The Morphology, Crystallization Behavior And Properties Of Biodegradable Poly(l-lactide)/octavinyl-polyhedral Oligomeric Silsesquioxanes Nanocomposites

Biodegradable poly(l-lactide)(PLLA)/octavinyl-polyhedral oligomericsilsesquioxanes (ovi-POSS) nanocomposites were prepared via solution andcasting method at low loadings of ovi-POSS in this work. Effect of ovi-POSSon the crystallization kinetics, spherulitic morphology, crystal structure,polymorphic crystallization behavior, multiple melting behavior and dynamicmechanical properties and thermal stability of PLLA in the nanocompositeswas investigated in detail. The main results are as follows:First, the nonisothermal and isothermal melt crystallization behaviors ofneat PLLA and PLLA/ovi-POSS nanocomposites were investigated in detail.It is found that the nonisothermal melt crystallization of PLLA in thenanocomposites is enhanced by the presence of ovi-POSS, and theenhancement becomes more significant with the increase of ovi-POSS content. For isothermal melt crystallization process, the crystallization mechanism andcrystal structure of PLLA remain unchanged despite the presence of ovi-POSS.However, the overall isothermal melt crystallization rates increase withincreasing ovi-POSS content.Secondly, isothermal and nonisothermal cold crystallization behaviors ofPLLA and PLLA/ovi-POSS nanocomposites were also investigated in thiswork. For nonisothermal cold crystallization, the crystallization process ofPLLA is accelerated by increasing both heating rate and the ovi-POSS loading.The activation energies of nonisothermal cold crystallization, calculated by theKissinger method, increase with increasing the ovi-POSS loading in thenanocomposites relative to neat PLLA. For both neat PLLA and itsnanocomposites, the overall isothermal cold crystallization rates increase withincreasing crystallization temperature (Tc); moreover, the overall isothermalcold crystallization rates of PLLA increase with increasing the ovi-POSSloading in the nanocomposites relative to neat PLLA, indicative of thenucleating agent effect of ovi-POSS. However, the presence of ovi-POSS doesnot modify the crystallization mechanism and crystal structure of PLLA.Finally, the polymorphic crystalline structure and multiple meltingbehavior of PLLA and its nanocomposite were studied by isothermal meltcrystallization at the Tcregion (80-130oC). For both neat PLLA and itsnanocomposite, the change of the crystallization half-time is discontinuous atthe Tcregion (100-110oC) due to the transition of α’-to-α-form of PLLA. The WAXD results indicate that α-form crystals generate when Tcis higher than110oC, while α’-form crystals forms when Tcis below110oC, causingsignificant change in lattice parameters. It was also found that α’-to-α-formtransition occurs prior to the dominant melting in both PLLA andPLLA/ovi-POSS nanocomposite crystallized at low Tc. With increasing Tc, themelting behavior of PLLA changes into usual melt-recrystallizationmechanism. All results suggest that the crystalline structures and kinetics arestrongly affected by Tc, but not by the incorporation of ovi-POSS. In addition,the storage modulus has been apparently improved in the PLLA/ovi-POSSnanocomposite with respect to neat PLLA. The thermal stability of PLLA inthe PLLA/ovi-POSS nanocomposites is reduced slightly relative to neatPLLA.