Characterization of the chemical properties of crumb rubber modified asphalt at the nanoscale using atomic force microscopy-based infrared spectroscopy

Nanoscopic chemical heterogeneity of crumb rubber modified asphalt (CRMA) significantly influences its performance. However, traditional characterization techniques such as AFM and ATR-FTIR are limited by spatial resolution or sample damage, making it challenging to analyze critical interfacial chemical mechanisms. This study systematically develops an AFM-IR-based method for nanoscopic chemical characterization of CRMA, enabling precise analysis of rubber phase, asphalt phase, and rubber-asphalt interface. By optimizing sample preparation and designing custom molds (high-temperature-resistant transparent glass substrate with silicone spacer rings), CRMA samples with 1 mm thick and 10 mm in diameter were obtained. Comparative studies of AFM-IR testing modes showed that the contact mode presented prominent peaks at 1460 cm⁻¹ (–CH₂– bending), 1378 cm⁻¹ (–CH₃ symmetric deformation), and 1030 cm⁻¹ (S=O stretching), with significantly enhanced signal intensity and high concordance with FTIR results. Infrared power studies revealed a 0.92%–1.74% IR power window balancing high signal-to-noise ratio and minimal sample damage (surface indentation < 32.5 nm), while >2.51% caused asphalt binder softening and spectral distortion. An innovative rubber particle localization method based on AFM-IR absorption mapping successfully identified rubber particles in CRMA by matching characteristic peaks with waste rubber powder FTIR spectra. The established AFM-IR measurement method provides critical technical support for elucidating CRMA interfacial crosslinking mechanisms, aging evolution dynamics, and targeted rejuvenator design.