Novel hyperbranched macromolecules as modifiers in biodegradable nanocomposites

Abstract

Dendritic macromolecules represent a novel class of structurally controlled macromolecules with highly branched structures and densely populated surface groups. Hyperbranched polymers are a subclass member, which can be made by a one-pot, single-step polymerization approach in large quantities and thus present economically great promising products for large-scale industrial applications. In this research, two types of novel biodegradable hyperbranched polymers containing a domination of either carboxylic acid groups (HBP-COOH) or hydroxyl groups (HBP-OH) at the peripheral surface were synthesized and characterized. Combining these organic nanostuffs to improve performances of bulk polymers has been undertaken. Via a solvent casting method, the nano-agents have successfully been integrated into aliphatic polyesters such as poly(L-lactic acid) (PLA) and poly(ε-caprolactone) (PCL) to achieve desired biodegradable composite materials. The incorporated modifiers have been comprehensively investigated with respect to their effects on physical, mechanical, thermal, and morphological properties of the end products. Herein, it can be seen that both of the hyperbranched products induce a positive effect on intrinsic ductility and toughness of the bulk polymers. An optimal content of the fillers added (~3 or 6% by weight) can give rise to improved mechanical properties of the materials significantly. In contrast to HBP-OH, HBP-COOH generally exhibits better effectiveness, which accounts for the end group effect in the composite systems. As revealed by the microscopic morphological study, the filler molecules can agglomerate to form well-defined spherical nanoclusters in the polymer matrixes. A debonding-initiated shear yielding mechanism is probably involved in the toughening of the composite materials. Besides, it is confirmed that all the specimens under study are fully biodegradable. The incorporation of the organic fillers does affect the bio-stability nature of PLA and PCL, but the degree is subjected to factors such as concentration, type and molecular weight of the fillers, and degradation conditions. In general, higher molecular weight filler or higher filler content would likely induce an increase in biodegradable rate for the end materials. The degradation is faster in an alkaline medium than in a neutral environment.