Effect of grafted Mesoporous silica [SBA-15-g-OLLA] additives on the hydrolytic degradation of poly (L-lactic acid) [PLLA]

Biodegradable PLLA/SBA-15 mixtures and PLLA/SBA-15-g-OLLA grafted composites were prepared by solution blending. The purpose was to determine the effect of the SBA-15 additives on the PLLA hydrolytic degradation and systematically characterize the degraded products. Before hydrolysis, all the composites were crystalline with defined crystal periodicity. In particular, there was a slight increase in the SBA-15 crystalline reflections with increasing SBA-15-g-OLLA concentration in PLLA/SBA-15-g-OLLA composites. Molecular contact (through C–O–C bonds) and miscibility enhancements were determined in proportion with the OLLA segment concentration in SBA-15-g-OLLA samples. The highest concentration of SBA-15-g-OLLA rendered the most hydrolysis and pH decrement. Specifically, hydrolytic degradation in buffer and body tempered conditions increased up to 13 wt % in the grafted PLLA/SBA-15-g-OLLA (10 wt%). Non-isothermal crystallization from the melt of the degraded products indicated heterogeneous nucleation using both SBA-15 and SBA-15-g-OLLA additives, and crystallization enhancement was observed using the degraded PLLA/SBA-15-g-OLLA (10 wt%). Depending on the SBA-15-g-OLLA concentration of the degraded samples, there was a gradual decrease in the intensity of the PLLA melt-recrystallization, which continued until the formation of a new melting endotherm. Two crystal habits and a morphological explanation were given to explain these results. Although unexpected, due to the lower molecular weight of the hydrolyzed products, isothermal crystallization experiments showed the enhancement of the first melting endotherm as a function of SBA-15-g-OLLA with 70 days degraded samples. This effect was demonstrated as related to the PLLA molecular weight. Isothermal bulk crystallization experiments of the hydrolyzed samples showed acceleration of the crystallization rate and changes in the crystal's geometry as a function of SBA-15-g-OLLA. The corresponding isothermal crystallization optical micrographs indicated highly nucleated patterns of such samples.