J. Phys. Chem. C 2009, 113, 97–104
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Polystyrene-Calcium Phosphate Nanocomposites: Preparation, Morphology, and Mechanical Behavior Selvin P. Thomas,†,‡ Sabu Thomas,*,† and Sri Bandyopadhyay§ School of Chemical Sciences, Mahatma Gandhi UniVersity, Priyadarshini Hills P.O., Kottayam, Kerala, India 686560, and School of Materials Science and Engineering, UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: NoVember 4, 2008
Calcium phosphate nanoparticles were prepared by matrix-mediated synthesis and were characterized using X-ray diffraction and transmission electron microscopy techniques. The particle size was found to be around 10 nm. Nanocomposites of polystyrene with calcium phosphate were prepared by a melt mixing technique. Mechanical properties of the nanocomposites such as tensile strength, tensile modulus, elongation at break, and impact strength were determined. Tensile strength and modulus showed 50 and 80% increase for 5% filled composites, respectively. Careful differential scanning calorimetric measurements were performed to evaluate the changes in heat capacity (Cp) as a function of filler loading. The ∆Cp values decreased dramatically as a function of filler loading indicating the existence of strong interaction between the nanofiller and polystyrene. A three-phase model consisting of a rigid amorphous phase has been proposed to illustrate the strong polymer-filler interaction. Introduction The use of inorganic fillers has been a common practice in the plastics industry to improve mechanical properties of thermoplastics. The addition of inorganic fillers has been reported to exhibit markedly improved properties as compared to the pure polymers or conventional particulate composites. These include increase of modulus and strength, improved barrier properties, increase in heat and solvent resistance, good optical transparency, etc. Further, these improvements are obtained at a very low loading of the nanofillers (1-10%) as compared to the conventional fillers, which require a high loading (25-40%).1,2 In nanocomposites one of the constituents has dimensions between 1 and 100 nm. Recent and ongoing research on polymer/inorganic nanocomposites has shown significant improvements in strength and physical and thermal properties of polymers, without compromising on density, toughness, or processibility. Major differences in behavior between conventional and nanostructured materials result from the fact that the latter have much larger surface (or interface) area per unit volume. Since many important chemical and physical interactions are governed by surfaces, a nanostructured material can have substantially different properties from a largerdimension material of the same foil thickness. Thus, the smaller these dimensions are, the larger is the surface area per unit volume.3,4 Many routes have been attempted for the synthesis of inorganic/organic nanocomposites.5,8 The effects of filler loading on mechanical and other properties of the composites strongly depend on the shape, size, surface characteristics and degree of dispersion. In general, the mechanical properties of the com* Corresponding author. Phone: +91-481-2730003. Fax: +91-4812731002. E-mail:
[email protected] or
[email protected]. † School of Chemical Sciences, Mahatma Gandhi University. ‡ Visiting Research Associate, School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia. § School of Materials Science and Engineering, University of New South Wales.
posites filled with micrometer-sized filler particles are inferior to those filled with nanoparticles of the same filler. In addition, the physical properties, such as surface smoothness and barrier properties, could not be achieved by conventional micrometersized particles. Further, it is known that the mechanical properties of the composites are, in general, related to the aspect ratio of the fillers. On the basis of this, various nanofillers are used as they have fairly large aspect ratio with respect to the conventional fillers. Therefore nanofillers such as clay, silica, and different types of inorganic particles like CaCO3, ZnO, Al2O3, etc., are being used to improve the mechanical properties of polymers.9,11 The nanoparticles can be prepared by several synthetic routes like, sol gel processing, in situ polymerization, hydrothermal process, matrix-mediated control of growth, forced hydrolysis approach, etc.12 Among these, matrix-mediated control of growth and morphology has received considerable attention in the recent years since it offers a novel route to material synthesis.13,14 Radhakrishnan and Saujanya prepared various inorganic particles using poly(ethylene oxide) (PEO) as the matrix.15,17 The matrix-mediated crystallization has several advantages over the conventional synthetic routes as (1) it is a simple precipitation technique which can be carried out at room temperature and (2) it is easy to handle the particles and process the powders. PEO-mediated growth of nano calcium phosphate was studied by several authors.18,21 As stated above, the effect of nanoparticles on the mechanical properties of polymers gained much attention in recent years. It was already reported that even at very low filler volume content such as 1-5% a considerable improvement of the mechanical and tribological properties could be achieved.22 In particular, several authors proved that ceramic and silica nanoparticles could effectively reinforce bulk polymers.22,23 Ji et al. made use of the tensile modulus values of montmorrillonite reinforced Nylon nanocomposites for theoretical modeling.24 Luo and Daniel attempted to study the effect of nanoclay on
10.1021/jp8065579 CCC: $40.75 2009 American Chemical Society Published on Web 12/08/2008
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SCHEME 1: Schematic Representation of the Nanoparticles Synthesis
TABLE 1: Characteristics of Polystyrene property 3
density (g/cm ) Poisson’s ratio solubility parameter (cal/cm3)1/2 water absorption (ASTM) softening temperature (°C) molecular weight (Mw)
value 1.04-1.065 0.33 18.6 0.05% 108 218000
epoxy resins, and they found that there is substantial increase in the mechanical properties.25 In the present article we have prepared and characterized the nanoparticles of calcium phosphate by the matrix-mediated growth method. These nanoparticles were characterized using different techniques, which include X-ray diffraction (XRD) and transmission electron microscopy (TEM). Further, these particles were incorporated into polystyrene by the melt blending technique. Various mechanical properties like Young’s modulus, tensile strength, and impact strength were characterized by standard methods. TEM and atomic force microscopy (AFM) measurements of the thin sections of the composites were also carried out. Finally, the ∆Cp values were carefully measured to analyze the polymer-filler interaction. Experimental Section The nanosized calcium phosphate filler particles were synthesized using an in situ deposition technique in the presence of PEO as follows: First, a complex of calcium chloride with PEO was prepared in desired proportions in methanol. An appropriate stoichiometric amount of trisodium phosphate in distilled water was added to the above complex slowly without stirring. The whole mixture was allowed to digest at room temperature for 24 h when both the chloride and phosphate ions diffused through the PEO and formed a white gel-like precipitate, which was filtered, washed, and dried. The concentrations of PEO-CaCl2 complex were varied from 2:1, 4:1, 8:1, 16:1, to 32:1. The yield of the calcium phosphate was recorded as 83 and 62% for 4:1 and 16:1 ratios, respectively. The method of nanoparticle synthesis is given in Scheme 1. The particle size was measured by X-ray diffractogram and TEM techniques. X-ray diffractograms of the particles were measured using a Philips 1140 X-ray irradiation apparatus using Cu KR radiation with λ ) 1.5406 Å at room temperature. Nanoparticles with ∼10 nm size were used for further studies (these were prepared by taking PEO and CaCl2 in the ratio 2:1). The nanoparticles were incorporated into atactic polystyrene, whose characteristics are given in Table 1, by a melt blending technique using a brabender plasticoder at 1800C with varying
filler amounts as 1, 3, 5, 7, and 10 wt %. The composites are designated as PS, CP1, CP3, CP5, CP7, and CP10, respectively. Tensile tests were carried out according to ASTM D 638 using rectangular samples of 10 × 1.2 × 0.2 cm3. Ten identical samples were used for measurements for each specimen, and the average value is reported for stress strain measurement. The sample was held tight by two grips in a tensile testing machine (TNE series 5T, India), the lower of which being fixed. The testing was performed at room temperature with a speed of 10 mm/min. From the stress-strain curves the tensile strength, Young’s modulus, and elongation at break were determined according to ASTM D638. The flexural properties were determined by applying the three-point bending load to a rectangular specimen, which was rested on a block at both the ends, at a crosshead speed of 10 mm/min at room temperature on (TNE series 5T, India) as per ASTM D790. Izod impact testing was done using an ITS AMP 105 (Italy) impact tester at room temperature. The impact strength was determined by striking the bar-shaped specimen with a hammer as per ASTM D256. The hammer energy was 5.5 J and the speed was 3.46 m/s. DSC studies were carried out on a pyris1 differential scanning calorimeter at a heating rate of 10 °C/min from 30 to 200 °C. The samples were first heated to 200 °C to remove any thermal history and cooled to 30 °C. The second heating curves were used for analysis. Tapping mode AFM measurements were carried out in air at ambient conditions (25 °C) with a Nanoscope III atomic force microscope, made by Digital Instruments Inc., USA. The experiments were carried out in tapping mode with constant amplitude, using microfabricated cantilevers. The scanning was done using an Olympus Tapping Mode etched silicon probe with a square pyramid shape. The characteristics of the probe are as follows: force constant (K), 42 N/m; nominal tip radius of curvature, less than 10 nm; cantilever length, 160 µm; tip height, 10 µm; cantilever configuration, rectangular substrate fits standard cantilever holder; reflective coating, aluminum; tip half angle, 17° on each side, 0° on front, and 35° back. Images were analyzed using Nanoscope imaging processing software. All images contained 256 data points. For AFM studies uniform ultrathin sections of the composite samples were prepared using a Reichert Ultracut E Ultramicrotome using a glass knife. TEM images of the particles and composites were measured by Philips CM200 field emission gun TEM using dispersions in alcohol and thin transparent sections. Results and Discussion Nanoparticle Characterization. White powders of calcium phosphate were obtained by the in situ deposition technique. Nanoparticles of calcium phosphate were characterized by XRD technique as well as TEM techniques. Figure 1 corresponds to the XRD patterns of nanoparticles obtained, (a) corresponds to the calcium phosphate without PEO and (b-f) for calcium phosphate with PEO concentration from 2:1 to 32:1 respectively. From the XRD patterns, the crystallite size was calculated using the Scherrer formula.
d(Å) ) kλ ⁄ ∆2θ cos θ
(1)
where k is the order of reflection, λ is 1.542 Å, θ is the diffraction angle, and ∆2θ is the full width at half-maximum (fwhm). In Figure 1, we can see a number of crystalline forms reported for calcium phosphate existing in both anhydrous and hydrated states. The calcium phosphate without any polymer shows three types of phases. Most of the peaks in diffraction
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Figure 1. XRD patterns for the calcium phosphate nanoparticles: (a) calcium phosphate without PEO; PEO:CaCl2 ratios in (b) 2:1, (c) 4:1, (d) 8:1 (e) 16:1, and (f) 32:1 (x axis, 2θ; y axis, intensity).
TABLE 2: XRD Values for the Calcium Phosphate Nanoparticles composition (PEO:CaCl2) fwhm (2θ), radians particle size (L), nm calcium phosphate (a) 2:1 (b) 4:1 (c) 8:1 (d) 16:1 (e) 32:1 (f)
0.00174 0.0134 0.0156 0.0162 0.0176 0.0181
82.4 10.65 9.24 8.96 8.02 7.82
patterns correspond to β-calcium orthophosphate (major phase) while the minor phase contains calcium orthophosphate primary along with a small amount of calcium metaphosphate hydrate. In the presence of PEO, with increase of PEO concentration, there occurs a major change in the diffraction patterns compared to curve a. A number of peaks were suppressed and peaks at Figure 3. Effect of PEO concentration on particle size of calcium phosphate.
Figure 2. TEM image of the calcium phosphate nanoparticles (PEO: CaCl2, (a) 2:1, (b) 4:1, (c) 32:1).
2θ of 31.9° broaden with increase of PEO concentrations from 42 to 89%. Only two peaks are clearly seen. The peak at 31.9° corresponds to all three states (Ca3(PO4), Ca3(H2PO4)2, Ca3(PO3)2 · 3H2O) and at 25.1° corresponds to the minor phase Ca3(H2PO4)2. It is interesting to note the extent of broadening from 42 to 89%. This reveals the small size of crystals, which could be associated due to good molecular mixing. Thus on comparison of curve a with curves b-f it is obvious that the effect of polymer plays a prominent role on the structure and
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Figure 4. TEM images of polystyrene nanocomposites: (a) CP3, (b) CP5, (c) CP7, and (d) CP10 (panels a and b magnification is 100 nm and panels c and d magnification is 200 nm).
growth behavior of calcium phosphate crystals. Since CaCl2 is first complexed with PEO, only certain crystalline phases of calcium phosphate are allowed to grow as compared to a large number of phases getting developed in normal free precipitation. The corresponding data for the nanoparticles is given in Table 2. We can see that the particle size decreases when we increase the concentration of PEO. The TEM images of the particles are shown in Figure 2, which are taken for 2:1, 4:1, and 32: 1 PEO:CaCl2, respectively. The particle size distributions for each concentration were calculated using image analyzer software. We can see that for each concentration there are a few particles with large size. However, the overall size distribution is found to be narrow. The particle size reduction against concentration of PEO is shown in Figure 3. The particle size reduction is attributed to the increase in the amount of oxygen atoms available from PEO matrix as we increase the concentration. This enables the suppression of the formed particles and reduces the size further. The particle size reduction can be mathematically expressed as a general equation, with respect to the concentration of PEO used, like
y ) -0.42x + 10.732
(2)
This linear equation gives a negative slope, which indicates that the concentration of PEO is directly related to the particle size reduction. TEM Images of the Composites. TEM images of the thin sections of the composites were analyzed to get an idea regarding the prticle dispersion in the matrix for the composites with 3, 5, 7, and 10%, respectively (Figure 4). In panels a and b the magnification is 100 nm and in panels b and d the magnification is 200 nm. TEM is required to tell if nanodispersion has been achieved. Generally, the low magnification images indicate how well dispersed is the filler, while the high magnification images permit the identification of an agglomerated structure in polymer nanocomposites. It is quite clear that the present filler is well distributed in PS as evidenced from the TEM images. Discrete particles of nanofillers were seen in the initial compositions (Figure 4, panels a and b)). However, it can be seen from the images that as the concentration of the filler increased, the efficient incorporation is lacking mainly due to the filler agglomeration. When we increase the loading of filler, the particle-particle contacts increase. This leads to
Figure 5. AFM images of polystyrene nanocomposites: (a) CP0, (b) CP3, (c) CP5, (d) CP7, and (e) CP10.
aggregation of the particles, and we can see agglomerated particles at higher loadings. AFM Images of the Composites. AFM images of the surfaces and sections of the composites were analyzed to understand filler dispersion behavior in the matrix. In Figure 5, panels a-d, the phase images of the composites with filler concentration 0, 3, 5, and 10% are given. Figure 5a shows the phase image of the neat matrix. We can see that the virgin polymer shows a smooth surface. The images of the filled composites show the particles incorporated in the matrix (Figure 5b-d). For the 3 and 5% filled systems isolated particles can be seen in the images. On increase of the filler amount, the particles get agglomerated as shown in the 10 wt % filled composites. A clear picture can be obtained from the threedimensional image of the 5 wt % filled composites as given in Figure 6. The dispersed nanofillers in the matrix is seen in the figure. Here also we can notice that particles “touch” each other. Stress-Strain Behavior of the Composites. Stress-strain behavior of the nanocomposites was studied under tension using rectangular samples with low speed. The stress-strain curves of the composites are given in Figure 7. We can see that all the curves show similar behavior. In the initial region, the stress increases linearly with the strain and thereafter shows a nonlinearity. We can also notice that the elongation at break is almost same for all the composites. Tensile Properties. The effect of wt % of calcium phosphate in tensile properties of the composites is given in Figure 8. The virgin polymer shows a tensile strength of around 30 MPa. All the composites show a higher value than the virgin polymer.
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J. Phys. Chem. C, Vol. 113, No. 1, 2009 101 TABLE 3: Elongation at Break and Impact Energy Values for the Calcium Phosphate Nanoparticles
Figure 6. 3-DAFM image of CP5 polystyrene nanocomposites.
Figure 7. Stress-strain behavior of polystyrene nanocomposites.
Figure 8. Tensile properties of polystyrene nanocomposites.
Tensile strength of the nanocomposites increases upon increasing the filler amount. The value increases initially, reaches a maximum at 5 wt %, and levels off. The composites having 5 wt % nanofiller show maximum tensile strength of approximately 41 MPa. For composites having 7 and 10 wt % filler loading, tensile strength is almost same. It is understood that higher levels of interaction between filler and matrix facilitate
wt % of nanofiller
elongation at break (%) ((SD)
impact energy (kJ/m) ((SD)
0 1 3 5 7 10
4.88 ( 0.0462 6.44 ( 0.0594 6.32 ( 0.0652 6.94 ( 0.0729 5.88 ( 0.0602 5.35 ( 0.0612
3.416 ( 0.00854 3.587 ( 0.00897 3.49 ( 0.00873 3.41 ( 0.00852 3.28 ( 0.0082 3.21 ( 0.00802
stress transfer to the filler phase.26 Present nanocomposites exhibit a maximum tensile strength at low filler loading (
