Polymerization compounding: epoxy-montmorillonite nanocomposites.
Over the past decade, layered-silicate based nanocomposites have attracted much attention as a simple and cost-effective method of enhancing polymer properties by addition of small amounts of properly designed fillers (1). Owing to their unique phase morphology and improved interfacial properties, polymer layered silicate nanocomposites exhibit dramatic increase in modulus, strength, barrier, flammability resistance, and heat resistance properties compared to conventional composites (2-4). With such a wide spectrum of properties, nanocomposites are more and more considered the material of choice for a variety of applications ranging from high barrier packaging for food and electronics to automotive applications.
Dispersion, wetting, and interactions between solid substrate and polymeric matrix are the critical factors in the design of composite materials. Polymerization compounding (PC), an approach in which the polymer to be used either as a matrix in the final composite or else as a special surface treatment of the reinforcement is forced to grow from the surface of the reinforcement, generally overcomes the above-cited limitations (5). This approach has been successfully applied by Salehi-Mobarekeh et al. for polyamide 66 (PA-66) grafted on DuPont Kevlar and glass fibers and then incorporated in a PA-66 matrix (6, 7). Wu et al. have used a similar method with carbon fibers (8).
In addition to interfacial properties, phase morphology plays an important role in the rheological and the mechanical behavior for all multiphase systems including nanocomposites. Few studies have reported systematic relationship between, on the one hand, phase morphology and interfacial properties, and on the other hand, mechanical properties and rheological behavior of epoxy/layered silicate nanocomposites (9-13). In this work, we extend the PC approach to epoxy/layered-silicate nanocomposites. Hydroxyl-functionlized montmorillonite will be used as a reinforcement. Tolylene 2,4-diisocyanate and bisphenol A will be used as reactants to produce novel epoxy-layered silicate nanocomposites. Dynamic mechanical properties and stress relaxation behavior below [T.sub.g] will be studied.
Bis(2-hydroxylethyl) methyl tallow ammonium mont-morillonite (1.34 TCN, Nanocor Inc.) is used as modified layered silicate. The quaternary ammonium surfactant is derived from natural tallow, which contains a mixture of alkanes ([C.sub.18]/[C.sub.16]/[C.sub.14] of 0.66/0.31/0.03). Tolylene 2,4-diisocyanate (TDI), bisphenol A (BA) and dibutylthindilaurate (DBTDL) were provided by Aldrich Chemicals. The epoxy resin was the diglycidyl ether of bisphenol A, EPON 828, provided by Shell Chemicals. Curing agent, Jeffamine D-230, a polyoxyalkylene diamine, was supplied by Huntsman Corporation. Acetonitrile (Sigma-Aldrich, HPLC grade), was used as received without further purification.
Synthesis of TDI and BA Modified 1.34 TCN (1.34-TDI-BA)
The reaction of 1.34 TCN with TDI was conducted in a brown bottle reactor with acetonitrile as solvent, using DBTDL as a catalyst. The second step of the reaction with BA was also conducted in acetonitrile under nitrogen to prevent side reaction with moisture. The product was filtered, washed two times with acetonitrile and two times with acetone, dried under vacuum at 80[degrees]C for 48 h, and crushed in a mortar. A modified 1.34 TCN (1.34-TDI-BA) powder was then obtained. It is worth mentioning that in the polymerization compounding approach, competition between polymerization on the surface of the solid substrate and the bulk polymerization must be avoided. This is why a two-step modification process was used.
Preparation of Epoxy-Clay Nanocomposites
Prior to curing, the epoxy resin was mixed with the desired amount of organophilic clay at 75[degrees]C for 2 h. A stoichiometric amount of the Jeffamine D-230 curing agent corresponding to 30 wt% of the epoxy resin content in the composite was added. The mixtures were degassed in a vacuum oven at 75[degrees]C and transferred into a DuPont Teflon mold for curing at 75[degrees]C for 3 h and, subsequently, 125[degrees]C for an additional 3 h.
Thermogravimetric analyses (TGA) were performed using a Mettler TG 50 equipped with Stare software at a scanning rate of 20[degrees]C/min and a nitrogen flowing rate of 200 ml/min from 60[degrees]C to 1000[degrees]C. FTIR was carried out on a Nicolet Magna 550 Spectrometer using KBr compressed pellets as samples. Small angle X-ray diffraction (XRD) patterns were obtained on a Siemens D5000 apparatus with Cu[K.sub.[alpha]] radiation ([lambda] = 0.154 nm). The clay powder was put into glass capillary having 0.5 mm diameter. Disc-shaped plates cut from cured epoxy nanocomposites were directly analyzed by XRD. The morphology was examined by means of transmission electron microscopy (TEM) using a JEOL TEM-1200EX apparatus with 80 kV accelerating voltage. The ultra-thin samples for TEM observations were cut at 25[degrees]C using an Ultracut E, Reichert and Jung ultramicrotome equipped with a diamond knife. Differential scanning calorimetry (DSC) tests were carried out on neat epoxy and epoxy/organoclay composites u sing a Perkin Elmer DSC-7 thermal analyzer. Samples were heated to 150[degrees]C and held at this temperature for 5 min and then cooled to 50[degrees]C at a scanning rate of 10[degrees]C/min under flowing nitrogen. The second scan was collected to determine the [T.sub.g] at the same condition as the first scan.
Dynamic mechanical analyses (DMA) of cured composite samples having dimensions of 48 X 3.0 x 6.0 mm were performed on a Rheometric Scientific Solid State Analyzer (RSA-II) using three-point bending fixture. Dynamic mechanical properties, obtained in the linear viscoelastic regime, were carried out for temperatures ranging from 25[degrees]C to 150[degrees]C at a constant frequency of 1 Hz and a scanning rate of 1[degrees]C/min. The static stress relaxation behavior was conducted at 60[degrees]C, a temperature lower than [T.sub.g] of composites and at a constant strain of 0.1% for 3 h (before testing, all samples were aged for 30 min at 60[degrees]C).
RESULTS AND DISCUSSION
Synthesis of 1.34-TDI-BA
TDI is widely used as a surface modification agent for either inorganic or organic reinforcements to improve interfacial properties in polymer composites (14). Hydroxyl groups on the surface and in the galleries of 1.34 TCN clay were made to react with TDI and BA according to the reaction scheme illustrated in Fig. 1. TGA analysis showed a 47% weight loss for the 1.34-TDI-BA organoclay in comparison with a 35% weight loss obtained with 1.34 TCN organoclay. These results indicate that TDI and BA have indeed reacted with the hydroxyl groups on the clay, and about 27% of hydroxyl groups on the clay have been involved in this reaction. Figure 2 shows the FTIR spectra of 1.34 TCN and 1.34-TDI-BA. New absorption peaks at 1720 [cm.sup.-1] and 1510 [cm.sup.-1] for 1.34-TDI-BA can be identified. These peaks are attributed to the vibration of -O-CO-NH- and benzene, indicating that the modification with TDI and BA occurred. The number of hydrogen bonds related to the hydroxyl groups (3350 [cm.sup.-1]) slightly decreased after modification. This is attributed to possible backbiting, in which one TDI molecule can react with two adjacent hydroxyl groups of the 1.34 TCN organoclay. The presence of these peaks is an indication that the reaction of TDI with the OH groups of the 1.34 TCN followed by the reaction of TDI's NCO groups with BA hydroxyl groups indeed occurs. XRD results, as shown in Fig. 3, indicate that the chemical reaction of TDI and BA with 1.34 TCN increased the (001) basal spacing from 1.95 to 3.22 nm, which further indicates that TDI and BA have reacted with the hydroxyl present in the clay galleries.
Structure of Epoxy-Nanocomposites
XRD was also used to determine the structure of the epoxy nanocomposites. As shown in Fig. 3, the d-spacings of epoxy nanocomposites with unmodified and modified 1.34 TCN are 3.68 and 4.42 nm, respectively, which indicates that both 1.34 TCN and 1.34-TDI-BA present a certain increase of basal spacing (from 1.95 to 3.68 nm and from 3.22 to 4.42 nm for 1.34 TCN and 1.34-TDI-BA, respectively). The increased basal spacing implies that during the preparation of epoxy nanocomposites, epoxy molecules diffused into the silicate layers for both modified and unmodified organoclay. Furthermore, the increase of basal spacing for 1.34-TDI-BA nanocomposites also indicates that there is no apparent crosslinking between silicate layers (see also FTIR spectra on Fig. 2). Crosslinking is indeed less probable since all reactants (TDI and BA) are used in excess. Typical TEM micrographs are shown in Fig. 4. From Fig. 4a and 4c, it can be seen that under low magnification, organoclay aggregates are still present, and mesoscopic ar rangements (10 nm and larger) are maintained. The higher-magnification micrographs shown in Fig. 4b and 4d, indicate that the face-to-face structure was formed within the clay crystallites and the layer-layer distance was about 3 to 5 nm, in agreement with the XRD results. The XRD and TEM results show that the intercalated structures were formed for both organoclays. Exfoliation of organoclay with epoxy resin depends on a number of parameters. These include the nature of the curing agent and the curing conditions (15), the stoichiometry of the curing agent (16) and the cation-exchange capacity of the clay (11), among others. All these parameters influence the competition between the extragallery reaction rate and the diffusion rate of the curing agent in the galleries of the epoxy swollen organoclay (15).
Thermal and Dynamic Mechanical Properties of Nanocomposites
The glass transition temperatures ([T.sub.g]) of epoxy and its nanocomposites are listed in Table 1. The results show that [T.sub.g] increases with increasing the amount of organoclay. This suggests that the layered silicates hinder the motion of molecules in the epoxy network at least in the vicinity of the silicate surface. It also indicates that the layered silicates were dispersed and wetted relatively well in the epoxy matrix. The data reported in Table 1 also indicate the TDI-BA modification of the 1.34 TCN organoclay results in higher values of [T.sub.g] in comparison with [T.sub.g] values obtained with 1.34 TCN organoclay. This is attributed to enhanced molecular interactions at the interface between layered silicates and epoxy matrix promoted by the hydroxyl groups of the BA, which are believed to participate to the formation of the epoxy matrix network. The effect of organoclay modification on the [T.sub.g] of composites is further confirmed by DMA results. As shown in Fig. 5 and Table 1, the result s of dynamic loss tangent (tan [delta]) for 5 and 10 wt% organoclay composites show the same trend as those of thermal analysis. The peak values of tan [delta] of epoxy/1.34-TDI-BA nonocomposites are found to be slightly lower than those of epoxy/1.34 TON nanocomposites, indicating that enhanced interfacial interaction can be present in the case of modified organoclay. The increase in tan [delta] temperature and the decrease in tan [delta] peak value in hybrid systems has also been reported for tetraethylenepentamine cured epoxy/silica nanocomposites (17).
Figures 6a, 6b show the effect of the different organoclay on the storage modulus (E') for 5 and 10 wt% clay loaded composites, respectively. In both cases, i.e., for composites with modified and unmodified organoclay, an improvement of the storage modulus observed in both the glassy state and the rubbery state (10, 13). 1.34-TDI-BA composites exhibit larger E' values mainly above the glass transition temperature. At room temperature, E' of nanocomposite with addition of 10 wt% 1.34 TCN showed a 25% increase compared to pure epoxy, and a 43% increase was found for nanocomposite with loading 10 wt% 1.34-TDI-BA. In rubbery state (120[degrees]C), the increases of E' for nanocomposites with 10 wt% of 1.34 TCN and 1.34-TDI-BA are 177% and 307%, respectively. These results further indicate that TDI-BA modification enhanced the interfacial interaction of layered silicate and epoxy matrix.
Figure 7a and 7b show the stress relaxation curves of epoxy nanocomposites. For all nanocomposites, the motions of epoxy network junctions slow down compared to pure epoxy (18). Again, the system reinforced with 1.34-TDI-BA has a larger relaxation modulus, confirming the temperature sweep results.
The interfacial interactions of filler and matrix are important in polymer/layered silicate nanocomposites. In this context, 1.34-TCN-BA was prepared by reacting 1.34 TCN with TDI and BA through addition reaction. The TGA, FTIR, and XRD results verified that this reaction was successfully carried out. By use of a commonly used two-stage cure process with diamine as curing agent, intercalated epoxy nanocomposites were prepared for both modified and unmodified organoclays. XRD and TEM results showed that the basal spacing of clay in nanocomposites was about 3 to 5 nm. DMA was used to evaluate the interfacial interaction between layered silicate and epoxy matrix. Modified organoclay composites are found to have enhanced storage moduli, particularly at temperatures higher than the glass transition temperature, [T.sub.g], of the matrix. The storage modulus was increased 3-fold in rubbery state with addition of 10 wt% modified organoclay, compared to 1.8-fold for unmodified organoclay with the same loading. Glass transition temperatures extracted from the linear viscoelastic data are found to be slightly higher for modified organoclay nanocomposites, indicating enhanced interactions between the modified organoclay and the epoxy matrix. [T.sub.g] of nanocomposites obtained from DSO showed the same trend as DMA results. These results imply that epoxy/modified 1.34 TCN nanocomposites have enhanced heat-resistant properties and improved stiffness, particularly in the rubbery state.
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Table 1 Glass Transition Temperature, [T.sub.g], as Obtained From DSC and Dynamic Mechanical Analysis (DMA) for Epoxy and Epox/Montmorillonite Nanocomposites. Composition (1) [T.sub.g] ([degrees]C) (wt% of clay) From DSC From [DMA.sup.2] Epoxy 0.0 77.5 [+ or -] 1.5 86.0 [+ or -] 1.0 1.34 TCN 5.0 80.0 [+ or -] 1.5 91.0 [+ or -] 1.0 10.0 83.5 [+ or -] 1.5 93.0 [+ or -] 1.0 1.34-TDI-BA 5.0 83.0 [+ or -] 1.5 93.0 [+ or -] 1.0 10.0 86.0 [+ or -] 1.5 95.0 [+ or -] 1.0 (1.) Refers to silicate mineral. (2.) Error based on duplicated data.
The authors acknowledge the financial support provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada and le Fonds pour la Formation de chercheurs et l'aide la recherche (FCAR) of the province of Quebec. We also wish to thank Nanocor Inc. and Huntsman Corporation for supplying us with the raw materials used in this study.
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ABBREVIATIONS AND SYMBOLS
BA: Bisphenol A
DMA: Dynamic Mechanical Analysis
DSC: Differential Scanning Calorimetry
PA-66: Polyamide 66
PC: Polymerization Compounding
TDI: Tolylene 2,4-diisocyanate
TGA: Thermogravimetric Analysis
XRD: X-ray Diffraction
[T.sub.g]: Glass transition temperature
tan [delta]: Dynamic loss tangent
E': Dynamic storage modulus
E: Young modulus
wt%: weight percentage
Wei Feng (1), Abdellatif Ait-Kadi (1), and Bernard Riedl (2) *
(1.) CERSIM/Departement de genie chimique Universite Laval Quebec, G1K 7P4, Canada
(2.) CERSIM/Departement des sciences du bois et de la foret Universite Laval Quebec, G1K 7P4, Canada
* To whom correspondence should be addressed. E-mail: firstname.lastname@example.org