Structural and rheological characterizations of nanoparticles of environment-sensitive hydrophobic alginate in aqueous solution

Amphiphilic polymers that form self-assembled structures in aqueous media have been investigated and used for the diagnosis and therapy of various diseases, including cancer. In our work, a series of environment-sensitive hydrophobic alginates (Ugi-Alg) with various weight-average molecular mass values (Mw   ~   6.7   ×   10 5 –6.7   ×   10 4   g/mol) were synthesized via Ugi reaction. The structure of Ugi-Alg was characterized by 1 HNMR spectrometer. The electrostatic self-assembly of different molecular weight (Mw) and composition (M/G ratio) of Ugi-Alg chain under various Na + concentrations, was investigated by dynamic light scattering, electron spin resonance experiments, and transmission electron microscopy. Result showed that by comparing to other Ugi-Alg, the mid-Mw Ugi-Alg (Mw   ~   2.8   ×   10 5   g/mol) could form stable and homogeneous nanoparticles in low Na + concentration solution. However, G/M values exerted no obvious effect on nanoparticles structure. Additionally, steady-shear flow, thixotropy and dynamical viscoelasticity tests were performed to characterize the rheological behavior of Ugi-Alg aqueous solutions as influenced by Mw and M/G ratio. All of the samples exhibited a non-Newtonian shear-thinning behavior above a critical shear rate ( γ ̇ c 2 ${\dot{\gamma }}_{\mathrm{c}2}$ ). The greater the Mw, the more sensitive the temperature-dependent behavior will be. The upward-downward rheograms showed that all of the systems evaluated in this study displayed a hysteresis loop, indicating a strong thixotropic behavior, and the thixotropic of mid-Mw Ugi-Alg was the strongest. The dynamical viscoelastic properties were characterized by oscillatory frequency sweep, revealing the gel-like viscoelastic behavior of mid-low Ugi-Alg and the fluid-like viscoelastic behavior of high-Mw Ugi-Alg.