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The influence of β-FeOOH nanorods on the thermal stability of poly(methyl methacrylate)
Polymer Degradation and Stability 92 (2007) 70e74 www.elsevier.com/locate/polydegstab
The influence of b-FeOOH nanorods on the thermal stability of poly (methyl methacrylate) Popovic´, Mirjana M. Novakovic´, Jovan M. Nedeljkovic´* Milena Marinovic´-Cincovic´, Maja C. Vinca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro Received 19 June 2006; received in revised form 23 August 2006; accepted 6 September 2006 Available online 1 November 2006
Abstract Colloidal dispersions consisting of b-FeOOH nanorods with three different aspect ratios (4, 75 and 120) were synthesized using thermal hydrolysis of FeCl3 solutions. After surface modification with oleic acid, the b-FeOOH nanorods were incorporated in poly(methyl methacrylate). Transmission electron microscopy and X-ray diffraction were applied for structural characterization of the b-FeOOH nanorods. The influence of inorganic phase on the thermal properties of PMMA matrix was studied using thermogravimetry and differential scanning calorimetry. Improvement of the thermal stability and increase of the glass transition temperature were found with the increase of content of inorganic phase and the increase of aspect ratio. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: FeOOH nanorods; Poly(methyl methacrylate); Nanocomposite; Thermal stability
1. Introduction Nowadays, polymer composites are widely used in many fields of technology [1,2]. The properties of composites mostly depend on size and shape of filler particles, their concentration, as well as the type of interaction with polymer matrix. Also, polymer composites take advantage of desired properties of host polymers such as possibility to be designed in various shapes, long-term stability and reprocess ability. Significant progress has been made in the preparation of a large number of metal oxide colloids consisting of particles of different chemical compositions, shapes and sizes. Iron oxides have been studied because of their applications in catalysis, gas sensing, magnetic storage, ferrofluids, magnetic refrigeration and color imaging. On the other hand, a stabilizing effect in polystyrene was observed after incorporation of 0.2e0.5 mm and 5e10 nm spherical a-Fe2O3 particles [3e5]. Also, stabilizing effects were observed in poly(vinyl chloride), chlorinated poly(vinyl chloride) and in their blends with
acrylonitrileebutadieneestyrene, in the presence of flame retarding/smoke-suppressing iron(III) compound FeOOH [6e9]. Poly(methyl methacrylate) (PMMA) is an important thermoplastic material with excellent transparency. However, its lower thermal stability restrains it from applications in higher temperature region. To improve the thermal properties of PMMA, fillers such as silica, titania, zirconia and alumina [10e16], as well as clay [17] were introduced into the PMMA. In this work, we present synthetic procedures for preparation of colloidal dispersions consisting of b-iron oxyhydroxide nanorods (b-FeOOH NRs) with three different aspect ratios. Also, we present synthetic methods for incorporation of the b-FeOOH NRs in the PMMA matrix. The b-FeOOH/PMMA nanocomposites were characterized using structural and thermal techniques and influence of concentration and aspect ratio of the b-FeOOH NRs on the thermal properties of the PMMA matrix is discussed in detail. 2. Experimental 2.1. Preparation of b-FeOOH/PMMA nanocomposites
* Corresponding author. Tel.: þ381 112453986; fax: þ381 113440100. E-mail address: [email protected] (J.M. Nedeljkovic´). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.09.012
Dispersions consisting of b-FeOOH NRs with different aspect ratios were prepared by ‘‘forced hydrolysis’’, i.e., thermal
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
hydrolysis of iron(III) chloride solution similar to the method described in the literature [18e20]. The b-FeOOH NRs (aspect ratio of about 4) were prepared by mixing 100 ml of 0.18 M FeCl3 with 100 ml of 0.01 M HCl. The solution was kept at 100 C for 24 h. The b-FeOOH NRs (aspect ratio of about 75) were prepared by mixing 100 ml of 0.45 M FeCl3 with 100 ml of 0.1 M HCl. The solution was kept at 100 C for 7 days. The b-FeOOH NRs (aspect ratio of about 120) were prepared by mixing 100 ml of 0.45 M FeCl3 with 100 ml of 0.01 M HCl. The solution was kept at 100 C for 10 days. In order to increase hydrophobicity of b-FeOOH NRs, 120 ml of oleic acid was added in dispersion, and than precipitate was washed out several times with water, filtered and finally dried. The b-FeOOH/PMMA nanocomposites were prepared by dispersing appropriate amount of b-FeOOH in xylene solution of commercially available Diakon CMG 314V PMMA (Mw ¼ 90,000; Mw/Mn ¼ 2.195). After evaporation of solvent, content of b-FeOOH NRs in PMMA matrices was 1.5, 3.5 and 6.5 mass%. 2.2. Apparatus Microstructural characterization of the b-FeOOH NRs was carried out on a transmission electron microscope (TEM) Philips EM-400 operated at 120 kV. Samples for microscopy analysis were deposited on C-coated Cu grids. The X-ray diffraction (XRD) measurements of b-FeOOH/ PMMA nanocomposites were performed on Philips PW 1710 diffractometer. Infra-red spectra of the pure PMMA polymer and the b-FeOOH/PMMA nanocomposite films were measured using PerkineElmer 983 G instrument.
The differential scanning calorimetry (DSC) measurements of the pure PMMA and b-FeOOH/PMMA nanocomposites were performed on a PerkineElmer DSC-2 instrument in the temperature range from 50 to 130 C. The heating rate was 20 C/min. In order to prepare samples with the same thermal history, prior to measurements samples were heated above the glass transition temperature and then cooled down (heating and cooling rates were 20 C/min). The thermogravimetric analysis (PerkineElmer model TGS-2) of the pure PMMA and b-FeOOH/PMMA nanocomposites was carried out under a nitrogen atmosphere in the temperature range from 30 to 550 C. The heating rate was 10 C/min. 3. Results and discussion Typical TEM images of the b-FeOOH NRs prepared in three different ways are shown in Fig. 1. The first dispersion (see Fig. 1A) consists of 75e125 nm in length and 23e27 nm in diameter b-FeOOH NRs (aspect ratio of about 4). The second dispersion (see Fig. 1B) consists of 250e350 nm in length and 3e5 nm in diameter b-FeOOH NRs (aspect ratio of about 75), while the third dispersion (see Fig. 1C) consists of 550e1000 nm in length and 5e8 nm in diameter b-FeOOH NRs (aspect ratio of about 120). Based on the particle size distribution data (see Fig. 1D), it is clear that it is more difficult to control length than the diameter of b-FeOOH NRs. Ability to control length of b-FeOOH NRs decreases with the increase of aspect ratio. Also, b-FeOOH NRs have tendency to array parallel to each other and to form bundle-like aggregates. The XRD spectrum of b-FeOOH NRs (aspect ratio of about 120) incorporated in PMMA matrix is shown in Fig. 2. The
B
diameter (nm)
A
C
D
30 20
sample A sample C
sample B
10 0
71
0
200
400
600
800
1000
length (nm) Fig. 1. Typical TEM images of b-FeOOH NRs with different aspect ratios: (A) 4, (B) 75, and (C) 120. (D) Size distribution of samples (A), (B) and (C).
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
(310)
(200)
(d) (c)
(b)
Cp (a. u)
(211)
(110)
72
(521)
(a)
40
60
80
100
120
140
temperature (°C) 10
20
30
40
50
60
2θ Fig. 2. Typical XRD spectrum of b-FeOOH/PMMA nanocomposite (aspect ratio of about 120).
XRD peaks exactly matched 110, 200, 310, 211 and 521 crystal planes of b-FeOOH with tetragonal crystal structure [21]. The XRD spectrum of the b-FeOOH/PMMA nanocomposite (aspect ratio of about 75) was exactly the same, while XRD spectrum of b-FeOOH/PMMA nanocomposite (aspect ratio of about 4) indicated amorphous nature of FeOOH NRs (XRD spectra are not shown). It is obvious that prolonged thermal treatment (7 and 10 days) at 100 C during synthesis induced development of crystal structure in the case of b-FeOOH NRs with aspect ratios of about 75 and 120, respectively. In order to determine if chemical bonding between b-FeOOH NRs and the PMMA matrix had taken place, IR measurements were performed. No difference between the IR spectra of the pure PMMA and b-FeOOH/PMMA nanocomposites was found (IR spectra are not shown). These results indicate that b-FeOOH/PMMA nanocomposites resemble solid solutions with weak interaction between the polymer matrix and nanofiller particles. The heat capacity curves of the pure PMMA and b-FeOOH/ PMMA nanocomposites (aspect ratio of about 120) with 1.5, 3.5 and 6.5 mass% of inorganic phase are shown in Fig. 3, while the data concerning nanocomposites prepared by using b-FeOOH NRs with aspect ratios of about 4 and 75 are collected in Table 1. A shift in the slope of the heat capacity curves towards higher temperatures was observed after incorporation of b-FeOOH NRs into the PMMA matrix. This slope, of course, corresponds to the glass transition temperature (Tg) of the polymer. It should be emphasized that the glass transition is not a true phase transition since the derivative of the heat capacity can be a continuous function of temperature. The different segmental motions lead to the glass transition spectrum. Therefore, the midpoint of the slope will be treated as the Tg. Accordingly, a significant shift in the Tg of PMMA towards higher temperatures (20e30 C) was observed after incorporation of b-FeOOH NRs. The increase of the Tg of more that 20 C was observed when content of b-FeOOH
Fig. 3. The heat capacity curves of the pure PMMA (a), and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with (b) 1.5, (c) 3.5, and (d) 6.5 mass% of inorganic phase.
NRs (all aspect ratios) was as low as 1.5 mass%. Further increase of the content of inorganic phase led to additional increase of the Tg. Also, for nanocomposites with the same content of inorganic phase increase of the Tg was observed with the increase of the aspect ratio of b-FeOOH NRs. Various nanofillers have sometimes completely opposite effects on the Tg of the PMMA. For example, the presence of 4 mass% of commercially available modified silica (AerosilÒ R805, R812 and R972) induced shift of the Tg by 18 C towards higher temperature [14,15], while the presence of 6 mass% of poly(dimethylsiloxane) surfactant-modified clay increased the Tg of PMMA by 3 C [17]. On the other hand, the presence of 0.5 mass% of 39 nm in diameter alumina particles decreased the Tg of PMMA by 25 C [13]. Based on this, it seems that b-FeOOH NRs have pronounced effect on the Tg of the PMMA matrix. The observed effect can be explained as a consequence of decreased molecular mobility of the PMMA chains due to adhesion of polymer segments onto the surface of b-FeOOH NRs. Because of that, the Tg of b-FeOOH/PMMA nanocomposites increased with the increase of the content of inorganic phase, and also with the increase of aspect ratio of NRs. The broadening of the glass transition region for the b-FeOOH/PMMA nanocomposites (all aspect ratios) was also observed. If we assume that the Tg corresponds to the motion
Table 1 Influence of content and aspect ratio of b-FeOOH NRs on the Tg ( C) of PMMA Content of inorganic phase (mass%)
0 1.5 3.5 6.5
Aspect ratio of b-FeOOH NRs 4
75
120
93 116 120 121
93 119 121 122
93 122 123 124
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
Table 2 Influence of content and aspect ratio of b-FeOOH NRs on the T50% ( C) of PMMA
A
mass (%)
80
Content of inorganic phase (mass%)
0 1.5 3.5 6.5
(d) (c)
40
(b) (a) 0 300
400
500
temperature (°C)
dm/dt (a.u.)
B
(d) (c) (b) (a)
300
400
73
500
temperature (°C) Fig. 4. Thermogravimetric (A) and differential thermogravimetric (B) curves of the pure PMMA (a), and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with (b) 1.5, (c) 3.5, and (d) 6.5 mass% of inorganic phase obtained in nitrogen atmosphere.
of segments with some average length, then the presence of filler particles will alter the distribution of segmental lengths and consequently induce changes in the glass transition region. It is obvious that the usage of fillers with large specific surface area leads to enhancement of this effect. The thermal stability of b-FeOOH/PMMA nanocomposites is compared with the thermal stability of the pure PMMA. The weight loss thermograms and differential thermograms of the pure PMMA and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with 1.5, 3.5 and 6.5 mass% of inorganic phase, obtained under the atmosphere of nitrogen, are shown in Fig. 4. The data concerning the thermal stability of nanocomposites prepared using b-FeOOH NRs with aspect ratios of about 4 and 75 are collected in Table 2. The obtained results indicate that incorporation of b-FeOOH NRs improved thermal stability of the PMMA matrix for 15e30 C. The increase of the content of inorganic phase led to improvement of the thermal stability of nanocomposites. Also, for samples with the same content of inorganic phase higher thermal stability was observed for nanocomposites prepared using NRs with larger aspect ratio. The extent of improvement of the thermal stability of PMMA upon incorporation of b-FeOOH NRs is comparable to improvement achieved using other oxide nanofillers such as SiO2 [15], SiO2/ZrO2 [16], TiO2 [22], and Fe2O3 [23].
Aspect ratio of b-FeOOH NRs 4
75
120
346 362 366 368
346 363 366 369
346 365 368 373
Three thermal decomposition stages for the PMMA and the composites can be recognized [24,25]. The first two decomposition steps correspond to the cleavage of head-to-head linkages and end-initiated vinyl-terminated PMMA. The third step corresponds to the random scission of PMMA chains, and it is the only degradation mechanism of b-FeOOH/PMMA nanocomposites since we used commercial PMMA synthesized in the presence of chain transfer agent. Although the enhanced stability as a result of adding b-FeOOH NRs to PMMA is open to debate, we believe that the possible reason for that can be assigned to reduced molecular mobility of polymer chains. Also, the inorganic component can restrain the attack of the free radicals. It is well known that Fe(III) ions are an effective free radical trap in solution for PMMA radicals. As pointed out in early work of Bamford et al. [26], Fe(III) ions behave as an ideal radical scavenger and consequently may be used to determine the rate of initiation in free radical polymerization. Acknowledgements The authors are grateful to Dr Natasˇa Bibic´ for critically reading the manuscript. Financial support for this study was granted by the Ministry of Science and Environmental Protection of the Republic of Serbia (Project 142066). References [1] Godovsky DY. Device applications of polymer-nanocomposites. Adv Polym Sci 2000;165:153. [2] Caseri W. Nanocomposites of polymers and metals or semiconductors: historical background and optical properties. Macromol Rapid Commun 2000;21:705. [3] Kuljanin J, Marinovic´-Cincovic´ M, Zec S, Comor MI, Nedeljkovic´ JM. Influence of Fe2O3-filler on the thermal properties of polystyrene. J Mater Sci Lett 2003;22:235. [4] Marinovic´-Cincovic´ M, Sˇaponjic´ Z, Djokovic´ V, Milonjic´ S, Nedeljkovic´ JM. The influence of hematite nano-crystals on the thermal stability of polystyrene. Polym Degrad Stab 2006;91:313. [5] Djokovic´ V, Nedeljkovic´ JM. Stress relaxation in hematite nanoparticlesepolystyrene composites. Macromol Rapid Commun 2000;21:994. [6] Carty P, White S. Anomalous flammability behaviour of CPVC (chlorinated polyvinylchloride) in blends with ABS (acrylonitrileebutadienee styrene) containing flame-retarding/smoke-suppressing compounds. Polymer 1997;38:111. [7] Lu LF, Price D, Milnes GJ, Carty P, White S. GC/MS studies of ABS/ CPVC blends. Polym Degrad Stab 1999;64:601. [8] Carty P, Price D, Milnes GJ. Chlorinated poly(vinylchloride) and plasticized chlorinated poly(vinylchloride) e thermal decomposition studies. J Vinyl Addit Technol 2002;8:227.
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[9] Carty P, White S. The effect of temperature on char formation in polymer blends: an explanation of the smoke suppressant FeOOH acting in ABS/ CPVC polymer blends. Polym Degrad Stab 2002;75:173. [10] Huang Z, Qiu KY. The effects of interactions on the properties of acrylic polymers/silica hybrid materials prepared by the in situ solegel process. Polymer 1997;38:521. [11] Chen WC, Lee SJ, Lee LH, Lin JL. Synthesis and characterization of trialkoxysilane-capped poly(methyl methacrylate)etitania hybrid optical thin films. J Mater Chem 1999;9:2999. [12] Chang TC, Wang YT, Hong YS, Chiu YS. Organiceinorganic hybrid materials. Dynamics and degradation of poly(methyl methacrylate) silica hybrids. J Polym Sci Polym Chem 2000;38:1972. [13] Lee LH, Chen WC. High-refractive-index thin films prepared from trialkoxysilane-capped poly(methyl methacrylate)etitania materials. Chem Mater 2001;13:1137. [14] Ash BJ, Schadler LS, Siegel RW. Glass transition behaviour of alumina/ polymethylmethacrylate nanocomposites. Mater Lett 2002;55:83. [15] Hu Y-H, Chen C-Y, Wang C-C. Viscoelastic properties and thermal degradation kinetics of silica/PMMA nanocomposites. Polym Degrad Stab 2004;84:545. [16] Wang H, Xu P, Zhong W, Shen L, Du Q. Transparent poly(methyl methacrylate)/silica/zirconia nanocomposites with excellent thermal stabilities. Polym Degrad Stab 2005;87:319.
[17] Jash P, Wilkie CA. Effects of surfactants on the thermal and fire properties of poly(methyl methacrylate)/clay nanocomposites. Polym Degrad Stab 2005;88:401. [18] Matijevic´ E, Scheiner PJ. Ferric hydrous oxide sols. J Colloid Interface Sci 1978;63:509. [19] Matijevic´ E. Monodisperse colloids: art and science. Langmuir 1986;2:12. [20] Matijevic´ E. Preparations and properties of uniform size colloids. Chem Mater 1993;5:412. [21] Smith NV. X-ray powder data files. Philadelphia: Am Soc for Testing and Materials; 1967. [22] Laachachi A, Leroy E, Cochez M, Ferriol M, Lopez-Cuesta JM. Use of oxide nanoparticles and organoclays to improve thermal stability and retardancy of poly(methyl methacrylate). Polym Degrad Stab 2000;89:344. [23] Laachachi A, Cochez M, Ferriol M, Lopez-Cuesta JM, Leroy E. Influence of TiO2 and Fe2O3 fillers on the thermal properties of poly(methyl methacrylate) (PMMA). Mater Lett 2005;59:36. [24] Marning LE. Thermal degradation poly(methyl methacrylate). Macromolecules 1989;22:2673. [25] Higashi N, Shiba H, Niwa M. Thermal degradation poly(methyl methacrylate). Macromolecules 1989;22:4652. [26] Bamford CH, Jenkins AD, Johnston R. Initiation of vinyl polymerization and the interaction of radicals with ferric chloride. Trans Faraday Soc 1962;58:1212.
The influence of b-FeOOH nanorods on the thermal stability of poly (methyl methacrylate) Popovic´, Mirjana M. Novakovic´, Jovan M. Nedeljkovic´* Milena Marinovic´-Cincovic´, Maja C. Vinca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro Received 19 June 2006; received in revised form 23 August 2006; accepted 6 September 2006 Available online 1 November 2006
Abstract Colloidal dispersions consisting of b-FeOOH nanorods with three different aspect ratios (4, 75 and 120) were synthesized using thermal hydrolysis of FeCl3 solutions. After surface modification with oleic acid, the b-FeOOH nanorods were incorporated in poly(methyl methacrylate). Transmission electron microscopy and X-ray diffraction were applied for structural characterization of the b-FeOOH nanorods. The influence of inorganic phase on the thermal properties of PMMA matrix was studied using thermogravimetry and differential scanning calorimetry. Improvement of the thermal stability and increase of the glass transition temperature were found with the increase of content of inorganic phase and the increase of aspect ratio. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: FeOOH nanorods; Poly(methyl methacrylate); Nanocomposite; Thermal stability
1. Introduction Nowadays, polymer composites are widely used in many fields of technology [1,2]. The properties of composites mostly depend on size and shape of filler particles, their concentration, as well as the type of interaction with polymer matrix. Also, polymer composites take advantage of desired properties of host polymers such as possibility to be designed in various shapes, long-term stability and reprocess ability. Significant progress has been made in the preparation of a large number of metal oxide colloids consisting of particles of different chemical compositions, shapes and sizes. Iron oxides have been studied because of their applications in catalysis, gas sensing, magnetic storage, ferrofluids, magnetic refrigeration and color imaging. On the other hand, a stabilizing effect in polystyrene was observed after incorporation of 0.2e0.5 mm and 5e10 nm spherical a-Fe2O3 particles [3e5]. Also, stabilizing effects were observed in poly(vinyl chloride), chlorinated poly(vinyl chloride) and in their blends with
acrylonitrileebutadieneestyrene, in the presence of flame retarding/smoke-suppressing iron(III) compound FeOOH [6e9]. Poly(methyl methacrylate) (PMMA) is an important thermoplastic material with excellent transparency. However, its lower thermal stability restrains it from applications in higher temperature region. To improve the thermal properties of PMMA, fillers such as silica, titania, zirconia and alumina [10e16], as well as clay [17] were introduced into the PMMA. In this work, we present synthetic procedures for preparation of colloidal dispersions consisting of b-iron oxyhydroxide nanorods (b-FeOOH NRs) with three different aspect ratios. Also, we present synthetic methods for incorporation of the b-FeOOH NRs in the PMMA matrix. The b-FeOOH/PMMA nanocomposites were characterized using structural and thermal techniques and influence of concentration and aspect ratio of the b-FeOOH NRs on the thermal properties of the PMMA matrix is discussed in detail. 2. Experimental 2.1. Preparation of b-FeOOH/PMMA nanocomposites
* Corresponding author. Tel.: þ381 112453986; fax: þ381 113440100. E-mail address: [email protected] (J.M. Nedeljkovic´). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.09.012
Dispersions consisting of b-FeOOH NRs with different aspect ratios were prepared by ‘‘forced hydrolysis’’, i.e., thermal
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
hydrolysis of iron(III) chloride solution similar to the method described in the literature [18e20]. The b-FeOOH NRs (aspect ratio of about 4) were prepared by mixing 100 ml of 0.18 M FeCl3 with 100 ml of 0.01 M HCl. The solution was kept at 100 C for 24 h. The b-FeOOH NRs (aspect ratio of about 75) were prepared by mixing 100 ml of 0.45 M FeCl3 with 100 ml of 0.1 M HCl. The solution was kept at 100 C for 7 days. The b-FeOOH NRs (aspect ratio of about 120) were prepared by mixing 100 ml of 0.45 M FeCl3 with 100 ml of 0.01 M HCl. The solution was kept at 100 C for 10 days. In order to increase hydrophobicity of b-FeOOH NRs, 120 ml of oleic acid was added in dispersion, and than precipitate was washed out several times with water, filtered and finally dried. The b-FeOOH/PMMA nanocomposites were prepared by dispersing appropriate amount of b-FeOOH in xylene solution of commercially available Diakon CMG 314V PMMA (Mw ¼ 90,000; Mw/Mn ¼ 2.195). After evaporation of solvent, content of b-FeOOH NRs in PMMA matrices was 1.5, 3.5 and 6.5 mass%. 2.2. Apparatus Microstructural characterization of the b-FeOOH NRs was carried out on a transmission electron microscope (TEM) Philips EM-400 operated at 120 kV. Samples for microscopy analysis were deposited on C-coated Cu grids. The X-ray diffraction (XRD) measurements of b-FeOOH/ PMMA nanocomposites were performed on Philips PW 1710 diffractometer. Infra-red spectra of the pure PMMA polymer and the b-FeOOH/PMMA nanocomposite films were measured using PerkineElmer 983 G instrument.
The differential scanning calorimetry (DSC) measurements of the pure PMMA and b-FeOOH/PMMA nanocomposites were performed on a PerkineElmer DSC-2 instrument in the temperature range from 50 to 130 C. The heating rate was 20 C/min. In order to prepare samples with the same thermal history, prior to measurements samples were heated above the glass transition temperature and then cooled down (heating and cooling rates were 20 C/min). The thermogravimetric analysis (PerkineElmer model TGS-2) of the pure PMMA and b-FeOOH/PMMA nanocomposites was carried out under a nitrogen atmosphere in the temperature range from 30 to 550 C. The heating rate was 10 C/min. 3. Results and discussion Typical TEM images of the b-FeOOH NRs prepared in three different ways are shown in Fig. 1. The first dispersion (see Fig. 1A) consists of 75e125 nm in length and 23e27 nm in diameter b-FeOOH NRs (aspect ratio of about 4). The second dispersion (see Fig. 1B) consists of 250e350 nm in length and 3e5 nm in diameter b-FeOOH NRs (aspect ratio of about 75), while the third dispersion (see Fig. 1C) consists of 550e1000 nm in length and 5e8 nm in diameter b-FeOOH NRs (aspect ratio of about 120). Based on the particle size distribution data (see Fig. 1D), it is clear that it is more difficult to control length than the diameter of b-FeOOH NRs. Ability to control length of b-FeOOH NRs decreases with the increase of aspect ratio. Also, b-FeOOH NRs have tendency to array parallel to each other and to form bundle-like aggregates. The XRD spectrum of b-FeOOH NRs (aspect ratio of about 120) incorporated in PMMA matrix is shown in Fig. 2. The
B
diameter (nm)
A
C
D
30 20
sample A sample C
sample B
10 0
71
0
200
400
600
800
1000
length (nm) Fig. 1. Typical TEM images of b-FeOOH NRs with different aspect ratios: (A) 4, (B) 75, and (C) 120. (D) Size distribution of samples (A), (B) and (C).
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
(310)
(200)
(d) (c)
(b)
Cp (a. u)
(211)
(110)
72
(521)
(a)
40
60
80
100
120
140
temperature (°C) 10
20
30
40
50
60
2θ Fig. 2. Typical XRD spectrum of b-FeOOH/PMMA nanocomposite (aspect ratio of about 120).
XRD peaks exactly matched 110, 200, 310, 211 and 521 crystal planes of b-FeOOH with tetragonal crystal structure [21]. The XRD spectrum of the b-FeOOH/PMMA nanocomposite (aspect ratio of about 75) was exactly the same, while XRD spectrum of b-FeOOH/PMMA nanocomposite (aspect ratio of about 4) indicated amorphous nature of FeOOH NRs (XRD spectra are not shown). It is obvious that prolonged thermal treatment (7 and 10 days) at 100 C during synthesis induced development of crystal structure in the case of b-FeOOH NRs with aspect ratios of about 75 and 120, respectively. In order to determine if chemical bonding between b-FeOOH NRs and the PMMA matrix had taken place, IR measurements were performed. No difference between the IR spectra of the pure PMMA and b-FeOOH/PMMA nanocomposites was found (IR spectra are not shown). These results indicate that b-FeOOH/PMMA nanocomposites resemble solid solutions with weak interaction between the polymer matrix and nanofiller particles. The heat capacity curves of the pure PMMA and b-FeOOH/ PMMA nanocomposites (aspect ratio of about 120) with 1.5, 3.5 and 6.5 mass% of inorganic phase are shown in Fig. 3, while the data concerning nanocomposites prepared by using b-FeOOH NRs with aspect ratios of about 4 and 75 are collected in Table 1. A shift in the slope of the heat capacity curves towards higher temperatures was observed after incorporation of b-FeOOH NRs into the PMMA matrix. This slope, of course, corresponds to the glass transition temperature (Tg) of the polymer. It should be emphasized that the glass transition is not a true phase transition since the derivative of the heat capacity can be a continuous function of temperature. The different segmental motions lead to the glass transition spectrum. Therefore, the midpoint of the slope will be treated as the Tg. Accordingly, a significant shift in the Tg of PMMA towards higher temperatures (20e30 C) was observed after incorporation of b-FeOOH NRs. The increase of the Tg of more that 20 C was observed when content of b-FeOOH
Fig. 3. The heat capacity curves of the pure PMMA (a), and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with (b) 1.5, (c) 3.5, and (d) 6.5 mass% of inorganic phase.
NRs (all aspect ratios) was as low as 1.5 mass%. Further increase of the content of inorganic phase led to additional increase of the Tg. Also, for nanocomposites with the same content of inorganic phase increase of the Tg was observed with the increase of the aspect ratio of b-FeOOH NRs. Various nanofillers have sometimes completely opposite effects on the Tg of the PMMA. For example, the presence of 4 mass% of commercially available modified silica (AerosilÒ R805, R812 and R972) induced shift of the Tg by 18 C towards higher temperature [14,15], while the presence of 6 mass% of poly(dimethylsiloxane) surfactant-modified clay increased the Tg of PMMA by 3 C [17]. On the other hand, the presence of 0.5 mass% of 39 nm in diameter alumina particles decreased the Tg of PMMA by 25 C [13]. Based on this, it seems that b-FeOOH NRs have pronounced effect on the Tg of the PMMA matrix. The observed effect can be explained as a consequence of decreased molecular mobility of the PMMA chains due to adhesion of polymer segments onto the surface of b-FeOOH NRs. Because of that, the Tg of b-FeOOH/PMMA nanocomposites increased with the increase of the content of inorganic phase, and also with the increase of aspect ratio of NRs. The broadening of the glass transition region for the b-FeOOH/PMMA nanocomposites (all aspect ratios) was also observed. If we assume that the Tg corresponds to the motion
Table 1 Influence of content and aspect ratio of b-FeOOH NRs on the Tg ( C) of PMMA Content of inorganic phase (mass%)
0 1.5 3.5 6.5
Aspect ratio of b-FeOOH NRs 4
75
120
93 116 120 121
93 119 121 122
93 122 123 124
M. Marinovic´-Cincovic´ et al. / Polymer Degradation and Stability 92 (2007) 70e74
Table 2 Influence of content and aspect ratio of b-FeOOH NRs on the T50% ( C) of PMMA
A
mass (%)
80
Content of inorganic phase (mass%)
0 1.5 3.5 6.5
(d) (c)
40
(b) (a) 0 300
400
500
temperature (°C)
dm/dt (a.u.)
B
(d) (c) (b) (a)
300
400
73
500
temperature (°C) Fig. 4. Thermogravimetric (A) and differential thermogravimetric (B) curves of the pure PMMA (a), and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with (b) 1.5, (c) 3.5, and (d) 6.5 mass% of inorganic phase obtained in nitrogen atmosphere.
of segments with some average length, then the presence of filler particles will alter the distribution of segmental lengths and consequently induce changes in the glass transition region. It is obvious that the usage of fillers with large specific surface area leads to enhancement of this effect. The thermal stability of b-FeOOH/PMMA nanocomposites is compared with the thermal stability of the pure PMMA. The weight loss thermograms and differential thermograms of the pure PMMA and b-FeOOH/PMMA nanocomposites (aspect ratio of about 120) with 1.5, 3.5 and 6.5 mass% of inorganic phase, obtained under the atmosphere of nitrogen, are shown in Fig. 4. The data concerning the thermal stability of nanocomposites prepared using b-FeOOH NRs with aspect ratios of about 4 and 75 are collected in Table 2. The obtained results indicate that incorporation of b-FeOOH NRs improved thermal stability of the PMMA matrix for 15e30 C. The increase of the content of inorganic phase led to improvement of the thermal stability of nanocomposites. Also, for samples with the same content of inorganic phase higher thermal stability was observed for nanocomposites prepared using NRs with larger aspect ratio. The extent of improvement of the thermal stability of PMMA upon incorporation of b-FeOOH NRs is comparable to improvement achieved using other oxide nanofillers such as SiO2 [15], SiO2/ZrO2 [16], TiO2 [22], and Fe2O3 [23].
Aspect ratio of b-FeOOH NRs 4
75
120
346 362 366 368
346 363 366 369
346 365 368 373
Three thermal decomposition stages for the PMMA and the composites can be recognized [24,25]. The first two decomposition steps correspond to the cleavage of head-to-head linkages and end-initiated vinyl-terminated PMMA. The third step corresponds to the random scission of PMMA chains, and it is the only degradation mechanism of b-FeOOH/PMMA nanocomposites since we used commercial PMMA synthesized in the presence of chain transfer agent. Although the enhanced stability as a result of adding b-FeOOH NRs to PMMA is open to debate, we believe that the possible reason for that can be assigned to reduced molecular mobility of polymer chains. Also, the inorganic component can restrain the attack of the free radicals. It is well known that Fe(III) ions are an effective free radical trap in solution for PMMA radicals. As pointed out in early work of Bamford et al. [26], Fe(III) ions behave as an ideal radical scavenger and consequently may be used to determine the rate of initiation in free radical polymerization. Acknowledgements The authors are grateful to Dr Natasˇa Bibic´ for critically reading the manuscript. Financial support for this study was granted by the Ministry of Science and Environmental Protection of the Republic of Serbia (Project 142066). References [1] Godovsky DY. Device applications of polymer-nanocomposites. Adv Polym Sci 2000;165:153. [2] Caseri W. Nanocomposites of polymers and metals or semiconductors: historical background and optical properties. Macromol Rapid Commun 2000;21:705. [3] Kuljanin J, Marinovic´-Cincovic´ M, Zec S, Comor MI, Nedeljkovic´ JM. Influence of Fe2O3-filler on the thermal properties of polystyrene. J Mater Sci Lett 2003;22:235. [4] Marinovic´-Cincovic´ M, Sˇaponjic´ Z, Djokovic´ V, Milonjic´ S, Nedeljkovic´ JM. The influence of hematite nano-crystals on the thermal stability of polystyrene. Polym Degrad Stab 2006;91:313. [5] Djokovic´ V, Nedeljkovic´ JM. Stress relaxation in hematite nanoparticlesepolystyrene composites. Macromol Rapid Commun 2000;21:994. [6] Carty P, White S. Anomalous flammability behaviour of CPVC (chlorinated polyvinylchloride) in blends with ABS (acrylonitrileebutadienee styrene) containing flame-retarding/smoke-suppressing compounds. Polymer 1997;38:111. [7] Lu LF, Price D, Milnes GJ, Carty P, White S. GC/MS studies of ABS/ CPVC blends. Polym Degrad Stab 1999;64:601. [8] Carty P, Price D, Milnes GJ. Chlorinated poly(vinylchloride) and plasticized chlorinated poly(vinylchloride) e thermal decomposition studies. J Vinyl Addit Technol 2002;8:227.
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