Recyclable Ethylene-Vinyl Acetate Covalent Adaptable Networks and Their Structure-Property Relationships

Ethylene-vinyl acetate (EVA) copolymers are widely used in a variety of applications from packaging to footwear owing to their balanced and tunable thermomechanical properties. However, thermoset EVAs face tremendous sustainability challenges. While sustainable alternatives in the form of recyclable EVA covalent adaptable networks (CANs) or vitrimers have been reported, no reports exist which study the relationships between the characteristics of EVAs and their CAN counterparts. This study investigates such structure-property relationships between EVAs and their corresponding CANs containing dialkylamine-disulfide-based dynamic covalent cross-links. We characterized these EVA CANs via differential scanning calorimetry, dynamic mechanical analysis (DMA), and swelling tests. Our results demonstrated that the crystallinity of EVA CANs decreased compared to their thermoplastic precursors. Swelling experiments revealed gel contents greater than 50%, with most EVA CANs showing values between 66% and 74%. DMA analysis indicated the robustness of the dynamically cross-linked EVAs; the effective cross-link density of EVA CANs increased with increasing vinyl acetate (VA) content yet decreased with increasing melt flow index (MFI) of the precursor EVAs, both results of which align with existing literature reports on thermoset EVAs. The dynamic covalent cross-linker BiTEMPS methacrylate facilitated the recyclability of each EVA CAN independent of the precursor EVA, and each recycled EVA CAN fully reproduced the thermomechanical properties of its predecessor. Our findings highlight the impact of VA content and MFI on the thermomechanical properties of EVA CANs and provide a foundation for optimizing the design of recyclable EVA materials with desirable thermomechanical properties.