New insights into the effects of molecular weight and end group on the temperature-induced phase transition of poly(N-isopropylacrylamide) in water

In an attempt to clarify issues related to the molecular weight dependence of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in water, we prepared a library of PNIPAM samples of well-controlled molecular weight (7000 to 45000 g/mol) bearing identical groups on each chain end. The polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) with a bifunctional chain tranfer agent and further end group modification. The effects of the end group chemical structure, hydroxyethyl (HE), propargyl (Pr), chloroethyl (CE), n-butyl (nBu), n-hexyl (nHe), and isobutylsulfanylthiosulfanyl (IBS) on the phase transition temperature of aqueous PNIPAM solutions were investigated by high-sensitivity differential scanning calorimetry (HS-DSC), yielding the enthalpy ΔH and the endotherm maximum temperature (T _M), and turbidimetry, providing the cloud point (T _CP) of each solution. The T _CP and T _M of the PNIPAM sample of lowest molar mass (M _n 7,000 g/mol, 0.5 g/L) ranged, respectively, from 38.8 to 22.5 °C and 42.2 to 26.0 °C, depending on the structure of the end-group, whereas ΔH showed no strong end-group dependence. The phase transition of all polymers, except α,ω-di(n-butyl-PNIPAM), exhibited a marked dependence on the polymer molar mass.