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The influence of some new montmorillonite modifier agents on the epoxy–montmorillonite nanocomposites structure
Applied Clay Science 50 (2010) 469–475
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
The influence of some new montmorillonite modifier agents on the epoxy–montmorillonite nanocomposites structure Sorina-Alexandra Gârea ⁎, Horia Iovu, Georgeta Voicu Polytechnic University of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Polymer Science and Engineering, 149 Calea Victoriei, 010072-Bucharest, Romania
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Article history: Received 20 July 2009 Received in revised form 7 August 2010 Accepted 21 September 2010 Available online 1 November 2010 Keywords: Montmorillonite Nanocomposites Epoxy Glass transition temperatures
a b s t r a c t Some new epoxy-amine adducts were used as modifier agents for montmorillonite in order to enhance the compatibility between inorganic and organic phases. The adducts were synthesized by reacting different epoxy resin types with a stoichiometric amount of polyetheramine which includes hydrophobic and hydrophilic units. The X-ray diffraction (XRD) proved that these epoxy-amine adducts can be intercalated within the layers of montmorillonite and thus lead to an increase of the basal distance. TEM and XRD analysis show that the final structure of epoxy–montmorillonite hybrids strongly depends on the agents used for montmorillonite modification. Dynamic mechanical analysis (DMA) results show that the glass transition temperature of the final nanocomposites depends on the montmorillonite modifier structure. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer–organoclay nanocomposites are considered a new class of advanced organic–inorganic materials. Nanocomposites based on polymer matrices exhibit much improved mechanical properties, thermal and chemical stability, compared to the neat polymers. The unmodified clays such as montmorillonite are hydrophilic and therefore incompatible with a wide range of hydrophobic polymers including epoxy resins. In order to increase the compatibility of the clay with the polymers one option is to modify the clay with some special agents which exhibit high hydrophobic character (Le Pluarta et al., 2004; Liu, 2007). The usual treatment of modification of a clay consists of inorganic (sodium or calcium) cation exchange with organic cations, which may include various substituents in their structures. The intercalation of clays with organic cations leads to an increase of hydrophobic character and thus the compatibility between the polymer and the clay is enhanced (Yoon et al., 2007; Betega de Paiva et al., 2008). The surface energy, basal spacing and thermal stability of modified clay are strongly influenced by the chemical structure, packing density and the cation type of the modifier agent (Lee and Kim, 2002; Di Gianni et al., 2009). The modifier agents most often used for clay are quaternary ammonium salts, phosphonium salts, alkylimidazolium salts, oligomers and polymer cations and oligomeric amine derivatives (Lagaly, 1994; Awad et al., 2004; Su et al., 2004a,b; Hedley et al., 2007; Leszczynska et al., 2007; Patel et al., 2007). ⁎ Corresponding author. Tel.: + 40 214022713; fax: + 40 214023844. E-mail address: [email protected] (S.-A. Gârea). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.09.010
The aim of this study was to investigate the influence of some new montmorillonite modifier agents on the nanocomposite structure using various techniques like Transmission Electron Microscopy (TEM), Dynamic Mechanical Analysis (DMA) and Thermogravimetric analysis (TGA). 2. Experimental 2.1. Materials The sodium montmorillonite (Cloisite Na–ClNa) with a Cationic Exchange Capacity (CEC) of 92.5 meq/100 g clay was supplied by Southern Clay Products (TX). Diglycidyl ether of resorcinol (DGER), Diglycidyl ether of bisphenol F (DGEBF), Diglycidyl ether of polydimethylsiloxane (DGEPDMS), Diglycidyl ether of 1, 4 cyclohexanedimethanol (DGECHM) and Triglycidyl ether of triphenylol methane (TGETPM) epoxy resin types were supplied by FLUKA and used as received. The polyetheramines Jeffamine XTJ 505 (XTJ 505) and Jeffamine D230 (D 230) were provided by Huntsman. Jeffamine D230 (D 230) was used as crosslinking agent. CHROMASOLV Tetrahydrofurane (THF) type was received from FLUKA. 2.2. Synthesis of epoxy-amine adducts The epoxy-amine adducts were synthesized by reacting epoxy resins with a stoichiometric amount of poyetheramine XTJ 505 (Scheme 1).
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kept at 80 °C for 3 h. Then the modified montmorillonites were isolated and washed with hot deionized water until no chloride ions were detected by one drop of 0.1 N AgNO3 solution. The product was dried for two days at 110 °C and then ground in a mill to produce powder. 2.4. Synthesis of epoxy–montmorillonite nanocomposites An amount of epoxy resin (5 g) was introduced into a glass vessel at 80 °C and then a certain amount of modified montmorillonite (5 wt.% to epoxy resin) was added and the mixture was sonicated using an ultrasound device for 3 h. Then, the mixture was degassed for a few minutes using an ultrasound bath and a stoichiometric amount of curing agent (D230) was added. The final mixture was degassed for 5 min and then was poured into a Teflon mould. It was cured for 24 h at room temperature and post-cured for 1 h at 100 °C. 2.5. Characterization methods 2.5.1. Fourier-transform infrared spectroscopy (FTIR) FTIR spectra were recorded on a SHIMADZU 8900 equipment using 40 scans and 4 cm−1 resolution. The samples were analyzed from KBr pellets. 2.5.2. Dynamic mechanical analysis (DMA) The glass transition temperature (Tg) of cured samples was established using Dynamic Mechanical Analysis (DMA). DMA tests were done on a Triton 2000 equipment (Triton Technology). The samples were run in single cantilever bending mode. The value of the glass transition temperature (Tg) was estimated from the maximum of tan δ–temperature curve. 2.5.3. X-ray diffraction (XRD) The XRD analysis was performed on a XRD 6000 SHIMADZU diffractometer. 2.5.4. Gel permeation chromatography (GPC) The molecular weight distributions of epoxy resins used as raw materials for adducts synthesis were found using a KNAUER Gel Permeation Chromatograph equipped with RI detector and Macherey Nagel column thermostated at 25 °C. Standards of polystyrene were used for calibration and tetrahydrofurane as eluent. The flow rate was 1 ml/min. Scheme 1. The synthesis reactions of epoxy-amine adducts.
Thus 0.1 mol of difunctional epoxy resins (DGEBF, DGECHM, DGEPDMS) and 0.1 moles of trifunctional epoxy resin (TGETPM) were dissolved in 10 ml THF in a 50 ml glass flask equipped with reflux condenser, magnetic stirrer and dropping funnel. The reaction mixture was heated at 60 °C. For all the resins a stoichiometric amount of amine was added. Thus for 0.1 mol of resin (DGEBF, DGECHM, DGEPDMS) and 0.1 mol of TGETPM the amount of Jeffamine XTJ 505 was 0.2 mol and 0.3 mol respectively. The reaction mixture was stirred for 5 h. The THF was removed from the final products using a rotavapor.
2.5.5. Transmission electron microscopy (TEM) TEM images were recorded on a Philips CM 120 ST equipment using an acceleration voltage of 100 kV. Thin specimens of about 50– 100 nm were cut from nanocomposite blocks using an ultramicrotome equipped with a diamond knife at ambient conditions. 2.5.6. Thermogravimetrical analysis (TGA) TGA tests were done on a TA Instrument Q 500. The samples of 10 mg were heated from 30 to 800 °C at 10 °C/min scanning rate under a constant nitrogen flow of 100 mL/min. 3. Results and discussion
2.3. Synthesis of modified montmorillonite 3.1. Synthesis and characterization of adducts The organophilization of montmorillonite was performed by a cationic exchange process between protonated epoxy-amine adducts and sodium montmorillonite. The epoxy-amine adducts were first protonated with 30% HCl in order to be used as modifiers for montmorillonite. Thus a certain amount of epoxy-amine adduct was dissolved in 12 ml mixture (THF = 3 ml; H2O = 9 ml) and 4.4 ml 30% HCl was added. After that the mixture was stirred for 30 min at 40 °C. The protonated adducts were added to 2.5 g ClNa already swollen in 200 ml hot deionized water for 1 h at 80 °C. The suspension was
An important goal of this work was to synthesize some new epoxyamine adducts for modifying the montmorillonite used as reinforcing agent in nanocomposites, the adducts being more compatible with the epoxy matrix. The epoxy-amine adducts synthesis started from epoxy resins which contain different concentrations of oligomers. Most of the commercial epoxy resins are provided as mixtures including fractions of oligomers obtained by opening of the functional epoxy groups during the synthesis procedure. The concentration of
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Table 1 Characteristics of epoxy resins used in epoxy-amine adduct synthesis obtained from GPC analysis. Epoxy resin type
Mna (n = 0) (g/mol)
Mn (n N 0) (g/mol)
Concentration of monomer (n = 0) (%)
Concentration of oligomers (n N 0) (%)
DGEBF (Gârea et al., 2006)
312 – – 460 – 257
– 410 719 – 754
80 – – 75.4 – 60 –
– 10.2 9.8 – 24.6 – 40
TGETPM (Gârea et al., 2006) DGECHM
373 a
Molecular weight.
the oligomeric fractions was calculated from the area of the peaks revealed in GPC curves at lower elution times than monomer (Table 1, Fig. 1). From Table 1, one may notice that DGECHM exhibits the highest amount of oligomers (40%). The GPC curve for DGEPDMS (Fig. 1) shows a single peak, very large which indicates that DGEPDMS contains a mixture of oligomeric fractions. The chemical structures of the epoxy-amine adducts were confirmed by FTIR analysis. Fig. 2 shows two examples of FTIR spectra for DGECHM-XTJ 505 and TGETPM-XTJ 505 epoxy-amine adducts. From Fig. 2 one may notice that the peak at 910 cm−1 corresponding to the stretching vibration of epoxy ring totally disappeared. This fact proves that all the epoxy groups were consumed in the reaction with XTJ 505. Similar FTIR spectra were recorded for the other epoxyamine adducts.
Fig. 2. FTIR spectra of 1-DGECHM-XTJ 505, 2-TGETPM-XTJ 505.
3.2. Characterization of modified montmorillonites 3.2.1. FTIR analysis The FTIR analysis was useful in order to detect some new peaks corresponding to the stretching and bending vibrations of the modifier agents molecules which were intercalated within the layers of montmorillonite after the cationic exchange reaction took place. From Fig. 3 one may observe the presence of some new peaks at 2974, 2931 and 2875 cm−1 which may be assigned to C–H stretching vibration of CH3 and CH2 groups and also the peak at 1454 cm−1 corresponding to C–H deformation vibration of CH2 groups from epoxy-amine adduct chain. In the FTIR spectra of aromatic epoxyamine adducts (TGETPM-XTJ 505 and DGEBF-XTJ 505) additional peaks appeared at 1600 and 1510 cm−1, these being assigned to C–C and C–H stretching vibrations from the benzene ring. 3.2.2. XRD analysis The basal distances of modified montmorillonites with epoxyamine adducts were determined from XRD spectra (Fig. 4, Table 2). The XRD results show an increase of basal distances with at least 6 Å for all the modified clays which is an argument that the cationic exchange process took place and therefore the new bulky adducts
Fig. 3. FTIR spectra of unmodified montmorillonite (1-ClNa) and modified montmorillonite with different agents: 2-Cl-DGEPDMS-XTJ 505, 3-Cl-DGECHM-XTJ 505, 4-ClTGETPM-XTJ 505, and 5-Cl-DGEBF-XTJ 505.
penetrate within the silicate gallery causing a significant increase of the basal distance. 3.2.3. TGA tests The thermal stability of the modifiers for montmorillonite is a significant factor in choosing the proper method for nanocomposite synthesis (in situ polymerization, melt polymerization etc.) and it may also influences the thermostability of the final nanocomposites. The most common modifier agents for montmorillonite are quaternary ammonium salts which exhibit a low thermostability. Park and Jana, (2004) showed that the starting temperature of degradation at which the weight loss is 5% (Tonset 5%) is 162 °C for montmorillonite modified with n-hexadecyl ammonium chloride in nitrogen atmosphere. By comparison, the thermal stability of the montmorillonite modified with our new epoxy-amine adducts is significant higher (Table 3).
Fig. 1. GPC curves of DGECHM and DGEPDMS.
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S.-A. Gârea et al. / Applied Clay Science 50 (2010) 469–475 Table 3 Data from TGA and DTG of different modified montmorillonite. Modified montmorillonite
Weight loss (%)
Tonseta (°C)
Tmaxb (°C)
Cl-DGEBF-XTJ 505 Cl-TGETPM-XTJ 505 Cl-DGECHM-XTJ 505 Cl-DGEPDMS-XTJ 505
44 46 34 32
247 256 190 200
269 270 210 208
a b
The temperature at which the weight loss is 5%. Temperature at maximum rate of weight loss.
Table 4 The thermostability of various hybrid materials.
Fig. 4. XRD spectra of ClNa and modified montmorillonite with different agents.
System
Tonset
DGER/D230 DGER/Cl-DGECHM-XTJ 505/D230 DGER/Cl-DGEPDMS-XTJ 505/D230 DGER/Cl-DGEBF-XTJ 505/D230 DGER/Cl-TGETPM-XTJ 505/D230
332 320 324 327 332
a b
From Table 3 one may observe that in all cases, the weight loss is higher than 31% which means that a significant quantity of organophilization agent was introduced within the silicate layers through the cationic exchange process. The organic cations based on protonated adducts show a different thermal stability which depends on the nature of epoxy resin used in the adduct synthesis. The epoxy-amine adducts based on aromatic epoxy resins (DGEBF, TGETPM) exhibit higher thermal stability in comparison with those based on cycloaliphatic epoxy resins. Thus the starting decomposition temperature (Tonset) exhibits a higher value for aromatic adduct (250 °C) than for aliphatic ones which allow the use of aromatic adducts for nanocomposites synthesis through a melt process.
3.3. Synthesis and characterization of new epoxy-modified montmorillonite nanocomposites New nanocomposites based on epoxy resin and different types of modified montmorillonite as reinforcing agent (Cl-DGEBF-XTJ 505, Cl-TGETPM-XTJ 505, Cl-DGEPDMS-XTJ 505, and Cl-DGECHM-XTJ 505) were synthesized.
a 5%
Tonset
b 10%
346 338 340 342 345
The temperature at which the weight loss is 5%. The temperature at which the weight loss is 10%.
3.3.2. XRD analysis From Fig. 5 one may notice that the best results were obtained in the case of epoxy systems which include modified clays with aromatic adducts (Cl-DGEBF-XTJ 505 and Cl-TGETPM-XTJ 505). In these cases the peak assigned to the basal distance totally disappeared which indicates that exfoliated structures are probably formed. This fact suggested that the presence of DGEBF and TGETPM molecules between Jeffamine XTJ 505 chains leads to an increase in the compatibility of the clay surface with the polymer matrix. In case of hybrids which include clays intercalated with aliphatic adducts (ClDGEPDMS-XTJ 505, Cl-DGECHM-XTJ 505) a small peak is still present and therefore intercalated structures are finally obtained. 3.3.3. TEM analysis Fig. 6a shows TEM images of the DGER/Cl-DGECHM-XTJ 505 system which gives intercalated structures, the silicate layers exhibiting a basal distance of about 50–60 Å. Even if the distance between the layers significantly increased, the layers still exhibit parallel orientation so that only intercalated structures are obtained due to the fact that the penetration of the polymer matrix (DGER) within the silicate layers is somewhat
3.3.1. TGA tests The influence of the modified montmorillonite with aromatic adducts (TGETPM-XTJ 505, DGEBF-XTJ 505) is extremely low for the system including Cl-DGEBF-XTJ 505 and does not exist for nanocomposites based on Cl-TGETPM-XTJ 505. A low decrease of thermostability is obtained for modified montmorillonite with aliphatic adducts (Table 4) but this does not affect the industrial applications of the nanocomposites. Similar results were reported by other authors concerning the thermostability of epoxy-modified montmorillonite nanocomposites (Wang and Pinnavaia, 1998; Becker et al., 2004).
Table 2 XRD results for modified clay with epoxy-amine adducts. Montmorillonite type
Basal distance (Å)
ClNa Cl-DGEBF-XTJ 505 Cl-TGETPM-XTJ 505 Cl-DGEPDMS-XTJ 505 Cl-DGECHM-XTJ 505
11.7 17 17.3 17.6 17.4
Fig. 5. XRD curves of hybrids based on DGER and modified montmorillonites with different agents.
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Fig. 6. TEM images of hybrid materials at different magnifications: a — DGER/Cl-DGECHM-XTJ 505/D230, b — DGER/Cl-DGEPDMS-XTJ 505/D230, c — DGER/Cl-DGEBF-XTJ 505/D230.
hindered by the high number of hydrogen bonds formed between the numerous hydroxyl groups from the oligomers of the modifier (DGECHM-XTJ 505). This is in good agreement with the GPC results which showed that the content of oligomers for DGECHM is much
higher than for DGEBF or TGETPM. Also the lower compatibility between aliphatic units of the modifier (DGECHM) and the aromatic polymer matrix (DGER) may contribute to the final results of intercalated nanocomposites.
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Fig. 7. TEM images of hybrid materials at different magnifications: a — TGETPM/Cl-DGEBF-XTJ 505/D230, b — DGEBF/Cl-DGEBF-XTJ 505/D230.
The TEM images of epoxy nanocomposites based on DGER and ClDGEPDMS-XTJ 505 (Fig. 6b) confirmed the formation of intercalated structures. In this case different zones with various distances between the silicate layers are observed, mainly one zone with a large distance (120 Å) and another one with a lower distance (80 Å). This is due to the large molecular weight distribution of DGEPDMS (Fig. 1) which means that different chain lengths cause various distances between the silicate layers when the clay is modified with the adduct based on DGEPDMS. Fig. 6c shows the TEM images for the epoxy system based on DGER and Cl-DGEBF-XTJ 505 as a reinforcing agent. One may observe
that the structure is a typically exfoliated one in good agreement with the XRD analysis (Fig. 5). Due to the higher compatibility between the epoxy resin and the modified montmorillonite with DGEBF-XTJ 505 adduct, the reactions within the silicate gallery are favoured such as the DGER homopolymerization catalyzed by the clay modifier and the DGER crosslinking process enhanced by the diffusion of crosslinking agent between the silicate layers. The homopolymer formation and/or the crosslinking process within the silicate gallery are both factors which determine the occurrence of exfoliated structures. Also for the hybrid system based on DGER and Cl-TGETPM-XTJ 505 as reinforcing agent an exfoliated structure was obtained. The silicate layers showed a basal distance higher than 100 Å. The TGETPM-XTJ 505 adduct was synthesized starting from TGETPM epoxy resin with a low content of oligomers so that a low number of hydrogen bonds are established within the silicate gallery between the hydroxyl groups and as a consequence the layers separation is not hindered. The modified montmorillonite with DGEBF-XTJ 505 was also introduced in other aromatic epoxy resin types with different viscosity values (DGEBF, TGETPM) in order to verify if the viscosity of polymer matrix influences the final structure of hybrids. Also in
Table 5 Tg values of epoxy–clay hybrids calculated from DMA.
Fig. 8. The dependence of tan δ against temperature for epoxy systems in the presence and in the absence of different modified montmorillonites.
Epoxy-modified montmorillonite hybrids
Tg (°C)
DGER/D230 DGER/Cl-DGEBF-XTJ 505/D230 DGER/Cl-TGETPM-XTJ 505/D230 DGER/Cl-DGEPDMS-XTJ 505/D230 DGER/Cl-DGECHM-XTJ 505/D230
85 71 73 78 79
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these cases the silicate layers were individually dispersed in the whole mass of the epoxy resin and exfoliated structures were obtained (Fig. 7b). Therefore it seems that the initial viscosity value of the epoxy resin is not an important factor which may influence the formation of the nanocomposites structure. Even in the case of TGETPM which exhibits a high viscosity value the silicate layers are individually dispersed (Fig. 7a). 3.4. The influence of modified montmorillonite on the glass transition temperature of hybrids The effect of modified montmorillonite on the glass transition temperature (Tg) of final nanocomposites has been extensively studied in the literature but the results are still contradictory. Many authors have shown an increase in the Tg (Koerner et al., 2006; Marras et al., 2009) but some authors have observed a decrease in Tg especially for epoxy-based nanocomposites (Lan and Pinnavaia, 1994; Wang and Pinnavaia, 1994; Becker et al., 2003; Park and Jana, 2004). The increase in Tg was explained by the restricted chain motion of the polymer which is confined in the silicate galleries (Messersmith and Giannelis, 1994). Thus for polystyrene-based nanocomposites reinforced by modified montmorillonite with azobisisobutyronitrile an increase of Tg with 10 °C against virgin polystyrene was noticed (Vyazovkin and Dranca, 2004). This explanation may be valid only for intercalated nanocomposites. As the composite becomes exfoliated, the silicate layers cannot hinder the motion of the polymer chains any more. However, in evaluation of the influence of modified montmorillonite on Tg, not only the contribution of the silicate architecture but also that of the modifier itself should be considered. Therefore our DMA results showing the decrease in Tg for the epoxy-based nanocomposites reinforced with various modified montmorillonite (Fig. 8, Table 5) can be attributed to the modifier which may act as a catalyst for both the homopolymerization process of the epoxy resin (Lan and Pinnavaia, 1994) and the desired crosslinking reaction between the epoxy groups and the crosslinking agent. As a consequence, the stoichiometry between the epoxy groups and the small molecules of crosslinking agent is changed, a significant quantity of the latter being not transformed which will act as a plasticizer together with the homopolymer thus leading to the decrease in the Tg (Park and Jana, 2004). DMA results may be correlated with TEM images. There is a dependence between the intercalation or exfoliation degree of layered silicate and the decrease of Tg. As the degree of exfoliation increases, the decrease of Tg is more significant. The system exhibiting the highest degree of exfoliation (DGER/Cl-DGEBF-XTJ 505/D230) gives the highest decrease of Tg in comparison with the reference (Fig. 8, Table 5). This is due to the high compatibility between the modified montmorillonite and the epoxy matrix which allows the penetration of epoxy matrix in large quantities and the homopolymerization process to occur within the silicate gallery, which will lead to a high degree of exfoliation. 4. Conclusions The use of epoxy-amine adducts based on epoxy resins which contain a low oligomeric fraction with hydroxyl groups (DGEBF and TGETPM) leads to a significant decrease of the hydrogen bonds and thus the exfoliated nanocomposites are more likely to be obtained. Therefore the best results were given by the adducts based on DGEBF and TGETPM. The epoxy resins including a high concentration of hydroxyl groups in the structure of oligomers lead to intercalated structures if they are used for synthesis of epoxy-amine adducts.
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The montmorillonite modified with DGEBF-XTJ 505 adduct is dispersed at a nanometric level within the polymer matrix regardless of the epoxy type used as thermoset matrix. The glass transition temperatures of hybrids reinforced with ClDGEBF-XTJ 505 exhibited lower values due to the catalytic activity of the modifiers used to treat the clay for increasing its compatibility with the epoxy matrix. In the case of hybrids in which the montmorillonite exhibit a better compatibility with the epoxy matrix (the silicate layers are considerable intercalated or exfoliated), the glass transition temperatures decreased with 10–20 °C in comparison with classical composites which means that there is a dependence between the intercalation or exfoliation degree of the final nanocomposite and the decrease in Tg. References Awad, W.H., Gilman, J.W., Nyden, M., Harris, R.H., Sutto, T.E., Callahan, J., Trulove, P.C., DeLong, H.C., Fox, D.M., 2004. 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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
The influence of some new montmorillonite modifier agents on the epoxy–montmorillonite nanocomposites structure Sorina-Alexandra Gârea ⁎, Horia Iovu, Georgeta Voicu Polytechnic University of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Polymer Science and Engineering, 149 Calea Victoriei, 010072-Bucharest, Romania
a r t i c l e
i n f o
Article history: Received 20 July 2009 Received in revised form 7 August 2010 Accepted 21 September 2010 Available online 1 November 2010 Keywords: Montmorillonite Nanocomposites Epoxy Glass transition temperatures
a b s t r a c t Some new epoxy-amine adducts were used as modifier agents for montmorillonite in order to enhance the compatibility between inorganic and organic phases. The adducts were synthesized by reacting different epoxy resin types with a stoichiometric amount of polyetheramine which includes hydrophobic and hydrophilic units. The X-ray diffraction (XRD) proved that these epoxy-amine adducts can be intercalated within the layers of montmorillonite and thus lead to an increase of the basal distance. TEM and XRD analysis show that the final structure of epoxy–montmorillonite hybrids strongly depends on the agents used for montmorillonite modification. Dynamic mechanical analysis (DMA) results show that the glass transition temperature of the final nanocomposites depends on the montmorillonite modifier structure. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer–organoclay nanocomposites are considered a new class of advanced organic–inorganic materials. Nanocomposites based on polymer matrices exhibit much improved mechanical properties, thermal and chemical stability, compared to the neat polymers. The unmodified clays such as montmorillonite are hydrophilic and therefore incompatible with a wide range of hydrophobic polymers including epoxy resins. In order to increase the compatibility of the clay with the polymers one option is to modify the clay with some special agents which exhibit high hydrophobic character (Le Pluarta et al., 2004; Liu, 2007). The usual treatment of modification of a clay consists of inorganic (sodium or calcium) cation exchange with organic cations, which may include various substituents in their structures. The intercalation of clays with organic cations leads to an increase of hydrophobic character and thus the compatibility between the polymer and the clay is enhanced (Yoon et al., 2007; Betega de Paiva et al., 2008). The surface energy, basal spacing and thermal stability of modified clay are strongly influenced by the chemical structure, packing density and the cation type of the modifier agent (Lee and Kim, 2002; Di Gianni et al., 2009). The modifier agents most often used for clay are quaternary ammonium salts, phosphonium salts, alkylimidazolium salts, oligomers and polymer cations and oligomeric amine derivatives (Lagaly, 1994; Awad et al., 2004; Su et al., 2004a,b; Hedley et al., 2007; Leszczynska et al., 2007; Patel et al., 2007). ⁎ Corresponding author. Tel.: + 40 214022713; fax: + 40 214023844. E-mail address: [email protected] (S.-A. Gârea). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.09.010
The aim of this study was to investigate the influence of some new montmorillonite modifier agents on the nanocomposite structure using various techniques like Transmission Electron Microscopy (TEM), Dynamic Mechanical Analysis (DMA) and Thermogravimetric analysis (TGA). 2. Experimental 2.1. Materials The sodium montmorillonite (Cloisite Na–ClNa) with a Cationic Exchange Capacity (CEC) of 92.5 meq/100 g clay was supplied by Southern Clay Products (TX). Diglycidyl ether of resorcinol (DGER), Diglycidyl ether of bisphenol F (DGEBF), Diglycidyl ether of polydimethylsiloxane (DGEPDMS), Diglycidyl ether of 1, 4 cyclohexanedimethanol (DGECHM) and Triglycidyl ether of triphenylol methane (TGETPM) epoxy resin types were supplied by FLUKA and used as received. The polyetheramines Jeffamine XTJ 505 (XTJ 505) and Jeffamine D230 (D 230) were provided by Huntsman. Jeffamine D230 (D 230) was used as crosslinking agent. CHROMASOLV Tetrahydrofurane (THF) type was received from FLUKA. 2.2. Synthesis of epoxy-amine adducts The epoxy-amine adducts were synthesized by reacting epoxy resins with a stoichiometric amount of poyetheramine XTJ 505 (Scheme 1).
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kept at 80 °C for 3 h. Then the modified montmorillonites were isolated and washed with hot deionized water until no chloride ions were detected by one drop of 0.1 N AgNO3 solution. The product was dried for two days at 110 °C and then ground in a mill to produce powder. 2.4. Synthesis of epoxy–montmorillonite nanocomposites An amount of epoxy resin (5 g) was introduced into a glass vessel at 80 °C and then a certain amount of modified montmorillonite (5 wt.% to epoxy resin) was added and the mixture was sonicated using an ultrasound device for 3 h. Then, the mixture was degassed for a few minutes using an ultrasound bath and a stoichiometric amount of curing agent (D230) was added. The final mixture was degassed for 5 min and then was poured into a Teflon mould. It was cured for 24 h at room temperature and post-cured for 1 h at 100 °C. 2.5. Characterization methods 2.5.1. Fourier-transform infrared spectroscopy (FTIR) FTIR spectra were recorded on a SHIMADZU 8900 equipment using 40 scans and 4 cm−1 resolution. The samples were analyzed from KBr pellets. 2.5.2. Dynamic mechanical analysis (DMA) The glass transition temperature (Tg) of cured samples was established using Dynamic Mechanical Analysis (DMA). DMA tests were done on a Triton 2000 equipment (Triton Technology). The samples were run in single cantilever bending mode. The value of the glass transition temperature (Tg) was estimated from the maximum of tan δ–temperature curve. 2.5.3. X-ray diffraction (XRD) The XRD analysis was performed on a XRD 6000 SHIMADZU diffractometer. 2.5.4. Gel permeation chromatography (GPC) The molecular weight distributions of epoxy resins used as raw materials for adducts synthesis were found using a KNAUER Gel Permeation Chromatograph equipped with RI detector and Macherey Nagel column thermostated at 25 °C. Standards of polystyrene were used for calibration and tetrahydrofurane as eluent. The flow rate was 1 ml/min. Scheme 1. The synthesis reactions of epoxy-amine adducts.
Thus 0.1 mol of difunctional epoxy resins (DGEBF, DGECHM, DGEPDMS) and 0.1 moles of trifunctional epoxy resin (TGETPM) were dissolved in 10 ml THF in a 50 ml glass flask equipped with reflux condenser, magnetic stirrer and dropping funnel. The reaction mixture was heated at 60 °C. For all the resins a stoichiometric amount of amine was added. Thus for 0.1 mol of resin (DGEBF, DGECHM, DGEPDMS) and 0.1 mol of TGETPM the amount of Jeffamine XTJ 505 was 0.2 mol and 0.3 mol respectively. The reaction mixture was stirred for 5 h. The THF was removed from the final products using a rotavapor.
2.5.5. Transmission electron microscopy (TEM) TEM images were recorded on a Philips CM 120 ST equipment using an acceleration voltage of 100 kV. Thin specimens of about 50– 100 nm were cut from nanocomposite blocks using an ultramicrotome equipped with a diamond knife at ambient conditions. 2.5.6. Thermogravimetrical analysis (TGA) TGA tests were done on a TA Instrument Q 500. The samples of 10 mg were heated from 30 to 800 °C at 10 °C/min scanning rate under a constant nitrogen flow of 100 mL/min. 3. Results and discussion
2.3. Synthesis of modified montmorillonite 3.1. Synthesis and characterization of adducts The organophilization of montmorillonite was performed by a cationic exchange process between protonated epoxy-amine adducts and sodium montmorillonite. The epoxy-amine adducts were first protonated with 30% HCl in order to be used as modifiers for montmorillonite. Thus a certain amount of epoxy-amine adduct was dissolved in 12 ml mixture (THF = 3 ml; H2O = 9 ml) and 4.4 ml 30% HCl was added. After that the mixture was stirred for 30 min at 40 °C. The protonated adducts were added to 2.5 g ClNa already swollen in 200 ml hot deionized water for 1 h at 80 °C. The suspension was
An important goal of this work was to synthesize some new epoxyamine adducts for modifying the montmorillonite used as reinforcing agent in nanocomposites, the adducts being more compatible with the epoxy matrix. The epoxy-amine adducts synthesis started from epoxy resins which contain different concentrations of oligomers. Most of the commercial epoxy resins are provided as mixtures including fractions of oligomers obtained by opening of the functional epoxy groups during the synthesis procedure. The concentration of
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Table 1 Characteristics of epoxy resins used in epoxy-amine adduct synthesis obtained from GPC analysis. Epoxy resin type
Mna (n = 0) (g/mol)
Mn (n N 0) (g/mol)
Concentration of monomer (n = 0) (%)
Concentration of oligomers (n N 0) (%)
DGEBF (Gârea et al., 2006)
312 – – 460 – 257
– 410 719 – 754
80 – – 75.4 – 60 –
– 10.2 9.8 – 24.6 – 40
TGETPM (Gârea et al., 2006) DGECHM
373 a
Molecular weight.
the oligomeric fractions was calculated from the area of the peaks revealed in GPC curves at lower elution times than monomer (Table 1, Fig. 1). From Table 1, one may notice that DGECHM exhibits the highest amount of oligomers (40%). The GPC curve for DGEPDMS (Fig. 1) shows a single peak, very large which indicates that DGEPDMS contains a mixture of oligomeric fractions. The chemical structures of the epoxy-amine adducts were confirmed by FTIR analysis. Fig. 2 shows two examples of FTIR spectra for DGECHM-XTJ 505 and TGETPM-XTJ 505 epoxy-amine adducts. From Fig. 2 one may notice that the peak at 910 cm−1 corresponding to the stretching vibration of epoxy ring totally disappeared. This fact proves that all the epoxy groups were consumed in the reaction with XTJ 505. Similar FTIR spectra were recorded for the other epoxyamine adducts.
Fig. 2. FTIR spectra of 1-DGECHM-XTJ 505, 2-TGETPM-XTJ 505.
3.2. Characterization of modified montmorillonites 3.2.1. FTIR analysis The FTIR analysis was useful in order to detect some new peaks corresponding to the stretching and bending vibrations of the modifier agents molecules which were intercalated within the layers of montmorillonite after the cationic exchange reaction took place. From Fig. 3 one may observe the presence of some new peaks at 2974, 2931 and 2875 cm−1 which may be assigned to C–H stretching vibration of CH3 and CH2 groups and also the peak at 1454 cm−1 corresponding to C–H deformation vibration of CH2 groups from epoxy-amine adduct chain. In the FTIR spectra of aromatic epoxyamine adducts (TGETPM-XTJ 505 and DGEBF-XTJ 505) additional peaks appeared at 1600 and 1510 cm−1, these being assigned to C–C and C–H stretching vibrations from the benzene ring. 3.2.2. XRD analysis The basal distances of modified montmorillonites with epoxyamine adducts were determined from XRD spectra (Fig. 4, Table 2). The XRD results show an increase of basal distances with at least 6 Å for all the modified clays which is an argument that the cationic exchange process took place and therefore the new bulky adducts
Fig. 3. FTIR spectra of unmodified montmorillonite (1-ClNa) and modified montmorillonite with different agents: 2-Cl-DGEPDMS-XTJ 505, 3-Cl-DGECHM-XTJ 505, 4-ClTGETPM-XTJ 505, and 5-Cl-DGEBF-XTJ 505.
penetrate within the silicate gallery causing a significant increase of the basal distance. 3.2.3. TGA tests The thermal stability of the modifiers for montmorillonite is a significant factor in choosing the proper method for nanocomposite synthesis (in situ polymerization, melt polymerization etc.) and it may also influences the thermostability of the final nanocomposites. The most common modifier agents for montmorillonite are quaternary ammonium salts which exhibit a low thermostability. Park and Jana, (2004) showed that the starting temperature of degradation at which the weight loss is 5% (Tonset 5%) is 162 °C for montmorillonite modified with n-hexadecyl ammonium chloride in nitrogen atmosphere. By comparison, the thermal stability of the montmorillonite modified with our new epoxy-amine adducts is significant higher (Table 3).
Fig. 1. GPC curves of DGECHM and DGEPDMS.
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S.-A. Gârea et al. / Applied Clay Science 50 (2010) 469–475 Table 3 Data from TGA and DTG of different modified montmorillonite. Modified montmorillonite
Weight loss (%)
Tonseta (°C)
Tmaxb (°C)
Cl-DGEBF-XTJ 505 Cl-TGETPM-XTJ 505 Cl-DGECHM-XTJ 505 Cl-DGEPDMS-XTJ 505
44 46 34 32
247 256 190 200
269 270 210 208
a b
The temperature at which the weight loss is 5%. Temperature at maximum rate of weight loss.
Table 4 The thermostability of various hybrid materials.
Fig. 4. XRD spectra of ClNa and modified montmorillonite with different agents.
System
Tonset
DGER/D230 DGER/Cl-DGECHM-XTJ 505/D230 DGER/Cl-DGEPDMS-XTJ 505/D230 DGER/Cl-DGEBF-XTJ 505/D230 DGER/Cl-TGETPM-XTJ 505/D230
332 320 324 327 332
a b
From Table 3 one may observe that in all cases, the weight loss is higher than 31% which means that a significant quantity of organophilization agent was introduced within the silicate layers through the cationic exchange process. The organic cations based on protonated adducts show a different thermal stability which depends on the nature of epoxy resin used in the adduct synthesis. The epoxy-amine adducts based on aromatic epoxy resins (DGEBF, TGETPM) exhibit higher thermal stability in comparison with those based on cycloaliphatic epoxy resins. Thus the starting decomposition temperature (Tonset) exhibits a higher value for aromatic adduct (250 °C) than for aliphatic ones which allow the use of aromatic adducts for nanocomposites synthesis through a melt process.
3.3. Synthesis and characterization of new epoxy-modified montmorillonite nanocomposites New nanocomposites based on epoxy resin and different types of modified montmorillonite as reinforcing agent (Cl-DGEBF-XTJ 505, Cl-TGETPM-XTJ 505, Cl-DGEPDMS-XTJ 505, and Cl-DGECHM-XTJ 505) were synthesized.
a 5%
Tonset
b 10%
346 338 340 342 345
The temperature at which the weight loss is 5%. The temperature at which the weight loss is 10%.
3.3.2. XRD analysis From Fig. 5 one may notice that the best results were obtained in the case of epoxy systems which include modified clays with aromatic adducts (Cl-DGEBF-XTJ 505 and Cl-TGETPM-XTJ 505). In these cases the peak assigned to the basal distance totally disappeared which indicates that exfoliated structures are probably formed. This fact suggested that the presence of DGEBF and TGETPM molecules between Jeffamine XTJ 505 chains leads to an increase in the compatibility of the clay surface with the polymer matrix. In case of hybrids which include clays intercalated with aliphatic adducts (ClDGEPDMS-XTJ 505, Cl-DGECHM-XTJ 505) a small peak is still present and therefore intercalated structures are finally obtained. 3.3.3. TEM analysis Fig. 6a shows TEM images of the DGER/Cl-DGECHM-XTJ 505 system which gives intercalated structures, the silicate layers exhibiting a basal distance of about 50–60 Å. Even if the distance between the layers significantly increased, the layers still exhibit parallel orientation so that only intercalated structures are obtained due to the fact that the penetration of the polymer matrix (DGER) within the silicate layers is somewhat
3.3.1. TGA tests The influence of the modified montmorillonite with aromatic adducts (TGETPM-XTJ 505, DGEBF-XTJ 505) is extremely low for the system including Cl-DGEBF-XTJ 505 and does not exist for nanocomposites based on Cl-TGETPM-XTJ 505. A low decrease of thermostability is obtained for modified montmorillonite with aliphatic adducts (Table 4) but this does not affect the industrial applications of the nanocomposites. Similar results were reported by other authors concerning the thermostability of epoxy-modified montmorillonite nanocomposites (Wang and Pinnavaia, 1998; Becker et al., 2004).
Table 2 XRD results for modified clay with epoxy-amine adducts. Montmorillonite type
Basal distance (Å)
ClNa Cl-DGEBF-XTJ 505 Cl-TGETPM-XTJ 505 Cl-DGEPDMS-XTJ 505 Cl-DGECHM-XTJ 505
11.7 17 17.3 17.6 17.4
Fig. 5. XRD curves of hybrids based on DGER and modified montmorillonites with different agents.
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Fig. 6. TEM images of hybrid materials at different magnifications: a — DGER/Cl-DGECHM-XTJ 505/D230, b — DGER/Cl-DGEPDMS-XTJ 505/D230, c — DGER/Cl-DGEBF-XTJ 505/D230.
hindered by the high number of hydrogen bonds formed between the numerous hydroxyl groups from the oligomers of the modifier (DGECHM-XTJ 505). This is in good agreement with the GPC results which showed that the content of oligomers for DGECHM is much
higher than for DGEBF or TGETPM. Also the lower compatibility between aliphatic units of the modifier (DGECHM) and the aromatic polymer matrix (DGER) may contribute to the final results of intercalated nanocomposites.
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Fig. 7. TEM images of hybrid materials at different magnifications: a — TGETPM/Cl-DGEBF-XTJ 505/D230, b — DGEBF/Cl-DGEBF-XTJ 505/D230.
The TEM images of epoxy nanocomposites based on DGER and ClDGEPDMS-XTJ 505 (Fig. 6b) confirmed the formation of intercalated structures. In this case different zones with various distances between the silicate layers are observed, mainly one zone with a large distance (120 Å) and another one with a lower distance (80 Å). This is due to the large molecular weight distribution of DGEPDMS (Fig. 1) which means that different chain lengths cause various distances between the silicate layers when the clay is modified with the adduct based on DGEPDMS. Fig. 6c shows the TEM images for the epoxy system based on DGER and Cl-DGEBF-XTJ 505 as a reinforcing agent. One may observe
that the structure is a typically exfoliated one in good agreement with the XRD analysis (Fig. 5). Due to the higher compatibility between the epoxy resin and the modified montmorillonite with DGEBF-XTJ 505 adduct, the reactions within the silicate gallery are favoured such as the DGER homopolymerization catalyzed by the clay modifier and the DGER crosslinking process enhanced by the diffusion of crosslinking agent between the silicate layers. The homopolymer formation and/or the crosslinking process within the silicate gallery are both factors which determine the occurrence of exfoliated structures. Also for the hybrid system based on DGER and Cl-TGETPM-XTJ 505 as reinforcing agent an exfoliated structure was obtained. The silicate layers showed a basal distance higher than 100 Å. The TGETPM-XTJ 505 adduct was synthesized starting from TGETPM epoxy resin with a low content of oligomers so that a low number of hydrogen bonds are established within the silicate gallery between the hydroxyl groups and as a consequence the layers separation is not hindered. The modified montmorillonite with DGEBF-XTJ 505 was also introduced in other aromatic epoxy resin types with different viscosity values (DGEBF, TGETPM) in order to verify if the viscosity of polymer matrix influences the final structure of hybrids. Also in
Table 5 Tg values of epoxy–clay hybrids calculated from DMA.
Fig. 8. The dependence of tan δ against temperature for epoxy systems in the presence and in the absence of different modified montmorillonites.
Epoxy-modified montmorillonite hybrids
Tg (°C)
DGER/D230 DGER/Cl-DGEBF-XTJ 505/D230 DGER/Cl-TGETPM-XTJ 505/D230 DGER/Cl-DGEPDMS-XTJ 505/D230 DGER/Cl-DGECHM-XTJ 505/D230
85 71 73 78 79
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these cases the silicate layers were individually dispersed in the whole mass of the epoxy resin and exfoliated structures were obtained (Fig. 7b). Therefore it seems that the initial viscosity value of the epoxy resin is not an important factor which may influence the formation of the nanocomposites structure. Even in the case of TGETPM which exhibits a high viscosity value the silicate layers are individually dispersed (Fig. 7a). 3.4. The influence of modified montmorillonite on the glass transition temperature of hybrids The effect of modified montmorillonite on the glass transition temperature (Tg) of final nanocomposites has been extensively studied in the literature but the results are still contradictory. Many authors have shown an increase in the Tg (Koerner et al., 2006; Marras et al., 2009) but some authors have observed a decrease in Tg especially for epoxy-based nanocomposites (Lan and Pinnavaia, 1994; Wang and Pinnavaia, 1994; Becker et al., 2003; Park and Jana, 2004). The increase in Tg was explained by the restricted chain motion of the polymer which is confined in the silicate galleries (Messersmith and Giannelis, 1994). Thus for polystyrene-based nanocomposites reinforced by modified montmorillonite with azobisisobutyronitrile an increase of Tg with 10 °C against virgin polystyrene was noticed (Vyazovkin and Dranca, 2004). This explanation may be valid only for intercalated nanocomposites. As the composite becomes exfoliated, the silicate layers cannot hinder the motion of the polymer chains any more. However, in evaluation of the influence of modified montmorillonite on Tg, not only the contribution of the silicate architecture but also that of the modifier itself should be considered. Therefore our DMA results showing the decrease in Tg for the epoxy-based nanocomposites reinforced with various modified montmorillonite (Fig. 8, Table 5) can be attributed to the modifier which may act as a catalyst for both the homopolymerization process of the epoxy resin (Lan and Pinnavaia, 1994) and the desired crosslinking reaction between the epoxy groups and the crosslinking agent. As a consequence, the stoichiometry between the epoxy groups and the small molecules of crosslinking agent is changed, a significant quantity of the latter being not transformed which will act as a plasticizer together with the homopolymer thus leading to the decrease in the Tg (Park and Jana, 2004). DMA results may be correlated with TEM images. There is a dependence between the intercalation or exfoliation degree of layered silicate and the decrease of Tg. As the degree of exfoliation increases, the decrease of Tg is more significant. The system exhibiting the highest degree of exfoliation (DGER/Cl-DGEBF-XTJ 505/D230) gives the highest decrease of Tg in comparison with the reference (Fig. 8, Table 5). This is due to the high compatibility between the modified montmorillonite and the epoxy matrix which allows the penetration of epoxy matrix in large quantities and the homopolymerization process to occur within the silicate gallery, which will lead to a high degree of exfoliation. 4. Conclusions The use of epoxy-amine adducts based on epoxy resins which contain a low oligomeric fraction with hydroxyl groups (DGEBF and TGETPM) leads to a significant decrease of the hydrogen bonds and thus the exfoliated nanocomposites are more likely to be obtained. Therefore the best results were given by the adducts based on DGEBF and TGETPM. The epoxy resins including a high concentration of hydroxyl groups in the structure of oligomers lead to intercalated structures if they are used for synthesis of epoxy-amine adducts.
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The montmorillonite modified with DGEBF-XTJ 505 adduct is dispersed at a nanometric level within the polymer matrix regardless of the epoxy type used as thermoset matrix. The glass transition temperatures of hybrids reinforced with ClDGEBF-XTJ 505 exhibited lower values due to the catalytic activity of the modifiers used to treat the clay for increasing its compatibility with the epoxy matrix. In the case of hybrids in which the montmorillonite exhibit a better compatibility with the epoxy matrix (the silicate layers are considerable intercalated or exfoliated), the glass transition temperatures decreased with 10–20 °C in comparison with classical composites which means that there is a dependence between the intercalation or exfoliation degree of the final nanocomposite and the decrease in Tg. References Awad, W.H., Gilman, J.W., Nyden, M., Harris, R.H., Sutto, T.E., Callahan, J., Trulove, P.C., DeLong, H.C., Fox, D.M., 2004. 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