Preparation and characterization of completely biodegradable polymer-titania nanocomposites

[摘要] PLA/PHBV, PLA/PCL and PHBV/PCL blends were prepared through melt-mixing in theabsence and presence of small amounts of titania (TiO2) nanoparticles. The effect of blending andthe presence of nanoparticles on the morphology, thermal degradation behaviour and kinetics,and the dynamic mechanical properties of the different blends and nanocomposites wasinvestigated. The dispersion and distribution of the TiO2 nanoparticles in the blends was studiedusing scanning electron microscopy (SEM) and transmission electron microscopy (TEM), andcontact angle measurements were used to assist in the explanation of the nanoparticle dispersionin the different blends. The thermal stabilities and degradation kinetics of the different sampleswere investigated using thermogravimetric analysis (TGA), and the Flynn-Wall-Ozawa methodwas 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 degradationproducts and the time taken for these products to be released from the sample. The storage andloss moduli, as well as the mechanical damping, of the blends and nanocomposites wereinvestigated using dynamic mechanical analysis (DMA), and these results were related to theeffect 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 the50/50 w/w blend compositions. In the PLA/PHBV system the nanoparticles were well dispersedin the PLA phase and on the interface between the two polymers, with a few large agglomeratesin the PHBV phase. The nanoparticles were found to be equally dispersed in both polymer phasesof PLA/PCL and PHBV/PCL, but some agglomerates were also observed. These observationswere explained through differences in the surface energies, interfacial tensions, molecularweights, viscosities, and crystallinities.For the PLA/PHBV blends the thermal stability of PHBV was improved through blendingwith PLA, while that of PLA was reduced due to the low thermally stable PHBV. The presence ofTiO2 nanoparticles improved the thermal stability of both polymers in the blends. Thedegradation kinetics results showed changes in the activation energy of degradation that couldhave been brought about by the nanoparticles catalysing the degradation process and/or retardingthe 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 bothpolymers, which was attributed to the incompatibility of the polymers. The presence of TiO2nanoparticles in these blends improved the polymers' thermal stabilities. This was also explainedin terms of the catalysis and immobilization effects of the nanoparticles. The thermal stability ofPHBV was improved when blended with the more thermally stable PCL, but the thermal stabilityof PCL decreased. The introduction of only 1 wt% of TiO2 nanoparticles observably improvedthe thermal stabilities of both polymers in the blend, but it is quite possible that the nanoparticlesonly retarded the evolution of the degradation products through their interaction with theseproducts.The storage modulus of the PLA/PHBV blends was higher than those of both PLA andPHBV in the temperature region below the glass transition of PHBV, but the PLA/PCL andPHBV/PCL bends did not show a similar feature. The E' values between the glass transitions ofPLA and PHBV depended on the blend compositions and morphologies. The presence of titaniananoparticles had little effect on the E' values of all the investigated blends. The coldcrystallization transition of PLA shifted to lower temperatures in the PLA/PHBV blends, andshifts in the Tgs of the two polymers indicated partial miscibility at the polymer-polymerinterfaces. This partial miscibility reduced the chain mobilities of these polymers, which could beseen in a reduction in the damping during their respective glass transitions. Blending andnanoparticle 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 higherand lower temperatures than those of the neat polymers, which is a somewhat abnormalobservation. The PCL glass transition peaks became broader as a result of blending, and this wasattributed to incompatibility between the polymers, because blending had no influence on thePCL crystallinity.

[发布机构] University of the Free State