Preparation and characterisation of biodegradable polymer nanocomposites with magnesium hydroxide (Mg(OH)2) and functionalized titania (f-TiO2) nanoparticles as fillers
Mukwada, Lesley Tsitsi
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Magnesium hydroxide (Mg(OH)2) (1, 3, 5 and 10 wt.%) and (3-aminopropyl)-trimethoxysilane functionalized titania (APTMS-TiO2) (1, 3 and 5 wt.%) nanoparticles were added to PLA, PCL and their blends (70/30, 50/50 and 30/70) to improve their morphology, thermal and thermomechanical properties. The nanocomposites were prepared via melt mixing, and various techniques were employed to investigate the effect of blending of the two polymers and the influence of the nanoparticles on those properties. The confirmation of the functionalization of titania was done using Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA)-FTIR. The morphology of the blend nanocomposites, the dispersion and localization of the Mg(OH)2 nanorods and APTMS-TiO2 nanoparticles were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), with help of melt flow tester for viscosities (melt flow indices (MFI)) of the polymers, and surface energy properties by surface energy evaluation system (SEES). With differential scanning calorimetry (DSC), the glass transition, cold crystallization, melting temperatures, melting and crystallization enthalpies of all the samples were studied. The effectiveness of these fillers as nucleating agents for crystallization was assessed and any other changes with the presence of both Mg(OH)2 and APTMS-TiO2 nanoparticles were noted. Through TGA, the thermal stabilities of PLA, PCL, their blends and nanocomposites were obtained, and their thermal degradation volatilization was studied using TGA-FTIR. The thermomechanical properties (storage modulus, loss modulus and damping factor (tan δ)) of the materials were determined via dynamic mechanical analysis (DMA). The functionalized TiO2 nanoparticles showed presence of a Si-O-Ti stretching vibration band at 919 cm-1 on an FTIR spectrum which was absent in neat TiO2 and the APTMS functionalizing agent, this indicated a newly formed bond between TiO2 and APTMS. A degradation step between 480.39 and 600 °C which was absent in the neat TiO2 was observed in functionalized TiO2. This indicated that the TiO2 nanoparticles were successfully functionalized, and the mass loss was due to the oxidative thermal decomposition of APTMS. A dispersion of Mg(OH)2 nanoparticles in both polymers was observed. Cracks/crevices were present at the interface of the filler with both polymers and within the agglomerates. In the APTMS-TiO2 nanocomposites, a good dispersion of the filler was seen, but agglomeration and a preferential location in the PLA phase was also observed. This was attributed to the surface energy, viscosity and degree of crystallinities of the polymers and fillers. Both Mg(OH)2 and APTMS-TiO2 nanoparticles acted as nucleating agents in the individual polymers and in some blend ratios. Decreased degrees of crystallinity were experienced in some nanocomposites due to poor dispersion and/or agglomeration of the nanoparticles especially at a higher content with both fillers. PLA and PCL were determined to be immiscible and the presence of the nanoparticles improved their miscibility slightly. Mg(OH)2 had an autocatalytic effect on the degradation of PLA, PCL and their blends. At lower temperatures (below 300 oC), the rate of degradation was faster and a shift to lower temperatures on the thermal degradation of the polymers was observed, due to the loss of water which is known to catalyse the thermal degradation of biodegradable polyesters. At temperatures above 300 oC, where only the stable MgO of Mg(OH)2 was left, the improvement in the degradation temperatures of the polymers was noticed. In the APTMS-TiO2 nanocomposites, an increase in the thermal stability of all the samples was observed even at low temperatures. Both fillers suppressed the release of the degradation volatiles which was seen with reduced intensities of the peaks on the FTIR spectra at different temperatures. In both single and blend nanocomposites with Mg(OH)2 and APTMS-TiO2, there was generally a decrease in the highest thermal degradation volatilization of PLA and PCL. Mg(OH)2 nanocomposites mostly had storage modulus between those of the neat polymers. However, a decrease was observed with the addition APTMS-TiO2 to PLA, PCL and their blends. Miscibility was improved between PLA and PCL in the presence of both Mg(OH)2 and APTMS-TiO2 nanoparticles. This was seen with the decrease in the temperature difference between the glass transition temperatures in the loss modulus curves. The loss factor (tan δ) was decreased in the presence of Mg(OH)2 nanorods and increased with APTMS-TiO2 in the single and blend nanocomposites of both Mg(OH)2 and APTMS-TiO2. This indicated that APTMS-TiO2 nanoparticles had a less restricting/stiffening effect than the Mg(OH)2 nanorods.