High Clarity, Thermally Stable and Mechanically Robust Titania

Article pubs.acs.org/IECR

High Clarity, Thermally Stable and Mechanically Robust Titania Nanocomposites: Probing the Role of Binding Agent Sonia Zulfiqar,†,‡,* S. Ismat Shah,§,∥ and Muhammad Ilyas Sarwar⊥,* †

Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan Institute of Polymers, Bulgarian Academy of Sciences, Acad. Georgi Bonchev Str. Bl.103, 1113 Sofia, Bulgaria § Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States ∥ Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States ⊥ Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan ‡

ABSTRACT: Highly transparent nanocomposites were produced by in situ generation of titania network into an aromatic polyamide through a sol−gel process. These nanocomposites were fabricated using two different strategies, system-I involved the interaction of hydroxyl groups present on the backbone of the matrix with tetraisopropylorthotitanate (TPOT), while in systemII, 3-isocyanatopropyltriethoxysilane (ICTS) was exploited for extensive bonding between titania and hydroxyl groups of aromatic polyamide derived from the condensation of different diamines and terephthaloyl chloride. Thin nanocomposite films were analyzed for their structural, optical, mechanical, thermal, and morphological analyses. FTIR data confirmed the formation of nanocomposites and interaction among the phases. Tensile properties of these materials were improved with increasing titania content relative to pure polyamide. FESEM micrographs clearly showed homogeneous distribution of titania at nanolevel in the matrix. Thermal decomposition and glass transition temperatures of nanocomposites were found in the ranges 450−550 °C and 285−328 °C, respectively, depicting high thermal stability and interactions between the two phases.

1. INTRODUCTION Nanocomposites offer the opportunity to tailor their properties by adjusting the proportions of both the organic and inorganic components.1−3 The uniform dispersion of inorganic phase within the polymer matrix and interaction between the phases eventually furnishes optimum properties. The compatibility of constituents depends on the nature of the interface among the components, as well as on the size and dispersibility of the inorganic phase. These materials find wide applications in electronics, optics, chemistry, and biomedicine4−6 and also possess superior thermal and mechanical properties.7−10 These outstanding properties of nanocomposites clearly make them exclusive over the conventional macro-composites. Sol−gel process is one of the most eminent routes to fabricate nanohybrid materials in which the reinforcement is dispersed well within the polymer matrix resulting in the formation of inorganic ceramic network. This technique typically involves the hydrolysis and condensation reactions of metal alkoxides to produce glasses and ceramics at relatively low temperature.11 The structure and properties of the hybrids are dependent on these reactions, which are controlled by pH, nature of solvent, and type of alkoxide. Titania (TiO2) is particularly in the spotlight due to its widespread motivating properties, such as high refractive index, photocatalytic activity, low birefringence, and of course low cost.12−14 Recently, numerous efforts have been made to produce titania nanocomposites for various applications such as gas separation membranes,15 for organic pollutant photodegradation16 etc. Incorporation of titanium oxide nanoparticles in thin films and coatings can significantly improve the surface hardness while self-cleaning and antimicrobial © XXXX American Chemical Society

properties can also be introduced based on the photocatalytic effect induced by some titanium oxide modification. Hybrid materials of various polymers with ceramics have been extensively studied.17−23 Sol−gel process has been used to synthesize hybrid materials based on polymers and TiO224−26 due to their refractive indices and good UV shielding properties for photoelectric applications.27−34 This process has also been used to introduce metal oxide networks in many commercially important polymers such as epoxies,35 polyacrylates,36 polyimides,37 polyamides,38−40 and poly(dimethylsiloxanes).41 The choice of polymeric matrix and the inorganic filler depends on the field of application as well as on the desired properties. Aromatic polyamides are modern high-temperature resistant polymers, especially fluorinated polyamides when thermal stability is considered. Polyamides are quite intractable materials, because they are infusible and insoluble in common organic solvents. Therefore, many attempts have already been carried out to modify the structure of aromatic polyamides in order to improve their solubility and processability without severely impairing their exceptional thermal properties.42−45 Previously, poly(trimethylhexamethyleneterephthalamide)-titania nanocomposites having no chemical bonding have been investigated.40 The properties of these hybrid materials were not improved much due to lacking of the interphase bonding. Recently, chemically bonded aramid-titania hybrids using isocyanatopropyltriethoxysilane have been reported.46 Phase Received: May 19, 2013 Revised: July 17, 2013 Accepted: July 23, 2013

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dx.doi.org/10.1021/ie401598d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Formation of Polyamide−Titania Nanocomposites Bonded through Hydroxyl Groups

separation occurred in these materials beyond 12 wt % titania with reduction in the property profile of the nanocomposites. So, we have chosen a particular diamine [2,2-bis(3-amino-4hydroxyphenyl) hexafluoro propane in order to produce bonding between the two phases or enhancement of properties in the resulting nanocomposites. The aim of present work is to generate titania network in polyamide matrix through sol−gel process and to investigate the effect of binding agent on the properties of resulting nanocomposites. Herein, we report two systems on the synthesis and characterization of soluble polyamide and its nanocomposites with titania. In the first system, hydroxyl groups of polymer chains were directly bonded with a titania network resulting from the hydrolytic condensation of tetraisopropyl orthotitanate (TPOT) using sol−gel process. Whereas, the second system presented the superior approach to create extensive interfacial bonding between the two phases by linking hydroxyl groups of polyamide and 3-isocyanatopropyl triethoxysilane (ICTS), thus yielding ethoxy pendant groups on the polymer backbone. Chemical bonding between the two phases was developed by subsequent hydrolysis and condensation of TPOT and pendant ethoxy groups of the matrix. Aromatic polyamide was synthesized by condensing a mixture of diamines namely 1,3-phenylenediamine, 1,4-phenylenediamine, and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoro propane with terephthaloyl chloride in N,N-dimethylacetamide (DMAc). Nanocomposite films were prepared with varying amount of titania in the matrix. These materials were characterized for their structural, optical, morphological,

mechanical and thermal profile using Fourier transform infrared spectroscopy (FTIR), UV−vis spectroscopy, field emission scanning electron microscopy (FESEM), tensile testing, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC).

2. MATERIAL AND METHODS 2.1. Materials. Various monomers such as terephthaloyl chloride (TPC), 1,3-phenylene diamine (m-PDA), 1,4-phenylene diamine (p-PDA), and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoro propane were procured from Fluka and used as received. Tetraisopropyl orthotitanate (TPOT) and (3isocyanatopropyl)triethoxysilane (ICTS) were purchased from Aldrich and used as such. N,N-dimethyl acetamide (DMAc) obtained by the courtesy of Aldrich was used after drying over molecular sieves before use. 2.2. Synthesis of Aromatic Polyamide Matrix. Polyamide chains were produced by the reaction of different aromatic diamines (0.05 mol) and terephthaloyl chloride (0.05 mol) at low temperature and under anhydrous conditions. In a typical reaction, 1,3-phenylenediamine, 1,4-phenylenediamine, and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoro propane were taken (60:30:10 mol ratio) in a flat bottom flask equipped with a magnetic stirrer. DMAc was used as solvent medium and added into the flask to dissolve these diamines. The agitation was continued until complete dissolution. To this diamines solution, terephthaloyl chloride was added with vigorous stirring at 0 °C. As the polymerization proceeds, the reaction mixture became highly viscous. The stirring was continued for B



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Scheme 2. Formation of Polyamide−Titania Nanocomposites Using ICTS as a Binding Agent

24 h to ensure completion of the reaction. This polymer solution served as a stock solution for the synthesis of nanocomposite materials. 2.3. Synthesis of Polyamide-Titania Nanocomposites. Polyamide-titania nanocomposites were synthesized using two different strategies, i.e., with and without binding agent. Thin nanocomposite films with different amounts of titania were obtained by solvent elution technique. 2.3.1. Bonded System-I. Typical synthesis of nanocomposite films involved the mixing of various amounts of polyamide solution with different proportions of TPOT. The mixture was thoroughly agitated for 30 min. Then a measured amount of water was added to carry out the hydrolysis and condensation

processes for in situ generation of titania network. The HCl produced from the polymerization reaction as byproduct served as catalyst for the sol−gel reaction. The concentration of TPOT was varied from 2.5 to 20 wt % in these nanocomposite films. The reaction mixture was further agitated for 6 h at room temperature in order to complete the sol−gel process. Thin and uniform nanocomposite films were cast by pouring each particular concentration into petri dishes placed on a leveled surface, followed by evaporating the solvent at 70 °C. Thin nanocomposite films were then soaked in water to get rid of the HCl produced during the polymerization reaction. These golden and highly transparent films were further dried at 70 °C under vacuum for 72 h (Scheme 1). C



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2.3.2. Extensively Bonded System-II. In this approach, reactive pendant alkoxy groups on the backbone of polyamide have been introduced using a binding agent (ICTS). The isocyanato groups of ICTS reacted with the hydroxyl groups on the polymer chain forming urethane linkage with pendant alkoxy groups. For this purpose, a stoichiometric amount of ICTS in DMAc was added to the polymer solution with continuous stirring for 4 h at 50 °C. These alkoxy groups were then exploited for further reaction with TPOT. For a particular nanocomposite concentration, measured amounts of both TPOT and polymer solutions were blended together at room temperature for 24 h. The concentration of TPOT in the nanocomposites was varied between 2.5 to 20 wt %. The subsequent hydrolysis and condensation reactions generated titania network in situ within a polyamide matrix. Thin films of controlled thickness were prepared by solvent elution technique. These films were then soaked in water to eliminate HCl and then further dried under vacuum at 70 °C to a constant weight (Scheme 2). 2.4. Characterization. FTIR spectra of nanocomposite films were recorded using Excalibur Series FTIR Spectrometer, Model No. FTSW 300 MX, manufactured by BIO-RAD. UV− vis transmission spectra of the pure polyamide and nanocomposite films were recorded using Shimadzu Spectrophotometer UV-1700 series. Tensile properties of rectangular strips of the nanocomposite films were measured according to DIN procedure 53455 at 25 °C using Testometric Universal Testing Machine M350/500. Average values obtained from five to seven different measurements in each case have been reported. For phase morphological studies, samples were cryogenically fractured in liquid nitrogen and the morphology was investigated by JEOL JSM 7400F field emission scanning electron microscope (FESEM). The thermal stability of polyamide and nanocomposites were determined using a Mettler Toledo TGA/SDTA 851e thermogravimetric analyzer using 1−5 mg of the sample in Al2O3 crucible heated from 25 to 900 °C at a heating rate of 10 °C/min under a nitrogen atmosphere with a gas flow rate of 30 mL/min. Thermomechanical properties of these materials were measured using a METTLER TOLEDO DSC 822e differential scanning calorimeter. For determining the glass transition temperatures, samples of 5−10 mg were encapsulated in aluminum pans and heated at a ramp rate of 10 °C min−1 under nitrogen atmosphere.

Figure 1. FTIR spectra of polyamide−titania nanocomposites; (a) bonded through hydroxyl groups (b) using ICTS as binding agent.

at 1402 and 1246 cm−1, respectively. A very important stretching vibration appeared at 1108 cm−1 for Ti−O−C, indicating the chemical interaction of the inorganic phase with the matrix through the hydroxyl groups and different bands in the range 435−781 cm−1 for Ti−O and Ti−O−Ti assigned to the TiO2 network formation. IR data for the hybrid films extensively bonded (system-II) through ICTS is given in Figure 1(b). The appearance of additional bands of urethane linkage, CO at 1733 cm−1, aliphatic C−H stretching at 2923 and 2856 cm−1 and Si−O−Ti at 967 cm−1, respectively, in system-II indicated a chemical interaction between the two phases. The presence of an aliphatic C−H band and urethane CO stretching in system-II and their absence in system-I again proved the interphase bonding in nanocomposites through ICTS. The rest of the bands for the formation of the TiO2 network appeared at their respective positions. These results verified formation of the hybrid materials and the interfacial interaction among the components of nanocomposites. 3.2. Optical Properties. Pure polyamide and its nanoccomposite films were also subjected for optical measurements. All the films were transparent and golden in color. More quantitatively, transmittance was measured in the region 200− 800 nm and transmission spectra of these nanomaterials are given in Figures 2 and 3. The color of films (system-I) became golden with increase of titania content and the films beyond 2.5 wt % titania were less transparent relative to the pure polyamide (Figure 2). Further titania loading displayed lower optical clarity indicating more scattering resulting in lower transparency. In system-II, transparency was improved up to 10 wt % titania in the matrix as compared to the neat polyamide (Figure 3) because the size of inorganic phase was smaller than the wavelength of light. Beyond this concentration, optical clarity decreased due to scattering, giving low transmittance. This system is more transparent as compared to system-I, reflecting a clear difference in employing a binding agent. The transparency of these nanocomposites depends on the size, size distribution, and spatial distribution of the titania network

3. RESULTS AND DISCUSSION Thin films obtained from the pure polyamide and composites were golden in color and transparent. The transparency was quantitatively scrutinized by UV−vis spectroscopy. The interphase bonding in nanocomposites was monitored by FTIR spectroscopy. Different analyses carried out for the characterization of these materials are described below. 3.1. FTIR Spectroscopy. FTIR analysis was performed to study the interfacial interactions between the two components of nanocomposites, especially the interaction of −OH groups on polymer chain with NCO-groups of binding agent along with TiO2 network being produced in the matrix. The IR spectrum of system-I bonded through hydroxyl groups are shown in Figure 1(a). This system gave various bands of matrix in the nanocomposites such as N−H stretching and bending at 3299 cm−1 and 1604 cm−1, aromatic C−H stretching at 3050 cm−1, aromatic CC stretching at 1512 cm−1 and 1482 cm−1, CO stretching at 1644 cm−1, and C−N and C−O stretching D



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Figure 2. UV−vis spectra of polyamide−titania nanocomposites bonded through hydroxyl groups.

Figure 4. Variation of maximum stress vs titania content in polyamide−titania nanocomposites.

Figure 3. UV−vis spectra of polyamide−titania nanocomposites using ICTS as binding agent.

Figure 5. Variation of maximum strain vs titania content in polyamide−titania nanocomposites.

within the polymer matrix. Films containing low titania content were transparent, presumably the average particle size was less than the wavelength of light, and the uniform dispersion of inorganic network. However, at high titania loading, agglomeration of the titania network took place that decreased homogeneity causing less transparency, thus scattering more light, reducing transmittance and optical clarity. 3.3. Tensile Testing. Mechanical profile of the polyamide− titania nanocomposites with different concentrations of titania in both the systems was measured. Maximum stress for pure aramid was found to be 86 MPa that increased up to 122 MPa with 5 wt % addition of titania in system-I. In system-II, maximum stress (124 MPa) was achieved with 10 wt % addition of titania and then it decreased with increase in titania content (Figure 4). The length at break of these materials increased up to 7.5 wt % of titania in both the systems (Figure 5). Young’s modulus of the nanocomposites increased from 6.7 GPa for pure polyamide to a maximum value of 8.1 GPa with 7.5 wt % of titania in system-I; and for system-II, an increase in the modulus was found to be 8.6 GPa with 10 wt % addition of

titania, beyond which it decreased as given in Figure 6. Tensile data revealed improvements in the mechanical properties of the nanocomposites relative to a pristine polyamide matrix due to the interfacial interactions between the two phases. The addition of titania in system-I beyond 7.5 wt % reduced the mechanical properties of the hybrid materials because only a part of the titania network becomes chemically linked with polyamide chains due to limited number of −OH groups available. As the inorganic network became extensive, the tendency for particle-size growth increased, and their agglomeration made their distribution in the matrix irregular. These particles would then form clusters, thus making the inorganic structure more porous and brittle, which would cause a decrease in the tensile strength and other associated mechanical properties of the hybrid materials. Toughness was measured by integrating the area under stress−strain curves. These values indicated a significant increase in this particular property of hybrids than pure polymide in both the systems with 7.5 wt % titania, as shown in E



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because at high concentrations, agglomeration may take place, which results in poor interfacial interactions between the two disparate phases, leading to brittleness and deterioration of mechanical properties. 3.4. Morphology. The morphology of the fractured samples was monitored by FESEM to investigate the distribution of titania in hybrid materials. Nanocomposites bonded through hydroxyl groups of polyamide containing 2.5 to 20 wt % titania are represented in Figure 8. The particle sizes within the hybrid films prepared by sol−gel process are small and of nanometer regime demonstrating that nanocomposites can be prepared with the current process. FESEM images clearly showed a fine interconnected or cocontinuous morphology, revealing that the titania network has rough surfaces and diffused boundaries. This cocontinuous morphology gave a better interfacial cohesion with improved efficiency of the stress transfer mechanism between the two components. That is why the tensile strength of the composite films improved for this system. Irrespective of enhanced cohesion, the elongation at break decreased since the interconnected titania phase hinder the plastic flow of the polyamide phase, preventing large deformations from occurring prior to fracture. An increase in titania concentration augmented the particle size and the large domains can result in light scattering giving low transmittance (See Figure 2). Therefore, these results are in accordance with % transmittance measurements of nanocomposite films with different titania concentrations. Similarly, the morphologies of the polyamide−titania nanocomposites obtained using ICTS as a coupling agent with 2.5 to 20 wt % of titania in the matrix are demonstrated in Figure 9. The micrographs of this system clearly showed a fine interconnected or cocontinuous morphology. This phenomenon also indicates that the titania network has a diffused network with a better interfacial cohesion, solving the stress transfer problem between the two components. This better compatibility between the inorganic network and polyamide in the composites resulted in improved tensile strength. Titania concentrations beyond 10 wt % gave large domains that might deteriorate the mechanical properties of the composites. At high loading of titania, the network may agglomerate thus decreasing homogeneity, scattering more light, reducing transmittance and optical clarity (Figure 3). 3.5. Thermal Stability. Thermogravimetric analysis on the composite systems was carried out under nitrogen atmosphere in the temperature range 25−900 °C. Thermograms for both systems are described in Figures 10 and 11. The initial weight loss in both the systems was probably due to the removal of water molecules generated from the conversion of uncondensed titania network structure at 150−200 °C. Then the polyamide matrix started to decompose in both systems near 450 °C. The thermal decomposition of system-I was found to be in the range 450−550 °C. System-II was decomposed slightly earlier than system-I, probably due to the decomposition of the binding agent added for interphase interactions among the two phases. Thus, the thermal stability of the materials was improved upon the addition of the titania relative to the pure polyamide. The amount of residue retained at 900 °C in both systems was roughly proportional to the titania content in the nanocomposites. 3.6. Glass Transition Temperatures. The glass transition temperatures (Tg) of polyamide−titania nanocomposites are presented in Figure 12. The data indicated a regular increase in the Tg values with increasing titania content in the polyamide

Figure 6. Variation of tensile modulus vs titania content in polyamide−titania nanocomposites.

Figure 7. The stress-bearing property of the inorganic condensed phase was associated with relatively less free

Figure 7. Variation of toughness vs titania content in polyamide− titania nanocomposites.

volume, whereas the organic phase has large free volume and is relatively less capable of bearing stress. The relative increase in toughness as compared to the pure polymer was much more pronounced in both systems studied. It was attributed to the chemical bonding between the two phases. In system-II, improvement was more pronounced due to the influence of a coupling agent in terms of extensively binding more inorganic network to the organic phase, particularly up to 10 wt %. The mechanical properties (stress, strain, and toughness) of systemII decreased more than system-I at high titania loading. This may be due to the greater agglomeration of the titania network growing at three points of ICTS (binding agent) yielding more brittleness and deterioration of mechanical properties. These interactions solved the stress transfer problem efficiently between the matrix and reinforcement, producing mechanically robust nanocomposites. However, this improvement in properties was limited only to low concentrations of reinforcement, F



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Figure 8. FESEM micrographs of polyamide−titania nanocomposites bonded through hydroxyl groups (a) 2.5%, (b) 5%, (c) 10%, (d) 15%, and (e) 20%.

matrix. Both systems gave maximum Tg values with 15 wt % titania, illustrating interaction between two disparate phases

relative to pure polymer. Further addition of titania decreased the Tg value because entire titania may not be linked with the G



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Figure 9. FESEM micrographs of polyamide−titania nanocomposites using ICTS as binding agent (a) 2.5%, (b) 5%, (c) 10%, (d) 15%, and (e) 20%.

This observation implied that more polyamide chains had linked with the titania phase. Therefore, the motion of polymer chains was restricted, thus increasing the Tg values of

polymer chains, reducing the interfacial interaction. Increased amount of titania impeded the segmental motion of the polymer chains, thus shifting the Tg toward high temperature. H



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Figure 10. TGA curves of polyamide−titania nanocomposites bonded through hydroxyl groups.

Figure 12. Variation in glass transition temperatures vs titania content in polyamide−titania nanocomposites.

nanocomposites. Tg’s of composites augmented relative to pure matrix. This behavior may be explicated due to mutual interaction of the two phases, which suppresses the mobility of the polymer segments near the interface. Tg values are less pronounced in System II due to plasticization of the ICTS as compared with System I.

compatibility between the two phases. The interfacial interactions through hydroxyl groups of polyamide and coupling agent promoted strong interphase bonding between the two disparate phases resulting in enhanced transparency, mechanical and thermal properties of the nanocomposites. The morphological investigations revealed a uniform distribution of titania network structure in the matrix at nanoscale. The thermal stability of hybrid materials was also increased by a corresponding increase in the titania content in the polyamide matrix. The shift in Tg values suggested good interaction between the two phases.

4. CONCLUSIONS High clarity nanocomposites, derived from polyamide and titania having chemical bonding either through hydroxyl groups of polymer backbone or ICTS, with an improved mechanical profile relative to the pure polyamide, were successfully prepared using a sol−gel process. The incorporation of titania network reinforced the polyamide matrix, depicting good

Figure 11. TGA curves of polyamide−titania nanocomposites using ICTS as binding agent. I



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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 0092-301-5017753. Fax: 0092-51-9247006. E-mail: soniazulfi[email protected]. *Tel.: 0092-51-90642132. Fax: 0092-51-90642241. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.I.S. is very thankful to Quaid-i-Azam University for financial support provided through the University Research Fund (URF 2012-13).

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