Effect of Urea on Phase Transition of Poly(N-isopropylacrylamide) Investigated by Differential Scanning Calorimetry

Abstract

The effect of urea on the phase transition of PNIPAM was studied using differential scanning calorimetry (DSC). For a certain urea concentration, the enthalpy change of phase transition of poly(N-isopropylacrylamide) (PNIPAM) aqueous solution increases with the number of DSC cycles, presumably due to the displacement of water molecules bound to the amide groups of PNIPAM by urea molecules at the temperature higher than the lower critical solution temperature (LCST) of PNIPAM and causes the decrease in the absolute value of the exothermic heat related to the dehydration of hydrophilic groups and interactions of hydrophilic residues to around 0. Moreover, the enthalpy change decreases with the urea concentration during the heating process of the first DSC cycle, indicating the replacement of water molecules around the apolar isopropyl groups by urea molecules at the temperature lower than LCST, and the endothermic heat caused by the dehydration of apolar groups decreases. Furthermore, the urea molecules which replace the water molecules at high temperature can be replaced again by water molecules at the temperature lower than LCST, but this process needs several days to complete.

Quartz crystal microbalance electrode modified with thermoresponsive crosslinked and non-crosslinked N-isopropylacrylamide polymers. Response to changes in temperature

“…Temperature of the volume phase transition appeared to be different in the heating and the cooling processes. Similar effect was observed in the investigation of the pNIPA volume phase transition by, e.g., differential scanning calorimetry [32]. It is presumably due to the formation of intra-and interchain hydrogen bonds in the collapsed state.…”

A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST

“…[19][20][21][22][23][24][25][26][27][28][38][39][40][41][42][43][44][45] Figure 4A shows the temperature-dependent transmittance of PNASME at different polymer concentration. It indicates that the LCST-type soluble-to-insoluble phase transition of the diluted PNASME 170 solution (0.10-0.20 wt%) occurs within a broad temperature range, whereas with the polymer concentration increasing above 0.30 wt% a very sharp soluble-to-insoluble phase transition takes place within a narrow temperature window of ~1 o C. This sharp soluble-toinsoluble phase transition of PNASME suggests that the PNASME is a qualified candidate of thermoresponsive sensors.…”

“…Poly(N-isopropylacrylamide) (PNIPAM) exhibiting an LCST at 32 o C, is the most widely studied secondary amide-based polyacrylamides (see the structure in Scheme 1). [19][20][21][22][23][24][25][26][27][28] High-frequency dielectric relaxation technique has shown that the water molecules could form hydrogen bonding with the secondary amide group of PNIPAM or other water molecules, producing hydrogen-bond bridges thus hydrating PNIPAM. 20 When the temperature was above LCST, the PNIPAM precipitated from the solution as a result of the hydrogen-bond bridges being broken.…”

“…21, 22 Ye et al employed differential scanning calorimetry (DSC) to study the effect of urea on the LCST of PNIPAM, and a similar result also was reported. 23 Poly(N,N-diethylacrylamide) (PDEAM), another thermoresponsive N-alkyl-substituted polyacrylamides containing a tertiary amide in the pendant (Scheme 1), exhibits an LCST at 35 o C. [38][39][40][41][42][43][44][45] Both static/dynamic light scattering and high-resolution magic angle spinning (HRMAS) NMR spectroscopy have revealed the tertiary amide groups of PDEAM still have the ability to interact with water or urea via hydrogen bonding by accepting the proton from them. 44,45 As such, all these results indicate that the amide groups, no matter the secondary amide group or the tertiary amide group, are the crucial component for the thermoresponse of polyacrylamides, and can be powerful tools for designing new thermoresponsive polymers.…”

“…As for the tertiary amide-based polyacrylamides, it is more difficult to equip the polymers with thermoresponse than the secondary amide-based polyacrylamides due to the complexity to achieve hydrophobic-hydrophilic balance. As a result, one can easily prepare thermoresponsive polymers of the secondary amide polyacrylamides, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] such as poly(N-methacryloyl-L -alanine methyl ester), 33 poly(N-acryloylglycine methyl ester) 34 , poly(N-acryloyl-Lproline methyl ester), 35 poly(N-acryloyl-L -phenylalanine methyl ester) 36 and poly(N-cyclopropylacrylamide), 37 In this work, we design and synthesize a thermoresponsive polymer, poly(N-acryloylsarcosine methyl ester) (PNASME, Scheme 1) with a tertiary amide group in the pendant. And the hydrophobic ester group was employed to counteract the hydrophilic tertiary amide group, making PNASME exhibiting a LCST-type phase transition behavior.…”