different mixing methods was observed using a Leica polarising transmission optical microscope. A drop of each mixture was placed on a glass slide covered with a slip, and the images were captured digitally with a Canon PhotoS camera. Particle size analysis was made using the computer program Mip 4
2.2.5. Mechanical tests The mechanical measurements were made using a universal testing machine, MTS Adamel Lhomargy DY-34, with load cell of 100 N. The crosshead displacement rate was set at 5 mm/min. The tensile strength and the elongation at break were measured at room temperature according to DIN 53504 and ASTM D638 standards. The specimens used for tensile testing were dog-bone shaped bars with a parallel region of rectangular cross-section with length 35 mm, width 6 mm, and thickness about 2 mm, this last being determined by the thickness of the cast sheet. Specimens were stamped directly from the cast sheet using a stamp of appropriate dimensions. At least five tests were performed for each of the clay contents used and for each method of preparation. 2.2.6. Scanning and transmission electron microscopy (SEM, TEM) Samples for SEM were cut from the fractured specimens after tensile testing. The fracture surfaces were coated with Au–Pd alloy and were investigated using a scanning electron microscope (Jeol 5610). Samples for TEM were prepared by ultra-microtomy under cryostatic conditions, giving sections of thickness approximately 50 nm. These sections were examined in a Jeol-2011 HRTEM electron microscope with an accelerating voltage of 200 kV.
Fig. 1. SAXS diffraction patterns for: (a) organically modified clay; (b) epoxy/clay, mechanically mixed, 10 wt.% MMT; (c) Jeffamine D-2000/clay, 2 wt.% MMT.
642
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
Fig. 2. Polarised optical micrographs of the dispersion of MMT (6 wt.% loading) in: (a) DER331, hand mixing; (b) Jeffamine D-2000, hand mixing; (c) Jeffamine D-2000, ultrasonic bath for 4 h. The scale bar is 100 lm.
Table 1 Number percentages of particles within the specified ranges of Deq (lm) for the Jeffamine D-2000/MMT and DER331/MMT mixtures with different wt.% MMT, either hand mixed (HM) or ultrasonically mixed (UM). The figures in bold relate to the SAXS results in Fig. 3. 2 wt.% HM
4 wt.% UM
HM
6 wt.%
8 wt.%
UM
HM
UM
HM
UM
Jeffamine D-2000/MMT mixtures 76.2 68.8 64.3 Deq 6 20 50 < Deq 2.6 2.7 5.5
76.2 1.5
62.3 9.0
79.1 2.3
47.3 14.5
80.6 2.3
DER331/MMT mixtures Deq 6 20 35.7 64.3 50 < Deq 20.6 10.8
66.6 8.1
36.2 30.8
38.1 28.9
34.2 30.0
28.3 36.5
30.7 28.6
Fig. 4. SAXS results for cured elastomeric nanocomposite samples: M1, DER331/ 6MMT/D-2000; M2 = M1 + 4hUS; M3, D-2000/6MMT/DER331; M4 = M3 + 4hUS; M5, DER331/8MMT/D-2000; M6 = M5 + 4hUS; M7, D-2000/8MMT/DER331; M8 = M7 + 4hUS.
Fig. 3. SAXS scattering patterns and densitometry traces obtained using the Statton camera for: (a) D-2000/2MMT/DER331/2HUS; (b) DER331/6MMT/D-2000.
3. Results and discussion 3.1. Intercalation Fig. 1 shows the SAXS diffraction patterns for the octadecyl onium ion modified clay before intercalation (Fig. 1a), after intercalation of epoxy resin (Fig. 1b), and after intercalation of Jeffamine D-2000 diamine (Fig. 1c). The diffraction pattern for the diamine/ clay mixture (Fig. 1c) shows a very strong and very broad scattering peak at 5.38 nm together with a peak of medium strength at 1.50 nm and the outermost ring corresponding to a d-spacing of 0.43 nm within the clay layers. The peak at 5.38 nm should be compared with the d-spacings of 2.09 nm for the modified clay
(Fig. 1a) and 3.75 nm for the intercalated epoxy (Fig. 1b), from which it can be seen that for the Jeffamine D-2000/MMT mixture, even though mixed only by hand, the 5.38 nm d-spacing is not only much higher than that for the intercalated epoxy, but also greater than the theoretical maximum spacing of 3.74 nm calculated for the vertically oriented octadecyl cation [17]. Furthermore, the appearance of an additional scattering peak at 1.50 nm, even though less strong, indicates the existence of a bimodal distribution of layer spacings when the diamine is intercalated into the clay galleries, one spacing being much larger than expected and the other being smaller than the layer spacing in the modified clay alone. This has implications for the subsequent exfoliation of the clay in the cured nanocomposite, as will be seen later in the TEM micrographs. 3.2. Dispersion To obtain significant property enhancement in the epoxy nanocomposite materials, it is essential to disperse the clay homo-
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
643
Fig. 5. TEM micrographs of sample D-2000/2MMT/DER331/2hUS at magnifications of (a) 600 and (b) 800.
Fig. 6. TEM micrographs of sample D-2000/2MMT/DER331/2hUS; regions of particle shown in Fig. 6b at magnifications: (a) 30,000, (b) and (c) 60,000.
geneously in the resin (or curing agent) prior to adding the crosslinking agent (or resin) and then to exfoliate the individual clay layers during the cure of the system. The dispersion of the clay depends strongly on the mixture preparation method, and it is noticeably better for the MMT mixtures prepared by swelling the clay in the Jeffamine D-2000 first, as can be seen by comparing Fig. 2a and b for mixtures containing 6 wt.% MMT and mixed by hand. Table 1 gives the percentages of the smallest (620 lm) and largest (>50 lm) particles in the mixtures prepared by mixing the clay first either with Jeffamine D-2000 or with epoxy resin. Two important conclusions can be drawn from these results: first, that mixing the clay with the diamine gives a significantly better dispersion compared with mixing the clay with the epoxy resin, whether mixed by hand or ultrasonically; and second, that ultrasonic mixing significantly improves the dispersion for the diamine/MMT mixtures, and particularly so when the clay content is higher. A typical illustration of the dispersion after ultrasonically assisted mixing is shown in Fig. 2c for a 6 wt.% loading of MMT in Jeffamine D-2000, for comparison with Fig. 2a and b. 3.3. X-ray scattering of cured nanocomposites
Fig. 7. TEM micrograph of sample D-2000/2MMT/DER331/2hUS, showing the existence of a bimodal distribution of clay layer spacings. The arrows indicate the layer spacings of about 1.5 nm.
The nanostructure of the cured nanocomposites can in part be assessed by SAXS, and this technique is used to examine the effect of the different preparation procedures discussed in the preceding section. Scattering patterns were obtained, and are shown in Fig. 3, for two samples which originally had very different clay dispersions: (a) D-2000/2MMT/DER331/2HUS, with 2 wt.% MMT and mixed first with the diamine using ultrasound for 2 h, has a good dispersion (see Table 1, figures in bold); (b) DER331/6MMT/
D-2000, with 6 wt.% MMT and mixed first with the epoxy and without ultrasound, has a very poor dispersion (see Table 1, figures in bold). It transpires that, despite the very different qualities of the original dispersions for the two samples, the cured nanocomposites appear from these SAXS results to have similar nanostructures: in particular, both show a rather broad range of clay layer spacings which give rise to significant scattering, and indicate that exfoliation is not complete in these systems.
644
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
Fig. 8. Tensile modulus (a) and tensile strength (b) as a function of clay content (wt.%) for the elastomeric nanocomposites prepared in different ways. The dashed lines represent linear fits to the data for DER331/MMT/D-2000 and D-2000/MMT/DER331/US, and passing through the point at zero MMT content.
A more quantitative study was made of cured samples with the highest clay contents of 6 wt.% MMT (samples M1–M4) and 8 wt.% MMT (samples M5–M8), prepared by mixing first with the epoxy resin (M1 and M5 without ultrasound (US), M2 and M6 with US), or with the diamine (M3 and M7 without US, M4 and M8 with US). The results are presented in Fig. 4. Here it can be seen that the scattering curves separate into two groups, one for samples M1–M4, for which there is a minimum in the scattering at very low angles, and the other for samples M5–M8, for which there is no minimum, implying that there is somewhat better exfoliation for the smaller clay content. Nevertheless, for all the samples there is significant scattering over the range of 2h from about 1.5° to 4.5°, corresponding to d-spacings of about 2.0–6.0 nm, which is clear evidence that the exfoliation is far from complete. Further confirmation of this aspect of the nanostructure can be seen in the TEM micrographs, presented below. 3.4. Transmission electron microscopy Typical micrographs at low magnification are shown in Fig. 5, for a sample prepared by mixing the clay (2 wt.% MMT) first with the diamine, using the ultrasonic bath for 2 h, before adding the epoxy resin and curing the nanocomposite. In Fig. 5a it can be seen that in the cured nanocomposite there remain some particles with sizes between 1 and 10 lm, though there was no evidence of any particles of greater size. This means that the particles of sizes greater than 20 lm which existed in the diamine–clay mixture before the addition of the epoxy resin and the cure process, a number percentage of approximately 30% (see Table 1), have been broken down during cure. One of the largest particles observed in the microtomed section examined here, shown in Fig. 5b, has a length of about 10 lm and a width of about 5 lm, thus an equivalent diameter somewhat less than 10 lm. There is therefore considerable nanostructural modification taking place during cure, including some exfoliation of the clay layers, as can be seen at higher magnifications of this same particle. In one particular region of the particle shown in Fig. 5b, for example, there is clear evidence, at higher magnification, of stacks of layers which have not been exfoliated and of substantial exfoliation, as can be seen in Fig. 6a. Indeed, in some regions the number of stacked layers is very large, as illustrated in Fig. 6b, these particular layers having a d-spacing of the order of 4 nm, within the range identified by SAXS. On the other hand, at the edges of the regions of layer stacking one can identify the extensive delamination and redistribution that is characteristic of exfoliation, with the separation between layers being as large as 50 nm, as shown in Fig. 6c.
Fig. 9. Plastic deformation energy per unit volume as a function of clay content (wt.%) for the elastomeric nanocomposites prepared in different ways. The dashed line is drawn to guide the eye.
A feature of the micrographs at higher magnification is the appearance, within the rather ordered stacks of clay layers, of a bimodal distribution of d-spacings. This is well illustrated in Fig. 7, where layer spacings of about 1.5 nm can be seen rather regularly interspersed within a larger layer spacing of about 5.4 nm. These dspacings correspond very closely to those found by SAXS of the diamine–clay mixture before the addition of the epoxy resin, as shown in Fig. 1c. The implication is that these regions of the nanocomposite remain unaltered from the diamine–swollen clay state, in other words that the epoxy resin has not penetrated to these regions, and no cure reaction has taken place here. This is presumably what occurs within the largest agglomerations of the original clay dispersion, and emphasises the importance of achieving a good dispersion if an exfoliated nanocomposite is to be obtained. 3.5. Mechanical properties Stress–strain curves for elastomeric nanocomposites containing 0, 2, 4, 6 and 8 wt.% MMT and prepared by the different methods discussed earlier were obtained by tensile testing. The values of modulus and tensile strength were determined for each of the at least 5 samples that were tested for each preparation condition, and the average values are shown in Fig. 8a and b, respectively, as a function of the clay content, where an approximately linear increase is observed in both cases. There is clear evidence of an increase in both modulus and tensile strength when ultrasonic mixing is used and, to a lesser extent, when the MMT is mixed first
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
645
Fig. 10. SEM micrographs of the fracture surface after tensile tests for nanocomposites prepared by mixing MMT first with DER331, and containing the following MMT contents: (a) 2 wt.%; (b) 8 wt.%.
with the diamine. Similar results and conclusions have been reported by several other workers concerning the effects of preparation method and dispersion, both for the same elastomeric system as studied here [4,8,18] as well as more generally in epoxy–clay nanocomposites, including rigid systems [e.g. 19–21]. The importance of the preparation method in the development of the optimum properties of layered silicate nanocomposites cannot be overstated. The work of fracture for the present elastomeric nanocomposites can be calculated from the area beneath the stress–strain curves, and is found to be slightly greater for the nanocomposites prepared by mixing the clay first with the diamine and to increase as the clay content increases; the increase is a factor of about three when the clay content increases from 0 to 8 wt.%. A large part of this work of fracture is stored as elastic strain energy per unit volume, according to the formula r2/2E, where r is the tensile strength and E is the tensile modulus; this elastic strain energy is recovered after fracture. The dissipated (plastic) energy is the difference between the total work of fracture and the elastic strain energy at fracture, and is a measure of the toughness of the material. Fig. 9 shows the dependence of the plastic deformation energy per unit volume as a function of the clay content for the nanocomposites prepared using different methods. A linear increase with the clay content is clearly seen, the increase being even more significant, a factor of approximately five for 8 wt.% clay, than the factor of approximately three that was found for the work of fracture. The implication is that the presence of the clay has an important and significant effect on the fracture behaviour of these elastomeric nanocomposites. This result is further supported by SEM micrographs of fracture surfaces obtained after the tensile tests, shown in Fig. 10. A significant increase in fracture surface roughness can be identified as the clay content increases from 2 to 8 wt.%, consistent with the tensile strength and plastic deformation energy results and indicating a toughening effect of the clay. 4. Conclusions Elastomeric epoxy–clay nanocomposites have been prepared with diglycidyl ether of bisphenol-A epoxy resin cross-linked with a long chain diamine, Jeffamine D-2000, and filled with an organically modified montmorillonite, MMT. It has been shown that the preparation method, in particular whether the clay was mixed first with the epoxy resin or with the diamine and whether or not ultrasonic mixing was used, as well as the clay content, up to 8 wt.%, both have a significant influence on the properties of the nanocomposite. The tensile modulus, tensile strength and the plastic deformation energy all increase with increasing clay content, while higher values of all these properties are found when the clay
is first mixed with the diamine and when ultrasonic mixing is used. The latter effect is attributed to an improvement in the dispersion of the clay, which has been demonstrated by particle size analysis of optical micrographs of the resin–clay or diamine–clay mixtures. The increase in plastic deformation energy correlates with a significant increase in the roughness of the fracture surfaces after tensile testing, as identified by SEM. The nanostructure of the cured nanocomposites, identified by SAXS and TEM, shows that substantial nanostructural modification takes place during cure, with a significant amount of exfoliation occurring. Nevertheless, there remain regions in which layer stacking persists, even after cure, with the same nanostructure as in the resin–clay or diamine–clay mixture before cure. These results, obtained for an elastomeric epoxy nanocomposite, may be interpreted also to indicate the importance, for rigid epoxy nanocomposites, of the preparation and curing procedure. Acknowledgements The authors are grateful to Nordmann–Rassmann GmbH for the provision of the modified clay, and to the Dow Chemical Company and the Huntsman Corporation for the provision of the epoxy resins and curing agents, respectively. This work was supported by the Spanish Ministry of Education and Science, Projects MAT 2008-06284-C03-03 and MAT2011-27039-C03-03. Magda Lipinska acknowledges a mobility grant from the Spanish Ministry of Education and Science (Ref. SB2005-0145). References [1] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mat Sci Eng 2000;28:1–63. [2] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641. [3] Becker O, Simon GP. Epoxy layered silicate nanocomposites. Adv Polym Sci 2005;179:29–82. [4] Lan T, Pinnavaia TJ. Clay-reinforced epoxy nanocomposites. Chem Mater 1994;6:2216–9. [5] Wang Z, Pinnavaia TJ. Hybrid organic–inorganic nanocomposites: exfoliation of magadiite nanolayers in an elastomeric epoxy polymer. Chem Mater 1998;10:1820–6. [6] Wang Z, Pinnavaia TJ. Nanolayer reinforcement of elastomeric polyurethane. Chem Mater 1998;10:3769–71. [7] Boukerrou A, Duchet J, Fellahi S, Kaci M, Sautereau H. Morphology and mechanical and viscoelastic properties of rubbery epoxy/organoclay montmorillonite nanocomposites. J Appl Polym Sci 2007;103:3547–52. [8] Ngo TD, Ton-That MT, Hoa SV, Cole KC. Reinforcing effect of organoclay in rubbery and glassy epoxy resins, Part 1: dispersion and properties. J Appl Polym Sci 2008;107:1154–62. [9] Hutchinson JM, Montserrat S, Román F, Cortés P, Campos L. Intercalation of epoxy resin in organically modified montmorillonite. J Appl Polym Sci 2006;102:3751–63. [10] Román F, Montserrat S, Hutchinson JM. On the effect of montmorillonite in the curing reaction of epoxy nanocomposites. J Therm Anal Calorim 2007;87:113–8.
646
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
[11] Hutchinson JM, Montserrat S, Román F. Nanostructure development and cure kinetics in epoxy-based PLS nanocomposites. J Nanostruct Polym Nanocompos 2008;4:13–20. [12] Montserrat S, Román F, Hutchinson JM, Campos L. Analysis of the cure of epoxy based layered silicate nanocomposites: reaction kinetics and nanostructure development. J Appl Polym Sci 2008;108:923–38. [13] Ton-That MT, Ngo TD, Ding P, Fang G, Cole KC, Hoa SV. Epoxy nanocomposites: analysis and kinetics of cure. Polym Eng Sci 2004;44:1132–41. [14] Miyagawa H, Rich MJ, Drzal LT. Amine-cured epoxy/clay nanocomposites. 1. processing and chemical characterization. J Polym Sci Part B Polym Phys 2004;42:4384–90. [15] Wang K, Chen L, Wu JS, Toh ML, He CB, Yee AF. Epoxy nanocomposites with highly exfoliated clay: mechanical properties and fracture mechanisms. Macromolecules 2005;38:788–800.
[16] Ratna D, Manoj NR, Varley R, Raman Singh RK, Simon GP. Clay-reinforced epoxy nanocomposites. Polym Int 2003;52:1403–7. [17] Lan T, Kaviratna PD, Pinnavaia TJ. Mechanism of clay tactoid exfoliation in epoxy–clay nanocomposites. Chem Mater 1995;7:2144–50. [18] Ngo TD, Ton-That MT, Hoa SV, Cole KC. Effect of temperature, duration and speed of pre-mixing on the dispersion of clay/epoxy nanocomposites. Comp Sci Tech 2009;69:1831. [19] Tolle T Benson, Anderson DP. Morphology development in layered silicate thermoset nanocomposites. Comp Sci Technol 2002;62:1033–41. [20] Dean D, Walker R, Theodore M, Hampton E, Nyairo E. Chemorheology and propertiesofepoxy/layeredsilicatenanocomposites.Polymer2005;46:3014–21. [21] Pustkova P, Hutchinson JM, Román F, Montserrat S. Homopolymerization effects in polymer layered silicate nanocomposites based upon epoxy resin: implications for exfoliation. J Appl Polym Sci 2009;114:1040–7.
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Elastomeric epoxy nanocomposites: Nanostructure and properties M. Lipinska a, J.M. Hutchinson b,⇑ a b
Institute of Polymer and Dye Technology, Technical University of Lodz, Stefanowskiego 12/16, 90-924 Lodz, Poland Centre for Research in NanoEngineering (CRNE) and Departament de Màquines i Motors Tèrmics, ETSEIAT, Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
a r t i c l e
i n f o
Article history: Received 6 October 2011 Received in revised form 12 January 2012 Accepted 15 January 2012 Available online 24 January 2012 Keywords: A. Nanoclays A. Nanocomposites B. Mechanical properties Exfoliation D. Transmission electron microscopy (TEM)
a b s t r a c t In polymer layered silicate nanocomposites, significant differences have been reported between the effects of the nano-reinforcement on rigid and elastomeric nanocomposites. In this paper, we have studied elastomeric nanocomposites based upon DGEBA epoxy resin filled with montmorillonite (MMT) and cured with a long-chain polyoxypropylene diamine, for comparison with analogous rigid nanocomposites. Ultrasonic mixing was used to disperse the MMT in the matrix to improve homogeneity and decrease the agglomerate size. Two different methods of nanocomposite preparation were used in which the MMT was first swollen with either the curing agent or the epoxy before the addition of, respectively, DGEBA or diamine. A better dispersion of the nanoclay in the matrix and a greater amount of intercalation occurred when the MMT was first swollen with the diamine. The effect of MMT concentrations up to 8 wt.% on the mechanical behaviour of the epoxy/MMT nanocomposites was investigated. It was found that the addition of MMT increased the tensile strength and modulus, although SAXS and TEM indicated that a significant fraction of the clay layers were not exfoliated. Nevertheless, the addition of the clay resulted in changes in the fracture surfaces, as indicated by SEM, consistent with the tensile results and indicative of toughening. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polymer layered silicate (PLS) nanocomposites based on thermosetting polymers are of interest as a new class of materials characterised by improved mechanical, thermal and physico-chemical properties. Such nanocomposites are generally obtained by first mixing the resin monomer with the organically modified clay, in the form of stacks of lamellae of thickness about 1 nm and several hundred nanometres long and wide [1,2]. After curing, the final properties of the nanocomposite are significantly influenced by the dispersion of the clay particles in the polymer matrix and by the degree of exfoliation of the clay layers. In fully exfoliated PLS nanocomposites, an ideal nanostructure that is in general difficult to achieve, the clay lamellae would be completely separated and uniformly distributed throughout the polymer matrix, and the property enhancement would be optimised. For epoxy-based PLS nanocomposites, the structure of the three-dimensional network formed can be significantly different, depending on the type of resin, curing agent and curing conditions used, which has an influence on the properties and performance of the cured materials [3]. Furthermore, by curing the epoxy with cross-linking agents of increased chain length, it is possible to
⇑ Corresponding author. Tel.: +34 93 739 8123; fax: +34 93 739 8101. E-mail addresses: [email protected] (M. Lipinska), [email protected]. edu (J.M. Hutchinson). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2012.01.012
obtain nanocomposites with rubber-like rather than rigid properties [4–8]. It has become clear that, in the case of rigid epoxy PLS nanocomposites, the procedure involved in the nanocomposite preparation and the curing conditions strongly influence the exfoliation of the clay layers [9–12]. For significant property enhancement in the nanocomposite materials, it is essential to disperse the clay homogeneously and then exfoliate the individual clay platelets. Unfortunately, in general it still remains a problem to achieve well-dispersed MMT particles in the epoxy matrix. Numerous methods, such as mechanical, ultrasonic and solvent mixing have been applied by many authors in an attempt to obtain a homogeneous dispersion of MMT in the nanocomposite [13–15]. Besides the effect of these different mixing methods, the procedure used for the preparation of elastomeric epoxy/organoclay nanocomposites in particular also plays an important role. One procedure is to swell the clay in the epoxy resin before adding the cross-linking agent and curing the system, while another is to add the clay directly to the mixture of epoxy and cross-linking agent [4,5]. As a further alternative, Boukerrou et al. [7] proposed the swelling of the MMT in the curing agent before the addition of the desired amount of the epoxy resin. This method of preparation resulted in a better dispersion of the clay and a greater extent of intercalation (larger d-spacing) of the curing agent in the MMT compared with that obtained by swelling the clay in the resin. The montmorillonite concentration is another important parameter to consider. In the case of elastomeric epoxy/clay
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
nanocomposites [4–8], the modulus increases rather linearly with clay content, and the enhancement is much more pronounced in comparison with rigid epoxy nanocomposites. Thus the objective of nano-reinforcement is achieved in these elastomeric PLS nanocomposites while it is generally not achieved in their rigid epoxy counterparts. There are many factors which could be important here, a principal one being the dispersion of the nanoclay in the composite, which is directly influenced by the nanocomposite preparation method. The objective of the present study was therefore to examine the effect of the preparation method on the nanostructure and properties of elastomeric epoxy nanocomposites, with a view subsequently to applying an improved understanding of the preparation–property relationship to achieve an enhanced performance in rigid epoxy PLS nanocomposites. 2. Experimental 2.1. Materials The epoxy resin was DGEBA (DER331, Dow Chemical Company) and the curing agent was a polyetheramine (Jeffamine D-2000, Huntsman Corp.). A stoichiometric ratio of DGEBA to Jeffamine D-2000 was used to prepare the nanocomposites. The nanoclay was octadecyl onium ion modified montmorillonite (Nanomer I.30E, Nanocor Inc.), with a cation exchange capacity (CEC) of 92 meq/100 g [16]. Clay contents from 2 wt.% to 8 wt.% were used.
641
Advanced (Consulting de Imagen Digital, S.L., Micron Spain, S.A.) for the five images taken for every sample. In all cases, the particle size was quantified in terms of an equivalent diameter, Deq, being the diameter of a circular particle of the same area. 2.2.3. Preparation and curing of epoxy composites After mixing the clay with the resin or with the Jeffamine D2000, the appropriate amount of the second reagent for the stoichiometric ratio of DGEBA to Jeffamine D-2000 was added to the clay mixture. The clay–amine–epoxide complex was stirred for 2 min at room temperature and then degassed under vacuum for 10 min to eliminate air bubbles before curing. The bubble-free mixture was then cast into a PTFE mould of dimensions approximately 200 mm by 300 mm, placed in an air circulating oven and subjected to two curing stages, first curing at 75 °C for 3 h and then post-curing at 125 °C for an additional 3 h. 2.2.4. X-ray diffraction (XRD) The intercalation of the Jeffamine D-2000 and/or the epoxy resin into the clay galleries was measured by small angle X-ray scattering (SAXS). X-ray diagrams were recorded on film under vacuum using a modified Statton camera (W.R. Warhus, Wilmington, DE). For the cured nanocomposites, a Bruker model D8 Advanced diffractometer with Bragg–Brentano geometry in reflection mode was also used; the measurements were taken in a range of 2h = 1–10° and were made on small samples cut from cast sheets with copper Ka radiation and scanning at 0.05°/s.
2.2. Methods 2.2.1. Methods of preparation of clay mixtures Two different preparation methods were used: one in which the MMT was first mixed with the epoxy before the addition of the curing agent, and the other in which the MMT was first mixed with the curing agent before the addition of the epoxy resin. For these purposes, the amount of either resin or curing agent was mixed directly with the amount of clay necessary to obtain the desired clay content, which was selected as 2, 4, 6 or 8 wt.%. The mixing was done at room temperature, either by hand or using an ultrasonic bath (Branson DTH-3510) with a heating power of 205 W; in the latter case, cooling was applied and different periods were used depending on the clay content: 2 wt.% clay for 2 h, 4 wt.% clay for 3 h, 6 and 8 wt.% clay for 4 h. 2.2.2. Analysis of the dispersion The dispersion of the clay in the resin and in the Jeffamine D2000 following the different mixing methods was observed using a Leica polarising transmission optical microscope. A drop of each mixture was placed on a glass slide covered with a slip, and the images were captured digitally with a Canon PhotoS camera. Particle size analysis was made using the computer program Mip 4
2.2.5. Mechanical tests The mechanical measurements were made using a universal testing machine, MTS Adamel Lhomargy DY-34, with load cell of 100 N. The crosshead displacement rate was set at 5 mm/min. The tensile strength and the elongation at break were measured at room temperature according to DIN 53504 and ASTM D638 standards. The specimens used for tensile testing were dog-bone shaped bars with a parallel region of rectangular cross-section with length 35 mm, width 6 mm, and thickness about 2 mm, this last being determined by the thickness of the cast sheet. Specimens were stamped directly from the cast sheet using a stamp of appropriate dimensions. At least five tests were performed for each of the clay contents used and for each method of preparation. 2.2.6. Scanning and transmission electron microscopy (SEM, TEM) Samples for SEM were cut from the fractured specimens after tensile testing. The fracture surfaces were coated with Au–Pd alloy and were investigated using a scanning electron microscope (Jeol 5610). Samples for TEM were prepared by ultra-microtomy under cryostatic conditions, giving sections of thickness approximately 50 nm. These sections were examined in a Jeol-2011 HRTEM electron microscope with an accelerating voltage of 200 kV.
Fig. 1. SAXS diffraction patterns for: (a) organically modified clay; (b) epoxy/clay, mechanically mixed, 10 wt.% MMT; (c) Jeffamine D-2000/clay, 2 wt.% MMT.
642
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
Fig. 2. Polarised optical micrographs of the dispersion of MMT (6 wt.% loading) in: (a) DER331, hand mixing; (b) Jeffamine D-2000, hand mixing; (c) Jeffamine D-2000, ultrasonic bath for 4 h. The scale bar is 100 lm.
Table 1 Number percentages of particles within the specified ranges of Deq (lm) for the Jeffamine D-2000/MMT and DER331/MMT mixtures with different wt.% MMT, either hand mixed (HM) or ultrasonically mixed (UM). The figures in bold relate to the SAXS results in Fig. 3. 2 wt.% HM
4 wt.% UM
HM
6 wt.%
8 wt.%
UM
HM
UM
HM
UM
Jeffamine D-2000/MMT mixtures 76.2 68.8 64.3 Deq 6 20 50 < Deq 2.6 2.7 5.5
76.2 1.5
62.3 9.0
79.1 2.3
47.3 14.5
80.6 2.3
DER331/MMT mixtures Deq 6 20 35.7 64.3 50 < Deq 20.6 10.8
66.6 8.1
36.2 30.8
38.1 28.9
34.2 30.0
28.3 36.5
30.7 28.6
Fig. 4. SAXS results for cured elastomeric nanocomposite samples: M1, DER331/ 6MMT/D-2000; M2 = M1 + 4hUS; M3, D-2000/6MMT/DER331; M4 = M3 + 4hUS; M5, DER331/8MMT/D-2000; M6 = M5 + 4hUS; M7, D-2000/8MMT/DER331; M8 = M7 + 4hUS.
Fig. 3. SAXS scattering patterns and densitometry traces obtained using the Statton camera for: (a) D-2000/2MMT/DER331/2HUS; (b) DER331/6MMT/D-2000.
3. Results and discussion 3.1. Intercalation Fig. 1 shows the SAXS diffraction patterns for the octadecyl onium ion modified clay before intercalation (Fig. 1a), after intercalation of epoxy resin (Fig. 1b), and after intercalation of Jeffamine D-2000 diamine (Fig. 1c). The diffraction pattern for the diamine/ clay mixture (Fig. 1c) shows a very strong and very broad scattering peak at 5.38 nm together with a peak of medium strength at 1.50 nm and the outermost ring corresponding to a d-spacing of 0.43 nm within the clay layers. The peak at 5.38 nm should be compared with the d-spacings of 2.09 nm for the modified clay
(Fig. 1a) and 3.75 nm for the intercalated epoxy (Fig. 1b), from which it can be seen that for the Jeffamine D-2000/MMT mixture, even though mixed only by hand, the 5.38 nm d-spacing is not only much higher than that for the intercalated epoxy, but also greater than the theoretical maximum spacing of 3.74 nm calculated for the vertically oriented octadecyl cation [17]. Furthermore, the appearance of an additional scattering peak at 1.50 nm, even though less strong, indicates the existence of a bimodal distribution of layer spacings when the diamine is intercalated into the clay galleries, one spacing being much larger than expected and the other being smaller than the layer spacing in the modified clay alone. This has implications for the subsequent exfoliation of the clay in the cured nanocomposite, as will be seen later in the TEM micrographs. 3.2. Dispersion To obtain significant property enhancement in the epoxy nanocomposite materials, it is essential to disperse the clay homo-
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
643
Fig. 5. TEM micrographs of sample D-2000/2MMT/DER331/2hUS at magnifications of (a) 600 and (b) 800.
Fig. 6. TEM micrographs of sample D-2000/2MMT/DER331/2hUS; regions of particle shown in Fig. 6b at magnifications: (a) 30,000, (b) and (c) 60,000.
geneously in the resin (or curing agent) prior to adding the crosslinking agent (or resin) and then to exfoliate the individual clay layers during the cure of the system. The dispersion of the clay depends strongly on the mixture preparation method, and it is noticeably better for the MMT mixtures prepared by swelling the clay in the Jeffamine D-2000 first, as can be seen by comparing Fig. 2a and b for mixtures containing 6 wt.% MMT and mixed by hand. Table 1 gives the percentages of the smallest (620 lm) and largest (>50 lm) particles in the mixtures prepared by mixing the clay first either with Jeffamine D-2000 or with epoxy resin. Two important conclusions can be drawn from these results: first, that mixing the clay with the diamine gives a significantly better dispersion compared with mixing the clay with the epoxy resin, whether mixed by hand or ultrasonically; and second, that ultrasonic mixing significantly improves the dispersion for the diamine/MMT mixtures, and particularly so when the clay content is higher. A typical illustration of the dispersion after ultrasonically assisted mixing is shown in Fig. 2c for a 6 wt.% loading of MMT in Jeffamine D-2000, for comparison with Fig. 2a and b. 3.3. X-ray scattering of cured nanocomposites
Fig. 7. TEM micrograph of sample D-2000/2MMT/DER331/2hUS, showing the existence of a bimodal distribution of clay layer spacings. The arrows indicate the layer spacings of about 1.5 nm.
The nanostructure of the cured nanocomposites can in part be assessed by SAXS, and this technique is used to examine the effect of the different preparation procedures discussed in the preceding section. Scattering patterns were obtained, and are shown in Fig. 3, for two samples which originally had very different clay dispersions: (a) D-2000/2MMT/DER331/2HUS, with 2 wt.% MMT and mixed first with the diamine using ultrasound for 2 h, has a good dispersion (see Table 1, figures in bold); (b) DER331/6MMT/
D-2000, with 6 wt.% MMT and mixed first with the epoxy and without ultrasound, has a very poor dispersion (see Table 1, figures in bold). It transpires that, despite the very different qualities of the original dispersions for the two samples, the cured nanocomposites appear from these SAXS results to have similar nanostructures: in particular, both show a rather broad range of clay layer spacings which give rise to significant scattering, and indicate that exfoliation is not complete in these systems.
644
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
Fig. 8. Tensile modulus (a) and tensile strength (b) as a function of clay content (wt.%) for the elastomeric nanocomposites prepared in different ways. The dashed lines represent linear fits to the data for DER331/MMT/D-2000 and D-2000/MMT/DER331/US, and passing through the point at zero MMT content.
A more quantitative study was made of cured samples with the highest clay contents of 6 wt.% MMT (samples M1–M4) and 8 wt.% MMT (samples M5–M8), prepared by mixing first with the epoxy resin (M1 and M5 without ultrasound (US), M2 and M6 with US), or with the diamine (M3 and M7 without US, M4 and M8 with US). The results are presented in Fig. 4. Here it can be seen that the scattering curves separate into two groups, one for samples M1–M4, for which there is a minimum in the scattering at very low angles, and the other for samples M5–M8, for which there is no minimum, implying that there is somewhat better exfoliation for the smaller clay content. Nevertheless, for all the samples there is significant scattering over the range of 2h from about 1.5° to 4.5°, corresponding to d-spacings of about 2.0–6.0 nm, which is clear evidence that the exfoliation is far from complete. Further confirmation of this aspect of the nanostructure can be seen in the TEM micrographs, presented below. 3.4. Transmission electron microscopy Typical micrographs at low magnification are shown in Fig. 5, for a sample prepared by mixing the clay (2 wt.% MMT) first with the diamine, using the ultrasonic bath for 2 h, before adding the epoxy resin and curing the nanocomposite. In Fig. 5a it can be seen that in the cured nanocomposite there remain some particles with sizes between 1 and 10 lm, though there was no evidence of any particles of greater size. This means that the particles of sizes greater than 20 lm which existed in the diamine–clay mixture before the addition of the epoxy resin and the cure process, a number percentage of approximately 30% (see Table 1), have been broken down during cure. One of the largest particles observed in the microtomed section examined here, shown in Fig. 5b, has a length of about 10 lm and a width of about 5 lm, thus an equivalent diameter somewhat less than 10 lm. There is therefore considerable nanostructural modification taking place during cure, including some exfoliation of the clay layers, as can be seen at higher magnifications of this same particle. In one particular region of the particle shown in Fig. 5b, for example, there is clear evidence, at higher magnification, of stacks of layers which have not been exfoliated and of substantial exfoliation, as can be seen in Fig. 6a. Indeed, in some regions the number of stacked layers is very large, as illustrated in Fig. 6b, these particular layers having a d-spacing of the order of 4 nm, within the range identified by SAXS. On the other hand, at the edges of the regions of layer stacking one can identify the extensive delamination and redistribution that is characteristic of exfoliation, with the separation between layers being as large as 50 nm, as shown in Fig. 6c.
Fig. 9. Plastic deformation energy per unit volume as a function of clay content (wt.%) for the elastomeric nanocomposites prepared in different ways. The dashed line is drawn to guide the eye.
A feature of the micrographs at higher magnification is the appearance, within the rather ordered stacks of clay layers, of a bimodal distribution of d-spacings. This is well illustrated in Fig. 7, where layer spacings of about 1.5 nm can be seen rather regularly interspersed within a larger layer spacing of about 5.4 nm. These dspacings correspond very closely to those found by SAXS of the diamine–clay mixture before the addition of the epoxy resin, as shown in Fig. 1c. The implication is that these regions of the nanocomposite remain unaltered from the diamine–swollen clay state, in other words that the epoxy resin has not penetrated to these regions, and no cure reaction has taken place here. This is presumably what occurs within the largest agglomerations of the original clay dispersion, and emphasises the importance of achieving a good dispersion if an exfoliated nanocomposite is to be obtained. 3.5. Mechanical properties Stress–strain curves for elastomeric nanocomposites containing 0, 2, 4, 6 and 8 wt.% MMT and prepared by the different methods discussed earlier were obtained by tensile testing. The values of modulus and tensile strength were determined for each of the at least 5 samples that were tested for each preparation condition, and the average values are shown in Fig. 8a and b, respectively, as a function of the clay content, where an approximately linear increase is observed in both cases. There is clear evidence of an increase in both modulus and tensile strength when ultrasonic mixing is used and, to a lesser extent, when the MMT is mixed first
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
645
Fig. 10. SEM micrographs of the fracture surface after tensile tests for nanocomposites prepared by mixing MMT first with DER331, and containing the following MMT contents: (a) 2 wt.%; (b) 8 wt.%.
with the diamine. Similar results and conclusions have been reported by several other workers concerning the effects of preparation method and dispersion, both for the same elastomeric system as studied here [4,8,18] as well as more generally in epoxy–clay nanocomposites, including rigid systems [e.g. 19–21]. The importance of the preparation method in the development of the optimum properties of layered silicate nanocomposites cannot be overstated. The work of fracture for the present elastomeric nanocomposites can be calculated from the area beneath the stress–strain curves, and is found to be slightly greater for the nanocomposites prepared by mixing the clay first with the diamine and to increase as the clay content increases; the increase is a factor of about three when the clay content increases from 0 to 8 wt.%. A large part of this work of fracture is stored as elastic strain energy per unit volume, according to the formula r2/2E, where r is the tensile strength and E is the tensile modulus; this elastic strain energy is recovered after fracture. The dissipated (plastic) energy is the difference between the total work of fracture and the elastic strain energy at fracture, and is a measure of the toughness of the material. Fig. 9 shows the dependence of the plastic deformation energy per unit volume as a function of the clay content for the nanocomposites prepared using different methods. A linear increase with the clay content is clearly seen, the increase being even more significant, a factor of approximately five for 8 wt.% clay, than the factor of approximately three that was found for the work of fracture. The implication is that the presence of the clay has an important and significant effect on the fracture behaviour of these elastomeric nanocomposites. This result is further supported by SEM micrographs of fracture surfaces obtained after the tensile tests, shown in Fig. 10. A significant increase in fracture surface roughness can be identified as the clay content increases from 2 to 8 wt.%, consistent with the tensile strength and plastic deformation energy results and indicating a toughening effect of the clay. 4. Conclusions Elastomeric epoxy–clay nanocomposites have been prepared with diglycidyl ether of bisphenol-A epoxy resin cross-linked with a long chain diamine, Jeffamine D-2000, and filled with an organically modified montmorillonite, MMT. It has been shown that the preparation method, in particular whether the clay was mixed first with the epoxy resin or with the diamine and whether or not ultrasonic mixing was used, as well as the clay content, up to 8 wt.%, both have a significant influence on the properties of the nanocomposite. The tensile modulus, tensile strength and the plastic deformation energy all increase with increasing clay content, while higher values of all these properties are found when the clay
is first mixed with the diamine and when ultrasonic mixing is used. The latter effect is attributed to an improvement in the dispersion of the clay, which has been demonstrated by particle size analysis of optical micrographs of the resin–clay or diamine–clay mixtures. The increase in plastic deformation energy correlates with a significant increase in the roughness of the fracture surfaces after tensile testing, as identified by SEM. The nanostructure of the cured nanocomposites, identified by SAXS and TEM, shows that substantial nanostructural modification takes place during cure, with a significant amount of exfoliation occurring. Nevertheless, there remain regions in which layer stacking persists, even after cure, with the same nanostructure as in the resin–clay or diamine–clay mixture before cure. These results, obtained for an elastomeric epoxy nanocomposite, may be interpreted also to indicate the importance, for rigid epoxy nanocomposites, of the preparation and curing procedure. Acknowledgements The authors are grateful to Nordmann–Rassmann GmbH for the provision of the modified clay, and to the Dow Chemical Company and the Huntsman Corporation for the provision of the epoxy resins and curing agents, respectively. This work was supported by the Spanish Ministry of Education and Science, Projects MAT 2008-06284-C03-03 and MAT2011-27039-C03-03. Magda Lipinska acknowledges a mobility grant from the Spanish Ministry of Education and Science (Ref. SB2005-0145). References [1] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mat Sci Eng 2000;28:1–63. [2] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641. [3] Becker O, Simon GP. Epoxy layered silicate nanocomposites. Adv Polym Sci 2005;179:29–82. [4] Lan T, Pinnavaia TJ. Clay-reinforced epoxy nanocomposites. Chem Mater 1994;6:2216–9. [5] Wang Z, Pinnavaia TJ. Hybrid organic–inorganic nanocomposites: exfoliation of magadiite nanolayers in an elastomeric epoxy polymer. Chem Mater 1998;10:1820–6. [6] Wang Z, Pinnavaia TJ. Nanolayer reinforcement of elastomeric polyurethane. Chem Mater 1998;10:3769–71. [7] Boukerrou A, Duchet J, Fellahi S, Kaci M, Sautereau H. Morphology and mechanical and viscoelastic properties of rubbery epoxy/organoclay montmorillonite nanocomposites. J Appl Polym Sci 2007;103:3547–52. [8] Ngo TD, Ton-That MT, Hoa SV, Cole KC. Reinforcing effect of organoclay in rubbery and glassy epoxy resins, Part 1: dispersion and properties. J Appl Polym Sci 2008;107:1154–62. [9] Hutchinson JM, Montserrat S, Román F, Cortés P, Campos L. Intercalation of epoxy resin in organically modified montmorillonite. J Appl Polym Sci 2006;102:3751–63. [10] Román F, Montserrat S, Hutchinson JM. On the effect of montmorillonite in the curing reaction of epoxy nanocomposites. J Therm Anal Calorim 2007;87:113–8.
646
M. Lipinska, J.M. Hutchinson / Composites Science and Technology 72 (2012) 640–646
[11] Hutchinson JM, Montserrat S, Román F. Nanostructure development and cure kinetics in epoxy-based PLS nanocomposites. J Nanostruct Polym Nanocompos 2008;4:13–20. [12] Montserrat S, Román F, Hutchinson JM, Campos L. Analysis of the cure of epoxy based layered silicate nanocomposites: reaction kinetics and nanostructure development. J Appl Polym Sci 2008;108:923–38. [13] Ton-That MT, Ngo TD, Ding P, Fang G, Cole KC, Hoa SV. Epoxy nanocomposites: analysis and kinetics of cure. Polym Eng Sci 2004;44:1132–41. [14] Miyagawa H, Rich MJ, Drzal LT. Amine-cured epoxy/clay nanocomposites. 1. processing and chemical characterization. J Polym Sci Part B Polym Phys 2004;42:4384–90. [15] Wang K, Chen L, Wu JS, Toh ML, He CB, Yee AF. Epoxy nanocomposites with highly exfoliated clay: mechanical properties and fracture mechanisms. Macromolecules 2005;38:788–800.
[16] Ratna D, Manoj NR, Varley R, Raman Singh RK, Simon GP. Clay-reinforced epoxy nanocomposites. Polym Int 2003;52:1403–7. [17] Lan T, Kaviratna PD, Pinnavaia TJ. Mechanism of clay tactoid exfoliation in epoxy–clay nanocomposites. Chem Mater 1995;7:2144–50. [18] Ngo TD, Ton-That MT, Hoa SV, Cole KC. Effect of temperature, duration and speed of pre-mixing on the dispersion of clay/epoxy nanocomposites. Comp Sci Tech 2009;69:1831. [19] Tolle T Benson, Anderson DP. Morphology development in layered silicate thermoset nanocomposites. Comp Sci Technol 2002;62:1033–41. [20] Dean D, Walker R, Theodore M, Hampton E, Nyairo E. Chemorheology and propertiesofepoxy/layeredsilicatenanocomposites.Polymer2005;46:3014–21. [21] Pustkova P, Hutchinson JM, Román F, Montserrat S. Homopolymerization effects in polymer layered silicate nanocomposites based upon epoxy resin: implications for exfoliation. J Appl Polym Sci 2009;114:1040–7.