<p>In this work, <i>n</i>EB-PEI₁₈₀₀ (<i>n</i> = 0.22, 0.29, 0.35), a property tunable polymer for CO₂ capture, is reported by partially modifying poly (ethylene imine) (PEI₁₈₀₀) with 1,2-epoxybutane (EB). The density and viscosity of CO₂-free and loaded aqueous <i>n</i>EB-PEI₁₈₀₀ solution are measured at the temperatures of (293.15 to 333.15) K and 0.1 MPa. The aqueous solutions with various weight fractions of <i>w</i> = (5, 10, 15, 20 and 30%) are investigated. According to the experimental data, thermal expansion coefficient (<i>α</i>_p), viscosity activation energy (<i>E</i>_<i>η</i>) and CO₂ diffusion coefficient (<i>D</i>_CO2), are calculated. The influence of temperature, <i>n</i>EB-PEI concentration, CO₂ loading and modification degree are discussed from the viewpoints of molecular interaction and steric effect. The results reveal that low temperature, high <i>n</i>EB-PEI concentration and high CO₂ loading lead to high density and viscosity. Modification degrees have a great impact on the physical properties of <i>n</i>EB-PEI₁₈₀₀ + H₂O systems and can reduce the density and viscosity. The density and viscosity values exhibit the decreasing sequence of <i>ρ</i>_{PEI + H2O} &gt; <i>ρ</i>_{0.29EB-PEI + H2O} &gt; <i>ρ</i>_{0.35EB-PEI + H2O} ≈ <i>ρ</i>_{0.22EB-PEI + H2O} and <i>η</i>_{PEI + H2O} &gt; <i>η</i>_{0.22EB-PEI + H2O} &gt; <i>η</i>_{0.35EB-PEI + H2O} &gt; <i>η</i>_{0.29EB-PEI + H2O}, respectively. All the results are attributed to the combined effects of hydrogen bonding, dispersion force and steric structure.</p>