Transformations of Thermosensitive Hyperbranched Poly(ionic liquid)s Monolayers

Abstract

We synthesized amphiphilic hyperbranched poly(ionic liquid)s (HBPILs) with asymmetrical peripheral composition consisting of hydrophobic n-octadecylurethane arms and hydrophilic, ionically linked poly(N-isopropylacrylamide) (PNIPAM) macrocations and studied low critical solution temperature (LCST)-induced reorganizations at the air−water interface. We observed that the morphology of HBPIL Langmuir monolayers is controlled by the surface pressure with uniform well-defined disk-like domains formed in a liquid phase. These domains are merged and transformed to uniform monolayers with elevated ridge-like network structures representing coalesced interdomain boundaries in a solid phase because the branched architecture and asymmetrical chemical composition stabilize the disk-like morphology under high compression. Above LCST, elevated individual islands are formed because of the aggregation of the collapsed hydrophobized PNIPAM terminal macrocations in a solid phase. The presence of thermoresponsive PNIPAM macrocations initiates monolayer reorganization at LCST with transformation of surface mechanical contrast distribution. The heterogeneity of elastic response and adhesion distributions for HBPIL monolayers in the wet state changed from highly contrasted two-phase distribution below LCST to near-uniform mechanical response above LCST because of the hydrophilic to hydrophobic transformation of the PNIPAM phase.

Weakly Ionically Bound Thermosensitive Hyperbranched Polymers

“…The values of V Oct /V PNIPAM decreased from 0.44 to 0.15 with increasing PNIPAM content. According to the results of our previous studies based on 13 C NMR, the starting polyester polyol HBP-OH has a degree of branching (DB) of 38.7%, 34 in good agreement with the literature data, which gives DB values of 36−43% for HBP-OH (Boltorn H40), which has a similar chemical structure to the starting HBP-OH in this study but with more terminal hydroxyl groups (64). 56 The determined DB values are valid for both intermediate hyperbranched polysulfonic acids and final HBPs (Table 1).…”

“…The high stability of the circular domains is attributed to the branched architecture and asymmetric chemical composition of S8P8 HBPs. It has been demonstrated that branched copolymers of asymmetric chemical composition with higher content of hydrophobic arms form stable circular domains at the air–water interface, which are preserved to high pressures. , Crowding of multiple hydrophobic arms tethered to a single joint point would favor the formation of highly curved, but stable interfaces, which allow the polymer morphology buried in a circular shape territory, making the transition from spherical to cylindrical morphologies unlikely to happen . For S16P16 HBPs with symmetric peripheral chemical composition, segregation of hydrophobic arms would lead to the formation of a less curved interface, making circular domains more vulnerable to pressure, which causes a morphological transition to a continuous monolayer during compression in the S16P16 monolayers (Figure b). , Above the LCST, the hydrophilic-to-hydrophobic transition of terminal PNIPAM chains promotes rearrangement of the whole HBP molecules.…”

“…Their morphologies were determined by the location and pH-dependent conformations of the P2VP segment (inner) and PAA segment (outer) . Introducing large counter ions of a different nature can alter the functionality of outer shells, affect hydrophilic/hydrophobic balance, and induce stimuli-sensitive behavior. − For example, ionic dendrimers functionalized with hydrophobic fatty acids as counter ions self-assembled into micelles, lamellae, or nanospheres with a lamellar structure depending on the number of fatty acids linked to the dendrimer core . In our recent study, we found that hyperbranched polymers (HBPs) with thermoresponsive poly­( N -isopropylacrylamide) (PNIPAM) macrocations showed temperature-induced phase transformation controlled by the lower critical solution temperature (LCST) of PNIPAM macrocations .…”

Ionic Liquid‐Based Stimuli‐Responsive Functional Materials

“…hydrophilic PNIPAM macrocations, were synthesized and their assembly behavior at the air-water interface as well as thermoresponsive behavior were investigated in the same group. [100] In addition, thermoresponsive PILs, poly(tetrabutylphosphonium styrenesulfonate)-based microgels copolymerizing with poly[di(ethylene glycol)ethyl ether acrylate] were synthesized and their microdynamic volume phase transition mechanism was studied. [101] The content of hydrophilic PIL moieties plays a significant role in the thermally induced phase transition behavior of microgel systems; that is, the microgels with fewer PIL moieties present a sharp transition behavior and a gel-like state (10%, w/v) in water.…”

“…Besides, another novel amphiphilic hyperbranched PILs, which consist of asymmetrical peripheral composition of hydrophobic octadecyl tails and hydrophilic PNIPAM macrocations, were synthesized and their assembly behavior at the air–water interface as well as thermo‐responsive behavior were investigated in the same group. [ 100 ]…”