A novel thermothickening phenomenon exhibited by a triblock polyampholyte in aqueous salt-free solutions

A novel thermothickening phenomenon exhibited by a water-soluble triblock copolymer in salt-free aqueous solutions has been investigated through rheological measurements and supported by dynamic light scattering. The copolymer is constituted of a long central poly(2-vinylpyridine) block endcapped by two shorter poly(acrylic acid) blocks, (PAA(135)-P2VP(628)-PAA(135)). At pH 3.4 of interest, the copolymer behaves as polyampholyte, bearing positively charged protonated 2VP and negatively charged AA moieties. In aqueous solutions where a physical gel is formed, rheological investigation showed a pronounced thermothickening behavior upon heating to 35 degrees C, followed by a weak zero-shear viscosity decrease. This unexpected temperature dependence has been interpreted by considering a competition between two antagonistic effects: (i) a remarkable swelling of the macromolecular chain upon heating, mainly due to the excluded-volume effect of the outer PAA blocks, that favors intermolecular interactions between oppositely charged blocks responsible for physical gelation and (ii) the thermal motion of molecules which speeds up the molecular dynamics and tends to weaken the rheological properties. The effect of the macromolecular swelling prevails at low temperatures while the influence of the thermal motion increases continually and predominates at high temperatures.