Novel preparation technique for polycarbonate/titanium dioxide nanocomposites
Bisphenol-A polycarbonate (PC) is a thermoplastic that is used in several different technological areas, e.g., in the construction, automotive, medical, and optoelectronic industries. The versatility of PC arises because of its excellent thermal stability, high transparency, ductility, and rigidity, as well as its good tensile strength.1 UV radiation, however, causes yellowing of PC because of the photo-oxidation of aromatic rings on the PC's chemical structure. The UV also simultaneously promotes polymer chain scission of the PC carbonyl groups, which in turn causes a reduction in the mechanical properties and esthetic integrity of the material.2, 3
To date, titanium dioxide (TiO2)—in its rutile crystalline phase—has been widely used as a white pigment and UV blocker for polymers (i.e., to prevent the photodegradation and yellowing problems experienced by several polymer systems) because of its ability to absorb UV radiation.4 For such uses, it is important for the TiO2 particles to be coated. Otherwise, the TiO2–UV interactions create reactive hydroxyl radicals that can speed up the degradation reactions in polymeric materials.5, 6 Coating the TiO2 thus prevents PC photodegradation and can be advantageous for the adhesion between the TiO2 particles and the polymer matrix in composite materials.7, 8
In this work,9 we have evaluated the use of electrospun TiO2 nanofibers—because of their geometry and large surface area—as an alternative method to improve the thermal stability and mechanical properties of PC. We thus obtained the TiO2 nanofibers by electrospinning (following the sol-gel method) a polyvinylpyrrolidone/TiO2 solution (at 18kV). We subsequently heat-treated the nanofibers to produce a sample of only the rutile phase. We then functionalized/dispersed these rutile fibers in a water–acetone mixture and ultrasonic bath, where we used [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AMS) as the coupling agent. This chemical modification of the TiO2 involves hydrolysis of AMS molecules and then a reaction between the silicon–hydroxyl and hydroxyl groups on the TiO2 surfaces, which leads to the formation of silicon-oxygen-titanium covalent bonds. In the final part of our methodology (see Figure 1), we conducted melt-blending in an internal mixer at 250°C (60–88rpm, 50% fill factor) to fabricate PC nanocomposites that contained 0–5wt% of the TiO2 nanofibers.
Scanning electron microscope (SEM) images of our uncoated TiO2 nanofibers—see Figure 2(a)—indicate that the fibers are not hollow after the electrospinning and thermal processes. The SEM image of the coated nanofibers in Figure 2(b), however, shows that there has been some surface modification caused by the AMS coupling agent. That is, a smooth film that covers the TiO2 grain boundaries on the nanofiber surfaces has developed, but this film is imperfect and a portion of the TiO2 nanofibers remained uncovered by the AMS.
Elastic (Young's) modulus measurements of our samples—see Figure 3(a)—show that the PC matrix modulus is not significantly enhanced by the inclusion of increasing TiO2 nanofiber concentrations. Furthermore, the elastic modulus values for all the samples generally fall in the 1.5–1.9GPa range. We attribute this small enhancement of the Young's modulus in the nanocomposites to the low transference of mechanical charges between the nanocomposite constitutents, because of their weak interfacial adhesion.10, 11 We confirmed this poor PC–TiO2 adhesion from the observation of interfacial voids that occur between the nanocomposite phases, and of holes in the PC matrix of all the samples. We illustrate these holes and voids in Figure 3(b), for the example of the AMS-coated sample that contained 2wt% ultrasonicated TiO2 nanofibers. We also find—see Figure 3(c)—that our nanocomposite samples have smaller average fracture elongations than the pure sample. However, the statistical error on the pure PC sample is quite large, which makes it impossible to confidently claim that the PC/TiO2 systems are actually more fragile than pure PC. Nonetheless, the inclusion of TiO2 nanofibers and the use of the AMS coatings does lead to a reduction in the fracture elongation error of the samples.
In addition to the mechanical tests, we have investigated the thermal stability of our nanocomposites (see Figure 4). Our thermogravimetric analysis (TGA) and the corresponding differential thermogravimetric (DTG) curves show that PC has an outstanding thermal stability, but that it starts to degrade at temperatures above 446±1°C. The AMS coating of our samples increases the onset of the degradation temperature (Tonset) to 456±1°C (i.e., by about 10°C). Furthermore, all our nanocomposites exhibit a Tonset above that of the pure PC sample, which indicates that the TiO2 nanofibers cause an increase to the thermal resistance of PC. The highest maximum degradation temperatures we measure from our samples (from those that contain 2 and 5wt% AMS-coated TiO2 and 2wt% electrospun TiO2 fibers) represent an enhancement of about 28–35°C over that of the pure PC (i.e., 484±1°C).
In summary, we have used a new fabrication method (involving a molten-state process) to produce nanocomposites of polycarbonate and electrospun TiO2 nanofibers. We have also shown that it is possible to use an ultrasonic routine to coat the surface of the TiO2 nanofibers with AMS. In the TiO2 concentration range that we investigated, we found that the PC thermal stability improved with the inclusion of both AMS and the nanofibers, and that the elastic modulus of the PC increased slightly with incorporation of electrospun nanofibers. In our future work we intend to study new methods and other processing parameters for the manufacture of this nanocomposite system.
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