Impact of Cellulose Nanocrystal Incorporation Stage during Polyurethane Synthesis on Composite Performance Variations

High-strength electrochemical corrosion-resistant materials hold significant application value in marine engineering and construction sectors. In this study, cellulose nanocrystals (CNCs) extracted from microcrystalline cellulose (MCC) via sulfuric acid hydrolysis were incorporated into polyurethane (PU) through in situ polymerization to systematically investigate the effects of CNC addition timing (soft segment, hard segment, chain extender, and prepolymer stages) on material properties. Low-field nuclear magnetic resonance (LF-NMR) relaxation analysis revealed that introducing CNC during the prepolymer stage most dramatically enhanced molecular chain rigidity, as evidenced by the most pronounced relaxation decay. Fourier transform infrared spectroscopy (FTIR) confirmed that CNC incorporation significantly increased the hydrogen bonding index (HBI), with later addition stages yielding greater HBI enhancement. Dynamic mechanical analysis (DMA) further demonstrated that prepolymer-stage CNC addition substantially improved dynamic cross-linking density, thereby endowing the composites with superior mechanical performance. Electrochemical impedance spectroscopy (EIS) measurements showed a remarkable 2–4 orders of magnitude increase in impedance modulus for all PU/CNC composites, validating CNC’s corrosion protection efficacy.