pH/Temperature control of interpolymer complexation between poly(acrylic acid) and weak polybases in aqueous solutions

The formation of interpolymer complexes (IPC) between poly(acrylic acid) (PAA) and poly(acrylamide) (PAM), poly(N,N-dimethylacrylamide) (PDMA), and statistical copolymers of acrylamide (AM) and N,N-dimethylacrylamide (DMA) has been studied as a function of pH, salt concentration and temperature (0-70 °C). The cloud points of dilute solutions were measured by turbidimetry and phase diagrams were determined as a function of temperature and pH in pure water and as a function of pH and salt concentration at room temperature. For each temperature and salt concentration a critical pH (pHcrit) below which IPC are observed was defined. In the case of PAA/PAM, pHcrit continuously decreased with increasing temperature, from pH 3.5 at 0 °C to pH 1.9 at 60 °C (UCST-type). In the case of PAA/PDMA, pHcrit, increased with temperature. The LCST-type behavior of the hydrogen-bonding complex formed between PAA and PDMA was attributed to the dimethyl substitution of amide groups that puts in hydrophobic interactions at high temperature. PAA and statistical copolymers P(AM-co-DMA) showed an intermediate behavior between PAA/PAM and PAA/PDMA with a continuous shift from UCST-type to LCST-type with increasing amount of DMA. This behavior can be attributed to changes in configurational entropy due to the IPC formation and (for PDMA) to the release of water molecules initially confined in hydrophobic hydration cages around DMA units. While at low salt concentration, the stability of PAA/PAM and PAA/PDMA complexes only slightly increases with the screening of ionized acrylic units, there is a sharp increase of pHcrit at high salt concentration in relation with the weakening of the solvent quality. In this regime, the complex formation of PAA/PDMA is greatly enhanced compared to PAA/PAM due to the interference of hydrophobic interactions.