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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 H showed no strong end-group dependence.The phase transition of all polymers,except,-di(n-butyl-PNIPAM),exhibited a marked dependence on the polymer molar mass.
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. These 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 (ethylhexyl) (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 (TM), and turbidimetry, providing the cloud point (T CP) of each solution. T CP and TM of the PNIPAM sample of lowest mola r mass (Mn 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, dependence. The phase transition of all polymers, except, -di (n-butyl-PNIPAM), labeled a marked dependence on the polymer molar mass.