Weakly Ionically Bound Thermosensitive Hyperbranched Polymers

Abstract

We synthesized novel amphiphilic hyperbranched polymers (HBPs) with variable contents of weakly ionically tethered thermoresponsive poly­(N-isopropylacrylamide) (PNIPAM) macrocations in contrast to traditional covalent linking. Their assembling behavior was studied below and above the lower critical solution temperature (LCST). The HBPs underwent a morphological transition under changing temperature and ionic strength due to the LCST transition of PNIPAM and the reduction in the ionization degree of terminal ionic groups, respectively. We suggest that, in contrast to traditional branched polymers, ionically linked PNIPAM macrocations can reversibly disassociate from the sulfonate groups and form mobile coronas, endowing the dynamic micellar morphologies. In addition, assembly at the air–water interface confined PNIPAM macrocations and resulted in the formation of heterogeneous Langmuir–Blodgett (LB) monolayers with diverse surface morphologies for different peripheral compositions with circular domains formed in the condensed state. The HBPs with 25% PNIPAM showed larger and more stable circular domains that were partially preserved at high compression than those of HBPs with 50% PNIPAM. Moreover, the LB monolayers showed variable surface mechanical and surface charge distribution, which can be attributed to net dipole redistribution caused by the behavior of mobile PNIPAM macrocations and core sulfonate groups.

Reconfiguration of Langmuir Monolayers of Thermo-Responsive Branched Ionic Polymers with LCST Transition

“…21,25 Additional absorption peaks at 1717−1736 cm −1 were observed due to ester and carboxylate groups. 21,25 Furthermore, the characteristic bands for the PNIPAM amide groups were apparent at ∼1649 and ∼1543 cm −1 for νC�O amide I and δN−H amide II, respectively. 21,26 At higher wavenumbers, bands due to νC−H of aromatic rings at 3075 cm −1 , urethane groups, amide, and ammonium cations at 3100−3700 cm −1 were observed.…”

“…21,26 At higher wavenumbers, bands due to νC−H of aromatic rings at 3075 cm −1 , urethane groups, amide, and ammonium cations at 3100−3700 cm −1 were observed. 21,26 Increasing the PNIPAM macro-cation content led to an increase in the intensity of the PNIPAMassociated peaks such as the amide I and amide II peaks as well as a decrease in the intensity of the C�O ester and carboxylate groups.…”

“…These include bands for νC−H of CH 2 at 2876, 2932, and 2974 cm −1 ; δC−H of CH 2 and δC−H asymmetric of CH 3 at 1457 cm −1 ; δC−H symmetric of CH 3 at 1367 and 1387 cm −1 ; and νC−H of CH 3 at 1090−1315 cm −1 . 21 Peaks associated with νC−O−C bonds (1000−1312 cm −1 ) of ester fragments overlap with the νC−H of CH 3 . 21,25 Additional absorption peaks at 1717−1736 cm −1 were observed due to ester and carboxylate groups.…”

“…21 Peaks associated with νC−O−C bonds (1000−1312 cm −1 ) of ester fragments overlap with the νC−H of CH 3 . 21,25 Additional absorption peaks at 1717−1736 cm −1 were observed due to ester and carboxylate groups. 21,25 Furthermore, the characteristic bands for the PNIPAM amide groups were apparent at ∼1649 and ∼1543 cm −1 for νC�O amide I and δN−H amide II, respectively.…”

“…Indeed, the ionically tethered macro-cation chains were able to desorb and dynamically hop between different anionic sulfonate sites of the polyester cores. 21 Here, we report on the synthesis and assembly of novel amphiphilic-branched polymers with weakly bonded (bond strength of ∼5−10 kcal/mol vs 80−100 kcal/mol for covalent bonds) 21 thermo-responsive terminal ionizable groups with symmetric and asymmetric peripheral hydrophobic/hydro- philic compositions (Scheme 1). Symmetric branched polymers composed of carboxylate polyester cores with 16 hydrophobic n-octadecyl urethane groups and 16 hydrophilic PNIPAM macro-cations and asymmetric polymers composed of the same cores and 8 hydrophobic alkyl chains and 24 hydrophilic PNIPAM macro-cations were synthesized and studied (Scheme 1).…”