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The effect of addition of acrylic acid and thioglycolic acid on the nanostructure and thermal stability of PMMA–montmorillonite nanocomposites
Applied Clay Science 47 (2010) 414–420
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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 e v i e r. c o m / l o c a t e / c l a y
The effect of addition of acrylic acid and thioglycolic acid on the nanostructure and thermal stability of PMMA–montmorillonite nanocomposites Adriana A. Silva, Karim Dahmouche, Bluma G. Soares ⁎ Macromolecular Institute, Federal University of Rio de Janeiro (UFRJ), 21945-970, Rio de Janeiro-RJ, Brazil
a r t i c l e
i n f o
Article history: Received 28 July 2009 Received in revised form 11 December 2009 Accepted 15 December 2009 Available online 4 January 2010 Keywords: PMMA Organo-montmorillonite SAXS Thermal stability Thermogravimetric analysis Nanocomposites
a b s t r a c t Poly(methyl methacrylate) (PMMA)/organophilic montmorillonite (OMMT) nanocomposites were synthesized by the in situ free radical bulk polymerization of methyl methacrylate. The effect of small amount of acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent on the nanostructure of these nanocomposites was investigated by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) analyses. No reflection was discernible in the XRD patterns of all samples. In SAXS experiments, a great amount of small clay montmorillonite aggregates (less than 40 Å) were observed. The acrylic acid used as the co-monomer increased the affinity between the montmorillonite and the polymer, resulting in better dispersion of the montmorillonite particles. The glass transition temperature of the nanocomposites was evaluated by differential scanning calorimetry (DSC) and the thermal stability was investigated by thermogravimetric studies (TGA). As expected, the presence of the montmorillonite increased the glass transition temperature. The thermal stability of nanocomposites was distinctly higher than of pure PMMA and was increased by the presence of small amounts of acrylic acid as the co-monomer. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Poly(methyl methacrylate) (PMMA) is an important thermoplastic material, which presents interesting characteristics such as high transparency, high strength and dimensional stability. Because of these outstanding properties, PMMA is widely used as transparent covers for several purposes including special devices for electronic industries. However, its poor heat resistance, brittleness and stress cracking in most organic solvents limit its application. The dispersion of small amounts of montmorillonite within the PMMA matrix has been considered a very promising strategy for improving thermal stability and barrier properties without losing its optical properties (Gilman, 1999). The enhancement of these properties is based on the ability of the polymer chains to penetrate into the interlayer spaces of the montmorillonite yielding separated platelets, giving rise to the ascalled nanocomposites with intercalated and/or exfoliated structures. This process is usually possible for several monomers and polymers because the cations which naturally reside within the interlayer space regions can be easily exchanged by large, organic cations, increasing the organophilic character of the montmorillonite (Tjong, 2006; Goettler et al., 2007; Liu, 2007; Pavlidou and Papaspyrides, 2008). The intercalated structure consists of a well ordered multilayered structure of silicate containing polymer chains inserted into the interlayer, whereas a complete exfoliated structure presents individ-
⁎ Corresponding author. Tel./fax: +55 21 25627207. E-mail address: [email protected] (B.G. Soares). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.12.010
ual silicate layers, homogeneously dispersed inside the polymer matrix. In practice, exfoliated structures are very rare even using strongly organophilic montmorillonite. The best way to prepare PMMA-based nanocomposites with a high degree of intercalation/exfoliation is through the in situ lamellar polymerization of methyl methacrylate (MMA). In this procedure, the clay minerals are swollen by the liquid monomer or its solution, whose polymerization inside the interlayer spaces expands the distance between the silicate layers and leads to an exfoliation of the particles. The in situ polymerization has been performed in bulk (Okamoto, et al., 2000, 2001; Su and Wilkie, 2003; Wei'an et al., 2003; Xie et al., 2003; Zhang et al., 2003; Qu et al., 2005; Stadtmueller et al., 2005; Ratinac et al., 2006; Yoon et al., 2007), in solution (Chen et al., 1999; Li et al., 2003; Wang et al., 2005), and emulsion (Lee and Jang, 1996; Choi et al., 2001; Essawy et al., 2004; Meneghetti and Qutubuddin, 2004; Yeh et al., 2004). The emulsion polymerization has been carried out in the presence of pristine montmorillonite (Lee and Jang, 1996; Choi et al., 2001) or alkyl ammonium and alkyl phosphonium montmorillonites (Yeh et al., 2004). The bulk and solution polymerization processes are usually performed in the presence of the organo-montmorillonite to improve its compatibility with the monomer and facilitate the diffusion of the monomer into the interlayer spaces. The degree of exfoliation can also be improved by using montmorillonite modified with a quaternary ammonium salt containing a free radical initiator moiety (Huang and Brittain, 2001), or a vinyl group, which acts as a co-monomer during the in situ interlamellar polymerization of MMA (Zhang et al., 2003; Ratinac et al., 2006). The
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use of a small amount of polar co-monomers in the in situ polymerization of MMA was also reported by Okamoto et al. (2001) as a way to improve the degree of dispersion and intercalation in the corresponding nanocomposites. They have used N,N-dimethylaminopropyl acrylamide, N,N-dimethyl-aminoethyl acrylate and acrylamide as the co-monomers and observed a greater exfoliation degree when acrylamide was used as the co-monomer. The use of acrylic acid as the co-monomer is also a good strategy for improving the interaction between the PMMA chains and the clay mineral surface, through hydrogen bonds. The carboxyl groups in the corresponding nanocomposite can also impart better compatibility with other polymer systems such as epoxy resins (Zaioncz et al., 2007). The present paper aims studying the effect of small amounts of acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent on the degree of intercalation/exfoliation of PMMAbased nanocomposites prepared by in situ interlamellar polymerization. For this purpose, a commercial organophilic montmorillonite (OMMT), Viscogel B8, was swollen by the monomers. The bulk polymerization was performed in the presence of an organic initiator. The degree of intercalation/exfoliation was investigated by wide angle X-ray diffraction (WAXS) and small-angle X-ray scattering (SAXS). A stacked-disc model was adapted from the literature (Porod, 1982; Hanley et al., 1997; Hernandez et al., 2007) to estimate the average size of the OMMT aggregates dispersed in the PMMA matrix. The thermal stability of PMMA-based nanocomposites was also discussed focusing the effect of the acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent. 2. Experimental 2.1. Materials The organoclay (OMMT) used in this work was Viscogel B8, supplied by Bentec Rheological Additives (Livorno, Italy). This is a montmorillonite modified with octadecylammonium salt. Methyl methacrylate (MMA) and acrylic acid (AA) (Fig. 1) were purchased from Methacryl do Brasil (São Paulo, Brazil) and were distilled under reduced pressure. Azo-bis-isobutyronitrile (AIBN) from Merck, used as free radical initiator, was recrystallized from methanol/water. Thioglycolic acid (TA) (Fig. 1) (from Aldrich) was used as the chain transfer agent without purification. 2.2. Synthesis of PMMA–OMMT nanocomposites Different amounts of OMMT were dispersed into 20 g (0.5 mol) of MMA (series M) or 20 g (0.5 mol) of MMA and 0.4 g (4 mmol) of AA (series A) under intensive stirring at room temperature for 24 h. After achieving complete dispersion of the OMMT, 0.61 g (1 mmol) AIBN was added under stirring and N2 atmosphere. The polymerization was performed at 50 °C for 24 h and completed at a temperature of 85 °C for 1.5 h. Nanocomposites consisted of lower PMMA molar mass were also prepared by adding 0.34 g (4 mmol) of thioglycolic acid in the reaction medium containing the organo-montmorillonite dispersed into MMA/AA monomers (series B). The resulting nanocomposites were dispersed in acetone, precipitated with methanol, filtered and dried under vacuum at 60 °C. (Table 1) The amount of montmorillonite in each sample was determined by thermogravimetry analysis.
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Table 1 Composition of PMMA–clay nanocomposites prepared by in situ bulk polymerization. Compound code M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
Feed composition (g) MMA
AA
TG
OMMT
100 97.5 95 90 100 97.5 95 90 100 97.5 95 90
0 0 0 0 2 2 2 2 2 2 2 2
0 0 0 0 0 0 0 0 1.7 1.7 1.7 1.7
0 2.5 5.0 10 0 2.5 5.0 10 0 2.5 5.0 10
OMMT content in the compositea (%) 0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
a OMMT content determined from the residue obtained from thermogravimetry analysis.
2.3. Characterization Molar mass and molar mass distribution were determined by Size Exclusion Chromatography (SEC) using a 600 module system from Water Associates equipped with a refractive index detector model 2414. Microstyragel columns with 102, 103 and 104 porosity were employed. The nanocomposites were extracted in a Sohxlet extractor for 48 h with tetrahydrofuran as the solvent. The extracted fraction in THF was precipitated with methanol and re-dissolved in tetrahydrofuran at a concentration of 0.1 g/100 mL. The molar mass of the extracted fraction was determined using a calibration curve obtained from polystyrene standards. X-ray diffraction patterns (XRD) were recorded on a DMAX-RC (Miniflex) X-ray diffractometer (Rigaku Co., Tokyo, Japan), equipped with Cu Kα radiation source operating at 40 kV and 100 mA. The scanning rate was 2 °C min− 1. The thermal stability was measured by thermogravimetry analysis in a TA Q50 thermogravimetric analyzer. Samples of 20 mg were heated to 600 °C at a heating rate of 20 °C/min under N2 atmosphere. The thermal properties of the composites were measured by differential scanning calorimetry (DSC) in a Perkin-Elmer DSC-7 calorimeter at a heating or cooling rate of 20 °C min− 1 under a N2 atmosphere between 35 and 160 °C. The samples were heated, cooled and re-heated again and the glass transition temperature (Tg) was recorded from the second heat scanning. The nanostructural investigation of the composites was performed by small-angle X-ray scattering (SAXS). The SAXS measurements were performed at room temperature using the beam line of National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. This beamline was equipped with an asymmetrically cut and bent silicon (111) monochromator that yielded a monochromatic (λ = 1.608 Å) and horizontally focused beam. The patterns were acquired for scattering vectors (q) in the range 10− 2 Å− 1 b q b 0.12 Å− 1. The scattering intensity I(q) was plotted as a function of the modulus of scattering vector, q = (4π/λ)sin (bthetaN/2), where bthetaN is the scattering angle. The scattering intensity was normalized by subtracting (background scattering) and sample thickness. Each SAXS pattern corresponds to a data collection time of 900 s. 3. Results and discussion 3.1. Structural investigation
Fig. 1. Chemical structures of the monomers and reagent.
The qualitative evaluation of the dispersion of the montmorillonite particles in PMMA–OMMT was performed by XRD. Fig. 2 compares the XRD patterns of the OMMT and the PMMA-based nanocomposites (Series M, A and B). The pure OMMT showed a basal reflection at 2θ = 3.52° corresponding to a basal spacing d2 = 25 Å. In addition, a
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Fig. 2. XRD patterns of the OMMT and the corresponding PMMA-based nanocomposites as a function of the OMMT content.
less intense reflection located at 7.0° indicating a basal spacing of d1 = 12.6 Å was assigned to the unmodified montmorillonite. In all patterns of the series M, A and B the former reflection was quite totally absent and the latter was very poorly defined, evidencing the high proportion of exfoliated platelets or small particles composed of a very small number of layers. This indicated the efficiency of intercalation of PMMA chains promoted by the in-situ polymerization of MMA. This effect is attributed to the large volume occupied by the polymer chains between the silicate layers, after the in situ lamellar polymerization of MMA. The good nanometric dispersion of the particles was also evidenced by the transparency of the films casted from the THF solution. To determine more quantitatively the dispersion of the montmorillonite particles within the PMMA matrix, small-angle X-ray scattering measurements (SAXS) was performed. This technique reveals the behavior of the entire sample, and sample preparation is much simpler than transmission electron microscopy. The SAXS patterns of all composites (Fig. 3) showed two interference peaks located around qmax = 0.2 Å− 1 and 0.4 Å− 1. Since no scattering was detected for OMMT-free samples, these peaks are associated to the interlayer spaces between the silicate layers. The interlayer distances determined from the position of the peak maxima (d = 2π/qmax) are presented in Table 2. The larger values of d1 and d2 compared to those of pure OMMT, determined from XRD, indicated the intercalation of the polymer chains in agreement with the XRD patterns. As expected, the
Fig. 3. SAXS patterns of PMMA-based nanocomposites as a function of the OMMT content, prepared in different conditions (Series M, A and B).
SAXS intensity of the peak located at larger q-values was lower than the other one, due to the lower amount of unreacted MMT particles. The existence of OMMT particles of nanometric size dispersed in the polymer matrix was confirmed by the additional scattering observed in SAXS spectra of all samples at low q-range (below 0.1 Å− 1). The presence of these particles was also confirmed by the value of the slope α of the linear regime observed in log–log plot for q b 0.07 Å− 1 (Fig. 3). According to the literature (Hernandez et al., 2007), the scattering intensity, I(q), for a three-dimensional network of linked platelets, corresponds to I(q) = q− α, with α around 3. The values of α (Table 2), located for all composites between 2.6 and 3, indicated the
Table 2 Values of clay interlayer spacing and of the exponent α obtained from XRD (for pure organoclay) and SAXS for PMMA–OMMT based nanocomposites. Sample
OMMT (-wt.%)
d1 (Å)
d2 (Å)
α
OMMT M1 M2 M3 A1 A2 A3 B1 B3
100 3 5 15.5 3 4.2 11.5 3.5 13
12.6 17.2 17 17.2 – 18.2 18.1 17.4 14.2
25 34.3 34.3 34.3 35.9 36.3 36.7 35.7 28.3
– 3 3 3 2.8 2.6 2.7 2.9 2.9
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presence of OMMT particles constituted of a significant number of layers. To have a more quantitative knowledge of the average size of the particles dispersed in the polymer matrix, the stacked-disc model was employed (Porod, 1982; Hanley et al., 1997). However, as mentioned by Hanley et al. (2003) the use of this model for composites containing particles with a wide distribution of thicknesses is questionable, since all particles contribute to the X-ray scattering. Furthermore, this model assumes a uniform dispersion of the particles but neither the exfoliated platelets nor thin montmorillonite particles were uniformly dispersed throughout the matrix. We have employed the methodology also adopted by Hernandez et al. (2007) who have used the aspect ratio of a plate, and considered a wide distribution of aggregate thicknesses. In a general way, the SAXS intensity I(q) is the product of a form factor F(q) (which contains informations about size, shape or surface state of an heterogeneity) and a structure factor S(q) (which contains informations about the position of the heterogeneity relatively to the others). The form factor P(q) of a flat particle of thickness t is given by the following law: PðqÞ =
. . 2 B 2 2 ðqt = 2Þ Δρ t sin 2 q
ð1Þ
qt
where B is a constant proportional to the surface of the particles and is the difference between the electronic densities of particles and the matrix (Porod, 1982). For a diluted system of particles, S(q) = 1 and consequently, the SAXS intensity I(q) = F(q). A distribution of thicknesses was assumed and the SAXS spectrum was considered as being the sum of contributions of several populations, with a fixed number of platelets in a stack constituting a population. For all samples, the best fit of SAXS intensity, performed up to 0.07 Å− 1, was obtained considering the weighted sum of the contributions arising from 4 entities of thickness ti. i = 4 = 1
IðqÞ = ∑i
. . 2 Bi 2 ðqti = 2Þ sin t i 2 q
qti
ð2Þ
The parameters Bi and ti were determined by applying a least square fitting procedure. The proportion Pi of the particles of size ti within the matrix was then estimated by the relation: Pi =
Bi
. ∑J Bj
ð3Þ
The populations of particles with different thicknesses determined from this procedure are summarized in Table 3. For all composites, the proportion of exfoliated particles (thickness around 10 Å) or very small OMMT aggregates formed by two lamellas (thickness around 40 Å) was N89%, which is consistent with XRD measurements and confirms the high degree of dispersion of MMT in all composites. The fraction of particles formed by three or more layers was larger in composites of series M than in series A and B, which is consistent with the larger values of the exponent α obtained for the materials of the family M and with the fact that the main interference peak in the SAXS spectra of these samples was better defined. These results showed that the introduction of AA promoted a better affinity between the OMMT and the polymer in composites of series A and B, resulting on a better dispersion of the montmorillonite particles. This is due to the increased polarity caused by the introduction of carboxylic groups in the polymer and the possibility to form hydrogen bonds between the carboxylic groups of AA and the ammonium groups of the modified MMT.
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Table 3 Area fraction of particles of thickness t in PMMA–OMMT based composites obtained from SAXS measurements. Sample
t b 40 Å
40 Å b t b 100 Å
100 Å b t b 200 Å
t N 200 Å
M1 M2 M3 A1 A2 A3 B1 B3
91.2% 90% 89.6% 98.5% 98.0% 97.0% 95.8% 97.0%
– 6.5% 6.0% – 1.5% 2.0% 2.2% 2.3%
7.2% 3.5% 4.4% 1. 3% 0.4% 0.9% 1.8% –
1.6% 0.1% – 0.2% 0.1% 0.1% 0.2% 0.7%
The increase in OMMT contents in samples of series M did not significantly change the degree of the OMMT dispersion, evidenced by the constancy of the fraction of around 90% of small particles. In series A and B, the fraction of exfoliated or very thin particles remained at a higher level, independently of OMMT content. Again, this trend confirmed the ability of AA to enhance the affinity between OMMT and the polymer. The incorporation of thioglycolic acid (TA) seems to inhibit the OMMT dispersion in the polymer matrix. This was attested by the smaller interlayer distances between the lamellae and the slightly higher proportion of thin particles in composites of series B compared to series A at low OMMT content. As the OMMT content in the composite increased, the degree of dispersion of B3 slightly increased, probably because of the decrease of the TA proportion related to the OMMT content. Another explanation should be that high contents of OMMT favor the probability of interactions of thiol groups with metal ions located at the clay mineral surface, promoting the OMMT dispersion (Bhuchar, 1961; Dutta et al., 1997; Liebeskind et al., 2002). 3.2. Molar mass and thermal properties of the nanocomposites Table 4 presents the main characteristics of nanocomposites as a function of the OMMT content. The polymers containing small amounts of AA (Series A) displayed lower molar masses, indicating a higher tendency to chain transfer reactions. The addition of thioglycolic acid (Series B) resulted in an additional decrease of molar mass, as expected, since this thiol-compound is a well-known chain transfer agent for free radical polymerization. The composites of series M exhibited higher values of the molar mass of the extracted polymers when compared to the pure PMMA, indicating that the OMMT protects the propagating free radical species against transfer or termination reactions. Moreover, the OMMT may trap some free radical initiator molecules, decreasing the initiator concentration for the MMA polymerization outside of the
Table 4 Main characteristics of PMMA–OMMT nanocomposites prepared by in-situ bulk polymerization. Compound code
Clay content in the compositea (%)
Mw × 104
Unextractable fraction (%)
Tg (°C)
M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
56.6 82.6 67.6 71.1 41.7 40.6 44.6 45.1 1.4 5.7 8.6 13.7
0 62 65 70 0 65 70 74 0 71 71 72
110 111 119 118 118 120 121 119 111 120 119 118
a OMMT content determined from the residue obtained from thermogravimetry analysis.
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interlayer spaces. Decreasing the initiator concentration resulted in an increase of the molar mass of the corresponding polymer. This phenomenon was more significant for the series B, containing thioglycolic acid. The difference of the molar mass between pure PMMA and those originated of the nanocomposites was larger and increased with the amount of the OMMT. This behavior may be attributed to the adsorption of thioglycolic acid molecules on the clay mineral surface and the interaction and/or reaction of the mercapto groups with the surface groups of OMMT. Therefore, fewer amounts of thioglycolic acid molecules were available for transfer reactions. This interpretation is consistent with the increase of the degree of dispersion of the OMMT particles with the OMMT content in series B as detected by SAXS. For series A, the values of molar mass of the composites were not significantly higher than of the pure polymer. The composites of series A presented a higher dispersion degree of the OMMT particles compared to series M, as shown by SAXS. These results suggest that the exfoliated or very small OMMT particles should not protect the free radical species against transfer or termination reactions. Despite of the high dispersion of OMMT particles in composites of series B, the decrease of the number of thioglycolic acid molecules available for termination or transfer reactions with increasing the OMMT content has a preponderant effect led to the observed increase of the molar mass. It is important to emphasize that these molar mass values are related to the extracted polymer fraction. The polymerization in the presence of OMMT produced a large amount of unextractable PMMA– OMMT adducts. This phenomenon was already observed (Li et al., 2003) and suggests strong interactions between the polymer chains and the clay mineral surface. Probably, the decomposition of the initiator produces some free radical well attached to the clay mineral surface, which may initiate the graft polymerization of MMA onto the surface. The effect of the OMMT on the glass transition temperature (Tg) is also summarized in Table 4. In series M and B, the presence of OMMT increased the Tg compared to the pure polymer, which may be attributed to the good dispersion of the OMMT particles, with a proportion N90% of exfoliated particles or small aggregates, determined by SAXS. Such high dispersion of montmorillonite particles promoted the interactions between the polymer chains and the clay mineral, as observed from the extraction experiments, and contributes to the decrease in polymer chain mobility. Despite of the high dispersion of OMMT particles, this phenomenon was not observed in nanocomposites of series A, whose Tg was mainly influenced by the insertion of AA. The presence of the carboxylic groups favors the intermolecular interactions through hydrogen bonds to the PMMA, explaining the higher Tg of the sample A0 compared to M0. Despite of the presence of AA, the B0 exhibited a lower Tg than A0, because the presence of thioglycolic acid decreased the molar mass. Except for the lowest OMMT content, the glass transition temperatures of series composites M, A and B were similar. This should be due to the competitive and opposite effects of the lower molar mass of PMMA in samples of A and B (which may induce a decrease of Tg) and the presence of AA leading to a higher dispersion degree of the particles in the families A and B(which may induce an increase of Tg). Despite the much lower molar mass of samples of series B, the similarity between Tg of composites A and B confirmed that a good particle dispersion is the determinant factor to promote polymer–clay mineral interactions, reducing the polymer chains mobility. This feature illustrates the key role of AA in promoting such interactions and better clay particle dispersion.
polymerization is technologically advantageous but its thermal stability is low because of some defects in the chain originated from termination reactions by coupling and disproportionation. One of the great objectives of incorporating layered clay minerals in PMMA is to achieve a better thermal stability without losing the optical clarity (Gilman, 1999). Considering series M (without acrylic acid) and series A (with acrylic acid) (Fig. 4, Table 5), the thermal decomposition temperature increased with the addition of OMMT. These results indicated that the thermal stability of PMMA was enhanced by the incorporation of the OMMT. In both M and A series, the temperature at 5 wt.% and 10 wt.% loss increased with the addition of OMMT but a significant difference to the pure polymer was observed in series A. The better thermal stability in both nanocomposite systems resulted from the unique ability of the OMMT to protect the polymer chain from external influence. According to the SAXS results, the nanocomposites of series A presented higher OMMT dispersion with a high amount of exfoliated or very small particles. Thus, this feature seems
3.3. Thermal stability The thermal stability of PMMA was somewhat limited and depended on the synthesis procedure. PMMA prepared by free radical
Fig. 4. Thermogravimetric analysis of PMMA-based nanocomposites as a function of the OMMT content.
A.A. Silva et al. / Applied Clay Science 47 (2010) 414–420 Table 5 Thermal degradation parameters related to PMMA–OMMT nanocomposites prepared by in-situ bulk polymerization. Compound code
M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
Clay content in the compositea (%)
Decomposition temperature (°C) At 5% weight loss
At 10% weight loss
1° stage N 200 °C
2° stage 200 °C to 350 °C
3° stage b350 °C
0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
248 271 292 272 198 280 273 280 313 309
284 288 321 287 277 293 287 340 335 331
301
326
177.2 – – – – – – – – – – –
300 301 300 298 296 318 333 – – – – 301
380 386 379 377 377 395 391 392 398 393 393 393
Maximum temperature decomposition (°C)
a
OMMT content determined from the residue obtained from thermogravimetry analysis.
to indicate that a better thermal stability is usually achieved with more exfoliated structures. The effect of OMMT on the decomposition process of PMMA can be better visualized by using derivative thermogravimetry curves (Fig. 5). PMMA prepared by free radical polymerization (curve M0)
419
displayed three main degradation peaks: the first around 190 °C is attributed to the weak head–head linkages in the main chain formed by termination by combination (Manring et al., 1989). The second peak around 290 °C is due to the terminal vinyl group decomposition originated from termination by disproportionation and the third peak at around 390 °C is related to the random scission of polymer main chain (Hirata et al., 1985; Manring, 1989). The presence of OMMT decreased or suppressed (at higher amounts) the first decomposition stage, indicating that the head– head linkage is almost absent in PMMA chains synthesized in the presence of OMMT. Thus, these results suggest that the termination by combination of two radical propagating species was not favored when the MMA polymerization was carried out in the interlayer space. Another interesting result observed in series M is the significant decrease of the second decomposition peak in the sample M2 (containing 5% of OMMT). This peak is related to the terminal vinyl group decomposition and its decrease may be attributed to the ability of the OMMT to act as free radical scavengers. The effect of the OMMT on the thermal decomposition of PMMA containing small amounts of acrylic acid was even more interesting. For the nanocomposites containing up to 5% of OMMT, the second and third decomposition peaks were shifted towards higher temperatures and for the sample containing as high as 11% of OMMT the second peak disappeared. As indicated by the SAXS results, the presence of acrylic acid in the PMMA chain favors the dispersion of the OMMT particles, increasing its specific surface area. Therefore, the reaction between the free radical propagating species and the clay mineral surface was enhanced, decreasing the probability of formation of vinyl end groups. The B series, which was carried out in the presence of thioglycolic acid, exhibited a different behavior. The pure PMMA displayed only one decomposition peak at a maximum temperature of 390 °C, corresponding to the degradation of the polymer main chain. In this case, the mercapto groups of the thioglycolic acid, acting as chain transfer agent, transfer hydrogen atoms to the free radical propagating species, avoiding the formation of PMMA chains end-capped by carbon–carbon double bonds. A similar behavior was also demonstrated in our earlier work involving the preparation of PMMA-based graft copolymers from EVA copolymer containing mercapto groups as macro-transfer agent (Moreira et al., 1997). The presence of OMMT in the nanocomposite decreased the thermal stability of the nanocomposites when compared to the pure PMMA, probably due to the interaction and/or reaction of the mercapto groups of the thioglycolic acid with the surface groups of the OMMT, decreasing the amount of mercapto groups in the system, available for protection. When higher amount of OMMT was used in the in situ polymerization, the corresponding polymer presented a second decomposition peak characteristic of the end-capped vinyl groups. The degradation behavior of the nanocomposites corresponding to the series B agreed with the increasing molar mass of the PMMA chains as the amount of OMMT increased. 4. Conclusion
Fig. 5. DTG curves obtained from thermogravimetry analysis of PMMA-based nanocomposites as a function of the OMMT content.
PMMA–OMMT nanocomposites with a high degree of OMMT dispersion and improved thermal properties were prepared by in-situ intercalative polymerization of methyl methacrylate. The control of the chemical composition of the polymer was the key parameter to promote an adequate dispersion of OMMT for the improvement of the properties. A better dispersion of OMMT inside the PMMA matrix was achieved when acrylic acid was employed as the co-monomer, as evidenced by SAXS experiments. A proportion as high as 97% of very small particles of size less than 40 Å or exfoliated particles was obtained even at high OMMT content (13 wt.%). In all PMMA–OMMT nanocomposites, the presence of OMMT increased the glass transition temperature. The onset degradation temperature was also greatly
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improved compared to PMMA and was higher by increasing the OMMT content in composites without thioglycolic acid (series M and A). PMMA–OMMT nanocomposites prepared in the presence of thioglycolic acid showed reduced thermal stability as the amount of OMMT increased. This behavior is attributed to the interaction between the thioglycolic acid and the clay mineral surface, decreasing the probability of the thioglycolic acid in acting as a chain transfer agent for the MMA polymerization. Acknowledgments This work has been sponsored by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). The authors would like to thank the Brazilian Synchrotron Light Laboratory (LNLS, Campinas — Brazil) for the financial and technical support in the SAXS measurements. References Bhuchar, V.M., 1961. Chromium, uranium, tungsten and nickel complexes of thioglycolic acid. Nature 191, 489. Chen, G., Yao, K., Zhao, J., 1999. Montmorillonite clay/poly(methyl methacrylate) hybrid resin and its barrier property to the plasticizer within poly(vinyl chloride) composite. J. Appl. Polym. Sci. 73, 425–430. Choi, Y.S., Choi, M.H., Wang, K.H., Kim, S.O., Kim, Y.K., Chung, I.J., 2001. Synthesis of exfoliated PMMA/Na-MMT nanocomposites via soap-free emulsion polymerization. Macromolecules 34, 8978–8985. Dutta, S., MacConville, D., Yiangs, W., Antonovichi, P., Blank, A., 1997. Reactivity of Mo– Ot terminal bonds toward substrates having simultaneous proton- and electrondonor properties: a rudimentary functional model for oxotransferase molybdenum enzymes. Inorg. Chem. 36, 2517. Essawy, H., Badran, A., Youssef, A., El-Fetoh, A., El-Hakim, A., 2004. Synthesis of poly (methyl methacrylate)/montmorillonite nanocomposites via in situ intercalative suspension and emulsion polymerization. Polym. Bull. 53, 9–17. Gilman, J., 1999. Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites. Appl. Clay Sci. 15, 31–49. Goettler, L.A., Lee, K.Y., Thakkar, H., 2007. Layered silicate reinforced polymer nanocomposites: development and applications. Polym. Rev. 47, 291–317. Hanley, H.J.M., Muzny, D.D., Butler, B.D., 1997. Surfactant adsorption on a clay mineral: application of radiation scattering. Langmuir 13, 5276–5282. Hanley, H.J.M., Muzny, D.D., Ho, D.L., Glinka, C.J., 2003. A small-angle neutron scattering study of a commercial organoclay dispersion. Langmuir 19, 5575–5580. Hernandez, M., Sixou, B., Duchet, J., Sautereau, H., 2007. The effect of dispersion state on PMMA-epoxy-clay ternary blends: in situ study and final morphologies. Polymer 48, 4075–4086. Hirata, T., Kashiwagi, T., Brown, J.E., 1985. Thermal and oxidative degradation of poly (methyl methacrylate): weight loss. Macromolecules 18, 1410–1418. Huang, X., Brittain, W.J., 2001. Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization. Macromolecules 34, 3255–3260. Lee, D.C., Jang, L.W., 1996. Preparation and characterization of PMMA–clay hybrid composite by emulsion polymerization. J. Appl. Polym. Sci. 61, 1117–1122. Li, Y., Zhao, B., Xie, S., Zhang, S., 2003. Synthesis and properties of poly(methyl methacrylate)/montmorillonite (PMMA/MMT) nanocomposites. Polym. Int. 52, 892–898.
Liebeskind, L., Srogl, J., Savarin, C., Polanco, C., 2002. Bioinspired organometallic chemistry. Pure Appl. Chem. 74, 115–122. Liu, P., 2007. Polymer modified clay minerals: a review. Appl. Clay Sci. 38, 64–76. Manring, L.E., 1989. Thermal degradation of poly(methyl methacrylate). 2. Vinylterminated polymer. Macromolecules 22, 2673–2677. Manring, L.E., Sogah, D.Y., Cohen, G.M., 1989. Thermal degradation of poly(methyl methacrylate). 3. Polymer with head-to-head linkages. Macromolecules 22, 4652–4654. Meneghetti, P., Qutubuddin, S., 2004. Synthesis of poly(methyl methacrylate) nanocomposites via emulsion polymerization using a zwitterionic surfactant. Langmuir 20, 3424–3430. Moreira, A.C.F., Oliveira, M.G., Soares, B.G., 1997. Thermal degradation studies of poly (EVAL-g-methyl methacrylate). Polym. Degrad. Stab. 58, 181–185. Okamoto, M., Morita, S., Tagushi, H., Kim, Y.H., Kotaka, T., Tateyama, H., 2000. Synthesis and structure of smectic clay/poly(methyl methacrylate) and clay/polystyrene nanocomposites via in situ intercalative polymerization. Polymer 41, 3887–3890. Okamoto, M., Morita, S., Kim, Y.H., Kotaka, T., Tateyama, H., 2001. Dispersed structure change of smectic clay/poly(methyl methacrylate) nanocomposites by copolymerization with polar comonomers. Polymer 42, 1201–1206. Pavlidou, S., Papaspyrides, C.D., 2008. A review on polymer-layered silicate nanocomposites. Prog. Polym. Sci. 33, 1119–1198. Porod, G., 1982. General Theory in Small Angle X-ray Scattering. Academic Press, London. Qu, X., Guan, T., Liu, G., She, Q., Zhang, L., 2005. Preparation, structural characterization, and properties of poly(methyl methacrylate)/montmorillonite nanocomposites by bulk polymerization. J. Appl. Polym. Sci. 97, 348–357. Ratinac, K.R., Gilbert, R.G., Ye, L., Jones, A.S., Ringer, S.P., 2006. The effects of processing and organoclay properties on the structure of poly(methyl methacrylate)–clay nanocomposites. Polymer 47, 6337–6361. Stadtmueller, L.M., Ratinac, K.R., Ringer, S.P., 2005. The effects of intragallery polymerization on the structure of PMMA–clay nanocomposites. Polymer 46, 9574–9584. Su, S., Wilkie, C.A., 2003. Exfoliated poly(methyl methacrylate) and polystyrene nanocomposites occur when the clay cation contains a vinyl monomer. J. Polym. Sci. Part A: Polym. Chem. 41, 1124–1135. Tjong, S.C., 2006. Structural and mechanical properties of polymer nanocomposites. Mat. Sci. Technol, R 53, 73–197. Wang, H., Shieh, C., Chang, K., Chu, H., 2005. Synthesis and dielectric properties of poly (methyl methacrylate)–clay nanocomposite materials. J. Appl. Polym. Sci. 97, 2175–2181. Wei'an, Z., Xiaofeng, S., Yu, L., Yue'e, F., 2003. Synthesis and characterization of poly (methyl methacrylate)/OMMT nanocomposites by γ-ray irradiation polymerization. Rad. Phys. Chem. 67, 651–656. Xie, T., Yang, G., Fang, X., Ou, Y., 2003. Synthesis and characterization of poly(methyl methacrylate)/montmorillonite nanocomposites by in situ bulk polymerization. J. Appl. Polym. Sci. 89, 2256–2260. Yeh, J.M., Liou, S.J., Lai, M.C., Chang, Y.W., Huang, C.Y., Chen, C.P., Jaw, J.H., Tsai, T.Y., Yu, Y.H., 2004. Comparative studies of the properties of poly(methyl methacrylate)– clay nanocomposite materials prepared by in situ emulsion polymerization and solution dispersion. J. Appl. Polym. Sci. 94, 1936–1946. Yoon, K., Sung, H., Hwang, Y., Noh, S.K., Lee, D., 2007. Modification of montmorillonite with oligomeric amine derivatives for polymer nanocomposite preparation. Appl. Clay Sci. 38, 1–8. Zaioncz, S., Silva, A.A., Sirqueira, A.S., Soares, B.G., 2007. Toughening of epoxy resin by methyl methacrylate/2-ethylhexyl acrylate copolymers: the effect of copolymer composition. Macromol. Mat. Eng. 292, 1263–1270. Zhang, W., Liang, Y., Luo, W., Fang, Y., 2003. Effect of clay-modifying agents on the morphology and properties of poly(methyl methacrylate)/clay nanocomposites synthesized via γ-ray irradiation. J. Polym. Sci. Part A: Polym. Chem. 41, 3218–3226.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y
The effect of addition of acrylic acid and thioglycolic acid on the nanostructure and thermal stability of PMMA–montmorillonite nanocomposites Adriana A. Silva, Karim Dahmouche, Bluma G. Soares ⁎ Macromolecular Institute, Federal University of Rio de Janeiro (UFRJ), 21945-970, Rio de Janeiro-RJ, Brazil
a r t i c l e
i n f o
Article history: Received 28 July 2009 Received in revised form 11 December 2009 Accepted 15 December 2009 Available online 4 January 2010 Keywords: PMMA Organo-montmorillonite SAXS Thermal stability Thermogravimetric analysis Nanocomposites
a b s t r a c t Poly(methyl methacrylate) (PMMA)/organophilic montmorillonite (OMMT) nanocomposites were synthesized by the in situ free radical bulk polymerization of methyl methacrylate. The effect of small amount of acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent on the nanostructure of these nanocomposites was investigated by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) analyses. No reflection was discernible in the XRD patterns of all samples. In SAXS experiments, a great amount of small clay montmorillonite aggregates (less than 40 Å) were observed. The acrylic acid used as the co-monomer increased the affinity between the montmorillonite and the polymer, resulting in better dispersion of the montmorillonite particles. The glass transition temperature of the nanocomposites was evaluated by differential scanning calorimetry (DSC) and the thermal stability was investigated by thermogravimetric studies (TGA). As expected, the presence of the montmorillonite increased the glass transition temperature. The thermal stability of nanocomposites was distinctly higher than of pure PMMA and was increased by the presence of small amounts of acrylic acid as the co-monomer. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Poly(methyl methacrylate) (PMMA) is an important thermoplastic material, which presents interesting characteristics such as high transparency, high strength and dimensional stability. Because of these outstanding properties, PMMA is widely used as transparent covers for several purposes including special devices for electronic industries. However, its poor heat resistance, brittleness and stress cracking in most organic solvents limit its application. The dispersion of small amounts of montmorillonite within the PMMA matrix has been considered a very promising strategy for improving thermal stability and barrier properties without losing its optical properties (Gilman, 1999). The enhancement of these properties is based on the ability of the polymer chains to penetrate into the interlayer spaces of the montmorillonite yielding separated platelets, giving rise to the ascalled nanocomposites with intercalated and/or exfoliated structures. This process is usually possible for several monomers and polymers because the cations which naturally reside within the interlayer space regions can be easily exchanged by large, organic cations, increasing the organophilic character of the montmorillonite (Tjong, 2006; Goettler et al., 2007; Liu, 2007; Pavlidou and Papaspyrides, 2008). The intercalated structure consists of a well ordered multilayered structure of silicate containing polymer chains inserted into the interlayer, whereas a complete exfoliated structure presents individ-
⁎ Corresponding author. Tel./fax: +55 21 25627207. E-mail address: [email protected] (B.G. Soares). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.12.010
ual silicate layers, homogeneously dispersed inside the polymer matrix. In practice, exfoliated structures are very rare even using strongly organophilic montmorillonite. The best way to prepare PMMA-based nanocomposites with a high degree of intercalation/exfoliation is through the in situ lamellar polymerization of methyl methacrylate (MMA). In this procedure, the clay minerals are swollen by the liquid monomer or its solution, whose polymerization inside the interlayer spaces expands the distance between the silicate layers and leads to an exfoliation of the particles. The in situ polymerization has been performed in bulk (Okamoto, et al., 2000, 2001; Su and Wilkie, 2003; Wei'an et al., 2003; Xie et al., 2003; Zhang et al., 2003; Qu et al., 2005; Stadtmueller et al., 2005; Ratinac et al., 2006; Yoon et al., 2007), in solution (Chen et al., 1999; Li et al., 2003; Wang et al., 2005), and emulsion (Lee and Jang, 1996; Choi et al., 2001; Essawy et al., 2004; Meneghetti and Qutubuddin, 2004; Yeh et al., 2004). The emulsion polymerization has been carried out in the presence of pristine montmorillonite (Lee and Jang, 1996; Choi et al., 2001) or alkyl ammonium and alkyl phosphonium montmorillonites (Yeh et al., 2004). The bulk and solution polymerization processes are usually performed in the presence of the organo-montmorillonite to improve its compatibility with the monomer and facilitate the diffusion of the monomer into the interlayer spaces. The degree of exfoliation can also be improved by using montmorillonite modified with a quaternary ammonium salt containing a free radical initiator moiety (Huang and Brittain, 2001), or a vinyl group, which acts as a co-monomer during the in situ interlamellar polymerization of MMA (Zhang et al., 2003; Ratinac et al., 2006). The
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use of a small amount of polar co-monomers in the in situ polymerization of MMA was also reported by Okamoto et al. (2001) as a way to improve the degree of dispersion and intercalation in the corresponding nanocomposites. They have used N,N-dimethylaminopropyl acrylamide, N,N-dimethyl-aminoethyl acrylate and acrylamide as the co-monomers and observed a greater exfoliation degree when acrylamide was used as the co-monomer. The use of acrylic acid as the co-monomer is also a good strategy for improving the interaction between the PMMA chains and the clay mineral surface, through hydrogen bonds. The carboxyl groups in the corresponding nanocomposite can also impart better compatibility with other polymer systems such as epoxy resins (Zaioncz et al., 2007). The present paper aims studying the effect of small amounts of acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent on the degree of intercalation/exfoliation of PMMAbased nanocomposites prepared by in situ interlamellar polymerization. For this purpose, a commercial organophilic montmorillonite (OMMT), Viscogel B8, was swollen by the monomers. The bulk polymerization was performed in the presence of an organic initiator. The degree of intercalation/exfoliation was investigated by wide angle X-ray diffraction (WAXS) and small-angle X-ray scattering (SAXS). A stacked-disc model was adapted from the literature (Porod, 1982; Hanley et al., 1997; Hernandez et al., 2007) to estimate the average size of the OMMT aggregates dispersed in the PMMA matrix. The thermal stability of PMMA-based nanocomposites was also discussed focusing the effect of the acrylic acid as the co-monomer and thioglycolic acid as the chain transfer agent. 2. Experimental 2.1. Materials The organoclay (OMMT) used in this work was Viscogel B8, supplied by Bentec Rheological Additives (Livorno, Italy). This is a montmorillonite modified with octadecylammonium salt. Methyl methacrylate (MMA) and acrylic acid (AA) (Fig. 1) were purchased from Methacryl do Brasil (São Paulo, Brazil) and were distilled under reduced pressure. Azo-bis-isobutyronitrile (AIBN) from Merck, used as free radical initiator, was recrystallized from methanol/water. Thioglycolic acid (TA) (Fig. 1) (from Aldrich) was used as the chain transfer agent without purification. 2.2. Synthesis of PMMA–OMMT nanocomposites Different amounts of OMMT were dispersed into 20 g (0.5 mol) of MMA (series M) or 20 g (0.5 mol) of MMA and 0.4 g (4 mmol) of AA (series A) under intensive stirring at room temperature for 24 h. After achieving complete dispersion of the OMMT, 0.61 g (1 mmol) AIBN was added under stirring and N2 atmosphere. The polymerization was performed at 50 °C for 24 h and completed at a temperature of 85 °C for 1.5 h. Nanocomposites consisted of lower PMMA molar mass were also prepared by adding 0.34 g (4 mmol) of thioglycolic acid in the reaction medium containing the organo-montmorillonite dispersed into MMA/AA monomers (series B). The resulting nanocomposites were dispersed in acetone, precipitated with methanol, filtered and dried under vacuum at 60 °C. (Table 1) The amount of montmorillonite in each sample was determined by thermogravimetry analysis.
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Table 1 Composition of PMMA–clay nanocomposites prepared by in situ bulk polymerization. Compound code M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
Feed composition (g) MMA
AA
TG
OMMT
100 97.5 95 90 100 97.5 95 90 100 97.5 95 90
0 0 0 0 2 2 2 2 2 2 2 2
0 0 0 0 0 0 0 0 1.7 1.7 1.7 1.7
0 2.5 5.0 10 0 2.5 5.0 10 0 2.5 5.0 10
OMMT content in the compositea (%) 0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
a OMMT content determined from the residue obtained from thermogravimetry analysis.
2.3. Characterization Molar mass and molar mass distribution were determined by Size Exclusion Chromatography (SEC) using a 600 module system from Water Associates equipped with a refractive index detector model 2414. Microstyragel columns with 102, 103 and 104 porosity were employed. The nanocomposites were extracted in a Sohxlet extractor for 48 h with tetrahydrofuran as the solvent. The extracted fraction in THF was precipitated with methanol and re-dissolved in tetrahydrofuran at a concentration of 0.1 g/100 mL. The molar mass of the extracted fraction was determined using a calibration curve obtained from polystyrene standards. X-ray diffraction patterns (XRD) were recorded on a DMAX-RC (Miniflex) X-ray diffractometer (Rigaku Co., Tokyo, Japan), equipped with Cu Kα radiation source operating at 40 kV and 100 mA. The scanning rate was 2 °C min− 1. The thermal stability was measured by thermogravimetry analysis in a TA Q50 thermogravimetric analyzer. Samples of 20 mg were heated to 600 °C at a heating rate of 20 °C/min under N2 atmosphere. The thermal properties of the composites were measured by differential scanning calorimetry (DSC) in a Perkin-Elmer DSC-7 calorimeter at a heating or cooling rate of 20 °C min− 1 under a N2 atmosphere between 35 and 160 °C. The samples were heated, cooled and re-heated again and the glass transition temperature (Tg) was recorded from the second heat scanning. The nanostructural investigation of the composites was performed by small-angle X-ray scattering (SAXS). The SAXS measurements were performed at room temperature using the beam line of National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. This beamline was equipped with an asymmetrically cut and bent silicon (111) monochromator that yielded a monochromatic (λ = 1.608 Å) and horizontally focused beam. The patterns were acquired for scattering vectors (q) in the range 10− 2 Å− 1 b q b 0.12 Å− 1. The scattering intensity I(q) was plotted as a function of the modulus of scattering vector, q = (4π/λ)sin (bthetaN/2), where bthetaN is the scattering angle. The scattering intensity was normalized by subtracting (background scattering) and sample thickness. Each SAXS pattern corresponds to a data collection time of 900 s. 3. Results and discussion 3.1. Structural investigation
Fig. 1. Chemical structures of the monomers and reagent.
The qualitative evaluation of the dispersion of the montmorillonite particles in PMMA–OMMT was performed by XRD. Fig. 2 compares the XRD patterns of the OMMT and the PMMA-based nanocomposites (Series M, A and B). The pure OMMT showed a basal reflection at 2θ = 3.52° corresponding to a basal spacing d2 = 25 Å. In addition, a
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Fig. 2. XRD patterns of the OMMT and the corresponding PMMA-based nanocomposites as a function of the OMMT content.
less intense reflection located at 7.0° indicating a basal spacing of d1 = 12.6 Å was assigned to the unmodified montmorillonite. In all patterns of the series M, A and B the former reflection was quite totally absent and the latter was very poorly defined, evidencing the high proportion of exfoliated platelets or small particles composed of a very small number of layers. This indicated the efficiency of intercalation of PMMA chains promoted by the in-situ polymerization of MMA. This effect is attributed to the large volume occupied by the polymer chains between the silicate layers, after the in situ lamellar polymerization of MMA. The good nanometric dispersion of the particles was also evidenced by the transparency of the films casted from the THF solution. To determine more quantitatively the dispersion of the montmorillonite particles within the PMMA matrix, small-angle X-ray scattering measurements (SAXS) was performed. This technique reveals the behavior of the entire sample, and sample preparation is much simpler than transmission electron microscopy. The SAXS patterns of all composites (Fig. 3) showed two interference peaks located around qmax = 0.2 Å− 1 and 0.4 Å− 1. Since no scattering was detected for OMMT-free samples, these peaks are associated to the interlayer spaces between the silicate layers. The interlayer distances determined from the position of the peak maxima (d = 2π/qmax) are presented in Table 2. The larger values of d1 and d2 compared to those of pure OMMT, determined from XRD, indicated the intercalation of the polymer chains in agreement with the XRD patterns. As expected, the
Fig. 3. SAXS patterns of PMMA-based nanocomposites as a function of the OMMT content, prepared in different conditions (Series M, A and B).
SAXS intensity of the peak located at larger q-values was lower than the other one, due to the lower amount of unreacted MMT particles. The existence of OMMT particles of nanometric size dispersed in the polymer matrix was confirmed by the additional scattering observed in SAXS spectra of all samples at low q-range (below 0.1 Å− 1). The presence of these particles was also confirmed by the value of the slope α of the linear regime observed in log–log plot for q b 0.07 Å− 1 (Fig. 3). According to the literature (Hernandez et al., 2007), the scattering intensity, I(q), for a three-dimensional network of linked platelets, corresponds to I(q) = q− α, with α around 3. The values of α (Table 2), located for all composites between 2.6 and 3, indicated the
Table 2 Values of clay interlayer spacing and of the exponent α obtained from XRD (for pure organoclay) and SAXS for PMMA–OMMT based nanocomposites. Sample
OMMT (-wt.%)
d1 (Å)
d2 (Å)
α
OMMT M1 M2 M3 A1 A2 A3 B1 B3
100 3 5 15.5 3 4.2 11.5 3.5 13
12.6 17.2 17 17.2 – 18.2 18.1 17.4 14.2
25 34.3 34.3 34.3 35.9 36.3 36.7 35.7 28.3
– 3 3 3 2.8 2.6 2.7 2.9 2.9
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presence of OMMT particles constituted of a significant number of layers. To have a more quantitative knowledge of the average size of the particles dispersed in the polymer matrix, the stacked-disc model was employed (Porod, 1982; Hanley et al., 1997). However, as mentioned by Hanley et al. (2003) the use of this model for composites containing particles with a wide distribution of thicknesses is questionable, since all particles contribute to the X-ray scattering. Furthermore, this model assumes a uniform dispersion of the particles but neither the exfoliated platelets nor thin montmorillonite particles were uniformly dispersed throughout the matrix. We have employed the methodology also adopted by Hernandez et al. (2007) who have used the aspect ratio of a plate, and considered a wide distribution of aggregate thicknesses. In a general way, the SAXS intensity I(q) is the product of a form factor F(q) (which contains informations about size, shape or surface state of an heterogeneity) and a structure factor S(q) (which contains informations about the position of the heterogeneity relatively to the others). The form factor P(q) of a flat particle of thickness t is given by the following law: PðqÞ =
. . 2 B 2 2 ðqt = 2Þ Δρ t sin 2 q
ð1Þ
qt
where B is a constant proportional to the surface of the particles and is the difference between the electronic densities of particles and the matrix (Porod, 1982). For a diluted system of particles, S(q) = 1 and consequently, the SAXS intensity I(q) = F(q). A distribution of thicknesses was assumed and the SAXS spectrum was considered as being the sum of contributions of several populations, with a fixed number of platelets in a stack constituting a population. For all samples, the best fit of SAXS intensity, performed up to 0.07 Å− 1, was obtained considering the weighted sum of the contributions arising from 4 entities of thickness ti. i = 4 = 1
IðqÞ = ∑i
. . 2 Bi 2 ðqti = 2Þ sin t i 2 q
qti
ð2Þ
The parameters Bi and ti were determined by applying a least square fitting procedure. The proportion Pi of the particles of size ti within the matrix was then estimated by the relation: Pi =
Bi
. ∑J Bj
ð3Þ
The populations of particles with different thicknesses determined from this procedure are summarized in Table 3. For all composites, the proportion of exfoliated particles (thickness around 10 Å) or very small OMMT aggregates formed by two lamellas (thickness around 40 Å) was N89%, which is consistent with XRD measurements and confirms the high degree of dispersion of MMT in all composites. The fraction of particles formed by three or more layers was larger in composites of series M than in series A and B, which is consistent with the larger values of the exponent α obtained for the materials of the family M and with the fact that the main interference peak in the SAXS spectra of these samples was better defined. These results showed that the introduction of AA promoted a better affinity between the OMMT and the polymer in composites of series A and B, resulting on a better dispersion of the montmorillonite particles. This is due to the increased polarity caused by the introduction of carboxylic groups in the polymer and the possibility to form hydrogen bonds between the carboxylic groups of AA and the ammonium groups of the modified MMT.
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Table 3 Area fraction of particles of thickness t in PMMA–OMMT based composites obtained from SAXS measurements. Sample
t b 40 Å
40 Å b t b 100 Å
100 Å b t b 200 Å
t N 200 Å
M1 M2 M3 A1 A2 A3 B1 B3
91.2% 90% 89.6% 98.5% 98.0% 97.0% 95.8% 97.0%
– 6.5% 6.0% – 1.5% 2.0% 2.2% 2.3%
7.2% 3.5% 4.4% 1. 3% 0.4% 0.9% 1.8% –
1.6% 0.1% – 0.2% 0.1% 0.1% 0.2% 0.7%
The increase in OMMT contents in samples of series M did not significantly change the degree of the OMMT dispersion, evidenced by the constancy of the fraction of around 90% of small particles. In series A and B, the fraction of exfoliated or very thin particles remained at a higher level, independently of OMMT content. Again, this trend confirmed the ability of AA to enhance the affinity between OMMT and the polymer. The incorporation of thioglycolic acid (TA) seems to inhibit the OMMT dispersion in the polymer matrix. This was attested by the smaller interlayer distances between the lamellae and the slightly higher proportion of thin particles in composites of series B compared to series A at low OMMT content. As the OMMT content in the composite increased, the degree of dispersion of B3 slightly increased, probably because of the decrease of the TA proportion related to the OMMT content. Another explanation should be that high contents of OMMT favor the probability of interactions of thiol groups with metal ions located at the clay mineral surface, promoting the OMMT dispersion (Bhuchar, 1961; Dutta et al., 1997; Liebeskind et al., 2002). 3.2. Molar mass and thermal properties of the nanocomposites Table 4 presents the main characteristics of nanocomposites as a function of the OMMT content. The polymers containing small amounts of AA (Series A) displayed lower molar masses, indicating a higher tendency to chain transfer reactions. The addition of thioglycolic acid (Series B) resulted in an additional decrease of molar mass, as expected, since this thiol-compound is a well-known chain transfer agent for free radical polymerization. The composites of series M exhibited higher values of the molar mass of the extracted polymers when compared to the pure PMMA, indicating that the OMMT protects the propagating free radical species against transfer or termination reactions. Moreover, the OMMT may trap some free radical initiator molecules, decreasing the initiator concentration for the MMA polymerization outside of the
Table 4 Main characteristics of PMMA–OMMT nanocomposites prepared by in-situ bulk polymerization. Compound code
Clay content in the compositea (%)
Mw × 104
Unextractable fraction (%)
Tg (°C)
M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
56.6 82.6 67.6 71.1 41.7 40.6 44.6 45.1 1.4 5.7 8.6 13.7
0 62 65 70 0 65 70 74 0 71 71 72
110 111 119 118 118 120 121 119 111 120 119 118
a OMMT content determined from the residue obtained from thermogravimetry analysis.
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interlayer spaces. Decreasing the initiator concentration resulted in an increase of the molar mass of the corresponding polymer. This phenomenon was more significant for the series B, containing thioglycolic acid. The difference of the molar mass between pure PMMA and those originated of the nanocomposites was larger and increased with the amount of the OMMT. This behavior may be attributed to the adsorption of thioglycolic acid molecules on the clay mineral surface and the interaction and/or reaction of the mercapto groups with the surface groups of OMMT. Therefore, fewer amounts of thioglycolic acid molecules were available for transfer reactions. This interpretation is consistent with the increase of the degree of dispersion of the OMMT particles with the OMMT content in series B as detected by SAXS. For series A, the values of molar mass of the composites were not significantly higher than of the pure polymer. The composites of series A presented a higher dispersion degree of the OMMT particles compared to series M, as shown by SAXS. These results suggest that the exfoliated or very small OMMT particles should not protect the free radical species against transfer or termination reactions. Despite of the high dispersion of OMMT particles in composites of series B, the decrease of the number of thioglycolic acid molecules available for termination or transfer reactions with increasing the OMMT content has a preponderant effect led to the observed increase of the molar mass. It is important to emphasize that these molar mass values are related to the extracted polymer fraction. The polymerization in the presence of OMMT produced a large amount of unextractable PMMA– OMMT adducts. This phenomenon was already observed (Li et al., 2003) and suggests strong interactions between the polymer chains and the clay mineral surface. Probably, the decomposition of the initiator produces some free radical well attached to the clay mineral surface, which may initiate the graft polymerization of MMA onto the surface. The effect of the OMMT on the glass transition temperature (Tg) is also summarized in Table 4. In series M and B, the presence of OMMT increased the Tg compared to the pure polymer, which may be attributed to the good dispersion of the OMMT particles, with a proportion N90% of exfoliated particles or small aggregates, determined by SAXS. Such high dispersion of montmorillonite particles promoted the interactions between the polymer chains and the clay mineral, as observed from the extraction experiments, and contributes to the decrease in polymer chain mobility. Despite of the high dispersion of OMMT particles, this phenomenon was not observed in nanocomposites of series A, whose Tg was mainly influenced by the insertion of AA. The presence of the carboxylic groups favors the intermolecular interactions through hydrogen bonds to the PMMA, explaining the higher Tg of the sample A0 compared to M0. Despite of the presence of AA, the B0 exhibited a lower Tg than A0, because the presence of thioglycolic acid decreased the molar mass. Except for the lowest OMMT content, the glass transition temperatures of series composites M, A and B were similar. This should be due to the competitive and opposite effects of the lower molar mass of PMMA in samples of A and B (which may induce a decrease of Tg) and the presence of AA leading to a higher dispersion degree of the particles in the families A and B(which may induce an increase of Tg). Despite the much lower molar mass of samples of series B, the similarity between Tg of composites A and B confirmed that a good particle dispersion is the determinant factor to promote polymer–clay mineral interactions, reducing the polymer chains mobility. This feature illustrates the key role of AA in promoting such interactions and better clay particle dispersion.
polymerization is technologically advantageous but its thermal stability is low because of some defects in the chain originated from termination reactions by coupling and disproportionation. One of the great objectives of incorporating layered clay minerals in PMMA is to achieve a better thermal stability without losing the optical clarity (Gilman, 1999). Considering series M (without acrylic acid) and series A (with acrylic acid) (Fig. 4, Table 5), the thermal decomposition temperature increased with the addition of OMMT. These results indicated that the thermal stability of PMMA was enhanced by the incorporation of the OMMT. In both M and A series, the temperature at 5 wt.% and 10 wt.% loss increased with the addition of OMMT but a significant difference to the pure polymer was observed in series A. The better thermal stability in both nanocomposite systems resulted from the unique ability of the OMMT to protect the polymer chain from external influence. According to the SAXS results, the nanocomposites of series A presented higher OMMT dispersion with a high amount of exfoliated or very small particles. Thus, this feature seems
3.3. Thermal stability The thermal stability of PMMA was somewhat limited and depended on the synthesis procedure. PMMA prepared by free radical
Fig. 4. Thermogravimetric analysis of PMMA-based nanocomposites as a function of the OMMT content.
A.A. Silva et al. / Applied Clay Science 47 (2010) 414–420 Table 5 Thermal degradation parameters related to PMMA–OMMT nanocomposites prepared by in-situ bulk polymerization. Compound code
M0 M1 M2 M3 A0 A1 A2 A3 B0 B1 B2 B3
Clay content in the compositea (%)
Decomposition temperature (°C) At 5% weight loss
At 10% weight loss
1° stage N 200 °C
2° stage 200 °C to 350 °C
3° stage b350 °C
0 3.1 5.0 15.5 0 3.2 4.4 11.4 0 3.5 7.3 13.0
248 271 292 272 198 280 273 280 313 309
284 288 321 287 277 293 287 340 335 331
301
326
177.2 – – – – – – – – – – –
300 301 300 298 296 318 333 – – – – 301
380 386 379 377 377 395 391 392 398 393 393 393
Maximum temperature decomposition (°C)
a
OMMT content determined from the residue obtained from thermogravimetry analysis.
to indicate that a better thermal stability is usually achieved with more exfoliated structures. The effect of OMMT on the decomposition process of PMMA can be better visualized by using derivative thermogravimetry curves (Fig. 5). PMMA prepared by free radical polymerization (curve M0)
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displayed three main degradation peaks: the first around 190 °C is attributed to the weak head–head linkages in the main chain formed by termination by combination (Manring et al., 1989). The second peak around 290 °C is due to the terminal vinyl group decomposition originated from termination by disproportionation and the third peak at around 390 °C is related to the random scission of polymer main chain (Hirata et al., 1985; Manring, 1989). The presence of OMMT decreased or suppressed (at higher amounts) the first decomposition stage, indicating that the head– head linkage is almost absent in PMMA chains synthesized in the presence of OMMT. Thus, these results suggest that the termination by combination of two radical propagating species was not favored when the MMA polymerization was carried out in the interlayer space. Another interesting result observed in series M is the significant decrease of the second decomposition peak in the sample M2 (containing 5% of OMMT). This peak is related to the terminal vinyl group decomposition and its decrease may be attributed to the ability of the OMMT to act as free radical scavengers. The effect of the OMMT on the thermal decomposition of PMMA containing small amounts of acrylic acid was even more interesting. For the nanocomposites containing up to 5% of OMMT, the second and third decomposition peaks were shifted towards higher temperatures and for the sample containing as high as 11% of OMMT the second peak disappeared. As indicated by the SAXS results, the presence of acrylic acid in the PMMA chain favors the dispersion of the OMMT particles, increasing its specific surface area. Therefore, the reaction between the free radical propagating species and the clay mineral surface was enhanced, decreasing the probability of formation of vinyl end groups. The B series, which was carried out in the presence of thioglycolic acid, exhibited a different behavior. The pure PMMA displayed only one decomposition peak at a maximum temperature of 390 °C, corresponding to the degradation of the polymer main chain. In this case, the mercapto groups of the thioglycolic acid, acting as chain transfer agent, transfer hydrogen atoms to the free radical propagating species, avoiding the formation of PMMA chains end-capped by carbon–carbon double bonds. A similar behavior was also demonstrated in our earlier work involving the preparation of PMMA-based graft copolymers from EVA copolymer containing mercapto groups as macro-transfer agent (Moreira et al., 1997). The presence of OMMT in the nanocomposite decreased the thermal stability of the nanocomposites when compared to the pure PMMA, probably due to the interaction and/or reaction of the mercapto groups of the thioglycolic acid with the surface groups of the OMMT, decreasing the amount of mercapto groups in the system, available for protection. When higher amount of OMMT was used in the in situ polymerization, the corresponding polymer presented a second decomposition peak characteristic of the end-capped vinyl groups. The degradation behavior of the nanocomposites corresponding to the series B agreed with the increasing molar mass of the PMMA chains as the amount of OMMT increased. 4. Conclusion
Fig. 5. DTG curves obtained from thermogravimetry analysis of PMMA-based nanocomposites as a function of the OMMT content.
PMMA–OMMT nanocomposites with a high degree of OMMT dispersion and improved thermal properties were prepared by in-situ intercalative polymerization of methyl methacrylate. The control of the chemical composition of the polymer was the key parameter to promote an adequate dispersion of OMMT for the improvement of the properties. A better dispersion of OMMT inside the PMMA matrix was achieved when acrylic acid was employed as the co-monomer, as evidenced by SAXS experiments. A proportion as high as 97% of very small particles of size less than 40 Å or exfoliated particles was obtained even at high OMMT content (13 wt.%). In all PMMA–OMMT nanocomposites, the presence of OMMT increased the glass transition temperature. The onset degradation temperature was also greatly
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