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pH Affects Sol−Gel Formation of Core−Shell Mesoporous Silica Coatings on Polyamide Junia N. M. Batista,† Emerson H. de Faria,† Paulo S. Calefi,† Katia J. Ciuffi,† Eduardo J. Nassar,† J. Mauricio A. Caiut,‡ and Lucas A. Rocha†,* †

Universidade de FrancaUNIFRAN-CP 82, Franca, SP 14404-600, Brazil Department of Chemistry, FFCLRP-USP, Ribeirão Preto, SP 14040-901, Brazil

‡

ABSTRACT: We synthesized a highly ordered mesoporous silica coating on polyamide (PA) by adding PA powder to the solution of silica precursors in different pH conditions. A pH equal to 7.0 and 8.0 afforded PA coated with spherical silica particles with an average diameter of 112 and 70 nm, respectively; a pH higher than 9.0 furnished a 150-nm thick coating consisting of hexagonally ordered mesostructured pores. The silica-coated PA samples presented lower melting points compared with the neat PA and are potentially applicable in many research fields such as rapid prototyping technology.

1. INTRODUCTION High-performance materials (HPM) must meet specific criteria, such as outstanding thermal resistance and/or mechanical strength, low specific density, high conductivity, excellent thermal, electrical, or sound insulation properties, and superior flame resistance.1 Polyamides (PAs) are strong, stiff, abrasionresistant thermoplastics that retain their physical and mechanical properties over a wide range of temperatures. The semicrystalline morphology and the intermolecular hydrogen bonds of the amide groups account for the properties of PAs.2−4 Laser sintering (LS) requires small polymer particles with appropriate morphology. However, most polymers cannot be directly produced in the powder form and have to be converted to an appropriate particle size prior to sintering. During LS, polymers are heated to high temperatures. As a result, the solid or glassy polymer existing at room temperature becomes softer and is ultimately converted to a viscous flowing melt. Several powdered inorganic additives, such as hydrated silicas, glassy oxides, fluoroplastics, and metallic stearates, can improve powder flow.5 Reinforcement with microsized inorganic fillers such as glass beads,6 hydroxyapatite,7 and Al2O3 particles8,9 can enhance the mechanical and physical properties of laser-sintered polymeric parts. Preparation of the starting material is key to obtaining polymers with uniform particle dispersion, good interfacial adhesion between the filler and the polymer, and suitable morphology and particle size.5 To produce reinforced polymers (including the commercially available materials), it is necessary to mechanically mix the filler with the neat base polymer. Unfortunately, this method rarely provides a uniform dispersion because the filler and the polymer usually have a different particle size (particularly when one powder is very small; e.g., nanofillers) and density (e.g., nylon and metal powders).5 Yan et al.10 employed a dissolution−precipitation process to produce a 3 wt % nanosilica/nylon (PA12) composite powder containing nanosilica uniformly dispersed within PA12. After conducting LS, the authors obtained a nanocomposite material © 2012 American Chemical Society

with 20.9, 39.4, and 9.54% larger tensile strength, tensile modulus, and impact strength, as compared with the neat PA12, respectively. The presence of certain nanoparticles (NPs) in the composite decreases the required laser energy because the NPs absorb the laser power more efficiently than the base polymer.5 Wang et al.11 reported that the addition of larger amounts of rectorite to a rectorite/PA12 nanocomposite reduced the laser power necessary to achieve the optimum tensile strength. Over the past decade, materials scientists have used the sol− gel chemistry to prepare mixed oxides for application as glasses, ceramics, and catalysts. Conventional sol−gel routes involve the hydrolysis and condensation of metal alkoxides in controlled pH conditions. The sol−gel chemistry has been geared to silicabased materials for several reasons: (i) tetracoordinated Si is flexible, giving rise to a great variety of structures; (ii) silica is less reactive, enabling precise control of the hydrolysis− condensation reactions; (iii) the resulting amorphous silica networks are thermally stable, avoiding crystallization during thermal treatment; (iv) silica allows for strong grafting of organic functions. Since the discovery of mesoporous silica and aluminosilicate molecular sieves in 1992, many researchers have been devoted to the design of mesotextured inorganic or hybrid phases that are potentially applicable in the fields of catalysis, optics, photonics, sensors, separation, drug delivery, sorption, and insulation.13,14 It is difficult to obtain homogeneity at the molecular level during the synthesis of heterometallic oxide and organic− inorganic hybrids because the various precursors display different reactivity during the hydrolysis and condensation steps.4,12 Selecting the organic template to spatially control mineralization in the mesoscale during the sol−gel process is a key issue in the synthesis of textured or porous materials. Received: Revised: Accepted: Published: 779

September 21, 2012 December 18, 2012 December 20, 2012 December 20, 2012 dx.doi.org/10.1021/ie302580q | Ind. Eng. Chem. Res. 2013, 52, 779−784

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Figure 1. Comparison of the SEM images of the silica/PA-12 nanocomposites obtained at different pH: (a) PA-12 powder before silica coating; nanocomposites synthesized at (b) pH = 7.0, (c) pH = 8.0, (d) pH = 9.0, (e) pH = 10.0, and (f) pH = 11.0.

Figure 2. Typical particle size distribution for the silica/PA-12 nanocomposites at (a) pH = 7.0 and (b) pH = 8.0. Line is just a guide for the eyes (n = 203). Inset-SEM image of the samples used to obtain the size distribution.

2.2. Characterization. X-ray diffraction (XRD) patterns were recorded on a Miniflex RIGAKU II instrument using Cu Kα radiation. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) were performed on a Thermal Analyst TA Instruments SDT Q 600 in air atmosphere, at a heating rate of 10 °C min−1, from 25 to 800 °C. Scanning electron microscopy (SEM) was conducted on a JEOL SEMFEG JSM 6330F apparatus at the Electron Microscopy Laboratory (LME) at the Brazilian Synchrotron Light Laboratory (LNLS). SAXS measurements were also conducted at the Brazilian Synchrotron Light Laboratory. The incident Xray monochromatic beam (k = 1.488 Å) was monitored with a photomultiplier and detected on a Pilatus detector (8 × 8 binning). The SAXS chamber parasitic scattering was also recorded (with bias and dark-noise subtraction) and subtracted from the sample pattern after sample attenuation correction.

Supramolecular arrays are usually the template of choice for mesoporous oxides. For example, micellar systems comprising surfactants or block copolymers impart larger pores and thicker walls, apart from being industrially available, hazard-free, and easy to remove from the mineral framework (by thermal treatment or solvent extraction).15−17 Here, we studied the properties of polymer samples coated with mesoporous silica obtained by the sol−gel process in the presence of a template. More specifically, we investigated how pH affected the sol−gel production of a mesoporous silica coating on the surface of PA12.

2. EXPERIMENTAL SECTION 2.1. Preparation of Mesoporous Silica Coating. All the reagents were purchased from Aldrich or Acros and used as received. The mesoporous silica coating was obtained by adding neat PA-12 powder to a solution containing deionized water (67 mL), tetraethylorthosilicate (TEOS, 11.2 mmol), and the structure-directing agent cetyltrimethylammonium bromide (CTAB, 1.4 mmol) in different pH conditions. The pH value was varied from 7.0 to 11.0 by the addition of aqueous ammonia hydroxide. The reaction was stirred for 24 h, at room temperature; the solid was isolated by centrifugation and washed three times with ethanol before being dried under vacuum.

3. RESULTS AND DISCUSSION The SEM micrograph of the neat PA-12 powder did not reveal any important details on the polymer surface (Figure 1a). The micrograph recorded for PA-12 coated with silica at pH 7.0 and 8.0 (Figures 1b,c, respectively) evidenced deposition of spherical particles instead of a continuous silica coating. Campos et al.4 observed a similar phenomenon when they prepared a protective coating on a PA-12 substrate using the 780

dx.doi.org/10.1021/ie302580q | Ind. Eng. Chem. Res. 2013, 52, 779−784

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Figure 3. SEM micrographs of the PA-12 powder after silica coating at pH = 9.0.

Figure 4. SEM micrographs of the PA-12 powder after silica coating at pH = 10.0.

Figure 5. SEM micrographs of the PA-12 powder after silica coating at pH = 11.0.

of 361 nm at high pH (11.72). The latter authors verified that silica particle size decreased from 242 to 30.6 nm with increasing ammonia concentration. According to Deng et al.21 and Rao et al.,22 higher pH accelerates both the polycondensation and the nucleation rates. However, nucleation has a stronger effect on particle size than polycondensation, resulting in a larger amount of nuclei and smaller particle size. The silica/PA-12 nanocomposite synthesized at pH 9.0 was almost completely coated with silica and had a mean thickness of around 150 nm (Figure 3). We were not able to estimate the thickness of the silica/PA-12 nanocomposites prepared at pH 10.0 and 11.0 (Figures 4 and 5, respectively), but the coatings were more homogeneous as compared with the nanocomposite

dip-coating technique. At pH values above 9.0, we obtained a homogeneous silica coating (Figures 1d,e,f), which shows that the affinity between the alkoxide and the PA-12 surface increases with rising pH values. According to the literature,18−20 increased ammonia concentrations accelerate hydrolysis and polycondensation, yielding larger silica particles. However, the silica-coated PA12 samples prepared here at pH 7.0 and 8.0 presented spherical particles with narrow size distribution: mean sizes of 112 and 70 nm, respectively (Figure 2), which coincides with the observations of Deng et al.21 and Rao et al.22 The former authors described highly monodisperse hybrid silica spheres with a mean size of 748 nm at low pH (10.6) and particle size 781

dx.doi.org/10.1021/ie302580q | Ind. Eng. Chem. Res. 2013, 52, 779−784

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cell parameter (a0 = 2d100/√3) increased from 65.42 to 42.90 Å ongoing from pH 7.0 to11.0. PAs degrade via hydrolysis of the amide bond and homolytic cleavage of the C−C, C−N, and C−H bonds, followed by cyclization and homolytic cleavage. The main gas products are cyclic monomers, hydrocarbons, CO2, CO, nitriles, NH3, and H2O.23 The thermogravimetric curve of neat PA (Figure 8a) evidenced two main mass loss events: between 298 and 518 °C and between 520 and 650 °C, corresponding to 92 and 8% mass loss, respectively. The second loss refers to oxidative degradation in the residual PAthe PA chains contain strong polar groups at regular intervals, which culminate in strong dipole−dipole interaction and extensive intermolecular hydrogen bonding in the hard block.24 Many works have described enhanced thermal stability upon incorporation of inorganic nanoparticles into polymeric blends.4,24−26 In contrast, we found that silica-coated PA-12 composites were less stable than neat PA-12, as evidenced by the lower temperature of degradation onset (Figure 8a). Generally, the inorganic matrix exerts opposing effects on the thermal stability of the hybrid organic/inorganic nanocomposites, the barrier effect enhances the thermal stability; the catalytic effect promotes polymer degradation. Improved thermal stability should arise from a combination of different effects: high nanoparticle surface volume, low permeability, reduced rate of volatile products formation, generation of highperformance carbonaceous silicate char on the nanoparticles surface (which insulates the underlying material), and slow escape of the volatile products generated during polymer decomposition.27 In our case, protonated silicate was probably generated on the surface of the nanocomposite, catalyzing PA12 degradation.27 Zong et al.28 also verified that nanoclay-functionalized PA starts to degrade at lower temperature than neat PA (350 and 363.5 °C, respectively). In addition, Dahiya et al.29 reported decreased thermal stabilility for PA in bentonite−PA nanocomposites; the authors argued that the organomodifier accelerates PA decomposition, probably generating a microcomposite instead of a nanocomposite during the melt point.30 The addition of a small amount of clay to the polymer determines good dispersion of the clay layers, favoring the barrier effect; increasing clay loading (4−5 wt %) enhances the catalyst effect of the clay and diminishes the thermal stability of the nanocomposite. The catalyst effect is accelerated by the presence of (i) hydroxyl groups on the edges of the polymer, (ii) active catalytic sites, and (iii) protonated silicate resulting from silica degradation, which together promote polymer degradation.31 The silica-coated PA-12 nanocomposites exhibited four main mass loss events (Figure 8a): (1) physisorbed solvent molecules desorbed between room temperature and ∼80 °C; (2) surfactant molecules decomposed and encapsulated water and solvent molecules were lost between ∼80 and 280 °C; (3) PA-12 degraded between ∼280 and 420 °C; (4) oxidative degradation in the residual PA occurred from ∼420 to 700 °C, due to structural changes and condensation of pore-surface silanol groups with subsequent water release. The samples prepared at different pH contained a distinct final residue mass, attributed to the silica. pH influenced the hydrolysis and condensation rates of the silica: higher pH accelerated condensation and increased the final mass. These findings corroborate the observations from the SEM micrographs

obtained at pH 9.0. The regions containing agglomerated particles in Figures 3, 4, and 5 are a consequence of excess TEOS in the precursor solution. The presence of cracks in all these coatings stems from a shrinkage effect during the gelling and aging steps of the nanocomposite synthesis. The XRD patterns of all the samples (Figure 6) displayed a similar profile and attest to the structural integrity of the

Figure 6. XRD patterns of neat PA-12 powder and coated samples synthesized at different pH values.

noncoated and silica-coated PA-12. A weak diffraction peak appeared around 2θ = 5°; an intense peak was evident between 2θ = 20° and 23°, which was broader for the silica-coated PA12 samples. Amorphous silica presents a similar profile; therefore, we attributed the broadened peak to the silica coating. The SAXS patterns of all the silica-coated PA-12 nanocomposites displayed correlation peaks ascribed to mesostructured pores (Figure 7). The coating synthesized at pH 11.0 presented higher structural order, with at least five correlation peaks typical of the p6mm 2D hexagonal mesostructure. The

Figure 7. SAXS patterns of the silica-coated PA-12 nanocomposites prepared at different pH values. 782

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Figure 8. TGA (a) and DSC (b) curves of the neat PA-12 and the silica-coated PA-12 nanocomposites prepared at different pH.

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(Figures 3, 4, and 5), which evidenced more homogeneous coatings for the samples prepared at higher pH. DSC measurements (Figure 8b) indicated that the silicacoated PA-12 nanocomposites melted at lower temperature than neat PA-12. Neat PA-12 presented a Tm of 183.0 ± 0.1 °C; Tm values of 177.8, 174.7, 175.4, 174.6, and 180.0 °C were recorded for the silica-coated PA-12 nanocomposites prepared at pH = 7.0, 8.0, 9.0, 10.0, and 11.0, respectively. This result points to a promising application of the silica-coated PA-12 in rapid prototyping technology, since it is possible to arbitrarily produce complex shapes with better mechanical properties using lower potency lasers (in the case of selective laser sintering) or lower temperature (in the case of fused deposition modeling).31

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4. CONCLUSION Polyamide powders coated with core−shell mesoporous silica exhibit improved thermal properties. Coating is successfully achieved by incorporating the polyamide powder in the precursors solution used to obtain the silica matrix, as attested by the SEM micrographs. The pH condition affects the homogeneity of the coatingsregular coatings with a thickness of about 150 nm are only obtained at pH higher than 9.0 DSC measurements revealed a lower melting point for the silica-coated polyamide as compared with the neat polyamide. X-ray diffraction confirmed that the structural integrity of the polyamide was well preserved after the coating process. Therefore, coating polyamides with silica by the sol−gel process is a promising methodology to improve the thermal properties of polymers.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge FAPESP, CNPq, and CAPES (Brazilian research funding agencies) for support of this work. LNLS is acknowledged through the projects D11ASAXS1-10614 and SEM-FEG-11121. We specially thank SAXS staff for support. 783

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dx.doi.org/10.1021/ie302580q | Ind. Eng. Chem. Res. 2013, 52, 779−784