Effect of the Plasticizer-to-Polymer Ratio on the Properties of In Situ-Prepared PCL/PEG-Based Polyurethane Elastomers

To investigate the effect of the plasticizer-to-polymer ratio on the structure and properties of in situ-prepared PCL/PEG-based polyurethane elastomers (iPCE elastomers), a series of iPCE elastomers with plasticizer-to-polymer ratios ranging from 0.8 to 1.2 were prepared using Bu-NENA as an energetic plasticizer. Their hydrogen-bonding structure, crosslinked network, segmental mobility, microphase structure, mechanical properties, and thermal decomposition behavior were characterized by Fourier-transform infrared spectroscopy (FTIR), low-field nuclear magnetic resonance (LF-NMR), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), uniaxial tensile testing, and coupled thermogravimetry–differential scanning calorimetry–Fourier-transform infrared spectroscopy (TG-DSC-FTIR). The results showed that, as the plasticizer-to-polymer ratio increased from 0.8 to 1.2, the fraction of hydrogen-bonded carbonyl groups decreased from 50.11% to 41.62%, exhibiting a strong negative linear correlation with the plasticizer-to-polymer ratio. Increasing the plasticizer-to-polymer ratio prolonged the transverse relaxation times, decreased the apparent crosslink density, and increased the average molecular weight between crosslinks, indicating a progressive reduction in the effective constraints imposed by the network on segmental motion. No distinct crystalline diffraction peaks were observed for any of the elastomers, whereas microphase separation was evident at all plasticizer-to-polymer ratios. The characteristic domain spacing increased from 4.39 to 4.65 nm, indicating an expansion of the characteristic periodicity of the microphase-separated structure. Owing to the combined effects of weakened hydrogen-bond association, reduced apparent crosslink density, and diminished interchain interactions, the tensile strength, elongation at break, and toughness decreased from 2.29 MPa, 166.80%, and 2.16 MJ·m−3 to 1.09 MPa, 62.32%, and 0.39 MJ·m−3, respectively. The Bu-NENA-containing iPCE elastomers underwent three principal mass-loss stages, corresponding to the decomposition of Bu-NENA, degradation of the polyurethane network, and further decomposition of the residual structures at elevated temperatures. The mass-loss fraction in each stage was broadly consistent with the relative proportions of Bu-NENA and polyurethane components. The plasticizer-to-polymer ratio primarily altered the relative mass loss associated with each stage, while exerting only a limited influence on the characteristic temperatures and composition of the principal decomposition stages.