Thermoresponsive Behaviors Of Thermoresponsive Hyperbranched Polyampholytes

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,which is neutral or positively-charged depending on the media p H.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.FTIR spectra and 1H-NMR characterization proved the successful preparation of HPEI-IBAm-SA.HPEI-IBAm-SA was also a LCST-type thermoresponsive polymer by turbidimetry measurements,which cloud point temperature(Tcp)is pH-dependent too.Compared with the precursor HPEI-IBAm,such hyperbranched polyampholytes showed the thermoresponsive property in a wide pH range.And HPEI-IBAm-SA had the lowest Tcp at isoelectric point.The isoelectric point of HPEI-IBAm-SA could be tuned by controlling the DS of SA units,and the more SA units had a lower isoelectric point.Four halide anions(F-,Cl-,Br-and I-)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.At pH above isoelectric point,the specific ranking of halide anions in reducing the Tcp of HPEI-IBAm-SA in water is I-<Br-<Cl-<F-,and the specific ranking orderings of these cations in reducing the Tcp of HPEI-IBAm-SA is Ca2+ >Mg2+ >Na+≈K+.At pH below isoelectric point,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+.The specific ranking orderings of these ions in reducing the Tcp of HPEI-IBAm-SA were opposite at pH below and above isoelectric point.The influences of these cations and anions on the Tcp of the HPEI-IBAm-SA were similar to their influences on the solubility of protein representative,ovalbumin at pH both below and above isoelectric points,implying that HPEI-IBAm-SA was a better protein model to mimic the interactions among ions and proteins with a different isoelectric point.