Adjacent UCST Phase Behavior in Aqueous Solutions of Poly(vinyl methyl ether): Detection of a Narrow Low Temperature UCST in the Lower Concentration Range

Abstract

We provide the first experimental evidence for the existence of a low concentration narrow UCST miscibility gap, which is the second of two narrow miscibility gaps constituting together adjacent UCST phase behavior. The existence of adjacent UCST miscibility phase behavior in aqueous polymer solutions has been theoretically predicted, but so far escaped experimental verification. We have been able to determine the low concentration narrow UCST miscibility gaps from the changes in apparent heat capacity observed in stepwise isothermal cooling modulated temperature differential scanning calorimetry (MTDSC) experiments. In wide-angle X-ray diffraction (WAXD) experiments it was unambiguously demonstrated that the MTDSC heat capacity changes are not due to crystallization. Together with earlier experimental results on the high concentration narrow UCST miscibility gap, the bimodal LCST miscibility gap and the sigmoidal melting line of ice the experimental results reported in this work complete the full experimental verification of the predicted complex phase behavior of aqueous solutions of PVME. As far as the authors are aware, this is the first system where adjacent UCST phase behavior is found. On the basis of the theoretical predictions the observed phase behavior could be more general and other systems may exist where similar behavior can be observed.

How to manipulate the upper critical solution temperature (UCST)?

“…Under these conditions, when the concentration reaches the overlapping concentration, intramolecular bonds are replaced by intermolecular bonding and the polymer redissolves. 29 Finally, it should be noted that there are several reports on poly(ethylene oxide), 3,35,36 poly(vinyl methyl ether)s, 37,38 polyvinylalcohols, 39 and poly(hydroxymethylacrylate)s 40 expressing UCST behavior. However, in most of the cases, the cloud points are either below 0°C or close to and above 100°C.…”

Doubly thermo-responsive ABC triblock copolymer nanoparticles prepared through dispersion RAFT polymerization

“…When the temperature increases above the LCST, a soluble-toinsoluble phase transition occurs and the polymer becomes insoluble. The second type is the thermo-responsive polymer showing an insoluble-to-soluble phase transition at the upper critical solution temperature (UCST), [15][16][17][18][19] in which the thermo-responsive behavior is different to that of the LCST-type. When two or more thermo-responsive polymeric segments are included in a single polymer chain, multi-thermo-responsive copolymers showing multiple LCSTs, UCSTs or one segment presenting the LCST whereas the other presents the UCST are obtained.…”

Thermo-responsive ABA triblock copolymer of PVEA-b-PNIPAM-b-PVEA showing solvent-tunable LCST in a methanol–water mixture

“…In recent years, thermo-responsive polymers have aroused much interest because of their wide applications including drug delivery and smart materials. 1,2 Generally, two kinds of thermoresponsive polymers, one of which shows soluble-to-insoluble phase transition at the lower critical solution temperature (LCST) [3][4][5][6][7] and the other shows insoluble-to-soluble phase transition at the upper critical solution temperature (UCST), [8][9][10] are classied. Of all the LCST-type thermo-responsive polymers, [3][4][5][6][7] poly(N-isopropylacrylamide) (PNIPAM) exhibiting LCST around 32 C in water has been one of the most widely studied thermoresponsive polymers, [11][12][13][14][15][16][17][18][19][20][21][22][23][24] since its LCST is close to the body temperature and thus it is suitable for many biological applications.…”