Effect of variation in chain segments and cross-linker stoichiometry on structure–property relationship of polyurethanes and their micro-silica composites

This study explores the structure–property relationships of long-chain and short-chain segmented polyurethanes (PUs) and their micro-silica composites by systematically varying the cross-linker stoichiometric ratios. The synthesized materials were characterized using FTIR, XRD, SEM, TGA, DSC, DMA, dielectric analysis, and mechanical testing. In XRD analysis, all samples exhibited a broad diffraction halo centered at 2θ = 20° and a weak diffuse feature in the 40–50° region, indicating predominantly amorphous structures with short-range segmental ordering rather than distinct crystalline and amorphous peaks. The degree of structural ordering further decreased with increasing cross-linker stoichiometric ratio and upon incorporation of in situ SiO₂ microparticles, reflecting enhanced cross-link density and disruption of interchain packing. SEM analysis reveals uniform distribution of agglomerate of micro-silica particles in PU matrix. Thermal analysis results shows increase in thermal stability with increase in cross-linker stoichiometric ratio in PU matrix and with incorporation of micro-silica particles in PU matrix. DSC analysis reveals increase in T_g with increase in cross-linker stoichiometric ratio as well as with incorporation of micro-silica particles likely due to strong H-bonding interaction as evident from FTIR results. From DMA studies, we have two T_g for neat PU due to long-chain and short-chain segments, and we have shifted single glass transition temperature (T_g) in silica-incorporated composites, indicating improved network uniformity due to hydrogen bonding. Mechanical property study showed PU2 (3.3) and its silica composition, and PU2-SiO₂ showed the best combination of properties having tensile strength (28.03 MPa), elongation at break (331.3%), and hardness (85 Shore A). Dielectric properties indicated a reduction in permittivity and tan δ with increase in cross-linker stoichiometric ratio as well as in silica-incorporated micro-composites, suggesting enhanced rigidity and reduced dipolar mobility. These results provide a pathway for designing polyurethane composites with tunable properties for specific applications, including coatings, elastomers, and dielectric materials.