Effects of Ionic Additives and Ionic Comonomers on the Temperature and Pressure Responsive Behavior of Thermoresponsive Polymers in Aqueous Solutions

Abstract

The cloud points of the aqueous solutions of poly(N-vinylisobutyramide) (PNVIBA) and poly(N-isopropylacrylamide) (PNIPAM) were measured down to a subzero temperature under elevated pressures, and the effects of ionic additives and the ionic comonomer were studied. The polymers with higher molecular weight showed lower transition pressure (P t) and smaller ellipsoids, due to larger absolute values of ΔC p and Δβ. Addition of 0.02% sodium dodecyl sulfate (SDS) into aqueous solutions of these polymers apparently extinguished the clouding phenomenon, but even under these conditions the dynamic light scattering measurement showed the chain collapse of the random coil preceding the interchain aggregations. P t and transition temperature (T t) were highly dependent on the addition of inert anions. Less lyotropic ions showed lower P t and T t, producing smaller ellipses as a result, while additions of SCN- and I- caused a slight increase in both P t and T t and therefore larger ellipses. Introduction of ionic comonomer (4-pentenoic acid) into PNIPAM (less than 20%) made the transition P−T curves pH dependent and P t was much more influenced by the charged comonomer than T t. Anionic residues less than 5% were sufficient to screen the cloud point (20 °C and <400 MPa).

Microcalorimetric Study of Aqueous Solution of a Thermoresponsive Polymer, poly(N-vinylisobutyramide) (PNVIBA)

“…Three samples of PNVIBA (M n = 11 kDa and M w /M n = 1.4, M n = 66 kDa and M w /M n = 1.6, and M n = 460 kDa and M w /M n = 2.4) and one sample of PNIPAM (M n = 49 kDa and M w /M n = 1.8) were studied. 12 Sodium dodecyl sulfate (SDS) and other chemical reagents were purchased from Nacalai Tesque (Kyoto, Japan) or Wako Pure Chemicals (Os-aka, Japan).…”

“…The T t observed at 0.04 (w/v)% (41.7 • C) was almost identical with that observed in turbidimetry. 12 With diluting polymer solutions, the endothermic peak shifted slightly to the higher temperature and the peak was 45.0 • C at 0.001 (w/v)%, though the thermogram was rather noisy with such a low concentration of polymer. The thermodynamic parameters calculated from these and other thermograms are compiled in Table I, together with the corresponding results from PNIPAM (M n = 49 kDa).…”

“…[4][5][6] Some of the poly(vinylamine) derivatives, such as poly(N-vinylisobutyramide) (PNVIBA), also show distinct thermoresponsive properties, 7-9 and we have studied the effects of pressure, salt, and surfactants on them, and compared with those of poly(acrylamide) derivatives. [10][11][12][13] PNVIBA and PNIPAM have the side chain amide bonds in inverted directions. Both are isomers of polyleucine and can be considered as a simple but relevant model of proteins.…”

Differences in pressure and temperature transitions of proteins and polymer gels

“…The apparent cloud point of this polymer is known to show an elliptic diagram on the temperature-pressure plane (10,12), and the T-or P-scanning in the present DLS measurements was performed at 25ºC or 0.1 MPa, respectively, up to the point just before the occurrence of clouding and precipitation.…”

“…Several synthetic polymers, including vinyl polymers, are also known to show distinct changes in their molecular level states in solution in response to temperature and pressure, as shown in several examples published by our group (10)(11)(12)(13)(14)(15)(16)(17)(18).…”