Please use this identifier to cite or link to this item:
Title: Double Critical Phenomena in (Water + Polyacrylamides) Solutions
Authors: Rebelo, L. P. N. 
Visak, Z. P. 
Sousa, H. C. de 
Szydlowski, J. 
Azevedo, R. Gomes de 
Ramos, A. M. 
Najdanovic-Visak, V. 
Ponte, M. Nunes da 
Klein, J. 
Issue Date: 26-Feb-2002
Publisher: American Chemical Society
Citation: Macromolecules. 35:5 (2002) 1887-1895
Abstract: Aqueous solutions of a copolymer derivative of a polyacrylamide showed very interesting behavior, that in which the system evolves from one kind of double criticality (pressure−hypercritical point) to another (temperature−hypercritical point) as polymer molecular weight decreases. While in the neighboring region of the former point one expects a change from contraction to expansion upon mixing with increasing pressure; in the latter, mixing should be accompanied by a change in the sign of the excess enthalpy as temperature increases. L−L equilibria studies were performed in a wide range of (T, p) experimental conditions (300 < T/K < 460, 0 < p/bar < 700). Poly(N-isopropylacrylamide), usually called PNIPAAM, and its copolymer derivative poly(N-isopropylacrylamide/1-deoxy-1-methacrylamido-d-glucitol), herein referred to as CP, were investigated for several chain lengths and compositions. An He/Ne laser light scattering technique was used for the determination of cloud-point (T, p, x) conditions. The experimental results were used to assist in the determination of computed values at temperatures beyond experimental accessibility, which are obtained by the application of a modified Flory−Huggins model. The model also estimates the excess properties of these solutions. Because of the intrinsic self-associating nature of these systems, all studied solutions show a lower critical solution temperature (LCST). Both modeling results and H/D isotope substitution effects suggest also the existence of upper critical solution temperatures (UCST) and therefore closed-loop-type phase diagrams. However, these upper-temperature branches are experimentally inaccessible. Pressure effects are particularly interesting. For a low-MW CP, experimental data display a tendency toward a reentrant T−p locus, which supports the conjecture that these systems are inherently of the closed-loop type. In the cases of PNIPAAMs and high-MW CPs, the T−p isopleths show extrema. The copolymer aqueous solutions under study in this work model a single chemical system where pressure−hypercritical behavior evolves to a temperature−hypercritical one as the chain length decreases.
ISSN: 0024-9297
DOI: 10.1021/ma011533a
Rights: openAccess
Appears in Collections:FCTUC Eng.Química - Artigos em Revistas Internacionais

Files in This Item:
File Description SizeFormat
Double Critical Phenomena in (Water + Polyacrylamides).pdf135.91 kBAdobe PDFView/Open
Show full item record


checked on May 20, 2024

Citations 1

checked on May 2, 2024

Page view(s) 50

checked on May 21, 2024


checked on May 21, 2024

Google ScholarTM




Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.