Real-Time Raman Spectroscopy of Photopolymerization Dynamics in Ethylene Glycol Methyl Ether Acrylate-g-Epoxidized Natural Rubber

Abstract

Natural rubber (NR) is a promising material for polymer electrolytes (PEs) due to its natural abundance and desirable properties, including soft and elastic texture at room temperature, adhesive qualities, and low glass transition temperature (T_g). Additionally, it is more sustainable compared to synthetic polymers. However, its low ion transport capability poses a challenge for electrochemical applications such as lithium-ion batteries. To address this limitation, ethylene glycol methyl ether acrylate (EGMEA) was grafted onto an epoxidized natural rubber (ENR-25) matrix at varying grafting ratios, optimizing the curing time. Photopolymerization was employed due to its rapid reaction kinetics, customizable monomer selection for tailored physical properties, and energy-efficient initiation via illumination. In situ Raman spectroscopy provided insights into the kinetics, mechanisms, and curing times of the reactions. The synthesis of EGMEA-g-ENR was confirmed through infrared (IR) and ¹H NMR analyses, which validated the successful grafting of EGMEA onto ENR-25. A notable decrease in the molecular weight and polydispersity index (PDI) was observed, approaching unity, indicating improved polymer homogeneity. This decrease in molecular weight is likely due to the degradation of ENR-25 during the ultraviolet (UV)-curing process, resulting in a lower T_g (<−43 °C) compared to that of ENR-25 (−35 °C). EGMEA itself has no T_g but exhibits a T_m of −42 °C and a T_c of −65 °C. This is favorable for enhancing the ion transport in polymer electrolytes. Additionally, a significant correlation was found between the grafting efficiency and different EGMEA grafting ratios. The grafting efficiency reached up to 61.95%, with a strong correlation between the grafting efficiency and grafting ratio (R² = 0.89). This research highlights the potential of rubber-based modification through photopolymerization for developing framework-based solid polymer electrolytes, particularly for applications in lithium-ion batteries.