Multiblock Copolymers for Compatibilizing and Recycling Polyesters and Polyolefins

Abstract

Plastics are ubiquitous in our daily life, but the currently low recycling rate, only 9% in the United States, led to most plastic waste in landfills, energy conversion, and leaking into the environment. Due to the immiscibility between polymers, the melting process of mixed plastics usually yields blends with poor mechanical properties. Therefore, presorting is often required before mechanical recycling, which translated into the added cost. In addition, multi-component products, such as multilayer films used in food packaging are not recycled at all. To tackle this recycling challenge, effective compatibilizers are needed to decrease domain sizes, increase interfacial adhesion, and thus improve the mechanical properties of recycled blends. In this thesis, we will focus on using multiblock copolymers (MBCPs) as compatibilizers to assist recycling of mixed plastics, specifically polyethylene (PE) and poly(ethylene terephthalate) (PET).First, I successfully synthesized and implemented PET-PE MBCPs for use as both adhesive tie layers in-between PET/PE multilayer films and compatibilizers for PET/PE blends. As adhesive tie layers, the PET-PE MBCPs led to films exhibiting adhesive strength comparable to that of commercially available adhesives. As compatibilizers, PET/PE (80/20 wt%) blends containing as low as 0.5 wt% PET-PE MBCP were melt mixed to mimic recycling mixed plastic waste, and they were found to exhibit mechanical properties better than neat PET. To understand the mechanisms responsible for the outstanding performance of these MBCPs, I systematically investigated the role of molecular architecture on compatibilization and transport. It is found that MBCPs are more efficient as compatibilizers in PET/PE blends than triblock copolymers (TBCPs) with comparable total molecular weights. It is believed that MBCP could form trapped entanglements or co-crystallization with homopolymers and therefore strengthen the interfaces to achieve tough blends. In addition, MBCPs showed significantly faster transport kinetics than TBCPs to the interfaces during static annealing. In addition, I explored the possibility of incorporating other PET miscible polyesters into an MBCP structure with PE. Degradable aromatic polyesters derived from salicylic esters, poly(salicylic glycolide) (PSG) and poly(salicylic methyl glycolide) (PSMG), are chemical structurally similar to PET and therefore of great interest. The miscibility of binary blends of PET with PSG and PSMG was systematically investigated by thermal and optical analyses, and we found both PET/PSG and PET/PMG are miscible over the entire composition range. We conclude that the miscibility originates from specific weak interactions between the polymer pairs. This new experimental finding may provide opportunities for the development of other BCPs containing aromatic polyesters and PE as compatibilizers.