Abstract
Long chain branched poly (lactic acid) (LCBPLA) was prepared via transesterification between high molecular weight poly (lactic acid) (PLA) and low molar mass monomer trimethylolpropane triacrylate (TMPTA) during melt blending in the presence of zinc oxide nanoparticles (nano-ZnO) as a transesterification accelerant in a torque rheometer. Compared with the traditional processing methods, this novel way is high-efficiency, environmentally friendly, and gel-free. The results revealed that chain restructuring reactions occurred and TMPTA was grafted onto the PLA backbone. The topological structures of LCBPLA were verified and investigated in detail. It was found that the concentration of the accelerants and the sampling occasion had very important roles in the occurrence of branching structures. When the nano-ZnO dosage was 0.4 phr and PLA was sampled at the time corresponding to the reaction peak in the torque curve, PLA exhibited a star-shaped topological structure with a high branching degree which could obviously affect the melt strength, extrusion foaming performances, and crystallization behaviors. Compared with pristine PLA, LCBPLA showed a higher melt strength, smaller cell diameter, and slower crystallization speed owing to the synergistic effects of nano-ZnO and the long chain branches introduced by the transesterification reaction in the system. However, severe degradation of the LCBPLAs would take place under a mixing time that was too long and lots of short linear chains generated due to the excessive transesterification reaction, with a sharp decline in melt strength.
Highlights
At present, the sustainable utilization of resources is arousing humans’ attention more and more
The environmentally friendly ZnO nanoparticles were applied as the efficient accelerant to enhance the transesterification reactivity of PLA with a multifunctional monomer to prepare a kind of star-shaped long chain branched poly during melt blending
The torque shows an obviously different trend when the PLA sample reacts with trimethylolpropane triacrylate (TMPTA) facilitated by ZnO nanoparticles
Summary
The sustainable utilization of resources is arousing humans’ attention more and more. Melt reactive processing for LCBPLA preparation mainly includes two methods: one is reactive processing via a melt chain extension reaction with an end group using multifunctional monomers as the branching center and the other is a radical reaction initiated by a peroxide initiator or high energy ray. The formation of PLA star branched macromolecules by transesterification with a small molecular multifunctional monomer has not been reported, the efficiency of transesterification is higher than that of an extension reaction It is one of the major reasons that the process conditions of the transesterification reaction between small molecules and macromolecular chains in a molten state is difficult to control and apply. The environmentally friendly ZnO nanoparticles were applied as the efficient accelerant to enhance the transesterification reactivity of PLA with a multifunctional monomer to prepare a kind of star-shaped long chain branched poly (lactic acid) during melt blending. The effect of the ZnO nanoparticles on the transesterification was discussed in our study
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