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

摘要

Effects of pressure on the phase separation and structure of weakly charged poly(N-isopropylacrylamide-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 pressure-temperature (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 deg C;for the solution) and (121.6 MPa,51.6 deg 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 deg 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.