Unexpected thermo-responsiveness of bisurea-functionalized hydrophilic polymers in water

Abstract

Hypothesis: Thermoresponsive polymers are important materials for various applications. However, the number of polymers that exhibit this property in the temperature range of interest remains limited. The development of novel rational design strategies through the understanding of the thermal transition's origin is therefore of utmost importance. Experiments: Bisurea-functionalized water-soluble polymers were synthesized by RAFT polymerization. After direct dissolution in water, the supramolecular assemblies were analyzed by cryoTEM and SANS. Their temperature-dependent water-solubility was characterized by various techniques, namely DLS, SANS, DSC, IR, to understand the origin of the temperature sensitivity. Findings: The supramolecular assemblies exhibit an unexpected temperature-dependent solubility. For instance, a cloud point of only 39 °C was measured for poly(N,Ndimethylacrylamide) assemblies. This property is not restricted to one specific polymer but is rather a general feature of bisurea-functionalized polymers that form supramacromolecular bottlebrushes in water. The results highlight the existence of two distinct transitions; the first one is a visually perceptible cloud point due to the aggregation of individual micelles, presumably driven by the hydrophobic effect. The second transition is related to the dissociation of intermolecular bisurea hydrogen bonds. Finally, we show that it is possible to widely tune the cloud point temperature through the formation of co-assemblies.

Effects of structural variation on the self‐assembly of bis‐urea based bolaamphiphiles

“…In contrast, solutions of more hydrophilic P8‐6 OMe and P12‐10 OMe remained transparent up to 80°C, the highest temperature measured (Figure 3(B)). We can conclude that the LCST behavior of OEG amphiphiles is strongly related to hydrophilic/hydrophobic balance, and more hydrophobic amphiphiles have lower LCST's, in line with observations of thermo‐induced aggregation in bis‐urea functionalized supramolecular bottlebrushes system 40 …”

“…We can conclude that the LCST behavior of OEG amphiphiles is strongly related to hydrophilic/hydrophobic balance, and more hydrophobic amphiphiles have lower LCST's, in line with observations of thermo-induced aggregation in bis-urea functionalized supramolecular bottlebrushes system. 40 Dynamic light scattering (DLS) was subsequently used to study aggregate behaviors in more detail. Bolaamphiphiles with apolar segments larger than 10 carbon atoms may form non-equilibrium aggregates, and the kinetics of exchange are further slowed down by hydrogen bonding of urea motifs.…”

Synthesis and Characterization of Temperature-Responsive N-Cyanomethylacrylamide-Containing Diblock Copolymer Assemblies in Water

“…Finally, at both sample concentrations (0.1 and 1 wt%), the initial size of the small aggregates was recovered when the samples were cooled back to 20 °C; the transition is thus reversible (Figure S10) as already indicated by turbidimetry. In view of theses combined analyses, the apparent LCST-type transition, might therefore be explained by the destabilization of the colloidal system [27]. At both concentrations, upon heating from 20 to 70 °C, the initial aggregates form larger aggregates through a secondary aggregation mechanisms, which are nanometric and remain colloidally stable at 0.1 wt%, but become micrometric and colloidally unstable at 1 wt%.…”

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

“…This specific interaction led to a significant salting-in effect (increasing LCST), competing with the typical salting-out effect (decreasing LCST), which could nearly restore the original LCST (27.9 1C compared to 30.2 1C of the salt-free LCST). 117 A couple of examples also revealed that by modifying a polymer with moieties that are prone to cause intermolecular non-covalent interactions, such as boronic acid 118 and urea, 119,120 could be applied for introducing and optimizing the range of thermoresponsiveness. It was illustrated that introduction of a bis-urea terminal group to poly(N,N-dimethylacrylamide) (PDMAc) enabled the temperature-sensitive characteristic to the polymer.…”

“…It was explained that the bis-urea terminal facilitated the hydrogen bond formation intermolecularly, causing stronger inter-chain interactions and temperature sensitivity. 120 Universally, polymers with pendant groups have been studied for thermoresponsive applications. These polymers benefit from modulation of responsive temperature range by simply changing monomer structures, polymer composition, and degree of polymerization.…”