Thermoresponsive Interpolyelectrolyte Complexation: Application to Macromolecular Assemblies

Abstract

pH and thermoresponsive polymers have been prepared by copolymerizing N-isopropylacrylamide (NIPAM) with various amounts of ionizable comonomers, either acrylic acid (AA) or N-[3-(dimethylamino)propyl]methacrylamide (MADAP). In aqueous solution, the LCST-type phase transition of these copolymers studied by differential scanning calorimetry is strongly influenced by the comonomer ratio. Under un-ionized conditions, the phase transition temperature progressively increases with MADAP content at pH 12 while it remains unchanged or slightly decreases with AA at pH 3 due to the formation of hydrogen bonds between AA and NIPAM units. When the copolymer chains are progressively charged by tuning the pH, the phase transition of PNIPAM-AA and PNIPAM-MADAP is shifted at higher temperature and is no longer observable below 60 °C when the ionic content exceeds 10%. By comparison with these single systems, where the association properties can be finely adjusted by coupling hydrophobic attractions and electrostatic repulsions, we also investigate the possibility to couple hydrophobic interactions with electrostatic attractions by mixing oppositely charged copolymers: PNIPAM-AA and PNIPAM-MADAP. This study was carried out with the copolymer pair containing 10 mol % of ionizable groups (A10 and M10) which was the most adaptable one from the point of view of responsivity. At pH 7, when AA and MADAP units are ionized, the copolymer chains are separately soluble in water in the whole temperature range while their mixture, also soluble at room temperature, phase separates upon heating. The original feature highlighted in this work is that the phase transition proceeds through a selective mechanism between complementary chains (formation of a reversible interpolyelectrolyte complex) and that this selectivity can be switched with the pH. Indeed, starting at room temperature with the copolymer mixture (A10 and M10), which remains homogeneous at all pH, we demonstrate that the association process can be switched by increasing the temperature and that the pH can be used to specifically address these associations: from A10/A10 at low pH to A10/M10 at pH 7 up to M10/M10 at high pH. Finally, the responsive precursors A10 were grafted onto a polyacrylamide backbone, and the viscoelastic properties of graft copolymers were studied in the semidilute regime and compared with calorimetric data. In this way, we show that the same set of interactions can be readily applied to more sophisticated macromolecular assemblies with the responsive formation of physical gels under pH and temperature control.

Study of the temperature-induced aggregation of polyoxometalate-poly(N,N-diethylacrylamide) hybrids in water

“…37,38 A partial dehydration of thermoresponsive monomer units during the transition has also been considered in the presence of vicinal hydrophilic or charged monomers. 40 Similar to these studies, an increased overall hydrophilicity due to the presence of the hydrophilic ammonium-surrounded POM units might be a reason for the observed low ∆H values of our NH 4 + hybrids.…”

Influence of topology of LCST-based graft copolymers on responsive assembling in aqueous media

“…Quite a lot of systems, mainly based on poly(N-alkylacrylamide) and poly(alkylene oxide) have been reported with various level of understanding of their structure/properties relationships [17][18][19][20][21][22][23]. For these copolymers the key parameters are the chemical nature of both LCST and hydrophilic sequences as well as the design of associating copolymers (distribution and relative ratio between hydrophilic and "hydrophobic" moieties) that directly M A N U S C R I P T…”

Dual Responsive Regulation of Host–Guest Complexation in Aqueous Media to Control Partial Release of the Host

“…Hence, in this work we aim to design new supramolecular polymerich ost-guest systemsf eaturing as ynergistic and switchable coil-to-globule transition in water with temperature and pH responsiveness based on ac opolymer (DNP-P)o fNisopropylacrylamide( NIPAm) and acrylic acid (AA) with am olar ratio around 90:10;t his composition was previously shownt o give the best synergyb etween pH and temperature leading to ap rogressive shift of the T c at pH values around the pK a of polyAA (pK a = 4.5). [14][15] Compared to our previously reported DNP based PNIPAm:BBox systems where all host subunits were released in aqueousm edia upon heatinga bove T c ,i nt he current work it is hypothesized to gain control over partial release of the hostf rom the polymer chain, resulting from partial collapse of the polymer in response to changes in pH and tem-perature.T he possibility to control supramolecular interactions either at ag iven temperature by tuning the pH or at ag iven pH by changing the temperature would greatlye xpand applications of such as upramolecular system in numerous fields including biological and multifunctional smart polymeric materials. Here, an ew class of supramolecular thermo-and pH-chromic polymericm aterialh as been developed andanew supramolecular communication concept allowing the controlled release of ac omplexed guest molecule at physiologically relevant temperaturea nd pH is proposed.…”