Visualizing Polymer Cross-linking Density: A Universal Fluorescence Strategy Based on Aggregation-Induced Emission

Cross-linking density is a critical parameter governing the performance of cross-linked polymers. Rapid and accurate determination of this parameter is essential for evaluating material states, assessing engineering applicability, and optimizing processing conditions. However, conventional methods suffer from drawbacks such as operational complexity, sample destructiveness, and high instrumentation costs. Herein, we propose a novel fluorescence-based visualization strategy that employs aggregation-induced emission (AIE) molecules as fluorescent probes for the rapid and quantitative visual detection of polymer cross-linking density. As the cross-linking density increases, the polymer forms a more constrained three-dimensional network, which restricts the intramolecular motion of the AIE probes and consequently enhances their fluorescence emission. Using cross-linked polyethylene (XLPE) as a model material, fluorescence images were captured above the melting temperature using a home-built hot-stage imaging apparatus, and gray values were extracted for quantitative analysis. The fluorescence intensity ratio exhibited a strong linear correlation with cross-linking density (R2 = 0.996), and the method achieved high accuracy with relative errors below 1.64% when benchmarked against nuclear magnetic resonance (NMR) results. Notably, the addition of 0.1 wt % AIE probes had negligible effects on the thermal, mechanical, electrical breakdown strength, and crystallization properties of XLPE. The general applicability of this strategy was further validated in vulcanized cis-1,3-butadiene rubber (V-BR), where excellent linearity (R2 = 0.990) and relative errors below 1.1% were obtained. This work establishes a simple, rapid, and high-precision visualization approach for characterizing polymer cross-linking density, offering a promising tool for real-time monitoring in industrial polymer processing and quality control.