Influence of aliphatic acids on the phase transition of thermoresponsive hyperbranched polymer

The influence of aliphatic acids on the phase transition temperature of thermoresponsive hyperbranched polyethylenimine possessing a large amount of isobutyramide and amine groups (HPEI-IBAm) was studied systematically. Nine saturated aliphatic acids including formic acid (C1), acetic acid (C2), n-pentanoic acid (n-C5), trimethylacetic acid (t-C5), hexanoic acid (C6), octanoic acid (C8), decanoic acid (C10), dodecanoic acid (C12) and hexadecanoic acid (C16) were used to measure their effects on the cloud point temperature (Tcₚ) of HPEI-IBAm in a mixture of H₂O/DMF (v/v = 9:1). For comparison, the effect of these aliphatic acids on the traditional thermoresponsive linear poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(N-isopropyl acrylamide) (PNIPAm) was also studied. The influence of aliphatic acids on different thermoresponsive polymers was different. The aliphatic acids with shorter carbon chains (C ≤ 8) increased the Tcₚ of HPEI-IBAm, and those with long hydrocarbon chains (C > 8) depressed the Tcₚ. Moreover, the effect of aliphatic acids with C ≤ 5 was similar, even though their configurations might be different. The specific ranking of aliphatic acids in raising the Tcₚ of HPEI-IBAm was as follows: C1 ～ C2 ～ n-C5 ～ t-C5 > C6 > C8 > C10 > C12 ～ C16. With respect of the linear PDMAEMA, all the aliphatic acids employed elevated its Tcₚ. The specific ranking of aliphatic acids in raising the Tcₚ of linear PDMAEMA was similar to that for dendritic HPEI-IBAm, but with a minor difference: C1 ～ C2 ～ n-C5 ～ t-C5 > C6 > C8 > C10 > C12 > C16. In the case of the linear PNIPAM, the aliphatic acids with C ≤ 6 had almost no influence on the Tcₚ. Raising the carbon number to be 8 or higher leads to the obvious Tcₚ depression. The different effect of aliphatic acids on the phase transition of these thermoresponsive polymers was discussed, and it was mainly attributed to their structural and topological difference.