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Polymers 2014, 6(7), 2037-2050; doi:10.3390/polym6072037
Abstract: This article reports about nanocomposites, which refractive index is tuned by adding TiO2 nanoparticles. We compare in situ/ex situ preparation of nanocomposites. Preparation procedure is described, properties of nanocomposites are compared, and especially we examine the applicability of two-photon polymerization (2PP) of synthesized nanocomposites. All prepared samples exhibit suitable optical transparency at specific laser wavelengths. Three-dimensional structures were generated by means of two-photon polymerization effect induced by a femtosecond laser.
Recently, the need for optical materials with high refractive index in the fields of ophthalmic lenses, filters, optical adhesives, highly reflective and antireflection coatings, as well as advanced optoelectric fabrications, are increasing [1,2,3,4,5,6,7,8]. High refractive index nanoparticles can be embedded into polymer matrix to tune the refractive index of nanocomposites. For example, PbS nanoparticles have been incorporated into gelation or poly(ethylene oxide) in order to study how the nanoparticles affect the overall refractive index of nanocomposites [9,10]. The preparation of poly(aryl ether sulfone)-based composites containing SiO2–TiO2–ZrO2 has also been reported . Trialkoxysilane-capped PMMA–TiO2 hybrid thin films with high refractive index were prepared using in situ sol-gel method . Photosensitive nanocomposites can be synthesized by adding TiO2 nanoparticles. In comparision with common photoresists these materials have been applied to fabricate microstructures by two-photon polymerization (2PP) [13,14,15,16]. 2PP processing is a polymerization process initiated by two-photon absorption (2PA). Using 2PP technology arbitrary 3D structures can be produced. The generated structures with high refractive index can be used, e.g., in optical tweezers, as the acting forces depend on the refractive index of materials [17,18,19]. In , 2PP fabrication of three-dimensional (3D) structures by means of 2PP was demonstrated in polymers with TiO2 nanoparticles. Firstly, the researcher wrote the 2PP structure in the polymer, and then produced the nanoparticles in the polymer matrix. Furthermore, Sakallari fabricated 2PP structures with refractive index 1.54 . We compare two different strategies of generation of 2PP structures in high refractive index nanocomposites: This article reports in situ and ex situ preparation of high refractive index polymer/TiO2 nanocomposites and analysis of structurability by 2PP when increasing the TiO2 nanoparticles concentration. We show that the refractive index of the synthesized nanocomposites increases with the TiO2 nanoparticle concentration dispersed in the polymer. The prepared samples exhibit suitable optical transparency at 2PP wavelength. 3D structures of nanocomposites have been produced by 2PP.
2. Experimental Section
2.1. In Situ Preparation of Nanocomposites
In situ synthesis of nanoparticles in a polymer matrix is a simple and effective route to prepare nanocomposites. This method allows one-step fabrication of nanocomposites with in situ generated nanoparticles from corresponding precursors. In this case, the nanoparticles can be grown inside the polymer matrix. The advantage of this route is that it prevents particle agglomeration while maintaining a good spatial distribution in the polymer matrix. The drawback of this method is that the unreacted educts of the in situ reaction might influence the properties of the final material. First of all, in this article we used in situ method to prepare the polymer/TiO2 nanocomposites, which is shown in Figure 1.
2.1.1. Synthesis of TiO2 Nanoparticles
The TiO2 nanoparticles used for in-situ preparation were synthesized in the laboratory using the sol-gel method [20,21,22,23]. Titanium (IV) isopropoxide (TTIP, 97.0%, Sigma Aldrich, St. Louis, MI, USA) was used as starting for synthesizing nanoparticles. The TiO2 nanoparticles were synthesized by hydrolysis of TTIP in water, which was formed by decomposition of ethanol. (1) A volume of 30 mL of TTIP was mixed with 100 mL of absolute ethanol. Ethanol is a useful solvent not only to disperse TTIP, but also for hydrolysis of TTIP. The mixture was vigorously stirred at room temperature. (2) A volume of 2.7 mL of hydrochloric acid (HCl, 37.0%) was used as a catalyst for alkoxide hydrolysis, and dropped into the solution. Resultant solution was stirred used a magnetical stirrer for 30 min, subsequently bath-sonicated another 30 min, the as-synthesized TiO2 sol is optically clear. (3) This sol-gel process is easily dried at room temperature to obtain TiO2 nanoparticles, TiO2 is prepared by hydrolysis of TTIP and condensation of titanium hydroxide. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) have been used to characterize the size of particles and dispersion in the polymer matrix, X-ray diffraction (XRD) was used to analyze the synthesized TiO2 nanoparticles.
2.1.2. Preparation of Photosensitive Nanocomposites Using TiO2 Sol
This TiO2 sol was used without further purification. TiO2 sol was added into 4 g polymer hybrid (Ormocers-Ormocore® b59 with 1.8% Photoinitiator Irgacure from Microresist technology GmbH (Berlin, Germany), the refractive index n = 1.554 after polymerized was measured by Abbe Refractometer (OP P/2, Carl Zeiss AG, Oberkochen, Germany) at a wavelength λ = 589 nm) to prepare the polymer/TiO2 sol nanocomposites with a concentration from 1.0 to 20.0 wt %. In addition, each TiO2 sol in this range was dried and the solvent was completely evaporated on the hotplate. TiO2 nanoparticles concentration in polymer was calculated with the linear fitting :
2.2. Ex Situ Preparation of Nanocomposites
Another important route for the preparation of polymer/TiO2 nanocomposites is to disperse pre-made nanoparticles directly into polymer to form composites. This approach is defined as the ex situ synthesis of nanocomposites (Figure 2). The ex situ synthesis method is more suitable for large-scale industrial applications than the in situ method. The key challenge for this method is to be able to prepare nanoparticles, which possess higher dispersibility in the polymer and have long-term stability against aggregation. In order to solve these problems sonication methods were used to disperse the nanoparticles in the polymer.
Preparation of High Refractive Index Polymer/TiO2 Nanocomposites
TiO2 nanoparticles with refractive index n ≈ 2.43−2.8  and diameter 20 nm were used (99.5% TiO2 with 80.0 wt % Rutile and 20.0 wt % Anatase from Evonik Degussa GmbH, Hanau, Germany) and dispersed into polymer matrix to adapt the composite’s refractive index. In contrast to the TiO2 nanoparticles used for in situ preparation, which were synthesized in the laboratory, the TiO2 nanoparticles used for ex situ preparation were purchased from a commercial supplier. Polymer/TiO2 composites with a concentration range between 0.001 and 3.0 wt % were prepared.
In this process the dispersion of the nanoparticles in polymer matrix becomes one critical issue in the successful preparation of these transparent hybrid nanocomposites. Particles agglomeration can significantly reduce the transparency of nanocomposite. In order to disperse the nanoparticles homogeneously in the polymer, the mixtures were first tip-sonicated for 8 min at room temperature, and subsequently the mixtures were sonicated again using a bath-type sonicator for 15 min at 20 °C.
2.3. 2PP of Nanocomposites
The 2PP process is initiated inside the composite by focusing a fesmtosecond laser beam into the photosensitive materials. By moving the laser focal point, arbitrary 3D structures can be directly written into the photosensitive materials. In the present work 2PP technology was used for photopolymerization of synthesized nanocomposite. In order to evaporate the residual solvent, before writing the structures, the liquid composite on the cover glass was pre-baked on the hotplate at 90 °C for 0.5 h.
We used a mode-locked frequency-doubled ytterbium-doped glass laser system at a wavelength of 515 nm with a pulse width of 240 fs as light source and a 100× oil immersion microscope objective lens (Plan Apochromat, N.A. = 1.4, Carl Zeiss AG, Oberkochen, Germany) was used to focus the laser beam into the composite (Figure 3). After polymerization, SEM was used to record images of fabricated 3D structures.
3. Results and Discussion
3.1. In Situ Process
3.1.1. X-ray Diffraction (XRD) Analysis
At room temperature XRD curves exhibit that TiO2 nanoparticles have amorphous structure . The refractive index of crystalline nanoparticles is higher than that of amorphous [15,24]. In order to obtain the TiO2 crystals, we investigated the nanoparticles at different temperature. XRD patterns of heat-treated TiO2 at different temperature are shown in Figure 4. It was observed that annealing at 250 °C leads to the formation of the anatase phase. XRD curves exhibit strong reflection peaks in the 2θ region of 20°–80° after treated at 250 °C. The identical peaks could be assigned to originate from reflection by (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), and (215) planes of TiO2 anatase crystals, matching well with its simulated anatase crystal structure .
We investigated the polymerized nanocomposites after heat-treated for 24 h at 250 °C, the reflection peaks (101) and (200) in Figure 5 were matched with standard anatase crystal. It can be confirmed that the TiO2-anatase crystal after heat-treated at 250 °C in vacuum oven can be obtained in the composites.
3.1.2. TEM/SEM Characterization
TiO2 nanoparticles synthesized using the in situ method as described in Section 2.1.1 was characterized by Transmission Electron Microscopy (TEM) (CM200, Philips, Eindhoven, Netherlands). The TiO2 sol was transparent, and the nanoparticles were dispersed in the sol (Figure 6a). Figure 6b shows the nanoparticles after the sol was completely dried. Figure 6c shows the TEM image of the TiO2 nanoparticles after the sol was completely dried. The average particle size of the TiO2 nanoparticles observed from TEM analysis was approximately 5 nm.
After the TiO2 particles in sol were incorporated directly into the polymer matrix at different concentrations, the resulting composites were polymerized under a UV lamp. The SEM images of samples at 0.16 and 0.64 wt % TiO2 nanoparticles at cross-section are shown in Figure 7.
3.1.3. Refractive Index Measurements
The refractive index of synthesized nanocomposites can be modified by changing the nanoparticles concentration. Figure 8 shows the variation of refractive index both at room temperature, as well as after heat-treated at 250 °C. First of all, the samples were prepared directly after the residual solvent was evaporated at 90 °C on a magnetic stirrer, as in the beginning the TiO2 sol was mixed with polymer, there are other lower refractive index solvents in the mixture, such as H2O (n = 1.33) and ethanol (n = 1.36), respectively.
Then, the polymerized samples with thickness of thin film 300 µm were measured using Abbe Refractometer (Figure 8). Furthermore, the polymerized samples were heat-treated in an oven at 250 °C for 2 h in order to obtain the TiO2 crystals, as the refractive index of TiO2 crystals is higher than that of amorphous TiO2 nanoparticles. Figure 8 shows the refractive index is increasing by increasing the concentration, after 250 °C heat-treated the refractive index of polymerized samples is higher than at room temperature.
3.1.4. Optical Characterization
In this article a method for the fabrication of 3D submicrostructures doped with TiO2 nanoparticles based on 2PP is presented. For 2PP processing, high transparency at the used wavelength (515 nm) is required to efficiently localize the focal point of the laser. Therefore the optical properties have to be measured and adapted. The transmission of synthesized composites was investigated and measured by using UV-Vis-NIR spectrometer, as the nanocomposites have higher transmission, the laser beam can be transmitted through the composite to write 3D structures. Figure 9a displays the transmission spectra of nanocomposite in the 480–560 nm area; Figure 9b shows the transmission at 2PP wavelength 515 nm. It is clearly that the transmission with 0.16–3.20 wt % of TiO2 nanoparticles is above 60%, and can potentially be used for 2PP structuring. It has been also shown that increasing TiO2 nanoparticle concentration indicates decreasing transmission.
3.1.5. 2PP Results
For demonstration of 3D structure fabrication, the woodpile structure was chosen. The 3D structures both at low (0.16 wt % TiO2 nanoparticles) and high (2.56 wt % TiO2 nanoparticles) concentrations are generated with a writing speed of 2.0 mm/s and with a laser power of 1.4 mW . The resulting 2PP-fabricated structures contain amorphous TiO2 nanoparticles, which were demonstrated by XRD analysis. The samples used in  were not heat treated. However, the refractive index of amorphous TiO2 is lower than that for crystalline TiO2. Therefore, samples in this study were thermally treated at 250 °C for 2 h. In Figure 10, nanoparticles on the surface of structures were observed. These nanoparticles are TiO2 anatase, which was generated through reaction between Ti4+ ions and H2O, can be confirmed by XRD. This result is clear evidence that using 2PP processing we have successfully generated the structures containing the nanoparticles in the polymer matrix.
3.2. Ex Situ Process
3.2.1. SEM Characterization
Figure 11 shows SEM images of nanoparticles dispersion at different concentrations: 0.001 (a), 0.1 (b) and 2.0 wt % (c). The nanoparticles are highly dispersed in polymer, the higher the concentration, the more the nanoparticles in polymer, and the nanoparticles were agglomerated; In this case it will be influenced on measurements in transmission and refractive index. Using 2PP processing to write structures, the laser beam will be scattered by agglomerated nanoaprticles at high concentration, which will distort the fabricated structures.
3.2.2. Refractive Index Measurements
The refractive index can be modified by changing the TiO2 concentration, as shown in Figure 12. The refractive index was measured by 8 min tip-sonication using an Abbe Refractometer. The refractive index increased with increasing TiO2 composite concentration, indicating that doping concentration is an important factor. The increase in refractive index clearly exceeded the precision of the refractive index measuring device (approximately 0.0025).
3.2.3. Optical Characterization
The transmission of synthesized composites was investigated and measured by using UV-Vis-NIR spectroscopy.
Figure 13a displays the transmission spectra of nanocomposite in the 480–560 nm area; Figure 13b shows the transmission at 2PP wavelength 515 nm. It is clearly that the transmission with 0.001–0.1 wt % of TiO2 nanoparticles is above 60%, which indicates higher optical transparency, and can potentially be used by means of 2PP to write structures. It has been also shown that increasing concentration indicates decreasing transmission. The transmission with 0.5–3.0 wt % of TiO2 nanoparticles is below 60%, the laser beam is difficult to transmit through the composite, therefore the laser beam will be strongly scattered by nanoparticles, which 3D structures cannot easily fabricate.
3.2.4. 2PP Results
Figure 14 shows the fabricated 3D structures by 2PP processing at 0.001 wt % (Figure 14a) and 0.5 wt % (Figure 14b). The inset of upper right structure (Figure 14a) is fabricated at laser power 10 mW and writing speed 1 mm/s, compared to the inset of upper right (Figure 14b) at laser processing power 7 mW and writing speed 1 mm/s. It can be clearly seen that, at low concentrations, the structure is smoother than at high concentrations; after the structures were fabricated, both of the structures and the surface of substrate were washed cleanly. At a concentration of 0.5 wt % the transparency is even below 20%, however, we have successfully fabricated the structures. We have tried to write the structures at higher concentrations than 1.0 wt %, the structures were heavily distorted depending on the low transparency, at which the laser beam could not transmit through the nanocomposite.
We have presented the results from investigations into high refractive index polymer/TiO2 nanocomposites, which can be successfully structured by 2PP. In the in situ process: We have synthesized high refractive index nanocomposites. All samples exhibit suitable optical transparency at the 2PP wavelength, in which the laser beam can transmit through the nanocomposite to write arbitrary 3D structures. In the beginning, the refractive index of synthesized nanocomposites is not too high, e.g., the refractive index at 2.0 wt % was approximately 1.557. After heat-treatment, the refractive index significantly increased, i.e., the refractive index at 2.0 wt % was approximately 1.563. The heat treatment process generated TiO2 anatase nanoparticles. In the ex situ process: sonication-tip was used to disperse the nanoparticles in the polymer. High refractive index nanocomposites have been directly synthesized, e.g., the refractive index at 2.0 wt % is about 1.565. Although this is higher than the refractive index obtained by the in situ procedure, it is not possible to write structures using lasers when the in situ procedure is used; this should be considered when choosing between the in situ and ex situ methods of polymer composite preparation for a particular application. The refractive index of both in situ and ex situ prepared nanocomposites can be tuned by increasing the TiO2 concentration. However, at higher concentrations, the optical transparency becomes unsuitable for the 2PP wavelength of 515 nm. In such a case, the laser beam scatters, and, as a result, cannot transmit through the nanocomposite to write 3D structures. The prepared samples exhibit suitable optical transparency for 2PP processing. Using 2PP technology, structures with high refractive index have also been produced. In this way, the synthesized composites can be used both as high refractive index and as structural polymers.
We would like to thank Deutsche Forschungsgemeinschaft (DFG) for the generous support of this article in the framework of project OS188/17-2 and acknowledge financial support by Bundesministerium für Bildung und Forschung (BMBF) in the frame of REMEDIS B2 (FKZ 03IS2081I).
Conflicts of Interest
The authors declare no conflict of interest.
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