Preparation and characterization of completely biodegradable polymer-titania nanocomposites
Mofokeng, Julia Puseletso
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PLA/PHBV, PLA/PCL and PHBV/PCL blends were prepared through melt-mixing in the absence and presence of small amounts of titania (TiO2) nanoparticles. The effect of blending and the presence of nanoparticles on the morphology, thermal degradation behaviour and kinetics, and the dynamic mechanical properties of the different blends and nanocomposites was investigated. The dispersion and distribution of the TiO2 nanoparticles in the blends was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and contact angle measurements were used to assist in the explanation of the nanoparticle dispersion in the different blends. The thermal stabilities and degradation kinetics of the different samples were investigated using thermogravimetric analysis (TGA), and the Flynn-Wall-Ozawa method was used to estimate the activation energies of degradation. Fourier transform infrared (FTIR) spectroscopy connected to the TGA was used to evaluate the nature of volatile degradation products and the time taken for these products to be released from the sample. The storage and loss moduli, as well as the mechanical damping, of the blends and nanocomposites were investigated using dynamic mechanical analysis (DMA), and these results were related to the effect of blending and the presence of nanoparticles on the glass transition temperature, miscibility, and compatibility of the polymers in the different samples. All three polymer pairs were immiscible and showed a co-continuous structure for the 50/50 w/w blend compositions. In the PLA/PHBV system the nanoparticles were well dispersed in the PLA phase and on the interface between the two polymers, with a few large agglomerates in the PHBV phase. The nanoparticles were found to be equally dispersed in both polymer phases of PLA/PCL and PHBV/PCL, but some agglomerates were also observed. These observations were explained through differences in the surface energies, interfacial tensions, molecular weights, viscosities, and crystallinities. For the PLA/PHBV blends the thermal stability of PHBV was improved through blending with PLA, while that of PLA was reduced due to the low thermally stable PHBV. The presence of TiO2 nanoparticles improved the thermal stability of both polymers in the blends. The degradation kinetics results showed changes in the activation energy of degradation that could have been brought about by the nanoparticles catalysing the degradation process and/or retarding the volatilization of the degradation products, depending on their localization and their interaction with the polymer in question. Blending of PLA and PCL reduced the thermal stabilities of both polymers, which was attributed to the incompatibility of the polymers. The presence of TiO2 nanoparticles in these blends improved the polymers’ thermal stabilities. This was also explained in terms of the catalysis and immobilization effects of the nanoparticles. The thermal stability of PHBV was improved when blended with the more thermally stable PCL, but the thermal stability of PCL decreased. The introduction of only 1 wt% of TiO2 nanoparticles observably improved the thermal stabilities of both polymers in the blend, but it is quite possible that the nanoparticles only retarded the evolution of the degradation products through their interaction with these products. The storage modulus of the PLA/PHBV blends was higher than those of both PLA and PHBV in the temperature region below the glass transition of PHBV, but the PLA/PCL and PHBV/PCL bends did not show a similar feature. The E’ values between the glass transitions of PLA and PHBV depended on the blend compositions and morphologies. The presence of titania nanoparticles had little effect on the E’ values of all the investigated blends. The cold crystallization transition of PLA shifted to lower temperatures in the PLA/PHBV blends, and shifts in the Tgs of the two polymers indicated partial miscibility at the polymer-polymer interfaces. This partial miscibility reduced the chain mobilities of these polymers, which could be seen in a reduction in the damping during their respective glass transitions. Blending and nanoparticle addition had little influence on the glass transition temperatures of PLA and PCL, but the glass transitions of PHBV and PCL in the PHBV/PCL blends were respectively at higher and lower temperatures than those of the neat polymers, which is a somewhat abnormal observation. The PCL glass transition peaks became broader as a result of blending, and this was attributed to incompatibility between the polymers, because blending had no influence on the PCL crystallinity.