Polylactide (PLA)-cationic (montmorillonite, MMT) and anionic (hydrotalcite, HT) clay micro- or nanocomposites based on semicrystalline and amorphous polymers and unmodified and organomodified as well as calcined or non-calcined clays at 5wt% concentration are produced by melt mixing. Depending on the clay type, presence of organomodifiers and calcination, micro- or nanocomposites are produced, as confirmed by XRD and SEM analysis.
Unfilled polymers and their composites are subjected to isothermal degradation in air at 180°C and 200°C. Degradation rate constants are calculated from novel equations incorporating IV. Results show that the thermal degradation rate constants of the amorphous PLA and its composites are lower than those of the semicrystalline PLA and its composites due to the higher initial MW of the amorphous PLA based on IV (Intrinsic Viscosity) measurements. Due to better filler dispersion in the polymer matrix the thermal degradation rates of the nanocomposites are significantly lower than those of the unfilled polymers under air. The thermal degradation rate constants of calcined clay composites are higher than those of the non-calcined clay composites due to the porous structure of the calcined clays. As per dynamic TGA data and thermal kinetics analysis organomodified nanofillers have a complex effect on the polymer thermal stability, whereas all unmodified fillers reduce polymer thermal stability. The thermal stability of calcined clay composites is higher than that of their non-calcined clay counterparts. It is shown that the effect of the unmodified cationic fillers on the polymer thermal stability depends on the heating rate by contrast to the heating rate independent effect of the unmodified anionic clays. In general, mathematical modeling based on random thermal scission equations was satisfactory for fitting the TGA experimental data.
Unfilled polymers and their composites subjected to accelerated hydrolytic degradation over a temperature range of 50°C-70°C show significant morphological differences after four weeks. Degradation rate constants were higher for the amorphous PLA and its composites than for semicrystalline PLA and its composites as a result of increased permeation through the amorphous domains. Since the effective pH of the cationic nanofillers and their hydrophilicity change through treatment with organomodifiers, the degradation rate constants of the nanocomposites are significantly higher than those of the unfilled polymers; by contrast, those of cationic microcomposites and anionic nanocomposites are lower or slightly lower than those of the unfilled polymers, possibly due to the reduction of the carboxylic group catalytic effect through neutralization with the hydrophilic alkaline filler. As a result of the stronger neutralizing ability of the anionic vs. the cationic clays, the hydrolytic degradation rates of the anionic clay composites were lower than those of the cationic clay composites and the unfilled polymers. In the case of semicrystalline PLA, calcined HT and MMT clays can reduce hydrolytic degradation rates through their stronger neutralization ability. Although the degradation rate constants increased with increasing temperature, based on the calculated activation energies the degradation kinetics did not differ significantly above and below the assumed Tg of 58°C~6O°C. From SEM examination of the degraded samples it appears that bulk hydrolytic degradation starts at the polymer/filler interface.
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