Study on the phase transition behaviors of thermoresponsive hyperbranched polyampholytes in water

Abstract

The amino groups of hyperbranched polyethylenimine (HPEI) were partially modified with isobutyric anhydride to introduce isobutyramide (IBAm) groups to HPEI, resulting in an LCST-type thermoresponsive HPEI-IBAm that is positively-charged or neutral depending on the media pH. Succinic acid (SA) units were further introduced to HPEI-IBAm through acylation with succinic anhydride, resulting in the hyperbranched polyampholyte, HPEI-IBAm-SA, with both amino and carboxylic acid groups. 1H NMR characterization proved the successful preparation of HPEI-IBAm-SA. Turbidimetry measurements showed that HPEI-IBAm-SA was also an LCST-type thermoresponsive polymer. The cloud point temperature (Tcp) of HPEI-IBAm-SA was pH-dependent. Compared with the HPEI-IBAm precursor, HPEI-IBAm-SA showed the thermoresponsive property in a wider pH range. At isoelectric point, HPEI-IBAm-SA had the lowest Tcp. The isoelectric point of HPEI-IBAm-SA could be tuned by controlling the amount of SA units, and the one with more SA units had a lower isoelectric point. Four halide anions with sodium as the counterion and four cations (Na+, K+, Mg2+ and Ca2+) with chloride as the counterion were used to measure the salt effects on the Tcp of HPEI-IBAm-SA. The specific ranking orderings of these ions in reducing the Tcp of HPEI-IBAm-SA were opposite at pH below and above isoelectric point. At pH above isoelectric point, the anion ranking is I−<Br−<Cl−<F−, and the cation ranking is Ca2+ > Mg2+ > Na+ ≈ K+. At pH below isoelectric point, and, the anion ranking is F−∼Cl−<Br−<I− in the low-salt-concentration region (≤10−2 M), while the cation ranking is Na+ ≈ K+ > Mg2+ ≈ Ca2+. All the specific salt ordering found in HPEI-IBAm-SA system could be also found in the system of the protein representative, ovalbumin, implying that HPEI-IBAm-SA was a better protein model to mimic the interactions among ions and proteins with a different isoelectric point.