Z-E isomerization of azobenzene based amphiphilic poly(urethane-urea)s: Influence on the dynamic mechanical properties and the effect of the self-assembly in solution on the isomerization kinetics

From nano to the macro: tuning hierarchical aggregation of thermoresponsive PEG/PCL-based polyurethanes via molar mass/composition control

Amphiphilic hyperbranched polyurethanes (HPUs) based on PEG and PCL are promising for several biomedical applications. However, the lack of control over the molar mass and composition hinders a deep understanding of the aqueous self-assembly of HPUs. In this paper, the control over the HPU molar mass and composition was provided by dynamic urea bond-mediated polymerization (DUBMP), enabling a careful evaluation of their aqueous self-assembly by 1H NMR, DLS, and Cryo-TEM. HPUs containing a single PCL block per chain self-assemble into nanoaggregates (Rh ≈ 10 nm) in water up to its cloud-point temperature (Tcp) of 34 °C. On the other hand, HPUs with more than one PCL block per chain self-assemble into nanoaggregates and their clusters below Tcp. In this case, the solution behavior can be tuned by the HPU molar mass. Increasing $$\overline{{\mathrm{M} }_{\mathrm{w}}}$$ M w ¯ from 4 to 19 kDa, HPUs of similar composition can form colloidally stable cluster suspensions ($$\overline{{\mathrm{M} }_{\mathrm{w}}}$$ M w ¯ = 4 kDa) and phase separate into a denser liquid aggregate–cluster phase ($$\overline{{\mathrm{M} }_{\mathrm{w}}}$$ M w ¯ = 7 kDa) or into a highly viscous aggregate-network phase ($$\overline{{\mathrm{M} }_{\mathrm{w}}}$$ M w ¯ = 19 kDa). This type of control over the hierarchical aggregation of HPUs was reported for the first time and is interesting for biomedical applications.

Graphical abstract

Self-aggregation, H-bonding, and photoresponse in film and solution states of azobenzene containing polyurea

We critically understand the hydrogen bonding interactions and electronic transitions occurring in a thin film as well as in solution of a photo-responsive polymer, azo-polyurea (azo-PU). We synthesize azo-PU by covalent attachment of the azobenzene chromophore to the main chain of polyurea. Azo-PU shows reversible photoisomerization between trans and cis states upon light exposure, the occurrence of which is typically analysed using the π-π* and n-π* electronic transition peaks in the UV-visible absorption spectrum. We find that the π-π* and n-π* bands undergo a redshift and blueshift respectively on dissolving azo-PU in DMF solvent, resulting in a single overlapped peak in the spectrum. However, upon UV irradiation, these bands split into two independent transitions that are characteristic of azo-PU solid films. These observations are explained based on the changes in polymer-polymer and polymer-solvent interactions through hydrogen bonding and self-aggregation tendency. The experimental findings are corroborated using DFT simulations which provide useful insights into electronic orbital transitions, electron distribution, and hydrogen bonding interaction through IR vibrational modes.

A one-pot, solvent-free, and controlled synthetic route for thermoresponsive hyperbranched polyurethanes

Hyperbranched polyurethanes (HPUs) are known for their multifunctionality and versatile properties.