Feifei Zheng

Four sets of dual-responsive statistical copolymers are synthesized from N,N-dimethylacrylamide and acrylamide comonomers bearing photoresponsive 4-aminoazobenzene (AzBn) side groups linked by various amino acid spacers. In aqueous solution, the copolymers exhibit lower critical solution temperature (LCST) behavior modulated by the E-to-Z photoisomerization of AzBn. Their cloud point temperatures (TCP) decrease markedly with increasing AzBn content and, at contents of 8 mol % and above, are counterintuitively lower in the—a priori—more polar-Z-state, with photoinduced shifts reaching –16 °C. Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) on three exemplary copolymers support the formation of compact, spherical single-chain nanoparticles (SCNPs), ascribed to hydrophobic association of AzBn. In most cases, photoisomerization induces only modest changes in global SCNP dimensions, but substantially alters the LCST behavior. These results indicate that the AzBn content primarily determines the overall photoresponsive trend, whereas spacer-dependent differences in local chemical and steric environment further modulate the thermoresponsive behavior.

Aqueous solutions from block copolymers with blocks featuring lower critical solution temperature behavior have attracted great interest for 3D bioprinting, as they are in the liquid state at room temperature but become a soft and tissue-like solid at body temperature by forming a high-water-content hydrogel. Here, we use small-angle neutron scattering (SANS) to resolve the temperature-dependent self-assembled structures of a triblock terpolymer featuring a hydrophobic and two thermoresponsive blocks with strongly different cloud point temperatures Tcp in D2O. Solutions having polymer concentrations of 1 and 15 wt % were studied around the transitions that occur at the cloud point Tcp, the gelation temperature Tgel (which is observed for the concentrated solution), and the phase separation temperature Tps. In dilute solution, the crew-cut spherical micelles formed at low temperatures convert to cylindrical micelles upon approaching Tcp. We attribute this shape change to the formation of hydrophobic channels when one of the thermoresponsive blocks collapses. Upon heating towards Tps, more isotropic aggregates from micelles are observed, and above Tps, the micelles are densely packed in large aggregates, which we attribute to the dehydration of the other thermoresponsive block. In concentrated solution, the same mechanism results in a sphere-to-cylinder transition as the system crosses from the cloudy liquid into the gel state, followed by the formation of an extended network with the cylindrical micelles acting as hydrophobic channels and physical cross-links. Further heating across Tps induces the collapse of the network and the reorganization into large, compact aggregates, consistent with the previously observed macroscopic phase separation. Thus, these results establish the relation between the dehydration of the thermoresponsive blocks, the size, shape, inner structure, and correlations of the self-assembled micelles and the mechanisms leading to hydrogel formation and phase separation, which helps to design tailor-made injectable thermoresponsive hydrogels for 3D bioprinting and biomedical applications.

In the present study, the effect of the distribution of repeated units in copolymers based on oligo(ethylene glycol) methyl ether methacrylate (A), n-butyl methacrylate (B), and di(ethylene glycol) methyl ether methacrylate (C) is reported and discussed for the first time. Several linear structures have been synthesized via group transfer polymerization, varying from statistical to gradient to block structures. Notably, among the other linear architectures, we report the one-pot synthesis of a forced gradient terpolymer and the investigation of its thermoresponsive and gelation properties. It is proven that the distribution of repeated units along the polymer chain governs the solubility and gelation of the copolymers, with the ABC and gradient structures being the best-performing. The self-assembled structures and aggregation mechanisms differ strongly for the ABC triblock and the gradient terpolymer, as revealed by synchrotron small-angle X-ray scattering. The promising results of the gradient structure open a new era for designing novel copolymers with potential applications in the biomedical sector.

The pressure-induced changes of the micellar structures of a thermoresponsive PMMA-b-PNIPAM diblock copolymer in a 90 : 10 v/v water/methanol mixture are characterized in the temperature–pressure frame using synchrotron small-angle X-ray scattering.