The effect of self-assembled nanofibrils on the morphology and microstructure of poly(L-lactic acid)

The morphologies and microstructures of neat 1,3:2,4-dibenzylidene-D-sorbitol (DBS) and DBS/ poly(L-lactic acid) (PLLA) samples have been investigated by polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The morphology of neat DBS samples prepared from solution had unspecific structures, and no fibrils formed. In comparison, DBS molecules self-assembled into fibrils with diameters ranging from 100 nm to 1 mm when samples were prepared from the melt. The DBS fibrils were also found in DBS/PLLA systems, but the average diameter was only around 20 nm. The DBS architectures could be well tuned by varying the DBS contents and PLLA crystallization temperatures. Micron-sized fibrillar rings or disks due to the aggregation of DBS nanofibrils were found using SEM in samples with DBS contents more than 3 wt% and crystallized above 120 C. Meanwhile, ‘‘concentric-circled’’ PLLA spherulites were observed by POM. The DBS nanofibrils largely formed at the circles, but some nanofibrils formed beyond the circles and were dispersed in the PLLA spherulites. These dispersed nanofibrils affected the orientation of PLLA lamellae and caused a change in birefringence, yet the growth rate of PLLA was not significantly influenced by the formation of DBS nanofibrils. In addition, porous PLLA structures could be obtained by solvent extraction of the DBS nanofibrils.