Complex coacervation, the liquid–liquid phase separation phenomenon, is commonly driven by electrostatic interactions. Typically, the interfacial tension of coacervate increases with a higher degree of polymerization (DP). In this study, we investigated coacervates synthesized from poly(allyl glycidyl ether) backbones functionalized with amine and sulfonate groups across varying DPs. Utilizing a surface forces apparatus (SFA), we quantified interfacial tension as a function of salt concentration. Notably, the DP20 coacervate exhibited an outstanding interfacial tension among them. This result is potentially driven by the high relative density of terminal phenyl groups in short-chain architectures, which may introduce significant non-electrostatic contributions. These findings suggest that coacervate interfacial properties can be strategically tuned by balancing DP and end-group chemistry, providing a robust framework for engineering high-performance adhesive interfaces.