Long-range inhomogeneities in sulfonated polystyrene ionomers

Abstract

Two Bonse-Hart ultra-small-angle X-ray scattering (USAXS) cameras were constructed and calibrated by using a polystyrene latex suspension and laser light scattering. The instruments were used to investigate the long-range inhomogeneities in a series of sulfonated polystyrene ionomers. The USAXS measurements were mainly centered in the region of g<0.1 nm-1 (where q is the magnitude of the scattering vector), which is beyond the reach of most conventional small-angle X-ray scattering (SAXS) instruments. All the scattering profiles of ionomers showed a small-angle upturn which was much higher than their parent pure polystyrene. The scattering behavior in this q region was not strongly dependent upon the counterions (except for the intensity change in amplitude), the compression-molding condition, temperature, and annealing. The small-angle upturn decreased with increasing ionic content for zinc sulfonated polystyrene ionomers, and increased with increasing ionic content for rubidium sulfonated polystyrene ionomers. By means of the Debye-Bueche model, an inhomogeneity length of around tens of nanometers was detected in the usual q range accessible by conventional SAXS (0.045-0.12 nm-1). However, the USAXS curves based on the Debye-Bueche model showed a consistent downward curvature in the light-scattering q range. A Guinier plot also failed to show a simple characteristic length. Surprisingly, the upturn could better be described by a power law with a ranging from 2.4 to 3.6 for the ionomers in this study, suggesting that, in the absence of long-range order, the structure could be very polydisperse and irregular. On the basis of USAXS results, the origin of the upturn is discussed. The majority of the upturn could result from chemical compositional inhomogeneities of the ionomers and the processing procedures used in forming the ionomer films for various physical measurements such as SAXS.

Sulfonated polystyrene ionomers containing 4-aminobenzoic acid studied by a small-angle X-ray scattering technique

“…8 Since partly crystalline ionomers have a wide range of application, extensive studies have been devoted to their morphology. [1][2][3]8 The morphology of polystyrene 7,[9][10][11][12][13][14][15][16][17][18][19][20][21][22] ionomers was also investigated widely using the SAXS technique.…”

Ultra-small-angle scattering studies of complex fluids

“…Power law behavior can also be obtained from a power law distribution of polydisperse spheres, rods, or platelets [31]. Thus, the authors suggest that there are inhomogeneities present that are very irregular and highly polydisperse, perhaps ranging from nanometers to microns in size [30]. The low q slope was not dependent on ionic strength, the nature of the cations, or annealing conditions.…”

“…Such systems show an upturn in the scattered intensity at low q in conventional SANS and SAXS studies, indicating some large-scale structure. An early USAXS study performed by Li et al [30] on sulfonated polystyrene shows that the low q scattering follows a power law, with exponents ranging from À2.4 to À3.6. Inasmuch as this range does not make sense physically for either mass fractals or surface scattering, the authors caution against interpreting these numbers as fractal dimensions [30].…”

“…An early USAXS study performed by Li et al [30] on sulfonated polystyrene shows that the low q scattering follows a power law, with exponents ranging from À2.4 to À3.6. Inasmuch as this range does not make sense physically for either mass fractals or surface scattering, the authors caution against interpreting these numbers as fractal dimensions [30]. Power law behavior can also be obtained from a power law distribution of polydisperse spheres, rods, or platelets [31].…”

Dynamic mechanical and morphological studies of styrene-co-methacrylate and sulfonated polystyrene ionomers containing aliphatic dicarboxylate salts

“…The small-angle X-ray scattering (SAXS) profiles of the ionomers containing DCA4 and DCA16 are shown in Figure 6. For the 7.1-MNa system, small angle upturns (SAUT's), probably representing the compositional heterogeneity of the ionomers, [22][23][24][25] are shown at very low angles. The position of the SAUT shifts to higher angles for the ionomers containing DCA's.…”