Small-Angle Neutron Scattering Investigation of Pressure Influence on the Structure of Weakly Charged Poly(N-isopropylacrylamide) Solutions and Gels

Abstract

Effects of pressure on the phase separation and structure of weakly charged poly(Nisopropylacrylamide-co-acrylic acid) (PNIPA-AAc) solutions and gels with 7 wt % were investigated by small-angle neutron scattering (SANS). The phase diagrams were determined in the pressuretemperature (P-T) plane for the solution and gel with a light scattering method, which were parabolic functions, having maxima at (P ) 38.1 MPa, T ) 37.1 °C; for the solution) and (121.6 MPa, 51.6 °C; for the gel). SANS experiments were carried out according to the P-T phase diagrams. At the low temperature and pressure region, where the systems were in one phase, the scattering intensity functions, I(q)s, were well described with an Ornstein-Zernike function. However, a scattering maximum appeared in I(q)s at elevated temperatures and pressures. The regions where a scattering maximum was observed at T ) 45 °C were P < 70 MPa and 200 MPa < P for the gel and P < 70 MPa for the solution. This scattering maximum is due to microphase separation as a result of antagonism between the electrostatic and hydrophobic interactions. In the case of the solution, an upturn in I(q) at the low q region was observed, indicating occurrence of macrophase separation in addition to microphase separation. For the polymer gels, on the other hand, such an upturn in I(q) was not observed as a result of pinning effect of crosslinks. The phase behavior and critical phenomena of PNIPA-AAc gels and solutions will be discussed from the viewpoint of pressure effects on the hydrophobic interaction.

Pressure‐ and Temperature‐Induced Phase Separation Transition in Homopolymer, Block Copolymer, and Protein in Water

“…The details of the convexity are discussed elsewhere. [10] Similar phase diagrams are also observed in protein solutions and regarded as P-denaturation and T-denaturation, respectively. Hawley regarded such a phase diagram as a part of an elliptical phase diagram and proposed a theory of reversible pressuretemperature denaturation.…”

Small-angle neutron scattering on polymer gels: phase behavior, inhomogeneities and deformation mechanisms

“…We performed a series of high-pressure SANS studies on polymer-aqueous systems to elucidate the pressure dependence of the hydrophobic interactions in PNIPA neutral gels, 17 PNIPA/AAc charged gels, 18,19 hydrophilic-hydrophobic block copolymer aqueous solutions [20][21][22] and globular proteins. 56 Figure 8 shows a pressure cell for SANS (PCI-400-SANS, Teramex, Kyoto, Japan).…”

“…61 We have discussed the reason for the convexity of the phase diagram in previous work. 18,19 According to the Clapeyron-Clausius equation, the derivative of pressure with respect to temperature is…”

“…Third, we review the applications of SANS to harsh environments, that is, phase behavior of polymeric systems under very high hydrostatic pressures. [17][18][19][20][21][22] Here, the roles of hydrophobic interactions in aqueous systems are discussed as a function of pressure. Fourth, the structural characterization of various types of super-tough gels will be shown by focusing on deformation mechanisms.…”

Aggregate formation of cellulose derivative-surfactant in aqueous solutions investigated by enhanced Rayleigh scattering spectroscopy

“…the size of the blobs at polymer chains) in a polymer solution or a gel network is correlated to the scattering light intensity and the scattering vector (q) by the Ornstein-Zernike equation (Collings et al 1999;Kosik et al 2008;Li et al 2010a, b;Nasimova et al 2004).…”